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Tectonic setting of the northern Okanagan Valley at Mara Lake, British Columbia Nielsen, Kent Christopher 1978

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TECTONIC SETTING OF THE NORTHERN OKAMAGAN VALLEY AT MARA LAKE, BRITISH COLUMBIA BY Kent Chr istopher Nielsen B . S c , Un i ve r s i t y of North Ca ro l i na , 1968 M.S., Un i ve r s i t y of North Ca ro l i na , 1972 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF ' THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY in THE FACULTY OF GRADUATE STUDIES (The Department of Geological Sciences) We accept th i s thes i s as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA March, 1978 e Kent Chr istopher N ie l sen . 1978 In presenting this thesis in partial fulfilment of the requirements for an advanced degree at the University of British Columbia, I agree that the Library shall make it freely available for reference and study. I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the Head of my Department or by his representatives. It is understood that copying or publication of this thesis for financial gain shall not be allowed without my written permission. Department of Geological Sciences The University of British Columbia 2075 Wesbrook Place Vancouver, Canada V6T 1W5 Date March 25, 1978 i i ABSTRACT Mara Lake, B r i t i s h Columbia, s t raddles the boundary between the Monashee Group on the east and the Mount Ida Group on the west. Both groups of rock have experienced four phases of deformation. Phases one and two are t i gh t and recumbent, trending to the north and to the west r e spec t i ve l y . Phases three and four are open to c lose and upr ight , trending northwest and northeast r e spec t i ve l y . Second phase deformation includes large sca le tec ton ic s l i de s which separate limbs of major f o l d s . These s l i d e surfaces are fo lded by t h i r d and fourth phase s t ructures and ou t l i ne domal outcrop patterns. Peak metamorphism accompanied and fol lowed phase two. Metamorphic grade i s re l a ted to pos i t i on wi th in the second phase s t ruc tu re , increas ing downward from greenschist to amphibol ite f a c i e s . Greenschist condit ions accompanied phase three whi le hydrothermal a l t e r a t i o n character izes phase four . B r i t t l e f r a c t u r i n g and loca l f a u l t i n g along a northeaster ly trend fol lowed phase four . Abrupt changes i n metamorphic grade found at the northern end of Mara Lake are re l a ted to these l a t e f a u l t s . Cor-r e l a t i o n of l i t h o l o g i e s across the southern end of Mara Lake and the s i m i l a r s t ruc tu ra l sequences i nd i ca te that no s t r a t i g r aph i c or s t ruc tura l d i s t i n c t i o n i s necessary between the Mount Ida Group and the Monashee Group. On a regional sca le s i m i l a r s t ruc tura l sequences are observed in other areas of the Shuswap Metamorphic Complex. Microscopic deformation features are common in many mineral phases in the Mara Lake area. Amphibole r a re l y shows evidence of p l a s t i c deformation. To examine th i s apparent high strength c h a r a c t e r i s t i c , f i f t y samples of hornblendite (AM-2) were deformed in a l a rge , s o l i d -medium Griggs-type apparatus at 700° to 1000°C at s t r a i n rates from - 4 - 6 10 /sec to 10 /sec and at 10 kb conf in ing pressure. T a l c , pyrophyl l i t e , and platinum jacket ing were used to yary water content. From 700° to 850°C both mechanical twins (toi) and t r an s l a t i on g l i de (100) were observed. Twin development appears to be favored over g l i de at higher conf in ing pressures, lower temperatures, and higher s t r a i n ra te . Above 850°C subgrain development and r e c r y s t a l l i z a t i o n occur j u s t p r i o r to melt ing. A flow law, i = % 1.5 x 10" 1 exp ( - 3 8 / R T)a 4 - 8 descr ibes steady s tate deformation from 750° to 910°C under wet con-d i t i o n s . Decreasing water and temperature are accompanied by increas ing ri values and perhaps increas ing a c t i v a t i o n energy. At 750°C unde dry condit ions an exponential r e l a t i o n s h i p , e = 53 exp (.23 a) best f i t s the data. From 910° to 950°C the amphibole s t ructure "hardens" such that s t r a i n rate remains constant f o r a given load. This hardening i s in terpreted to be re l a ted to ox idat ion and d i s -t o r t i o n with in the l a t t i c e . Uncerta inty regarding the a c t i v a t i o n energy precludes e f f e c t i v e ext rapo la t ion of the data to "geo log ic " s t r a i n ra tes . A tenta t i ve comparison of amphibole and quartz data reveals an order of magnitude d i f f e rence i n flow s t r e s s , suggesting that quartz w i l l y i e l d p l a s t i c a l l y before amphibole. TABLE OF CONTENTS Page GENERAL INTRODUCTION 1 STRUCTURAL AND METAMORPHIC RELATIONSHIPS BETWEEN THE MONASHEE AND MOUNT IDA GROUPS NEAR MARA LAKE, BRITISH COLUMBIA 3 ABSTRACT 3 INTRODUCTION 5 LITHOLOGIC UNITS 10 GENERAL DISTRIBUTION 10 SILVER CREEK FORMATION 12 Unit I 12 TSALKOM FORMATION 13 Unit II 13 Unit III 14 EAGLE BAY FORMATION 15 Unit IV •••••• 1 5 SICAMOUS FORMATION 15 Unit V 15 Unit VI 16 Unit VII 16 INTRUSIVE UNITS 16 STRUCTURAL EVOLUTION 18 GENERAL SEQUENCE 18 STRUCTURAL MAP 20 DOMAIN ANALYSIS 22 F i r s t Phase 22 Second Phase 28 Third Phase 34 Fourth Phase 45 F i f t h Phase 53 STRUCTURAL SUMMARY 53 METAMORPHISM • 57 DISCUSSION 69 MARA LAKE AREA 69 REGIONAL TRENDS 74 DEFORMATION MECHANISMS AND FLOW LAW EQUATIONS FOR CALCIC AMPHIBOLE , 84 ABSTRACT 84 INTRODUCTION 85 PART I: CHEMISTRY, STRENGTH AND DEFORMATION MECHANISMS . . . 89 STARTING MATERIAL 89 EXPERIMENTAL APPARATUS 92 STRENGTH CHARACTERISTICS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93 DEFORMATION FEATURES 97 Mineral Or ientat ion As a Function of Nominal S t ra in . . . . . . . . . . . 97 Deformation Mechanisms . . 100 Brecc ia t ion 100 Sharp Kink Bands and Undulose Ex t inc t ion . . . 101 Subgrains . . 114 R e c r y s t a l l i z a t i o n 116 PART II; FLOW EQUATION 121 THEORY 121 CONSTANT STRAIN RATE EXPERIMENTS 122 RELAXATION EXPERIMENTS 124 CONSTANT LOAD EXPERIMENTS 126 DISCUSSION . 134 CONCLUSIONS 141 BIBLIOGRAPHY 143 vi LIST OF FIGURES Figure Page 1 Area of Study . . . . . 6 2 General Geology and S t r a t i graphic Corre la t ions 8 3 D i s t r i bu t i on and Cor re la t ion of L i tho log ies at Mara Lake, B.C. 11 4 Complexly Folded Aplit.es in Unit II 17 5 Pegmatites Associated with Second Phase in Unit II . . 17 6 F i r s t Phase Folding at Mara Lake, B.C 23 7 F i r s t Phase. Fold Elements along East Side of Black Point 26 8 F i r s t Phase Fold Elements from Mara Lake Area, P r i n c i p a l l y Domain IX 27 9 Second Phase Folding at Mara Lake, B.C 29 10 Second Phase Fold Elements and Geologic Map from Domain X 32 11 Second Phase Fold Elements from Domain VI and VII Showing E f fec t of Th i rd Phase Deformation 33 12 Th i rd Phase Folding at Mara Lake, B.C 36 13 Th i rd Phase Fold Elements from North and West of Mara Lake, B.C. 39 14 Geologic Map and Th i rd Phase Cleavage Development in Domain I 40 15 Deformation of Second Phase F o l i a t i o n about Th i rd Phase Axes 42 16 Deformation of Second Phase L ineat ions West and East of Mara Lake, B.C 43 17 Fourth Phase Folding at Mara Lake, B.C 46 18 Fourth Phase Folding of Compositional Layering in High Grade Rocks near Mara Lake, B.C .48 19 Fourth Phase Folding of Compositional Layering in Low Grade Rocks West of Mara Lake, B.C. 49 20 Fourth Phase Deformation of E a r l i e r Cleavages 51 21 A Deformation of Th i rd Phase L ineat ions 52 B " F i f t h " Phase Elements 52 22 Sketch of Proposed Deformational Sequence 54 23 Sample L o c a l i t i e s and Primary Mineral Assemblages fo r P e l i t i c Samples 58 vi i 24 Sample L o c a l i t i e s and Primary Mineral Assemblages f o r Amphibolites and Other Mafic Units 61 25 Sample L o c a l i t i e s and Primary Mineral Assemblages f o r C a l c - S i l i c a t e Units 64 26 Univar iant Reaction Curves fo r Mineral Assemblages at Mara Lake, B.C 67 27 Second Phase Fold Trends of Fyson (1970) and the "Older" Deformation of Jones (1959) as Compared to Mara Lake Area 75 28 Area! D i s t r i bu t i on of T r i a s s i c Units West df Okanagan Va l ley and The i r Relat ion to Th i rd Phase Fold Elements . 77 29 D i s t r i bu t i on of T r i a s s i c Units from Shuswap Lakes to Kootenay Lake, B.C 79 30 Ideal ized Amphibole Structure and Crys ta l l og raph ic Elements 86 31 Composite of Dehydration Reactions f o r Py rophy l l i t e and Ta lc 94 32 Composite of S t re s s - S t r a in Curves fo r Various S t ra in Rates 94 33 Optic Axis Or ientat ion as a Function of Percent S t ra in 99 34 Brecc ia t ion of Large Grains with Segments Approximately Normal to C_ Crysta l lographic Axis 102 35 Occurrence of Broad Warping and Sharp Kinks in the Same Grain 102 36 Sharp Kink Development at 70° to P r inc ipa l Compressive Stress 106 37 Lamellae Development Associated with Undulose Ex t inc t ion 106 38 Lamellae and Mechanical Twin Development in a S ingle Grain 108 39 (100) Growth Twin at 45° to a Uniaxia l Compressive Stress 109 40 The poles of the "Sharp" Kink Bands C lus ter about the [101] D i rec t ion f o r A l l Experimental Conditions I l l 41 Changes in S l i p Mechanisms Associated with Temperature and S t ra in Rate 112 42 S l i p System (100) [010] 113 vi i i 43 Composite of Constant S t ra in Rate Experiments in Relat ion to Temperature, S t ra in Rate, and Percent S t ra in . . . . . . . . . . . . 117 44 A R e c r y s t a l l i z a t i o n of Amphibole along Grain and Subgrain Boundaries 118 B Fine Linear "Bubble" Trains Contained in the (101) Plane 118 45 Color Change Associated with Temperature 120 46 Strength Data at 5% S t ra in P lot ted to Test Power Law Relat ionship 123 47 High Temperature Data from Figure 46 F i t by a Three Var iab le L inear Regression 123 48 Four Relaxation Runs Showing the Change i'rvn_ Values as a Function of Temperature 125 49 Fourteen Relaxation Runs Relat ing n to Temperature 125 50 Va r i a t i on of Q with Temperature from Six Constant Load Experiments 128 51 Three kb Constant Load Data Showing the Small Change S t ra in Rate Associated with 950 C 129 52 Comparison of Ca lcu lated and Experimental Strength Values 131 53 Results of Regression on 5 kb Creep Data and Strength Data f o r the Temperature Interval 900° to l O O O Y 131 54 Discrepancy between Isotherms of Figure 53 and Data from Creep Runs Less than 3 kb 133 55 Thermal Expansion of Tremol i te a f t e r Sueno, et a l . , 1973 136 56 Ca l cu la t ion of Strength Values at "Geologic S t ra in Rates" 140 ix LIST OF TABLES Table I C h a r a c t e r i s t i c Deformational Fabrics Associated . 19 II Comparison o f Composition of Experimental ly 91 III , 96 IV Development of Mechanical Twins as a Function of 104 V C r i t i c a l Resolved Shear Stress fo r Twinning and ...115 LIST OF PLATES Plate T Geology of the Mara Lake Area, B.C (-PoGk-et-) II Tectonic Map of Mara Lake Area, B.C (-PoGk-et-)- [ III V e r t i c a l Cross Sections of Mara Lake Area, B.C. . . . -(-Poeket) IV S t ructura l Analyses near the Western Margin (Wo? of the Shuswap Terra in (-Roc-k-et) X ACKNOWLEDGEMENTS Many people have contr ibuted to the progress of th i s study. Dr. J . V. Ross, as p r i nc i pa l superv i sor , suggested the i n i t i a l problem and a d i sconcer t ing array of ideas. Dr. W. C. Barnes and Mr. I. J . Duncan were of immeasurable he lp, prov id ing wel l - t imed support and wide-ranging background informat ion. Dr. G. L. S t i rewa l t introduced the author to the operat ion of the Griggs-type apparatus. Drs. R. L. Armstrong, H. J . Greenwood, and E. P. Meagher c r i t i c a l l y read the manuscript as well as introducing the author to many new concepts. Very personal thanks i s extended to Dr. M. A. Barnes, Ms. M. E l l i o t t , Mr. P. King, Mr. S. Hyde and a l l o f the s t a f f in the Department of Geoscience, Un iver s i t y of B r i t i s h Columbia, each of whom has helped more than they r e a l i z e . F inanc ia l support from the National Research Council o f Canada (Grant # A-2134)) and from the Geological Society of America (Penrose Grant # 1935-74)• is g r a t e f u l l y acknowledged. 1 GENERAL INTRODUCTION This study was designed to re l a te experimental rock deformation to bas ic s t ruc tura l f i e l d research. The development of the Griggs-type hot creep apparatus has provided a r e l a t i v e l y easy, q u a l i t a t i v e tool fo r examining rock-forming minerals at high temperatures and pressures f o r long periods of time (Griggs, 1967). By experimental ly approximating the condit ions envis ioned f o r various metamorphic t e r -r a i n s , deformation mechanisms, flow c h a r a c t e r i s t i c s , and the re su l t i n g deformation f abr i c s can be studied and compared to those observed in the f i e l d . This comparison provides greater understanding of the deformational environment. As the techniques evo lve, quant i t a t i ve values of s t r e s s , s t r a i n r a te , and temperature w i l l be assigned to deformed rocks. The polydeformed and metamorphosed rocks of the Shuswap Meta-morphic Complex were se lected f o r th i s study. Located in south-cen-t r a l B r i t i s h Columbia, these high grade rocks form the southern c r y s -t a l l i n e core of the Eastern C o r d i l l e r a Fold Bel t (Campbell, 1973) (F i g . 1) and extend from south of the Internat ional Border to approx i -mately 350 km north. The sch i s t s and gneisses of the Shuswap Complex are of uncertain age, even though complexly deformed Paleozoic units o f the Kootenay Arc to the east can be traced into the Shuswap Complex (Ross, 1970; Fy le s , 1970a). The western margin of the Shuswap Complex genera l ly corresponds to the present Okanagan Va l l ey . Several workers have provided a s t ruc tu ra l sequence f o r the southern port ion of th i s va l l ey (Ross, 1973; C h r i s t i e , 1973; Ryan, 1973; Ross, 1974; Medford, 1976; So lberg, 1976). As an extension to th i s 2 developing regional p i c t u r e , an area at the northern end of the Okanagan Va l ley bordering Mara Lake and near Sicamous, B r i t i s h Columbia, was chosen. Within the area , the boundary between the Monashee Group and the lower grade Mount Ida Group i s exposed (Jones, 1959). The nature of th i s boundary, the s t ruc tu ra l sequence, and the metamorphic condi t ions are the subject of the f i e l d work. Experimental deformation studies on common crus ta l minerals by previous workers have inc luded p r imar i l y quartz and c a l c i t e with add i -t i ona l work on p l a g i oc l a se , b i o t i t e , and amphibole. Because both the Mount Ida Group and the Monashee Group contain large amphibol i tes, a ser ies of experiments on amphibole was conducted to amplify the e x i s t -ing knowledge. Amphibole i s p a r t i c u l a r l y i n te re s t i n g i n that even in h igh ly deformed environments, the grains seldom show p l a s t i c de fo r -mation features (Carter and Rale igh, 1969). This apparent high r e l a t i v e strength was the subject o f i n v e s t i g a t i o n . The two approaches of eva luat ing the condit ions during defor -mation require d i f f e r e n t techniques and y i e l d d i f f e r e n t data; there -fore the presentat ion w i l l be in two par t s . F i r s t , the s t ruc tu ra l and metamorphic set t ings of the Mara Lake area are descr ibed. Second, the experimental data f o r amphibole are out l ined and comparison to con-d i t i on s e x i s t i n g at Mara Lake d iscussed. 3 STRUCTURAL AND METAMORPHIC RELATIONSHIPS BETWEEN THE MONASHEE AND MOUNT IDA GROUPS NEAR MARA LAKE, BRITISH COLUMBIA ABSTRACT Mara Lake, B r i t i s h Columbia, i s located at the northern end of the Okanagan Va l ley and s traddles the boundary between the Monashee Group on the east and the Mount Ida Group on the west. Repe-t i t i o n of units across the southern end of Mara Lake ind i ca te . • l i t h o l o g i c con t inu i t y between the two groups. Both groups of rock have experienced four phases of deformation. Phases one and two are t i gh t and recumbent, trending to the north and to the west respec- . t i v e l y . Phases three and four are open to c lose and upr ight , trending northwest and northeast r e spec t i ve l y . Second phase deformation includes large sca le tec ton ic s l i de s which separate areas of cons i s -tent vergence thought to be-limbs of major. fo lds ! S I ide ' sur faces are fo lded by t h i r d and fourth phase s t ructures and ou t l i ne domal outcrop patterns. Metamorphic grade increases from north to south along the west s ide of Mara Lake. C a l c - s i l i c a t e react ions invo lv ing the f o r -mation of d iops ide are c h a r a c t e r i s t i c . From west to east increas ing grade i s evident in the react ion of muscovite + quartz producing s i l l i m a n i t e . These prograde react ions are re la ted to r e l a t i v e pos i t i on in the second phase s t ruc ture . The highest grade i s located near the lowest s l i d e sur face. Greenschist condit ions accompanied phase three deformation with formation of ep idote , muscovite, and c h l o r i t e . Fourth phase i s character ized by hydrothermal a l t e r a t i o n , b r i t t l e f r a c t u r i n g , and loca l f a u l t i n g . Abrupt changes in metamorphic grade found at the northern end of Mara Lake are re la ted to these l a te f a u l t s . F i r s t phase deformation appears to be pre -Uppe'r T r i a s s i c while second and t h i r d phase are Upper T r i a s s i c and Lower Ju ra s s i c . Fourth phase i s part of a regional T e r t i a r y event. On a regional sca le s i m i l a r s t ruc tura l sequences are observed in other areas of the Shuswap Complex. 5 INTRODUCTION Mara Lake i s located j u s t southeast of the Shuswap Lakes, at the northern end of the Okanagan Va l l e y , a near ly continuous depression extending from south of the Internat ional Border (119°30'W, 49°N) 210 km north-northeast (119°W, 50°45'N) (F i g . 1). Like much of the Okanagan Va l l ey , the Mara Lake approximates the geologic boundary between low grade or unmetamorphosed rocks on the west and higher grade rocks on the east. The Shuswap Complex i s composed of po ly -deformed and metamorphosed sch i s t s and gneisses with associated i n t r u -sions which make up the core of the Eastern' Cordi11eran Fold Bel t (Campbell, 1973) (F i g . 1). Located with in the broadly def ined Shuswap T e r r a i n , Mara Lake i s on the western f lank of the orogenic b e l t and east of the low grade metasediments and metavolcanics o f the Intermontaine Zone (Wheeler, 1970). Study of the Shuswap Lake area began with Dawson (1898) who introduced the name, Shuswap Se r i e s , f o r the metamorphic rocks. Daly (1912, 1915, 1917) extended the area of the Shuswap Terra in and i n t e r -preted the shal lowly dipping f o l i a t i o n as a product o f load meta-morphism. On the other hand, Brock (1934) and G i l l u l y (1934) re l a ted the metamorphism to, dynamic mechanisms. Jones (1959) i d e n t i f i e d two separate deformational events. Fyson (1970), r e l y i n g p r imar i l y on Jones' mapping and on an inves t i ga t i on of small sca le s t ruc tu re s , con-cluded that four generations of f o l d ing a f fec ted rocks of the Shuswap Lake area. He (Fyson, 1970) proposed a model invo lv ing both time and depth of deformation to descr ibe the various f o l d s t y l e s . Using mineral f a b r i c s , Fyson (1970) concluded that r e c r y s t a l l i z a t i o n occurred during three of the deformation episodes. BRITISH COLUMBIA ! - N i l % -MAP AREA\jj|| v \ SHUSWAP \ TERRAIN ry . S ->i-<t~ U N I T E D S T A T E S FIGURE I AREA OF STUDY SCALE 5 KILOMETERS 10 PLEISTOCENE AMD RECENT ALLUVIUM MOUNT IDA GROUP MONASHEE GROUP 7 S t r a t i graphic re l a t i onsh ip s wi th in the Shuswap Complex and the Mount Ida Group are uncerta in. Both Dawson (1898) and Daly (1915) assigned Precambrian ages to the metamorphic rocks. Cairnes (1939) bel ieved that the complex inc luded rocks of Precambrian to Mesozoic age, a l l of which experienced a common Mesozoic metamorphism. Cairnes (1939) a l so suggested the existence of gradational contacts between the low grade rocks to the west and the high grade rocks of the Complex. Jones (1959) returned to the proposed Precambrian age f o r the Shuswap rocks and d iv ided these into three s t r a t i graphic groups: the Monashee Group, the Mount Ida Group, and the Chapperon Group. The Monashee Group cons i s ts p r imar i l y of high grade sch i s t s and gneisses in the area between the Columbia River drainage and the Okanagan Va l l ey . This area corresponds to the core zone of the orogenic be l t mentioned above. The Mount Ida Group cons i s t s of lower grade and 1 i t h o l o g i c a l l y more d i s t i n c t metasediments and metavolcanics found near the Shuswap Lakes. These lower grade rocks were shown to be in f a u l t contact with the Monashee Group and placed s t r a t i g raph ica l l y above the Monashee Group (Jones, 1959) (F ig . 1). Jones (1959) sub-d iv ided the Mount Ida Group into s ix formations (F ig . 2). The Eagle Bay Formation i s a c h l o r i t e and s e r i c i t e s ch i s t with add i t iona l l i t h o l o g i e s . Underlying the Eagle Bay i s a d i s t i n c t i v e f laggy l ime-stone, the Sicamous Formation. The remainder of the group cons i s t s o f a l t e rna t ing qua r t z i t e and s ch i s t units (Jones, 1959). Permian rocks to the west were thought to o v e r l i e uncomformably th i s whole sequence. Recent e f f o r t s by the Geological Survey of Canada have helped to c l a r i f y the s t r a t i graphic re l a t i on s wi th in the Mount Ida Group 8 MESOZOIC and TERTIARY intrusive and extrusive rocks UPPER TRIASSIC NICOLA GROUP volcanic rocks pelitic rocks SICAMOUS FORMATION imeslone PRE-UPPER TRIASSIC _MOUNT I OA GROUP quartzite,greenstone, marble, gneiss EAGLE BAY FORMATION r-.'-.V-vl phy 11 its, guartxite, greenstone,argillite l^ eej i^ limestone CACHE CREEK GROUP I' • • l l greenstone, argil I ite, minor limestone LITHOLOGIC CONTACT HIGH ANGLE FAULT LOW A N G L E FAULT 20 KILOMETERS JONES (1959) MOUNT IOA GROUP $$8^ EAGLE 8AY FORMATION li '; '' I || SICAMOUS FORMATION MARA FORMATION TSALK0M FORMATION SILVER CREEK FORMATION CHASE CREEK FORMATION ESZ3 |o 6 i a, CAMPBELL AND OKULITCH (1973) = SICAMOUS FORMATION PiU TSALKOM FORMATION SILVER (CHASE) CREEK FORMATION EAGLE BAY— 1 FIGURE 2 GENERAL GEOLOGY (after Okulitch and Cameron, 1 9 7 6 ) AND STRAT1GRAPHIC CORRELATIONS 9 (Okul i tch and Cameron, 1976; Oku l i t ch , et a l , 1975; Oku l i t ch , 1974; and Campbell and Oku l i t ch , 1973). On the basis of m i c ro fo s s i l s and regional l i t h o l o g i c c o r r e l a t i o n i t has been shown that the Mount Ida Group contains both Paleozoic and Mesozoic un i t s . F i g . 2 shows the most recent i n t e r p r e t a t i o n of the general geology and compares the sequence of Jones (1959) to that of Campbell and Okul i tch (1973). The Eagle Bay and the Sicamous Formations have received the most a t ten t i on . On the basis o f z i rcon dates from in t rus i ves and f o s s i l evidence from l imestones, the Eagle Bay i s be l ieved to range from middle Paleozoic to ea r l y Mesozoic. This changes the r e l a t i v e pos i t i on o f the Eagle . Bay from youngest (Jones, 1959) to poss ib ly o ldest (F ig . 2) . Evidence from conodonts i d e n t i f i e d the Sicamous Formation as Upper T r i a s s i c and probably younger than the Eagle Bay Formation (Okul i tch and Cameron, 1976). The Sicamous Formation i s thought to l i e unconformably on the Eagle Bay Formation on the basis o f f o s s i l evidence found east of Vernon (Fig. 2) and on the basis of l i t h o l o g i c c o r r e l a t i o n to observed unconformities (Campbell and Oku l i t ch , 1973). A l so , the Mara, S i l v e r Creek, and Chase Formations of Jones, (1959) have been combined and c a l l e d the S i l v e r Creek Formation ( F i g . 2 ) . For th i s study the formation names of Campbell and Okul i tch (1973) w i l l be used and co r -r e l a t i o n i s based on the sketch map of Okul i tch (1974). F i n a l l y , con-t r a ry to Jones (1959), both Fyson (1970) and Okul i tch and Cameron (1976) have shown units o f the Mount Ida Group e x i s t i n g on e i t h e r s ide of the Okanagan Va l l ey . The imp l i ca t ion is that there i s no s t r a t i -graphic separat ion of the Monashee Group and the Mount Ida Group. The purpose of the present study is an attempt to reso lve the boundary re l a t i onsh ip s between the Mount Ida Group and the Monashee Group. To th i s end, l i t h o l o g i c mapping was conducted to t race potent ia l markers across Mara Lake and the groups were studied with emphasis on the metamorphic h i s t o r i e s . LITHOLOGIC UNITS GENERAL DISTRIBUTION Within the map area eleven map units were i d e n t i f i e d (Plate I). Cor re la t ion across the lake reduced the number to seven major l i t h o r l o g i c un i t s . F i g . 3 shows the d i s t r i b u t i o n of these un i t s , the s t ruc tu ra l success ion, and the c o r r e l a t i o n across the lake. As mentioned above, the formation names are the same as those used by Okul i tch (1974) and are based on a comparison of the present map (F ig . 3) to his sketch map (Oku l i t ch , 1974). Cor re l a t i on o f units i s based p r imar i l y on the r e p e t i t i o n of d i s -t i n c t i v e l i t h o l o g i e s across the southern end of the lake (F i g . 3). In p a r t i c u l a r , large amphibo l i t i c layers were traced along the west s ide of Mara Lake and down to the shore l i n e . In the r a i l r o a d cuts on the west s i de , a sequence of a l t e rna t ing amphibo l i t i c and micaceous units i s succeeded by calcareous units f a r ther south (F i g . 3). This same pattern i s repeated on the east s ide of Mara Lake (F ig . 3). As a r e su l t o f th i s apparent r e p e t i t i o n , units in the higher grade sect ion were redef ined in terms of the three p r i nc ipa l l i t h o l o g i e s observed west o f the lake. P late I contains the o r i g i n a l breakdown of units as mapped; while F i g . 3 shows the proposed c o r r e l a t i o n . Change in meta-morphic grade i s respons ib le fo r the d i f f e r e n t appearance of the rock types across the lake. Three react ions appear to account f o r the d i f f e r e n t mineralogy. The appearance of s i l l i m a n i t e on the east 11 A pegmatite \ steeply dipping fault shallowly dipping fault (slide) FIGURE 3 DISTRIBUTION AND CORRELATION OF LITHOLOGIES • • ALLUVIUM VII SILVER PHYLLITE GREEN PHYLLITE BLACK PHYLLITIC LIMESTONE CHLORITE SCHIST AND AMPH1B0LITE WEST SIDE CALCAREOUS QUARTZITE 8 SCHIST -AMPHIBOLITE MICACEOUS QUARTZITE AND MAFIC SCHIST QUARTZ-MUSCOVITE SCHIST EAST SIDE . CALCAREOUS QUARTZITE VARIABLE HORNBLENDE! GNEISS a AMPHIBOLITE SILLIMANITE GNEISS 8 SCHIST SICAMOUS FORMATION EAGLE BAY FORMATION (?) TSALKOM FORMATION SILVER CREEK FORMATION 12 s ide of Mara Lake ind icates increas ing grade. From north to south , along the west s ide epidote disappears and the anor th i te content r i s e s . F i n a l l y , t remol i te reacts with c a l c i t e and quartz so that i n the southwest corner o f the map area d iops ide i s observed rep lac ing t remo l i te . Therefore metamorphic grade increases from north to south on the west s ide o f Mara Lake and from west to east across the lake. The formations w i l l be presented from o ldest to youngest and each un i t described f i r s t on the west s ide and then on the east. Reference should be made to F i g . 3 and Plate I. SILVER CREEK FORMATION Unit I The S i l v e r Creek Formation i s in terpreted as the s t r u c t u r a l l y lowest and o ldest rock un i t (F i g . 3). It i s a medium to coarse-grained quartz muscovite s ch i s t with var iab le qua r t z i t e i n t e r l a y e r s , ranging up to 25 cm in th ickness. This un i t i s exposed in road cuts along the Trans-Canada Highway from the west-centra l edge of the map to the second large turn-out over looking Shuswap Lake, a d is tance of 3 km. The un i t i s less r e s i s t an t than surrounding units and makes up the saddle in the r idge l i n e west of Mara Lake. More mafic port ions are found l o c a l l y and garnets appear i r r e g u l a r l y near the northern contact. Also near the northern contact, a zone of pegmatite dikes i s exposed (F i g . 3). These pegmatites form a discontinuous band which can be traced from,the Trans-Canada Highway over the r idge and down to Mara Lake (a trend of ^ 1 2 4 ° ) . Within th i s dike sequence a set o f v e r t i c a l dikes i s cut by moderately dipping d i k e si(^ 35°SEJ. Across Mara Lake Unit I contains s i l l i m a n i t e . Var i a t ion in l i t h o -logy includes s i l l i m a n i t e b i o t i t e gneiss , s i l l i m a n i t e s c h i s t , and ., s i l l i m a n i t e b i o t i t e garnet gneiss. The best outcrops of s i l l i m a n i t e b i o t i t e gneisses are found on the east s ide of Mara Lake (Highway 97-A), immediately north of Hummingbird Creek (see Plates I and II f o r geographic names). S i l l i m a n i t e s ch i s t i s found in th in layers on the northern peninsu la, Black Point and in t i g h t l y fo lded zones j u s t south of Hummingbird Creek. These s i l l i m a n i t e gneisses are in part co r re l a ted with sect ions of Unit II as well as Unit I (see southern port ion F i g . 3). TSALKOM FORMATION S t r u c t u r a l l y above the S i l v e r Creek Formation, th i s formation i s cons iderably more va r i ab le . Two major l i t h o l o g i c types are def ined. The lower sect ion is quartzose and genera l ly mafic in composit ion. The upper port ion i s ca lcareous. Unit II Character ized by dark micaceous q u a r t z i t e s , Unit II includes th in s ch i s t layers o f muscovite or b io t i te -hornb lende s c h i s t . Large d i s -t i n c t i v e amphibol ite layers and an intervening f l a t tened metacon-glomerate are useful markers. Near the contact with Unit I along the Trans-Canada Highway many s l i g h t l y d i scordant , t i g h t l y fo lded a p l i t i c dikes are seen in the dark micaceous qua r t z i t e (F ig . 4). These a p l i t e s are d i s t r i b u t e d throughout the un i t . East of Mara Lake Unit II i s a var i ab le hornblende gne iss , con-ta in ing l i t t l e or no s i l l i m a n i t e . The composition range includes amphibol i te, hornblende-garnet gneiss , hornb lende-b iot i te gneiss , and hornblende-feldspar gneiss. The l a t t e r l i t ho l ogy i s poorly f o l i a t e d and appears s i m i l a r to a hornblende d i o r i t e . Loca l l y mafic concen-t ra t i ons in the gneiss resemble f l a t tened pebbles. Thick amphibo-l i t e s were found at several locat ions such as j u s t south of Hummingbird Creek. The r e p e t i t i o n of amphibolites and o f Unit III across the southern end of Mara Lake led to the c o r r e l a t i o n proposed above. Unit III This un i t cons i s t s o f calcareous q u a r t z i t e , calcareous s c h i s t , greenish gray q u a r t z i t e , and an occasional massive buff to brown weathering l imestone which contain t i g h t l y fo lded s i l i c e o u s l ayer s . The contact with Unit II i s usua l ly from a more mafic uni t to a calcareous qua r t z i t e and appears to be sharp and conformable. Unit III crops out only in a short i n te rva l along the Trans-Canada Highway Much l a rger exposures are found at the south end of Mara Lake. The mass of calcareous rocks found on the steep slopes north of the Trans Canada Highway i s thought to be part o f the same sequence (F ig . 3). The c o r r e l a t i v e l i t ho l o gy east of Mara Lake includes greenish gray q u a r t z i t e s , calcareous q u a r t z i t e s , l i g h t co lored l imestones, and d i s t i n c t i v e granular d i op s i d i c c a l c - s i l i c a t e s . It i s th i s granular brown weathering c a l c - s i l i c a t e that crops out along the east s ide of Black Point. It i s a lso seen in the road cuts south of Hummingbird Creek and again at the south end of the lake. Jones (1959) equated the calcareous rocks north of Sicamous with those cropping out on Black Point. This c o r r e l a t i o n appears reasonable; however, th i s 15 author bel ieves that the calcareous rocks on the west s ide of Mara Lake should be inc luded. EAGLE BAY FORMATION Unit IV Unit IV i s confined to the northern port ion of the area, cropping out at the top of the c l i f f north of Sicamous. These rocks are . s i l i c e o u s p h y l l i t e s , s c h i s t s , and gneisses which d i f f e r from the ca r -bonates of Unit III which make up the s lope. The contact appears gradational and the f o l i a t i o n in both Units III and IV i s conformable. The dominant rock type in Unit IV i s a ch lor i te -muscov i te p h y l l i t e with occasional prophyroblasts o f f e ld spa r , garnet or amphibole. In the northeast corner of the map area there are blue-green hornblende sch i s t s and gneisses, as well as b i o t i t e - g a rne t s c h i s t . This mixture of l i t h o l o g i e s was not subdivided and has been cor re l a ted to the Eagle Bay Formation on the basis o f s i m i l a r rock descr ip t ions and the sketch map of Okul i tch (1974). SICAMOUS FORMATION Unit V The most d i s t i n c t i v e rock type in the area i s a black p h y l l i t i c l imestone with white c a l c i t e veins which give the rock a white and black banding. These white layers are f requent ly t i g h t l y fo lded and reveal s i g n i f i c a n t t ranspos i t i on of l a ye r i ng . This l i t ho l o gy var ies from massive limestone to p h y l l i t e but has a cons i s tent black color-? ing . Extensive outcrops of the Sicamous Formation can be seen along the Trans-Canada Highway for 2-1/2 km west of Sicamous (F ig . 3). North of Sicamous, the d i s t i n c t i v e p h y l l i t i c l imestone i s found along shore l i n e and in small outcrops along the steep s lopes. Foss i l evidence c o l l e c t e d outside of the map area has led Okul i tch and Cameron (1976) to place the Sicamous Formation as Upper T r i a s s i c , equivalent to the Nicola Group found fa r ther west. Although Okul i tch and Cameron (1976) suggest an unconformity below the Sicamous Formation, no c r i t i c a l evidence f o r an uncon-formity below Unit V was found in the Mara Lake area. The angular discordance between Unit V and Units 11-111 observed approximately 3 km southwest of Sicamous i s re l a ted in part to tec ton ic movement (F ig . 3). Covered at most l o c a l i t i e s , port ions of the contact are exposed on the steep slopes j u s t west of Mara Lake (F ig . 3; Plate I). In th i s area, there i s a t r a n s i t i o n zone in which Unit V.and Unit III are t i g h t l y in fo lded and the f o l i a t i o n s are e s s e n t i a l l y conformable. Consequently, regional studies (Okul i tch and Cameron, 1976) i nd i ca te an unconformable contact while l o c a l l y evidence f o r a tec ton ic contact e x i s t s . North of Sicamous, the shore l i n e exposures of Unit V grade upward into dark green p h y l l i t e , Unit VI, which in turn grades upward into s i l v e r brown p h y l l i t e , Unit VII. These p h y l l i t e s represent the uppermost units in the area and are bel ieved to be uppermost Sicamous Formation. Units VI and VII are in f a u l t contact with Units III and IV to the north (Plate I). INTRUSIVE UNITS Intrus ive rocks found in the area inc lude p r e - , syn - , and post-kinematic dikes and s i l l s . Although a large pegmatit ic body crops out, no large pluton was observed. The e a r l i e s t igneous rocks are a F i g . 4 Complexly fo lded ap l i t e s in Unit II, near contact with Unit I along Trans-Canada Highway. F i g . 5 Pegmatites associated with second phase in Unit II east of Mara Lake. 18 complex sequence of ap l i t e s which pre-date the second deformational event (F i g . 4 ) . These in t rus ions are located p r imar i l y in Unit II. In the gnessic rocks east of the lake pegmatite bodies are associated with second phase fo lds (F ig . 5). These coarse-grained bodies have weakly developed f o l i a t i o n p a r a l l e l to the second phase ax ia l surfaces and are f requent ly p a r a l l e l to second phase tec ton ic s l i d e s . . . A second set of pegmatites fol lows the general trend of t h i r d phase ax ia l sur faces. It i s th i s l a t t e r age group which accounts f o r the very large pegmatit ic masses found along the Trans-Canada Highway. These nearly v e r t i c a l dikes are in turn cut by a ser ies of shal lowly dipping pegmatites. F i n a l l y , the youngest igneous rocks are andesite porphyry dikes which c l o se l y fo l low fourth phase trends and are associated with extensive c h l o r i t e m ine ra l i z a t i on . These dikes appear s i m i l a r to those descr ibed f a r the r south which have been dated at approximately 50 my (Ross, 1974). STRUCTURAL EVOLUTION GENERAL SEQUENCE In order to e s tab l i sh the above l i t h o l o g i c success ion, i t was f i r s t necessary to unravel the s t ruc tu ra l sequence. At each outcrop, the r e l a t i v e ages of the f o l d s , c leavages, f r a c t u r e s , and compositional l ayer ing was determined. From th i s data a to ta l o f f i v e deformational events i s proposed. Although the s t y l e of fo lds and the nature of cleavages var ies with l oca t i on in the tec ton ic p i l e and with l i t h o l o g y , the deformation sequence appears cons i s tent and f o l d trends vary in a ra t iona l pat tern. Table I i s a l i s t i n g of the deformational events by r e l a t i v e age and c h a r a c t e r i s t i c f a b r i c elements. No 19 TABLE I; CHARACTERISTIC DEFORMATIONAL FABRICS ASSOCIATED WITH THE PROPOSED DEFORMATION SEQUENCE Event Qescr ipt ion ,_ Nomenclature Dr ^ Open f rac tu re set or iented in Fg the northeast quadrant D. -Symmetric, upr ight , open F^ f o l d s * with rounded limbs and hinges; usua l ly a s s o c i -ated with f r a c tu r i n g of one l imb; N-S trend -Fracture cleavage and crenu- S l a t i o n cleavage i n northern port ion of area 4 Do - S l i g h t l y asymmetric, upr ight , Fo - open to c lose fo lds with sharp hinges and planar limbs in micaceous un i t s . More rounded s t ructures in massive un i t s ; NW trend - I r r e g u l a r l y developed crenu- S~ l a t i o n and f rac tu re cleavage - C h l o r i t e and mica edge L~ l i nea t i ons D£ Ubiquitous, recumbent, t i gh t F2 to i s o c l i n a l fo lds with rounded hinges and planar l imbs; strong development of s l i d e s ; E-W trend. -Well developed s c h i s t o s i t y S2 - S i l l i m a n i t e , amphibole, and L2 mica edge l i nea t i on s D-, Recumbent, t i gh t to i s o c l i n a l F-| f o l d s , very appressed with sharp hinges and planar l imbs; N-S trend ( va r i ab le ) . - S c h i s t o s i t y usua l ly p a r a l l e l S-| to compositional l a ye r ing . -Amphibole and mica edge l i n e a t i o n L-j DQ -Compositional l ayer ing SQ * Interl imb angles are descr ibed according to terminology of F leuty (1964a). primary sedimentary or vo lcan ic features have been preserved; conse-quent ly, only reference to compositional l ayer ing w i l l be made. STRUCTURAL MAP A s t ruc tura l map has been prepared f o r the Mara Lake area (Plate II). The f o l d ax ia l traces have been drawn and the plunge of the fo lds shown where known. The most obvious pattern i s produced by the i n te r sec t i on of northeast - t rending fourth phase and west-northwest-trending t h i r d phase fo lds (P late II). Extensive f r a c tu r i n g occurs at several o r ienta t ions approximating the norther ly trend of fourth phase s t ruc tures . A b r i e f f i e l d inves t i ga t ion of r e l a t i v e ages of f r ac tu re sets ind icates a progress ive ly more eas ter ly o r i en ta t i on f o r younger set s . This age re l a t i on sh ip i s observed on Black Point where sharp wedge-shaped blocks are found in h ighly f ractured pegmatites. The suggested f i f t h phase (050° -070°) , in f a c t , may represent the youngest set of th i s f r ac tu re pat tern. Fractur ing and f a u l t i n g are apparent at the south end of Mara Lake; whi le c renu la t ion cleavage and f o ld ing are more common fourth phase s t ructures at the north end. Th i rd phase ax ia l traces change from an east-west o r i en ta t i on in the gne i s s ic rocks east o f the lake to a west-northwest o r i en ta t i on west of the lake " , . (P late II). The change i s i n terpreted as a r e su l t of i n te r fe rence between the l a t e r f o l d phases. The ax ia l traces of recumbent f i r s t and second phase fo lds are more complex and are c l o s e l y re l a ted to major second phase s l i d e surfaces |PI ate II). A s i i d e is def ined as a f a u l t formed in c lose connection w i t h h o l d i n g and i s conformable with the f o l d limb or ax ia l sur face. It i s accompanied by th inning and/or disappearance of the 21 fo lded beds (F leuty , 1974b). The most important second phase features are near Black Point (Plate II). Here several ax ia l traces are shown in conjunction with two tecton ic s l i d e s . The upper s l i d e (west of Black Point) i s contained e n t i r e l y wi th in the lower grade rocks. This s l i d e separates northward verging second phase fo lds from southward verging fo lds (P late III, Sect ion B-B ' ) . Several fo lds were noted above the s l i d e surface in the d i s t i n c t i v e amphibol ite of Unit II (F ig . 3 or P late I). The lower s l i d e i s character ized by the jux tapos i t i on of t i g h t l y fo lded and strongly l i neated s i l l i m a n i t e s ch i s t in the lower p late and amphibole gneiss and c a l c - s i l i c a t e in the upper p l a te . The f a u l t contact i s sharp where i t i s exposed along Highway 97-A and the vergence i s souther ly (P late III, Sect ion C - C ) . The same s i l l i -manite un i t i s found on Black Point and the s l i d e i s domed by i n t e r -ference of t h i r d and fourth phase f o l d s . North of Hummingbird Creek d e l t a , the s l i d e surface is less d i s t i n c t ; however, a zone of pegma-t i t e separates souther ly verging s t ructures on top from norther ly verging s t ructures below. Local s l i d e s were observed (F ig . 9 ) ; there -f o r e , the s l i d e t race was drawn through th i s area (Plate II). The other second phase ax ia l t race i s located at the extreme northern end of the area where l i t h o l o g i c r e p e t i t i o n out l ines the f o l d . F i r s t phase f o l d ing is prominent on the east s ide of Mara Lake (Plate II). Here Units I and II are contained predominantly between the two s l i d e surfaces mentioned above (F ig . 3). The units are near ly hor izonta l with a cons i s tent southward second phase vergence. Mapping revealed a large sca le r e p e t i t i o n of l i t h o l o g i e s , and vergence of small sca le fo lds supports a f o l d model. One ax ia l t race trends 22 southward from Sicamous Creek across Hummingbird Creek and continues south, cross ing Mara Lake midway between the southern end and Black Point. On the west s ide the development of f i r s t phase s t ructures is less pronounced and the pro jec t ion of th i s ax ia l t race was not pos s ib le . However, the imp l i ca t ion i s that r e p e t i t i o n of l i t h o l o g i e s on the west s ide i s re la ted to f i r s t phase f o l d i n g . On Black Point a carefu l examination of Unit III revealed congruent p a r a s i t i c fo lds above and below the d i s t i n c t i v e c a l c - s i l i c a t e . The trace of th i s f o l d i s shown along the east s ide of Black Po in t f (P l a te II). DOMAIN ANALYSIS Eleven domains were se lected p r i n c i p a l l y on the basis of t h i r d and fourth phase trends (F i g . 8). To examine f i r s t phase s t ructures i t was necessary to r e s t r i c t ana lys i s to r e l a t i v e l y small areas; e i t he r to ind iv idua l fo lds or to an area along the east s ide of Black Point. Second phase s t ructures are s t rong ly developed near the s l i d e sur faces ; therefore Domains VI, VII and X are se lected to avoid c ros s -ing l a t e r f o l d t races . Th i rd phase i s well developed along the west s ide of the lake and Domains III, IV, VI and part of I cross over t h i r d phase t races . Conversely, fourth phase i s more apparent in the higher grade un i t s ; therefore Domains V, IX, and XI examine data across these f o l d t races . Fourth phase data on the west s ide of Mara Lake i s analyzed in Domains II, IV, and VIII. This d i scuss ion w i l l progress from the f i r s t recognizable events to the most recent. F i r s t Phase F i r s t phase deformation i s penetra t ive , with the development of a pervasive s c h i s t o s i t y , root less i n t r a f o l i a l f o l d s , and s i g n i f i c a n t F i g . 6 F i r s t phase f o l d i n g at Mara Lake, B r i t i s h Columbia A. Mesoscopic recumbent f i r s t phase fo lds in Unit I along High-way 97-A north of Hummingbird Creek. B. Tight recumbent f i r s t phase f o l d in the lower limb of the megascopic f i r s t phase, synform along Hummingbird Creek (see P late II). C. Rootless i s o c l i n a l f i r s t phase f o l d in Unit II south of Hummingbird Creek. D. Domal outcrop pattern of f i r s t phase fo lds in N-S trending v e r t i c a l . f r a c t u r e , east of Mara Lake. 25 t ranspos i t ion of l a ye r i ng . F i g . 6 i s a composite of several phase one f o l d s . These s t ructures are f a r more common in the higher grade rocks. Two ax ia l traces have been shown in Plate II. The large recumbent synform east of Mara Lake i s out l ined by Unit II (F i g . 3). The trend i s s l i g h t l y east of north based on measurements of small sca le fo lds which can be seen in Hummingbird Creek and which have been ind icated on Plate I. These fo lds show a cons i s tent westward vergence on the lower f o l d l imb; while the upper limb appears to be h ighly attentuated with few p a r a s i t i c fo lds observed. The ax ia l t race along the east s ide of Black Point i s constructed on the basis o f outcrop pattern (Unit II I). A carefu l examination of small sca le s t ructures ind icates a con-gruent p a r a s i t i c f o l d system above and below Unit III. F ig 7 shows f i r s t phase f o l d measurements with sense of ro ta t ion above and below Unit III. Compositional l ayer ing data taken below the second phase s l i d e are presented in stereonets A and B_ (F ig . 7). These are above and below Unit III r e spec t i ve l y . From th i s data i t can be seen that the f i r s t phase f o l d i s very t i gh t in that limb measurements over lap. F i g . 7C shows the f i r s t phase cleavage data as well as the bearing and plunge of f i r s t phase f o l d s . In summary, Unit III forms the core of an i s o c l i n a l recumbent synform which has a cons i s tent north-south t rend. Addi t iona l f i r s t phase data i s presented in F i g . 8. Poles to S-j (39) are scattered along a great c i r c l e whose normal trends 025° , p a r a l l e l to fourth phase f o l d axes. This f o l d data comes from the / ' high grade un i t s , p a r t i c u l a r l y along the s t re tch from Sicamous Creek to Hummingbird Creek. In th i s region (Domain IX) fourth phase fo ld ing i s well developed. The geometry of these f i r s t phase fo lds ranges AND LOWER LIMBS OF FIRST PHASE FOLD KEY: • (_L) S 0 (LIMBS OF FOLDS) • (JL) S, o L, 0 CALC. FA. o o ° 025°/0° <9 <h 4 4 FIGURE 8 FIRST PHASE FOLD ELEMENTS FROM MARA L A K E MAP AREA, PR INC IPALLY DOMAIN IX 28 from t r u l y i n t r a f o l i a l i s o c l i n a l to c lose fo lds with in ter l imb angles of 54°. Three examples of the more open s t ructures are shown in the p lo t (F ig . 8 ) . The limbs and the r e l a t i v e pos i t i on of the ax ia l sur -face are shown. In each case, the fo lds are asymmetric with the ax ia l surface more near ly p a r a l l e l to the shallow dipping l imb. L ineat ions (L-j) are d i s t r i b u t e d about the fourth phase axis (small c i rc le . ? ) . The lack of evidence f o r t i g h t , recumbent, second-phase f o l d -ing probably r e f l e c t s sampling e r r o r . If the s l i p d i r e c t i o n f o r second phase was approximately p a r a l l e l to the o r i g i n a l L^  o r i en ta t i on (north-northeast ) , then l i t t l e or no r e - o r i e n t a t i o n of the L-| elements would be apparent in the planar limbs of the second phase f o l d s . Major change i n o r i en t a t i on would be found i n the r e l a t i v e l y small hinge zones. The lack of measurements from the hinges y i e l d s the apparent simple d i s t r i b u t i o n of L^ (Ramsay, 1967, p. 470-474). Second Phase Second phase s t ructures are the most common f o l d form in the area. These fo lds are seen on a l l scales and are t i gh t to i s o c l i n a l . The hinges are rounded and limbs r e l a t i v e l y p lanar. F i g . 9 i l l u s t r a t e s the various s ty le s and the s trong ly developed ax ia l plane cleavage. This f o l i a t i o n can be observed anywhere in the area. Whereas f i r s t phase cleavage i s commonly p a r a l l e l to compositional l a ye r i n g ; second phase usua l ly i n te r sec t s compositional l ayer ing \a t a low angle ( 1 0 ° - 1 5 0 ) . Mineral l i nea t i on s are common inc lud ing s i l l i m a n i t e , mica, and amphibole p a r a l l e l to second phase axes. A general trend fo r these axes i s wester ly , plunging approximately 25°. Major l i t h o l o g i c d i s -t r i bu t i on s on the west s ide of the lake can be re l a ted to second phase F i g . 9 Second phase f o l d ing at Mara Lake, B r i t i s h Columbia A. T ight recumbent second phase f o l d in Unit II west of Mara Lake. B. Small s l i d e surface found along Highway 97-A north of Hummingbird Creek. C. Complex recumbent i s o c l i n a l second phase fo lds in Unit III ( c a l c - s i l i c a t e ) along the east s i de ,o f Black Point. D. .T ight recumbent second/phase f o l d re fo ld ing i s o c l i n a l f i r s t phase f o l d . 31 f o l d s . For example, the units in the southwest port ion of the area are confined to one limb of a recumbent second phase synform (Plate I, F i g . 3) which i s cut by a s l i d e west of Black Point. North of th i s large s l i d e , second phase s t ructures give a cons i s tent northward vergence, suggesting the upper limb of a large recumbent ant i form. North of Sicamous, there i s a r e p e t i t i o n of p h y l l i t i c units with a few i n d i -cat ions of the opposite vergence, r e s u l t i n g in the i n fe r red second phase synform. On the east s ide of Mara Lake, the second phase s t r u c -tures show a cons i s tent southward vergence. F i g . 10 presents the data in Domain X which i s located south of the major t h i r d phase a n t i c l i n a l trace and straddles the lower s l i d e sur face. A geologic .map shows the observed sense of ro ta t ion associated with mespscopic second phase f o l d s . The dominant vergence is souther ly above the s l i d e s . As the s l i d e i s approached there are loca l reversa l s and evidence of t i gh t f o l d i n g . Not enough data was c o l l e c t e d to examine a s ing le large second phase f o l d ; however several mesoscopic fo lds were analyzed and th i s data i s combined with readings throughout the domain. 65 data points are contoured and reveal a broad maximum in which the maximum SQ concentrat ion corresponds to maximum S2 concen-t r a t i o n . The average in ter l imb angle i s approximately 4 2 ° , d i s t r i -buted symmetrical ly about S 2 , and the ca l cu la ted f o l d axis i s 260°/26°. Measured phase two l i nea r s agree well with th i s ca l cu l a ted ax i s . One o r i en ta t i on of the major s l i d e surface i s p lo t ted to show the c lose re l a t i on sh ip to the southwesterly dipping limb p o s i t i o n . These s l i de s d i s rupt second phase fo lds and are associated with zones of intense f o l d i n g . Th i rd phase fo ld ing deforms the upper s l i d e sur face; yet the FIGURE 10 SECOND P H A S E FOLD ELEMENTS AND GEOLOGIC MAP FROM DOMAIN X FIRST PHASE FOLDS ARE OPEN ARROWS DOMAIN VI (JL)S0 (201) (22 Max.) C.l.=>0,>l,>2,>4,>8% per 1% area! sharp nature of the lower s l i de s suggests at l eas t some l a t e r movement associated with less d u c t i l e cond i t ions . Consequently, the time of the s l i de s i s thought to be l a te second phase while temperatures were s t i l l h igh; these surfaces were probably react ivated during the t h i r d phase. Domain VII represents data from Black Point where several meso-scopic second phase fo lds were measured (F ig . 11). Poles to SQ (130) are descr ibed by two great c i r c l e s and L 2 data p lo t near the more westerly trending ax i s . Poles to S 2 mimic the SQ d i s t r i b u t i o n about the 330°/20° ax i s . The p a r a l l e l i s m between S 2 and SQ r e f l e c t s the t i g h t i -nearly i s o c l i n a l second phase f o l d ing in th i s domain. The northwest trending axis i s a ca l cu la ted t h i r d phase ax i s . Domain VI i s on the west s ide of Mara Lake and includes a sTide surface (F ig . 11). The poles to S 2 and the o r i en ta t i on of L 2 show cons iderably more s ca t te r than seen in Domains VII or X. Contoured poles to SQ (201) are contained in a wide zone d i s t r i b u t e d about a northwest trending ax i s . F i e l d data f o r second phase f o l d limbs y i e l d s the ca l cu l a ted axis of 284°/05° which does not agree with the great c i r c l e d i s t r i b u t i o n of SQ ( F i g . 11). The measured in ter l imb angles were less than 60° and again do not agree with the observed S Q d i s t r ibut ion-whieh is bel ieved to be re la ted to th ird;phase deformation Considering the s ca t te r observed in poles to S 2, t h i r d phase becomes inc reas ing ly s i g n i f i c a n t from east to west (Figs." 10 and 11). Th i rd Phase Th i rd phase fo lds are of two s t y l e s . In the lower grade, more micaceous un i t s , the fo lds are open to c l o s e , asymmetric and have sharp hinges with planar l imbs. The i r vergence i s toward the south. A poorly developed conjugate f o l d system i s found l o c a l l y in the shallow northeast -d ipp ing u n i t s , p a r t i c u l a r l y in the northern port ion of the area. In the more massive high grade rocks east of Mara Lake, the fo lds are s l i g h t l y asymmetric and are open with round hinges and limbs (F ig . 12). The cleavage associated with these t h i r d generation fo lds i s nonpenetrat ive. For the micaceous units th i s i s a s t r a i n - s l i p cleavage in which th in d i s c re te zones of movement have re -o r ien ted the e a r l i e r f o l i a t i o n s (Ramsay, 1967). Consequently, a common l i n e a t i o n associated with t h i r d phase i s a c renu la t ion l i n e a t i o n fo l lowing the trace of these s l i p surfaces in the compositional l a ye r -ing (F ig . 12). More massive units have healed f r a c t u r e s , usua l ly f i l l e d with c h l o r i t e or quartz . Th i rd phase s t ructures occur on several sca les . The l a rges t observed wavelength was a f o l d which extends from one end of Mara Lake to the other (M6 km). Gently northeast dipping uni ts in the north give way to southwesterly dipping units in the south. The ax ia l t race of th i s f o l d passes from Hummingbird Creek j u s t south of Black Point and continues west through the high point in the r idge west o f Mara Lake (Plate II). This a n t i c l i n e has an approximate trend i- i' of 300°. In the shal lowly dipping northeast limb of th i s large a n t i -c l i n e , several second order fo lds were observed. These fo lds reveal the asymmetric s t y l e while the large sca le f o l d i s more rounded and open. The development of minor t h i r d phase s t ructures i s cons iderably less apparent on the southwest limb where only weakly developed cleavage and open congruent fo lds were observed. F i g . 12 Th i rd phase fo ld ing at Mara Lake, B r i t i s h Columbia A. Upright c lose asymmetric t h i r d phase f o l d in a p i l i t e of Unit II west of Mara Lake, B. Th i rd phase cleavage in the mixed l i t h o l o g i e s of Unit II \ west of Mara Lake. C. Complex t h i r d phase kink bands in Unit VII along the shore l i n e of Shuswap Lake north of Sicamous. D. Shallowly dipping kink band d i s rupt ing second phase f o l d in Unit VI north of Sicamous, E. Open.upright t h i r d phase synform in gne i s s ic units along the east s ide of Black Point. V Domains I, III, IV, and VI from the lower grade units west of Mara Lake show the va r i a t i on in the asymmetric t h i r d phase s tructures (F ig . 13). Of these, Domain III i s c l o ses t to the envis ioned " i d e a l " t h i r d phase s t ructures seen in micaceous un i t s . These fo lds are open to c lose ( in ter l imb angle 9 7 ° ) , s l i g h t l y asymmetric with a long r e l a t i v e l y planar northeast -d ipping limb (average dip 36°) . The hinges are sharp and the southwest-dipping limb is short curv ip lanar with dips increas ing from 30° to 60° . The average S 3 o r i en ta t i on is v e r t i c a l with a trend of 128° (F ig . 15B). In Domain IV, poles to SQ have a continuous d i s t r i b u t i o n such that the hinge zone i s less marked (F i g . 13). The in ter l imb angle i s approximately 120°, r e f l e c t i n g the change in the major sync l ine up plunge. The t i g h t e r fo lds o f the core (Domain III) become more open lower in the s t ructure (Domain IV). Domain VI i s near the hinge of the t h i r d phase a n t i c l i n e and as discussed e a r l i e r the SQ data ind icates northwest trending f o l d axis (296°/14°) and an in ter l imb angle of 95° (F ig . 13). The trend of the : t h i r d phase cleavage is 130° and the bearing of Lg approximately 300°. Domain l i s a more complex area (F ig . 13). The data i s d i s -t r ibu ted along a great c i r c l e with an axis o f 317°/4°. The maximum i s located in the southwest quadrant and i s s p l i t . Most of these readings are from the southwest port ion of the domain where the r e p e t i t i o n in p h y l l i t i c l i t h o l o g i e s was observed (F ig . 14). Two f au l t s are mapped in Domain I (F i g . 14). One i s s teeply dipping with a northwest t rend. This f a u l t places dark green p h y l l i t e s (Unit VI) and the p h y l l i t i c limestones (Unit V) .in.'contact with the carbonate units of Unit III. The movement i s northeast s ide up. The second f a u l t i s sub -hor i zon ta l , 39 DOMAIN I (JL) S 0 (107) (14 Max.) ==>0,>3,>6,>9,>I2% per 1% area DOMAIN III LL) S 0 (87) (15 Max.) C.l. 8 >0,>I,>2I>5,>I0% per 1% area DOMAIN IV LL) S 0 (173) (32 Max.) C.I.=>0,>I,>4,>7,>I0,>I3% per 1% area DOMAIN VI LL) S 0 (201) (22 Max.) C.l.=>0,>l,>2,>4,>8% •per 1% area 296°/l4' o 0 FIGURE 13 THIRD PHASE FOLD ELEMENTS FROM NORTH AND WEST OF MARA LAKE B.C. FIGURE 14 GEOLOGIC MAP AND THIRD PHASE C L E A V A G E D E V E L O P M E N T IN DOMAIN I UNITS ARE THE SAME AS FIGURE 3 nearly p a r a l l e l to the p r i nc ipa l f o l i a t i o n , SQ, and the upper p late movement i s northward. Both f au l t s cut second phase elements and fourth phase fo lds d i s t o r t p a r t i c u l a r l y the shallower sur face. There-f o r e , th i s system appears to be a conjugate f a u l t set near the core of a t h i r d phase a n t i c l i n e (Plate III, Sect ion A-A^). Cleavage measure-ments in Domain I show a complex pattern as well (F i g . 14). One cleavage concentrat ion is v e r t i c a l and another i s subhor izonta l . The age re l a t i on sh ip between the various kink o r ien ta t i ons i s c o n f l i c t i n g ; however, the youngest set i s most f requent ly the s teeply dipping kinks with a trend of 135°-150°. Shear surfaces dip exc lu s i ve l y . towards the northeast and p lo t c lose to-second phase cleavage o r ienta t ions ( F i g . 14). Several second phase fo lds show l a t e r move-ments re la ted to kink s t ructures (F ig . 12) and the shallow shear sur -faces are thought to be re la ted to reac t i va t i on of favorably or iented Third phase deformation of S 2 produces a d i f f e r e n t f o l d geometry (F i g . 15). F i g . 15A shows poles to S 2 from the west s ide of the lake. The fo ld ing i s open with rounded hinge and l imbs. The ca l cu l a ted f o l d axis i s 307°/03°. F i g . 15B shows the d i s t r i b u t i o n of Sg data with the great c i r c l e from F i g . 15A. The deformation of S 2 i s a r e f l e c t i o n of the overa l l t h i r d phase f o l d envelope; while the d i s -t r i b u t i o n of poles to SQ r e f l e c t the second order fo lds observed in the various un i t s . The sharpest and t i gh te s t fo lds are those seen in the p h y l l i t e s . Deformed second phase l i nea t i ons r e f l e c t the d i f f e r e n t f o l d mechanisms of t h i r d phase deformation. F i g . 16A represents the A. (JL) S 2 (94) (19 Max.) C.I.=>0,>4,>8,>I2,>I6% KEY: © CALC. F.A. FIGURE 15 DEFORMATION OF SECOND PHASE FOLIATION ABOUT THIRD PHASE AXES WEST (A) AND EAST (B) OF MARA L A K E , B.C. 44 d i s t r i b u t i o n of L 2 measurements west of the lake. A great c i r c l e f i t suggests a simple inhomogeneous shear mechanism for t h i r d phase defor -mation on the west s ide of Mara Lake. The average Sg o r i en ta t i on in ter sec t s the locus of deformed l i nea t i on s to give an a^  d i r e c t i o n of shear, 315°/30° (Ramsay, 1967, p. 470). This o r i en ta t i on i s nearly p a r a l l e l to the trend of t h i r d phase f o l d axes but with a steeper plunge. F i g . 16B i s a p lo t o f L 2 data from east of the lake. Rather than a great c i r c l e d i s t r i b u t i o n , L 2 i s scat tered about a small c i r c l e (25°) with an east-west axis (260°) . The average S 3 passes north of th i s ax i s ; however, the impl ied f l exura l s l i p mechanism i s favored fo r the open t h i r d phase fo lds observed in the gne i s s ic rocks. Another and probably preferab le explanation of th i s va r i a t i on in L 2 d i s t r i b u t i o n s i s f l exura l f o l d ing on both s ides of the lake fol lowed by f l a t t e n i n g on the west s ide (Ramsay, 1967, p. 466). This mechanism i s d i s t i n c t l y poss ib le p a r t i c u l a r l y in the northern areas where t h i r d phase fo lds appear t i g h t e r . Folding can be examined as a combination of buckl ing and shear components upon which compressive s t ra in s exert a modifying i n f luence . " I f the shear component of f o l d ing i s s t rong ly developed and e n t i r e l y l a t e r than the buckl ing component" then the great c i r c l e d i s t r i b u t i o n of l i nea t i on s i s achieved and the shear d i r e c t i o n (a 3 ) can be at any o r i en ta t i on (Ramsay, 1967, p. 482). This assumes that compressive s t ra in s are f a i r l y homogeneous and that the p r i nc i pa l e longat ion i s nearly p a r a l l e l to the s l i p d i r e c t i o n . Inhomogeneous s t r a i n in which the p r i n c i p a l e longat ion does not co inc ide with a^ could produce reo r i en ta t i on of the s l i p d i r e c t i o n c l o se r to the f o l d axis (Ramsay, 1967, p. 485). Fourth Phase Fourth phase fo lds are near ly symmetric open s t ructures with rounded hinges and limbs which are f requent ly associated with s teeply dipping f au l t s (F i g . 17). The trend of both fo lds and f au l t s i s s l i g h t l y east of north. The conf i gurat ion of the present va l l ey is re la ted to a major fourth phase sync l ine which trends northeast and whose trace passes j u s t west of Black Point and j u s t east of the steep b lu f f s in the Larch H i l l s (P late II). Fractures re l a ted to phase four are i r r e g u l a r l y developed and are genera l ly f i l l e d with c h l o r i t e or c a l c i t e . A strong f r ac tu re set p a r a l l e l s th i s norther ly trend and i s i nva r i ab l y coated by c h l o r i t e or shows signs of a l t e r -a t i on . In the northern port ion of the area, c renu la t ion cleavage takes the place of the f r a c tu re s . The mafic dike sequence (andesite porphyry) i s or iented p r e f e r e n t i a l l y along a north-south trend. F i n a l l y , ac t i ve gas venting (COv,?) i s observed along the lake shore approximately p a r a l l e l to the fourth phase trends. Unl ike t h i r d phase, fourth phase deformation is bet ter developed in the gne i s s i c un i t s . F i g . 18 includes Domains V, IX, and XI. The broad s ca t te r of SQ out l ines the open and rounded nature of fourth phase f o l d s . The trend i s east o f north or southwesterly. The change from a 20° southwesterly plunge in Domain XI to an 8° nor th-eas ter l y plunge in Domain IX i s re l a ted to the superpos i t ion of fourth phase fo lds on the ex i s t i n g t h i r d phase a n t i c l i n e . F i g . 17 Fourth phase fo ld ing at Mara Lake, B r i t i s h Columbia A. B r i t t l e character of fourth phase fo ld ing in Uni t I near Sicamous Creek along the east s ide of Mara Lake. B. Broad warp in Unit III at the northern end of Black Point. C. Strongly developed north trending f r ac tu re set in Unit I near Sicamous Creek along the east s ide of Mara Lake. Fractures show a l t e r a t i o n along borders and are f i l l e d with c h l o r i t e . FIGURE 18 FOURTH PHASE FOLDING OF COMPOSITIONAL LAYERING IN HIGH GRADE ROCKS NEAR MARA L A K E , B.C. I ; - , . DOMAIN (JJ S 0 (45) KEY: o 0 CALC. FA. DOMAIN IV (_L) S 0 (173) (32 Max.) C.l.= >0,>I,>4,>7,>I0,>I3% per 1% area DOMAIN VIII (_L) S 0 (153) (II Max.) C.l.= >0,>2,>4,>6% per 1% area FIGURE 19 FOURTH PHASE FOLDING OF COMPOSITIONAL LAYERING IN LOW GRADE ROCKS WEST OF MARA LAKE, B.C. GREAT CIRCLES REPRESENT POSSIBLE BROAD WARPS SUPERIMPOSED ON THE MORE STRONGLY DEVELOPED NW TRENDING THIRD PHASE FOLDS A s i m i l a r change is seen on the west s ide (F i g . 19). F i g . 19 i s the S Q data from Domains II, IV, and VIII. Domain II was se lected because i t s t raddles the fourth phase a n t i c l i n e . The general s ca t te r can be descr ibed by a great c i r c l e whose normal i s 354°/40° which agrees with the few measurements. Domain IV as discussed prev ious ly r e f l e c t s p r imar i l y t h i r d phase f o l d i n g ; however, the spreading of contours along the se lec ted great c i r c l e i s i n terpreted as warping of the northeast -d ipping l imbs. Domain VIII represents a s i m i l a r s i t u a t i o n . The p r i nc i pa l s ca t te r i s from northeast to southwest r e f l e c t i n g t h i r d phase deformation. Here too the pro-nounced sca t te r in the northern hemisphere i s thought to be. re l a ted to fourth phase deformation. The e f f e c t o f fourth phase on e x i s t i n g f o l i a t i o n s i s shown i n F i g . 20. F i g . 20A i l l u s t r a t e s the warping of S-| produced on the east s ide of Mara Lake. S 2 data, from the east s ide i s a lso broadly warped, but about a souther ly trending ax i s , 180°/02° (F i g . 20B). The average o r i en ta t i on of S^ (both cleavages and f rac tures ) from the east s ide i s ^010°/85° NW (F ig . 20C). Both S y and S 2 f o l d axes f a l l near th i s t rend. F i g . 20D summarizes the Sg data from the en t i r e area and reveals an open f o l d geometry with a nearly v e r t i c a l f o l d ax i s . This i n te rp re ta t i on i s based on the broad f o l d s t y l e of phase four . In comparison with the t i gh t concentrat ion in F i g . 20C, S^ from the west s ide i s f a r more scattered and the p r i nc i pa l concen-t r a t i o n i s v e r t i c a l with a trend s l i g h t l y west of north (F ig . 20E). Deformed Lg are inconc lus ive with regards to fourth phase mechanism. F i g . 21A shows t h i r d phase l i nea r s from the western domains. These readings c l u s t e r in the northwest and southeast A.(_L) S, (39) KEY: © CALC. F.A. B. U)S 2 (94 ) ( I6 Max.) C.I.=>0,>4,>8,>I2% per 1% area C. U) S 4 (128)(25 Max.) C.I. = >0,>4,>8,>I2,>I6% per 1% area East of Mara Lake l80°/2° D. U ) S 3 (I96)(23 Max.) >0,>2,>4,>6,>8,>I0% per 1% area, i E.U)S 4(I92)(I8 Max.) C.l.= >0,>2,>4,>6,>8% per 1% area West of Mara Lake F I G U R E 20 FOURTH PHASE DEFORMATION OF EARLIER. CLEAVAGES SEE TEXT FOR DISCUSSION POSSIBLE SMALL CIRCLE DISTRIBUTION OF THIRD PHASE LINEATIONS RELATED TO AVERAGE FOURTH PHASE CLEAVAGE WEST OF MARA LAKE B. "FIFTH" PHASE U ) S 5 o (JL) MAFIC FIFTH . PHASE ELEMENTS FROM MARA LAKE AREA • • •$ FIGURE 21 53' quadrants. The average fourth phase planar o r i en ta t i on i s located clockwise of these c l u s t e r s , suggesting a poss ib le small c i r c l e d i s -t r i b u t i o n . I n su f f i c i en t Lg data was measured east of the lake to pursue the poss ib le f l exura l s l i p mechanism. F i f t h Phase F i f t h phase deformation was e n t i r e l y b r i t t l e . Several l a te stage f rac tu re sets cross the area. The most d i s t i n c t i v e is a northeast trend which produces p a r a l l e l open f r ac tu re s . These f rac tures are genera l ly barren. The pronounced l i n e a r nature of Hummingbird Creek and the general trend of the Eagle Va l ley northeast of Sicamous are thought re la ted to th i s northeast system. Some of the broad warps observed in the compositional l ayer ing are a lso in terpreted as f i f t h phase events. A p lo t o f f i f t h phase elements includes f r a c t u r e s , f au l t s surf faces and two mafic dikes (F i g . 21B). Because of the occurrence of these dikes in both fourth and f i f t h phase trends, the timing of the two events i s thought c l o s e l y r e l a ted . These data are by no means a complete coverage of l a te stage f rac ture development, but do repre -sent a cons i s ten t l y younger set of f rac tures observed throughout the area. STRUCTURAL SUMMARY The f i r s t three deformational phases are i l l u s t r a t e d in F i g . 22. Only four units are presented with Units B and I serv ing as markers. The thickness and r e l a t i v e geographic pos i t ions are diagramatic. For a more accurate i l l u s t r a t i o n re fe r to the s ix v e r t i c a l s t ruc tura l sect ions (Plate II I). Three sect ions are drawn approximately nor th -55 south (A-A 1 , B-B', C - C ) and show second and t h i r d phase geometries. The l a s t three sect ions are or iented east-west (D-D 1 , E - E ' , F -F ' ) and ou t l i ne the f i r s t and fourth phase s t y l e . F i r s t phase deformation produced t i gh t to i s o c l i n a l recumbent fo lds with a norther ly trend (F ig . 22). Cross sect ions E-E' and F-F ' show the f o l d geometry and the i n fe r red cont inu i t y of s t ructures across the Mara Lake. In order to expla in the outcrop pattern of Unit D (Plate I) west of Mara Lake, a f i r s t phase f o l d i s proposed and projected into cross sect ion F-F ' (P late II I). Minor f i r s t phase fo lds were found in units west of Mara Lake; however, they are scattered and no d e f i n i t e congruent f o l d set was observed. Imp l i c i t in Section F-F ' i s the idea that F^  i s not found in the Sicamous Formation (Unit I). No c l e a r cut phase one fo lds were found in Units I-,-J,K. The i n t e n s i t y of second phase produced obvious t ranspos i t ion of l a ye r i ng . Many root less -folds are found a l l of which appear re l a ted to the second deformation. Both mineral l i nea t ions and c renu la t ion axes are best descr ibed as re la ted to second and t h i r d phase elements in the underlying un i t s . No e a r l i e r l i nea t ions were recorded. On the basis of these observations f i r s t phase is thought to pre-date Units I, J , and K (Plate I). Second phase i s o c l i ne s trend at approximately r i gh t angles to the f i r s t phase fo lds (F ig . 22). A f t e r i n i t i a l f o ld ing and th icken ing , second phase deformation is character ized by l a r ge . tec ton i c s l i de s (F ig . 22). These s l i de s resu l ted in extension in a north and south d i r e c t i o n . The s l i p d i r e c t i o n (a 2 ) fo r second phase i s bel ieved to be contained in the f i r s t phase ax ia l surface and to be nearly p a r a l l e l 56 to the f i r s t phase axes (F ig . 22). The major s l i d e surfaces are shown fo lded by t h i r d phase (Section B-B') and fourth phase (Section E -E ! ) (Plate II I). Th i rd phase deformation forms a ser ies of open to c lose north-west-trending fo lds (F ig . 22). The shape of the fo lds in Sections B-B1 and C - C (Plate III) i s determined from the d i s t o r t i o n of (F ig . 15). These are open fo lds with rounded hinges which occur in the southcentral port ion of the area. In the northern po r t i on , these fo lds are t i gh te r and are fau l ted with the north s ide up. The t i gh te r f o l d i n Section A-A' (P late III) resu l ted from pro jec t ing the syn-c l i n e from Section B-B 1 and accounting f o r the dominant southward vergence o f t h i r d phase s t ructures seen north of Sicamous. The a n t i c l i n a l t race passes near the point where react ivated second phase s t ructures show northward vergences in what would be the over-turned limb (Plate II; P late III, Section A - A ' ) . The ca l cu la ted s l i p d i r e c t i o n i s plunging ^30° northwest and i s be l ieved to post date an i n i t i a l f l exu ra l s l i p f o l d ing which i s s t i l l evident in the higher grade rocks. Fourth phase f o l d ing produced a f au l ted sync l ina l trough which accounts fo r the present va l l ey trend (Plate II). The major sync l ine can be seen in Section F-F 1 (Plate II I). In the southern port ion of the area, increased f a u l t i n g i s apparent (sect ion E-E' and D-D 1, Plate II I). The r idge west of the lake corresponds to the hinge of an a n t i c l i n e whose trace can be fol lowed to the c l i f f north of Sicamous (Plate II). Here units are traced from nearly hor izonta l to nearly v e r t i c a l . The f o l d i s asymmetric with the ax ia l surface (cleavage) nearly p a r a l l e l with the s teeply dipping un i t s . METAMORPHISM The mixed l i t h o l o g i e s of the Mara Lake area provide an oppor-tun i ty f o r i nves t i ga t ing the condit ions associated with metamorphism. The most obvious change i s d isp layed by the assemblages in p e l i t i c rocks (pr imar i l y Unit I). F i g . 23 shows p e l i t i c sample locat ions and gives the primary mineral assemblages. P e l i t i c assemblages on the west s ide of Mara Lake are character ized by the coexistence of musco-v i t e -qua r t z and muscov i te - ch lo r i te -quar tz , i nd i ca t i n g greenschist fac ies cond i t ions . In con t ra s t , . the east s ide of the lake has coex i s t ing o r t h o c l a s e - s i l l i m a n i t e and g a r n e t - b i o t i t e , i nd i ca t i ng tem-peratures greater than that f o r the react ions (1) muscovite + c h l o r i t e + quartz = almandine + b i o t i t e + H 20 (2) muscovite + quartz = or thoc lase + s i l l i m a n i t e + H^O. The f i r s t react ion was i n f e r r e d from petrographic observations by Thompson and Norton (1968). The second react ion has been examined by Evans (1965) and Kerr ick (1972) (F i g . 26). In a recent review, Greenwood (1976) l i s t e d f i v e experimental s tudies on un ivar iant e q u i l i b r i a in c h l o r i t e . Four of these react ions involve the f o r -mation of c o r d i e r i t e , which was not observed at Mara Lake. The f i f t h react ion produces almandine by the reac t i on . (3) F e - c h l o r i t e + quartz = almandine and i s shown in F i g . 26 ( a f te r Hsu, 1968). The presence of muscovite F i g . 23 Sample l o c a l i t i e s and primary mineral assemblages fo r p e l i t i c samples (pr imar i l y Unit I). Abbreviat ions are quartz ( q t z . ) , p lag ioc lase (p lag . ) , K-feldspar (K - spar . ) , muscovite (muse ) , garnet ( g rn t . ) , c h l o r i t e ( c h i . ) , s i l l i m a n i t e ( s i l l . ) , b i o t i t e (b io t . ) Sample 8 - - q t z . , p lag. (An36) K-spar., m u s e , grnt. 1 8 - - q t z . , m u s e , c h i . , grnt. 2 3 - - q t z . , K-spar., s i l l . , m u s e , grnt. 3 7 - - q t z . , p l a g . , K-spar., m u s e , c h i . 6 1 - - q t z . , p lag. (An34), b i o t . , s i l l . , g rn t . , K-spar. 68- - q t z . , s i l l . , p l a g . , K-spar., (a l tered) 69- - q t z . , p lag. (An48), b i o t . , g rn t . , K-spar. 7 1 - - q t z . , p l a g . , b i o t . , g rn t . , K-spar., s i l l . 7 3 - - q t z . , p l a g . , b i o t . , g rn t . , K-spar., s i l l . 59 and quartz reduces c h l o r i t e s t a b i l i t y . Reactions (2) and (3) r e f l e c t the changes from west to east across Mara Lake. Amphibolites and other mafic units (p r imar i l y Unit II) provide evidence of increas ing grade from north to south along the west s ide (F i g . 24). In the northwest sect ion samples inc lude a c t i n o l i t e -b i o t i t e - c l i n o z o s i t e - quartz - p lag ioc lase (An^g) - sphene - c a l c i t e . The southwest port ion of the area has a s i m i l a r assemblage, except that primary c l i n o z o i s i t e i s f a r less common, the anor th i te content increases to An^g ^ and a few samples had d i op s i d i c pyroxene in a s soc ia t ion with the amphibole. The change in anorth i te content (>An^Q) appears to correspond to the upper s l i d e surface (F i g . 24). Although the l oca t i on of the react ion boundary i s uncer ta in , tempera-ture condit ions in the southwest corner were high enough f o r the react ion (4) c l i n o z o i s i t e + a c t i n o l i t e + quartz = d iops ide + anorth i te + H 2 0.. Recently Hoye (1976) examined c a l c - s i l i c a t e gneisses near Kootenay Lake and was able to draw a T-X^Q g r id fo r th i s r eac t i on . He con-cluded that increas ing anor th i te content r e f l e c t e d increas ing grade. E a r l i e r Wenk and Ke l l e r (1969) were able to r e l a te increas ing anorth-i t e content in amphibolite to increas ing metamorphic grade. With th i s p o s s i b i l i t y in mind but without the composition of a l l the phases, three anorth i te composition contours have been drawn in F i g . 24. The highest anorth i te (>Angg) content was found in the centra l port ion of the map area. Diopside appearance in the southwest corner i s accom-panied by i t s common occurrence in the southeast corner. The assem-blage d i o p s i d e - b i o t i t e - p l a g i o c l a s e - g a r n e t i s found on the east s ide of F i g . 24 Sample l o c a l i t i e s and primary mineral assemblages fo r amphibo-l i t e s and other mafic units (Unit II). Addi t iona l abbrev i - ' at ions are a c t i n o l i t e ( ac tn , ) , amphibole (amph.), c a l c i t e ( c a l c ) , epidote ( ep id . ) , microc l ine (micr . ) , pyroxene (py r . ) , sphene (sph.) , z i r con ( z i r . ) . Sample 6 - - q t z . , m i c r . , p lag. (An35), a c t n . , e p i d . , sph. , b i o t . , c a l c , : z i r . 7 — q t z . , p l a g , , a c t n . , b i o t . , e p i d . , c a l c , sph. 9 — q t z . , K-spar., p lag. (An43), muse 11— - q t z . , p lag. (An66), py r . , amph., sph. 12— p l ag . , K-spar., amph., b i o t . , grnt. 1 3 — q t z . , p l a g . , a c t n . , b i o t . , e p i d . , c a l c , sph. 1 9 - - q t z . , fe ldspar (brecciated) 21- - q t z . , K-spar., grnt. (a l tered) 2 2 — q t z . , amph., p laq. (An54), b i o t . , sph. 24— q t z . , p l a g . , K-spar., m u s e , c h l . 25— - q t z . , a c t n . , b i o t . , p lag. (An39), sph. 2 6 — q t z . , p lag. (An39), b i o t . 2 9 — q t z . , p l a g . , b i o t . , a c t n . , ep id . 3 3 - q t z . , p lag. (An62), amph., g rn t . , biot,., K-spar. 34- - q t z . , b i o t . , p lag. (An45). 35- - q t z . , b i o t . , m u s e , p lag. (An49), epid.,, c h l . (a l tered) 36- - q t z . , p lag. (An33), K-spar., m u s e , b i o t . 38— - q t z . , p l a g . , K-spar., b i o t . , e p i d . , muse 3 9 — q t z . , p l a g . , K-spar., m u s e , b i o t . 4 0 — q t z . , m u s e , c h l . a f t e r b i o t . , fe ld spar . 41— - q t z . , p lag. (An50), K-spar., c h l . (a l tered) 4 2 — q t z . , b i o t . , m u s e , p lag. (An40), K-spar. 4 3 — q t z . , b i o t . , a c t n . , p lag. (An42), e p i d . , K-spar. 45— - q t z . , b i o t . , a c t n . , py r . , p l a g . , e p i d . , K-spar., muse 4 6 — q t z . , b i o t . , m u s e , p l a g . , K-spar. 4 7 — q t z . , p lag. (An39), b i o t . , K-spar. 4 9 — q t z . , a c t n . , p lag. (An72). 5 4 - - q t z . , a c t n . , py r . , p lag. (An73), c a l c , sph. 5 6 - - q t z . , p lag. (An50),.actn. ( f ine-gra ined) 6 0 - - q t z . , p lag. (An58), K-spar., amph., pyr. a 6 2 - - q t z . , p lag. (An60), amph., py r . , b i o t . , grnt. 6 4 — q t z . , p lag. (An59), b i o t . , p y r . , K-spar. 6 5 — q t z . , p lag. (An54), b i o t . , py r . , g rn t . , sph. 6 6 — q t z . , K-spar., p lag. (An46), b i o t . , grnt. 67— - b i o t . , g rn t . , q t z . , p lag. (An61), '.K-spar. 7 0 - - q t z . , p lag. (An43), b i o t . , amph., sph. , g rn t . , ep id , 7 2 — b i o t . , amph., p lag. (An83), q t z . , K-spar. 74--p lag. (An90), amph., g rn t . , q tz . 62 63 Mara Lake north of the l a te stage northeast trending f a u l t (F i g . 24). An add i t iona l mineralogic change associated with mafic units i s a change in pleochroism of b i o t i t e and amphibole. B i o t i t e s change from greenish co lo r ing to a br ight red brown approximately mid-way along the west s ide of Mara Lake. Amphibole changes from green to brownish green in the same in te rva l and f i n a l l y a l i g h t brown va r ie ty was viewed in the north-eastern gne i s s i c un i t s . S imi la r react ions are observed in the c a l c - s i l i c a t e sequences (pr imar i ly Unit II I). In add i t ion to react ion (4), the presence of c a l c i t e permits the react ion with a c t i n o l i t e to form diops ide (5) a c t i n o l i t e + c a l c i t e + quartz = d iops ide + C0 2 + H^O. F i g . 25 shows the sample l o c a l i t i e s f o r c a l c - s i l i c a t e s and the p r i -mary assemblages. The f i r s t appearance of d iops ide i s at l o ca t i on #52 and i s commonly observed rep lac ing a c t i n o l i t e in sect ions from the southwest corner. Therefore i t appears that react ion (5) occurs p r i o r to react ion (4). Phlogopite i s observed in sect ions from the southwest corner i nd i c a t i n g condit ions s u f f i c i e n t f o r the reac t ion . (6) t remol i te ( a c t i n o l i t e ) + c a l c i t e + K-feldspar = phlogopite + diopside + C0 2 + H 2 0. Reactions (5) and (6) are complete in the southeast region because the assemblage diops ide - b i o t i t e - K-feldspar - p l ag ioc la se (>An 6 0) + quartz + c a l c i t e i s s t ab le . In c a l c - s i l i c a t e units north of Sicamous, no a c t i n o l i t e was observed which suggests that the low temperature react ion (7) quartz + dolomite + H 90 = a c t i n o l i t e + c a l c i t e + CO, F ig . 25 Sample l o c a l i t i e s and primary mineral assemblages f o r c a l c -s i l i c a t e units (pr imar i l y Unit H I ) . Addi t iona l abbrev i -at ions are c l i n o z o i s i t e ( c l i n o . ) , d iopside (d iop . ) , phlogopite (ph lg . ) , t remol i te (trem.). Sample 5- - t rem., c a l c , q t z . , p lag. (An47), ep id . 1 7 - - c a l c , c h l . , q t z . , fe ldspar 27- - q t z . , K-spar., c a l c , d i op . , (epid. a l t e r : ) 2 8 — K-spar., d i o p . , sph. , q t z . , (epid. a l t e r ; ) 3 0 - c a l c , c l i h o . , p l a g . , q t z . 31- - q t z . , c a l c , e p i d . , p lag. (An32), fe ldspar 32- - c a l c , py r . , q t z . , t rem., K-spar., sph. 4 8 — q t z . , . c a l c . , p lag. (An32), d i o p . , t rem., sph. 50- - q t z . , p lag. (An34), py r . , a c t n . , c a l c , sph. 5 1 — q t z . , c a l c , p lag. (An40), t rem., d i o p . , sph. 5 5 — q t z . , plag (An62), c a l c , ph l g . , d i op . , trem., K-spar,, (twining in pyr. ) 57- - c a l c . , q tz . 58- - q t z . , p lag. (An70), d i o p . , b i o t . , K-spar. 59- - q t z . , p lag. (An80), K-spar., p y r . , sph. , (epid. + c h l . a l t e r . ) 6 3 — c a l c , q t z . , d i o p . , p l a g . , sph. 7 7 - - q t z . , b i o t . , m u s e , p lag. (An39). 8 0 — c a l c , q t z . , fe ldspar (a l tered) 8 1 — c a l c , b i o t . , q t z . , phlg. 8 3 — c a l c , m u s e , q t z . , fe ldspar . 65 66 may not have occurred. However, Unit IV, north of Sicamous, includes assemblages suggestive of s i m i l a r grade to Unit II southwest of Sicamous (F i g . 25). Because c l i n o z o i s i t e i s a common phase in the lower grade un i t s , the f l u i d phase i s thought to have been water - r i ch (Greenwood, 1976, p. 247). As a r e s u l t , the temperature i s h ighly uncerta in. The a c t i n o l i t e react ions (5 & 6) apparently increased the molecular f r a c t i o n of CG^ enough so that g ro s su l a r i te was not formed in the higher grade un i t s . On the basis of th i s negative evidence, i t i s i n fe r red that the concentrat ion of C0 2 was enough to l i m i t the temper-ature range f o r react ion (5) to 50°C (Skippen, 1972). A general intermediate un ivar iant curve fo r the react ion t remol i te + c a l c i t e + quartz shown in F i g . 26 (Winkler, 1974). A maximum and minimum temperature fo r the react ion would be at l ea s t + 25°C. A c t i n o l i t e s o l i d so lu t ion would increase th i s poss ib le e r r o r . In summary, the temperature condit ions increase from <550° in the northwest port ion of the area to above 600°C in the eastern ha l f (F i g . 26). Below the second phase s l i d e on the western s ide of Mara Lake, the react ions forming diopside i nd i ca te temperatures in the range of 550°-600°C. Q u a l i t a t i v e l y , both anor th i te content and p leochro ic changes agree with th i s sense of increas ing temperature. In p a r t i c u -l a r , the highest anorth i te content in the mafic units co inc ides with the lower s l i d e sur face. Therefore temperature gradients appear to be re l a ted to the second phase s t ruc tu re . Conf ining pressure e s t i -mates are are even less constra ined. The absence of anda lus i te or kyanite suggest pressures in excess of 2 kb and less than 6 kb east 67 FIGURE 26 UNIVARiANT REACTION CURVES FOR MINERAL ASSEMBLAGES AT MARA LAKE , B,C. REACTIONS: I) HOLDAWAY, 1971, 2) EVANS, 1965, 3) WINKLER, 1974, 4) HSU, 1968, 5) LAMBERT, et al, 1969 68 of the lake, depending on the pos i t i on of the melting curve (F ig . 26). Moderate pressures of 3 to 5 kb are bel ieved co r rec t . In the previous sect ion a s t ruc tura l succession was out l ined in which second and t h i r d phase f o l d ing account f o r the major mappable f o l d s . A ser ies o f nappe- l ike s t ruc tu re s , each separated by a s l i d e , character ize second phase deformation. The s t r u c t u r a l l y highest o f these fo lds i s in the northern port ion of the area while the lowest i s in the centra l por t ion . The pattern of increas ing metamorphic grade can be re la ted to th i s s t ruc tu ra l p i c tu re . Second phase f o l d -ing i s s t rong ly developed in a l l rock types, producing a pervasive cleavage and l i n e a t i o n . Consequently, primary mineral paragneiss is i s a t t r i bu ted to th i s deformational event. Retrograde metamorphism i s of two types. Amphibole and b i o t i t e rims are seen on some pyroxenes in the southern area and secondary epidote and c h l o r i t e are widespread. The epidote react ion appears to involve p l a g i oc l a se , poss ib ly by the react ion proposed by Liou (1973) (8) C a l c i t e + anor th i te + H 20 = c l i n o z o i s i t e + C0 2 In some cases d iops ide a l so was seen in contact with the epidote. These samples are found in the southeastern area and on Black Point. Ch l o r i t e react ion rims and masses are usua l ly associated with l a te f ractures which may a lso be f i l l e d with c a l c i t e . One area of intense c h l o r i t i z a t i o n i s north of Sicamous near the trace of the fourth phase a n t i c l i n e (Plate II). F i e l d occurrence of c h l o r i t e f i l l i n g in fourth phase f ractures i s common. The amphibole-and. b i o t i t e react ions with pyroxene are an ear ly retrograde expression thought to be re l a ted to t h i r d phase deformation. Th i rd phase cleavage f requent ly 69 i s out l ined by euhedral muscovite and c h l o r i t e which d i f f e r s from the l o c a l i z e d a l t e r a t i o n associated with l a t e r f r ac tu re s . An est imat ion of condit ions during f i r s t phase deformation was inconc lus ive because of the pervasive nature of the second phase. F i r s t phase mineral l i nea t i ons inc lude amphibole, mica, and d iops ide ; however, these could be mimetic overgrowths. No separat ion of l i nea t i ons by mineralogy was observed. Therefore, f i r s t phase involved reo r ien ta t i on and poss ib le c r y s t a l l i z a t i o n of grains p a r a l l e l to f o l d axes, probably at l ea s t in greenschist cond i t ions . DISCUSSION MARA LAKE MAP AREA Three p r i nc i pa l conclusions can be made from th i s study, (1) Units o f the Mount Ida Group are c o r r e l a t i v e to units in the Monashee Group.. (2) The s t ruc tu ra l sequence is the same in the Mount Ida Group and the Monashee Group.-. (3) The metamorphic grade i s re l a ted to r e l a t i v e pos i t i on in the second phase f o l d s t ruc ture . (1) L i t ho l og i c r e p e t i t i o n of c a l c - s i l i c a t e s and amphibolites across Mara Lake are evident at the southern end (Plate I). However, Jones (1959, p. 54) concluded that "the Mount Ida Group i s apparently not equiva lent to any part of the Monashee Group." On th i s basis he drew a major f a u l t in the Okanagan Va l l ey . The fau l ted boundary found in the Mara Lake area i s in f ac t a ser ies of discontinuous o f f se t s re la ted to fourth and f i f t h phase deformation. Because the Mount Ida Group had " su f fe red " less metamorphism, he concluded that these units were higher in the s t r a t i g r aph i c column. Later wr i ter s have suggested that units can be traced across Mara Lake (Fyson, 1970; Okul i tch and Cameron, 1976). Fyson i d e n t i f i e d the d i s t i n c t i v e marble on Black Point as a marker which could be traced along the southeast shore l ine of Mara Lake (Fyson, 1970, p. 108). The present mapping agrees w i th ' the p ro jec t ion of the "marble" (Unit G) across Mara Lake; but th i s un i t i s then truncated by the lower s l i d e J sequence (P late I). Exposures of Unit G in the southeast corner are in terpreted as f o l d r e p e t i t i o n above the s l i d e . Okul i tch (1974) showed the Tsalkom Formation cross ing the southern end of Mara Lake as part o f an eastward trending b e l t (Oku l i t ch , 1974, p. 27). Equat-ing Units B, C, D, F, and G (Units II and III of F i g . 3) to the Tsalkom, the present mapping agrees with the pro jec t ion except that s t r i k e and dip measurements in the southeast corner have a d e f i n i t e southerly trend re la ted to l a te stage f o l d i n g . Within the gne i s s i c t e r r a i n , the d i s t r i b u t i o n of l i t h o l o g i e s can be explained as fo lded repet i t i ons of the three p r i nc i pa l l i t h o l o g i e s observed west o f Mara Lake. (.2) A l l workers in the area have concluded that the Mount Ida Group and the Monashee Group have experienced s i m i l a r deformation h i s t o r i e s (Jones, 1959; Fyson, 1970; Oku l i t ch , 1974; Okul i tch and Cameron, 1976). Whereas Jones d iv ided the deformation into "o lder " and "younger," Fyson out l ined a sequence e s s e n t i a l l y the same as presented here (Fyson, 1970, p. 110). The f i r s t two events are t i gh t to i s o c l i n a l and recumbent. In the Mara Lake area trends are norther ly fo r f i r s t phase and westerly for second. Th i rd phase is character ized by c renu la t ion cleavage at high angles to compositional l a ye r ing . The average trend i s northwesterly. Fyson bel ieved that there were two generations of t h i r d phase: one near ly p a r a l l e l to second phase, and a younger, northwest t rend. Fyson (1970) a l so assigned amphibole and s i l l i m a n i t e l i nea t i ons to t h i r d phase. Neither the separate f o ld sequence nor the high grade mineral l i nea t i ons were recorded in th i s study. The trends of t h i r d phase f o l d change from an east-west trend in the gneiss to a northwest trend west of the lake and are bel ieved re l a ted to fourth phase r o t a t i on . A s i m i l a r ro ta t ion i s seen in second phase f o l d axes which s h i f t from a ^260° east of the lake to ^280° west of the lake. The most s i g n i f i c a n t ro ta t i on of second phase was found north of Sicamous where axes have bearings of 330° near fourth phase s t ructures and t h i r d phase deformation generated a great c i r c l e d i s t r i b u t i o n of second phase elements. Although l oca l var i a t ions in trends are pro-nounced, the trend of the large second phase s t ructure changes system-a t i c a l l y west o f the lake. The present study shows that large sca le s t ructures are mappable in the Mount Ida Group. In p a r t i c u l a r , second phase f o l d ing involves a sequence of s l i d e s separat ing p lates of near ly cons i s tent in terna l vergence. The upper p late has dominantly northward verging second phase s t ruc tu re s , the middle p la te has southerly vergence, and the l im i ted exposures of the lower p la te have northward verging f o l d s . Total second phase vergence i s unclear in that the s l i de s appear to involve movement in both norther ly and souther ly d i rec t i on s (Plate III, Section B-B 1 ) . The middle p late accounts f o r most of the exposures along the east s ide of Mara Lake. Here, fo r approximately 14 km, second phase s t ructures are cons i s ten t l y souther ly verging. Both Fyson (1970) and Jones (1959) commented on the large area southeast of Sicamous in which a dominant souther ly vergence can be observed. Frequent ly, recumbent second phase fo lds are truncated on the lower l imb. Jones (1959) in terpreted th i s s t y l e to be "smearing" in which layers s l ipped over one another without much buck l ing , e s s e n t i a l l y laminar flow. The present study suggests that large sca le fo lds have been strongly attenuated with s l i d e development p a r a l l e l to the ax ia l sur faces. If th i s f o l d s t y l e appl ies on a regional s c a l e , then a f o l d model invo lv ing s t ruc tu ra l th ickening f o l -lowed by large sca le s l i de s might descr ibe regional s t ruc tu ra l evo-l u t i o n . Th i rd phase deformation has warped these r e l a t i v e l y f l a t l y i ng s l i d e surfaces in to large open f o l d s . At Mara Lake the wave-length of the major t h i r d phase a n t i c l i n e i s at l ea s t 16 km. Inter-ference of t h i r d phase (NW-trending) and fourth phase (NE-trending) fo lds produces broad domal outcrop patterns (Plate I). Timing of these s t ruc tu ra l events i s l oose ly constra ined by both f o s s i l and i s o top i c dates. The Sicamous Formation (Unit V, F i g . 3) has been i d e n t i f i e d as Upper T r i a s s i c and equiva lent to the Nicola Group found f a r the r west (Okul i tch and Cameron, 1976). These authors a lso suggest a regional unconformity below the Sicamous Formation. Within the Mara Lake area second phase fo ld ing has s i g n i f i c a n t l y deformed the Sicamous Formation while f i r s t phase deformation appears t o pre-date th i s un i t . Reg iona l ly , Oku l i t ch , et a l . (1975) found evidence fo r Devonian plutonism. Ross and Barnes (1972) described a Paleozoic unconformity in the southern Okanagan Va l l ey . To the east , 73 Read (1975) found evidence of Paleozoic deformation and eros ion which are part of a long Paleozoic event (Ross, 1970). Consequently, f i r s t phase i s be l ieved to be Pa leozo ic . Second and th j rd phases are both thought to be Upper T r i a s s i c and Lower Juras s i c events. K-Ar dates on l oca l plutons and gneisses y i e l d dates of 127-140 my (Wanless, 1969). Near Vernon, a recent hornblende K-Ar mineral age of 178 my suggests coo l ing by that time (Solberg, 1976). Fourth phase deformation i s accompanied by extensive a l t e r a t i o n and the in t rus ion of andesite d ikes. These c h a r a c t e r i s t i c s are common along the whole length of the Okanagan Va l ley (Ross, 1974). This thermal event has been examined in de ta i l near Okanagan Lake and i s dated by a va r ie ty of methods as T e r t i a r y (^50 my) (Medford, 1976). (3) Contrary to Jones' (1959) idea of abrupt change in meta-morphic grade, the units of the Mara Lake area i nd i ca te a gradual increase in grade from north to south along the west s ide . Across the southern end of Mara Lake, the change in grade i s not appre-c i ab le with react ions (4) and (5) accounting fo r the complete d i sap-pearance of a c t i n o l i t e . The disappearance of c l i n o z o i s i t e , the increas ing anorth i te content, the change in pleochroism and the appearance of s i l l i m a n i t e and garnet are re l a ted to pos i t i on in the second phase f o l d s t ruc ture . The lowest s l i d e surface corresponds to the highest grade. In cont ras t , t h i r d phase deformation involved c r y s t a l l i z a t i o n of muscovite and c h l o r i t e along cleavage t races . The va r i a t i on in f o l d s t y l e between the gneisses and the lower grade sch i s t s imply lower temperature condit ions than second phase. Fyson (1970) a t t r i bu ted some of his amphibole and s i l l i m a n i t e l i nea t i on s in the eastern gneisses to t h i r d phase deformation. The p a r a l l e l i s m of r~2 and trends in the gneisses may account fo r some confus ion; however, the present study revealed no high grade mineral l i nea t i ons associated with t h i r d phase. REGIONAL TRENDS The large sca le f o l d s , the f o l d sequence, and the high tempera-tures associated with deformation at Mara Lake can be re l a ted to regional trends. Fyson (1970) ou t l i ned a deformational sequence invo lv ing four phases of deformation between Vernon and Shuswap Lake, B r i t i s h Columbia. His second and t h i r d phases are the same as described here and his data from the Mara Lake area agree with the observed f o l d trends. Jones (1959) d iv ided his deformational h i s to ry into " o lder " and "younger" f o l d s . His " o l de r " fo lds correspond in s t y l e and trend to the well developed second phase s tructures at Mara Lake (Jones, 1959). F i g . 27 i s a compi lat ion of second phase fo lds trends as mapped by Fyson (1970, p. I l l ) and the "o lder " deformational trend of Jones (1959, F i g . 2). The "o lder " fo lds of Jones (1959) are shown as bars represent ing the average f o l d axis o r i e n t a t i o n . Fyson's (1970) second phase data are shown as small arrows and are conf ined to the western port ion of the area in the lower grade uni ts (F i g . 27). Three stereonets are shown from the Mara Lake area; two show con-toured data from the gne i s s i c uni ts and one, west of Mara Lake, shows the s ca t te r of second phase cleavage. The L 2 data are p lo t ted as small open c i r c l e s . These l i nea r s change o r i en t a t i on across the lake. The southwest trend ovserved on the east cn 76 s i de , s h i f t s to a northwest trend on the west s ide . The SQ data changes from a t i g h t f o l d s t y l e ( in ter l imb 42°) to an i s o c l i n a l f o l d s t y l e in the western most gneisses. For these two areas the second phase cleavage data p lo t as f a i r l y t i gh t c lu s te r s (F igs. 10 and 11). In comparison, west of Mara Lake, S 2 i s d i s t r i bu ted about a t h i r d phase f o l d ax i s . This change in f o l d s t y l e corresponds to the change in metamorphic grade. The strong expression of second phase is seen in the gneisses whi le t h i r d phase deformation s i g n i f i c a n t l y d i srupts the second phase trends in the lower grade un i t s . On a regional sca le th i s pattern is well demonstrated (F i g . 27). From Arrow Lake to the Okanagan Va l l e y , Jones (1959) found an impress ively cons i s tent east-west f a b r i c fo r his " o lder " deformation. West of the Okanagan Va l l ey , both Jones (1959) and Fyson (1970) recorded a more var i ab le second phase f o l d trend (F i g . 27). The change in average o r i en t a t i on i s s i m i l a r to that observed at Mara Lake, from east-west to northwesterly. Local va r i a t i ons are very pronounced with ind iv idua l readings at high angles to the northwest t rend. These var ia t ions are bel ieved to be re la ted to the large t h i r d phase s t ruc tures . For example, along a l i n e from the southwest end of the Shuswap Lakes to the northern end of Okanagan Lake, the s ca t te r i s be l ieved re la ted to a major t h i r d phase an t i f o rm. (F i g . 27). Regional t h i r d phase deformation west of the Okanagan Va l ley i s examined in F i g . 28. The Sicamous • Formation and the c o r r e l a t i v e Upper T r i a s s i c N ico la Group are shown as a marker uni t (Okul i tch and Cameron, 1976). These T r i a s s i c un i t s form an a n t i c l i n a l outcrop pattern with a northwesterly trend (F ig . 28). Small arrows ind ica te KEY: 3rd PHASE FOLD TRENDS FYSON, 1970 "PROPOSED REGIONAL THIRD PHASE FOLD TRENDS FAULT TRIASSIC UNITS U ) S 0 (201) (22 Max.) C.I. = >0, >l,>2,>4, > 8 % per 1% area FIGURE 28 ARE A L DISTRIBUTION OF TRIASS IC UNITS WEST OF OKANAGAN V A L L E Y AND THEIR RELATION TO THIRD PHASE FOLD E L E M E N T S FROM MARA L A K E AREA AND FROM FYSON, 1970 (MODIFIED FROM FYSON, 1970) 78 mesoscopic t h i r d phase fo lds (Fyson, 1970). For comparison, data from west of Mara Lake i l l u s t r a t e s the Fg deformation of SQ (F i g . 28; a lso s e e ' F i g . 13). The major t h i r d phase anti formal t race observed at Mara Lake is inc luded. To the north a major t h i r d phase synformal t race was out l ined by Fyson (1970) (F i g . 28). On the basis o f these two f o l d t r aces , the cons i s tent northwest trend of mesoscopic t h i r d phase f o l d s , the pattern of second phase f o l d axes, and the d i s t r i b u t i o n of the T r i a s s i c units a major ant i formal trace has been drawn southeastward from Chase (F ig . 28). This f o l d has been re fer red to as the Chase A n t i c l i n e by Okul i tch (personal comm., 1975). Okul i tch and Woodsworth (1977) extended the T r i a s s i c uni ts eas t -ward to form a discontinuous be l t across the Shuswap Complex (F i g . 29). Units which have been equated inc lude the Sicamous Formation, the N i co l a , the S locan, and the Rossland Groups. This c o r r e l a t i o n i s based on Upper Tr iass ic "conodont i d e n t i f i c a t i o n (Okul i tch and Cameron, 1976) and on l i t h o l o g i c a l s i m i l a r i t i e s . The e f f e c t of such a c o r r e -l a t i o n is to t i e the proposed t h i r d phase Chase A n t i c l i n e to the T r i a s s i c units of the Slocan Sync l ine , in other words, a f o l d sequence extending 200 km along trend. The Slocan Syncl ine has been examined at Upper Arrow, Slocan and Kootenay Lakes (Hyndman, 1968; Ross and K e l l e r h a l s , 1968). Ross (1970) descr ibes the Slocan Group as well-bedded " sha les , a rg i l l aceous l ime-stones, sandy l imestones, q u a r t z i t e , and greywacke" with poorly pre -served T r i a s s i c f o s s i l s . The Slocan Group near kootenay Lake crops out in the centra l port ion of the Kootenay Arc., a ' s t ructura l be l t extending from Revel stoke along Kootenay Lake southwestwards into Washington" REVELSTOKE FIGURE 29 DISTRIBUTION OF TRIASSIC UNITS FROM SHUSWAP LAKES TO KOOTENAY LAKE, B.C. AFTER OKULITCH AND WOODSWORTH, 1977 (Ross, 1970, p. 55) (F i g . 29). S im i l a r to the Shuswap Complex, the Kootenay Arc i s a polydeformed and metamorphosed area i n which three phases of deformation are observed. The f i r s t two are coax ia l and c l o s e l y approximate the nor ther ly trend of the a r c , which d i f f e r s from the east-west f a b r i c observed i n the Shuswap Complex (Jones, 1959; Reesor and Moore, 1971). F i r s t phase s t ructures are i s o c l i n a l " s i m i l a r " fo lds with an ea s te r l y vergence. Second phase fo lds are " s i m i l a r " more open asymmetric fo lds with ax ia l surfaces d ipping p r i -mar i ly southwesterly or s teep ly to the east . Near the Slocan Syncl ine t h i r d phase i s character ized by widespread s t r a i n - s l i p cleavage with f o l d axes plunging shal lowly to the northwest (Ross and K e l l e r h a l s , 1968). Ross and Ke l le rha l s (1968, p. 863) fee l that the Slocan Syncl ine i s a large t h i r d phase s t ruc ture with a c losure southeast of Kaslo, B r i t i s h Columbia. Okul i tch and Woodsworth (1977) show time equiva lent units scat tered southwestward from Kaslo (F ig . 29). Crosby (1968) observed "west-plunging open cross fo lds with assoc iated non-penetrat ive ax ia l plane f o l i a t i o n " as c h a r a c t e r i s t i c o f t h i r d phase near Kootenay Lake. Fyles and Hewlett (1959) documented t h i r d phase open cross fo lds and kink bands, plunging gently west or southwest near Salmo, B r i t i s h Columbia, approximately 50 miles south of Kaslo (F i g . 29). In Washington, the Kootenay Arc i s a l so character ized by three d i s t i n c t phases o f deformation (M i l l s and Nordstrom, 1973). Th i rd phase ax ia l surfaces s t r i k e northwesterly and dip shal lowly to the southwest (M i l l s and Nordstrom, 1973, p. 195). Therefore, i t appears that t h i r d phase deformation i s widespread and r e l a t i v e l y con-s i s t e n t i n o r i en t a t i on across the Shuswap Complex. ' Along the western margin of the Shuswap Complex, deformation sequences, s t y l e s , and trends are s i m i l a r to the Mara Lake area (Plate IV). Five areas have been examined and in each case four or f i v e phases o f deformation have been observed. F i r s t phase fo lds are norther ly t rend ing , i s o c l i n a l , and usua l ly roo t l e s s . Only a few mega-scopic phase one fo lds have been descr ibed (Ross and C h r i s t i e , 1969; C h r i s t i e , 1973; Ryan, 1973) (Plates I,II). Far more common are phase two s t ructures (P late IV). These fo lds occur on a l l sca les and are i s o c l i n a l , recumbent, and f requent ly associated with mylonites or tec ton ic s l i d e s (Plate IV). The peak metamorphic event i s be l ieved re la ted to phase two. Phase two fo lds vary from northwesterly trends in the southern Okanagan Va l ley to near ly east-west in the northern Okanagan Va l ley (Plate IV). Phase three fo lds are v a r i a b l e , ranging from open to t i gh t and upright to overturned. These fo lds are character ized by c renu la t ion or s t r a i n - s l i p c leavages; are usua l ly asymmetric; and are f requent ly associated with a conjugate ax ia l su r -face (P late IV). The d i f f e rence between phases two and three ind ica te a common change in rheo log ica l behavior and, consequently, in i n fe r red pressures and temperatures. Phase three i s thought to be genera l ly lower grade. Phase three i s a lso in terpreted as accounting fo r the Chase A n t i c l i n e and the Slocan Syncl ine (F igs. 28 and 29). The l a t e r phases of deformation are b r i t t l e , invo lv ing open f o l d s , f r a c t u r e s , mafic d ikes , and hydrothermal a l t e r a t i o n . A l l of these features have norther ly trends (Plate IV). The r e p e t i t i v e pattern of deformation suggests regional orogem'c events; however, the t iming of these events i s poorly def ined. In the Mara Lake area, second and t h i r d phases appear to be Upper T r i a s s i c 82 and Lower Ju ra s s i c . In the Slocan Sync l ine , the second phase may predate the Upper T r i a s s i c (Ross and K e l l e r h a l s , 1968); however, Hyndman (1968) ind ica ted that second phase d id deform Upper T r i a s s i c units near Arrow Lake (F ig . 29). In the southern Okanagan Va l l e y , phases one to three are thought to be middle Paleozoic ( C h r i s t i e , 1973). This i n te rp re t a t i on i s based p r imar i l y on f o s s i l evidence from unde-formed limestones above an unconformity (Ross and Barnes, 1972). Ryan's (1973) o ldest Rb-Sr i s o top i c date was 170 my and C h r i s t i e (1973) ind icated a minimum age f o r phase three of 144 my. Medford (1973) con-cluded that 200 my was the minimum age f o r termination of phase three. Fourth phase deformation i s assoc iated with a regional T e r t i a r y ther -mal event (50 my) (Ross, 1974). This thermal event has s t rong ly a f fec ted the i s o top i c systems along the western margin of the Shuswap Complex (Ryan, 1973; Medford, 1976; Solberg, 1976). Consequently, the o lder ages may be s i g n i f i c a n t l y re se t . A regional tec ton ic model w i l l depend on c l a r i f i c a t i o n of these age r e l a t i on sh i p s . However, that model w i l l have to account f o r the pronounced east-west f a b r i c associated with phases two and three, the tec ton ic s l i de s associated with phase two, and the regional extent of phase three fo lds (F i g . 29). A l so , there i s strong evidence that the p r i n c i p a l deformation in the Shuswap Complex s i g n i f i c a n t l y pre-dated the l a t e Mesozoic thrus t development in the Rocky Mountains (Ba l l y , et a l . , 1966). The changes in f o l d s t y l e associated with phases two, three and four represent a progression from d u c t i l e to b r i t t l e behavior. Theo-r e t i c a l f o l d studies have shown an analogous change from " s i m i l a r " to concentr ic geometry associated with decreasing temperature (Pa r r i sh , et a l . , 1976). The t r a n s i t i o n from d u c t i l e to b r i t t l e behavior i s i n f l u -enced not only by decreasing temperature but a lso by decreasing c o n f i n -tr ing pressure, increas ing s t r a i n r a t e , and the presence o f pore f l u i d s (Heard, 1960). These var iab les i n te r ac t so that b r i t t l e behavior i s conf ined to r e l a t i v e l y shallow depths in the ear th ' s c rus t . The con-d i t i ons envis ioned fo r the Shuswap Complex (500° - 600°C and 3-5 kb) are high enough fo r most common s i l i c a t e minerals to deform by i n t r a -c r y s t a l l i n e ^ s l i p , twinning, or other mechanisms re l a ted to d i f f u s i o n (Carter , 1976). To succes s fu l l y model f o l d development Pa r r i sh , et a l . (1976) r e l i e d on experimental ly determined nonl inear flow laws f o r quartz and marble. These flow laws compare favorably to the steady s tate creep theor ies o f Weertman (1968). Although flow laws are ava i l ab le f o r quar tz , c a l c i t e , and dolomite no such law ex i s t s f o r amphibole. At Mara Lake, amphibol i te cons t i tu tes a s i g n i f i c a n t rock type (P late I). Although p r e f e r e n t i a l l y o r i en ted , the amphibole grains seldom show signs of d u c t i l e deformation while quar tz , c a l c i t e , mica, and p l ag i o -c lase are s i g n i f i c a n t l y deformed. The lack of a flow law and the apparent high strength of amphibole motivated the fo l lowing e x p e r i -mental study. With the r e s u l t s , comparisons can be made to other s i l i -cates and eventual ly used to evaluate to ta l rock behavior. 84 DEFORMATION MECHANISMS AND FLOW LAW EQUATIONS FOR CALCIC AMPHIBOLE ABSTRACT Fifty samples of hornblendite (Am-2) were deformed in a large solid-medium Griggs-type apparatus at 700° to 1000°C, at strain rates - 4 ' -6 from 10 /sec to 10 /sec and at 10 kb confining pressure. Talc, pyrophyll ite, and platinum jacketing were used to vary water content. From 700° to 850°C both mechanical twins (T'Ol) and translation g l i d -ing (100) were observed. Twin development appears to be favored over gliding at high confining pressures, lower temperatures, and higher strain rate. Above 850°C subgrain development and recrystal l izat ion occur just prior to melting. A flow law, e = ^1.5 x 10" 1 exp (-38/RT) a 4 , 8 describes steady state deformation from 750° to 910°C under wet condi-tions. Decreasing water and temperature are accompanied by increasing revalues and perhaps increasing activation energy. At 750°C and under dry conditions an exponential relationship, i = 53 exp (.23a) best f i t s the data. From 910° to 950°C the amphibole structure "hardens" such that strain rate remains constant for a given load. This hardening is interpreted to be related to oxidation and distortion within the l a t t i ce . Uncertainty regarding the activation energy precludes effective extrapolation of the data to "geologic" strain rates. J 85 INTRODUCTION Amphiboles represent a s i g n i f i c a n t mineral group in both igneous and metamorphic c rus ta l environments. Although common in intermediate igneous rocks, amphiboles are found throughout the composition range of p lu ton ic rocks, being less important in vo lcan ic rocks. During metamorphism, amphiboles c r y s t a l l i z e over a wide range of pressure and temperature cond i t ions . An thophy l l i t e , cummingtonite, t r emo l i t e , glaucophane, and hornblende are commonly assoc iated with metamorphic rocks and can be dominant minerals in s ch i s t s and gneisses. Because of th i s wide d i s t r i b u t i o n , presence of amphiboles contr ibutes to the mechanical response of c rus ta l rocks under d e v i a t o r i c s t re s s . An explanation of tec ton i c processes must inc lude fundamental understand-ing of rock and mineral behavior. To th i s end rock-forming s i l i c a t e s such as quar tz , pyroxene, o l i v i n e and to some extent amphibole have been studied in compression experiments. This study continues the inves t i ga t i on of the mechanical propert ies of amphiboles. The amphibole s t ructure i s character ized by i n f i n i t e l y long paired chains of t e t r ahedra l l y coordinated ca t ions . A l te rna t ing pa i r s are inverted with respect to one another, forming a l ayer of t e t r a -hedra. F i g . 30A i l l u s t r a t e s these layers and shows the incomplete l ayer o f j o i n i n g cat ions (Ml, M2, M3, M4) which are p r imar i l y o c t a -hedral in coord inat ion . The vacant port ions shown adjacent to the octahedral layers are the potent ia l A s i t e s . Or ient ing the £ axis v e r t i c a l l y and pro jec t ing onto (010) (F i g . 30B), the monocl inic form i s seen. The two common space groups are a lso shown. On the l e f t i s the body centered, I 2/m, space grouping used by Warren (1929) in his 86 Figure 30 I dea l i zed amphibole s t ruc tu re and c r y s t a l ! o g r a p h i c elements. See tex t f o r d i s c u s s i o n . 87 o r i g i n a l s t ruc tu ra l ana l y s i s . On the r i gh t i s the more commonly used C 2/m space grouping. Notably, the (001) in I 2/m grouping becomes (-101) in C 2/m. For th i s d i scuss ion and f o r those referenced, the C 2/m space group i s used. In C 2/m the " l a y e r i n g " i s p a r a l l e l to (100), the op t i c axis i s approximately normal to (101),.and the pole to (001) i s approximately 30° away from z. F i g . 30C shows a Wulff net p ro jec t ion looking down the c_ axis with the [T01] and [001] d i rec t i on s shown as well as the d i s t i n c t i v e pr i smat ic cleavage. Previous deformation studies have been d i rec ted towards de s c r i b -ing and determining the mechanisms of p l a s t i c f low. In order to accom-modate s t r a i n without loss of cohes ion, the amphibole s t ructure has been observed e i t h e r to twin mechanical ly by simple shear along the (T01) plane or to kink by s l i p along c l o se l y spaced (100) planes. In s ing le c r y s t a l experiments, the most f requent ly documented feature has been twinning on (101) (Buck, 1970; Rooney, et a l . , 1970; Rooney and Riecker, 1973; Rooney, et a l . , 1974; Rooney, et a l . , 1975). The bulk of these experiments was on s ing le c r y s t a l s compressed p a r a l l e l to [001] at various conf in ing pressures (5-20 kb), temperature (20 -1200°C), and at s t r a i n rates o f 1 0 " 5 / s e c . When o r i en ta t i on was v a r i e d , decreases i n strength were observed as well as a decrease or lack of twinning on (T01) (Rooney, et a l . , 1975). A decrease of (T01) twinning i s a lso descr ibed above 800°C. However, Buck (1970) found (T01) twinning in samples of var i ab le o r i en ta t i on when compressed at 10~^/sec and at room temperature. T rans la t ion g l i d i n g on the system T = (100), t = [001] has been observed as a secondary mechanism in s ing le c r y s t a l s (Rooney, et a l . , 1975). Amphibole aggregates reveal twinning on (101) or complex kink bands related to the system T = (100), t = [001] (Rooney, et a l . , 1975). Dollinger and Blacic (1975) suggested that slip occurs exclu-sively on (100) in the aggregate they studied and that the high resol*' ved shear stress necessary for (101) twinning occurs only in favorably oriented grains ([001] parallel to a-|). Recent transmission electron microscopy is in agreement with slip on the system T = (100), t = [00l] with additional possible systems of (010), [001]; (010), [100]; and (001), [100] (Morrison-Smith, 1976). Plastic deformation features in naturally deformed amphibolites are rarely observed (Carter and Raleigh, 1969). However, (101) twin-ning has been described in shock regions associated with nuclear explosions (Borg, 1972) or meteor impact (Chao, 1967). Other occur-rences include xenoliths in kimberlite-bearing diatremes of the Colo-rado Plateau (Gavasci, 1973) and in the highly deformed rocks of the Ivrea-Verlano zone in the southwestern Alps (Rooney, et a l . , 1975). Slip on the system T = (100),t = [001] has been recently observed in a zone of imbricate thrusting in the northern Cascades of Washington (Dollinger and Blacic, 1975). The objective of this study is to clarify the mechanical proper-ties of a relatively pure hornblendite including the contributions of mechanical twinning, translation gliding, and recrystallization. This is done by experimental deformation experiments in which either strain rate or load is held constant while temperature is varied. The range of temperature is from 700° to 1000°C (0.7 to 1.0 T . homologous1 temperature) and the confining pressure is 10 kb except for a few runs at 15 kb. All experiments were "wet" except at lower temperatures (700° to 800°C). The data will be presented in two parts. The f i rst part describes the chemistry and structure of this amphibole and compares i t to earlier studies. Relative strength characteristics are outlined and the resulting deformation fabrics are correlated with nominal strain, strain rate, and temperature conditions. The second part combines data from constant strain rate and constant load experiments to determine coefficients of empirical flow equations. These equations are then related to the various fabric elements. PART I: CHEMISTRY, STRENGTH, AND DEFORMATION MECHANISMS STARTING MATERIAL The amphibole aggregate selected for these experiments is an igneous calcic amphibole collected near Hope, British Columbia and will be referred to as Am-2. This hornblendite is a part of an ultra-mafic body within the Spuzzum intrusion and occurs as a "dike" with both sharp and gradational contacts (Richards and McTaggart, 1976). The material has a foliation in which z_ optic axes l ie in a plane. Cores were cut parallel to this foliation. The other optic axes are variable and indicate a correspondingly variable c crystallographic axes. Samples examined in thin section are exceptionally pure, with <10% (commonly <5%) impurities consisting of chlorite, plagioclase, quartz, and opaques. The average grain size is approximately 0.4 nm in width and 0.9 mm in length. Generally, the pleochroism is light brown to greenish brown (x = light brown, y = reddish brown, z = greenish brown); however, a greenish-blue rim is seen in some cases. This variation may be related to a later amphibole growth, as proposed 90 by Vining (1976). Growth twins on (100) are very common. The extinc-optical direction lies essentially parallel to [101]. A representative composition from eight electron microprobe analyses of separate grains in two samples is Variation in the major components Na, Ca, Mg, Fe, Al , and Si is less than 0.10 formula units. The total weight percent for each sample was ^97%. From Ernst (1968), this composition falls in the Ca-series and approximates a magnesiohastingsite. Although l i t t l e work has been done on the phase relationships of magnesiohastingsite, a recent study has shown pargasite to be stable up to 1060°C at 8 kb water pressure (Holloway, 1973). The addition of iron to the pargasite composition reduces the stability (Ernst, 1968). Such a reduction is in agreement with the present experiments where partial melting was observed at 950°C and extensive melting occurred above 1000°C at 10 kb confining pressure. Probable site occupancy includes incomplete saturation of the tetrahedral sites (Tl) with respect to aluminum (Papike, et a l . , 1969) and additional aluminum located preferentially in M2 (Ernst, 1968). The M4 site is dominated by Ca and approximately half of the A sites are f i l led, primarily with Na (refer to Fig. 30A). Table II is a comparison of the compositions of three experimen-tally deformed amphiboles. Although all three are dominated by Ca in the M4 site, there are greater amounts in the amphiboles of Riecker and Rooney and the present study. Other distinctive features of Am-2 tion angle is close to 15° (2V = 75° - 80°) which means that the z A ( K 0 5 N a > 4 5 ) ( C a 1 > 7 6 Na < 2 4)(Mg Al TABLE II: COMPARISON OF COMPOSITION OF EXPERIMENTALLY DEFORMED AMPHIBOLES 1) (Na < 3 / > 1 3 ) (Ca 1 > 7 4 Na ^ 5 > Q 3 A l ^ S i ^ 2) (Na > 4 9 K > 0 4 Ca 1 < 4 9 ) (Mg 3 < 1 0 Fe ; 2 4 6 T i J 5 A1 > 6 5 Cr > 0 1 Mn > 0 3 ) 5 A Q A l l i 2 5 S 1 6 > 7 5 3> ( N a .45 K .05 ) ( C a 1.76 N a .24) (%.12 F e 1.30 T i .22 A 1 .50 M n .02 ) 5.16 A 11.48 S i6.52 Source: 1) Rooney, et a l . ; Science, 169, 173-175, 1970. 2) Dollinger, G. and J.D. Blacic; Earth Planet. Sci. Let., 26, #3, 409-416, 1975. 3) This study. are (1) more aluminum, particularly in the tetrahedral sites, and (2) more of the large A sites are f i l led. The presence of Al produces significant kinking of the tetrahedral chains in order to f i t the larger Al-rich tetrahedra to the octahedral strips. Also the presence of Al in the tetrahedral sites produces a charge-balance displacement of Na along the crystallographic b direction, away from the mirror plane, to more closely approach the Al-rich (Tl) tetrahedra (Papike, et a l . , 1969). EXPERIMENTAL APPARATUS Experiments were conducted in a modified solid-medium Griggs-type apparatus (Griggs, 1967). Basically a piston-cylinder design, this machine is capable of high confining pressures (20 kb), high tempera-tures (>1200°C), and either constant strain rate or constant load conditions for long periods of time. For this study three experimental conditions were employed. First, constant strain rate experiments were used to evaluate the temperature and strength relationships. Second, relaxation experiments, in which the motor is stopped and the machine is allowed to rebound elastically, were used to evaluate the stress and strain rate relationships. Third, constant load experiments were used to evaluate temperature and strain rate relationships. Confining pressure was held at 10 kb for all experiments except for five runs at 15 kb. The test specimens are right cylinders (6.3 x 19.0 mm) and are enclosed in a relatively weak solid confining medium of either talc or pyrophyllite. Pyrophyllite dehydration occurs at lower tempera-tures than talc (Fig. 31) and was used to add water to runs below 800°C. Because of the potential errors related to the strength of the confining medium (Blacic, 1971; Edmond and Paterson, 1972), flow stress was taken at 5% strain and is believed to be accurate to within 0.5 kb at the higher stress levels. This low value of strain was chosen to eliminate as much as possible the resistance effect of talc due to sample bulging. Following the discussion of Blacic (1971, p. 52 - 56), the shear stress exerted by the talc on the sample is assumed to be effectively negated by the friction of the talc on the inner wall of the pressure vessel. Five percent strain is beyond the yield point and in the plastic region of the stress-strain curve (Fig. 32). At 700°C strain hardening is apparent; while the curves at 800°C and above show more nearly steady-state conditions. Temperature is monitored by a Pt/Pt-10%Rh thermocouple which is placed next to the center of the sample using a modified assembly design suggested by Carter (personal comm., 1971). Because of the large thermal gradient associated with the tungsten-carbide piston at the ends of the specimen, strength values reflect deformation seen in the central third of the sample and optical work is confined to that area except in the observations concerning changes in pleochroism. STRENGTH CHARACTERISTICS The f i rst series of experiments was designed to establish strength characteristics over a range of temperatures and strain rates. Fig. 32 is a composite of stress-strain curves and is not intended to reflect more than the general temperature effects beyond 5% strain. The most apparent strength relationship is the drop associated with the interval 800° to 850°C. Riecker and Rooney (1969, 1973) reported similar weak-400 600 800 TEMP , °C Figure 31 Composite of dehydrat ion react ions f o r p y r o p h y l l i t e and t a l c . P y r o p h y l l i t e ( P y ) -> Kyanite(Ky) + quartz(Q) + H?0 vapor (V) ; Wall and Essene, 1972. Ta l c (T ) -> Ens ta t i te (En ) + quartz(Q) + H 20 vapor(V) ; K i t aha ra , et a l . , 1966. % STRAIN Figure 32 Composite of s t r e s s - s t r a i n curves f o r var ious s t r a i n ra tes . erring phenomena in experiments using anhydrous confining mediums such as boron nitride, AlSiMag ceramic, sodium chloride. Three different explanations were proposed: (1) dehydration of hornblende causing a rise in pore pressure, (2) "hydrolytic" weakening as described by Griggs (1967), and (3) loss of structural hydroxy! units. Because the present experiments were conducted with either a talc or pyrophyl-l ite confining medium, no clarification can be provided as talc break-down occurs in this temperature range at 10 kb (Kitahara, et a l . , 1966; Fig. 31) and pyrophyllite dehydrates below this temperature (Wall and Essene, 1972; Fig. 31). Of 30 constant strain rate experiments, those at a'.strain rate of 10"^/sec are compared to other published and unpublished data (Table III). Although the experimental apparatus precludes accurate strength measurements, the large difference between the present study and those of earlier workers is apparent. At low temperature (700° and 800°C), Am-2 is actually stronger than single crystal experiments of Riecker and Rooney. Some of this strength difference can be explained by the different apparatus. Riecker and Rooney used a solid medium apparatus but of a smaller design. Blacic (1971) found that below the dehydra-tion point of talc, the strengths generated on the large apparatus appear high when compared to the small apparatus or the apparatus using a gas confining medium. The amphibole of Dollinger and Blacic has negligible strength above 850°C. At higher temperature (900° and 950°C), the strength of Am-2 approachs that of the amphibole used by Riecker and Rooney and both amphiboles melt above 1000°C. To explain this high strength aspect of Am-2, three ideas are proposed. First, kinking of tetrahedral chains related to the high TABLE I I I : COMPARISON OF STRENGTH DATA (e = 10" /sec, P = 10 kb, Talc confining medium) TEMP. FLOW STRESS1 SAMPLE2 SOURCE (3%) (10%) 600 13.7 17.3 A1-54T RR 4.3 8.5 A1-92T RR 3.5 7.4 AB1-90T RR 13.7 13.5 W-17 DB 700 3.6 6.3 A1-83T RR 5.9 7.7 A1-84T RR 2.7 5.8 A1-91T RR 4.6 7.5 AB1-121T RR 3.1 5.2 (6%) AB1-153T RR 18.2 25.9 (8%) GR-31 N 750 6.6 8.1 W-14 DB 9.2 10.3 W-16 DB 8.1 11.0 W-20 DB 21.9 26.3 GY-115 N 800 9.1 10.8 A1-56T RR 4.9 9.3 AB1-91T RR 2.1 4.7 AB1-97T RR 3.6 4.4 W-ll DB 3.2 4.2 W-15 DB 14.7 16.0 GY-83 N 850 1.0 3.0 W-13 DB 9.9 12.2 GY-93 N 8.0 9.7 GY-85 N 900 2.2 5.5 (6%) AB1-92A RR 6.6 7.4 GR-5 N 5.2' . - GY-95 N 950 2.9 5.1 GR-33 N 2.1 GY-86 N 1000 0.1 0.2 AB1-94T RR 1) RR flow stress at 2% and 10%. 2) A l l are t a l c runs except AB1-92A which i s AlSiMag. 3) RR = Rooney and Riecker, Erivir. Res. Pap., #430, A.F.C.R.L., 1973. DB = Dollinger and B l a c i c , Earth & Planet. S c i . Let., 26, 1975. N = This study. aluminum content and the high occupancy of the A sites may present barriers to simple glide mechanisms. Second, the presence of other mineral species has significantly affected the strength measurements in earlier aggregate experiments. The Bamle amphibolite (AB-1) con-tains 70% hornblende (Rooney and Riecker, 1973); and the igneous horn-blendite of Dollinger and Blacic (1975) was estimated to be greater than 70% hornblende. The companion phases include plagioclase, quartz, and biotite. Rooney, et a l . , (1975) note that chlorite and mica appear to accommodate large amounts of strain while the hornblende is rela-tively undeformed. Because Am-2 is particularly pure, the strength values would be expected to be higher. Third, the foliation observed in Am-2 is such that the (100) plane is preferentially oriented approx-imately parallel to the core axis. With the major plane of weakness in an orientation of low shear stress, the grain would also be stronger. DEFORMATION FEATURES Using the 30 constant strain rate experiments, including 4 runs at 15 kb confining pressure, an optical survey was conducted to establish (1) the relationship of strain and mineral orientation and (2) the deformation mechanisms related to temperature and strain rate conditions. Extensive use was made of a four axis universal stage in conjunction with standard stereonet plotting techniques. Mineral Orientation as a Function of Nominal Strain To describe the mineral orientations in a sample, 75 grains were examined within the central third of the sample. Optical X,, Y_> and 7 axes were determined, plotted and contoured on a lower hemisphere equal area net. Fig. 33 is a composite of three samples. Two deformed samples (10% and 20% strain) are compared to an undeformed one. Both of the deformed samples were tested under the same conditions: 850°C, -5 10 kb confining pressure and 10 /sec strain rate. The starting material has a preferred orientation. This fabric can be seen in the f i r s t row of Fig. 33. 1 axes are contained in a great c i rc le oriented parallel to the core axis. Two concentrations are apparent: one near the center of the diagram and one approximately 60° away at the "north" pole. Y_ axes are scattered; however, a great c i rc le str iking 110° describes most of the data with a concentration plunging steeply to the southwest. axes also are scattered with a broad concentration along the east northeast margin. The £ crystal -lographic direction l ies in the X-l plane inclined 15° to 1. Using the above data, small c ircles (^15°) have been constructed around each of the Z, axis concentrations. The resulting picture shows £ oriented both parallel . to, perpendicular to, and at 45° to the maximum compressive stress. The major plane of weakness in amphibole is (100) and this plane contains the £ and b_ crystallographic directions. Remembering that b_ is parallel to X i n the monoclinic system, the general orientation of (100) in the undeformed state can be roughly described by superimposing 1 and Y_ in Fig. 33. From such an operation, i t can be seen that in the undeformed material there is a tendency for (100) to be parallel to the core axis with lesser development of other orientations. A preferred orientation of (100) in a low shear stress environment would contribute to the high strength values for Am-2. At 10% stra in, the concentration of optic axes seems to be more clearly defined. The 1 axes cluster about the principal compressive gure 33 Opt i c ax is o r i e n t a t i o n as a func t ion o f percent s t r a i n . Great c i r c l e s descr ibe planes of concent ra t ion (see text ) Small c i r c l e s in upper l e f t i l l u s t r a t e pos s ib le c axes con cent ra t ions in undeformed s t a t e . Arrows mark core ax is which i s the axis of maximum compressive s t r e s s . Contour i n t e r v a l s are >1%, >4%, >7%, >10% per 1% a rea . Counting template was a f t e r S t i r e w a l t , et a l . (1967). 100 stress axis. The Y_ axes are spread more uniformly along a great c i rc le which is striking again ^110°. The X axes concentrate at the east and west margins and also suggest a great c i rc le distr ibution. At 20% stra in, extensive reorientation is apparent. 2 axes dis-tribute fu l l y around the diagram with a strong concentration at ^70° to the principal stress. Y_ axes f a l l on a great c i rc le inclined only s l ight ly to the equatorial plane and almost include the ,Z maximum. In this last instance X. axes are s ignif icantly scattered with possible concentrations in a zone running from the northeast to the southwest quadrants. Therefore, with increasing strain 2 axes rotate away whereas the X axes rotate towards the maximum compressive stress. The X axes maintain nearly the same position. With continued strain i t could be expected that 2 and Y_ axes (approximately (100)) would l i e in a plane normal to the principal stress while X would cluster around that stress direction. This rotation is s ignif icant in that the major plane of weakness (100) appears to rotate away from the prin-cipal stress towards the plane of the least stress. Deformation Mechanisms Six fabric elements were observed in conjunction with the rotation of grains. These elements are brecciation, sharp kink bands, broad undulose extinction, subgrains, recrysta l l izat ion, and melting a l l of which can be assigned to temperature, strain rate, and strain environments. As before, observations were confined to the central portion of the sample. v Brecciation occurs both as intergranular abrasion and as fragmen-tation of large grains. At low strain (^ 5%) the samples display mottled birefringence and undulose extinction with l i t t l e fragmentation. At 5% and 10% strain, grains begin to rotate and interact such that small angular portions between grains fragment, reducing overall grain size. In high strain conditions (>15%) large grains rotate away from the principal stress and break into serial sections, frequently parting on the (101) and (001) planes, possibly following earlier kink or twin boundaries (Fig. 34). Although brecciation is seen under all strain rate conditions, faster rates produced more intense milling. Separa-tion along through-going surfaces is seen in high strain (>17%) runs -4 o at 10 /sec. These surfaces develop as shear zones at 850 C and are not apparent above 900°C. Through-going shears were not commonly -5 -6 s present at rates of 10 /sec or 10" /sec. The result of brecciation is a texture of fragmented large grains surrounded by a matrix of finer-grained material. Sharp kink bands and undulose extinction are associated with brecciation. These features are observed in all strain rate environ-ments and indicate a temperature dependence. The kinks and broad warp-ing should be considered as separate mechnisms. Fig. 35 shows the occurrence of both elements in the same grain with an angular differ-ence of approximately 30°. The principal stress axis for the sample (6R-56, 700°C, 10~*Vsec) is oriented northeast-southwest. Consequently, the long direction, [001], of the grain is oriented in a plane of high shear stress. In a few instances the age relationship between these features was_established. The sharp kinks appear to precede the development of the undulose extinction. Sharp kinks also were observed at low strain in grains oriented with [001] nearly parallel to the principal stress-.whereas,the undulose pattern developed only when [001] and the (100) plane were in a zone of high shear stress. Because of Figure 34 B recc ia t i on of large grains with segments approximately normal to c c ry s ta l l og raph ic ax i s . Figure 35 Occurrence o f broad warping and sharp kinks i n the same grain with angular d i f fe rence of 30 . .ML the init ial preferred orientation, in which (100) is parallel to the core axis development of sharp kinks f irst is consistent with grain rotation related to strain. The sharp kinks are very similar in appearance to the mechanical (101) twins described by Rooney, et al. (1975). These twins formed as the dominant ductile deformation mode when singled crystals were compressed parallel to [001]. In Fig. 36 the principal stress axis is located northeast-southwest such that the twin plane is inclined at 70° to the compression axis and [001] is nearly parallel. In addition to orientation, four other variables are associated with these mechanical twins. First, twin development is a function of con-fining pressure. Two experiments at 15 kb confining pressure (800°C, -5 10 /sec) reveal a significantly greater number of sharp kinks in favorably oriented grains. Second, temperature affects twin develop-ment. Table IV compares five runs at 10 kb confining pressure in which 50 grains were examined for easily visible twins. Temperature increase is accompanied by decreased twinning, suggesting that some other mechanism is favored. These observations agree in part with those of Rooney, et al. (1975) in which they found that mechanical twins occur only below 800°C and the same twinning is enhanced at 15 kb confining pressure. In the present study, twins are found at higher -4 temperature, but in less quantity. FinaVTy, fast strain rate (10 / sec) experiments were highly fractured in the temperature range of 700° to 800°C. Frequently fractures paralleled twin directions and in some cases portions of sharp kinks were observed on one side of the fracture. Therefore, sharp kinks form at low strains and lose cohesion with increased strain (>15%). Because of fracturing, effect of strain TABLE IV: DEVELOPMENT OF MECHANICAL TWINS AS A FUNCTION OF TEMPERATURE Sample f. ' %e Temp. # Twins GR-47 10" 4 / sec 18.4% 700°C 20* GR-56 1 0 ' 6 / s e c 8.3% 700°C 31 GR-48 10" 4 / sec 21.1% 900°C 5 GY-104 10" 6/.sec 12.3% 900°C 13 GR-54 10" 6 / sec 8.0% 950°C 6 (1) Number of twins per 50 grains * GR-47 is highly f ractured and many of the f ractures p a r a l l e l would be twin d i r ec t i on s . o 4=» 105 rate was not clearly established; however,' sharp kinks develop more readily at higher strain rates and low strains. Undulose extinction and bending of grains are common in all of the experimental conditions. Degree of bending is related to nominal strain and nature of strain accommodation. Higher strain rate and brigh temperature conditions tend to produce narrow ductile zones in which bending of <25° is seen. With continued deformation these grains break. Associated with undulose extinction are fine lamellae which are characterized by alternating light and dark birefringences. Fig. 37 shows the lamellae strongly developed with extinction bands normal to the lamellae direction. A (100) growth twin can be seen in the center of the grain suggesting that lamellae are in fact approximately parallel to (100). Dollinger and Blacic (1975) documented similar lamellae in their experimentally deformed hornblende. Lamellae defini-tion becomes sharper with decreasing strain rate such that the best examples were seen at 10"6/sec and at temperatures of 700° to 800°C. Recent work with transmission electron microscopy shows small platelets of exsolved material nucleating parallel to dislocation lines in deformed amphibole (Morrison-Smith, 1976). Increased definition of lamellae at lower strain rates would be compatible with initiation and growth of a new phase related to deformation. Electron microscopy confirms optical observations suggesting that these lamellae are defor-mation lamellae related to movement of dislocations along a slip plane. Like mechanical twins, deformation lamellae are less apparent at higher temperatures. Although there is some indication that twins are favored at high strain rate and definitely high confining pressures, both lamellae and 106 Figure 36 Sharp kink development at 70° to principal compressive stress. Figure 37 Lamellae development associated with undulose extinction. Lamellae are approximately parallel to (100) growth twin. twins occur together. Fig. 38A displays a single grain in which mechanical twins are formed on one side of a growth twin while defor-mation lamellae are produced on both sides. Fig. 38B illustrates the crystallographic elements of Fig. 38A. To explain this association, Fig. 39 shows the C 2/m system with (100) as & twin plane. A compres-sive stress axis is drawn at 45° to (100). This same axis is 60° to the (101) plane on one side of the twin and 30° on the other. If the grain rotates, one of these potential twins could become active while the other would actually experience a decrease in shear stress. To explain the apparent earlier development of mechanical twins, the stress axis could be placed init ia l ly parallel to the c. axis. If the grain rotates in a counterclockwise direction,;the (101) plane of the "untwinned" portion will be at 45° to^a-j after 30° rotation. If the grain continues to rotate, the (100) slip plane would become active producing the secondary warping. (This assumes that the critical resolved shear stress is great enough for both systems to operate). An opposite pattern would develop with clockwise rotation of the "untwinned" portion. Crystallographic measurements were made to determine the active slip systems. Using the pole to the kink band and the axis of external rotation, both slip plane and slip direction can be determined (Raleigh, 1968). In cases involving broad warps rather than well defined kink bands, the boundary trend was used in conjunction with the external rotation axis to approximate a "kink band" pole. The resulting ele-ments for each grain were rotated so that the c axis of the undeformed portion was located in the center of the stereonet. In this way several grains can be compared, but the specific stress orientations are Lamellae and mechanical twin development in a single grain. Mechanical twins are con-fined to one side of a growth twin. Orientation of crystallographic elements for apne A X E S 9 r a - | n s n o w n a bove. Slip system (100) [001] is OPT IC A X E S a c t i v e o n b Q t h s i d e s Q f £ 1 0 Q j g r ( ) w t h ll Y (101) mechanical twins only occur on one side •- z (at higher shear stress orientation). 109 F igure 39 (100) growth twin at 45 to a un i ax i a l compressive s t r e s s . The po ten t i a l mechanical twin planes at at 60 and 30 . variable in each diagram. Changes in s l ip mechanisms are associated with temperature and strain rate (Fig. 41), remembering that the pole to the kink band approximates the s l ip direction and that the s l ip plane is defined by this pole and the axis of external rotation. Three conditions were examined: (a) 700° and 10" 5/sec, (b) 700° and 10" 6/sec, and (c) 900° -6 and 10" /sec. In a l l cases, the poles of sharp kink bands plotted around the [101] direction (Fig. 40). This feature is very easy to establish in this amphibole as the z optic direction is close to [101]; consequently, the grain goes to extinction as the twin boundary is rotated paral lel to the cross hairs. The s l ip system associated with ! -5 o deformation lamellae is more variable. At 10 /sec and 700 two dist inct systems were observed (Fig. 41). Primarily, s l ip was accom-modated on the (100) plane and in the [001] direction; however, four measurements gave a s l ip direction of [010]. Fig. 42 is a photo of one such instance in which undulose extinction bisects the obtuse angle of intersecting cleavages, the (010) plane. At 10" 6/sec and 700°C axes of external rotation s t i l l cluster about [010]; but there is con-siderable scatter suggesting operation of some other s l ip planes. Poles to kink boundaries are s t i l l clustered around [001] and spread out along the (100) plane to include three near the [010] direction. At 10" 6/sec and 900°C both rotation axes and kink band poles are scattered indicating s l ip on a variety of planes. Therefore, increased temperature is accompanied by more variable s l ip systems. Similar measurements made by Dollinger and Blacic (1975) on samples deformed at 10" 5/sec and 600°-750°C as well as 800°-850°C show a well defined system, (100)[001], at the lower temperature and a scatter of the Figure 40 The poles o f the " sharp" kink bands c l u s t e r about the [101] d i r e c t i o n f o r a l l experimental cond i t i on s . 112 Doo] Figure 41 Changes" i n s l i p mechanisms as soc ia ted with temperature and s t r a i n r a t e . external rotation axes along the (100) plane with an implied variation of slip direction in the same plane at the higher temperature. Occurrence of both kinks and broad warping in grains with large growth twins provides a unique opportunity for examining shear stresses necessary for each mechanism. Three such cases were investigated in which kinks occur only .on one side of the growth twin while deformation lamellae are seen on both sides. Fig. 38B shows crystal-lographic data for the grain pictured in Fig. 38A; GY-105, 800°C, -6 10" /sec and 6.8% strain. Using the angles for twin and glide elements as well as a stress difference (a D) measured at the transition from elastic to ductile behavior, "yield point", a critical resolved shear stress was calculated for each system. Because twins occurred on only one side of the growth twin, a minimum value of shear stress can be proposed. Table V gives the results for the three grains. The values of shear stress are comparable for both systems at 700°C. However, at 800°C and 900°C, that for (101) twinning appears to be higher than the corresponding slip system. The difference is reflected in the average resolved shear stress coefficient (SQ). Keeping in mind that these numbers are approximations, slip is energetically favored at higher temperatures. Therefore, thermal effects aid in dislocation propagation. Subgrains were observed at higher temperature conditions and pre-cede recrystallization. Strain rate affects the development of sub-grains. At 10"4/sec f i rst subgrains were observed at 850°C and 20% strain; whereas, at 10"6/sec they were observed at 800°C and 10% strain. Strain effects were not clearly established as there were insufficient runs at low strain conditions. However, two runs at TABLE V: CRITICAL RESOLVED SHEAR STRESS FOR TWINNING AND GLIDE SYSTEMS Sample Temp. Strain rate (100)[0011 (Tor) 2 ( T o i ) 3 GR-55 700°C 10" 5/sec 4.3 kb 4.5 kb 2.5 kb GY-105 800°C 10" 6/sec 5.0 kb 6.9 kb 2.7 kb GY-104 900°C 10" 6/sec 1.5 kb 1.8 kb .9 kb S S S 0 0 0 .39 .46 .23 1) T c = (a-, " o3) cos a cos b; where a = a^Aslip plane and b = 0.|A s l ip direction 2) (101) twin developed 3) (101) twin undeveloped 800°C, 10"5/sec and different strains (23% versus 16% strain) suggest that increased strain slightly lowered the temperature for subgrain development. Fig. 43 is a plot of data on a temperature, strain rate, and strain diagram. Strain rate dependence of subgrains formation is shown. Recrystallization occurs in this amphibole only in a small temperature interval prior to melting. Fig. 43 shows the strain _4 rate effect on recrystallization. At 10 /sec recrystallization is seen only in the sample run at 950°C in which small melt pockets were -fi apparent. At 10" /sec the zone of recrystallization included those runs at 900°C. Therefore, both recrystallization and subgrain develop-ment appear strain rate dependent, an observation which is most likely related to diffusion rates in amphibole. Fig. 44A shows beginning of recrystallization occurring along grain.and subgrain boundaries. Fig. 44B shows fine linear features parallel to (101) with associated "bubble"-like structures near the subgrain boundary. This relationship is interpreted to be a diffusion phenomenon parallel to the a^  crystal-lographic direction. Because of the proximity of the melt boundary and the recrystallization zone, no whole-scale recrystallization texture was observed. Melting began uniformly above 950°C. Finally, two other features were recorded. First, the effect of water seems to accentuate the deformation mechanism in each temperature and strain rate condition. At low temperature (700° and 800°C) and at 10" /sec use of pyrophyllite assemblies with the associated increase of water significantly weakened samples, and the texture exhibits intense brecciation and loss of cohesion. In a talc run at 825°C, 10"4/sec, 19.3% strain, the zone of deformation was characterized by 117 Figure 43 Composite of constant s t r a i n ra te experiments i n r e l a t i o n to temperature, s t r a i n r a t e , and percent s t r a i n . Figure 44A R e c r y s t a l l i z a t i o n of amphibole along grain and subgrain boundaries. a small barrel shape related to the dehydration halo. At high temper-atures and 10~ 4/sec (^20% s t r a i n ) , deformation was more homogeneous. The best example of lamellae development i s seen in a "wet", 700°C, -fi ' 10" /sec run. The second feature i s a pronounced change in pleo-chroism associated with temperature. I n i t i a l l i g h t brown and greenish brown colors change to reddish brown and brown. Fig. 45 shows a run conducted at 10" 6/sec and 950°C. The center (900° to 950°C) i s di s -t i n c t l y darker than the ends (600° to 700°C). Therefore, at 850° to 900°C chemical or structural variation affects the pleochroism. In summary, Am-2 i s a very strong c a l c i c amphibole displaying an abrupt reduction i n strength over the 250°C temperature range p r i o r to melting. Addition of water below the t a l c dehydration temperature lowers strength s i g n i f i c a n t l y . Brecciation i s a common textural fea--4 ture which i s strongly developed at 10 /sec, >15% s t r a i n , and at lower temperature. Two duc t i l e deformation mechanisms were observed: (101) mechanical twinning and translation g l i d i n g primarily on the (100) plane. Twinning i s favored at high s t r a i n rate and higher con-f i n i n g pressure;-whereas glide i s favored at high temperature (800° and 900°C). The s l i p direction and s l i p plane for glide become more varia-ble with increased temperature. Both ductile mechanisms become less apparent above 850°C. Coincident with this temperature subgrains are observed. Near 900GC a change i n pleochroism takes place and re c r y s t a l -l i z a t i o n i s observed j u s t prior to melting along subgrain boundaries and twin planes. Both r e c r y s t a l l i z a t i o n and subgrains show a s t r a i n rate dependence, being more apparent at low s t r a i n rates. Melting begins above 950°C in a l l cases. 120 Figure 45 Color change associated with temperature. Temperature varies from 950 C at the bottom of the photograph to 650 C at the top. PART I I : FLOW EQUATION THEORY In order to compare, various s i l i c a t e s in a given tectonic environ-ment, i t i s necessary to quantify material behavior by means of empiri-cal equations. Ductile behavior i n s i l i c a t e s has been successfully described by the relationship e = A exp ( " Q c ^  f(a) (1) R ' T where e i s the steady-state s t r a i n rate, A i s a material constant, Q„ J C i s activation energy for creep, R i s the gas constant, and f(o) i s a function of d i f f e r e n t i a l stress (a^ = - a^) (Carter and Ave'Laile-mant, 1970; Raleigh and Kirby, 1970; Post, 1973; Raleigh, et a l . , 1971; Heard, 1972). The function of stress providing the best f i t to the data has been a power r e l a t i o n , a n , in which n_ varies from 3 to 6, as predicted i n the models of Weertman (1970). Other stress functions have been observed, p a r t i c u l a r l y at higher stress l e v e l s . Typically in these s i t u a t i o n s , e i s proportional to exp (ba) (Dorn, 1954; Garo-. f a l o , 1965). A single equation has been proposed to describe both situations in which e i s proportional to (sinhaa)" (Garofalo, 1965). Assuming a power relat i o n s h i p , equation (1) can be studied in detail by p l o t t i n g experimental data on a log stress versus log s t r a i n rate plot. The effect i s to transform equation (1) to the following form, log £ = log A - (Q c/RTlnlO) + nlog a (2) To examine the rate relationships, three types of experiments were conducted. F i r s t , constant s t r a i n rate tests were used to evalu-ate strength as a function of temperature. Second, following the procedures described by Raleigh and Kirby (1970), relaxation experi-ments in which a , Ty and e are known were used to determine an appar-ent activation energy directly by the simplified equation from Dorn (1954), Qc = V i R l n U ^ ) (3) T 2 - T1 With Qc and n_ determined, an equation constant A can be calculated and thereby a complete flow equation proposed. CONSTANT STRAIN RATE EXPERIMENTS Stength data from Fig. 32 (5% strain) were plotted on a log stress versus log strain rate plot in order to test the power law relationship (Fig. 46). Reference to Fig. 32 reveals that only runs above 800°C approach steady state conditions. Consequently, straight lines, iso-therms, were f i t to the three highest temperature conditions using a two variable regression on a Hewlett Packard desk calculator. Increas-ing temperature is accompanied by an increasingly steep slope or a decreasing n_ value. The low temperature runs (700° to 825°C) have irregular patterns of strength values, perhaps reflecting the uncer-tainty of water activity. A line through the two points for 700°C (dry conditions) suggest a very flat slope, high revalue, and a decreased sensitivity of the eao nrelationship >. (Fig. 46). If a single mechanism of deformation is presumed to operate from o o 850 to 950 , then a multivariable linear regression can be performed on the 11 data points of this temperature interval. Reference to equation (2) shows that three variables o , T, and e can be entered and best values for n_, Q„, and A generated. Fig. 47 shows results 123 • . o o o-TOO " »-800 Figure 46 Strength data at 5% strain plotted to test power law rela-tionship. See text for discussion. A-850 o- 900 Ill • - 990 it , o v A \ * CO CD UJ •-n • 4.62 CC h-co 3 CD o 4 3 6 7 \ S ^ -LOG STRAIN RATE ^ \ Figure 47 High temperature data from Figure 46 f i t by a three vari-. able linear regression. Although a poor f i t (r 2 = .64) the slope does give a feeling for the n_ value. of such a regression. In this case the f i t i s poor (r = .64); how-ever, i t does give a feeling for the n_ value (4.82 ± 2.62) (1 standard deviation). RELAXATION EXPERIMENTS To c l a r i f y the trend of n_ values as a function of temperature, a series of relaxation experiments were conducted s i m i l a r to those of Raleigh and Kirby (1970). In these experiments samples were strained (^10%) such that they were deforming in'steady state (except) those runs below 800°C and dry which were s t i l l s t r a i n hardening) and then the motor was turned of f . Knowing the " s t i f f n e s s " or modulus, k, of the appratus, incremental stresses and s t r a i n rates can be calculated as the machine rebounds e l a s t i c a l l y against the sample. Fig. 48 i s a plot of four such runs showing the changing slope {n values) as a function of temperature and, i n d i r e c t l y , t a l c dehydra-tion. Each run i s characterized by one continuous slope. In total 14 relaxation runs were made. These data are summarized on an n_ versus temperature plot (Fig. 49). Two lines have been sketch-ed i n to i l l u s t r a t e approximately the effect of water on n_ values. The upper l i n e traces the t a l c dehydration. At 750°C and 10 kb con-fi n i n g pressure, the sample i s assumed to be dry on the basis of phase e q u i l i b r i a work (Kitahara, et a l . , 1966; and Fig. 31). At or near 800°C dehydration of t a l c begins. By 850°C the reaction i s presumed complete. The completion of this reaction i s v e r i f i e d when a thin section of the sample i s cut. A d i s t i n c t halo can be seen i n the t a l c near the center of the sample. As the 800°C isotherm migrates up and down from the center of the sample (with increasing tempera-125 n - 12.5 5 6 7 - LOG STRAIN RATE Figure 48 Four r e l a x a t i o n runs showing the change in n_ values as a f unc t i on of temperature and, i n d i r e c t l y , t a l c dehydrat ion, 12 10 T a l c 700 800 900 1000 Temp. (°C) Figure 49 Fourteen r e l a x a t i o n runs r e l a t i n g n_ to temperature. The upper curved l i n e approximates the e f f e c t o f t a l c dehy-d r a t i o n . The lower curve descr ibes "wet" c o n d i t i o n s . E r r o r bars are one standard d e v i a t i o n . ture) the amount of water increases. Therefore, by 900°C the center of the sample i s s i g n i f i c a n t l y "wetter". The lower l i n e i s based on three runs which were designed to be wet when compared to t a l c runs. (Fig. 49). The experiment at 700°C used a pyrophyllite assembly which has a lower breakdown temperature (Wall and Essene, 1972; and Fig. 31). The 800° and 900°C runs were jacketed with platinum and additional water included. The relationships shown here suggest that water lowers the value at the lower temperatures and that the n_ value drops to a somewhat lower value than the corresponding t a l c run. The very high value of r\_ at 750°C was of interest and the data were replotted as stress versus log st r a i n rate. A s l i g h t l y better f i t (r = .95 vs. .93) was obtained, implying that below 800 C and in dry conditions an exponential relationship of the form e = A exp (bo) describes the flow behavior s l i g h t l y better where b i s the slope of the resulting curve (Dorn, 1954, p. 105). The coefficients i n this case are A = 53.9 and b = .23. CONSTANT LOAD EXPERIMENTS To arrive at a value of activation energy, Q c > s i x temperature raising experiments were run i n which load was held constant. This approach was f i r s t used by Dorn (1954) and l a t e r applied to o l i v i n e by Raleigh and Kirby (1970) and Post (1973) and to orthopyroxene by Raleigh, et a l . (1971) and Ross and Nielsen (1976). Assuming that the deformation process remains constant (A and a are constant), then a small change i n temperature and a corresponding change i n st r a i n rate can be used to calculate Qc by equation (3). 127 Results of these temperature raising experiments are shown in Fig. 50. A single activation energy (38 + 5.5 kcal/mole) (la) can be assigned to the temperature range 750° to 910°C under wet condir tions (pyrophyllite). The single point for "dry" conditions (talc) at 800°C suggests a higher activation energy (50 kcal/mole). With the talc assemblies a Targe peak occurred between 825° and 850°C corresponding to dehydration and the abrupt addition of water to the sample. The weakening effect of water is reflected in a signi-ficant increase in strain rate. There is a high value generated for this temperature interval, because equation (3) is sensitive to large changes in strain rate by the relationship In eg/e-i' Above 830°C the values with talc assemblies approximate those with pyro-phyllite. Above 910°C there is a pronounced drop. In order to show this drop a l i t t le more clearly, a second plot shows all of the 3 kb data on ln e 2/e-| versus 10 /T axes (Fig. 51). The slope. of this curve mimics the activation energy, Qc/R« In both cases o 11 the low point occurs at 950 C. Activation energy tends to increase as melting is approached in anhydrous minerals (Raleigh, et a l . , 1971; Heard, 1963; Heard and Raleigh, 1972; Post, 1973; Ross and Nielsen, 1976; Parrish and Ross, 1976). Perhaps the drop shown here is related to the loss of structural OH" and the beginning of breakdown in the amphibole structure or to some other structural change. If so, the basic premise of constant structure makes this type of experiment invalid for this mineral in this temperature range and does not reflect the true activation energy. With values for Qc and n_, the equation constant A can be calcu-lated. Using n values of 8.1 for 800° to 850°C and 4.8 for 900° to 128 F igure 50 V a r i a t i o n of Q with temperature from s i x constant load experiments. C Q i s c a l c u l a t e d using equat ion #3 i n tex t . 1.0 h — ' ' 1 1 ' t - 1 1 I I t . I t .78 .80 .82 .84 .88 .88 Temp. ( I0 3 /T°K) F igure 51 Three kb constant load data showing the small change s t r a i n ra te a s soc ia ted with 950 C. 950 C and Qc values of 50.7 kcal/mole for the lower interval and 38 kcal/mole for the higher, two separate constants are available: -4 -1 A800 = 5'"' x 1 0 a n d A900 = 1 , 9 x 1 0 * T n e s e c o n s t a n t s apply to strengths derived with t a l c assemblies. Similar calculations for pyrophyllite at 800°C y i e l d an A value of 8.9 x 10~ 2 which i s within a factor of two or the higher temperature wet conditions. To test the resulting flow equations, e = 5.1 x 10* 4 exp(-50.7/RT)o 8 , 1 (A) e = 1.9 x 10"1 exp(-38/RT)a 4 , 8 (B) temperature and s t r a i n rate values were chosen and the resulting stress value compared to experimental results from constant s t r a i n rate tests (Fig. 52). The lower temperature group prediction for 825° and 850° i s f a i r l y close to the observed. The 800°C i s dis- ' t i n c t l y low. Also evident are the anomalously high strengths for the 10~*7sec s t r a i n rate. High values are attributed to diffusion of water away from the hot central portion as a function of time. Such drying out would be p a r t i c u l a r l y noticeable near the breakdown temperature of t a l c . Higher temperature predictions appear to have the proper slope; however, spacing of the isotherms i s too close. This spacing i s controlled by the^activation energy term, suggesting that the Q between 900° and 950°C i s higher than the 38 kcal/mole c chosen. In agreement, the m u l t i v a r i a t e regression performed on the strength data (Fig. 47) yielded a calculated activation energy of 144 ± 48 kcal/mole (To). A plot of strength data and 5 kb creep data for the interval 900° to T0O0°C produced a Qc = 102 ± 16 kcal/ mole (la) (Fig. 53). The implication of both of these plots i s that the value of Q i s considerably higher than the apparent activation 131 • - 800 -LOG STRAIN RATE Figure 52 Comparison of c a l c u l a t e d and experimental s t rength values using equations (A) and (B) in the t e x t . o-900 Figure 53 Results o f regres s ion on 5 kb creep data and strength data f o r the temperature i n t e r v a l 900° to 100(rC. energy resulting from the temperature raising experiments. Again, the drop shown for Q i s r e a l l y related to a decreasing rate of • change i n s t r a i n rate, suggesting that there i s a hardening mechanism balancing the softening ef f e c t of temperature. An additional point r e l a t i n g to the structure i s the high s t r a i n rate observed for low stress creep tests. Fig. 54 shows the prev-ious isotherms, with the additional points for runs less than 3 kb ( a D ) . - Choosing 9 0 0 ° C , .calculations, for A were repeated for a l l s i x tests. The small inset i n Fig. 54 shows stress versus log A. With decreasing stress there i s a corresponding increase i n A, implying that the structure of this amphibole i s responding to variables other than j u s t temperature at 900°C. In summary, the data for wet conditions i n the temperature range 750° to 910°C indicate a Q C = 38 kcal/mole and an JI value of 4 . 8 . The complete equation i s e = H . 5 ) x 1 0 " 1 exp ( - 3 8/RT ) a 4 , 8 . With decreasing water the r± value increases to as high as 8 with a possible increase i n activation energy of approximately 10 kcal/mole. Below 800°C in dry conditions an exponential relationship e = 54 exp ( .23a) best f i t s the data. Above 910°C and in wet conditions the amphibole structure appears to change so that s t r a i n rate remains f a i r l y con-stant up to 950°C. At higher temperatures, j u s t prior to melting, s i g n i f i c a n t increases i n s t r a i n rate are seen. 133 X J Figure 54 Discrepancy between isotherms of F igure 53 and data from creep runs les s than 3 kb (p-.). S t ress d i f f e r e n c e versus Log A suggest s t r a i g h t l i n e r e l a t i o n s h i p at 900 C. DISCUSSION The dir e c t application of a flow law to one of the observed deformation mechanisms i s d i f f i c u l t i n that the observational data overlap. However, considering that brecciation i s more strongly developed at low temperatures, (700° to 800°C), high s t r a i n rates, and high s t r a i n s ; and that this amphibole exhibits high strength i n this same temperature gegion, then the application of the exponen-t i a l law i s suggested. Both twin and glide mechanisms appear to operate, but at high stresses, grains fragment before s t r a i n can be accommodated p l a s t i c a l l y . With increasing temperature (800° to 900°C) and with the addition of water, the power law re l a t i o n begins to operate, favoring glide with continued temperature increase. The amount of water inversely affects both the ji and the Qc values. In f a i r l y saturated conditions and through the temperature range 750° to 900°C, the power law r e l a -tion given above applies. Primary mechanisms are glide and, i n favorably oriented grains, mechanical twinning. Between 850° and 900°C subgrain development i s observed, the s l i p systems become more variable, the pleochroism changes, and small features, possibly d i f -fusion-related, are v i s i b l e . In the interval 900° to 950°C, s t r a i n rate ceases to increase with temperature, suggesting some hardening mechanisms. The activation energy for this high temperature i n t e r -val i s uncertain as the structure appears to be changing. F i n a l l y , r e c r y s t a l l i z a t i o n appears j u s t p r i o r to melting. The combination of these high temperature effects suggests interaction of thermal expansion ( l a t t i c e d i s t o r t i o n ) , d i f f u s i o n , and charge balance disruption. Thermal expansion i n tremolite i s accomplished primarily by di s t o r t i o n of the M-polyhedra (octahedral s t r i p s ) and the A si t e s (Sueno, et a l . , 1973 and Fig. 55). The bond lengths change with mean M-0 bonds being M4>M2>M1>M3. The volumes of these polyhedra vary s i m i l a r l y except that M2 actually increases s l i g h t l y more than M4. The total effect i s to bend the tetrahedral chain about the c axis such that apices are pushed away from the A s i t e and 0(4) oxygens of T2 tetrahedra serve as the "pivot" point. Site occupancy i n c a l c i c amphiboles i s highly variable. The M4 +2 +1 s i t e , the largest and most distorted, i s f i l l e d with Ca and Na (Fig. 55). F e + 2 and Mg + 2 are found in Ml, M2, and M3 with F e + 2 enriched i n Ml and M3 (Burns, 1970, p. 116). Semet (1973) found +3 +2 that Fe and Fe are possible occupants of Ml and M3 and are l i k e l y to be located in neighboring M2-M1 or M3-M2 s i t e s . Assuming s i m i l a r thermal expansion and s i t e occupancy in the present c a l c i c amphibole, the implication of pleochroism changes near 900°C appears s i g n i f i c a n t . Pleochroism can be attributed to three mechanisms: 1) tra n s i t i o n metal ions i n low-symmetry or distorted coordination s i t e s ; 2) charge transfer between neighboring ions (Burns, 1970); and 3) d i r e c t l y related, oxidation state of i r o n , manganese, or titanium. Thermal expansion does d i s t o r t the otherwise regular M s i t e s , as mentioned above. Transition metal ions are common cations i n these octahedrally coordinated s i t e s . Charge transfer between neighboring ions i s favored when the neighboring elements are capable +2 +3 +2 of existing in different oxidation states ( i . e . , Fe - Fe , Mn Mn + 3, or T i + 3 - T i + 4 ) (Burns, 1970, p. 68), a l i k e l y s ituation i n 136 Figure 55 Thermal expansion of tremolite after Sueno, et a l . , 1973. Tetrahedral chains are bent about the £ axis such that apices are pushed away from the A s.ite. c a l c i c amphiboles. Using optical absorption spectra, Faye and Nickel (1920) proposed that charge-transfer of 0"2 -*• F e + 3 as well as F e + 2 -> +3 Fe are important agents for pleochroism in t h e i r c a l c i c amphibole. +3 Because Fe i s p r i n c i p a l l y found i n the M2 s i t e , these authors con-clude that excess charge on 0(4) and the r e l a t i v e l y short bond length, P - 2 + 3 1.85A, promote s i g n i f i c a n t charge-transfer from 0^ -> Fe (M2). +? With substitution of aluminum i n some T2 tetrahedral s i t e s and Ca • in M4, the excess electronic charge on 0(4) i s increased. The s i g -n i f i c a n t expansion of M2 with temperature would probably influence this key M-0 bond and the pleochroism. Oxidation state of iron i s c r i t i c a l to color changes i n amphi-bole. Barnes (1930) concluded that color changes i n hornblende are caused by oxidation of ferrous to f e r r i c iron. Also he concludes that " . . . i n dehydration, hydrogen and not water (except water that i s not a constituent part of the space l a t t i c e ) i s given o f f , and the oxygen remains in the mineral, . . . oxidizing ferrous to f e r r i c iron. . ." p. 417. More recently, Semet (1973) used a v a r i -ety of buffers i n production of synthetic mangesiohastingsite. +3 Noting that changes in color are d i r e c t l y related to Fe content and therefore to oxidation state at formation, he maintained that the best explanation for f e r r o u s - f e r r i c relationships i s excess pro-tons in the structure at low oxygen fugacity. In e f f e c t , these protons would form excess hydroxy! groups by combining with under-bonded tetrahedral oxygen. "The oxygen 0(4) i s a l i k e l y candidate. . 'i . ." p. 492. Consequently, oxidation would involve diffusion of extra protons without disrupting structural OH" in the 0(3) s i t e and would leave 0(4) with excess charge. 138 A synthesis of these ideas"regarding pleochroism includes (1) lattice expansion and distortion, (2) possible isomorphous substitu--2 +3 tion, (3) change in charge-transfer 0^ Fe (M2), (4) diffusion of excess protons (H+), (5) oxidation of ferrous to ferric iron, and (6) +3 +2 change in charge-transfer between neighboring Fe , and Fe . The observation that these effects are not reversed upon cooling indi-cates a fundamental structural change. In response to these distor-tions and charge imbalances, temporary barriers to glide (particu-larly on the (100) plane) may effectively "harden" the amphibole structure and account for the observed strain rates. Because this drop in apparent activation energy is seen in amphi-bole and not in pyroxene, involvement of the structural OH" is implied. Based on infrared spectra, Semet (1973) saw no evidence for loss of normal 0(3)-H hydroxyls related to oxidation state. Perhaps the distortion and associated thermal vibration above the 850°C temperature of Semet's experiments are sufficient to break -structural OH" and to create excess electronic charge in the center of the octahedral strip. Another possibility is that iron content of the orthopyroxene experiments (Ross and Nielsen, 1976) was too low to reflect high temperature oxidation and the proposed strain rate effects. Extrapolation of the present equations to "geologic" strain rates is uncertain, because of the uncertainty regarding apparent activation energy at high temperature (and low stress) conditions. The value of 38 kcal/mole describes energies related to active twin-ning and glide mechanisms in wet conditions. The activation energy associated with recovery processes such as subgrain development and r e c r y s t a l l i z a t i o n i s unclear. However, a review of high temperature work on o l i v i n e shows values >80 kcal/mole (Weertman and Weertman, 1975). Orthopyroxene has a high temperature activation energy of 70 kcal/mole (Raleigh, et a l . , 1971; Ross and Nielsen, 1976). The combination of data i n Fig. 53 y i e l d s an activation energy of 102 ±16 (la) kcal/mole which i s in keeping with the increases observed in other s i l i c a t e s . The ji value of 4.78 ±0.18 (1?) was obtained from relaxation experiments at 900°C (Fig. 48) and data in Fig. 53 yiel d s ji = 4.84 ± 0.68 (la) from 900°C to 1000°C. These n_ values are close to the 4.5 value predicted by Weertman's (1968) dislocation climb-controlled creep. Therefore i n extrapolation, the slope l i e s near 4.8, but the spacing and intercept of the isotherms i s very uncertain. For i l l u s t r a t i o n both 38 kcal/mole and 102 kcal/mole were used to -12 -15 calculate isotherms for the 10 /sec to 10 /sec s t r a i n rate environment (Fig. 56). The variation i n strength prediction i s s i g -n i f i c a n t . The smaller activation energy lowers isotherms by approxi-mately 250°C for a given stress difference or lowers the stress difference by 3 kb at a given temperature. An equation for quartz e = 5.82 x 10" 7 a 3 ! 6 4 at 500°C and 10" 1 2/sec (Balderman, 1974) yields a low strength of ( 26 bars. Using the lower activation energy (38 kcal/mole), amphi-bole has a predicted strength of 792 bars, a difference of one order of magnitude. This difference i s i n agreement with the apparent r a r i t y of amphibole deformation features in crustal rocks. Although the higher activation energy values are favored, further c l a r i f i c a -tion of high temperature behavior i s necessary. F i n a l l y , occurrence of mechanical (101) twinning in amphibole 140 F igure 56 C a l c u l a t i o n of s t rength values at " geo log i c s t r a i n r a t e s " . Dashed l i n e s are c a l c u l a t e d using Q = 38 kca l /mole and s o l i d l i n e s are Q = 102 kca l /mole. These two a c t i v a t i o n energies represent po s s i b l e extremes f o r high temperature behav ior . may be related to chemical composition. Dollinger and Blacic (1975) found no twins i n the igneous amphibole they studied. On the other hand, both the present study and that of Rooney, et a l . (1975) found (101) twins. Amphiboles which exhibit twins (Table II) have three possibly s i g n i f i c a n t t r a i t s : 1) higher aluminum saturation of tet r a -hedral s i t e s , 2) higher calcium, and 3) higher A s i t e occupancy. With the structure more nearly f i l l e d and the tetrahedral chain s l i g h t l y more distorted, glide along (100) could be retarded enough to allow the c r i t i c a l resolved shear stress to increase such that (101) twinning occurs. CONCLUSIONS 1) Am-2 shows a change from exponential to power law behavior r e l a -ted to temperature and to the addition of water. The tr a n s i t i o n between these two mechanisms appears gradational with the r± value {iaan) dropping from 8.1 to 4.8. Water lowers the n_ value at lower temperatures and appears to lower the activation energy by approxi-mately 10 kcal/mole. 2) Above 910°C the amphibole structure changes such that s t r a i n rate remains f a i r l y constant f o r 50°C i n t e r v a l . This "hardening" i s thought to be related to oxidation and d i s t o r t i o n occurring within the l a t t i c e . 3) Two primary p l a s t i c deformation mechanisms are observed. (101) mechanical twins develop in favorably oriented grains and are also favored at higher confining pressures (15 kb) and lower temperatures (700° to 800°C). Faster s t r a i n rates and low strains favor twins. 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