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Cooling and exhumation in the footwall of the Dangardzong fault, Thakkhola graben, west central Nepal Brubacher, Alex 2018

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Cooling and exhumation in thefootwall of the Dangardzong fault,Thakkhola graben, west central NepalbyAlex BrubacherB.Sc. Hons., Memorial University of Newfoundland, 2016A THESIS SUBMITTED IN PARTIAL FULFILLMENT OFTHE REQUIREMENTS FOR THE DEGREE OFMASTER OF SCIENCEinThe College of Graduate Studies(Earth and Environmental Sciences)THE UNIVERSITY OF BRITISH COLUMBIA(Okanagan)April 2018© Alex Brubacher, 2018The following individuals certify that they have read, and recommend to the College ofGraduate Studies for acceptance, a thesis/dissertation entitled:Cooling and exhumation in the footwall of the Dangardzong fault,Thakkhola graben, west central Nepalsubmitted by Alex Brubacher in partial fulfilment of the requirements of the degree ofMaster of ScienceDr. Kyle Larson, I. K. Barber School of Arts & SciencesSupervisorDr. John Greenough, I.K. Barber School of Arts & SciencesSupervisory Committee MemberDr. Yuan Chen, I.K. Barber School of Arts & SciencesSupervisory Committee MemberDr. Lori Kennedy, Faculty of Science, UBC VancouverUniversity ExamineriiAbstractLate-stage E-W orogen-parallel extension is well documented in the Tibetan plateau, how-ever, the nature and timing of extension related features remains unclear. In order to investigatethe temporal development of the graben that accommodate this extension, 40Ar/39Ar and U-Th/He thermochronology were used to characterise the evolution of the Thakkhola grabenin west-central Nepal, a major N-S striking, E-W extensional structure that cuts across theHigh Himalaya. The Mugu and Mustang granites occur in the footwall of the Dangardzongfault, which defines the western boundary of the graben. Monazite U-Th/Pb geochronology isused to constrain the minimum crystallization age of these granites to ⇠22-20 Ma. 40Ar/39Argeochronology of micas date the cooling of the granites through calculated Ar migration closuretemperatures (580-344 °C) to ⇠17 Ma. Zircon and apatite U-Th/He dating yield ages of ⇠15-9Ma and ⇠12-4 Ma, respectively, corresponding to cooling of the granites through respective Hemigration closure temperatures of ⇠220-140 °C and ⇠116-44 °C. Microstructures in rocks col-lected in this region show that E-W extensional structures were contemporaneous with coolingthrough muscovite closure to Ar migration at ⇠17 Ma, providing a minimum age of initiationof extension. Detailed low-temperature cooling paths, based on U-Th/He data extracted fromthe undeformed intrusive bodies, record inflections in cooling rates at ⇠13-8 Ma and at ⇠5 Ma.These two inflections mark deceleration of cooling in the footwall, which is interpreted to re-flect a reduction in the spreading rate of the graben. The slowdowns are coeval with increasedgraben development across the Tibetan plateau and as such are interpreted to reflect strainbeing partitioned into these newly generated structures. This study adds to a growing bodyof evidence for a major kinematic shift in the Himalaya occurring around ⇠13 Ma potentiallylinked with the flow of material eastward out of the Tibetan plateau.iiiLay SummaryThe Himalayan mountain chain is developing because of N-S convergence between India andAsia. Interestingly, the Tibetan plateau is characterized by graben that accomodate active E-W extension. The timing of mountain chain-parallel extension in an actively converging regionis a key aspect of understanding the development of such features. The Thakkhola graben inNepal, which is the result of E-W extension, is determined to have been active by ca. 17 Ma,based on microstructural analysis and radiometric dating carried out in this study. Furthermore,multiple di↵erent radiometric ages from the same rock in the Thakkhola graben define a majorreduction in the rate of cooling at around 8-13 million years ago. This change in cooling rate iscontemporaneous with the initiation of movement on several other E-W extensional structuresin southern Tibet and a major shift in how the Himalayan system is evolving.ivTable of ContentsAbstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iiiTable of Contents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . vList of Tables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . viiList of Figures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . viiiAcknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xChapter 1: Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.1 Overview of the Himalaya . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.2 Geological Setting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51.2.1 Thakkhola graben . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51.2.2 This Study . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7Chapter 2: Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102.1 Rock Classification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102.1.1 Petrography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102.2 Geochronology and Thermochronology . . . . . . . . . . . . . . . . . . . . . . . . 102.2.1 Geochronology overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102.2.2 U-Th/Pb Geochronology . . . . . . . . . . . . . . . . . . . . . . . . . . . 112.2.3 40Ar/39Ar Thermochronology . . . . . . . . . . . . . . . . . . . . . . . . . 122.2.4 U-Th/He Thermochronology . . . . . . . . . . . . . . . . . . . . . . . . . 18Chapter 3: Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 253.1 Petrology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 253.2 Geochronology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 343.2.1 U-Th/Pb in Monazite . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 343.2.2 40Ar/39Ar Thermochronology in Mica . . . . . . . . . . . . . . . . . . . . 403.2.3 U-Th/He Thermochronology in Zircon and Apatite . . . . . . . . . . . . . 453.2.4 Summary of Geochronologic Constraints . . . . . . . . . . . . . . . . . . . 553.3 Cooling Paths . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57vTABLE OF CONTENTSChapter 4: Discussion and Conclusions . . . . . . . . . . . . . . . . . . . . . . . . 624.1 E-W extension in the Thakkhola graben . . . . . . . . . . . . . . . . . . . . . . . 624.2 Potential sources of kinematic shift . . . . . . . . . . . . . . . . . . . . . . . . . . 674.3 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 684.4 Future Work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70Appendix . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81Appendix A: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82Appendix B: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84Appendix C: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94viList of TablesTable 3.1 40Ar/39Ar ages and closure temperatures . . . . . . . . . . . . . . . . . . . 41Table 3.2 U-Th/He ages and elemental data . . . . . . . . . . . . . . . . . . . . . . 46Table 4.1 E-W Extensional Structures . . . . . . . . . . . . . . . . . . . . . . . . . . 66Table C.1 40Ar/39Ar Isotopic data I . . . . . . . . . . . . . . . . . . . . . . . . . . . 94Table C.2 40Ar/39Ar Isotopic data II . . . . . . . . . . . . . . . . . . . . . . . . . . . 95Table C.3 40Ar/39Ar Isotopic data III . . . . . . . . . . . . . . . . . . . . . . . . . . 96Table C.4 40Ar/39Ar Isotopic data IV . . . . . . . . . . . . . . . . . . . . . . . . . . 97Table C.5 40Ar/39Ar Isotopic data V . . . . . . . . . . . . . . . . . . . . . . . . . . . 98Table C.6 Zircon U-Th/He Elemental and Age Data . . . . . . . . . . . . . . . . . . 99Table C.7 Apatite U-Th/He Elemental and Age Data . . . . . . . . . . . . . . . . . 100viiList of FiguresFigure 1.1 Map of south Asia: study area, political boundaries, and current structures 2Figure 1.2 Simplified geology of the Himalaya . . . . . . . . . . . . . . . . . . . . . . 3Figure 1.3 Geological map of the Himalaya and study area . . . . . . . . . . . . . . 9Figure 2.1 Ideal Ar spectra . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16Figure 2.2 Excess Ar spectra . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17Figure 3.1 Study area geology, sampling locations, and radiometric ages . . . . . . . 27Figure 3.2 Outcrop photograph: Ghami . . . . . . . . . . . . . . . . . . . . . . . . . 28Figure 3.3 Outcrop photograph: Ghar Gompa . . . . . . . . . . . . . . . . . . . . . 29Figure 3.4 Outcrop photograph: Dhanggna Khola . . . . . . . . . . . . . . . . . . . 30Figure 3.5 Thin section photographs: Ghami and Ghar Ghompa . . . . . . . . . . . 31Figure 3.6 Thin section photographs: Dhanggna Khola . . . . . . . . . . . . . . . . . 32Figure 3.7 IUGS QAP ternary diagram . . . . . . . . . . . . . . . . . . . . . . . . . 33Figure 3.8 U-Th/Pb ages: Ghami . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36Figure 3.9 U-Th/Pb ages: Ghar Ghompa . . . . . . . . . . . . . . . . . . . . . . . . 37Figure 3.10 U-Th/Pb ages: Dhanggna Khola . . . . . . . . . . . . . . . . . . . . . . . 38Figure 3.11 40Ar/39Ar spectra: Ghami . . . . . . . . . . . . . . . . . . . . . . . . . . 42Figure 3.12 40Ar/39Ar spectra: Ghar Ghompa . . . . . . . . . . . . . . . . . . . . . . 43Figure 3.13 40Ar/39Ar spectra: Dhanggna Khola . . . . . . . . . . . . . . . . . . . . . 44Figure 3.14 Zircon Logratio and Isochron diagrams: Ghami . . . . . . . . . . . . . . . 49Figure 3.15 Zircon Logratio and Isochron diagrams: Ghar Ghompa . . . . . . . . . . 50Figure 3.16 Zircon Logratio and Isochron diagrams: Dhanggna Khola . . . . . . . . . 51Figure 3.17 Apatite Logratio and Isochron diagrams: Ghar Ghompa . . . . . . . . . . 53Figure 3.18 Apatite Logratio and Isochron diagrams: Dhanggna Khola . . . . . . . . 54Figure 3.19 Age summary diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56Figure 3.20 Cooling paths: Ghami . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59Figure 3.21 Cooling paths: Ghar Ghompa . . . . . . . . . . . . . . . . . . . . . . . . 60Figure 3.22 Cooling paths: Dhanggna Khola . . . . . . . . . . . . . . . . . . . . . . . 61Figure 4.1 Cooling paths: Dhanggna Khola . . . . . . . . . . . . . . . . . . . . . . . 65viiiLIST OF FIGURESFigure B.1 Field specimens GG1A and GG1B . . . . . . . . . . . . . . . . . . . . . . 84Figure B.2 Field specimens GG10 and GG11 . . . . . . . . . . . . . . . . . . . . . . 85Figure B.3 Field specimens GG12 and GG13 . . . . . . . . . . . . . . . . . . . . . . 86Figure B.4 Field specimens DK14 and DK15 . . . . . . . . . . . . . . . . . . . . . . 87Figure B.5 Field specimens DK16 and GH17B . . . . . . . . . . . . . . . . . . . . . 88Figure B.6 Mica separates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89Figure B.7 Zircon separates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90Figure B.8 Zircon separates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91Figure B.9 Apatite separates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92Figure B.10 Apatite separates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93ixAcknowledgementsI would like to thank my supervisor, Dr. Kyle Larson, first and foremost, for providingsupport and guidance through every stage of this project, from booking the flights to crossingthe 'I's and dotting the 'T's. Dr. Larson has pushed me to work hard at becoming a better andmore inquisitive geologist and scholar, while providing thoughtful insight and feedback whenneeded. I am also thankful for the support of the rest of my supervisory committee, Dr. JohnGreenough and Dr. Yuan Chen. I am indebted to Dr. William Matthews, Dr. Alfredo Camacho,and Dr. John Cottle, along with their laboratory assistants, who have helped me understandthe laboratory methods used and made sure I obtained meaningful data. I am grateful for thefield preparation and guidance of Pradap and Teke Tamang, and for Pasang, Kailash, Balbadur,and Nima Tamang for field logistics. I am also thankful for the field assistance of Iva Lihter, andthe guidance and advice of rest of the Himalayan tectonics group at UBCO; Sudip, Mathieu,and Kumar. I am also extremely grateful for my family and friends, who have supported methrough these 5.5 years of university, and who have been very understanding when I neverreach for the bill. This project was funded, in part, by an NSERC Discovery grant awarded toDr. Kyle larson, and by an NSERC Canada Graduate Scholarships-Masters grant, GraduateEntrance Scholarship, and University Graduate Fellowship awarded to myself.xChapter 1IntroductionThe Himalayan mountain chain spans for ⇠2500 km, crossing the borders of Pakistan,India, Nepal, China, and Bhutan (Fig. 1.1). It is one of the few currently active continentalcollision zones, and this, combined with its broad extent and excellent exposure, has resultedin it becoming the type locality of continental collisional boundaries (Hodges, 2000; Yin, 2006).Geologists have been studying the Himalaya since as early as the mid-19th century (Upreti,1999) and despite political and topographic obstacles (Hodges, 2000), have continued to do sofor more than a century as they seek to understand the inner workings of large, hot orogens.1.1 Overview of the HimalayaThe Himalaya began to form ⇠55 Myr ago with the collision of the Indian and Eurasiancontinents (DeCelles et al., 2001; Mascle et al., 2012; Rowley, 1996). Sediments deposited on thenorthern passive margin of the Indian plate were scraped o↵ of the downgoing plate, deformedand metamorphosed, and are now exposed in the Himalaya (Bollinger et al., 2004; Cottle et al.,2015; DeCelles et al., 2001; Mascle et al., 2012; Myrow et al., 2003). Four major lithotectoniczones are recognized within the Himalayan belt, separated or bound by five major structuraldivisions (Fig. 1.2; Cottle et al., 2015; Le Fort, 1975), all of which are remarkably continuousalong the length of the range (Kohn, 2014; Montomoli et al., 2013; Wu et al., 1998; Yin, 2006).The Main Frontal thrust (MFT) is the youngest and southernmost of the major N-dippingstructures in the Himalaya. It separates the Terai (lowlands) in its footwall from the Siwaliks(foothills) in its hanging wall (Hodges, 2000; Mascle et al., 2012; Yin, 2006). The Siwaliks(or Subhimalaya) is the southernmost lithotectonic zone within the Himalaya (Cottle et al.,2015; Hodges, 2000), consisting of Neogene to Quaternary unmetamorphosed molasse deposits,deformed into a south-verging fold and thrust belt (Cottle et al., 2015; Mascle et al., 2012;Hodges, 2000). The thickness of the unit varies from ⇠2 km in the south to ⇠10 km in thenorth (Hodges, 2000).The Main Boundary thrust (MBT) separates the Siwaliks from the overthrust Lesser Hi-malayan sequence (LHS; Fig. 1.2; Mascle et al., 2012). The LHS consists of 8-10 km of phyllite,impure quartzite, marble, metabasalt, and orthogneiss, metamorphosed at lower greenschistto (locally) lower amphibolite facies (Hodges, 2000; Montomoli et al., 2013). The LHS can befurther divided into upper and lower units, separated by a major unconformity (Hodges, 2000;11.1. Overview of the HimalayaFigure 1.1: Map of south Asia, showing Himalayan topography and currently active E-Wextension-related N-S trending normal faults, and associated strike-slip faults, from Styron etal. (2011). The location of the study area and figure 1.3 shown in black boxes.21.1. Overview of the HimalayaFigure 1.2: Major lithology and structures of the Himalaya, modified after Larson et al. (2010)and Hauck et al. (1998). A) Simplified geological map showing major lithotectonic units andfaults. B) Simplified cross section of the Himalaya, indicating location of major faults at depth.31.1. Overview of the HimalayaUpreti, 1999). The Lower LHS is Mesoproterozoic in age, while Upper LHS is Cambrian inage and locally unconformably overlain by Carboniferous-Permian strata (Hodges, 2000). Theupper boundary of the LHS is marked by the Main Central thrust zone (MCT).The MCT is a north dipping, thrust-sense ductile shear zone that, like the other majorfaults previously mentioned, can be mapped across the entire range (Fig. 1.2; Montomoli et al.,2013; Searle et al., 2008; Yin, 2006). There has been much controversy over how the structuresis mapped in recent years (see Searle et al., 2008; Martin, 2017). Searle et al. (2008) definesthe MCT as “the base of the large-scale zone of high strain and ductile deformation, commonlycoinciding with the base of the zone of inverted metamorphic isograds, which places Tertiarymetamorphic rocks of the Greater Himalayan sequence over unmetamorphosed or low-graderocks of the Lesser Himalaya.” (p. 532). This definition accounts for along-strike variations inthe exact location of metamorphic gradients and lithological di↵erences, which have been thefoci of debate in the past (see Searle et al., 2008; Martin, 2017, for full discussion).The Main Frontal thrust, Main Boundary thrust, and Main Central thrust are all believedto sole onto a N-dipping decollement surface beneath the Himalaya, commonly called the MainHimalayan thrust (MHT; Fig. 1.2; Mascle et al., 2012; Yin, 2006). Various geophysical surveyshave identified the MHT at depth, which is believed to defined the location of the underthrustingIndian plate (Bollinger et al., 2004). Seismic data showing recent earthquake foci along thisthrust suggest that the MHT is currently accommodating convergence (Bollinger et al., 2004).The Greater Himalayan Sequence (GHS; Fig. 1.2) - also called the High Himalaya Crys-tallines, Higher Himalayan gneisses, Central Crystallines, or the Tibetan Slab - (Cottle et al.,2015; Hodges, 2000) lies structurally above the LHS and consists mainly of high-grade micaschist, paragneiss, calc-silicate, and migmatite (Hodges, 2000; Montomoli et al., 2013; Yin,2006). Once believed to have represented the crystalline basement to the overlying TethyanSedimentary Sequence (TSS; Le Fort, 1975), more recent studies have shown that the GHShas a sedimentary protolith, and likely represents units deposited in the Indian passive marginstratigraphically above those that formed the LHS (Hodges, 2000; Searle et al., 2006). TheGHS ranges in age from Neoproterozoic to Ordovician (DeCelles et al., 2001; Yin, 2006) and inthickness along strike from as little as 2 km in western Nepal, up to 50 km in NW India andBhutan, with an average thickness of 20-30 km (Montomoli et al., 2013, 2015). Numerous largeleucogranite plutons occur within the uppermost GHS, which are believed to be derived frompartial melting of the GHS itself (Hodges, 2000; Searle et al., 2006; Wu et al., 1998).The top of the GHS is marked by a network of low-angle normal-sense faults, called theSouth Tibetan detachment system (STDS; Fig. 1.2; Hodges, 2000; Kohn, 2014; Mascle et al.,2012; Yin, 2006). This boundary, which separates the unmetamorphosed to low-grade TSSabove from the high metamorphic grade GHS below, often occurs at the crest of the Himalaya,such that many of the great peaks (e.g. Everest, Dhaulagiri, Cho Oyu) actually contain thestructure. It has been suggested that the STDS may either sole onto the MHT at depth, or that41.2. Geological Settingit shallows to the north, merging with the Great Counter thrust (Fig. 1.2; Burchfiel and Royden,1985; Kohn, 2014; Yin, 2006). Movement on the STDS initiated in the Miocene (Hodges, 2000)and is widely viewed to be at least broadly contemporaneous with that on the MCT (Beaumontet al., 2004; Godin et al., 2001, 2006; Hodges et al., 1992; Jamieson et al., 2004; Kellett andGrujic, 2012; Vannay and Hodges, 1996).The structurally highest lithotectonic zone in the Himalaya is the TSS (Fig. 1.2) - alsoknown as the Tibetan Sedimentary Sequence, or Tethys Himalaya - (Hodges, 2000). It consistsof largely unmetamorphosed clastic and carbonate sedimentary rocks that locally reach loweramphibolite facies (Hurtado, 2002; Montomoli et al., 2013; Yin, 2006). The protolith to thiszone is often considered to be laterally equivalent to the GHS (Searle et al., 2006). Ages in theTSS range from Proterozoic to Eocene (Yin, 2006; Liu and Einsele, 1994), and based on detritalzircon and Nd isotope data, Searle et al. (2006, 2008) suggest that the entire LHS-GHS-TSSsequence corresponds to a semi-continuous series of sediments deposited on the northern passivemargin of India prior to collision.The E-W trending north Himalayan anticline crops out in structural windows through theTSS in southern Tibet and northern Nepal (Fig. 1.3). It is marked by a series of structural cul-minations cored by leucogranite and/or high-grade metamorphic rock of GHS anity, typicallysurrounded by low grade metasediments (Hodges, 2000; Lee et al., 2000, 2006; Larson et al.,2010).The Indus-Tsangpo suture zone (ITSZ; Figs. 1.2 and 1.3; also called India-Asia, Indus-Yarlung, or Yarlung-Tsangpo suture zone) marks the boundary between the Indian and Eurasianplates and is the northern boundary of the Himalaya (Hodges, 2000; Mascle et al., 2012). Thesuture zone is marked by flysch deposits and ophiolites related to the closure of the TethysOcean (Hodges, 2000; Montomoli et al., 2013).1.2 Geological Setting1.2.1 Thakkhola grabenThe Tibetan Plateau is a high, mostly flat plateau with an average elevation of ⇠5000 m(Hodges, 2000; Mascle et al., 2012). Relief within the plateau is dominated by late Miocene toPliocene graben that accommodate E-W extension in the plateau (Fig. 1.1; e.g. Ratschbacheret al., 2011; Yin et al., 1999). The E-W extension generating these structures is thought toreflect a shift from S-directed to orogen parallel crustal movement, which has been documentednot only in the upper crust, but also in the midcrust (Cottle et al., 2009), and in the lower crust(Clark and Royden, 2000). This shift in orogen-wide kinematics may be driven by a change inconvergence rate and direction between India and Asia (e.g. Molnar and Stock, 2009). Someof these structures extend southward across the crest of the Himalaya, incising deep valleysand forming sedimentary basins. The Thakkhola graben, in the Mustang region of west-central51.2. Geological SettingNepal, is one such structure and is the focus of this study.The present day geomorphology of the Mustang region is dominated by the Kali Gandakiriver valley, which is one of the deepest valleys in the world, incised to an elevation of ⇠2500m between the peaks of Dhaulagiri (8167 m) in the west and Annapurna (8091 m) in the east.The upper reaches of the river run through the Thakkhola graben. The Thakkhola grabenextends southward almost to the STDS (Fig. 1.3), while the northern edge of the graben,which is poorly defined in most geological maps, ends south of the Indus-Tsangpo suture zone(Hurtado et al., 2001; Hurtado, 2002). The eastern boundary of the graben is defined by aseries of unnamed, steeply west-dipping normal faults, while the west side is defined by theN-NE striking, steeply east-dipping Dangardzong normal fault. It is believed to be the maingraben-forming structure (Fig. 1.3; Colchen, 1999; Fort et al., 1982; Hurtado et al., 2001). TotalE-W extension accommodated within the graben is estimated to be ⇠30 km (Hurtado, 2002).As mapped, the footwall of the Dangardzong fault includes low-grade to unmetamorphosedrocks of the TSS and two granitic bodies, the (Dolpo-) Mugu batholith, a large elongate NW-SEtrending body, and the smaller Mustang granite to the north (Fig. 1.3; e.g. Hurtado, 2002; LeFort and France-Lanord, 1994). The Mugu and Mustang granites have mineralogy and texturessimilar to other plutons that occur near the crest of the Himalaya, which are interpreted asbeing derived from partial melting of a sedimentary source rock (i.e., S-type granites; Le Fortet al., 1987; Deniel et al., 1987; Searle et al., 2009). Initial age data determined by monaziteID-TIMS, date the Mugu batholith as 20.8 ± 0.7 Ma, and the Mustang granite as 23.4 ± 0.2Ma (Hurtado, 2002). These ages overlap with those from many other Himalayan leucogranites,including the nearby Manaslu pluton (23-19 Ma; Harrison et al., 1999), the Makalu pluton, ineastern Nepal (⇠24-22 Ma; Scha¨rer, 1984), and even those at the western end of the Himalayain Zanskar, India (22-19 Ma; Noble and Searle, 1995).The hanging wall block of the Dangardzong fault comprises five sedimentary formations,which show evidence of syn-kinematic deposition (Garzione et al., 2003). The oldest sedimentaryunit within the graben is the Tetang formation, consisting of a 150 m thick succession of siltygravel and breccia, conglomerate, sandstone, and tu↵aceous sediments (Fort et al., 1982). Thedeposition of this lowest unit has been dated at ⇠11 to ⇠9.6 Ma (Garzione et al., 2003). TheThakkhola formation overlies the Tetang formation, separated from it by a slight angular un-conformity, and consists of a 700 m thick succession of red conglomerate, sandstone, limestone,and clay deposits (Fort et al., 1982; Hurtado et al., 2001). The Sammargaon and Marpha for-mations both lie disconformably atop the Thakkhola formation, and have been dated as upperPliocene to upper Pleistocene (Hurtado et al., 2001). The Sammargaon formation consists ofunsorted angular breccia, and only occurs in the western slopes of the graben, while the Marphaformation is made up of >200 m thick glaciolactustrine sediments (Hurtado et al., 2001). Theuppermost sedimentary unit in the Thakkhola graben is the Kali Gandaki formation, consist-ing of terraced fluviatile and lacustrine deposits, limited to the sides of the Kali Gandaki river61.2. Geological Setting(Fort et al., 1982; Hurtado et al., 2001). Hurtado et al. (2001) used the chronology of terraceformation to bracket the bulk of the Kali Gandaki sedimentation to between 17.2 and 7.5 ka.A series of ⇠N-S trending syn-sedimentation normal faults cross cut the graben-fill sediments,providing evidence of continued E-W extension (Colchen, 1999; Hurtado et al., 2001).The age of the Thakkhola graben is only loosely constrained. As mentioned previously, theearliest sediments dated in the valley are ⇠11 Ma (Garzione et al., 2003), which give a potentialminimum age. Approximately 40 km east of the graben, hydrothermal muscovite from a N-Strending brittle fracture, which was interpreted to represent the earliest deformation related toE-W extension in the Thakkhola graben, was dated as ⇠14 Ma, extending the minimum ageconstraint for the inception of E-W extension in the region (Coleman and Hodges, 1995).1.2.2 This StudyThe majority of studies in the Himalaya have focused on the results of N-S compression,namely the development of the High Himalaya and the southward extrusion of the Himalayanmetamorphic core. However, late-stage E-W extension across the region has resulted in numer-ous N-S trending graben and related strike-slip structures in Tibet, some of which cut acrossthe High Himalaya (Fig. 1.1). The mechanisms and timing behind orogen-parallel (E-W) ex-tension in an actively N-S converging orogen are poorly understood. This study uses detailedgeochronologic data to characterize the timing of opening and rate of development of the N-Strending Thakkhola graben (Fig. 1.3).During preliminary work in the Thakkhola graben Hurtado (2002) suggested that part of itscooling history was imparted by movement on the Dangardzong fault. This project seeks to char-acterize the plutons exposed in the footwall of the fault using petrology, U-Th/Pb geochronol-ogy, and single-grain 40Ar/39Ar and U-Th/He thermochronology, in an attempt to: 1) isolateany potential contributions of pre-Thakkhola graben cooling/exhumation from that associatedwith the opening of the structure, and 2) compare the detailed cooling history of the Thakholawith data from graben investigated across Tibet. These new data will help constrain the timingand rate of cooling in the footwall of the Dangarzong fault, which when paired with regionaldata, can be used to elucidate the kinematic importance of similar structures across the orogen.Fieldwork in the Upper Mustang region of west-central Nepal was conducted in October2016. Three field locations were visited over the course of a two week trek; Ghami (GH), GharGompa (GG), and Dhannga Khola (DK; Fig. 1.3). Fieldwork was originally planned to compriseup to 8 days of jeep-assisted structural mapping and specimen collection along the western edgeof the graben. Due to logistical setbacks, however, the entire trek was completed on foot, andhence only allowed for specimen collection with limited structural data collected in metamor-phosed TSS rocks to be used in a separate study. Ten leucogranite specimens were collected forpetrographic analysis and radiometric dating by methods as outlined in the following chapter.Four geochronologic systems with di↵erent closure temperatures are used to constrain detailed71.2. Geological Settingcooling paths of the specimens; U-Th/Pb in monazite, 40Ar/39Ar in muscovite and biotite, andU-Th/He in both zircon and apatite. Note that specimens are referred to throughout as per Fig.1.3, where the initial letters represent sampling location (GH = Ghami, GG = Ghar Ghompa,DK = Dhanggna Khola). Mineral abbreviations used throughout this thesis follow Whitneyand Evans (2010).81.2. Geological SettingFigure 1.3: Geological map of the central Himalaya modified from Hodges (2000) and referencestherein. Inset: geology of the Upper Mustang region, based on Hurtado (2002), showing majorlithologies and sampling locations from this study.9Chapter 2Methods2.1 Rock Classification2.1.1 PetrographyPolished thin sections were cut from 7 specimens (GG01A, GG10, GG12, DK14, DK15,DK16, and GH17B; Appendix B-II) for petrographic observations and scanning electron micro-scope (SEM) analysis. Point-counting was conducted in each thin section under a petrographicmicroscope to estimate modal mineral abundances and classify the specimens under the In-ternational Union of Geological Sciences (IUGS) system for granitic rocks. Thin sections werefurther examined for the calculation of An% using energy dispersive spectrometry of backscat-tered electron images on the scanning electron microscope (SEM) in the Fipke Laboratory forTrace Element Research at the University of British Columbia, Okanagan. The remaining threespecimens (GG01B, GG11, and GG13) were texturally and mineralogically similar to theirneighbours in hand sample, and thus were only used for thermochronology.2.2 Geochronology and Thermochronology2.2.1 Geochronology overviewGeochronometers within minerals record the time when a radioactive clock began ticking.At high temperatures, which are specific to the decay system, the radioactive decay productsused for dating are not retained within a mineral but instead are released to equilibrate withtheir surroundings (Dodson, 1973). The temperature below which all of the decay product isretained is that isotopic systems closure temperature (Tc), which varies depending on the struc-tural and chemical properties of the mineral being dated, and more crystal-specific propertieslike crystal size and di↵usion behaviour (e.g. Dodson, 1973; Harrison et al., 1985; Hames andBowring, 1994; Kelley, 2002b). Since geochronometers have varying closure temperatures, theymay record di↵erent thermal events. Higher temperature systems, such as the U-Th/Pb system,have closure temperatures close to or above the crystallization temperature of many igneousrocks and, therefore, are used to date the formation of igneous rocks from a cooling magmaor lava (e.g. Harrison and Zeitler, 2005; Faure and Mensing, 2009). Other systems, such as theK/Ar system and its derivative, the 40Ar/39Ar system, have much lower closure temperatures102.2. Geochronology and Thermochronology(⇠300-400 °C for common micas), and will therefore record when a rock cooled below a rela-tively low temperature (Dodson, 1973). The practice of using this and other low-temperaturegeochronometers, such as fission-track dating or U-Th/He dating, is commonly referred to asthermochronology. These lower temperature systems are useful for determining cooling ages andrates, which are often interpreted with respect to timing and rate of exhumation (McDougalland Harrison, 1999).This study employs the use of four geochronologic systems in order to develop a detailedcooling history (U-Th/Pb in Mnz, 40Ar/39Ar in Ms and Bt, U-Th/He in Ap and Zrn). These an-alytical methods are outlined in the following sections, including principles behind the 40Ar/39Arand U-Th/He systems. For a review of the U-Th/Pb method see Faure and Mensing (2009)and references therein, and for a more formal review of the 40Ar/39Ar and U-Th/He methods,see McDougall and Harrison (1999), or Harrison and Zeitler (2005), respectively, and referencestherein.2.2.2 U-Th/Pb GeochronologyThe U-Th/Pb dating method is an intensively studied and widely used system for datingthe crystallization, sedimentation, and metamorphism of rocks. Based on the radioactive decayof 238U, 235U, 234U and 232Th, it is a complex system involving several separate decay schemes(Faure and Mensing, 2009). Because the most commonly used mineral for U-Th/Pb dating, zir-con, is highly resistant to weathering and is refractory during melting, often only relict/inheritedgrains are found in S-type granites, thus monazite (Mnz) is a better choice for dating the crys-tallization of such rocks (Parrish, 1990; Scha¨rer et al., 1986). Monazite 208Pb/232Th dates areused in this study, partly due to the abundance of Th in monazite, and partly to avoid problemsassociated with monazite incorporating excess 230Th during crystallization, leading to extra-neous 206Pb which would give an erroneously old 206Pb/238U age (Cottle et al., 2009; Parrish,1990; Scha¨rer, 1984).The seven samples (GG01A, GG10, GG12, DK14, DK15, DK16, and GH17B) were anal-ysed for Mnz U-Th/Pb geochronology at the University of California Santa Barbara (UCSB).Specimens were crushed and ground using standard mechanical methods, and heavy miner-als separated using a Rogers Gold™ table, heavy liquids (Methyl iodide, MI, 3.35g/cm3), andFrantz™ magnetic techniques. Separated monazite grains were mounted in epoxy, polished andanalysed using laser ablation multi-collector inductively coupled plasma mass spectrometry(LA-MC-ICP-MS), which simultaneously collects U, Th, and Pb isotopic ratios. Additionalprocedures and instrument parameters are outlined in Appendix A-I.112.2. Geochronology and Thermochronology2.2.3 40Ar/39Ar ThermochronologyPrincipleThe 40Ar/39Ar dating method is a derivative of the K/Ar method, with the 40Ar/39Armethod quickly becoming popular after its initial description by Merrihue and Turner (1966).One main advantage of the 40Ar/39Ar over the K/Ar system is that it does not require separatemeasurement of K and Ar, but instead measures 39Ar as a proxy for K from the same specimenbeing analyzed for radiogenic 40Ar (McDougall and Harrison, 1999), which allows for moreaccurate information to be gathered from smaller samples.The K/Ar and 40Ar/39Ar methods are based on the radioactive decay of the 40K isotope.40K has an atomic abundance of 0.012% of naturally occurring K, and a half-life of 1250 Ma(McDougall and Harrison, 1999). It has a forked-decay scheme, with 89.5% of 40K becoming40Ca by beta emission, and 10.4% becoming 40Ar by either electron capture (10.3%) or positronemission (<1%; Gillot et al., 2006). Potassium-bearing rocks will produce 40Ar, which is eas-ily released out of the mineral at higher temperatures, but upon cooling to suciently lowtemperatures, becomes trapped within crystal lattices of K-bearing minerals (McDougall andHarrison, 1999). The amount of trapped 40Ar and parent K can then be measured, and withknowledge of the decay rate and mineral-specific di↵usion rates, can provide the date at whichthat specific mineral reached its closure temperature (Dodson, 1973; Kelley, 2002a).In the 40Ar/39Ar method the specimen of interest is first irradiated in a nuclear reactorby fast-neutron bombardment. This changes a percentage of the 39K to gaseous 39Ar by the39K(n,p)39Ar reaction, where n is the incident neutron, and p is the expelled proton (Merrihueand Turner, 1966). The amount of 39Ar formed during this process depends on the neutronflux in the reactor. This can be determined by co-irradiating a specimen of known age withthe unknown specimen, in close proximity and for a fixed amount of time, using the followingequation (Mitchell, 1968):39Ar =39KTZ✏✏d✏ (2.1)Where ✏ = neutron energyT = duration of irradiation✏ = neutron flux at energy ✏✏ = neutron capture cross-section at energy ✏122.2. Geochronology and ThermochronologyEquation 2.1 can be more easily incorporated into decay equations by creating the dimen-sionless parameter J using equation 2.2 below (Mitchell, 1968):J =etm  1(40Ar⇤/39Ar)m(2.2)Where  = total decay constant of 40Ktm = age of flux monitor(40Ar⇤/39Ar)m = measured isotopic ratio of flux monitorBy incorporating this parameter into a general equation for the 40K/39Ar decay scheme weget:t =1ln[1 + J(40Ar⇤/40K)S ] (2.3)Where t = age of specimen = total decay constant for 40KJ = dimensionless irradiation parameter from eq. 2.240Ar⇤ = amount of radiogenic 40Ar in specimen40K = amount of 40K in specimen, determined by measurement of 39Ar produced from theunknown during irradiation (McDougall and Harrison, 1999).Thus, it is possible to calculate an age from measuring only the 40Ar/39Ar ratio in a singlemineral separate, avoiding problems associated with measuring K and Ar separately, such asinhomogeneous distribution in a specimen (Faure and Mensing, 2009).The most common minerals used for 40Ar/39Ar dating are muscovite, biotite, K-feldspar,and hornblende. Muscovite and biotite have high K content at ⇠8% and approximate closuretemperatures of 400 and 350°C, respectively (Harrison and Zeitler, 2005). Muscovite tends to beless likely to incorporate excess Ar, and also breaks down less easily during heating, making it amore reliable chronometer (Harrison and Zeitler, 2005). K-feldspar has K contents up to ⇠17%and a fairly low closure temperature (⇠130°C), while hornblende has very low K content (⇠0.3-1%) and a higher closure temperature (⇠500°C; Harrison and Zeitler, 2005). Only muscoviteand biotite were used in this study.CorrectionsThere are a number of corrections required before a measured 40Ar/39Ar ratio can be usedto calculate an age. Since Ar makes up ⇠1% of earths atmosphere, it can readily di↵use intominerals during or after formation (Faure and Mensing, 2009). Thus, an atmospheric Ar cor-rection (or air correction) is applied to account for non-radiogenic 40Ar. Mass discrimination ismonitored by measuring atmospheric 40Ar/36Ar every day, and comparing that measured ratio132.2. Geochronology and Thermochronologyto a standard of 295.5 (Steiger and Jager, 1977) to get a correction factor. Since all 36Ar in thesample is assumed to be atmospheric, we can use the known isotopic ratios of Ar to determinea base level of atmospheric 40Ar read by the machine, and subtract this amount from the totalmeasured 40Ar (Renne et al., 2009).During the irradiation process, several other isotopes beside the desired 39Ar are formedthat may interfere with measurements, the most important of which are those formed from Caisotopes via 42Ca(n,↵)39Ar and 40Ca(n,n↵)36Ar reactions, because they will interfere with 39Arreadings and atmospheric corrections (Merrihue and Turner, 1966; Faure and Mensing, 2009).Naturally occurring 37Ar is generally negligible, and so all measured values of this isotope areassumed to be a byproduct of the irradiation process (McDougall and Harrison, 1999). By mea-suring the 37Ar in a monitor, the amounts of neutron-induced 39Ar and 36Ar can be determinedby comparison with known isotopic ratios. Other reactions also occur during irradiation, buttheir e↵ect is negligible and are not discussed here. For more detail on interfering isotopes, seeDalrymple and Lanphere (1971), who developed an expression to relate the measured 40Ar/39Arratio to all necessary corrections. Line-blanks are also measured before every analysis to deter-mine the base reading of each Ar isotope. Since the mass spectrometer measures the currentassociated with each isotope, a simplified equation outlining all corrections can be written asfollows:Imeasured = IAr⇤ + Iatm Ar + Ineutron Ar + Iblank Ar (2.4)Where Ar⇤ = radiometric Ar value used for age calculationI = currentAtm Ar = atmospheric ArNeutron Ar = neutron reaction induced ArBlank Ar = base Ar readingSimilar corrections are made for all other Ar isotopes.40Ar/39Ar age interpretationIn the simplest case, if an igneous rock cools and crystallizes quickly and remains at lowtemperatures until the rock is sampled and analyzed, the age provided may represent thecrystallization age of that rock (e.g. Kelley, 2002b; McDougall and Harrison, 1999). If the rockcools more slowly, the 40Ar produced in the early stages of cooling will not accumulate in thecrystal, and the date obtained will be younger than the crystallization age. Similarly, if the rockwas heated after initial cooling (to approximately greenschist facies or higher conditions), someor all of the trapped 40Ar may be released, e↵ectively ‘resetting’ the radiometric clock (e.g.Dodson, 1973; McDougall and Harrison, 1999). If the resetting is complete, i.e. all accumulated40Ar was released, then the age obtained will be the age of cessation of that resetting thermal142.2. Geochronology and Thermochronologyevent, but if the system is only partially reset, i.e. not all accumulated 40Ar escapes the rock,then the age found will be intermediate between the initial crystallization age and that ofthe resetting thermal event (e.g. Kelley, 2002b; McDougall and Harrison, 1999). In such casesthe age obtained may be of no use, as there is no definite way to tell what proportion of themeasured 40Ar was produced before the resetting event and what proportion was produced after.However, this problem can often be overcome by use of the step-heating technique (Lanphereand Dalrymple, 1976), the results of which are usually portrayed in an age spectrum plot (Fig.2.1).In this application, an aliquot or single-crystal is heated to incrementally higher tempera-tures using a furnace (multiple-crystal) or higher power using a laser (single- or multiple-crystal),measuring the 40Ar/39Ar ratio at each step (Lanphere and Dalrymple, 1976; McDougall andHarrison, 1999). Di↵usion of Ar from crystal sites happens from rim locations first, with greatertemperatures or greater time periods at elevated temperatures needed to release Ar from corelocations (Harrison and Zeitler, 2005). Thus, it is assumed that upon step heating, the gastrapped in the outer portions of a crystal is released first at low temperature, and each subse-quent higher temperature step releases gases from deeper lattice sites (Fig. 2.1; e.g. Harrisonand Zeitler, 2005). By this method, meaningful dates may be gleaned from a partially resetcrystal, where gas from partially reset rim portions of the crystal is released at low temper-atures, giving one date, and subsequent heating steps release gas from inner portions of thecrystal where resetting didn’t occur, giving a minimum date of the last heating event, whetherit was initial crystallization or later metamorphism (Harrison and Zeitler, 2005). If the step-heating method yields little variation in the age obtained from each step, this forms a ‘plateau’in the age-spectrum diagram, and the corresponding date of that plateau is called a plateauage. A large plateau typically indicates a simple cooling history, involving little to no thermaldisturbance after crystallization (e.g. McDougall and Harrison, 1999).A common problem with 40Ar/39Ar dating, especially in Himalayan biotite, is the presenceof ‘extra’ 40Ar in the crystal after corrections are made for non-radiogenic 40Ar. Extraneous40Ar can enter a crystal lattice due to high partial pressure of Ar in the environment, or by beingincluded within a xenolith, fluid inclusion, or sometimes will remain after imperfect resettingfrom a thermal event (e.g. Kelley, 2002a). The presence of excess 40Ar can cause the releasespectrum to appear ‘saddle shaped’ (Fig. 2.2), with early and late steps releasing the extra40Ar from the rim or inclusions, thus giving an older, perhaps inaccurate age (Kelley, 2002a).In this ideal situation the excess Ar is easy to detect, and meaningful information may stillbe obtained from the specimen, either by reading the center plateau age as a minimum age,or by using an inverse isochron plot (e.g. McDougall and Harrison, 1999). However, excess Arin biotite may also yield perfect plateaus at improbably old ages, and biotite that experiencedAr loss can show flat or convex upward spectra (McDougall and Harrison, 1999). Neverthelessbiotite regularly provides meaningful dates as it is usually possible to determine the reliability152.2. Geochronology and ThermochronologyFigure 2.1: Idealized mineral 40Ar/39Ar concentration (top) and age spectra (bottom) for a)specimen that has not undergone subsequent heating since crystallization and shows a flat‘plateau’, b) recent Ar loss, c) previous Ar loss with subsequent build up. Modified from Harrisonand Zeitler (2005).162.2. Geochronology and ThermochronologyFigure 2.2: Typical ‘saddle’ shaped Ar release spectra diagram, showing excess Ar at first andlast increments (right and left edges), and a small plateau in the middle. From (Kelley, 2002b).172.2. Geochronology and Thermochronologyof an 40Ar/39Ar date in biotite by interpreting it in conjunction with other dates from di↵erentsystems such as 40Ar/39Ar muscovite dates (McDougall and Harrison, 1999).ProcedureBefore crushing, micas were scraped o↵ and hand-picked from all specimens, except DK16,under a stereoscopic microscope using a scalpel and tweezers. The outcrop of Mustang Gneissat the Dhanggna Khola station was badly weathered, and hence a specimen was retrieved froma float-block of the same material, assumed to be locally derived that was less badly weathered,however, cooling data obtained from a float block may not be representative of the cooling of theoutcrop, so it was excluded from 40Ar/39Ar analysis. Scaled digital photographs were obtainedto measure an average grain size for each specimen (Appendix B-II). Biotite and muscovitegrains were then selected that had no visible inclusions or unusual colouration. Specimens wereirradiated at the Oregon State University TRIGA reactor along with standards to determine theneutron fluence parameter, J. After a cool-down period, specimens were sent to the Universityof Manitoba for 40Ar/39Ar analysis. Crystals were step-heated using a CO2 laser and the gasreleased measured in a multi-collector Thermo Fischer Scientific ARGUSVI mass spectrometer.Mass discrimination was monitored by online analysis of air pipettes for use in the atmosphericAr correction, and corrections were applied for other irradiation-induced particles. Age calcu-lation and age spectra diagrams were done on site using the MassSpec software (Deino, 2013).Closure temperatures were calculated separately for each crystal using equation 23 of Dodson(1973) and iterative determinations of cooling rate using Isoplot (Ludwig, 2003). Equipment,di↵usion, and geometric parameters are outlined in Appendix A-II. 40Ar/39Ar isotopic data aredetailed in Appendix C.2.2.4 U-Th/He ThermochronologyPrincipleHelium retained within minerals was first explored as a geologic dating method in theearly 1900s (Zeitler et al., 1987, and references therein), and again several decades later byHurley and Goodman (1941). These and similar early studies found that He ages obtained weremuch younger than U-Th/Pb ages from the same rock, a discrepancy which was attributedto incomplete He retention in the minerals being studied (Hurley and Goodman, 1941, 1943;Hurley, 1950). This method wasn’t explored again until 1987, when it was recognized as apotential thermochronometer (Zeitler et al., 1987). It has since been developed into a usefullow-temperature thermochronometer for several minerals including apatite (Ap), zircon (Zrn),and titanite (e.g. Farley et al., 1996). The method continues to evolve, with more accuratethermal modelling (Flowers et al., 2009; Ketcham et al., 2007, 2011; Shuster et al., 2006), andadvances in technology such as the in-situ ‘double-dating’ (U-Th)/(He-Pb) method contributing182.2. Geochronology and Thermochronologyto a growing field of use (Reiners et al., 2002).Helium is a decay product of the radioactive breakdown of U, Th, and to a lesser extent Sm.Uranium and Thorium-bearing minerals will retain daughter He within their crystal lattice afterthe mineral has cooled below its closure temperature (e.g. Dodson, 1973; Farley, 2000; Zeitleret al., 1987). The amount of He produced during decay can be calculated via the followingformula (e.g. Faure and Mensing, 2009):4HeV = 22.41383 ⇤ [0.0336 ⇤ U(e238t  1)+0.00021173 ⇤ U(e235t  1)+0.0258577 ⇤ Th(e232t  1)](2.5)Where 4HeV = volume of radiogenic 4He at STP in µL/gU,Th = concentrations of U and Th in µg/g = respective radioactive decay constantt = timeThis equation is the basis for He dating. The volume of He, and concentrations of 238U, 235U,and 233Th are measured from a crystal, then the equation is solved for the variable t to obtainan age.The main advantage of this technique stems from the unique low closure temperature of Heretention in apatite and zircon, approximately 70°C (Farley et al., 1996) and 200°C (Reinerset al., 2002) respectively, allowing geologists to examine very recent cooling histories. Heliumalso has advantages over other noble gas chronology such as 40Ar/39Ar dating in that He isfar less abundant in the atmosphere, hence contamination from modern atmosphere duringanalysis is easier to avoid (Harrison and Zeitler, 2005). Since He is produced at a high ratefrom U and Th decay, it is useful for even very young samples, despite U and Th themselvesgenerally appearing in low quantities within minerals (Harrison and Zeitler, 2005).Helium is extracted from apatite and zircon either by in situ-methods, where He, U, and Thare all measured simultaneously from a single point on a crystal, or by whole-grain methods,where the crystal is heated by furnace or laser to extract the He and the grain is later digested inacid to obtain U and Th concentrations via isotope dilution mass spectrometry (e.g. Faure andMensing, 2009; Harrison and Zeitler, 2005). After these volumes and abundances are determined,an initial date can be calculated, however, some corrections must be applied to improve itsaccuracy.CorrectionsInclusions in apatite, if they are U or Th bearing, can contribute additional He to the crystallattice. If the parent U or Th to this He is not accounted for - for example a zircon inclusionwill not dissolve in nitric acid typically used during the isotope dilution process for apatite, so192.2. Geochronology and ThermochronologyU and Th inside will not be measured - then the He that was produced is called ‘parentless’and any age measured will be older than the real age (e.g. Vermeesch et al., 2007). Vermeeschet al. (2007) explored the e↵ects of U-bearing inclusions in apatite, and found that if a singleinclusion is less than a few percent of the length, width, and height of the crystal, the parentlessHe it contributes to the lattice is probably negligible. The authors suggest a method to accountfor parentless He in samples with numerous small inclusions, or with inclusions larger thanthe suggested limit, whereby the entire grain is digested in hydrofluoric acid (HF) instead ofa weaker acid which is the norm. Hydrofluoric acid will dissolve the apatite and the includedzircon, and thus the otherwise parentless He will not skew the age (Vermeesch et al., 2007).This method may make it possible to date specimens with apatites that are otherwise unusable,however, the more aggressive acid technique is more time consuming and expensive, limitingits practical use. Inclusions in zircon tend to be even less of an issue, as the few mineralsthat commonly occur as inclusions within zircon contain little to no U, and the hot HF bathused to digest zircon will certainly dissolve any inclusion within - eliminating the possibility ofparentless He.Alpha particles (He) ejected from U and Th during decay travel some distance within acrystal lattice (⇠20µm), and if that distance is greater than the distance from the source to theedge of the crystal, it may be ejected out of the lattice, thus any measured He is assumed tobe less than what was produced (Farley et al., 1996). Farley et al. (1996) determined that theretentivity of alpha particles varies with parent nuclide and host mineral, and depends largelyon crystal size and morphology. This He loss can be corrected for using an ‘FT’ or ‘alphaejection’ factor (‘FT’ from Farley’s original retentivity equation derivations). This correctioncan be applied easily using a computer program if the total U and Th concentrations, size, andmorphology of the grain is known. The morphology information needed for apatite is simplywhether the crystal analyzed had 0, 1, or 2 complete terminations. The authors also providea minimum grain-size necessary for analyses, as any grain with 1 or more dimensions <60µmwould result in an unacceptably large FT correction (Farley et al., 1996). FT corrections assumean even distribution of U and Th within the crystal, but if there is zoning the amount of alphaejection from the edge of the crystal could be greater or lesser than in a homogeneous crystal,causing the FT correction to be less accurate (Meesters and Dunai, 2002).Closure temperature and thermal modellingClosure temperature for the ApHe system was first calculated to be ⇠105 ± 30 °C byZeitler et al. (1987). In the years following several authors conducted step-heating di↵usionexperiments and calculated closure temperatures varying from 68 °C to 130 °C (e.g. Farley,2000; Lippolt et al., 1994; Wolf et al., 1996). The closure temperature concept, however, isa simplification of reality. In nature, the transition from temperatures above which no Heis retained within a crystal lattice, to temperatures below which all He is retained within a202.2. Geochronology and Thermochronologycrystal lattice may not happen immediately (e.g. McDougall and Harrison, 1999). This windowof temperatures over which a gradual shift from total loss to total retention occurs is referredto as the partial retention zone (PRZ; McDougall and Harrison, 1999). Recently, Shuster et al.(2006), and Flowers et al. (2009) examined the e↵ect of radiation damage on the di↵usion ofHe in apatite, and found that the presence of radiation damage tracks (both alpha-ejection andfission tracks) in slowly cooled samples will act as ‘traps’, and initially increase He retentivityuntil the tracks are so numerous as to create an open channel to the edge of the crystal, atwhich point retentivity decreases. The amount of time spent in the radiation damage PRZ (arange of T similar to that of the He PRZ) depends on cooling rate and will a↵ect the formationand annealing of radiation damage (Flowers et al., 2009; Shuster et al., 2006). Using e↵ectiveuranium (eU = U + 0.235⇤Th), and parameters derived from fission track dating to modelannealing behaviour, the authors determined an e↵ective proxy for radiation damage and haveoutlined a dynamic Tc for ApHe, varying with total eU and cooling rate, ranging from 44 °C to116 °C (Shuster et al., 2006). Furthermore, this model has been incorporated into the thermalmodelling software HeFTy (Ketcham, 2005), where the software compares model ages based oneU and grain dimensions to measured ages in order to obtain cooling paths that are sensiblegiven user-input thermal constraints (Flowers et al., 2009).The closure temperature for the zircon U-Th/He (ZrnHe) system was originally determinedto range from 200-220 °C, but the authors also recognized that the Tc can vary over thelife of a crystal (Reiners et al., 2002). Wolfe and Stockli (2010) examined ZrnHe di↵usionbehaviour from a drill hole in Germany and determined that the system behaves similarly tolaboratory experiments, however more recent studies (Guenthner et al., 2013) have shown acomplex relationship between He di↵usion kinetics and radiation damage within zircon, similarto that in apatite. Radiation damage in zircon initially decreases di↵usivity (increases retention)within the crystal, until enough damage paths are formed that interconnected ‘fast-pathways’form for He to escape the crystal, at which point di↵usivity greatly increases (Guenthner et al.,2013). Instead of damage tracks creating ‘traps’ as is the case proposed for apatite by Shusteret al. (2006), Guenthner et al. (2013) have an alternative interpretation, where the radiationdamage act as ‘roadblocks’ that block otherwise fast c-axis di↵usion paths in zircon, forcing theHe into slower di↵usion paths. Thus, the tortuosity of He di↵usion paths increases with radiationdamage, until a level of interconnectedness allows fast He di↵usion (Guenthner et al., 2013).From this model, the ZrnHe system has an e↵ective Tc in the range of 140-220 °C (Guenthneret al., 2013). The authors also apply their di↵usion model to the thermal modelling softwareHeFTy, using fission-track annealing and eU to approximate radiation damage in zircon, inorder to determine geologically reasonable cooling paths (Guenthner et al., 2013).Helium di↵usion in apatite and zircon is most complex in slowly cooled samples that spenda long time in the He PRZ and/or radiation damage PRZ, and in samples with high eU (e.g.Flowers et al., 2009; Guenthner et al., 2013). The e↵ect of radiation damage is probably negligi-212.2. Geochronology and Thermochronologyble in young specimens that cooled quickly, and samples with low eU, but this will, nonetheless,be accounted for when modelling cooling paths using HeFTy.ProcedureSpecimens sent to UCSB (GG01A, GG10, GG12, DK14, DK15, and Gh17B) were crushedusing a Fritsch jaw crusher and sieved to 355µm. The <355µm portion was triple-rinsed beforebeing separated into heavy and light portions by hand gold-panning. The heavy portion wasfurther separated using MI, to extract zircon. The MI ‘lights’ were further separated to bet-ter isolate apatite using bromoform (⇠2.89 g/cm3 density). Specimens were then run 3 timesthrough a Frantz magnetic separator at 0.5, 1.0, and 1.5 amps, with a 15° sideways tilt anda 5° backwards tilt. Specimens GG01B, GG11, and GG13 underwent a similar process at theUniversity of Calgary. They were crushed using a Braun Chipmunk jaw crusher and a BICOpulverizer (disk grinder) before being run through a water table to separate into light, inter-mediate, and heavy portions. The water table ‘heavies’ were further separated using the samebromoform/MI process as above, before magnetic separation in the Frantz instrument.Typically apatite should appear in the non-magnetic portion of the separates, however inthis case this was only true for DK14. For the other specimens, apatite was mostly in the 1.0and 1.5 amp magnetic portions. This is not believed to be caused by the Frantz, but rather aresult of the nature of the crystals. One possibility is that they were not completely physicallyseparated from neighbouring magnetic crystals, and hence were ‘pulled’ into the wrong trackof the Frantz. Unusual composition is also a possibility; geochemical data is not available forindividual apatite grains.Separates containing apatite and zircon were hand-picked in petri dishes under a stereoscope.Desirable crystals had few to no inclusions, smooth and unchipped surfaces, two completeterminations, and every dimension >60µm. Not all of these requirements could always be metfor these samples, instead these factors were considered together to choose the best crystals. Allcrystals with one or more dimension <60µm were excluded, as this produces an alpha-ejectioncorrection that is unacceptably large (Farley et al., 1996), as discussed previously. Apatitecrystals with large and/or numerous inclusions were excluded, however, since small inclusions(even those containing U or Th) tend to have a negligible e↵ect on obtaining accurate datesin (Vermeesch et al., 2007), some crystals with small inclusions were used if they had otherdesirable qualities. Inclusion-free crystals with 0-1 terminations were, however, chosen over thosewith inclusions and 2 terminations, as this imperfection can be corrected for in alpha ejectioncalculations, whereas an inclusion with unknown composition cannot be corrected for withoutcompletely dissolving all crystals in HF (Vermeesch et al., 2007), an impractical practice formost apatite studies. Inclusion-free zircons are also preferred over those containing inclusions,however, this is mainly so the volume calculations are accurate and the di↵usion behaves asexpected, opposed to the possibility of parentless He as is the case with apatite.222.2. Geochronology and ThermochronologyFive specimens (GG01A, GG10, GG12, DK14, and Gh17B) contained apatite suitable foranalysis, and 4 specimens (GG01B, GG11, DK14, and Gh17B) contained usable zircons. An-notated digital photographs of the selected grains are in Appendix B-III and B-IV. Retrievingsuitable crystals from the heavy mineral separates was challenging because they contained lownumbers of apatite and zircon, and the majority were of poor quality. Crystals commonly con-tained multiple inclusions, were badly broken, or too small. However, it was possible to retrievea total of 29 suitable apatites from the 5 specimens, with relatively few, very small inclusions,and minimal surface irregularities or chips (Appendix B-IV). Similarly, a total of 18 suitablezircons across the 4 specimens were picked for analysis, using the same criteria as for apatite.Once suitable crystals were selected, they were measured and photographed, then placedby hand into small Nb tubes. Niobium tubes are used instead of the traditional Pt tubes toavoid interference during isotope dilution mass spectrometry. Ar added in the nebulizing processcontributes an atomic mass of 40, which, when added to Pt (atomic mass 195), can interferewith readings of 235U. Since Nb has an atomic mass of only 92, this interference is avoided.The capsules were crimped perpendicularly on each end to create an air pocket inside to avoiddamage to the crystal during transport. Three ‘hot blanks’ (empty Nb packets), and 4 Durangoapatite shards (for Ap) or Fish Canyon Tu↵ zircon (for Zrn) were placed in Nb packets to beused as monitors to measure any drift in base He readings during heating.Packets were then loaded into an ASI Alphachron He system at the University of Calgary,which uses a Prisma-Plus quadrupole mass spectrometer for He readings. After calibrating themachine to the packet locations and pumping down the vacuum, a sequence was set with hotblanks, standards, and line blanks (readings of 3He straight from tank) dispersed among thesamples to calculate drift during the analysis. Packets containing apatite were heated by thelaser for 5 minutes each, at 9 amps, after which a re-extraction was done to ensure all the Hewas released. Zircon packets were heated by the laser for 15 minutes each, at 12 amps, plus are-extraction.After He extraction was complete, apatite packets were loaded into vials and dissolved for4 hours in nitric acid along with a U-Th spike, a U-Th stock solution, and water. Ten stock-solution-only and 9 Sm-spike-vials were run interspersed with the packets to later calculatedrift due to evaporation. Since the Sm solution is based on concentration, it drifts over timeas water evaporates from the solution. The U-Th spikes on the other hand are based on theratio of U and Th in the solution, so as water evaporates the ratio remains constant even if theconcentration changes. The interspersed vials are analyzed using an Agilent 7700x quadrupolemass spectrometer. Data reduction for apatite was done with a series of spreadsheets, and anFT correction factor calculated based on total U and Th present in the crystal and its size andshape using a freely distributed alpha ejection program written by Gautheron et al. (2011).After degassing, zircon crystals were sent to the University of Colorado, Boulder, for digestionand isotope dilution. Zircon packets were placed in Savillex vials, to which a 235U/230Th spike232.2. Geochronology and Thermochronologyand 200 µL HF were added. The vials were heated at 220 °C for 72 hours, and after cooling,uncapped and dried on a hot plate. A second round of dissolution in HCl was then done toremove fluorine salts created during the first digestion. 200 µL of HCl were added to the vials,which were then baked at 200 °C for 24 hours before again cooling and drying. After a thirdround of dissolution in 200 µL of HNO3:HF mixture in an oven at 90 °C for 4 hours, samples,along with standards and blanks, were diluted with deionized water and analyzed by the ThermoElement 2 magnetic sector mass spectrometer (ICP-MS) at the Colorado University Institutefor Arctic Alpine Research. Data reduction and age calculations for zircon were done in customspreadsheets based on the methods of Ketcham et al. (2011).Cooling PathsSeveral geo-thermochronometers from the same locality can be used to determine the coolingpath of a package of rocks. If closure temperatures are known for each mineral system dated, andplotted against ages, a simple linear regression between points would outline a basic coolingpath. However, computer modelling may be used to quickly compute many more plausiblepaths, hence apatite and zircon thermochronologic data were entered into the thermal modellingsoftware HeFTy (Ketcham, 2005), along with temperature-time constraints from U-Th/Pb and40Ar/39Ar dating. The reverse-modelling function of the program was used, where HeFTy testedten thousand random cooling paths, calculating a model age for the entered grains (based ondimensions and eU), and comparing that to measured crystal ages to find a best-fit path thatmatches all ages. For apatite grains, the radiation damage accumulation and annealing modelparameters of Flowers et al. (2009) were used, and for zircon, the parameters of Guenthneret al. (2013).The HeFTy program requires specific U and Th abundances, as well as an e↵ective radiiand the uncorrected age from a single crystal of apatite and/or zircon. One cooling diagramwas created for each field site, using the zircon or apatite crystal that yielded a correctedU-Th/He age closest to that of the preferred mean age from the same location. The other,manually entered, age constraints used in modelling were the U-Th/Pb and 40Ar/39Ar agesfrom the same specimen where available, and from a neighbouring specimen otherwise. Coolingrates are determined graphically from the completed cooling path diagrams. The steepest andshallowest positive lines that can be fit between two constraints without exiting the envelopethat surrounds all good paths are the maximum and minimum cooling rates for that timeinterval, respectively (see section 3.3).24Chapter 3Results3.1 PetrologyPetrology ResultsField sampling locations are shown in Fig. 3.1. Field relationships at each site are highlightedin Figures 3.2, 3.3, and 3.4 and are detailed below. Point-counting of minerals in thin-sections(Figs. 3.5 and 3.6) was used to classify the specimens using the International Union of GeologicalSciences (IUGS) ternary diagram (Fig. 3.7). In this scheme, plagioclase with anorthite percent<5% is qualified as an alkali feldspar, hence scanning electron microscope analysis of plagioclasewas used to determine how much plagioclase should be classified as alkali feldspar. Grain-sizesmentioned in the following descriptions are as per Winter (2010), and mineral abbreviationsare after Whitney and Evans (2010).The outcrop at Ghami (Fig. 3.1) exposes ⇠1 m wide undeformed Mugu leucogranite dikesand dikelets that cross-cut the foliation of the host TSS (Fig. 3.2). Specimen GH17B is asample of one of the dikes. It is a medium-grained Qz + Kfs + Pl + Bt granite with 12.5%of its plagioclase in the An05 range (see QAP diagram Fig. 3.7). Feldspar in thin section ischaracterized by local development of perthitic texture (Fig. 3.5).Specimen collection at Ghar Ghompa was more extensive, with 6 specimens sampled fromMugu batholith material (Fig. 3.1). Specimens GG10, GG11, and GG12 were sampled fromoutcrops of the contact between the main body of the Mugu batholith and the TSS (Fig. 3.3).Specimens GG01A, GG01B, and GG13 are also assumed to be from the main body of theMugu batholith, however, their contact with the TSS was not observed. Specimen GG01A isa medium-grained Qz + Kfs + Pl + Ms + Bt granite (Fig. 3.7) in which feldspar exhibitsperthitic texture locally and minor sericitic alteration in the plagioclase. 40% of plagioclasefrom specimen GG01A has a composition of An05. GG10 is a fine-grained equigranular alkali-feldspar granite (Fig. 3.7) that contains Qz + Kfs + Pl + Ms + Tur ± Grt with 100% of theplagioclase in the An05 range. GG12 is a fine- to medium-grained Qz + Kfs + Pl + Ms + Turgranite (Fig. 3.7) with minor perthitic texture in the feldspar and minor sericitic alteration inthe plagioclase (Fig. 3.5). None of the plagioclase in GG12 is within the An05 range. No thinsections were made from specimens GG01B, GG11, and GG13, but from hand sample they areall identified as medium-grained granites senso lato, consisting of Qz + Kfs + Pl + Ms + Tur± Ms ± Grt. The TSS at Ghar Ghompa is locally metamorphosed to staurolite grade, and253.1. Petrologyappears to quickly decrease in grade with distance away from the granites.Both Mugu batholith and Mustang gneiss material crop out in the Dhanggna Khola area(Figure 3.1); TSS material was not present. At this location, badly weathered leucocratic or-thogneiss (DK16) is cross-cut by a vertical ⇠1+ m wide coarse-grained, undeformed granitedike, which in turn is cut by two near-horizontal, narrower (⇠15 cm wide) undeformed fine-grained leucogranite dikes (DK15; Fig. 3.4). DK15 is a fine-grained granite containing Qz + Kfs+ Pl + Ms + Tur and minor perthitic texture in some feldspar, with none of the plagioclasefalling in the An05 range. DK16 is a fine- to medium-grained porphyritic Qz + Kfs + Pl + Bt +Ms granitic orthogneiss (Fig. 3.7), with large Kfs porphyroclasts, and minor sericitic alterationseen in the plagioclase (Fig. 3.6). This specimen contains no plagioclase that falls in the An05range. Specimen DK16 is the only one collected in this study that exhibits deformation, whichis manifested in thin section in the form of sutured quartz grains with weak subgrain boundariesand aligned biotite flakes (Fig. 3.6). Specimen DK14 is a medium-grained to pegmatitic Qz +Kfs + Pl + Ms + Tur alkali-feldspar granite (Fig. 3.7), with 100% of the plagioclase falling inthe An05 range. This specimen is inequigranular and contains large Ms books and perthiticKfs that exhibits minor sericitic alteration (Fig. 3.6).263.1. PetrologyFigure 3.1: Sampling locations and respective U-Th/Pb (Mnz - minimum crystallization age),40Ar/39Ar (Ms & Bt - pleateau age), and U-Th/He (Ap & Zr - central ages) dates (in Ma)for each location. Details about di↵erent types of ages are discussed in section 3.2. Symbolshapes represent sampling location: triangle for Ghami, squares for Ghar Ghompa, and starsfor Dhanggna Khola. Symbol colouring is unique to each specimen and consistent throughoutthis document.273.1. PetrologyFigure 3.2: Outcrop at the Ghami location, showing leucogranitic Mugu dikes cross-cuttingthe bedding of the TSS.283.1. PetrologyFigure 3.3: Outcrop at the Ghar Ghompa location showing intrusive contact between the fine-grained granitic Mugu batholith and unmetamorphosed TSS. Red dashed lines outline the traceof bedding in the TSS.293.1. PetrologyFigure 3.4: Outcrop at the Dhanggna Khola location, showing two fine-grained leucogranitedikes horizontally cross-cutting both a larger coarse-grained granite vertical dike, and badlyweathered granitic orthogneiss. Hammer is approximately 30 cm long.303.1. PetrologyFigure 3.5: Representative digital photographs of thin-sections from the Ghami and GharGhompa sites. Mugu dike specimen GH17B (top) and Mugu specimen GG12 (bottom) in cross-polarized light, exhibiting typical mineralogy and textures seen in the granites.313.1. PetrologyFigure 3.6: Representative digital photographs of thin-sections from the Dhanggna Kholaarea. Mustang granite specimen DK14 (top) in crossed-polars, showing perthitic K-feldsparmegacryst and abundant tourmaline. Mustang gneiss specimen DK16 (bottom) in crossed-polars, exhibiting sutured quartz grain boundaries and weakly aligned biotite.323.1. PetrologyFigure 3.7: IUGS QAP diagram. Compositions determined by thin-section point-counting tech-nique.333.2. GeochronologyPetrology InterpretationsThe TSS is locally metamorphosed to staurolite-grade at Ghar Ghompa, which appears tobe due to contact metamorphism, as the grade shows a relationship with proximity to the Mugubatholith. No other evidence of TSS metamorphism was observed in this study, contrary to theobservations of Hurtado (2002), who, in the same area, identified a unit classified as a blackfine-grained gneiss.Eight of the ten specimens collected are from Mugu batholith material (Fig. 3.1). Thesespecimens are all fine- to medium-grained undeformed granites or alkali feldspar granites (Fig.3.7), consisting of Qz + Fsp ± Ms ± Bt ± Tur. The remaining specimens (DK14 & DK16),are interpreted as belonging to a separate granitic body, commonly called the Mustang granite,which is observed in the field to have both orthogneiss and granite components.The Mugu batholith is an undeformed granite broadly consistent with other Oligocene-Miocene Himalayan plutonic rocks, interpreted as S-type granites (Deniel et al., 1987; Le Fortet al., 1987; Searle et al., 2009). In contrast to previous work, which interpreted it to be adeformed Himalayan leucogranite, the Mustang intrusion as investigated in this study is con-sidered a pervasively deformed orthogneiss. An undeformed, cross-cutting pegmatitic granitesampled from within the Mustang orthogneiss (specimen DK14) could represent a late-stagemelt related to the metamorphism of the body. A more detailed structural and geochemicalstudy focusing on the Mustang gneiss is needed in order to properly investigate the relation-ship between the orthogneiss, the pegmatitic granite, and their respective contacts with thesurrounding TSS, which as of yet, has not been observed.3.2 GeochronologyThree geochronologic systems were used to date the crystallization and cooling of the Mugubatholith and Mustang gneiss; U-Th/Pb in monazite, 40Ar/39Ar in muscovite and biotite, andU-Th/He in zircon and apatite. All ages are plotted with their respective sampling locations inFig. 3.1, and are detailed in the following sections.3.2.1 U-Th/Pb in MonaziteU-Th/Pb Geochronology Results208Pb/232Th monazite ages are shown in weighted mean diagrams below (Figs. 3.8, 3.9, and3.10). Box heights are 2. Plots and ages were generated using the program Isoplot (Ludwig,2003). The 208Pb/232Th system is preferred as monazite commonly contain weight% Th and itavoids potential complications of unsupported 206Pb derived from the decay of 230Th (Scha¨rer,1984).343.2. GeochronologyForty monazite crystals from specimen GH17B were dated (Fig 3.1) yielding a narrow rangeof ages from 21.1 ± 0.8 Ma to 25.0 ± 1.1 Ma (Fig. 3.8).Three specimens were dated from the Ghar Gompa location (GG01A, GG10, and GG12;Figs. 3.1 and 3.9). The 40 monazite grains dated from GG01A yield a range of ages from 20.7± 0.8 Ma to 85.4 ± 8.5 Ma, while specimen GG10 yielded a range of monazite ages from 18.9± 1.1 Ma to 46.1 ± 6.0 Ma, over the same number of grains. 40 monazite grains from specimenGG12 were also analyzed, with ages ranging from 18.1 ± 0.9 Ma to 78.4 ± 18.3 Ma.Three more specimens were dated from the Dhanggna Khola location (Fig. 3.1). SpecimenDK16, a sample of the Mustang orthogneiss, and yields a wide range of ages from 24.3 ± 1.6Ma to 535.1 ± 35.0 Ma over 39 grains analyzed (Fig. 3.10). DK14 is a medium-grained topegmatitic dyke. It yields a wide range of ages over the 40 monazite grains analyzed, from 23.7± 0.9 Ma to 488.8 ± 18.2 Ma. Finally, monazite from specimen DK15, a Mugu-anity dikethat cross-cuts the foliation of the Mustang orthogneiss, yields a range of monazite ages from21.3 ± 0.9 Ma to 38.7 ± 2.0 Ma over the 40 grains dated.353.2. GeochronologyFigure 3.8: Weighted mean diagram of U-Th/Pb monazite dates from Ghami. Interpretedminimum crystallization age in inset, uncertainty indicated by width of black box.363.2. GeochronologyFigure 3.9: Weighted mean diagram of U-Th/Pb monazite dates from Ghar Ghompa. Inter-preted minimum crystallization age in inset, uncertainty indicated by width of black box.373.2. GeochronologyFigure 3.10: Weighted mean diagram of U-Th/Pb monazite dates from Dhanggna Khola.Interpreted minimum crystallization age in inset, uncertainty indicated by width of black box.383.2. GeochronologyU-Th/Pb Geochronology InterpretationsFollowing Lederer et al. (2013), and Larson et al. (2017), minimum crystallization ages areinterpreted as the weighted mean age of the youngest subjectively significant monazite agepopulation subset for a specific specimen (Fig. 3.8 - 3.10). Following this procedure, the Mugugranite specimen from Ghami (GH17B), yields an interpreted minimum crystallization age of21.58 ± 0.24 Ma, (MSWD = 0.39; Fig. 3.8).Three specimens of the Mugu granite from the Ghar Ghompa area were dated (Fig. 3.1).Specimen GG01A has an interpreted minimum crystallization age of 21.23 ± 0.16 Ma, (MSWD= 0.84), GG10 an age of 20.30 ± 0.23 Ma (MSWD = 1.07), and GG12 an age of 20.90 ± 0.19Ma (MSWD = 0.61). Together, these specimens yield a combined weighted mean minimumcrystallization age of 20.40 ± 0.22 Ma. (Fig. 3.9).At the Dhanggna Khola site, specimen DK14, a medium-grained to pegmatitic granitethought to be of Mustang anity, yields a minimum crystallization age of 26.06 ± 0.30 Ma(MSWD = 1.2). Specimen DK15, from the fine-grained Mugu-anity dike that cross-cuts thefoliation of the Mustang orthogneiss, yields an age of 22.23 ± 0.37 Ma (MSWD = 0.98). Spec-imen DK16, collected from the orthogneiss itself, yields a minimum age of 24.92 ± 0.68 Ma(MSWD = 0.37), which is interpreted to reflect recent metamorphism. Specimens DK14 andDK16 both contain a significant older population, ranging from ⇠350 to ⇠500 Ma.Monazite U-Th/Pb geochronology from all Mugu and Mustang granite specimens yield lateOligocene- early Miocene age populations, which are interpreted as crystallization ages in thegranite specimens and as a metamorphic age in the gneiss specimen (Figs. 3.8 - 3.10). The olderpopulations reported in both DK14 and DK16 from the Dhanggna Khola site (Fig. 3.10) mayprovide further information on these specimens. The oldest population, which ranges from 445± 25 Ma to 535 ± 35 Ma in specimen DK16 is interpreted to represent xenocrystic materialthat records the age of the igneous protolith. A similar range of ages in DK14, from 447 ±13 Ma to 489 ± 18 Ma, is likely inherited from DK16 as a product of partial melting duringmetamorphism. Intermediate ages, between 109 ± 9 and 445 ± 25 Ma in specimens DK16 andDK14 may reflect partial lead-loss from the xenocrystic grains and/or sampling of multipledomains. The younger of the pre-crystallization populations, ranging from 30 ± 2 Ma to 53 ±3 Ma and appearing in both specimens DK16 and DK14, might represent the earliest stagesof Himalayan metamorphism during initial underplating and burial of the underriding Indianplate, as other authors have dated an early stage of metamorphism across in the Annapurna-Dhaulagiri region at this time. Larson and Cottle (2015) interpreted structural deformation inthe Annapurna region to be coeval with metamorphism in the GHS, occurring as early as ⇠48Ma, dated by 232Th/208Pb in monazite.The undeformed Mugu granite dike (DK15) provides the final crystallization age for theDhanggna Khola site. Therefore, the age of this specimen was used as the upper temperature-time constraint in the cooling paths created for this site.393.2. Geochronology3.2.2 40Ar/39Ar Thermochronology in Mica40Ar/39Ar Thermochronology Results40Ar/39Ar step-heating results are presented in Figs. 3.11 - 3.13. Calculated 40Ar/39Ar agesand closure temperatures are reported in Table 3.1. All specimens analysed returned relativelyflat age spectra that define a narrow range of dates from 15.7 ± 0.1 Ma to 17.2 ± 0.1 Ma,averaging 88.0% 39Ar released. 40Ar/39Ar isotopic data are detailed in Appendix C.403.2. GeochronologyTable 3.1: 40Ar/39Ar plateau ages and corresponding closure temperatures.413.2. GeochronologyFigure 3.11: 40Ar/39Ar age spectra diagram for Ghami location. Symbol corresponds to locationon Fig. 3.1.423.2. GeochronologyFigure 3.12: 40Ar/39Ar age spectra diagrams for Ghar Gompa site. Symbols correspond tolocations on Fig. 3.1.433.2. GeochronologyFigure 3.13: 40Ar/39Ar age spectra diagrams for Dhanggna Khola site. Symbols correspond tolocations on Fig. 3.1.443.2. Geochronology40Ar/39Ar Thermochronology InterpretationsSince 40Ar/39Ar ages are consistently younger than the corresponding U-Th/Pb monaziteages, they are all interpreted as cooling ages. In each specimen, the well-defined plateau and theapparent simplicity of the cooling history precluded any need for analysis of isochron diagrams.Although the ‘saddle’-shape in specimen DK14 likely indicates the presence of excess Ar at grainmargins (Lanphere and Dalrymple, 1976), the good agreement in age with the other specimensfrom the region indicates the middle portion of the spectra yields a meaningful age.3.2.3 U-Th/He Thermochronology in Zircon and ApatiteZircon and Apatite U-Th/He Thermochronology ResultsCalculated ages, U and Th content, e↵ective U, and e↵ective radii for individual zircon andapatite grains are summarized in Table 3.2. Complete elemental data for zircon and apatite U-Th/He analysis is available in Appendix C. Photographs and grain measurements of all zirconand apatite grains selected for analyses are available in Appendix B-IV and B-V.453.2. GeochronologyTable 3.2: U-Th/He age, U and Th concentrations, eU, and e↵ective radii (eR) for individualzircon and apatite grains.463.2. GeochronologyZircon U-Th/He Thermochronology InterpretationsSince all U-Th/He ages are younger than their corresponding 40Ar/39Ar ages, they areconsidered valid cooling ages. Zircon and apatite U-Th/He thermochronologic analyses yieldoverdispersed cooling ages, therefore, to aid in interpretation, grain data have been plottedusing both isochron (Vermeesch, 2008) and logratio plots (Vermeesch, 2010). Where available,the central age (bold text in Figs. 3.14 - 3.18) is the preferred age for such overdispersed data. Ingeneral, the central age method will provide greater confidence in any geological interpretationbased on the data, as it is a more accurate representation of the true age than is an arithmeticmean (Vermeesch, 2008). An exception to this is the zircon cooling data from the Ghamilocation, where only 2 crystals yield successful analyses, and the preferred age is thus thegeometric mean. Ellipse colouring in the logratio plots (Figs. 3.14 - 3.18) indicates the e↵ectiveuranium (eU = U + 0.235⇤Th in ppm) concentration in each crystal, allowing for a qualitativecheck for possible age bias due to alpha-ejection track concentrations (Shuster et al., 2006).The isochron diagrams show a ‘pooled age’ (red line, Figs. 3.14 - 3.18) of all successful crystalsfrom that location (Vermeesch, 2008).Zircons GH17-2, GH17-6, and GG14-1 were lost during the isotope dilution process. In ad-dition, zircons GG01B-3, GG11-1 and GG11-2 released negligible He, and thus gave no mean-ingful results. These three grains yielded a reasonable amount of U and Th during chemistryand, therefore, the problem must solely lie with He data collection. The most likely cause ofthis failure is an error in the sequence in which the laser fired, resulting in the laser reheatingan already degassed crystal, or moving to a hot-blank instead of a new packet location. Theremaining successful zircons were numerous enough to obtain meaningful age data from eachlocation.Four successful zircon crystals from the Mugu granite dike (GH17B) at the Ghami sitedefine a central age of 10.31 ± 1.78 Ma, and an isochron age of 18.3 ± 1.2 Ma (Fig. 3.14). They-intercept of the ‘pooled age’ line (Vermeesch, 2008) on the isochron diagram of -139 ± 21 isfar from the expected value of 0, which indicates that the age obtained by the isochron methodis unreliable. The central age from the logratio plot is the preferred age for this location andis more reasonable given the 16.1 ± 0.1 Ma muscovite 40Ar/39Ar age from the same specimen.The random arrangement of eU concentration in Fig. 3.14 indicates no age bias for the Ghamizircon specimens.Two zircons picked from Ghar Ghompa Mugu granite (grains GG01B-1 and GG11-3) yieldedsuccessful and reasonable U-Th/He ages. Ages from the logratio and isochron plots are inagreement within error (Fig. 3.15), at 11.27 ± 0.32 and 10.81 ± 0.27 Ma, respectively. Theisochron age may be reliable, as indicated by the y-intercept value of 2.0 ± 0.9, however, thismethod is strongly a↵ected by the low number of data. As there are only two crystals, TheHelioplot program cannot calculate a central age on the logratio plot, so instead a geometricmean is used as the preferred zircon U-Th/He age at this location. The lack of data from this473.2. Geochronologylocation also precludes any useful interpretation of age bias in the logratio plot based on eUconcentrations. Zircon GG01B-2 yields an anomalously old U-Th/He age of 43.57 ± 3.58 Mabefore alpha-ejection correction, and is thus discarded as it indicates a cooling age greater thanthe crystallization age from the same specimen.Five zircons from from the Dhanggna Khola pegmatitic Mustang granite (specimen DK14)were analyzed. The central age (12.39± 3.76 Ma) and isochron age (16.31± 0.21 Ma) marginallyoverlap within uncertainty, however, the isochron age is interpreted as less reliable due to thelow y-intercept value of -11.8 ± 0.7, thus the central age is again the preferred age for thislocation (Fig. 3.16). Ellipse colouring indicates no clear age bias based on eU concentration.483.2. GeochronologyFigure 3.14: Zircon U-Th/He dates from Ghami site. A) Logratio plot. Ellipses indicate indi-vidual ages and associated errors. Ellipse colouring indicates eU content. Preferred age in boldtype. B) Isochron diagram. Y axis P value is a function of U and Th concentration (Vermeesch,2008).493.2. GeochronologyFigure 3.15: Zircon U-Th/He dates from Ghar Gompa site. A) Logratio plot. Ellipses indicateindividual ages and associated errors.Ellipse colouring indicates eU content. Preferred age inbold type. B) Isochron diagram. Y axis P value is a function of U and Th concentration(Vermeesch, 2008).503.2. GeochronologyFigure 3.16: Zircon U-Th/He dates from Dhanggna Khola site. A) Logratio plot. Ellipsesindicate individual ages and associated errors. Ellipse colouring indicates eU content. Preferredage in bold type. B) Isochron diagram. Y axis P value is a function of U and Th concentration(Vermeesch, 2008).513.2. GeochronologyApatite U-Th/He Thermochronology InterpretationsThe specimen of the dike (GH17B) collected from the Ghami site contained no useableapatite crystals.Apatites GG12-2 and GG12-6, retrieved from the Ghar Ghompa specimen GG12, releasednegligible He, leading to a geologically meaningless 0 Ma age. Apatite GG12-2 also yieldedlow U and Th concentrations, 2 or more orders of magnitude lower than crystals that gavegeologically reasonable ages, indicating perhaps that this was a non-U-bearing mineral pickedby mistake. The lack of complete terminations and otherwise poorly defined crystal shape ofGG12-2 are consistent with that interpretation (Appendix B-IV). Apatite GG12-6, however,contained U and Th concentrations in a similar range to the successful crystals. This lattersituation would be consistent with a crystal that fell out of the Nb packet, preventing fromproper degassing, but that remained in the vial so was still dissolved during digestions. A morelikely possibility is that an error was made when preparing the laser sequence, and instead ofvisiting the desired crystal site, the laser instead re-fired on an already degassed packet, or a hotblank, and hence returned no He. The 15 analyses that yield reasonable ages from all granitespecimens containing useable apatites from the Ghar Ghompa site (specimens GG01A, GG10,and GG12) are combined into logratio and isochron diagrams (Fig. 3.17). The central age (7.84± 1.25 Ma) and the isochron age (11.91 ± 0.26 Ma) do not overlap within uncertainty, anddue to the non-zero y-intercept value of -1.53 ± 0.11 and the loose grouping of ages aroundthe pooled age line in the isochron diagram, the central age is the preferred apatite U-Th/Heage for this location. Ellipse colouring reveals no age bias based on eU content of the crystalsanalyzed.Eleven successful apatite ages were obtained from granite specimens DK14 and DK15 fromthe Dhanggna Khola site. They yield overlapping central and isochron ages at 5.63 ± 0.78 and5.04 ± 0.06 Ma, respectively. Despite the very good fit on the isochron diagram (y-intercept0.10 ± 0.01), the central age is again taken as the preferred age at this location due to themore conservative error calculated by this method (Fig 3.18). There is no apparent correlationbetween eU and age at this location. Apatite DK15-6 yields an anomalously old U-Th/Heage of 25.9 ± 0.58 Ma before alpha-ejection corrections. This age is discarded because it isunreasonable to expect an apatite cooling age greater than the crystallization age from thesame specimen.523.2. GeochronologyFigure 3.17: Apatite U-Th/He dates from Ghar Gompa site. A) Logratio plot. Ellipses indicateindividual ages and associated errors. Ellipse colouring indicates eU content. Preferred agein bold type. B) Isochron diagram. Y-axis P value is a function of U and Th concentration(Vermeesch, 2008).533.2. GeochronologyFigure 3.18: Apatite U-Th/He dates from Dhanggna Khola site. A) Logratio plot. Ellipsesindicate individual ages and associated errors. Ellipse colouring indicates eU concentration.Preferred age in bold type. B) Isochron diagram. Y-axis P value is a function of U and Thconcentration (Vermeesch, 2008).543.2. Geochronology3.2.4 Summary of Geochronologic ConstraintsA summary of radiometric ages is presented in Figure 3.19. This diagram includes all apatite,zircon, muscovite, and biotite cooling ages from this study, as well as the subset of monazite agesthat were selected to represent the minimum crystallization age for each Mugu batholith-relatedspecimen. As the Mustang orthogneiss and granite specimens (DK16 and DK14, respectively)are significantly older than the cross cutting Mugu dike at the Dhanggna Khola location (DK15),their monazite ages are not considered representative of the final crystallization in that areaand hence are not included in the figure.553.2. GeochronologyFigure 3.19: Age summary diagram. Symbols and colours correspond to sampling locations(Fig. 3.1). Mineral systems indicated by bars on right axis. Only monazite analyses used forminimum crystallization age determination shown. All other mineral specimens plotted. Errorbars are 2.563.3. Cooling Paths3.3 Cooling PathsThree cooling path diagrams (Fig. 3.20 - 3.22) were created using the thermal modellingsoftware HeFTy (Ketcham, 2005), one for each location previously detailed. These models areused to interpret the combined thermochronology data presented in the previous sections.Temperatures used for the thermal constraints include 700 ± 50 °C for U-Th/Pb ages, basedon typical crystallization temperatures for Himalayan leucogranites (Copeland et al., 1990;Ayres and Harris, 1997; Visona` and Lombardo, 2002) and calculated closure temperatures forthe mica used in a particular model (Table 3.1). HeFTy requires grain-specific U-Th/He inputdata for zircon and apatite in order to calculate cooling paths that are compatible with alphaejection and radiation damage modelling (Shuster et al., 2006; Flowers et al., 2009; Guenthneret al., 2013). These factors are based on grain size and eU, hence individual grains were selectedto represent each location. The zircon and apatite grains that had an U-Th/He age closest to thecentral age from their respective sites were chosen to represent each location. These grains arebolded in Table 3.2. The lowest temperature thermal constraint used in HeFTy was estimatedat 10 ± 10 °C based on multiple modern-day yearly averages for the Upper Mustang region.Cooling rates are estimated between the weighted mean path inflection points on the coolingpath diagrams, by averaging the highest and lowest straight-line slopes that are entirely withinthe ‘good’ (pink) path envelope between the two constraint boxes.The cooling path from Ghami (Fig. 3.20) was created using the crystallization age for thislocation (Fig. 3.8), the biotite 40Ar/39Ar age and associated closure temperature from specimenGG17B (Table 3.1), and zircon data from grain GH17B-4 (Table 3.2), as its individual age wasclosest to the central age for the specimen. This location has no apatite data, which is the maincontributor to the wide range of ‘good’ and ‘acceptable’ paths found during modelling. At thislocation there is a steady decrease in cooling rate with time, most notable in the slope changesof the weighted mean path (bold blue line). The cooling rate from 22 Ma to 16 Ma is 61.3 ±10.0 °C/Ma, decreasing to an indistinguishably lower rate of 37.5 ± 18.2 °C/Ma from 16 Ma to10 Ma, and again to a distinctly lower rate of 15.7 ± 1.1 °C/Ma from 10 Ma to present.The cooling path for Ghar Ghompa (Fig. 3.21) was created using the crystallization age ofspecimen GG10 (Fig. 3.9), the muscovite 40Ar/39Ar age and associated closure temperature ofspecimen GG11, the zircon data from grain GG11-3, and apatite data from grain GG10-1. Theinitial cooling rate at this location is 61 ± 18 °C/Ma from 20 Ma to 17 Ma, which changes toan indistinguishably lower rate of 51.1 ± 9.6 °C/Ma from 17 Ma to 9 Ma. This rate furtherdecreases after the zircon U-Th/He constraint to 22.3 ± 13.0 °C/Ma from 9 to 6.5 Ma, andagain to 4.4 ± 4.4 °C/Ma from 6.5 Ma to present.The cooling path for Dhanggna Khola (Fig. 3.22) was created using the minimum crystal-lization age for that location (DK15; Fig. 3.10), the muscovite 40Ar/39Ar age and associatedclosure temperature of specimen GG15, the zircon data from grain GG14-5, and apatite data573.3. Cooling Pathsfrom grain GG15-2 (Table 3.2). The initial cooling rate at this location is 45 ± 11 °C/Ma from22 Ma to 17 Ma, increasing indistinguishably within error to 74.9 ± 19.6 °C/Ma from 17 Mato 12 Ma, then decreasing to 13.0 ± 4.4 °C/Ma from 12 Ma to 7 Ma, and decreasing again toan indistinguishable rate of 6.7 ± 0.8 °C/Ma from 7 Ma to present.583.3. Cooling PathsFigure 3.20: HeFTy-derived cooling paths of specimens from Ghami site. Bold line is weightedmean path. (Upper) Pink paths are considered ‘good’, green ‘acceptable’. (Lower) Envelopessurrounding all ‘good’ and ‘acceptable’ paths.593.3. Cooling PathsFigure 3.21: HeFTy-derived cooling paths of specimens from Ghar Ghompa site. Bold lineis weighted mean path. (Upper) Pink paths are considered ‘good’, green ‘acceptable’. (Lower)Envelopes surrounding all ‘good’ and ‘acceptable’ paths.603.3. Cooling PathsFigure 3.22: HeFTy-derived cooling paths of specimens from Dhanggna Khola site. Bold lineis weighted mean path. (Upper) Pink paths are considered ‘good’, green ‘acceptable’. (Lower)Envelopes surrounding all ‘good’ and ‘acceptable’ paths.61Chapter 4Discussion and Conclusions4.1 E-W extension in the Thakkhola grabenThe initiation of brittle E-W extension related to the Thakkhola graben was previously esti-mated to be ⇠14 Ma (Coleman and Hodges, 1995), based on dating of hydrothermal muscovitein a ⇠N-S striking, vertical brittle structure, ⇠40 km east of the Dangardzong fault. Garzioneet al. (2000) later dated the Tetang formation of the Thakkhola graben using magnetic polaritymeasurements and sedimentation rates, and determined that the oldest sediments in the grabenwere between ⇠11 and ⇠9.6 Ma.Preliminary microstructural data from the metamorphosed TSS at the margin of the Mugubatholith in the Ghar Ghompa area (Fig. 3.1) shows ductile, top-to-the-east sense shear (Larsonet al., Prep). Moreover, quartz c-axis-derived deformation temperatures from the same rocksoverlap muscovite closure temperatures calculated in this thesis (Larson et al., Prep), includingthat of specimen GG10 (473 ± 32 °C; Table. 3.1), which is spatially the closest granite specimento the metamorphic specimens used for quartz fabric analysis. These data indicate that ductileE-W extension in the region was ongoing by at least 16.7 ± 0.2 Ma.Other regions of the Himalaya and Tibet also record the initiation of E-W extension inthe early Miocene. Cooper et al. (2015) document that E-W extension in the Yadong Crossstructure (YCS) was ongoing by ⇠14 Ma (Fig. 4.1a), Mitsuishi et al. (2012) interpreted ductileE-W extension to have initiated with granite emplacement at ⇠19 Ma in the Kung Co areaof southern Tibet, and Murphy and Copeland (2005) provided evidence of E-W extensioncontemporaneos with N-S shortening in the Gurla Mandhata, and date movement on structuresthat accommodate the E-W component to 15 Ma (Fig. 4.1a). Moreover, in NW India, Thiedeet al. (2006) interpret rapid exhumation of the Leo-Pagril dome beginning at ca. 16 Ma to reflectthe initiation of E-W extension. While some have argued against these previous estimatesof a mid-Miocene age of E-W extension in the Himalaya and Tibet, due to the ambiguousrelationship between N-S striking dikes and veins that have been dated, which may also becaused by N-S compression (e.g. Mahe´o et al., 2007), the new data from this work explicitlydates E-W extension in the Thakkhola region to a minimum of 16.7 ± 0.2 Ma.In addition to providing information on the initiation of the Thakkhola graben, this studyalso presents the first low temperature (zircon and apatite U-Th/He) cooling constraints fromthe Upper Mustang region. These new data, along with the 40Ar/39Ar and U-Th/Pb ages624.1. E-W extension in the Thakkhola grabenpresented, facilitate the computation of complete cooling paths previously only possible be-tween higher temperature (40Ar/39Ar and U-Th/Pb) constraints (e.g. Hurtado, 2002; Vannayand Hodges, 1996). The cooling paths developed can be used as a proxy to investigate theexhumation of the region if cooling is assumed to be driven by exhumation.Since movement on the STDS in the Annapurna/Mustang region had ceased by ⇠22 Ma(Godin et al., 2001), cooling rates determined in this study are inferred to relate directly toexhumation driven by E-W extension. The cooling rates calculated at the three locations inthis study (Ghami, Ghar Ghompa, Dhanggna Khola; Fig. 3.1) are steady from ⇠22 to ⇠13Ma, which is demonstrated by the indistinguishable slopes in the monazite-to-mica (55 ± 7°C/Ma average) and mica-to-zircon (58 ± 9 °C/Ma average) sections of the cooling paths (Figs.3.20 - 3.22, 4.1a). A distinct change in cooling rates at all three field locations between ⇠13and ⇠8 Ma (16 ± 1 °C/Ma average) marks a slowing in the cooling rate and likely relatedE-W extension at that time (Figs. 3.20 - 3.22, 4.1b). This slower cooling rate could be linkedto 1) the initiation of brittle extension within the Thakkhola graben, or 2) the initiation ofextensional structures elsewhere in the Himalaya and the Tibetan plateau that accommodateE-W extension, partitioning deformation away from Thakkhola region at this time.The age of the oldest sediments found in the Thakkhola graben are between ⇠11 and ⇠9.6Ma (Garzione et al., 2003). If these sediments are truly the first deposited in the graben, thereis a possibility that brittle E-W extension in the region only began at this time. However, therapid cooling rates from ⇠22 Ma to ⇠13 Ma (Figs. 3.20 - 3.22) suggest tectonic exhumationwas already active at this time, and although the oldest exposed sediments preserved are from⇠11 Ma, they may not be the oldest sediments generated during graben formation.The second possibility is that the change in cooling rate in the Thakkhola graben is dueto initiation of E-W extension structures elsewhere in the Himalaya and Tibetan plateau. Theinitiation of graben structures in southern Tibet is shown in Figure 4.1 and summarized in Table4.1. Eleven studies document the regional initiation of ductile and brittle E-W extensionalstructures at ⇠13-8 Ma (Table 4.1) at 8 di↵erent locations (Fig. 4.1b). Another 4 studiesdocument a second period of extension in brittle structures from the same locations at ⇠6-4Ma (Table. 4.1; Fig. 4.1c). The development of extensional graben in the plateau increasesthrough time with some evidence for progressive south-to-north development (Fig. 4.1).Previous authors have recognized a multi-stage history for the development of E-W extensionin Tibet. Kali et al. (2010) recognize a period of ductile E-W extension in the Ama Drime regionat ⇠12 Ma, and a later initiation of related brittle extension in the same area at ⇠6-4 Ma (Table4.1). Northwest of the Kung Co rift, in the Tangra Yum Co rift (Fig. 4.1b), Dewane et al. (2006)outline two periods of E-W extension based on cooling path inflections at ⇠13 Ma and ⇠6 Ma.Ratschbacher et al. (2011) provide a summary of rifting in southern Tibet and show that earlyductile E-W extensional structures consistently initiate at ⇠13-8 Ma, coincident with a decreasein the cooling rate of the Thakkhola graben, while a second population of brittle E-W structures634.1. E-W extension in the Thakkhola grabeninitiate around ⇠4-5 Ma, the same time as the second inflection point in the Thakkhola coolingcurves (Figs. 3.20 - 3.22).There is, thus, regional evidence that the inflection points in the cooling history of theDangardzong footwall in this study (⇠13-8 Ma and ⇠5 Ma; Figs. 3.20 - 3.22) are related tothe progressive initiation of E-W extension elsewhere in the Himalaya and Tibetan plateau.Although it is not possible to uniquely identify the source of the shift of cooling rate in theThakkhola graben from the data presented herein, the inflection points at ⇠13-8 Ma and ⇠5Ma (Figs. 3.20 - 3.22) add to a growing body of evidence consistent with a major shift inorogen-wide kinematics took place at this time (e.g. Cottle et al., 2009; Leloup et al., 2010;Zhang and Guo, 2007).644.1. E-W extension in the Thakkhola grabenFigure 4.1: Cooling paths (this study) and E-W extensional structures (modified from Styronet al., 2011). Study area is indicated by the star. Left: weighted mean cooling paths for thethree field locations from this study, with time period of interest highlighted in red. Right: mapof the Himalaya and Tibetan plateau indicating E-W extensional structures active in di↵erenttime periods (red lines). Abbreviations: AD - Ama Drime massif, GB - Gyirong basin, GM -Gurla Mandhata dome, LP - Leo Pagril dome, LU - Lunggar rift, SH - Shuang Hu graben, TG- Thakkhola graben, TY - Tangra-Yumco rift, XD - Xainza-Dinggye rift, YC - Yadong Crossstructure, YG - Yadong-Gulu rift.654.1. E-W extension in the Thakkhola grabenTable 4.1: Location and age of various N-S trending brittle and ductile structures in the Hi-malaya and Tibetan plateau.664.2. Potential sources of kinematic shift4.2 Potential sources of kinematic shiftWhile there is considerable variation in the timing of movement on the STDS along theHimalaya (e.g. Cottle et al., 2007) at most locations in the western Himalaya movement hadceased by at least 19 Ma (Liu et al., 2017). The STDS in the Annapurna-Dhaulagiri Himalaya,a region which extends over the southern extent of the Thakkhola graben, has been studied indetail and is believed to have ceased by ⇠22 Ma (Godin et al., 2001). Locations in the easternHimalaya show a much younger cessation of movement on the STDS, perhaps as young as ⇠11-13 Ma at Wagye La and Khula Kangri (Edwards and Harrison, 1997; Kellett et al., 2009; Wuet al., 1998). While the cessation of movement on the STDS in the Mustang region (Annapurna-Dhaulagiri) predates an approximately linear cooling rate between the monazite U-Th/Pb andzircon U-Th/He constraints (this study, Figs. 3.20 - 3.22), the end of STDS movement farthereast in the Himalaya is contemporaneous with the inflection point on the same cooling curvesbetween ⇠13 and ⇠8 Ma (Edwards and Harrison, 1997; Kellett et al., 2009; Wu et al., 1998),and with the initiation of numerous E-W extensional structures throughout the Himalaya andthe Tibetan plateau discussed above.Several authors have recognized a major kinematic shift in the Himalaya during middleMiocene, however, the cause of this shift is unknown. There is a transition in S-directed thrust-ing at this time, moving from the MCT farther into the farland and the MBT (e.g. Meigset al., 1995), which is believed to be linked to increased intensity of the Asian monsoon (Sunand Wang, 2005) and Himalayan erosion (Clift et al., 2008). In addition, Cottle et al. (2009)suggest activity on the faults bounding the Ama Drime massif in the middle Miocene reflecta shift from S-directed to orogen parallel mid-crustal flow. That is compatible with data fromZhang et al. (2004) who document present-day eastward ‘escape’ of crustal material in easternTibet via global positioning system data, the initiation of which has been dated to ⇠15-10 Ma(Kirby et al., 2002). Some authors further interpret the current eastward movement of surfacematerial to reflect partial coupling with an eastward flowing lower-crust (Clark and Royden,2000; Royden et al., 1997). Finally, Leloup et al. (2010) suggest a link between cessation ofmovement on the STDS, the transition of thrusting from the MCT to the MBT, the initiationof E-W dominated extension, and a change in convergence direction and rate between Indiaand Asia. Large scale tectonic movements such as a change in convergence rate or directioncould trigger a multitude of complex kinematic adjustments within an orogen. While the exactcause and e↵ect relationships of the above phenomena are unknown, the temporal similaritybetween them are suggestive of a genetic link. Further work is required to parse exactly whatthe dominant driving forces may be.674.3. Conclusions4.3 ConclusionsThis study presents the first low-temperature (U-Th/He) constraints for the Thakkholagraben, and together with 40Ar/39Ar dating of muscovite and biotite, and U-Th/Pb dating ofmonazite, outline a detailed cooling history in the footwall of the Dangardzong fault. Coolingrates in the footwall show rapid cooling from ⇠22 Ma to ⇠13 Ma, and two inflection pointsindicating a decrease in cooling rate at ⇠13-8 Ma and again at ⇠5 Ma (Figs. 3.20 - 3.22).These inflection points are interpreted to correspond to decreases in E-W extension-drivenexhumation in the Thakkhola graben at these times, which may be related to the initiation ofE-W extensional structures elsewhere in the orogen that partition strain away from the graben(Fig. 4.1). The development of such structures coincides with significant changes in the regionalkinematics of the orogen.4.4 Future Work1) This study has revealed that the plutonic body in the Upper Mustang region, commonlyreferred to as the Mustang granite, is in fact a granitic orthogneiss with a Paleozoic protolithand a Himalayan metamorphic and anatectic overprint. Further field-based research is needed inorder to investigate the contact between the orthogneiss and the surrounding TSS. This contactmay be structural and represent an exposure of the STDS similar to that in culminations alongthe North Himalayan anticline.2) Understanding of the relationship between the thermal evolution of rocks in the up-permost Himalaya and E-W extensional structures in the Tibetan plateau may benefit fromfurther detailed thermochronologic studies, based on rocks from E-W extensional zones in boththe plateau and the Himalaya. Many of the studies mentioned in previous sections that attemptto date the onset of E-W extension use only one chronometer. By applying multiple chronome-ters, as done in this study, the evolution of cooling rates across Tibet may be better constrained.These data are necessary to better resolve the regional process(es) responsible for the develop-ment of E-W extensional structures and the transition from N-S dominant kinematics.3) Hurtado (2002), using LANSAT imagery of the Upper Mustang, interpreted two portionsof the Mustang granite, one on the western side of the graben (visited in this study), and oneon the eastern side. Field work in the northeast Thakkhola graben is required in order toinvestigate the nature of this second pluton, and its relationship to the Mustang gneiss andgranite as observed in this study. If it can be demonstrated that this was originally a singularbody that was bisected during graben initiation (Hurtado, 2002), then this could be used toprecisely constrain the amount of E-W separation experienced in the graben and contributeto estimates of total E-W extension experienced in the Tibetan plateau, which is as of yetunknown.4) Apatite U-Th/He data are missing from the Ghami location of this study, and zircon684.4. Future WorkU-Th/He data from the Ghar Ghompa location is sparse. Replacing the missing data wouldincrease confidence in cooling ages and improve the accuracy of cooling paths from these loca-tions. A return visit to the Ghami and Ghar Ghompa locations to sample the TSS surroundingthe granite may yield the missing apatite and zircon.69BibliographyAyres, M. and Harris, N. B. W. (1997). REE fractionation and Nd-isotope disequilibrium duringcrustal anatexis: constraints from Himalayan leucogranites. 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Cenozoic tectonic evolution of the Himalayan orogen as constrained by along-strike variation of structural geometry, exhumation history, and foreland sedimentation.Earth-Science Reviews, 76(1-2):1–131. ! pages 1, 4, 5Yin, A., Kapp, P. A., Murphy, M. A., Manning, C. E., Harrison, T. M., Grove, M., Lin, D.,Xi-Guang, D., and Cun-Ming, W. (1999). Significant late Neogene east-west extension innorthern Tibet. Geology, 27(9):787–790. ! pages 5Zeitler, P. K., Herczeg, A. L., McDougall, I., and Honda, M. (1987). U-Th-He dating of apatite:A potential thermochronometer. Geochimica et Cosmochimica Acta, 51(10):2865–2868. !pages 18, 19, 20Zhang, J. and Guo, L. (2007). Structure and geochronology of the southern Xainza-Dinggyerift and its relationship to the south Tibetan detachment system. Journal of Asian EarthSciences, 29(5-6):722–736. ! pages 64Zhang, P. Z., Shen, Z., Wang, M., Gan, W., Bu¨rgmann, R., Molnar, P., Wang, Q., Niu, Z., Sun,J., Wu, J., Hanrong, S., and Xinzhao, Y. (2004). Continuous deformation of the TibetanPlateau from global positioning system data. Geology, 32(9):809–812. ! pages 6780Appendix81Appendix AI - U-Th/Pb analytical procedures and parametersU-Th/Pb dating was conducted in collaboration with Dr. John Cottle and the University ofCalifornia Santa Barbara. Procedures outlined after Cottle et al. (2009) and Braden et al. (2017).Monazite separates were obtained by standard crushing and grinding methods followed by heavymineral separation using a gold table, heavy liquids (Methyl Iodide), and Franz™ magnetictechniques. Monazite separates and reference materials were rinsed in alcohol, 2% nitric acidand water before being mounted on double-sided tape on an epoxy resin disc. Before analysis thelaser beam was rastered on the specimens using a large spot size (⇠100 µm) at low fluence (⇠0.6J/cm2), to further clean the samples and standards (Cottle et al., 2009). Dates were obtainedusing laser ablation multi-collector inductively coupled plasma mass spectrometry (LA-MC-ICP-MS), which collects all isotopes of interest simultaneously. Specifically, a Photon Machines193 nm ArF Excimer laser ablation system, connected by a split stream to a multi-collectorNu Plasma. Reference monazites, 44096, FC1, and Bananeira were analysed periodically, andreproduce to within 2% (2) of the accepted value. Isotopic data were collected using an 8-10µm spot, 90 shots, at a frequency of 3 Hz and laser energy of 2.5 mJ.II - 40Ar/39Ar analytical procedures and parameters40Ar/39Ar analyses were completed by collaborator Dr. Alfredo Camacho at the Universityof Manitoba (UMB). All 40Ar/39Ar analytical work was performed at UMB using a multi-collector Thermo Fisher Scientific ARGUSVI mass spectrometer, and a stainless steel ThermoFisher Scientific extraction/purification line and Photon Machines (55 W) Fusions 10.6 CO2laser. Faraday and discrete dynode (CDD) detectors were used to measure Argon isotopes. Thesensitivity for argon measurements is ⇠6.312 x 1017 moles/fA as determined from measuredaliquots of Fish Canyon Sanidine (Daze´ et al., 2003; Kuiper et al., 2008).All specimens were irradiated in the Cadmium-lined, in-core CLICIT facility of the OregonState University TRIGA reactor. The duration of irradiation was 7 hours, using the Fish Canyonsanidine (28.2 Ma; Kuiper et al., 2008) and GA1550 biotite (98.5 Ma; Spell and McDougall,2003) standards. Standards and unknowns were placed in 2 mm deep wells in 18 mm diameteraluminium disks, with standards strategically placed in order to evaluate the lateral neutron fluxgradients across the disk. Planar regressions were fit to the standard data, and the 40Ar/39Arneutron fluence parameter, J, interpolated for the unknowns. Uncertainties in J are estimatedat 0.1 - 0.2% (1), based on Monte Carlo error analysis of the planar regressions (Best et al.,1995).Irradiated samples were placed in a Cu sample tray with a KBr cover slip, and baked with aninfrared lamp for 24 hours in a stainless steel high vacuum extraction line. Single crystals were82Appendix A.either fused or step-heated using the laser. Reactive gases were removed, after ⇠3 minutes,by three GP-50 SAES getters (two at room temperature and one at 450 °C) prior to beingadmitted to an ARGUSVI mass spectrometer by expansion. Five argon isotopes were measuredsimultaneously over a period of 6 minutes. Measured isotope abundances were corrected forextraction-line blanks, which were determined before every sample analysis and averaged ⇠2.7fA for mass 40 and ⇠0.01 fA for mass 36.Mass discrimination was monitored by online analysis of air pipettes, which gave D =1.0087 ± 0.0003 per amu based on 68 aliquots interspersed with the unknowns. A value of295.5 was used for the atmospheric 40Ar/39Ar ratio (Steiger and Jager, 1977) for the purposesof routine measurement of mass spectrometer discrimination using air aliquots, and correctionfor atmospheric argon in the 40Ar/39Ar age calculation. Corrections are made for neutron-induced 40Ar from potassium, 39Ar and 36Ar from calcium, and 36Ar from chlorine (Roddick,1983; Renne et al., 1998; Renne and Norman, 2001). Data collection was performed usingPychron (Ross, 2017). Data reduction, error propagation, and age calculation and plottingwere performed using MassSpec software (version 8.091; Deino, 2013).III - U-Th/He analytical procedures and parametersU-Th/He analyses were completed at the University of Calgary with collaborator Dr. WilliamMatthews. Helium extraction was done using an ASI Alphachron helium line connected to aPrisma-Plus quadrupole mass spectrometer. Durango apatite and Fish Canyon zircon wereused as standards for apatite and zircon U-Th/He runs respectively. Uranium and Thoriumconcentrations were measured by isotope dilution using an Agilent 7700x quadrupole massspectrometer. Data reduction for helium and actinide concentrations are done in a custom setof spreadsheets.Zircon and titanite were dissolved using Parr large-capacity dissolution vessels in a multi-step acid-vapor dissolution process. Grains (including the Nb tube) were placed in Ludwig-styleSavillex vials, spiked with a 235U/230Th mixture, and mixed with 200 µL of Optima grade HF.The vials were then capped, stacked in a 125 mL Teflon liner, placed in a Parr dissolution vessel,and baked at 220 °C for 72 hours. After cooling, the vials were uncapped and dried down on a90 °C hot plate until dry. The vials then underwent a second round of acid-vapor dissolution,this time with 200 µL of Optima grade HCl in each vial, and baked at 200 °C for 24 hours.Vials were then dried down a second time on a hot plate. Once dry, 200 µL of a 7:1 HNO3:HFmixture was added to each vial, which was subsequently capped and cooked on the hot plateat 90 °C for 4 hours. Following either the apatite or zircon/titanite dissolution process, thesamples were diluted with 1 to 3 mL of doubly-deionized water and analyzed by the ICP-MSat the Colorado University Institute for Arctic and Alpine Research. Sample solutions, alongwith standards and blanks, were analyzed for U, Th, and Sm content using a Thermo Element2 magnetic sector mass spectrometer.83Appendix BFigure B.1: Hand specimens GG01A and GG01B, from Ghar Ghompa.84Appendix B.Figure B.2: Hand specimens GG10 and GG11, from Ghar Ghompa.85Appendix B.Figure B.3: Hand specimens GG12 and GG013, from Ghar Ghompa.86Appendix B.Figure B.4: Hand specimens DK14 and DK15, from Dhanggna Khola87Appendix B.Figure B.5: Hand specimens DK16 and GH17B, from Dhanggna Khola and Ghami, respectively.88Appendix B.Figure B.6: Mica separates from all granite specimens. Single grain was selected from separatesfor analysis.89Appendix B.Figure B.7: Zircon crystals selected from specimens GG01B, GG11, and DK14 for U-Th/Heanalysis. All measurements in micrometers.90Appendix B.Figure B.8: Zircon crystals selected from specimen GH17B for U-Th/He analysis. All measure-ments in micrometers.91Appendix B.Figure B.9: Apatite crystals selected from specimens GG010 and GG12 for U-Th/He analysis.All measurements in micrometers.92Appendix B.Figure B.10: Apatite crystals selected from specimen DK14 and DK15 for U-Th/He analysis.All measurements in micrometers.93AppendixCTableC.1:40Ar/39ArIsotopicdataI94Appendix C.TableC.2:40Ar/39ArIsotopicdataII95Appendix C.TableC.3:40Ar/39ArIsotopicdataIII96Appendix C.TableC.4:40Ar/39ArIsotopicdataIV97Appendix C.TableC.5:40Ar/39ArIsotopicdataV98Appendix C.TableC.6:ZirconU-Th/HeElementalandAgeData99Appendix C.TableC.7:ApatiteU-Th/HeElementalandAgeData100

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