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Evolution and late stage deformation of the Himalayan metamorphic core, Kanchenjunga region, eastern… Buckingham, Heather Marie 2014

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    EVOLUTION AND LATE STAGE DEFORMATION OF THE HIMALAYAN METAMORPHIC CORE, KANCHENJUNGA REGION, EASTERN NEPAL   by  HEATHER MARIE BUCKINGHAM  B.Sc., The University of British Columbia, 2012    A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF  THE REQUIREMENTS FOR THE DEGREE OF  MASTER OF SCIENCE   in   THE COLLEGE OF GRADUATE STUDIES  (Environmental Science)      THE UNIVERSITY OF BRITISH COLUMBIA  (Okanagan)      December 2014  © Heather Marie Buckingham, 2014  ii Abstract  Understanding the recent history of the Himalayan orogen not only helps elucidates near-surface convergence accommodation processes, but also provides constraints for geometric modification of earlier midcrustal structures. New 40Ar/39Ar and fission track (FT) data from the former Himalayan metamorphic core exposed in the Kanchenjunga region of eastern Nepal help constrain the evolution and low temperature uplift history of this portion of the orogen. Within the Lesser Himalayan Sequence (LHS), new apatite FT dates, combined with existing apatite and zircon FT dates from the region, define general younging trends towards the north - up structural section - of ~2.9 to 1.3 Ma and ~6.2 to 4.6 Ma respectively. There appears to be a significant jump in apatite FT dates from 1.3 Ma to 2.4 Ma that is coincident with an abrupt change in existing muscovite 40Ar/39Ar ages from the Proterozoic to the Cenozoic. This break in ages is consistent with the mapped location of the Main Central thrust (MCT) fault in the area. In structurally lower rocks in the Greater Himalayan Sequence (GHS), north of the MCT, trends in both muscovite 40Ar/39Ar and apatite FT continue to decrease to the north. These trends are interpreted to be consistent with the exhumation and uplift of these rocks associated with the growth of a duplex system within the LHS developed through underplating. Cooling rates across the mapped area indicate fast cooling in the GHS in early to mid Miocene, coupled with very slow cooling in the LHS. In the late Miocene to Pleistocene, cooling rates slow down in the GHS and increase in the LHS, such that they are similar. This is consistent with development of late-stage duplexing within the LHS at this time and the coupled exhumation of the GHS.  iii Biotite 40Ar/39Ar dates may indicate a complex history across the study area. Some biotite dates (~24-16 Ma) are older than nearby 232Th-208Pb monazite melt crystallization dates (~18-16 Ma). Previous studies have attributed similar old biotite dates to excess argon. It is possible, however, the old biotite dates indicate crystallization along the retrograde path prior to final melt crystallization. iv Preface  I conducted the majority of the research and writing associated with this thesis, including field work, sample collection and descriptions, apatite fission track dating specimen preparation, 40Ar/39Ar specimen preparation and dating, and the interpretation of results and conclusions drawn. The majority of this work was conducted at the UBC Okanagan under the supervision of Dr. Kyle Larson. Dr. Larson provided directional aid, guidance for field, thin section, and regional interpretations, as well as editing and proof-reading support. Apatite fission track dating was conducted at Dalhousie University in Halifax, Nova Scotia under the direction of Dr. Isabelle Coutand. Dr. Coutand was responsible for counting the fission tracks and measuring the final ages for those samples. 40Ar/39Ar dating was conducted at the University of Manitoba in Winnipeg, Manitoba under the direction of Dr. Alfredo Camacho. Dr Camacho was responsible for the calculation of the final dates based on the isotopic data collected. Tyler Ambrose aided with sample collection and interpretation in the field.           ! v!TABLE!OF!CONTENTS!!Abstract…………………………………………………………………………………………...………………….ii!Preface………………………………………………………………………………………….......………………..iv!Table!of!Contents………………………………………………………………………………………………….v!List!of!Tables…………………………………………………………………………………....………………...vii!List!of!Figures……………………………………………………………………………………………………viii!Acknowledgements…………………………………………………………………………………………......ix!CHAPTER!1! Introduction…………………………………………………………………………………...1!1.1 Overview!of!the!Himalaya…………………………………....…………………….1!1.2 PresentJday!Geology!and!Structures!of!the!Himalaya…………………..3!1.3 Shortening!Process!in!the!Himalaya...……….………………………………...7!1.3.1 Convergence!Accommodation!in!the!MidJCrust……………..7!1.3.2 Convergence!Accommodation!Processes!in!the!Shallow!Foreland…………………………………………………………………….10!1.4 This!Study.………………………………………………………....……………………12!CHAPTER!2! Methods………………………………………………………………………………….........15!! ! 2.1!Apatite!Fission!Track!Dating…………………………………………………….16!! ! ! 2.1.1!Introduction!and!Theory…………………………………………….16!! ! ! 2.1.2!Specimen!Selection……………………………....…………………….21!! ! ! 2.1.3!Preparation………………………………………………………………..22!! ! 2.2!40Ar/39Ar!Dating………………………………………………………………….......28!! ! ! 2.2.1!Introduction!and!Theory…………………………………………….28!! ! ! 2.2.2!Excess!Argon……………………………………………………………...34!! ! ! 2.2.3!Specimen!Selection………………………………………………..…...37!! ! ! 2.2.4!Procedure……………………………………………………………….....38!CHAPTER!3! Thermochronology!Data!and!Results……………………………………………..40!! ! 3.1!Previous!work…………………………………………………………………………40!! ! 3.2!Results……………………………………………………………………………………44!! ! ! 3.2.1!Apatite!Fission!Track………………………………………………….44!! ! ! 3.2.2!40Ar/39Ar……………………………………………………………………47!! vi!CHAPTER!4! Discussion!and!Interpretations!………………………………………………...…..53!! ! 4.1!Apatite!Fission!Track!…………………………………………………………...…53!! ! 4.2!40Ar/39Ar…...!………………………………………………………………..................55!! ! ! 4.2.1!Old!Biotite!Ages……………………………………………………...….55!! ! ! 4.2.2!Biotite!Ages!versus!Monazite!Rim!Crystallization……......56!! ! ! 4.2.3!Interpretation!of!40Ar/39Ar!Results……………………………..58!! ! 4.3!Cooling!Rates……………………………………...…………………………………..59!! ! 4.4!Structural!Implications………………………...………………………………….63!! ! 4.5!Regional!Implications……………………….......…………………………………65!! ! 4.6!Old!Biotite!Grains………………………………...………………………………….66!CHAPTER!5!! Conclusions!and!Future!Work.…………………...………………………………….70!! ! 5.1!Conclusions……………..…………………………...…………………………………70!! ! 5.2!Future!Work…...!…………………………………………………………..................71!5.2.1!Old!Biotite……………………………………………………...................71!5.2.2!Further!Data………………………………………………………………71!REFERENCES…………………………………………………………………..................................................73!APPENDICES.…………………………………………………………….........................................................87!! ! Appendix!A:!Grain!descriptions!and!thin!section!photos!for!!specimens!selected!for!40Ar/39Ar!dating…………...………………….86!! ! Appendix!B:!Field!photos!for!specimens!selected!for!40Ar/39Ar!!dating………………………………………………………………………………107!! ! Appendix!C:!Photos!of!mica!grains!for!40Ar/39Ar!dating………..……….120!! vii!LIST!OF!TABLES!!Table!1.1!40Ar/39Ar!and!UJThJPb!calculated!cooling!rates…………………..…………………..4!Table!3.1!Apatite!fission!track!results...…...………….....…………………………………………….45!Table!3.2!40Ar/39Ar!results…………………...………………………………………………………….....48!Table!4.1!40Ar/39Ar!closure!temperatures………………….….........………………………………57!Table!4.2!40Ar/39Ar!and!UJThJPb!monazite!calculated!cooling!rates……………………..61!!!!! viii!LIST!OF!FIGURES!!Figure!1.1!Overview!of!Study!Area.…………………………………………………………………….....2!Figure!1.2!Cross!Section!of!the!Himalaya……….………………………………………………………5!Figure!1.3!Exhumation!of!Greater!Himalayan!Sequence…………..……………………………..9!Figure!1.4!Duplexing………………………………………………………………............…………............11!Figure!1.5!Geologic!Map!of!Study!Area………………………………………………………………...13!Figure!2.1!Elevation!profile!and!specimens!dated!in!this!study…………………………….23!Figure!2.2!Mounting!and!polishing!slides!for!apatite!fission!track!dating………………26!Figure!2.3!“UJshaped”!40Ar/39Ar!profile……………………………………………………………….33!Figure!3.1!Compilation!of!dates!in!Eastern!Nepal!Himalaya………………………………….41!Figure!3.2!Overview!of!age!results………………………………………………………………………46!Figure!3.3!Biotite!40Ar/39Ar!age!spectra.……………………………………………………………...49!Figure!3.4!Muscovite!40Ar/39Ar!age!spectra.………………………………………………..……….50!Figure!4.1!Apatite!Fission!Track!dates,!elevation,!and!rainfall!data…………….…………54!Figure!4.2!Cooling!Rates…………………………………………………………………………..…………60!Figure!4.3!Evolution!of!the!Kanchenjunga!Region……………………………………..………....64!Figure!4.4!Pseudosections………………………………………………………………………..…………68!!!!!ix#Acknowledgements   In addition to those mentioned in the Preface, I would like to acknowledge Dr Santa Man Rai, from the Tribhuvan University, Kathmandu for assisting in logistics of field work in Nepal; Teke Tamang, and Pradap Tamang, from Pike Peak Trekking for organizing the trek and porters in the Kanchenjunga region of Nepal, and David Arkinstall for assisting in SEM work at UBC Okanagan. I would like to thank UBC Okanagan for supporting my thesis work, as well as NSERC and GSA for scholarships and research grants that have helped fund my research. Finally, the Geology and Tectonics group at UBC Okanagan for their support and advice throughout the entire project.                       Chapter 1: Introduction  1.1 Overview of the Himalaya  The Himalaya and adjacent Karakoram host the tallest mountains on Earth. They are the physiographic manifestation of the continued collision of India with Eurasia. These and other related ranges form a broad, ~2500 km long topographic arc along the southern boundary of the Tibetan plateau (Mascle et. al, 2012) that trends approximately WNW to ESE between syntaxes at Nanga Parbat in the far west and Namche Barwa in the far east (Figure 1.1).  The Himalayan range is commonly regarded as the type-example of a mountain belt formed through continent-continent collision. The ongoing collision between India and Eurasia initiated approximately 50 Mya (eg. Green et al., 2008), culminating in the geology and structures we see in the field today. The collision of two continents creates a fundamental space problem. Traditionally, textbook treatments of collisional orogenesis have drawn a simplified picture of plates colliding and forming mountains, often with one continental plate sliding neatly beneath another plate (e.g. Marshak, 2007; Reynolds et al., 2008; Smith and Pun, 2006; Tarbuck et al., 2005). The reality of this process, which we are only now starting to understand in detail, is much more complicated. A significant amount of the convergence between the two continents may be accommodated through processes not easily relatable to typical, rigid plate tectonics. Indeed, much recent research has been conducted in an effort to unravel the details of these processes and how they may reflect convergence accommodation throughout the collisional history of the orogen (e.g. Larson and Cottle, 2014, 2010a, 2010b; Carosi et al., 2013; Montomoli et al., 2013; Sakai et al., 2013; Webb et al., 2013; Streule et al., 2012; Kali et al., 2010). 1Figure 1.1: a) Map showing the syntaxes of Nanga Parbat and Namche Barwa that confine the Himalaya and related mountain ranges within southern Asia. The red box indicates the location of a regional map of the eastern Nepal Himalaya and environ (Figure 3.1). The yellow box indicates the study area (Figure 1.5). Base map is modified from Hodges 2000. CHINA (TIBET)NEPAL SIKKIM (INDIA)MYANMARINDIAPAKISTANÜ( Ü( Ü(Ü1Nanga ParbatNamche BarwaWestern HimalayaEastern HimalayaCentral HimalayaTibetan Plateau (Hinterland)Indo-Gangetic Plain(Foreland)0  350  700   1050   1400 km BHUTANBANGLADESH2 1.2. Present-day geology and structures of the Himalaya  The convergence that has led to the Himalaya is also reflected in the rise of the Tibetan Plateau. The continental crust currently beneath southern Tibet has been thickened to approximately 75-80 km (e.g. Zhang and Klemperer, 2005), which has led to an average topographic elevation of > 4500 m above sea level. The elevation of much of India to the south, in contrast, which has a more typical thickness crust at ~35 km, is within a few hundred metres of sea level. The difference in crustal thickness between India and Tibet forms a natural taper and a considerable horizontal lithostatic pressure gradient between the orogenic hinterland and foreland. This gradient and the associated gravitational potential is one of the main forces driving deformation within the Himalayan system, moving material from higher to lower potential energy, or from the hinterland towards the foreland, and an equilibrium state. The main structures along the Himalayan arc have accommodated deformation associated with the development of the orogenic wedge and the constant creep towards equilibrium (Hodges 2000, Hodges et al., 2001). While there are a number of large-scale structures recognized along the Himalaya the names and definitions of these structures can vary significantly (e.g. Searle et al., 2008). For clarity in how they are defined in this thesis, the various structures discussed are outlined in Table 1.1, drawn in Figure 1.2, and are discussed below.  There are four major north-dipping, orogen-parallel fault systems commonly mapped along the Himalaya (Figure 1.2). From north to south these include: the South Tibetan detachment system (STDS), Main Central thrust (MCT), Main Boundary thrust (MBT), and Main Frontal thrust (MFT) (Figure 1.2). The three thrust systems are 3Table 1.1: Names and definitions of structures and zones within the Himalayan range    North                                       South Tibetan Plateau High elevation plateau north of the Himalaya. Variably referred to as the orogenic Hinterland. The crust beneath the plateau is thickened due to the continued collision of India with the Eurasian plate.  Tethyan Sedimentary Sequence (TSS) Sediments of the pre-existing Tethys ocean between Tibet and India predating the initial collision of the Himalaya circa 50 Ma. Now preserved mainly along southern margin of the Tibetan Plateau. South Tibetan Detachment System (STDS) System of top-to-the-north sense shear structures commonly found near the transition from the Tibetan Plateau to the Himalaya. Juxtaposes the TSS to the north in its hanging wall against the high-grade metamorphic rocks of the Greater Himalayan Sequence to the south in its footwall. Greater Himalayan Sequence (GHS) Exhumed, former midcrustal high-grade metamorphic core of the orogen. Bound above and below by the STDS and MCT (see below) respectively. Main Central Thrust (MCT) “Base of the large-scale zone of high strain and ductile deformation, commonly coinciding with the base of the zone of inverted metamorphic isograds” (Searle et al., 2008) Lesser Himalayan Sequence (LHS) Unmetamorphosed-to-low metamorphic grade rocks, within the footwall of the MCT and the hanging wall of the MBT Main Boundary Thrust (MBT) Thrust fault separating the rocks of the LHS in its hanging wall from the Sub Himalaya / Siwaliks in its footwall Sub Himalaya (SH) Also referred to as the Siwaliks, this zone represents material that was eroded off of the evolving mountain front, accumulated in the Himalayan foreland basin, and is now being overridden and incorporated into the deforming orogenic wedge. Main Frontal Thrust (MFT) Thrust fault separating the SH to the north in the hanging wall, and the Indo-Gangetic Plain to the south in the footwall Main Himalayan Thrust (MHT) Sub-surface thrust decollment that is assumed to connect the MFT, MBT, and MCT down towards the Hinterland Indo-Gangetic Plain The foreland basin built on the Indian Craton. The crust is thinner beneath the Indo-Gangetic Plain than beneath the Tibetan Plateau, creating a lithostatic gradient from north to south.  4MOHOTethyan SedimentarySequenceSub-Himalayaor Siwaliks  Indo-Gangetic Foreland Main Central ThrustMain Boundary ThrustMain Frontal ThrustSouth North50kmDepth (km)  02040Main Himalayan Thrust      Lesser     Himalayan                SequenceGreater      Himalayan                SequenceSouth Tibetan Detachment SystemFigure 1.2:  Cross-section of the Himalaya detailing major structural features, adapted from Hodges et al., 2001. Half arrows indicate relative movement directions across faults.605 interpreted root down structurally toward the north to a common detachment, the Main Himalayan thrust (MHT).  The STDS comprises a series of top-to-the-north sense ductile stretching faults and low-angle normal faults that were active between c. 23 Ma and 15 Ma (Kellett et al., 2013; Harris 2007; Godin et al., 2006). The Tethyan sedimentary sequence (TSS) occurs north of the STDS, in its hanging wall, and represents the supracrustal hinterland of the orogen (eg. Larson et al., 2010a). The TSS is juxtaposed across the STDS against the subjacent Greater Himalaya Sequence (GHS). Rocks in the GHS represent the exhumed former midcrustal core of the orogen and typically record evidence of kyanite to sillimanite grade metamorphism and anatexis (Larson et al, 2011). The GHS comprises the hanging wall of the MCT, a top-to-the-south sense shear zone that was at least partially coeval with the movement across the STDS between c. 22 Ma and 12 Ma (Harris, 2007, Godin et al., 2006). The MCT juxtaposes the GHS over the Lesser Himalayan sequence (LHS; Figure 1.2). The LHS consists of unmetamorphosed to low metamorphic grade sedimentary and meta-sedimentary rocks that are intercalated with occasional orthogneiss lenses (Searle et al., 2008). The LHS occurs in the hanging wall of the MBT (Figure 1.2), which was actively overthrusting the Sub-Himalayan (SH) zone, or Siwaliks, in its footwall between c. 10 and 3 Ma (Robert et al, 2013). The Siwaliks, which consist of the detrital mollass shed off of the growing mountain range, are carried in the hanging wall of the presently active MFT (Robert et al, 2013). The MFT marks the leading edge of shortening in the Himalaya (Figure 1.2), and separates the SH zone from the Indo-Gangetic Plain to the south.   6 1.3 Shortening Processes in the Himalaya Orogens are continuously evolving as they shorten, extend, uplift, and erode in response to both local and regional changes in convergence rate, climate, internal rheology, etc. Moreover, rocks can translate laterally or vertically within the orogen through time in response to the various structures and processes that may be active. This movement of material through an orogen means that it may be subject to different processes that dominate at different structural levels as the orogen evolves. For example, the GHS, which was metamorphosed and deformed at high temperatures and pressures in the mid-crust, is now found at the surface where it is subject to low-temperature/surface processes (eg. Wobus et al., 2008). It is, therefore, important to consider the history of the orogen as a whole when attempting to analyze what was happening at a particular location or structural level and what it might reflect in terms of processes. In keeping with this, in order to understand the earlier midcrustal evolution of the orogen recorded in the GHS we must first work backwards from the present geometries. Understanding the recent history and its potential effect on previously imparted geologic characteristics is critical to elucidating the whole evolution of the orogen.      1.3.1 – Convergence Accommodation in the Mid-Crust The structural geometries observed at the surface in the Himalaya today (Figure 1.2) have largely resulted from convergence accommodation processes affecting the GHS and the LHS (eg. Larson et al. 2010b). Rocks that comprise the GHS may have accommodated convergence, at least partially, through lateral midcrustal flow in response to the horizontal lithostatic gradient between Tibet and India (e.g. Beaumont et al., 2004; 7 Godin et al., 2006). During such a process, rocks originating within the mid-crust are thought to have weakened significantly as a result of in situ partial melting. The melt-weakened mid-crust was then driven horizontally across the lithostatic gradient between the thick crust of the Tibetan plateau and the relatively thin crust of the Indian craton. Thermo-mechanical models of lateral midcrustal flow may also employ focused surface denudation coupled with a low-viscosity midcrustal ‘channel’ to enhance the gradient and promote associated exhumation (Beaumont et al., 2001). In these variants, surface erosional processes effectively act as a ‘release valve’ removing material from the orographic front of the range, while advecting higher temperature isotherms towards the surface. Research, however, indicates that the rocks were not being exhumed to the surface at the time the two faults bounding the GHS were active (DeCelles et al., 2004; Martin et al. 2014). The extruding mid-crust appears to have cooled as the material moved out from beneath Tibet and perhaps up a crustal ramp to the south. This cooling affected the rheology of the midcrustal material, making it less ductile as it approached shallower depths (Larson and Cottle, 2014). As lateral flow of this cooled material ceased, it appears that convergence was taken up through imbrication of the extruded mid-crust (Figure 1.3; e.g. Carosi et al., 2013; Montomoli et al., 2013; Larson and Cottle, 2014). The accommodation of convergence in the GHS also appears to have involved at least some out-of-sequence thrusting (eg. Ambrose, 2014; Regis et al., 2014; Grujic et al., 2011) further complicating the structural and kinematic history. Therefore, the final structures observed in and data extracted from the GHS today, do not reflect a single process, but a complicated history of multiple events; adjacent rocks may record very 8~33 Ma~16 MaPresent DayMCTMohoSTDSTibetMohoGreater HimalayaTibetLesser        HimalayaSTDSMCTMFT MBTMohoMCT  ? STDSFigure 1.3: Model for the development and exhumation of the Greater Himalayan Sequence. Modified from Larson and Cottle, 2014. See text for discussion.9 different histories depending on how and when they were incorporated into the GHS (eg. Larson and Cottle 2014, Ambrose, 2014)  1.3.2 – Convergence Accommodation Processes in the Shallow Foreland In order for midcrustal characteristics to be observed at the surface today, the mid-crust must have been exhumed. The exhumation of the GHS appears to have been driven by horizontal shortening and vertical thickening of the subjacent LHS (McQuarrie 2008, Bollinger et al., 2006) with paired erosion. There are two commonly debated models proposed to describe the deformation within the LHS (Figure 1.4) including 1) the development of a hinterland-dipping duplex (e.g. McQuarrie 2008, Robinson et al., 2006) and 2) the development of a foreland-dipping duplex by underplating along a relatively stationary ramp (e.g. Avouac, 2003; Bollinger et al, 2004, 2006; Herman et al., 2010; Webb et al., 2013).  A hinterland-dipping duplexing develops through in-sequence thrust propagation towards the foreland within the LHS (Figure 1.4a). In this model, the ramp associated with the MHT moves towards the foreland as thrusts systematically incorporate new material into the duplex. A foreland-dipping duplex, in contrast, is developed through underplating, which occurs when material from the down-going plate is transferred up across the thrust boundary and added to the hanging wall (Herman et al., 2010). The various slices are progressively moved up and towards the foreland along a relatively stationary ramp (Figure 1.4b) during which time their geometry is modified such that they become foreland dipping.   10Figure 1.4: Schematic representation of duplex models proposed to explain deformtion within the Lesser Himalayan Sequence and exhumation of the overlying Greater Hima-laya. A) In-sequence thrusting where thrust (1) occurs first, followed by thrusts progres-sively propagating towards the foreland, ending in thrust (3). B) Underplating processes where earliest wedges (1) are incorported into the LHS towards the Hinterland, rotated and moved towards the foreland as further material is underplated (7). See text for discussion.BDistance (km)Taplejung WindowElevation (km)02.55.07.510.00                10                20                30                40                50                60                70                80                90               100                        LHS GHSGHSMCTMCTMCTMBTMHT23MHTMCT1AYoung OldDistance (km)Taplejung WindowElevation (km)02.55.07.510.00            10            20            30            40            50            60            70            80            90            100            LHS GHSGHSMCTMCTMCTMBTMHTMHTMCT1 2 3 4 576NSNS11 1.4 This Study The area for this study is in the Kanchenjunga region of far northeast Nepal (Figure 1.5). The area affords excellent, semi-continuous exposure along a natural cross section through exhumed LHS and GHS rocks. Unlike most previous work in the area, which has typically examined the evolution of the mid-crust, this study attempts to gain information about the shallower, more recent evolution of the orogen through the examination of the thermochronologic history of the exhumed metamorphic rocks and the structures that bound and expose them. There has been little research conducted in the Kanchenjunga region relative to other areas of Nepal. Goscombe and Hand (2000) and Goscombe et al. (2006) mapped the basic geology of the area and carried out some preliminary investigation of the P-T-t history of the midcrustal rocks exposed. Imayama et al., 2012 conducted research in the area on the partial melting and cooling histories in the GHS using U-Pb and K-Ar methods. Sakai et al. (2013) recently reported limited new thermochronologic data from within and around the Taplejung window, a structural window through the GHS into the LHS (Figure 1.5), with the goal of reconstructing paleotectonic correlations. There have been no systematic studies, however, that have examined the thermochronologic history of the Kanchenjunga region in the context of how it may reflect the processes that resulted in the exhumation of the mid-crust, and what implications it might have for the shallower, more recent development of the orogen and the present day geometries in the region.  This study presents new apatite fission track (FT) and 40Ar/39Ar data from across the Kanchenjunga region; a spatially expansive suite of specimens was collected along a 12MCTMCTSchist/gneiss, locally migmatitic QuartziteSandy PhylliteOrthogneissPhyllitic schistMigmatite/gneissGEOLOGYwith leucogranite lenses2000200020002000200040004000400040005200Figure 1.5: Simplified geologic map of the study area. Location is shown as a yellow rectangle in Figure 1.1.YanmaNEPALSIKKIM(INDIA)TIBET (CHINA)KanchenjungaChaukiGufa PokhariBasantapurTaplejungDobhanChirwaSekathumOlanchangolaDeurali0 108642 30 kms20Main Central Thrust DiscontinuityTrail, roadVillageSummitStream, riverTopographic contour(in meters)Contour interval: 400m4000LEGENDN13 generally SSW-NNE trending transect from Basantapur to Yanma (Figure 1.5). It aims to elucidate the most recent thermochronologic events, and their implications for existing overlying geometries. For example, the duplexing processes described above and outlined in Figure 1.4 may potentially be distinguished based on spatial cooling histories. Duplexing in the LHS by foreland propagating, in-sequence thrusting may be characterized by younger ages towards the foreland and older ages towards the hinterland as thrusts propagate southward from hinterland to foreland (Figure 1.4a). Duplexing by underplating may be characterized by older ages towards the foreland and younger ages towards the hinterland as deformation drives exhumation towards the hinterland within the LHS as older structures are translated south towards the foreland (Figure 1.4b).  Data from this study can be combined with existing data from eastern Nepal and adjacent regions (e.g. Doon 2010; Gong et al. 2011, Hodges et al. 1998, Hubbard and Harrison 1989, Imayama 2012, Kali et al. 2010, Kellet et al. 2013, Krummenacher et al. 1978, Sakai et al. 2013, Sakai et al. 2005, Streule et al. 2012), where the spatial distribution of ages can be used to inform a more comprehensive understanding of the late stage exhumation processes and kinematic history across the region.  Because the Himalayan system is often used to help interpret older orogens such as the Meso-Cenozoic Canadian Cordillera, the Paleozoic Appalachains, and the Proterozoic Trans-Hudson (eg. Nabelek and Liu 2004, Brown and Gibson 2006, Pawlak et al. 2010), it is critically important to understand the processes operating and controlling the development of this ongoing continent-continent collision. In doing so we gain not only a better understanding of how the collision of India and Asia has evolved, but also gain potential insight into how older, inactive orogenic systems formed. 14Chapter 2: Methods   The age of materials can be determined by measuring the ratio between the parent and daughter isotopes, using known decay rates. Each radioactive isotope has a specific decay rate and is constant. Some of the most common radiometric dating methods in geology are U-(Th)-Pb, K-Ar (40Ar/39Ar), Lu-Hf, U-Th-He, and fission track. These methods are typically performed on specific target minerals that contain parent isotopes and that are retentive of the daughter isotopes. The age interpreted from these techniques often reflects the age of a mineral at a specific range of temperature in time. These specific temperatures are dependent on a range of parameters, and are referred to as a “closure temperature,” which typically represents the temperature at which the mineral is effectively ‘closed’ to diffusion, meaning that daughter isotopes remain in the system. At temperatures above the closure temperature, radioactive decay still occurs, but the daughter products are often lost (diffuse) from the system, or tracks such as in fission track methods (see below) are annealed.  Closure temperatures of individual minerals are determined based on cooling rate, grain size, geometry, and diffusion kinematic properties (Dodson, 1973). For some minerals, like monazite and zircon, the effective closure temperatures are so high that the dates obtained are typically interpreted to represent the time of crystallization. For other minerals, closure temperatures can be estimated for each specimen based on the measured size of the grains analyzed, diffusion geometry, activation energy, frequency factor of the mineral, and the inferred cooling rate (A. Camacho., Pers. Comm., 2014). Studies have shown that in some systems various factors such as fluids present, chemical!15composition of the mineral, grain defects, and new crystal growth make the effective closure temperatures difficult to interpret (Villa 1998).   In this study, and in nearby geochronological studies, both apatite fission track (FT) and 40Ar/39Ar radiometric dating methods are used. The details of these methods are discussed below. In addition to these methods, zircon FT, K-Ar, and U-Pb dating methods have been used in existing geochronological studies in nearby regions. All these methods are collectively used to interpret the late stage deformation and exhumation history of the Eastern Nepal Himalaya.  2.1 Apatite Fission Track Dating 2.1.1 Introduction and Theory Apatite Fission Track (FT) dating is a technique used to constrain the time at which an apatite grain within a rock specimen cooled through a temperature below which fission tracks no longer heal, referred to as a closure temperature. It is a radioactive isotopic dating method that is based on the decay of the 238U isotope. In most other radioactive isotopic dating techniques, such as 12C/14C, 39Ar/40Ar, or U-Th-Pb, the ratio of parent to daughter isotopes is measured and compared to a known decay constant or half-life in order to determine the age of a particular specimen. Apatite FT dating however, looks only at the parent isotope and its decay, and does not take into account the daughter isotope. Apatite contains trace amounts of 238U, which is a naturally occurring radioactive isotope. 238U typically decays through α-decay, emitting a 4He nucleus. For approximately every 2 million α-decay emissions, however, 238U will undergo spontaneous fission, breaking into 2, sometimes 3, nuclei as well as 2 or 3 high-energy 16neutrons (Donelick et al., 2005). These high-energy neutrons are capable of leaving scars or tracks in their wake as they destroy the crystal structure within the apatite grains. The tracks that result from this fission can be counted using an optical microscope and related back to how long they have been accumulating for as the decay constant for 238U is known. Tracks in apatite only accumulate after the mineral grain has a cooled through a ‘closure temperature’ of approximately 70-110 ˚C, with variation dominated by cooling rate. The closure temperature is the temperature at which tracks created through fission do not anneal, but remain within the grains to be counted and recorded by an observer. For this reason, the apatite FT dating method is a useful indicator of low temperature (70-110 ˚C) thermochronologic history.  The number of fission tracks left behind by the fission decay of 238U are proportional to the number of daughter particles gained/parent material lost, but they do not give information about the initial concentration of parent material or the amount of 238U remaining in the system. 238U and 235U have a known natural ratio that is assumed to be the same in everything at any one point in history (Chen and Wasserburg, 1980; Weyer et al, 2008). The amount of 235U in the specimen can, therefore, be used to determine the abundance of 238U. In order to assess the concentration of 235U, mounted apatite grains have a sheet of muscovite mica placed over them. The specimens are then irradiated in a nuclear facility. During irradiation, 235U fission is induced, generating tracks in the mica sheet. Once returned to the lab, and sufficiently cooled (radioactively), the mica sheets can be separated from the grain mount. Tracks recorded in the mica sheet reflect the concentration of 235U in the apatite, which can then be used, in comparison to a standard of known U concentration, to estimate the concentration of 238U. Induced fission 17tracks are also recorded in the apatite grains, but by using the mica sheet as a reference it is possible to determine the relative amount of 238U tracks in the apatite for counting purposes. The tracks in both the muscovite and apatite are analyzed using an optical microscope. This method of apatite FT dating using irradiation is commonly referred to as the external detector method (EDM). Alternatively, U concentrations can be measured using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) (Donelick et al., 2005). The LA-ICP-MS method of measuring uranium concentrations uses a laser to ablate the material, which is then fed into a plasma stream, and measured by the mass spectrometer. This method avoids potential bias associated with counting tracks, however, it damages the grain, creating a pit from the laser beam (Donelick et al., 2005). The specimens examined in this study were done so using the EDM as discussed below. Preparation of a specimen for irradiation for the EDM takes approximately 2 months, and apatite grains can only be confidently recognized after etching, the final step in the preparation process. Preparation includes separating the apatite grains from the specimen via physical crushing, heavy liquid separation, magnetic separation, and hand picking. Careful preparation of the specimen is required in order to not contaminate nor lose any of the grains. This is followed by approximately 2 more months of irradiation and cooling. Finally, a minimum of 2-6 months is required for the technician’s track counting calibration (if not completed previously) before the actual dating process can begin.  Apatite grains used in apatite FT dating should be pure, and typically between 50 µm and 300 µm, though it can be performed on smaller grains (Donelick et al, 2005). 18Whole or broken grains can be used. Significant information can be gleaned from relatively few grains; large numbers of track-bearing apatite are not required. Because the tracks are counted and measured by eye, and are sometimes difficult to interpret correctly, there is potential for human error. In an attempt to minimize error, it is important that the technician counting the tracks has had sufficient time to train using standard specimens with known ages in order to calibrate his or her counting abilities. Calibration is a critical step to apatite FT dating, wherein the observer must practice counting on standard apatite grains of known age until the counting age they retrieve for the standard is consistent. If the observer is consistent, but off the actual age of the standards, a unique correction factor must be applied to their count in order to determine the correct age. Because counting tracks is based on human-eye interpretation, it is difficult to obtain 100% accuracy, and some bias or inconsistencies may occur depending mainly on the amount of calibration that the observer has undergone.  After counting and analyzing the spontaneous tracks in apatite and induced tracks in the muscovite sheet, the resultant data are incorporated into an equation to determine the age of the specimen. A glass dosimeter is used to monitor the amount of ionizing radiation that has occurred. The age equation is as follows (for a more detailed explanation on the fission track age equation, see Donelick et al., 2005):  ! =1!!ln 1+ !!!"!!!!!!  ! = !"#!!"!!ℎ!!!"#$%&!!! = !"#$%!!"#$%&#%!!"#!!"#!!! = !"#$%&"'$()!!"#$%&!!"#$%!!"!!"#!!"!!"##"$%!!"#$%!!"#!!"!"#!$#%!!! = !"#$%&$'#(!!!"#$%!!"#$%&'!!! = !"#$%&#!!"#$%!!"#$%&'!!! = !"#$%&'&(!!"#$$!!"#$%!!"#$%&'  19Apatite FT dating has been limited in the Himalaya due largely to a history of poor quality and quantity of apatite grains obtained in the region, particularly in the Lesser Himalayan sequence (I. Coutand, pers. Comm., 2013). The potential information gained from apatite FT dating, however, is invaluable in interpreting the low-temperature uplift history of the Lesser and Greater Himalaya. Moreover, it is critical to understanding the recent deformation recorded in the orogen, which itself is necessary to properly unravel what portion of the observed kinematics are attributable to recent processes and what portion likely relate to older, midcrustal processes. Ruling out the apatite FT method based solely on previous challenges in finding grains could result in foregoing compelling and important scientific data.  FT dating has been successfully employed in the Himalaya in a number of recent studies (for example, Sakai et al., 2013, Herman et al., 2010, Burbank et al., 2003, Schlup et al., 2003, and Jain et al., 2000).  It is often paired with other low temperature dating methods such as U-Th/He, or 40Ar/39Ar in order to broaden the temperature-time history data provided by single geochronologic method. For example, Sakai et al. (2013) combined FT analyses and U-Pb dating techniques to constrain the metamorphic and exhumation history of the Dumri Formation in western Nepal. Herman et al. (2010), meanwhile, used FT dating combined with both 40Ar/39Ar thermochronology and (U-Th)/He dating to interpret the low temperature exhumation history of the central Himalaya as a test for crustal shortening models. Finally, apatite FT dating has been employed to estimate denudation rates in the Himalaya (Burbank et al., 2003). These few examples highlight the versatile potential applications of the apatite FT method.  20The (U-Th)/He dating method is often paired with apatite FT dating. Both methods can be applied to apatite for determining low temperature thermochronologic histories. As previously mentioned, the closure temperature for apatite in apatite FT dating is 70-110˚C, while the closure temperature for apatite in (U-Th)/He is 40-70˚C. Inconsistencies have been described between the two techniques with respect to ages (eg. Green et al., 2006), where the (U-Th)/He method yields older than expected ages. If a population of the same specimen is split between (U-Th)/He and apatite FT techniques, ages derived from the (U-Th)/He method should be younger as the apatite grains ‘close’ to He loss at lower temperatures. In practice, however, this is often not the case (Green & Duddy, 2006, Flowers et al., 2009). Causes for these discrepancies may include helium retention, radiation damage, zoning (heterogeneous distribution of uranium, thorium, and samarium), variation in grain size, and other extraneous factors. The majority of these discrepancies, however, are related to the (U-Th)/He method, and not the apatite FT method (Green et al., 2006)   2.1.2 Specimen Selection   Specimens were selected for apatite FT dating based on rock type and mineral content. Although it is nearly impossible to definitely determine if apatite is present from hand specimens, candidates were selected based on the suspected likelihood of having apatite present. Unfortunately, apatite content is only determined after preparation of the specimens, and etching is complete (see Preparation below). Rock types selected for apatite FT dating in this study included (in no particular order): granite, leucogranite, orthogneiss, gneiss, phyllitic schist, schistose phyllite, and meta-psammite. In addition to 21rock-type, specimens were also selected based on their spatial distribution across the mapped area. Ten specimens in total were chosen for dating, with six spread across the Greater Himalaya, and four spread across the Lesser Himalaya within the Taplejung window. Only two specimens of those ten yielded sufficient apatite for dating, one in the Taplejung window, and one structurally higher in the Greater Himalaya (Figure 2.1).   2.1.3 Preparation Rock specimens KA036 and KA058 were crushed and had heavy and light minerals separated using heavy liquid separation at a density of 3.3kg/L. This allowed the isolation of zircon grains for a companion project using the U-Th-Pb dating method (Ambrose 2014), and was completed at the University of California, Santa Barbara (UCSB). The remaining light and heavy fractions were sent to Dalhousie University in Halifax, Nova Scotia for further processing. Specimens KA005, KA012 (A, B, and C together), KA014, KA018A, KA025, KA048B, KA049, and KA066C were prepared at CF Mineral Research Lab in Kelowna, BC. The specimens were crushed and separated using heavy liquids, 3.3kg/L liquid to remove zircon and 2.8kg/L liquid to isolate apatite. All fractions were sent to Dalhousie University in Halifax, Nova Scotia for further preparation. At Dalhousie University, specimens received from UCSB were further separated using heavy liquids including sodium polytungstate (SPT) and lithium metatungstate (LMT). The density of the liquids is ~2.85 kg/L, which allows apatite grains to sink and quartz grains to float. Where the specimen was too large to mix and separate all at once, it was done in stages. The grains were poured into the heavy liquid and mixed every ~10 22Figure 2.1: Elevation profile of specimens collected for apatite fission track analyses, and muscovite and biotite 40Ar/39Ar thermochronology along the transect from SSW to NNE in the Kanchenjunga region of northeast Nepal. Also shown are the location of reported monazite U-Th-Pb rim ages (from Ambrose, 2014).Distance (km)Elevation (km)02.55.07.510.00              10              20              30              40               50              60              70              80              90             100                       LHS GHSGHSMCTMCTMCTMBTMHTMHTMCTTaplejung WindowKA010KA004KA018KA014KA013KA028KA025KA024KA031KA031KA039KA036KA037KA047 KA057KA055KA055KA051 KA063KA064KA061(KA066)40Ar/39Ar muscovite40Ar/39Ar biotiteapatite fission trackU-Th-Pb monazite rim NNESSWKA007KA034KA044 KA05823minutes for at least 2 hours per specimen or portion of specimen. Apatite and other heavy minerals were collected for further mineral separation. Both heavy and light fractions were saved.  The heavy fractions from UCSB were sieved using a standard US sieve at 250µm and a mesh sieve at 75µm. Grains smaller than 75µm would likely be too small to date, and grains larger than 250µm were likely not apatite in these specimens. Grains between 75 and 250µm were selected for further separation. The heavy fractions from CF Mineral Research were sieved using a standard US sieve at 250µm and a mesh sieve at 104µm (the 104µm sieve was used here instead of the 75µm sieve as the specimen was larger, and sufficient material was more likely to be gained from a smaller range of grain size). Grains between 104 and 250µm were selected for further separation. All specimens were then subjected to magnetic Frantz separation, removing magnetic material in 3 stages. Specimens were first run through the Frantz at a current of 0.4A, followed by runs of 0.8A and 1.2A respectively, at various tilt angles depending on how the grains travelled through the machine (in order to maximize the effectiveness of the separation). At the end of all three runs, the remaining non-magnetic fraction contained any apatite present. These fractions are referred to as 1.2A non-magnetic. All fractions were saved separately for each specimen as Frantz @0.4A magnetic, Frantz @0.8A magnetic, Frantz @1.2A magnetic, and Frantz @1.2A non-magnetic. KA036B and KA058C 1.2A non-magnetic fractions were then hand picked under a microscope as they were not yet pure enough in apatite grains. All other specimens yielded pure enough apatite fractions with the final 1.2A Frantz step. Selected grains from these two specimens were placed on double-sided tape on a glass slide with grains 24aligned approximately 10 grains by 12. Where possible, grains were placed on the tape with the c-axis oriented north-south (Figure 2.2a). Two slides were prepared for each specimen in this manner.  An epoxy-hardener mixture was prepared with a mass ratio of 100:17, epoxy:hardener. Epoxy used was Sylmasta Araldite (AY103), and the hardener was Sylmasta Araldite Hardener (HY956). The remaining 1.2A non-magnetic specimens that did not require hand-picking were mounted on glass slides within a patch of the epoxy mix, with about 100-150 grains per mount, spread evenly throughout. The grains tend to sink in the epoxy, leaving them close to the glass slide (Figure 2.2a). The epoxy cures at 70 ˚C for 30 min, but these slides were cured at 40 ˚C and left overnight. The long airing time and low heat helped to remove potential bubbles from the epoxy.  The slides mounted on tape  (KA036B and KA058C) required additional set up due to the tape applied. They had the epoxy mix applied over the grains that were placed on the tape, then had another glass slide applied on top. Between the two glass slides on either side of the tape/grain/epoxy mix, were a few sheets of paper in order to ensure that the epoxy only touched one glass slide, and did not ooze off the tape onto the other glass slide (Figure 2.2a). This step allowed for the epoxy to harden to the new glass slide. Once hardened, the initial glass slide with tape on it was removed, and the grains remain, oriented, within the epoxy mounted on the second slide. In this method, the grains are removed from the tape, and are situated at the top of the epoxy on the new slide as opposed to the bottom of the non-handpicked slides. The handpicked slides, therefore, required less grinding and polishing to reach the appropriate thickness for track counting (Figure 2.2b). 25actual mounted slide sticky notesslide to be peeled off double sided tapegrainsslideepoxya) Mountingb) PolishingepoxyslidegrainFigure 2.2: Sample preparation for apatite fission track dating: a) showing a normal grain mount and a hand-picked grain mount, and b) illustrating the various grinding and polishing steps of the apatite grains in order to achieve best results for analysis Normal Grain MountHand-Picked Grain Mountc-axis26Grinding and polishing was completed in 5 steps for each specimen. The first step was hand-grinding the specimens using silicon carbide grinding paper, Buehler Carbirnet 2 abrasive paper 400/P800 wet, using a block to move the slide against the paper, alternating equal amounts of clockwise and counter-clockwise turns around the paper, orienting the block at 0˚, 90˚, 180˚ and 270˚ to ensure even grinding. The aim of this first step was to remove excess epoxy and expose the majority of the grains within the slide. A second, similar grinding step was performed with finer grained silicon carbide grinding paper, 600/P800 wet, with similar grinding patterns. The aim of this step was to grind the exposed grains so that approximately 2/3rds or less of the individual grains remain, but no less than half (Figure 2.2b). Grinding was followed by polishing using a Struers Planopol-V polishing machine. Polishing on the machine was completed in three stages. The first used a polishing cloth, Struers DP-DAC, with Struers Diaduo 9µm diamond suspension and lubricant liquid. This step removed some of the larger scratches from the grains and epoxy on the mount. The next polishing cloth used was also a Struers DP-DAC, with Struers Diaduo 3µm diamond suspension and lubricant liquid. This step removed most of the scratches on the grains. The final polishing step used a Struers OP-Chem polishing cloth with Buehler micropolish II, 0.3µm deagglomerated alpha alumina powder mixed with distilled water. This step removed all remaining scratches on the grains and epoxy. Between each step of grinding and polishing, all specimens were washed with soap and distilled water, and soaked for a minimum of 10 minutes in a sonic bath to avoid any contamination between steps. Solutions used in polishing were also soaked for a minimum of 15 minutes in a sonic bath in order to fully mix the suspended material within the liquid. At the end of grinding and polishing, the aim was to have the 27majority of the grains cut in half and exposed in a smooth polished surface within the epoxy for viewing under a microscope.  The slides were each etched in 5.5M HNO3 acid for exactly 20 seconds, before being rinsed in 3 different beakers of water. This helped etch the fission tracks so that they were easier to see under a microscope. Good polishing was essential as any existing scratches may also be etched, potentially confusing or obscuring fission tracks. It is possible to see the depth of tracks in a grain by adjusting the focus on an optical microscope, which is how the tracks are identified.  Once polished and etched, specimen slides were reduced in size in order to fit in the reactor. Once at an appropriate size, the specimens were covered with a sheet of mica, re-labeled, and taped together and wrapped in cling-wrap. The specimens were then packaged and shipped to a reactor (for this project in Germany) for irradiation. Once sufficiently cooled, specimens were returned to Dalhousie University where they were dated by Dr Isabelle Coutand.   2.2 40Ar/39Ar dating 2.2.1 Introduction and Theory 40Ar is derived from the radioactive decay of 40K. 40Ar will naturally accumulate in any mineral containing K, and this process can therefore be used to date such specimens. Originally, minerals were dated separately analyzing 40K by flame photometry and 40Ar by mass spectrometry (eg. Halas, 2012; Aldrich and Nier, 1948). Because this process requires large amounts of material and 40K and 40Ar are measured in separate aliquots, the method has since stopped being used.  28The 40Ar/39Ar method is a variant of the K-Ar dating technique and is now the dating method of choice for many applications. A sample is irradiated along with known age standards (flux monitors) with fast neutrons in the core of a nuclear reactor. This process converts another isotope of potassium (39K) to gaseous 39Ar and thus 39Ar acts as a proxy for 39K, and therefore, 40K (as the relative abundances of 39K and 40K are known; McDougall and Harrison, 1998). This enables the simultaneous measurement of both the parent (39ArK) and daughter (40Ar) isotopes from the same aliquot, thus avoiding many potential systematic errors (Kelley, 2002b). The main advantage of 40Ar/39Ar dating is that it allows much smaller samples to be dated, measurements are very precise and very accurate, and more age and composition (e.g. Ca/K) information can be obtained from each sample.  The 39K/40K ratio in nature is known (Emery et al. 2009), so the 39Ar derived from 39K can be related to parent 40K and thus compared to daughter 40Ar.  The efficiency of 39Ar production is a function of the duration of irradiation, neutron flux, and neutron capture cross-section of the specific specimen being measured. A specimen of known age is irradiated in close proximity to the specimens with unknown ages in order to monitor the dose of irradiation, and determine the J-factor that relates production of 39Ar from 39K during irradiation (McDougall and Harrison, 1999). Micas including muscovite and biotite are common minerals used in 40Ar/39Ar dating as they are K-bearing and commonly found in many types of rocks. Muscovite typically has a significantly higher closure temperature than biotite due to slower diffusivity of Ar, closing around 350˚C, while biotite closes around 300˚C (eg. Harrison et al., 1985; Hames and Bowring, 1994; McDougall and Harrison, 1999; Fleet, 2003), 29although it is not always very well constrained. By dating both muscovite and biotite specimens from the same rock using the 40Ar/39Ar dating method it is possible to estimate a cooling rate for that rock.  There are two methods commonly used to acquire age information from 40Ar/39Ar dating: traditional step-heating and in situ laser ablation. Step heating involves heating a grain or collection of grains from a given specimen in a series of steps by using either a furnace or a laser (McDougall & Harrison, 1998). As the specimen is heated, it releases argon, which is then measured in a mass spectrometer. The initial steps of heating are typically interpreted to yield information about the rim of the grain (shortest diffusion path), while higher temperature steps are interpreted as de-gassing towards the centre of the grain (longest diffusion path) (eg. Harrison and Zeitler, 2005). This is possible in micas for example as diffusion is parallel to the basal cleavage plane and not perpendicular to it. In theory, a grain could record multiple events elucidated by using step-heating from most recent to oldest until the entire grain is fused or melted. If multiple grains are being analyzed at the same time from the same specimen, it is important that they are of similar size such that they have similar total diffusion lengths. Moreover, the closure temperature of mica is strongly related to grain size. If a mica population of interest is heterogeneous, of mixed sizes or ages, or contains intergrowths with other minerals or distinct phases, the step-heating method has no means to spatially control where the argon is released from, which may result in a homogenized, meaningless age for the specimen. This potential limitation can be overcome using in situ 40Ar/39Ar dating, wherein a laser is used to release gas from a specific grain or area within the grain in a specimen (Kelley, 2002a). In this way, it is 30possible to date different parts of a single grain or a heterogeneous population. This method, however, is considerably more expensive than traditional step-heating, and there is less equipment currently available to use this technique, which therefore limits its use in the scientific community. Moreover, the relatively small sampling volume extracted by the laser means that it works best in older, more radiogenic material rather than younger, less radiogenic material. Step heating methods may produce age results that do not appear to make meaningful sense, or do not have good plateau ages. In situ methods may potentially overcome these issues by identifying zones within grains.  The 40Ar/39Ar dating method has an incredibly wide range of geological dating applications, from dating inclusions in primitive iron meteorites almost as old as the solar system, (eg. Niemeyer, 1979), to mafic volcanic rocks as young as the Holocene (eg. Lanphere, 2000, Guillou et al 1997). It can be applied to igneous, metamorphic and sedimentary rocks examining crystallization, cooling, or detrital ages. It has successfully been used in the Himalaya to interpret the low temperature rock histories including determining exhumation rates of the Ama Drime range (Kali et al., 2010), examining the rapid cooling of rocks subjacent to the STDS in Sikkim, India (Kellett et al., 2013), and interpreting late-stage channel flow in Yadong, Eastern Himalaya (Gong et al., 2011).  This study employs 40Ar/39Ar dating on muscovite and biotite, two commonly used K-bearing minerals. Mica grains were hand picked and selected based on rock type and purity of the grains. They were then irradiated in a nuclear reactor in order to induce production of 39Ar from 39K. The specimens were heated incrementally using a photon machines CO2 laser, and the released Ar isotopes were measured using an ARGUS VI mass spectrometer. After step heating, if a consistent age plateau was achieved (i.e. 31Figure 2.3), 40Ar/39Ar ratios were interpreted from the well-behaved steps. These values are input into an age equation to determine the age of the specimen. The age equation is as follows ! =1! ln 1+ !!!"!"∗!!"!"! ! = !"#!!"!!ℎ!!!"#$%&!! = !! + !! = 5.43 ∗10!!"!" !! =!!"∗ − 1!!"!"∗!!"!"!!t ∗!= !"#!!"!!ℎ!!!"#$%#&%!!!"!"∗!!"!"!= !"#$%&"'  Constants !, !!!!and!!! from Steiger and Jager, 1977, and the irradiation parameter, J is determined through the irradiation process.  The 40Ar/39Ar dating method can be both accurate and precise. It requires no specimen splitting as it only looks at a single element and age information is derived directly from the ratio of 40Ar/39Ar. There are, however, some factors that need to be taken into account that affect accuracy. One potential complication of the 40Ar/39Ar dating method (and its predecessor, the K-Ar method) is the potential for excess argon, which may significantly affect results and potentially skew the dates of minerals analyzed. 320           20          40           60          80          100Cumulative % 40Ar released50403020100Apparent age (Ma)Ü& Ü&Ü&Ü&Ü&Ü&Ü&Ü&Ü&Ü&Ü&biotiteplateau ageexcess argon excess argonFigure 2.3: example of a typical “U-shaped” 40Ar release spectra. The plateau age yields the most accurate age estimation for the sample. See text for discussion.332.2.2 Excess Argon Excess argon can be defined as “parent-less radiogenic argon incorporated into a mineral during crystallization, introduced into the mineral lattice by subsequent diffusion or occluded within fluid or melt inclusions within the mineral” (Kelley, 2002a). Measured argon in a particular specimen can be derived from five different components including air, potassium, calcium, chloride, and excess argon (Villa 1990). Argon is a component of Earth’s atmosphere, and as such, an air correction for atmospheric 40Ar is required when using the 40Ar/39Ar or K-Ar dating methods (Renne et al., 2009). The atmospheric ratio of 36Ar/40Ar has changed throughout time, though the ratios are relatively well known. Current ratios of atmospheric 36Ar/40Ar are 298.6, and are the highest they have ever been (Pujol et al., 2013). It is usually assumed that all 36Ar found in a mineral or rock specimen prior to irradiation is atmospheric in origin, and thus the initial 40Ar can be calculated from that (Renne et al., 2009).  Radiogenic 40Ar is typically derived from irradiation of 40K. Neutron activation is non-selective, however, which means that during irradiation of the specimen, Ar isotopes may be derived from nuclides other than K such as Ca or Cl. Aside from non-selective irradiation, excess argon can be incorporated into a system by a high partial pressure of 40Ar in circulating fluids or by degassing of adjacent rocks or minerals at the time/temperature of closure of the analyzed minerals. Excess argon can be incorporated in a mineral through diffusion across grain boundaries during complex thermal histories (Treloar et al. 2000; Maluski et al. 1988; Ruffet et al. 1995). Deformation can create voids or defects in mineral grains. Excess argon from circulating fluids or degassing of adjacent rocks or minerals may become localized in these defects, often associated with 34grain boundaries, or in potassium voids within the grain (Ruffet et al. 1995). Localized sites containing excess argon are often also recognized as inclusions or alterations containing an excess of non-radiogenic 40Ar (Treloar et al. 2000). Biotite over other minerals is noted to be particularly susceptible to excess argon due to its crystal chemistry properties such as density, which affect diffusivity within the grain (Treloar et al. 2000). Moreover, because it has a low closure temperature compared to other minerals (eg. muscovite), biotite also has more time during cooling in which to accumulate excess Ar. Excess argon is not uncommon in low-potassium bearing minerals or rocks (Kelley, 2002a), and will often yield a “U-shaped” step-heating release spectrum (Figure 2.3), with older initial apparent ages released at low temperature steps, leveling out to a more accurate representation of age for the specimen in the “saddle,” or plateau, before yielding older ages again at the highest temperature steps or fusion temperatures (Lanphere and Dalrymple, 1976). The low and high temperature release of excess argon is best explained by fluid or melt inclusions, rather than the diffusion of argon across grain boundaries during metamorphism, although this may also be a contributing factor (Kelley, 2002a). These inclusions may contain excess argon in amounts exceeding 10,000 times that found within the mineral lattice by weight (Kelley 2002a). Because excess argon is associated with excess daughter isotope, its presence will yield older than appropriate age results, where argon loss (which occurs through similar processes as argon gain, only reversed) will yield younger than appropriate age results.  Previous thermochronological studies in the Himalaya have included 40Ar/39Ar, and/or K-Ar dating. Excess argon is a commonly cited problem in the Himalaya 35especially with biotite grains (e.g. Hubbard and Harrison, 1989, Treloar, et al 2000), although the problem is not inherently unique to biotite. In the Everest region of Nepal, Hubbard and Harrison (1989) attribute many unreliable, older than expected biotite ages in both their study and in Krummenacher et al., (1978) to excess argon. Maluski et al. (1998) describe large amounts of excess argon being a problem with biotite separates in particular, and some muscovite separates in orthogneiss specimens taken from the Lhagoi Kanhgri Massif, in south central Tibet. They also note, however, that some specimens of muscovite may have experienced argon loss. Other Himalayan studies describe excess argon as a problem in cyclosilicates such as tourmaline and beryl, but not with biotite (Villa 1990 and references therein). MacFarlane (1993) describes excess argon being likely in hornblende specimens in the Langtang region of central Nepal. Excess argon is difficult to isolate and differentiate from other forms of argon as it is not characteristically unique (Villa, 1990). Because excess argon is impossible to separate from measured results, a series of theoretically unsolved issues has arisen (Villa, 1990). The problem of excess argon may be partially overcome in 40Ar/39Ar dating through step heating by looking at the partial plateau age (Maluski et al. 1988), and ignoring initial and/or fused results as they may be most significantly affected by either excess argon or argon loss. Certain domains within grains, such as grain edges or inclusions, may yield older ages that can potentially be identified in preliminary or final steps of heating. Furthermore, in situ laser ablation may be used to investigate patchily zoned grains that may not be distinctly zoned from rim to core. Individual grains within a rock specimen may also be differently affected through metamorphic cooling, resulting in disparate ages. It can be useful to use multiple geochronometers 36to compare ages, which may help in determining whether excess argon is a problem. For example, if mica (biotite or muscovite) 40Ar/39Ar dates (which should close around ~350-450˚C respectively) are older than nearby specimens dated through other geochronometer techniques such as monazite rim ages in U-Th-Pb dating, which are known to close at much higher temperatures (~750˚C), it can be assumed that the mica 40Ar/39Ar date is not representative of a cooling age, and excess argon may be a possible explanation.  While excess argon has often been used to explain anomalously old biotite ages, those ages may actually provide useful timing information on timing constraints for the metamorphic evolution of the mid-crust. Research has shown that retention of argon in a system, and therefore the resulting age, in addition to temperature, can also be affected by fluids present, chemical compositions, grain defects, and new crystal growth (Villa 1998). Biotite grains that have yielded 40Ar/39Ar dates older than nearby muscovite grains or 232Th-208Pb monazite rim dates, and have previously been disregarded and attributed to excess argon, could actually represent a crystallization age as opposed to a cooling or deformation age. Thus, while this is a complicating factor, and work is still being done to understand this process, the significance of the ages dated through 40Ar/39Ar processes must be read with room for interpretation.  2.2.3 Specimen Selection  Specimens were selected for 40Ar/39Ar dating based on availability of visible mica grains in specimens as well as their spatial distribution across the mapped area. They were also selected based on the quality of the mica grains, with preference given to inclusion-free, large, and unaltered grains. Eighteen rock specimens in total were selected 37for dating, which include 16 specimens of biotite and 7 specimens of muscovite. There are 14 specimens spread across the Greater Himalaya, and 9 spread across the Lesser Himalaya within the Taplejung Window (Figure 2.1). Grains were hand picked from whole rock or crushed specimens, using tweezers or other implements, and a stereomicroscope. Care was also taken to ensure that the grains did not contain other mineral phases, were not altered, were relatively the same size, and from the same population (in specimens with multiple mica populations, based primarily on grain size and orientation with respect to foliation). Where appropriate, separate populations of both biotite and muscovite grains were picked from each rock specimen.   2.2.4 Procedure All 40Ar/39Ar analytical work was performed at the University of Manitoba. Standards and unknowns were placed in 2 mm deep wells in circular configurations on 18 mm diameter aluminum disks, with standards placed strategically so that the lateral neutron flux gradients across the disk could be evaluated. Planar regressions were fit to the standard data, and the 40Ar/39Ar neutron fluence parameter, J, interpolated for the unknowns. Uncertainties in J are estimated at 0.2 – 0.4 % (1s), based on Monte Carlo error analysis of the planar regressions (Best et al., 1995). Samples were irradiated in the CLICIT facility of the Oregon State University TRIGA reactor for 7.5 hours and using the Fish Canyon sanidine (Renne et al., 2010) and GA1550 biotite (Spell and McDougall, 2003) standards. Irradiated samples were placed in a Cu sample tray in a high vacuum extraction line and were either fused or step-heated using a 100 W CO2 laser. Sample viewing 38during laser fusion was by a video camera system and positioning was via a motorized sample stage. Reactive gases were removed by three GP-50 SAES getters (two at room temperature and one at 450˚C) prior to being admitted to an ARGUS VI mass spectrometer by expansion. Five argon isotopes were measured simultaneously over a period of 7 min. Measured isotope abundances were corrected for extraction-line blanks. A value of 295.5 was used for the atmospheric 40Ar/36Ar ratio (Steiger and Jager, 1977) for the purposes of routine measurement of mass spectrometer discrimination using air aliquots, and correction for atmospheric argon in the 40Ar/39Ar age calculation.   39Chapter 3: Thermochronology Data and Results 3.1 Previous Work The various low temperature chronometers employed in nearby studies include apatite and zircon dating through FT analysis, 40Ar/39Ar muscovite and biotite dating, and biotite dating using the K-Ar method (Figure 3.1; Krummenacher et al. 1978; Hodges et al. 1989; Hubbard and Harrison 1998; Sakai et al. 2005, 2013; Doon 2010; Kali et al. 2010; Streule et al. 2012; Gong et al. 2011; Imayama 2012; Kellett et al. 2013).    Published muscovite dates from the LHS exposed in both the Taplejung and Ranjit Windows are significantly older than the Eocene onset of the Himalayan collision; 1674-1558 Ma in the Taplejung window (Sakai et al., 2013), and 1784-343 Ma in the Ranjit window (Figure 3.1; Doon, 2010). Existing apatite and zircon FT dates from the Taplejung window, in contrast, are young, ranging from 2.9 to 1.3 Ma and from 6.2 to 4.6 Ma respectively. Both apatite and zircon FT dates appear to young towards the northwest (Figure 3.1).   The specimens north of the Taplejung window in east Nepal, one within the MCT zone, and two farther north within the GHS (Figure 3.1) yield apatite and zircon FT dates that overlap with those from within the window to the south, at 2.4 – 1.4 Ma and 5.4 - 5.0 Ma respectively. Muscovite 40Ar/39Ar dates from similar locations range between 13.8 and 11.0 Ma, while biotite 40Ar/39Ar dates are older than those from muscovite, between 25.0 and 21.0 Ma, perhaps reflecting excess argon contamination (Sakai et al., 2013).  FT and 40Ar/39Ar dates have also been extracted from within the GHS in nearby regions of western Bhutan, northern Sikkim, and the Everest and Makalu regions of Nepal. In western Bhutan two specimens from the ~middle of the GHS yield muscovite!406RXWK7LEHWDQ'HWDFKPHQW6\VWHP67'60DLQ&HQWUDO7KUXVW0&7*UHDWHU+LPDOD\DQ6HTXHQFH/HVVHU+LPDOD\DQ6HTXHQFHGHSLHSSHTSS6XE+LPDOD\D7HWK\DQ6HGLPHQWDU\6HULHV'LVFRQWLQXLW\&RXQWU\%RUGHU0XVFRYLWH40$U39$UDJH0D%LRWLWH40$U39$UDJH0D=LUFRQ)7DJH0D$SDWLWH)7DJH0D*(2&+521$*(6/(*(1'%LRWLWH.$UDJH0D7KLVVWXG\Figure 3.1: Compilation of dates determined in the eastern Nepal Himalaya. Dates are compiled from Doon (2010); Gong et al. (2011),  Hodges et al. (1998), Hubbard and Harrison (1989), Imayama et al. (2012), Kali et al. (2010), Kellet et al. (2013), Krumme-nacher et al. (1978), Sakai et al (2013), Sakai et al. (2005), Streule et al. (2012).Û( Û( Û(Û1Û1GHSLHSTSSMCT0W(YHUHVWMakalu.DQFKHQMXQJDSTDSAma'ULPHSH1(3$/6,..,0,1',$7,%(7&+,1$-7DSOHMXQJ:LQGRZ5DQJLW:LQGRZ17.819.047.318.717.018.013.410.59.816.039.514.09.05.53.48.517.052.53014.414.412.149.616.513.717.612.210.921.68.425.516.616.517.116.75.44.85.05.44.65.2 6.22.32.42.22.42.52.32.921.525.021.011.01562167413.81669164215581.72.22.02.33.8 7.19.729.1 30.52.72.42.3 3.11.41.31.91.30.91.41.51.43.85.26.616.38.86.913.06.57.07.613.313.012.413.216.726.820.216.220.19.041ages of 16 and 13 Ma, and biotite dates of 28 and 11 Ma respectively (Gong et al., 2011; Figure 3.1). In northern Sikkim, Kellett et al. (2013) report apatite FT and muscovite 40Ar/39Ar dates in which the apatite dates are older close to the footwall of the STDS (13.0 Ma), and younger, structurally lower, to the south (7.6 - 6.5 Ma). Muscovite ages within the STDS and its footwall show no obvious trend and range from 13.3 to 12.4 Ma (Figure 3.1).  Several studies have been conducted in the Everest and Makalu regions of east-central Nepal (Krummenacher et al., 1978; Hubbard and Harrison, 1989; Hodges et al., 1998; Sakai et al., 2005; Streule et al, 2012). Three apatite specimens from below the MCT, ~SSE of Makalu (Streule et al., 2012) yielded FT dates from 1.4 to 0.9 Ma. To the NNW, above the MCT and towards the Makalu massif, apatite FT dates range between 3.1 and 0.9 Ma. One zircon FT date (8.8 Ma) was reported just south of the Makalu massif (Figure 3.1).  In the nearby Everest region the structurally lowest apatite and zircon FT dates published are from within the middle of the GHS (Figure 3.1) yielding dates of 1.7 and 3.8 Ma respectively. FT dates from both apatite and zircon grains increase in age to the north, ranging from ~2.0 to 3.8 Ma for apatite and ~5.2 to 6.6 Ma for zircon. Both zircon and apatite FT dates increase farther north towards the Everest massif with zircon FT dates jumping to 16.3 Ma and 14.4 Ma and apatite FT dates increasing with elevation in the region from 7.1 Ma near the base of Mt. Everest to 30.5 Ma near its peak (Figure 3.1). Apatite FT dates from the nearby, lower elevation Tibetan plateau to the east of Mt. Everest decrease to between 2.7 and 2.4 Ma. Muscovite and biotite dates from specimens taken from the lower portion of the Everest massif are 16.7 Ma and 17.1 Ma respectively 42(Hubbard and Harrison, 1989), though the older biotite date may reflect excess argon contamination. Structurally higher to the north in Tibet, near the STDS, muscovite and biotite specimens yield similar dates of 16.6 and 16.5 Ma respectively (Hodges et al., 1998). 40Ar/39Ar dates from the Ama Drime range (Kali et al. 2010) are variable. On the west side of the range, it appears that younger biotite dates generally occur to the north and older dates occur in the south - ranging from 10.9 Ma to 25.5 Ma (Figure 3.1). On the east side of the range, however, no distinct pattern is observed, and at least one biotite date (49.6 Ma) is interpreted to be strongly affected by excess argon (Kali et al. 2010). There are only two muscovite dates from the region, one in the southwest of the Ama Drime range at 8.4 Ma, and the other to the north, which is older at 17.6 Ma. This study aims to build on this previous work and create a comprehensive transect of low temperature thermochronometry across the Eastern Nepal Himalaya. These new data will not only help fill in a significant gap in the current spatial distribution of dates from the region, but also, with previously published data, help enable the interpretation of the large-scale processes that may have influenced the spatial distribution of the low temperature dates. This study presents new apatite fission track, and muscovite and biotite 40Ar/39Ar ages from samples collected across most of the exhumed, former midcrustal core of the Himalaya in the Kanchenjunga region of far east Nepal.   433.2. Results 3.2.1 Apatite Fission Track   Unfortunately, apatite can be difficult to find in sufficient quantities within the LHS rocks to date using FT methods (e.g. van der Beek et al., 2006, Patel et al., 2007, I. Coutand, Pers. Comm., 2013). The size of the rock specimens collected in the field can also be a limiting factor; larger specimens could potentially have yielded more apatite. Collecting larger specimens during fieldwork, however, was not feasible. The size of the rock specimens was limited by the fact that the trek was completed on foot, most specimens had to be carried over 100 km, and in only 3 weeks at high elevations. Only two of the ten specimens selected for apatite FT dating, KA025 and KA036B yielded enough apatite grains to use for dating.   KA025 yielded 7 apatite grains in which tracks were counted yielding a representative age of 1.30 ± 0.43 Ma (± 1σ). Specimen KA036B (separated into KA036B-1 and KA036B-2) had 12 and 13 apatite grains respectively that were dated, yielding ages of 1.38 ± 0.39 Ma and 1.50 ± 0.31 Ma (± 1σ). The error bounds reflect the relatively small number of grains that were available for dating. Results are shown in Table 3.1 and Figure 3.2.   44Table 3.1: Apatite fission track results, dated by Dr Isabelle Coutand at Dalhousie University in Halifax, Nova Scotia. Only two of the ten samples collected for AFT dating yielded significant results, specimens KA025 and KA036B (separated into KA036B-1 and KA036B-2).    Apatite fission-track results (Dalhousie University AFT Lab)  Sample Number of grains Spontaneous Track Density rs x 106 cm-2 (Ns) Induced Track Density ri x 106 cm-2 (Ni) Dosimeter Track Density rd x 106 cm-2 (Nd)  P(c2)   (%) Central Age ± 1σ  (Ma) U   (ppm) KA025 7 0.0205 (9) 4.2790 (1879) 1.4546 (5291) 88.8 1.30 ± 0.43 39.59 KA036B-1 12 0.0145 (13) 2.8425 (2550) 1.4662 (5291) 56 1.38 ± 0.39 25.35 KA036B-2  13  0.0207 (23)  3.7575 (4184)  1.4679 (5291)  81   1.50 ± 0.31  33.82     !! Operator: I. Coutand, Zeta = 370.6 ± 5.0 !45Schist/gneiss, locally migmatitic with leucogranite lensesQuartziteSandy PhylliteOrthogneissPhyllitic schistMigmatite/gneissGEOLOGYMCTMCTChaukiGufa PokhariDobhanChirwaSekathumOlanchangolaYanmaTaplejungBasantapuraTaplejung WindowNEPALSIKKIM(INDIA)TIBET (CHINA)N0 10                      20          30 kms86421.3 ± 0.4 1.3 ± 0.42.9 ± 0.22.3 ± 0.22.5 ± 0.42.4 ± 0.32.2 ± 0.22.4 ± 0.22.3 ± 0.26.2 ± 0.65.4 ± 0.34.6 ± 0.45.2 ± 0.44.8 ± 0.45.4 ± 0.25.0 ± 0.21558 ± 81642 ± 251669 ± 51562 ± 131674 ± 1513.8 ± 0.611.0 ± 0.325.0 ± 0.421.0 ± 0.721.5 ± 0.615.4 ± 0.425.0 ± 0.419.7 ± 0.412.4 ± 0.2 16.4 ± 0.211.3 ± 0.4meaningless18.4 ± 2.419.1 ± 0.9meaningless9.5 ± 0.110.2 ± 0.512.5 ± 0.319.7 ± 0.714.3 ± 0.318.3 ± 0.5VillagePathRiverMain Central Thrust DiscontinuityThis studyImayama et al., 2012Sakai et al., 2013AFT ageBiotite 40Ar/39Ar  ageBiotite K-Ar age Muscovite 40Ar/39Ar age ZFT ageTHERMOCHRON AGES (Ma)16.7 ± 0.626.8 ± 0.620.2 ± 0.316.2 ± 0.420.1 ± 0.59.0 ± 0.3Distance (km)S NTaplejung WindowElevation (km)01562340             10             20             30             40             50             60             70             80             90             100       LHS GHS05.010.015.020.025.0Age (Ma)MCTMCT170016001500GHSdiscontinuityKA004KA014KA025KA028KA031KA036KA039KA066KA063KA061KA057KA055KA051KA047This studyImayama et al., 2012Ambrose et al., 2014disregarded from      interpretationSakai et al., 2013 40Ar/39Ar muscovite40Ar/39Ar biotitezircon fission trackmonazite rim U-Pbapatite fission trackK-Ar biotiteFigure 3.2: A) Overview of age results from this study, Sakai et al. (2013) and Imayama et al. (2012). Includes geologic interpration and identified structures within the study area of the Kanchenjunga region of far east Nepal. B) Cross section interpretation of results overlaid on representative elevation along the profile. Previously identified discontinui-ties are also displayed. This figure also includes monazite melt crystallization ages from a companion study by Ambrose (2014).  AB463.2.2 40Ar/39Ar   24 mica separates were picked from 18 different specimens collected from across the mapped area: 17 biotite and 7 muscovite. Results are shown in Table 3.2 and Figures 3.3 and 3.4. The spatial distribution of ages is shown is Figure 3.2. Plateau ages were determined when a plateau was defined by measured spectra from the step-heating process. If a plateau was not apparent after step-heating, an integrated age, which is the sum of all the steps, equivalent to a total fusion age, was determined. Ar release spectra and behaviour are discussed below. For a full description of the mica in each specimen see Appendix I. Two 40Ar/39Ar dates were acquired south of the Taplejung window (Figure 3.2). The southern-most, and structurally lowest specimen dated within the GHS, KA004, is a garnet + biotite schistose gneiss. Biotite from this specimen yielded a poorly behaved age spectrum, meaning that a plateau age was not achieved, and instead defines an integrated age of 14.3 ± 0.3 Ma over 8 steps of heating (Figure 3.3A). The other specimen from south of the Taplejung window is KA014 (Figure 3.2), a garnet + biotite + muscovite schistose gneiss, selected for muscovite dating. It too yielded a poorly behaved age spectrum (Figure 3.4A) that did not define a plateau. The associated integrated age for the specimen is 18.3 ± 0.5 Ma over 10 steps. Only one specimen was dated from within the LHS in the Taplejung window (Figure 3.2). KA025 is a biotite + muscovite orthogneiss, that was selected for biotite dating. The biotite yielded a poorly behaved age spectrum (Figure 3.3B) that did not define a plateau. The integrated age of the specimen is 19.7 ± 0.7 Ma over 8 steps.  47Table&3.2:&40Ar/39Ar&dating&results&including&both&biotite&and&muscovite&grains,&showing&plateau&age&and&integrated&age&±1σ &&Sample # Grain Plateau Age (Ma) ±1σ Integrated Age (Ma) ±1σ KA004 Biotite !14.3 ± 0.3 KA014 Muscovite !18.3 ± 0.5 KA025-1 Biotite  19.7 ± 0.7 KA028 Biotite  68.1 ± 0.3 KA028 Muscovite 12.53 ± 0.31  KA031B Biotite  69 ± 0.7 KA031B Muscovite  10.24 ± 0.05 KA036A Biotite  9.5 ± 0.1 KA039 Biotite 11.27 ± 0.38  KA047 Biotite  16.4 ± 0.2 KA051* Biotite 12.4 ± 0.2 12.0 ± 0.2  KA055 Biotite 19.70 ± 0.36  KA057 Biotite  25.0 ± 0.4 KA057 Biotite !25.6!±!0.4!KA061B* Biotite 19.07 ± 0.93  KA063A* Bitoite 15.35 ± 0.37  KA066B Biotite 18.4 ± 2.4   ! ! ! !! !* Preliminary age  !&48  KA063A   KA061BKA057   KA055KA047KA039   KA036KA031BKA0250.0 0.2 0.4 0.6 0.8 1.0 KA0280.0 0.2 0.4 0.6 0.8 1.0 KA0040.0 0.2 0.4 0.6 0.8 1.0 282420160 0102030400204060801004812KA066BPlateau age = 15.35 ± 0.37 Ma Plateau age = 18.4 ± 2.4 Ma Plateau age = 19.07 ± 0.93 Ma Plateau age = 19.70 ± 0.36 Ma Plateau age = 11.27 ± 0.38 Ma Integrated age = 9.5 ± 0.1 Ma Integrated age = 69.0 ± 0.7 Ma Figure 3.3: 40Ar/39Ar age spectra for all biotite specimens dated. Each step is indicated by DUHGRUEOXHER[SODWHDXVWHSVDUHUHGUHMHFWHGSODWHDXVWHSVDUHEOXH5HVXOWVVKRZıneglecting error in J. Age (Ma) Age (Ma) Age (Ma) Cumulative 39Ar Fraction Cumulative 39Ar Fraction Cumulative 39Ar Fraction0.0 0.2 0.4 0.6 0.8 1.0 0.0 0.2 0.4 0.6 0.8 1.0 0.0 0.2 0.4 0.6 0.8 1.0 Cumulative 39Ar Fraction Cumulative 39Ar Fraction Cumulative 39Ar Fraction0.0 0.2 0.4 0.6 0.8 1.0 0.0 0.2 0.4 0.6 0.8 1.0 0.0 0.2 0.4 0.6 0.8 1.0 Cumulative 39Ar Fraction Cumulative 39Ar Fraction Cumulative 39Ar Fraction0.0 0.2 0.4 0.6 0.8 1.0 0.0 0.2 0.4 0.6 0.8 1.0 0.0 0.2 0.4 0.6 0.8 1.0 Cumulative 39Ar Fraction Cumulative 39Ar Fraction Cumulative 39Ar Fraction20016004080120Age (Ma) 201604812Age (Ma) 24201604812Age (Ma) 30262218261014Age (Ma) 2824201604812Age (Ma) 403020010Age (Ma) 2824201604812Age (Ma) 3020010Age (Ma) 403020010Age (Ma) Integrated age = 14.3 ± 0.3 Ma Integrated age = 19.7 ± 0.7 Ma Integrated age = 68.1 ± 0.3 Ma Integrated age = 16.4 ± 0.2 Ma Integrated age = 25.0 ± 0.4 Ma A B CD E FG H IJ K L49Figure 3.4: 40Ar/39Ar age spectra for all muscovite specimens dated. Each step is indicated by a red or blue box; plateau steps are red, rejected plateau steps are blue. Results show 2ıQHJOHFWLQJHUURULQ-KA014KA028KA031BABC0.0 0.2 0.4 0.6 0.8 1.0 Cumulative 39Ar Fraction201604812Age (Ma) 0.0 0.2 0.4 0.6 0.8 1.0 Cumulative 39Ar Fraction24201604812Age (Ma) 0.0 0.2 0.4 0.6 0.8 1.0 Cumulative 39Ar Fraction403020010Age (Ma) Plateau age = 12.53 ± 0.31 Ma Integrated age = 18.3 ± 0.5 Ma Integrated age = 10.24 ± 0.05 Ma 50The majority of the 40Ar/39Ar data presented in this study comes from structurally above the MCT and north of the Taplejung window (Figure 3.2). Specimen KA028, a kyanite + staurolite + garnet + biotite + muscovite gneiss collected ~ 18 km north of the MCT (Figure 3.2), yielded both muscovite and biotite. The muscovite yielded a well-behaved age spectra, with an interpreted plateau age of 12.53 ± 0.31 Ma that comprises 7 out of 10 total heating steps and 91.1% of total Ar (Figure 3.4B). The biotite from the specimen yielded a poorly behaved age spectra (Figure 3.3C) with an integrated age of 68.1 ± 0.3 Ma over 14 steps. KA031B was collected ~4 km north of KA028 (Figure 3.2). It is a biotite + garnet + muscovite schist. It yielded a poorly behaved muscovite age spectra with an integrated muscovite age of 10.24 ± 0.05 Ma over 9 steps (Figure 3.4C) and poorly behaved biotite age spectra with no discernable plateau and an integrated age of 69 ± 0.7 Ma over 10 steps (Figure 3.3D).  KA036 (Figure 3.2; 3.3E) is a quartz + biotite ± muscovite quartzite collected ~17 km north of the MCT. It yielded a poorly behaved biotite age spectra with an integrated age of 9.5 ± 0.1 Ma over 10 steps.  KA039 (Figure 3.2; 3.3F) is a garnet + biotite gneiss. It yielded a well-behaved biotite age spectra with an interpreted plateau age of 11.27 ± 0.38 Ma. The plateau comprises 6 out of 11 total heating steps and 61% of total argon released. Specimen KA047 (Figure 3.2; 3.3G) is a sillimanite + kyanite + biotite gneiss collected ~20 km north of the MCT. It produced a poorly behaved age spectra, and an integrated biotite age of 16.4 ± 0.2 Ma over 13 steps. KA055 (Figure 3.2; 3.3H) is a garnet + biotite gneiss ~4 km north of KA051. It gave a well-behaved biotite age spectra with a plateau age of 19.70 ± 0.36 Ma. The plateau comprises of 7 out of 11 total heating steps and 71.2% of total argon released. The structurally higher, KA057 (Figure 3.2; 3.3I) 51is a kyanite + garnet + biotite gneiss. Two biotite separates from this specimen each yielded poorly behaved age spectras (only one shown), with integrated ages of 25.0 ± 0.4 Ma and 25.6 ± 0.5 Ma over 10 and 9 steps respectively. Specimen KA061B (Figure 3.2; 3.3J), which was collected ~46 km north of the MCT, is a sillimanite + biotite paragneiss. It has a well-behaved biotite age spectra, yielding a plateau age of 19.07 ± 0.93 Ma for 5 of 11 steps and 71.8% of total Ar released. The structurally higher KA063A (Figure 3.2; 3.3K) is a sillimanite + biotite gneiss. It gave a well-behaved biotite age spectra, with a plateau age of 15.35 ± 0.37 Ma over all 11 steps and 99.14% of Ar released. Finally, the structurally highest specimen collected, KA066B (Figure 3.2; 3.3L), is a quartzite with minor biotite partings. It produced a well-behaved biotite age spectra with an interpreted plateau age of 18.4 ± 2.4 Ma for 6 of 9 total steps and 83.1% of total Ar released.       52Chapter 4: Discussion and Interpretations 4.1 Apatite FT Apatite FT dates from this study are significantly younger than those reported by Sakai et al. (2013) from nearby locations (Figure 4.1). This may reflect either an abrupt break in cooling ages across the MCT (Figure 4.1), or possible calibration discrepancies between the studies. Discrepancies could result from a difference in etching procedures, dosimeter standards, and external detectors. This study utilizes the “standard” etching procedures described in Donelick et al. (2005), whereas the Sakai et al. (2013) used chemical concentrations, temperatures, and etching times that differ. Additionally, the CN5 dosimeter standard was used in this study while Sakai et al. (2013) used NIST-SRM612. Finally, a uranium-free mica slab was used as the external detector for the present study whereas Sakai et al. (2013) used a DAP plastic detector.  While it is possible that the procedural differences between this study and previous work could systematically affect apatite FT dates from each, the Zeta calibrations should account for relative differences between those procedures and the operators. Therefore, the differences in ages between the studies are attributed to geologic processes. When integrated with the apatite FT ages from Sakai et al. (2013) the ages appear to young from south to north within the LHS (Figure 4.1). Structurally higher, and farther north, at the MCT, the trend breaks abruptly and apatite FT dates are significantly older. Continuing to the north and structurally higher, a similar trend to that noted in the window is observed with ages younging to the north once more (Figure 4.1). The youngest apatite FT date from the GHS north of the window coincides with highest estimated rainfall amounts (Burbank and Bookhagen, 2006; Figure 4.1), which could!530.52.53.53.02.01.51.0MCTApatite Fission Track Ages South  LHS    GHS  North0km 10km 20km 30km 40kmAge (Ma)Elevation (m)Rainfall (mm/yr)400050000100020003000LHSGHSMCTTaplejung window2.52.32.92.22.41.32.42.31.4A BFigure 4.1: a) Map view of apatite fission track dates across the Taplejung window in the Kanchenjunga region of eastern Nepal, and b) cross section showing the same results plotted against topography and estimated rain fall amounts. Rainfall data are modified from Bookhagen and Burbank, 2006. Apatite fission track ages are from this study and from Sakai et al., 2013. Specimens from this studySpecimens from Sakai et al. (2013)54have resulted in local differences in erosion and exhumation rates and potentially contributed to a younger age (assuming that past rainfall patterns were similar to present day). Additionally, the cluster of relatively older ages in the LHS (Figure 4.1) from Sakai et al., 2013, may be partially affected by increased incision and related uplift near the confluence of two large rivers in the valley bottom (Figure 3.2). Such affects are considered to likely be minor, however, and even if significant, would not be expected to result in a sharp break in ages. A break, such as that observed across the MCT (Figure 4.1), would be consistent with minor reactivation/offset across the structure at ca. 2.5 Ma, the apatite FT age from the immediate hanging wall.   4.2 40Ar/39Ar 4.2.1 Old biotite ages Biotite grains from two specimens in this study, KA028 and KA031B yielded extraneously old ages. KA028 yielded an integrated age of 68.1 ± 0.3 Ma, which has been discarded as meaningless. It is much older than expected for a biotite grain at that structural level given that the collision between India and Eurasia initiated only ~ 50 Myr ago. Examination of biotite grains in thin section show that they are characterized by irregular grain boundaries, contain inclusions of quartz, previously formed biotite, and staurolite, and are themselves being replaced by muscovite, garnet, and chlorite. For a full description see Appendix I. Specimen KA031B also yielded a similarly old age of 69 ± 0.7 Ma. It too appears meaningless. Thin section analysis demonstrates that some biotite grains in this specimen appear to be locally breaking down to chlorite and contain inclusions of quartz which may affect the resulting age.  554.2.2 Biotite ages versus monazite rim crystallization At the map-area scale, biotite and muscovite 40Ar/39Ar age results can be separated into two distinct categories: 1) dates that are younger than nearby 232Th-208Pb monazite rim ages at similar structural levels, and 2) dates that are older than nearby 232Th-208Pb monazite rim ages at similar structural levels (Figure 3.2). The monazite rim ages are interpreted to reflect melt crystallization along retrograde pressure-temperature paths (Ambrose, 2014). Based on phase equilibria models (see Ambrose, 2014) the temperature at which final melt crystallized and monazite rim growth occurred is interpreted to be 765 ± 60 ˚C. In comparison, closure temperatures calculated for muscovite grains in this study using the equations of Dodson (1973) range from 448 ± 23 ˚C for small muscovite grains to 467 ± 13 ˚C for larger grains. Closure temperatures for biotite grains in this study calculated using the same method range from 343 ± 17 ˚C at the low end for small biotite grains to 403 ± 14 ˚C for larger grains. Initial cooling rates used for closure temperature calculations were estimated based on the dates of different mineral phases and their interpreted associated temperatures at similar structural levels. These data included monazite rim dates and temperatures derived from Ambrose (2014) paired with muscovite and/or biotite 40Ar/39Ar dates from this study and from Sakai et al. (2013) and predicted initial closure temperatures (350 ± 50˚C biotite; 450 ± 50˚C muscovite). The unknown variables in the calculation were then iterated based on previous results until they remained constant (Table 4.1).  If the mica ages are interpreted as cooling ages they would be expected to be younger than spatially equivalent monazite rim dates. Biotite in specimens KA047, KA055, KA057, and KA061B, however, all yielded dates that are either older or very 56Table&4.1:&Select&40Ar/39Ar&specimens&and&U;Th;Pb&monazite&specimens&at&similar&structural&levels&and&their&respective&closure&temperatures,&ages,&and&cooling&rates&&&Sample # Mineral Closure Temp (˚C) Error Age (Ma) Error ± 2σ Rate (˚C/Ma) Error KA004 Biotite 369 14 14.3 0.3 84 11 KA007 Monazite Rim 730 15 18.6 0.4 KA031 Muscovite 467 23 10.24 0.05 86 20 KA031 Monazite Rim 765 60 13.7 0.3 KA036 Biotite 399 14 9.5 0.1 57 10 KA037 Monazite Rim 765 60 15.9 0.4 KA039 Biotite 343 17 11.27 0.38 87 16 KA044 Monazite Rim 765 60 16.1 0.4 KA063 Biotite 375 14 15.35 0.37 93 16 KA064 Monazite Rim 760 20 19.5 0.6 &57similar to nearby monazite rim dates (specimens KA044, KA055, KA058, and KA064 respectively; Figure 2.1, 3.2). Because they do not fit within the expected age range, these specimens have been disregarded from interpretation (Figure 3.2).   4.2.3. Interpretation of 40Ar/39Ar results  Only one 40Ar/39Ar biotite date obtained in this study is located south of the Taplejung window. Not much information can be inferred from this, except that the date of 14.3 ± 0.3 Ma appears to be a meaningful cooling age as it is younger than U-Th-Pb monazite rim ages at similar structural levels.  There was also one muscovite specimen dated south of the Taplejung window, from a different area than the biotite-bearing specimen (Figure 3.2), and it yielded a date consistent with early Miocene cooling.  A single biotite date was extracted from within the LHS of the Taplejung window. As there is an absence of other chronometers at similar structural levels in the area, it is difficult to assess the accuracy of the 19.7 ± 0.7 Ma date. Dates from five muscovite specimens within the Taplejung window were reported by Sakai et al. (2013), all of which are Proterozoic in age ranging from 1672 to 1558 Ma (Figure 3.2). This may indicate that rocks at this structural level were not subject to temperatures exceeding the closure temperatures for muscovite (~350-450˚C) for any appreciable length of time during the development of the orogen, but did reach those high enough to affect biotite. In such an interpretation temperature would be the controlling factor in Ar diffusion at the grain scale. Structurally higher, across the MCT to the north, muscovite ages decrease drastically, consistent with the MCT forming a boundary between rocks that were 58significantly involved in Cenozoic heating and cooling and those that were not (e.g. Searle et al., 2008). Structurally above the MCT to the north, this study adds two new muscovite 40Ar/39Ar dates to the existing data from Sakai et al. (2013). Sakai et al. (2013) report two muscovite dates that decrease northward above the MCT from 13.8 ± 0.6 Ma to 11.0 ± 0.3 Ma (Figure 3.2). Specimens here yielded dates of 12.53 ± 0.31 Ma and 10.24 ± 0.05 Ma, also decreasing up structural section northward from MCT. When combined, the muscovite dates appear to decrease northward from 13.8 ± 0.6 Ma to 10.24 ± 0.05 Ma (Figure 3.2).  Farther north, and structurally higher, muscovite disappears while biotite increases in abundance. Biotite 40Ar/39Ar data from just north of the muscovite specimens yields an age of 9.5 ± 0.1 Ma (Figure 3.2). From there, biotite dates increase northward up structural section towards the northern margin of the studied transect (Figure 3.2).  4.3 Cooling Rates The existence of spatially proximal data from different chronometers with various associated closure temperatures allows estimation of cooling rates (Figure 4.2). They were calculated based on thermochronologic dates and respective predicted closure temperatures with all errors integrated. Monazite U-Th-Pb rim ages and estimated crystallization temperatures are from the companion study of Ambrose (2014). The monazite data provide the highest temperature constraints, followed by muscovite and biotite 40Ar/39Ar dates from this study (actual closure temperatures calculated; Table 4.2) and from the Sakai et al. (2013) study (closure temperatures estimated at 450 ± 50 ˚C for!59Distance (km)S NTaplejung WindowElevation (km)01562340           10           20           30           40           50           60           70           80           90           100       LHS GHS05.010.015.020.025.0Age (Ma)MCTMCT170016001500GHSdiscontinuitydiscontinuitydiscontinuity123 4Age (Ma)Age (Ma)Age (Ma) Age (Ma)Age (Ma)7HPSHUDWXUHÛ&7HPSHUDWXUHÛ&7HPSHUDWXUHÛ&7HPSHUDWXUHÛ&7HPSHUDWXUHÛ&This studyImayama et al., 2012Ambrose et al., 2014Sakai et al., 201340Ar/39Ar muscovite40Ar/39Ar biotitezircon fission trackmonazite rim U-Pbapatite fission trackK-Ar biotite0800600200400051015200600200400051015200200400051015AB200600600200400051015CABC DD080060020040005101520EEFigure 4.2: Five cooling rate profiles calculated across the LHS and GHS within the study area. Profiles A and B are located within the Taplejung window of the exposed LHS. Profile C shows cooling rates within the MCT. Profiles D and E show cooling rates within the GHS to the north of the MCT.60Table&4.2:&Biotite&and&Muscovite&specimens&selected&for&40Ar/39Ar&dating&and&their&respective&closure&temperatures.&Biotite&parameters&are&from&Grove&and&Harrison&(1996),&muscovite&parameters&are&from&Hames&and&Bowring&(1994).&&&Sample'#''Mineral''Rock'Type''Grain'Size'(mm)''Closure'Temperature'(˚C)''+/>'(˚C)''KA004 Biotite& Garnet&biotite&schistose&gneiss& 0.7& 369& 14&KA010 Muscovite& Kyanite&garnet&migmatitic&schist&gneiss& 1.2& 455& 23&KA013 Muscovite& Quartzite/Schistose&Phyllite& 1.0& 448& 23&KA014 Biotite& Garnet&biotite&phyllitic&schist& 0.6& 355& 14&KA014 Muscovite& Garnet&biotite&phyllitic&schist& 2.5& 484& 23&KA018B Biotite& biotite&phyllite& 0.3& 334& 16&KA024 Muscovite& Phyllitic&Schist& 2.5& 484& 23&KA025-1 Biotite& Orthogneiss& 0.5& 349& 15&KA025-2 Biotite& Orthogneiss& 2.5& 403& 14&KA028 Biotite& Kyanite&staurolite&garnet&biotite&gneiss& 0.7& 360& 14&KA028 Muscovite& Kyanite&staurolite&garnet&biotite&gneiss& 1.5& 464& 23&KA031B Biotite& Biotite&garnet&schist& 0.7& 360& 14&KA031B Muscovite& Biotite&garnet&schist& 1.2& 467& 23&KA036A Biotite& Quartzite& 2.0& 399& 14&KA036A Muscovite& Quartzite& 2.5& 484& 23&KA039 Biotite& Garnet&biotite&gneiss& 0.3& 347& 17&KA047 Biotite& Sillimanite&kyanite&biotite&gneiss& 0.7& 360& 14&KA051A Biotite& Kyanite&sillimanite&garnet&gneiss& 0.5& 349& 15&KA055 Biotite& Garnet&biotite&gneiss& 0.6& 355& 14&KA057 Biotite& Kyanite&Garnet&biotite&gneiss& 0.6& 355& 14&KA061B Biotite& Sillimanite&biotite&metapelitic&paragneiss& 1.5& 385& 14&KA063A Biotite& Garnet&cordierite&metapelitic&paragneiss& 0.8& 375& 14&KA066B Biotite& Quartzite/Metapelite/Leucogranite& 0.5& 349& 15&&61muscovite and 350 ± 50 ˚C biotite), biotite K-Ar dates from Imayama et al. (2012) (estimated closure temperature of 350 ± 50 ˚C), zircon FT dates from Sakai et al. (2013) (estimated closure temperature of 240 ± 50 ˚C, and finally, lowest temperature constraints are provided by apatite FT dates from both this study and from Sakai et al. (2013) (estimated closure temperature of 100 ± 20 ˚C). Predicted closure temperatures and ages of the various chronometers were compared at similar structural locations across the study area (Figure 4.2). While not every geochronometer could be used at each location, representative cooling rates were determined using as many geochronometers as possible.   Two cooling rate profiles were determined within the LHS (Figure 4.2A, B). In the middle of the Taplejung window (Figure 4.2A), two cooling rates were interpreted: 9.3 ± 5.1 ˚C/Ma from 16.7 – 5.4 Ma, increasing to 53 ± 16 ˚C/Ma from 4.6 to 2.5 Ma. Only one, generalized cooling rate was interpreted in the other profile within the window of 15 ± 3 ˚C/Ma from 19.7 to 1.3 Ma (Figure 4.2B). Two different cooling rate segments were determined for a profile near the MCT: 13 ± 8 ˚C/Ma from 13.8 to 5.0 Ma, increasing to 54 ± 21 ˚C/Ma from 5.0 to 2.4 Ma (Figure 4.2 C). Two cooling profiles were calculated from within the GHS to the north of the MCT (Figure 4.2 D, E). For the first, (Figure 4.2 D) three cooling rate segments were calculated, 86 ± 20 ˚C/Ma from 13.7 to 10.3 Ma, decreasing to 47 ± 11 ˚C/Ma from 10.3 to 5.4 Ma, and remaining consistent at 45 ± 17 ˚C/Ma from 5.4 to 2.3 Ma. Structurally higher, two more cooling rate segments were determined (Figure 4.2 E), 57 ± 10 ˚C/Ma from 15.9 to 9.5 Ma, decreasing to 37 ± 4 ˚C/Ma from 9.5 to 1.3 Ma.  The rocks within the LHS and the immediately adjacent MCT appear to record slow cooling rates of ~9 – 13 ˚C/Ma from the mid to late Miocene, which increases to 62~55 ˚C/Ma in the Pliocene to Pleistocene. Within the GHS to the north, cooling rates are fastest at structurally lower locations in the early to mid Miocene (Figure 4.2) with rates across the GHS appearing to stabilize between the mid to late Miocene and Pleistocene around 40-50 ˚C/Ma; similar to those in the LHS and MCT.   4.4 Structural Implications Figure 4.3 depicts a schematic model constrained by the distribution of ages and cooling rates observed across the study area. It illustrates the development of structures seen in Figure 1.4 B as data supports that model, as opposed to Figure 1.4 A. There is no interpreted cooling prior to the oldest 40Ar/39Ar dates at highest structural levels. Previous studies, however, have interpreted lateral extrusion of the GHS along the MCT at this time  (eg. Larson and Cottle, 2014; Beaumont et al., 2004, 2001; Figure 4.3 A). Initial exhumation of the GHS up the MCT ramp is interpreted to occur at ca.18 to 15 Ma (Figure 4.3 B) consistent with the oldest 40Ar/39Ar dates at highest structural levels. Once movement on the MCT ceased, deformation propagated toward the foreland and was taken up within the LHS, where duplexing began at ca. 15 to 13 Ma (Figure 4.3 C). This timing is consistent with the oldest muscovite 40Ar/39Ar at lowest structural levels within the GHS and north of the MCT. As duplexing by underplating continued and material was accreted from the footwall to the hanging wall of the MHT the overlying GHS was continuously uplifted and exposed (with paired erosion) as reflected in the biotite dates from across the middle portion of the GHS (Figure 4.3 D). Moreover, as a result of the duplex formation processes, the older underplated material was transferred up and towards the foreland, as the younger slices were accreted (Figure 4.3 E). This process is 63Elevation (km)02.55.07.510.0MCTElevation (km)02.55.07.510.0LHSMCTMHTMHT1Distance (km)Taplejung WindowElevation (km)02.55.07.510.00            10            20            30            40            50            60            70            80            90            100            LHS GHSGHSMCTMCTMCTMBTMHTMHTMCT1 2 3 4 5Elevation (km)02.55.07.510.0MCTDistance (km)Elevation (km)02.55.07.510.00            10            20            30            40            50            60            70            80            90            100            LHSMCTMHTMHT12234GHSGHS76Distance (km)0            10            20            30            40            50            60            70            80            90            100            Distance (km)0            10            20            30            40            50            60            70            80            90            100            Distance (km)0            10            20            30            40            50            60            70            80            90            100            ABCDE~18 - 15 Ma~15 - 13 Ma~10 MaFigure 4.3: schematic tectonic evolution of the Kanchenjunga region. See text for detail.64consistent with the decrease in both apatite and zircon FT dates observed within and just above the Taplejung window.  The cooling rates estimated across the study area also support this model. During the early to mid Miocene, fast cooling rates in the GHS are coupled with slow cooling rates in the LHS (Figure 4.2). This is consistent with exhumation of the GHS as it moved up a ramp at this time while there was no exhumation of the LHS as underplating had not yet begun. This is followed by a decrease in cooling rates in the GHS and an increase in rates in the LHS in the Late Miocene/Pliocene to Pleistocene (Figure 4.2). At this time, the cooling rates of the GHS and LHS are very similar, consistent with their exhumation driven by a single process, duplexing within the LHS (Figure 4.3 D).   4.5 Regional Implications When combined with previously published data, the results of this study comprise the most complete thermochronologic transects across the GHS in the region. Unfortunately, the gaps in data for nearby areas make it difficult to compare the interpretations made herein. More spatially complete data are needed to make meaningful regional interpretations and develop a thorough understanding of the exhumation and deformation history of the exhumed metamorphic core across the eastern Nepal Himalaya. The results of this study are, however, consistent with observations of similar recent studies in Bhutan and the Annapurna region of central Nepal (Figure 1.1). McQuarrie et al. (2014) recently conducted an investigation across the Bhutan Paro window, a structural window into the LHS, similar to the Taplejung window. Results 65from their work indicate a decrease in Zircon-He (ZHe), apatite FT, and retrograde monazite ages northward within the LHS exposed in the Paro window in (McQuarrie et al., 2014 and references therein). From south to north within the window, retrograde monazite dates decrease from 14.3 ± 0.7 Ma to 11.3 ± 1.6 Ma, ZHe dates decrease from 6.42 ± 0.2 Ma to 8.1 ± 0.3 Ma, and apatite FT dates decrease from 3.5 ± 0.6 Ma to 2.6 ± 0.6 Ma. The results of this study are interpreted to be consistent with duplex development in the LHS. The authors do not, however, identify whether the duplex was formed through foreland propagation or underplating processes. Our results are consistent with their duplex development and favour similar underplating processes occurring at the same structural level in both locations. There is no structural window in the Annapurna region of west central Nepal. In their study Martin et al. (2014) found that muscovite 40Ar/39Ar dates generally increase with increasing structural level within the GHS, from 7.2 ± 1.3 Ma just below the MCT to 16 ± 0.5 Ma in the mid-GHS. Such observations are consistent with the general trend in biotite dates observed in this study (Figure 3.2).   4.6 Old Biotite Grains As discussed above, some biotite grains yielded ages older than nearby monazite rim ages. Petrography does not provide an explanation for the older than expected ages (See Appendix I). However, the old ages may in fact be meaningful. There are three possible explanations for why some biotite grains yielded ages older than nearby monazite rim ages at similar structural levels. If the precision of the dates is over estimated, then the mica and monazite rim specimens may be similar in age, and this 66would indicate very rapid cooling and exhumation. The second possibility is that micas incorporated excess argon. Excess argon in 40Ar/39Ar dates has commonly been called on to explain older than expected biotite and muscovite dates in many studies across the Himalaya (eg. Martin et al., 2014; Sakai et al. 2013; Kali et al., 2010; Treloar et al., 2000; Maluski et al., 1998; Hubbard and Harrison, 1989; Krummenacher et al., 1978). As explained in Chapter 2, excess argon (40Ar) can be incorporated into biotite structures during deformation through diffusion, grain defects, and melt inclusions, etc. A third explanation for old dates suggests that they may not represent diffusion related cooling ages, but are recrystallization ages (Villa 1998; 2012). Villa (1998) argues that temperature is not the main factor controlling diffusion rates; fluid circulation, deformation, and associated re-crystallization are fundamental factors.   In this study, the “old” biotite grains that were older than expected and therefore excluded from interpretation, were located at similar structural levels (Figure 2.1) to rocks that had pseudosections created for them from a companion study (Ambrose, 2014); KA044 (near specimen KA047 from this study) and KA064A (near specimen KA057 from this study). The pseudosection for specimen KA044 predicts that the prograde P-T path crosses the biotite-out line while the retrograde or return path passes through increasing biotite isomodes from 0 – 6 % as melt crystallized and the solidus is reached (Figure 4.4 A). Monazite growth during final melt crystallization occurred at 16.1 ± 0.4 Ma at 765 ± 60 ˚C (Ambrose, 2014). The biotite age of specimen KA047 is interpreted to be 16.4 ± 0.2 Ma. The ages of the monazite and biotite in this example are identical (within error). Because biotite crystallization occurred during melt crystallization and the growth of associated monazite, it is possible that the “old” biotite 671 N654322015121087............Bulk(1)= NA(2.56)CA(.76)K(5.34)FE(5.61)MG(1.91)AL(18.86)SI(59.08)TI(0.68)H(5.13)F3(.06)O(?)700 750 800 850 900Temperature [C]4000600080001000012000Pressure [Bar]      Pl Liq Bt Sil I lm +Crd   Pl Liq Bt Ky I lm Pl Bt Sil I lm Pl Bt Ky I lm Pl Bt Ky I lm MsPl Bt KyI lm Ms Liq Pl Bt Ky I lm Ru MsPl Bt Ky I lm Ru Ms LiqPl Liq Sil I lm Pl Liq Sil I lm RuPl Liq Sil I lm Ru BtPl Liq I lm Ru BtPl Liq Ky I lm Ru Liq Ky I lm RuBt Liq Ky I lm RuLiq Ky I lm Ru? C(all !elds + Qz + Gr t + Kfs)K A064Rsolidusbiotite growthKA064 monazite melt crystallization age “0DDW“Û&KA057 biotite age              “0DDW“Û&1N65432Bulk(1)= NA(2.23)CA(1.73)K(2.97)FE(8.08)MG(2.72)AL(24.68)SI(55.30)TI(0.81)H(1.32)F3(0.08)O(?)700 750 800 850 900Temperature [C]6000800010000120001400016000Pressure [Bar]CPl I lm Liq Sil RuPl Bt I lm Liq Sil RuPl  Bt I lm Liq  Ky RuPl I lm Liq Ky RuPl  Liq Ky RuPl Liq Ms Ky RuPl I lm Liq SilPl  I lmLiq Sil BtPl I lm Sil BtPl  I lmSil Bt RuPl I lmKy Bt RuPl  I lm Ky Bt Ru M uPl I lm k y Bt Ru Liq MsPl  I lm Ky Ru MsPl Ky Bt Ru MsPl Ky Ru MsPl  I lm Ky Bt+CrdK A044(all !elds + Qz + Gr t + Kfs)Rsolidusbiotite growthKA044 monazite melt crystallization age “0DDW“Û&KA047 biotite age             “0DDW“Û&Figure 4.4: Pseudosections from companion study, Ambrose (2014) for specimens KA044 and KA064. The thick black line in each represents the solidus. The green arrows represent the pressure-temperature path followed. The blue lines represent biotite isomodes.AB68age represents a crystallization age, as opposed to, the more traditional, cooling age affected by excess argon.   The pseudosection for specimen KA064A predicts a similar P-T path to that described above for KA044, with biotite breakdown on the prograde portion and biotite crystallization on the retrograde portion (Figure 4.4 B). The monazite growth associated with final melt crystallization is at 19.5 ± 0.6 Ma and 760 ± 20 ˚C (Ambrose, 2014). Biotite in specimen KA057 yields a date of 25.0 ± 0.4 Ma. The biotite in this example is significantly older than the melt crystallization age for the monazite at a similar structural level. Again, however, the biotite date could represent a meaningful crystallization age for the grain as biotite was predicted to be crystallizing before final melt crystallization.   If the interpretation of Villa (1998) is correct, and temperature is not the rate limiter for diffusion, this would imply that the rocks were very dry with no fluid available to enhance diffusion rates, and that the biotite grains are little deformed. Both of these are consistent with thin section observations and phase equilibria models (Ambrose, 2014). While the possibility of these old ages being meaningful exists, further work is required to determine if the dates acquired in this study are actually due to crystallization or from excess argon.  !69!Chapter 5: Conclusions and Future Work 5.1 Conclusions Apatite FT dates from this study and Sakai et al. (2013) outline a younging trend towards the northwest, which is consistent with duplexing by underplating affecting the lesser Himalaya. The recent duplexing within the LHS has uplifted the overlying GHS and rotated structures therein. The 40Ar/39Ar dates from this study and from Sakai et al. (2013) within the Taplejung window indicate that metamorphic temperatures in the window were not sufficiently high for long enough to reset muscovite Ar systematics, but may have affected biotite dates therein. The 40Ar/39Ar dates from the GHS outline a cooling history interpreted to reflect exhumation related to both movement up a thrust ramp and uplift driven by duplexing of the LHS below. Similarly, the biotite 40Ar/39Ar data suggest two populations with one recording older and higher temperature crystallization ages, and the other recording younger more traditional cooling or deformation ages.  The results from this study within the Taplejung window and the exposed LHS, interpreted as being a result of duplexing by underplating, are consistent with those seen in the Paro window in Bhutan (McQuarrie et al., 2014). Moreover, 40Ar/39Ar dates from the GHS in the present study area are consistent with results from the Annapurna region of central Nepal (Martin et al., 2014). The similarity between the study area and other regions indicates that the active processes are likely regional in scale, however, gaps in data remain, making further correlations tenuous. More apatite FT and 40Ar/39Ar dating in the surrounding areas would help develop a more complete picture of processes that occurred within the exhumed metamorphic core of the eastern Nepal Himalaya.  705.2 Future work 5.2.1 Old biotite  Further work is required to determine if old biotite dates are a result of excess argon, or represent meaningful (re)crystallization ages. If Ar is diffusing out of the system it would be expected that Ar profiles across grains have a ‘bell’ shaped distribution. Moreover, Ar distribution within a specimen could be used to determine if mineral grains are zoned. Some parts of a grain may yield older or younger ages compared to the other areas of the grain, and may have been affected by local fluid alteration, deformation, or some other spatially restricted process. Moreover, patchily zoned specimens may be indicative of partial melting and recrystallization (eg. Villa, 2012).  Additionally, it would be useful to compare 40Ar/39Ar results from this study with results from Rb-Sr dating of specimens at similar structural levels. These results would be helpful in determining if the dates acquired through 40Ar/39Ar are in fact the result of argon inheritance and recrystallization processes. If a specimen dated through 40Ar/39Ar methods yielded an age that was older than expected, and a specimen at a similar structural level dated through Rb-Sr methods also yields an age that is older than expected, it would suggest that both Ar and Sr isotopes were relatively immobile and that the results are in fact (re)crystallization ages (Villa, 2012).   5.2.2 Further data In addition to work completed during this study in the Kanchenjunga region of far-east Nepal, it would be useful to obtain more data, both through successful apatite FT 71dating and through 40Ar/39Ar methods. More apatite FT dates from south of the Taplejung window, in the structurally low GHS, to within the LHS, and across the MCT into the GHS above the Taplejung window would also allow for a more complete interpretation of what structures and what processes occurred within the LHS were. It would allow us to better define the mechanism of duplexing, whether by in sequence thrusting or by underplating.  More 40Ar/39Ar dates to fill in the gaps in this study would be useful to identify structures within the GHS above the Taplejung window, and perhaps give a wider understanding of the differences between the young and old biotite ages. An extension of the study to the north towards the STDS would be useful to help identify potential discontinuities within the GHS, and better understand the uplift and exhumation processes that occurred there.    !72References  Aldrich, L.T., & Nier, A.O., 1948. Argon 40 in potassium minerals. Physical Review, 71, 876-877. Ambrose, T.K., 2014. Ductile extrusion, underplating, and out-of-sequence thrusting within the Himalayan metamorphic core, Kanchenjunga, Nepal. Masters Thesis, UBC Okanagan.  Avouac, J.P., 2003. Mountain building, erosion and the seismic cycle in the Nepal Himalaya. Advances in Geophysics, 46, 1-80 Beaumont, C., Jamieson, R.A., Nguyen, M.H., & Lee, B., 2001. 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Journal of Geophysical Research: Solid Earth, 110, 1-14. doi:10.1029/2004JB003139. 85mmmmQ uar tzQ uar tzQ uar tzBiotiteBiotiteBiotite**Biotite**mmBiotite ** grains are brown in plain and polarized light, and do not change colour with rotation. Possibly thicker grains or 2 phases of biotite growth?KA004Specimen KA004 is a garnet-biotite-schistose-gneiss. Biotite from this specimen likely occurs in only one population. Biotite grains are medium sized. Some grains are brown in plain and polarized light, and some do not change colour with rotation. These may possi-bly be thicker grains or 2 phases of biotite growth, however it is not common throughout the thin section. Garnet is seen with intergrowths of both quartz and biotite and is replac-ing biotite. In some cases, there is minor biotite growing on garnet rims, some quite dirty looking. Most grains look relatively well behaved, uniform in appearance and with clear, distinct grain boundaries. Grains appear to be striated. Of note, some (minor) grains appear to have light and dark laths intergrown, though it seems to be an anomaly. KA004 has an Integrated age of 14.3 ± 0.3 Ma.AppendicesAppendix A: Grain descriptions and thin section photos and interpretations for   specimens selected for 40Ar/39Ar dating86Q uar tzQ uar tzZirconKyaniteM uscoviteM uscoviteKA010mmmm mmQ uar tzM uscoviteQ uar tzQ uar tzmmmmmmQ uar tzQ uar tzKyaniteM uscoviteM uscoviteKyaniteKyaniteM uscoviteSpecimen KA010 is a kyanite-garnet-migmatitic-schistose-gneiss. Muscovite from this specimen occurs in 2 distinct populations. The first population is sparsely located through the slide with no obvious orientation with foliation. It appears well behaved, although is not commonly found. Grains from this population are uniform in appearance, with clear, distinct grain boundaries. The second population of muscovite occurs in fractures, and is roughly oriented with foliation. Much of this muscovite contains kyanite intergrowths with some zircons, sericite, and alterations evident. It generally appears to be of poor quality, with irregular grain boundaries. It is coarser grained, and is likely what was picked. It does not appear to be well behaved, as it is distinctly altered, sericite-rich, and breaking down. Garnet is breaking down to sericite. No date was acquired for this speci-men.87mm mmmmQ uar tzQ uar tzQ uar tzM uscoviteM uscoviteK A010mmmm mmM uscoviteQ uar tzBiotiteQ uar tzQ uar tzQ uar tzS er icitemmmm mmM uscoviteQ uar tzS er iciteQ uar tzQ uar tzQ uar tzS er iciteFeldspar?Feldspar?KA01088Specimen KA013 is a quartzite/schistose phyllite. Muscovite from this specimen occurs in only one obvious population. Mineral grains are strongly aligned with foliation. There are potentially coarser-grained muscovite-rich bands. Fine to medium grained muscovite is common throughout. The grains have clear, distinct boundaries, and no apparent altera-tions.Slide is quite dirty, and contains bubbles that appear as inclusions. No date was acquired for this specimenKA013200 umbiotitequar tzquar tzquar tzbiotite200um200um89mmmmQ uar tzBiotiteBiotite?Edge of slidemmmm mmmmQ uar tzQ uar tzM uscoviteM uscoviteBiotiteG ar netSpecimen KA014 is a garnet-biotite-phyllitic schist. Biotite and muscovite were both analyzied in this specimen. There appears to be only one population of biotite. It is fibrous, and typically intergrown with muscovite. It is strongly aligned with foliation, and occurs in distinct bands. It is seen on the rims of muscovite, and therefore may potentially be younger. The biotite has low birefringence, and does not appear typical. There also appears to be only one population of muscovite. It is strongly aligned with foliation, and also occurs in fibrous bands. The muscovite contains many inclusions. Only a biotite age was aquired for this specimen, an integrated age of 18.3 ± 0.5 Ma.KA01490mmmmmmQ uar tzBiotiteStretched quar tz  in ser iciteStretched quar tz  in ser iciteKA018Bmmmm mmQ uar tz  veinStretched quar tz  in ser iciteStretched quar tz  in ser icite(folded and wav y fabr ic)Specimen KA018 is a biotite phyllite. There are potentially three different populations of biotite in this specimen. The first population is matrix biotite that is strongly aligned with foliation. It is primarily blade-like, medium grained, and typically has clear, distinct grain boundaries, although not always. This biotite contains minor inclusions, primarily quartz. The second population of biotite is replacing or being replaced by quartz, and forms elongated grains, but less blade-like. These appear to be tranistioning/alterations of half biotite-half quartz. The third potential population of biotite grains are not elongated, and occur at an angle to the foliation. All biotite appears greenish in appearance. No date was acquired for this specimen.91KA025Specimen KA025 is an orthogneiss. There are two distinct populations of biotite, and possibly three. The first population of grains are parallel to and strongly aligned with foliation. They occur in well defined biotite-rich bands. Grains are medium sized and well behaved. The second population of grains are perpendicular to foliation, occur in bands strongly aligned with foliation, though individual grains are striated and weakly aligned with foliation. Grains are coarse grained and well behaved. Some have inclusions of biotite, quartz, and sericite. Some coarser grained biotite in this population is wavy. The third population of biotite is matrix biotite, which is fine grained with no distinct orientation with foliation. It is typically well behaved with clear grain boundaries, minor chlorite alterations. Muscovite is also present in this sample, growing with biotite and quartz in a muscovite-rich band. Sericite is also present. KA025 yielded an integrated biotite age of 19.7 ± 0.7 Ma. biotite quar tzquar tzbiotite2 mm2 mm2 mm92KA028mmmm mmM uscoviteM uscoviteS omething     Funk yQ uar tzS er iciteQ uar tzQ uar tzSpecimen KA028 has an integrated biotite 40Ar/39Ar age of 56.8 ± 0.2 Ma, which does not fit with what is expected, and is likely a meaningless age. The specimen comes from a kyanite-staurolite-garnet-biotite gneiss. There are two distinct populations of biotite identified in thin section within the specimen. The first population appears to be well-behaved biotite, finer grained, associated with the matrix, and in line with the foliation fabric of the rock. The majority of these grains have clear grain boundaries with few to no inclusions, and are indicative of a retrograde metamorphic path. The second popula-tion consists of larger grained biotites, and therefore was more likely to have been picked for 40Ar/39Ar dating. They do not occur in concordance with the foliation fabric of the rock, have very irregular grain boundaries, contain inclusions of quartz, biotite, and staurolite, and are being replaced by muscovite, garnet, and chlorite. These grains appear to be older than the first population of biotite, and significantly less well behaved. Addi-tionally, there may potentially be a third older population of biotite that is included in garnets at a different fabric angle than the primary foliation of the rock. This garnet is further being replaced by staurolite. These grains may be indicative of a prograde meta-morphic path. In conclusion, there are two main problems with the biotites being dated in specimen KA028, 1) multiple episodes of biotite growth, and 2) chlorite overgrowing biotite in some grains leading to excess argon. For these reasons, the biotite age associ-ated with this specimen has been discarded as meaningless. This specimen also yielded a muscovite plateau age of 12.53 ± 0.31 Ma. 93mmmm mmQ uar tzM uscoviteBiotite?M uscoviteM uscoviteQ uar tzStrange    fabr ic in     M uscovitemmmm mmM uscoviteBiotiteBiotiteM uscoviteM uscovite Q uar tzS er iciteKA02894mm mmmmM uscoviteQ uar tzS er iciteQ uar tzS er iciteM uscoviteM uscoviteS er icitizingM uscoviteS er icitemmmmmmS er iciteQ uar tzM uscoviteQ uar tzBiotiteFibrolite?Sillimanite???M uscoviteKA031BSpecimen KA031B is a garnet-biotite schist, and contains both muscovite and biotite. There appears to be only one population of biotite that is well aligned with foliation. The biotite shows chlorite alterations on rims of some grains, although most appear well behaved, uniform in appearance, with clearly defined grain boundaries. Biotite is medium grained, and is sometimes intergrown with muscovite. It is possibly growing on rims of muscovite, though is only minor, not common. Muscovite also only occurs in one popula-tion, strongly aligned with foliation. Muscovite is coarse grained. Most appear well behaved, with clear grain boundaries, uniform in appearance. Some grains appear to be breaking down, and have inclusions of quartz. Some grains are intergrown with biotie. The biotite from this specimen yielded an anomalously old integrated age of 69 ± 0.7 Ma, which has been disregarded as a meaningless age for similar reasons as for specimen KA028. This specimen also yielded an integrated muscovite age of 10.24 ± 0.05 Ma.  95mmmmmmQ uar tzFeldsparM uscovite/    S er iciteFeldsparQ uar tzM uscovite/    S er iciteKA036ASpecimen KA036A is a quartzite and contains both biotite and muscovite. Biotite possi-bly occurs in two populations. The first population of biotite is medium-fine grained, needle-like and strongly aligned with foliation. The second population of biotite is coarser grained, less blade-like, and has a weaker alignment with foliation. There is not an obvi-ous difference between the populations. Biotites are typically well behaved, and have clearly defined grain boundaries. Few grains have minor alterations on rims, possibly chlorite. Biotite is replacing muscovite in places. This sample contains less biotite than other samples at similar structural levels – biotite is sparse throughout the slide. Musco-vite also occurs in two populations, though there is a greater distinction between the two. The first population of muscovite is associated with feldspar, and has no alignment with foliation. It is poorly behaved, with irregular grain boundaries replacing feldspar, and is often associated with sericite. The second population of muscovite is less common, and has a weak alignment with foliation. It is fine-medium grained, and is well behaved with clearly defined grain boundaries. Muscovite is generally sparse throughout the slide, and of poor quality. KA036A yielded an integrated biotite age of 9.5 ± 0.1 Ma.96mmmmmmS er icitereplacingfeldsparQ uar tzFeldsparQ uar tzQ uar tzFeldsparS er icitereplacingfeldsparKA036Ammmm mmQ uar tzFeldsparS er iciteQ uar tzQ uar tzFeldsparBiotitemmmmmmS er iciteQ uar tzFeldsparQ uar tz97KA039Specimen KA039 is a garnet biotite gneiss. Biotite from this specimen apears in two distinct populations. The first, most likely picked, is younger, finer grained matrix biotite that occurs in bands and is oriented along foliation. It typically appears well behaved, athough some grains have irregular boundaries. The second population of biotite is older, coarser grained, and associated with garnets, not with the rock fabric. An old large garnet is breaking down and being replaced by biotite and quartz. This biotite is being replaced by muscovite in many places, with possible chlorite alterations. KA039 has a plateau age of 11.27 ± 0.038 Ma and a preliminary integrated age of 9.81 ± 0.15.200 um200 um200 umbiotitequar tzfeldsparquar tzfeldsparbiotite200 um200 um200 umbiotitequar tzfeldsparbiotite biotite98mmmm mmQ uar tzM uscoviteFeldsparFeldspar/Q uar tz/  Sillimanite??Fibrous messQ uar tzmmmm mmQ uar tzQ uar tzM uscovite intergrowthsKA047Specimen KA047 is a sillimanite kyanite biotite gneiss. Biotite from this specimen appears to occur in only one population, and is well aligned with a wavy foliation in bands that are rich in biotite and sillimanite. Biotite grains are very red in colour and are sometimes intergrown with ilmenite. Many grains are intergrown with sillimanite, and these have irregular grain boundaries, contain many inclusions and are poorly behaved. Grains that are located away from sillimanite appear to be better behaved, with clear grain boundaries and few to no inclusions, although the majority is of poor quality. Biotite is replacing garnet. KA047 has a preliminary integrated age of 16.45 ± 0.16 Ma, and an integrated age of 16.4 ± 0.2 Ma. 99mm mmmmBiotitePlagioclaseQ uar tzPlagioclase/Sillimanite/Fibrolite??BiotiteBiotitemmmmQ uar tzQ uar tzFeldspar BiotiteM yr mek iteQ uar tz and FeldsparBiotiteBiotitemmKA047100mm mmmmQ uar tz Q uar tzQ uar tzBiotiteFibrolite?BiotiteSpecimen KA051A is a kyanite sillimanite garnet biotite gneiss. Biotite from this speci-men occurs in one population, and is found in well developed foliation. This specimen contains more biotite than other structurally high specimens. Biotite is medium-grained, and sillimanite replacing biotite is common. Some biotite grains are well behaved with no inclusions and distinct grain boundaries, although most have irregular grain boundar-ies, and are breaking down to sillimanite. Biotite is replacing garnet, and sillimanite is replacing biotite. KA051A has a preliminary plateau age of 12.4 ± 0.2 Ma and a prelimi-nary integrated age of 12.0 ± 0.2 Ma.KA051A101mmmmmmQ uar tzBiotiteQ uar tzSpecimen KA055 is a garnet biotite gneiss. Biotite from this specimen may possibly occur in two biotite populations. The first population is matrix biotite, which is finer grained, while the second population is the biotite that is replacing garnet, and is coarser grained. This specimen contains less biotite than other structurally high specimens. The biotite is typically well behaved, with clearly defined grain boundaries. Some older biotite located near garnet is not as well behaved. KA055 has a plateau age of 19.7 ± 0.36 Ma, and a preliminary integrated age of 19.72 ± 0.15 Ma.KA055102KA057Specimen KA057 is a kyanite garnet biotite gneiss. Biotite from this specimen may possibly occur in two populations. The first population is finer grained matrix biotite, with weak to no alignment with the foliation fabric, appearing generally well behaved. The second population occurs in a single, slightly curved band of medium grained biotite, quartz and feldspar. Grains appear well behaved with distinct grain boundaries and no inclusions. Fabric is not well developed. This specimen contains less biotite than other structurally high specimens. Biotite is replacing garnet. There is minor late-stage musco-vite of poor quality. Minor chlorite is intergrown with and replacing biotite, and may be a potential issue. This may be an anomaly however, and is not associated with most grains. KA057 has a preliminary plateau age of 23.97 ± 0.14 Ma, and integrated ages of 25.6 ± 0.4 Ma and 25.0 ± 0.4 Ma. 500 umquar tzbiotiteser icitequar tzbiotite500 um500 um103KA061B500 um500 um500 um500 um500 umbiotitequar tzbiotitequar tz500 umbiotitequar tzquar tzchlor itealterationschlor itealterationsSpecimen KA061B is a sillimanite biotite metapelitic paragneiss. Biotite from this speci-men occurs in two populations. The first population has no distinct visible lineation or mineral alignment. Biotite has a feathery fabric, with some growth in fractures and biotite inclusions in feldspar. It is also associated with muscovite, sericite, and sillimanite. The biotite has irregular grain boundaries and is poorly behaved. It also displays high birefrin-gence. The second population of biotite consists of coarser grained clots. These appear better behaved than the first population, although many irregular grain boundaries are still apparent. There are some muscovite intergrowths and minor chlorite alterations, and sericite. Many grains have high birefringence. In a large clot in the corner of the slide, grain boundaries are better defined, and biotite appears to be better behaved than other populations. There are some isotropic remnants that remain black in plain and polarized light. Some grains here appear to be more typical, brown in polarized light. There are minor apatite/zircon inclusions, bubbles too. Some muscovite growth on rims of biotites. KA061B has a preliminary plateau age of 19.07 ± 0.93 Ma.  104KA063ASpecimen KA063A is a garnet cordierite metapelitic paragneiss. Biotite from this speci-men may possibly occur in two populations. The first population is matrix biotite that is parallel to well developed foliation, and is most common. The second population of biotite is cross-cutting foliation, with undulose extinction, and is only minor in the speci-men. The specimen contains more biotite than other structurally high specimens. Biotite is mostly medium grained, with minor sericitization. Most grains are of good quality, not many abnormalities, and with few inclusions. Biotite is greenish in colour. Biotite is replacing garnet. There is almost complete dissolution of garnet. KA063A has a prelimi-nary plateau age of 15.35 ± 0.37 Ma, and a preliminary integrated age of 13.0 ± 0.2 Ma.500 um500 um500 umbiotitequar tzchlor itebiotitequar tz105mmmm mmQ uar tzM uscoviteQ uar tzFeldsparFeldsparmmmm mmQ uar tzBiotiteEdge of   SlideKA066BSpecimen KA066B is a quartzite. Biotite grains from this specimen are weakly aligned, with little to no obvious alignment with foliation. There is only one population of biotite. This specimen contains less biotite than other structurally high specimens. Biotite is fine grained, and is sometimes alone or late stage replacing other grains. There is very minor sericitization and chloritization. Most grains are elongated with generally well-defined grain boundaries. KA066B yielded a plateau biotite age of 18.4 ± 2.4 Ma.106KA010-Poor quality outcrop-Folded and deformed-Highly quartz-rich, interbedded with mud layers-Quartz is often lensoidal, possible large c' fabrics cutting foliation, shearing top to the southeast-Outcrop also includes more micaceous, finer layers with minor melt towards the north -Boudins (quartz and feldspar melt leucozones), direction is not helpful, as lineations are into the rock-Few poor s lineation fabrics present-Small semi ductile fault about 10m north of waypoint. -Migmatitic-Few garnets. -Foliation 091/37, 106/22, 068/32, 025/33 -Lineation: 18-->144, 22-->117-C' fabric(?) 055/58-Fault oriented at 325/60 to the NE EESEESEfaultfoliationboudinsESEfoliationa bc -Photos showing      (a) large lenses and foliation, viewed to the east     (b) fault, viewed to the east south east     (c) foliations, close-up of boudins, viewed to the east south east. Appendix B: Field photos for specimens selected for 40Ar/39Ar dating107KA013-Thin, poorly exposed outcrop -Phyllite schist intterbedded with mud -Foliation 211/28, 207/29, and 184/12-Lineation 10->240-Outcrop includes large quartz melt pods, oxidized-Quartz-rich, less mud interbedded between layers -Shear sense top to the west-Small, isoclinal folds repeated throughout outcrop are not local-S fabrics are visible in sigmoidal clasts-Photo shows a small non isoclinal fold, viewed to the NNW-About 40m to the ENE, secondary foliation, more finely spread, and steeply dipping, forming an intersection lineation-Quartz rich and oxidized-Primary foliation is flat, 000/00-Lineation of 3->317, 3->137 -Secondary foliation/cleavage is at 325/42, and is steeply dipping Photo showing secondary and primary foliation, viewed to the North -About 100m to the ENE, there is a small rock outcrop on the trail with foliation 209/20-Lineation 12->348 (crenulation cleavage) -Same rock type, quartz rich with thin metapelitic layers-Minor chlorite, garnet/staurolite.NNWprimary foliationsecondary foliation108KA014W-Micaceous phyllitic schist/schistose phyllite-Highly muscovite-rich, with garnets throughout-Foliation is variable, not always forming distinct planes-Much is polished-Foliaiton is 210/11, 220/23, 240/16, and 245/29-Lineations 22->033 for tourmaline, 6->045 for mica shape, and 0->189-Few s and c' fabrics-Few oriented tourmaline crystals defining lineation shear sense top to the southwest-S fabric 265/25, c' fabric 242/26-Photo showing s and c' fabrics, viewed to the westS fabricC’ fabric109KA018W-Outcrop may be in place - not clear-Low grade, sheared-Phyllite, quite broken-Potentially some quartzite-Quartz blebs and boudins present-Abundant c' and s fabrics indicate top to the south shear sense-Contains quartz, chlorite, muscovite, biotite?, is very muscovite and quartz rich-Foliation 270/33, 249/26-Lineation 23->340 of oriented mineral grains, muscovite, biotite, and chlorite-Layers distinct, some quartz sand rich, some muscovite-rich-Fold axial plane 270/40, possibly slumped-Larger fold - Crenulation fabric: 5->281-Fold hinge (lineation) 20->303-Minor folds same event as crenulation-Larger fold possibly different event (passive fold)-Photos      (a) showing foliation, c' and s fabrics, viewed to the west     (b) showing a larger fold, viewed to the westWfoliationS fabricC’ fabrica bfoldfoliation110-Large outcrop cut by river-Foliation highly variable-Garnets present - possibly entering higher grade rocks-Foliation not as distinct as previous outcrops due to weathering(?)-Phyllite-schist, still contains quartz veins, blebs and lenses-Possibly minor muscovite-Possible melt lenses? -Minor chlorite-Foliation 195/39, 234/30, 260/36, 220/36-Photo showing variable foliation viewed looking down, top of photo is SSEKA024SSENNWvariablefoliation111KA025E-Just past/above town of Mitlung-Foliation 303/54, 315/45, 275/47-Foliation is much less variable than previous outcrops-Orthogneiss, contains biotite, quartz, feldspar-Almost L tectonite, highly lineated-Some augens, possibly contains zircon and monazite-Distinct lineations of biotite, quartz, and feldspar, 49->008-Shear sense top to the south. S and c' fabrics-Potenitally different ages of biotite - large and small grains-Photos showing     (a) augens, c' and s fabrics, viewed to the east     (b) continuing along trail about 100-200m, same rocks, strung out xenoliths, isoclinal fold(?) with fold axis parallel to foliation - hinterland origin, mid-crustal deformation, viewed to the east.ES’ fabricC’ fabric112KA028N-Large outcrop by bridge crossing a small tributary river, before town of Chirwa-Schist, with finer grained quartz, biotite, and muscovite minerals aligned-Small pink garnets throughout-Few small quartz lenses-Metapelitic-C' and s planes, step up features suggest shear sense top down to the northwest - differ-ent than previous -Photo showing c' and s fabrics, viewed to the northS’ fabricC’ fabric113KA031S-Outcrop up a small creekbed-Contains quartz, muscovite, biotite, feldspar, and garnets -Foliation is 259/43, 265/47, and 270/61 -Joint fabric is 358/72-Biotite minerals are weakly aligned, possible tourmaline-Schist-Possibly kyanite in country rock, sillimanite in melt-Significant melt close to boundary of kyanite/sillimanite --> latent heat of crystallization: as melt crystalizes, it gives off heat into surrounding country rock-Mica shape lineation 49->353Photos     (a) showing close up of melt/sillimanite viewed to the south     (b) showing overall outcrop viewed to the south     (c) showing kyanite blade, viewed to the southabcSSkyanitemelt country rock114KA036NW-Outcrop, possibly slightly slumped, but relatively in place - same stuff visible on oppo-site side of the river-Quartz-rich, notably contains muscovite-Much is quartzite, also contains melt-St foliation 336/44-Mineral lineations of muscovite, micas: 44->061, almost down dip-Some granite sections with quartz, biotite, feldspar, muscovite -No visible garnets-Crosscuts quartzite-Photo showing melt cross cutting quartzite viewed to the NW115KA039?-St foliation 355/31, 338/39-Quartz, biotite, garnets common, feldspar -Foliation fairly consistent-Gneiss-Lineation is mineral alignment and elongation, 35->076-Metasedimentary package - gneissic layers and quartzite layers-Photo showing gneiss/quartzite banding (light and dark), viewed to the ?? 116KA055NNW-Just crossed some very large scree slopes down by the river -Outcrop is quartz and biotite-rich, garnets common, light pink, 1-3mm-Some melt-quartzo-feldspathic-Foliation highly variable, 210/16-Photo showing weird melt, viewed to the NNWmelt117-Large outcrop with cross cutting dykes in various directions-St foliation 244/43, highly variable -Crenulation 360/30, foliation 225/41-Base of outcrop - foliation is highly deformed-Possibly massive feldspar, with altered biotite patches - chlorite(?), green in colour, some still biotite-Melt-Also includes biotite rich metapelitic paragneiss-Increased amount of tourma-line in melt towards Yanma as well as finer grained granite(?) containing quartz, biotite and feldspar. Note lack of garnet, sillimanite(?)-lack of in situ melt - more dykes and intrusions - either rock composition doesn't allow melt (quartzite) or rocks not deep/hot enough to melt? KA061a bde f-Photos showing      -(a) an overview of the outcrop, viewed to the NNW     -(b) feldspar melt(?) viewed to the north     -(c) alterted biotite and feldspar(?) viewed to the north     -(d) a fold, viewed to the north (fold axis 38->320, upper limb: 029/41, lower limb: 337/64)     -(e) deformed layers, viewed to the NW     -(f) deformed layers, viewed to the NWNNW NNNNW NWmeltcountry rockcfold axis:    38 -> 320029/41337/64118KA063badcPhotos showing      -(a) local melt and folding, viewed to the NW     -(b) overall outcrop to the WNW     -(c) overall outcrop to the W     -(d) overall outcrop to the SWNWSWWWNW-St foliation 011/21, 349/15-Paragneiss, metapelitic - highly quartz and biotite rich-Foliation is variable-Minor quartz melt in gneiss-Sillimanite-Melt is variable, locally up to about 20%(?)-Melt is almost entirely sillimanite in places, with minor quartz, biotite and a green mineral-Gneiss layer in between quartzite layers potentially about 20-50m thick(?)119Appendix C: Photos of mica grains for 40Ar/39Ar dating     KA004 Biotite    KA010 Muscovite 120  K013 Muscovite    KA014 Muscovite 121  KA014 Biotite    KA018B Biotite 122  KA024 Muscovite    KA025 Biotite 123  KA025 Biotite    KA028 Muscovite 124  KA028 Biotite    KA031 Muscovite 125  KA031 Biotite    KA036A Muscovite 126  KA036A Biotite    KA039A Biotite 127  KA047A Biotite    KA051A Biotite 128  KA055 Biotite    KA057 Biotite 129  KA061B Biotite    KA063A Biotite 130  KA066B Biotite  131

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