Oligocene–Miocene Duplexing Along the India-Asia Suture Zone, Lazi Region, Southern Tibet

Gangdese culmination model: Duplexing along the India-Asia suture zone Gangdese culmination model: Oligocene–Miocene duplexing along the India-Asia suture zone, Lazi region, southern Tibet

Andrew K. Laskowski1,2,†, Paul Kapp1, and Fulong Cai3 1Department of Geosciences, University of Arizona, Tucson, Arizona 85721, USA 2Department of Earth Sciences, Montana State University, Bozeman, Montana 59717, USA 3Key Laboratory of Continental Collision and Plateau Uplift, Institute of Tibetan Plateau Research, Beijing 100101, China

ABSTRACT hinterland-dipping duplex beneath the Tibet (Yin, 2006; Hu et al., 2016), and along- Gangdese mountains, of which the Gang- strike variations (Replumaz et al., 2010; Leary The mechanisms for crustal thickening dese thrust is a component, is kinematically et al., 2016b; Webb et al., 2017). The signifi- and exhumation along the Yarlung (India- linked with a foreland-dipping passive roof cance and timing of Cenozoic fault systems Asia) suture in southern Tibet are under duplex along the Yarlung suture zone, the along the ~1300-km-long Yarlung (India-Asia) debate, because the magnitudes, relative Great Counter thrust system. The spatial suture in southern Tibet (Fig. 1), however, re- timing, and interaction between the two and temporal convergence between the pro- main a subject of debate. Juxtaposition of deeply dominant structures—the Great Counter posed duplex structures along the Yarlung exhumed magmatic arc rocks of the southern thrust and Gangdese thrust—are largely suture zone and the South Tibetan detach- Lhasa terrane against Indian passive-margin unconstrained. In this study, we present ment system indicate that they may be kine strata, as well as thermochronologic data and new geologic mapping results from the Yar- matically linked, though this relationship field mapping, led to the discovery of a north- lung suture zone in the Lazi region, located is not directly addressed in this study. Our dipping mylonitic shear zone—the Gangdese ~350 km west of the city of Lhasa, along with interpretation, referred to as the Gangdese thrust—that carried magmatic arc rocks south- new igneous (5 samples) and detrital (5 sam- culmination model, explains why the Gang- ward in its hanging wall (Yin et al., 1994, 1999). ples, 474 ages) U-Pb geochronology data to dese thrust system is only locally exposed Primarily documented southeast of the city of constrain the crystallization ages of Jurassic– (at relatively deeper structural levels) and Lhasa, the Gangdese thrust was interpreted as Paleocene Gangdese arc rocks, the prov- provides a structural explanation for early a crustal-scale structure that was active by late enance of Tethyan Himalayan and Oligo- Miocene crustal thickening along the Yar- Oligocene to early Miocene (27–23 Ma) time, cene–Miocene Kailas Formation strata, and lung suture zone, relief generation along the based on 40Ar/39Ar thermochronology data, with the minimum age (ca. 10 Ma) of the Great modern Gangdese Mountains, early Miocene a minimum displacement of 46 ± 9 km (Har- Counter thrust system. We supplement these Yarlung River establishment, and creation rison et al., 1992; Yin et al., 1994; Copeland data with a compilation of 124 previously of the modern internal drainage boundary et al., 1995). However, this structure is appar- published thermochronologic ages from along the southern Tibetan Plateau. The pro- ently not exposed along strike to the west of Gangdese batholith, Kailas Formation, and gression of deformation along the suture zone Lhasa, leading others to call into question its Liuqu Formation rocks, revealing a domi- is consistent with tectonic models that impli- significance and along-strike continuity (Aitchi- nance of 23–15 Ma cooling contemporaneous cate subduction dynamics as the dominant son et al., 2003). The dominant structures along with slip across the Great Counter thrust sys- control on crustal deformation. the Yarlung suture west of Lhasa are a system tem and other potentially linked structures. of south-dipping reverse faults called the Great These data are systematically younger than INTRODUCTION Counter thrust (Heim and Gansser, 1939; Yin 98 additional compiled thermochronologic et al., 1999; Murphy and Yin, 2003), which ages from the northern Lhasa terrane, re- Documentation of the structural style and typically places Indian passive-margin rocks on cording mainly Eocene cooling. Structural timing of crustal thickening that produced the suture zone mélange, mélange on Cretaceous and thermochronologic data were combined ~5 km average surface elevation of the Tibetan forearc basin strata, and forearc basin strata on with regional geological constraints, includ- Plateau is key to understanding the response of Oligocene–Miocene conglomerate, from south ing International Deep Profiling of Tibet and continental crust to intercontinental collision to north. A lack of hanging-wall cutoffs and the Himalaya (INDEPTH) seismic reflec- and recognizing feedbacks among climate, sur- no clear thermochronological date differences tion data, to develop a new structural model face processes, and tectonics (e.g., Quade et al., across individual fault splays render constraints for the Oligocene–Miocene evolution of the 2003; Harrison et al., 1992; Beaumont et al., on the timing and magnitude of Great Counter Tethyan Himalaya, Yarlung suture zone, and 2001; Whipple, 2009). It is also critical to as- thrust activity tenuous, but most studies agree southern Lhasa terrane. We propose that a sessing the viability of lithospheric-scale tec- that it was active by late Oligocene–early Mio- tonic models (e.g., DeCelles et al., 2011; Las- cene time (Quidelleur et al., 1997; Harrison †Present address: Department of Earth Sciences, kowski et al., 2017; Webb et al., 2017), which et al., 2000; Yin et al., 1999; Wang et al., 2015), Montana State University, Bozeman, Montana 59717, have developed significantly with increasing temporally overlapping or closely following ac- USA; andrew.laskowski@montana.edu. understanding of the geology and geophysics of tivity on the Gangdese thrust. Despite the close

GSA Bulletin; July/August 2018; v. 130; no. 7/8; p. 1355–1376; https://doi .org /10 .1130 /B31834 .1 ; 10 figures; Data Repository item 2018068; published online 23 February 2018.

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87ºE 88ºE persisted during India-Asia collision (Kapp et al., 2007). The Gangdese Mountains (Fig. 1) Indus 30ºN 35º JPg in southern Tibet (also called the Trans-Hima- ASIA C′ KPlv laya) are composed mostly of Gangdese mag- Himala detail JPg matic arc rocks. The Gangdese Mountains de- gure 10 30º Yarlung i yas F fine the northern boundary of the Yarlung River Lhasa JPg INDIA Kl watershed, in southern Tibet, and the southern 95º 80º boundary of the internally drained portion of the JPg Kl JPg GANGDESE MTNS. KPlv JPg Tibetan Plateau. JPg Kl Along the southern flank of the Gangdese OMk Mountains (Fig. 1), an Oligocene–Miocene, Dogxu OMk ng River JPg conglomerate-rich, continental unit, referred Kx Ngamring GCT L Kx 61012AL2,3 to as the Kailas (Gangrinboche) Formation, is oph Kx GCT exposed in buttress unconformity atop Gang- mlg Yarl oph ung River Ml dese arc rocks (Gansser, 1964; Aitchison et al., 7712AL2 mlg GCT Figure 2 Ml 2002; DeCelles et al., 2011, 2016; Leary et al., 62211PK3 Lazi

29ºN THS 2016b). Nonmarine strata of similar composi- THS 62211PK5 tion and structural position are continuous, with No es rth H n Dom some variations in sedimentary facies, for over Hlako imalaya THS GHS L 1300 km along the Yarlung suture zone (Leary Peak L L GHS GHS et al., 2016b). Some workers have interpreted THS Tingri L the Kailas Formation as the product of con- Mabja tractional deformation, associated with a litho- TETHYAN HIMALA THS YA spheric flexure during a late stage of India-Asia collision (Aitchison et al., 2007), flexural fore- THS land basin deposition related to the Great Coun- L ter thrust system (Wang et al., 2015), or wedge- L THS top sedimentation related to out-of-sequence STDS C L Great Counter thrust system activity (Yin et al., L GHS L 1999). However, recent investigations of the L Qomolangma L Kailas Formation sedimentology, fossil assem-

28ºN HIMAL AYA STDS (Everest) GHS 50 km blages, and basin architecture indicate that the L Kailas Formation was deposited in an extensional basin bounded by a north-dipping normal Paleogene-Neogene Oligocene-Miocene Cretaceous L OMk oph leucogranite Kailas Formation Xigaze Ophiolite fault, perhaps related to Oligocene–Miocene Jurassic-Paleogene Cretaceous Cambrian-Paleocene JPg Kl THS rollback or peeling-back of the subducted Great Gangdese batholith Lhasa terrane Tethyan strata (undiv.) Indian slab (DeCelles et al., 2011, 2016; Wang Cretac.-Paleogene Cretaceous Neoproterozoic Greater KPlv Kx GHS Linzizong Volcanics Xigaze Forearc Himalaya Sequence et al., 2013; Leary et al., 2016a). Miocene Sedimentary-matrix The Cretaceous–Paleogene Xigaze forearc Ml mlg Liuqu Fm. mélange (undiv.) basin (Einsele et al., 1994; Dürr, 1996; Wang Figure 1. Tectonic map of the Himalaya, Tethyan Himalayan physio- et al., 2012; An et al., 2014; Orme and Las- graphic zone, and southern Lhasa terrane in central-southern Tibet. kowski, 2016) is exposed to the south of the Geology is modified after Orme et al. (2015). The Lazi region study area Kailas Formation across a splay of the Great is shown in the box, and the cross section line refers to the cross sections Counter thrust system. Xigaze forearc basin in Figure 10. Inset map is adapted from Guillot et al. (2008). STDS— strata were deposited atop serpentinite mélange South Tibetan detachment system, GCT—Great Counter thrust. (Orme and Laskowski, 2016)—exposed along its southern margins in the Lazi region—suggesting that the 132–122 Ma Yarlung suture zone spatial and temporal relationship between the To the north of the Yarlung suture zone, there ophiolites (Hébert et al., 2012; Chan et al., 2015) Great Counter thrust system and the Gangdese is a belt of calc-alkaline plutonic rocks that are were in a suprasubduction-zone position at the thrust (where the Gangdese thrust is exposed), dominantly Cretaceous to Paleogene in age onset of forearc basin deposition ca. 110 Ma the crosscutting or branching relationships be- (Schärer et al., 1984), referred to as the Gang- (Huang et al., 2015; Orme and Laskowski, tween them are not known. The possibility that dese batholith, and related volcanic and vol 2016). Another conglomerate unit—the Liuqu the Gangdese thrust is an orogen-scale structure caniclastic rocks that are dominantly Paleocene Formation—is locally exposed to the south of that accommodated significant crustal shorten- to Eocene in age, referred to as the Linzizong the Xigaze forearc basin, where it was deposited ing during Cenozoic time, and the nature of its Formation (Lee et al., 2009). Collectively, these mainly atop serpentinite-matrix mélange dur- relationship to the more prominently exposed rocks compose the Gangdese magmatic arc, ing early Miocene time (Li et al., 2015; Leary Great Counter thrust system (Fig. 1) are open which developed along the southern Lhasa ter- et al., 2016a) or late Paleocene time (Ding et al., questions with major implications for Hima rane (Asian) margin during northward subduc- 2017). The Yarlung suture zone ophiolites and layan-Tibetan tectonics. tion of Neo-Tethyan oceanic lithosphere and serpentinite-matrix mélanges structurally overlie

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a belt of subduction-accretion, shale- and sand- In this study, we present a structural model exposed to the north of the Tethyan Himalayan stone-matrix mélange with (meta-) sedimentary for the Oligocene–Miocene evolution of the sequence. We correlate these rocks to the Pomu- and (meta-) basalt blocks (e.g., Cai et al., 2012). southern Lhasa terrane and Yarlung suture zone nong mélange (unit JKp), which composed of The sedimentary-matrix mélange separates based on new regional-scale geologic mapping Late Jurassic to Early Cretaceous rocks, based rock units that were located along the southern of an ~1500 km2 area north of the town of Lazi, on radiolarian fossils (Zhu et al., 2005; Cai margin of the Lhasa terrane from the Cambrian Tibet (Figs. 1 and 2A), encompassing three de- et al., 2012). Farther north, a unit of shale- and to Paleogene, the (meta)sedimentary Tethyan tailed (~1:50,000 scale) geologic mapping lo- lithic-sandstone-matrix mélange, with blocks of Himalayan sequence, the majority of which was cales (Figs. 2B–2D). In addition, we present five chert, basalt, and gabbro, is exposed between deposited in a passive-margin setting along the igneous U-Pb ages from the Gangdese arc and the Pomunong mélange and serpentinite-matrix northern margin of India (Gaetani and Garzanti, younger intrusive rocks and five detrital zircon mélange. We correlate this unit to the Tangga 1991; Liu and Einsele, 1994; Garzanti, 1999). U-Pb samples from sedimentary rocks in the mélange, composed of Late Triassic to Early Thermochronology data from Gangdese Lazi region. We interpret that the Great Counter Cretaceous rocks, based on radiolarian fossils batholith rocks exposed along the Yarlung su- thrust system is a passive roof duplex associated (Ziabrev et al., 2003; Zhu et al., 2005), and is ture zone (Copeland et al., 1987, 1995; Harri- with crustal-scale, hinterland-dipping duplex- equivalent to the Bainang terrane of Aitchison son et al., 1992; Dai et al., 2013; Sanchez et al., ing—equivalent to the Gangdese thrust. This et al. (2000) and the radiolarian intra-ophiolitic 2013; Carrapa et al., 2014; Wang et al., 2015; model explains the occurrence of imbricated, thrust sheet of Tapponnier et al. (1981). The Ge et al., 2016; Li et al., 2016; Laskowski et al., foreland-dipping thrust sheets in the absence Tangga and Pomunong mélanges likely formed 2017) reveal an orogen-scale Oligocene–Mio- of an emergent, hinterland-dipping detachment in an accretionary wedge setting during Neo- cene exhumation event, possibly associated with horizon. The new structural model is discussed Tethyan oceanic subduction beneath the south- one or more tectonic, climatic, and erosional in the context of lithospheric-scale tectonic modern Lhasa terrane (Cai et al., 2012). factors (Carrapa et al., 2014, 2017). Low-tem- els that invoke subduction dynamics as the prin- In the southern portion of the study area perature thermochronology data from the Kailas cipal driver of crustal deformation, providing an (Fig. 2A), a sandstone- and chert-clast, pebble- Formation reveal a preponderance of Miocene explanation for episodes of internal shortening to-cobble conglomerate unit containing interbeds (19–15 Ma) cooling ages, which have been in- (duplexing) along the Yarlung suture zone. of sandstone and shale was deposited in buttress terpreted to record efficient erosion associated unconformity atop chert- and matrix-dominated with drainage reorganization and establishment GEOLOGY OF THE LAZI REGION portions of the Tangga mélange, the Pomunong of the Yarlung River during early Miocene time mélange, and between the Tethyan Himalayan (Carrapa et al., 2014; Lang and Huntington, Field Methods sequence and serpentinite-matrix mélange, from 2014). Based on these data, the Gangdese mag- west to east (Fig. 2A). We correlate this unit to matic arc appears to have experienced semicon- We report data collected during field work in the Liuqu Formation (Yin et al., 1980), which tinuous exhumation throughout the period in 2012 and 2014 from the Yarlung River valley, was deposited in a contractional setting as part which it has variably been interpreted to be in ~10 km north of the city of Lazi, Tibetan Auton- of a fluvial and alluvial-fan depositional sys- the hanging wall of the Gangdese thrust (e.g., omous Region, China. The city of Lazi, which tem (Leary et al., 2016a). The preponderance of Yin et al., 1994), in the footwall of the Great is also referred to as Lhatse, Quxar, Quxia, or geochronological and thermochronological data Counter thrust system, and/or in the hanging Chusar, is located ~350 km west-southwest of from the Liuqu Formation suggest that it was de- wall of a north-dipping normal fault that created Lhasa City and ~35 km north-northeast of the posited during a short interval between 20 and the accommodation space for burial by up to Mabja Dome (Lee et al., 2004, 2006), crowned 19 Ma (Li et al., 2015; Leary et al., 2016a). How- ~4 km of Oligocene–Miocene nonmarine strata by Hlako Peak (~6500 m; Fig. 1). Mapping was ever, Ding et al. (2017) argued that the Liuqu (the Kailas Formation; DeCelles et al., 2011; conducted at ~1:100,000 scale across the study Formation was deposited during latest Paleocene Wang et al., 2015; Leary et al., 2016b). Further area (Fig. 2A) and at 1:50,000 scale in three time based on U-Pb geochronology of interbed- complication arises from the broad range of areas (Figs. 2B–2D) atop topographic maps ded tuffs. More work is needed to assess these explanations for relief generation between the generated from 3-arc-second Shuttle Radar competing hypotheses. In the Yarlung River val- presently high-standing Gangdese Mountains Topography Mission data with draped Land- ley, the Liuqu Formation was deposited in angu- (Figs. 1–2) and the low-lying Yarlung suture Sat orthoimagery. Contacts were interpolated lar unconformity atop chert blocks in the Tangga zone (Fig. 1), including flexure in a foreland between traverses using both Google Earth and mélange, and it is dominated by red chert clasts basin setting (Wang et al., 2015), crustal thick- LandSat orthoimagery. Cross sections were (Fig. 4D). The maximum preserved thickness ening driven by the Great Counter thrust (San- drawn from our structural and mapping data of the Liuqu Formation is ~2 km, near the town chez et al., 2013) or Gangdese thrust (e.g., Yin through the eastern and western portions of the of Liuxiang (Li et al., 2015), whereas the maxi- et al., 1994), and fluvial incision influenced by study area (Figs. 2A and 3). mum thickness of the Liuqu Formation in the the strengthening Asian monsoon coupled with Lazi region is ~200 m (Leary et al., 2016a). renewed Indian underthrusting (Carrapa et al., Rock Units and Correlations A belt of serpentinite- and gabbro-block 2014). No structural model exists that recon- mélange with a serpentinite-dominated matrix ciles the roles of the Gangdese thrust and Great The southernmost rocks in the Yarlung su- is exposed north of the Liuqu Formation, and Counter thrust system, provides context for ture zone are low-grade metasedimentary rocks, on both the south and north sides of the Tangga Oligocene–Miocene sedimentary basin devel- dominated by slate and well-cemented quartz mélange (Fig. 2A). We correlate these rocks to opment along the suture zone, explains relief arenite in the Lazi region (Fig. 2A), which we the laterally extensive and variably tectonized generation of the Gangdese Mountains, and correlate to the Tethyan Himalayan sequence. south Tibetan ophiolites, which formed between provides a mechanism for Yarlung River estab- In the western third of the map area (Fig. 2A), 132 and 122 Ma along strike (Hébert et al., lishment while maintaining compatibility with shale-matrix mélange, with blocks of sand- 2012; Chan et al., 2015). However, zircon U-Pb thermochronometric data. stone, limestone, chert, and volcanic rocks, is ages from a gabbro block and a fine-grained,

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L. Jurassic-E. Cretaceou Pomunong sed. mélange L. Jurassic-E. Cretaceou Ta Cretaceous Xigaze forearc strata, undi v. Te undivided Radiolarian chert bloc ophiolitic mélange

0 55 77

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cht oph 42 THS 36 - re and locations sample geochronology with area, study region Lazi the of map Geologic (A) pages ). three Figure 2following and this ( on in the referenced and geographical features stereonet, lower-hemisphere (Kx) on an equal-area, the Xigaze forearc sults, structural data for Yarlung indicated (detail maps 1–3). (B–D) Detailed geologic mapping locales of (B) the northern text. Detailed geologic mapping locales are (detail map 2), and (D) the detailed mapping area valley River Yarlung (detail map 1), (C) the southern detailed mapping area valley River (detail map 3). Jiwa detailed mapping area

000 000 60 000 60

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wn 0 iangular arrow shows dip of fault Inclined, overturned and vertical bedding 5

4 Detrital zircon sample U-Pb zircon Multiple chronometers applie Tr To solid lines, well located; dashed lines Foliation, arrow shows trend and plunge diamond arrow shows trend and plunge of striae on fault surface Lithologic (thin) and fault (thick) contacts poorly located or inferred; dots, covered, red: active or recently faul Thrust fault; triangle on hanging wal of lineation 55 Oligocene-Miocene Kailas Formation Miocene Liuqu Formation THS Quaternary alluviu Eocene (?) hypabyssal felsic intrusive rock Jurassic-Paleogene Gangdese arc rocks Neogene leucogranit 37 55 30 Geochronology Ei Ng Ml Qal OM k JP g 10 ′

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87°44′ E 87°40′ E 87°42′ E Gonggangcun 4000 B Kx Kx Qal 50 55 Qal Kx 64

35 60 40 70 69 50 Kx 4000 35 Yarlung River 56 29°14′ N Kx 55 42 78 30

4500 47 35 Jiandacun 48 oph 72 77 oph oph 60 37 37 oph cht 64 55 37 47 60 35 85 58 75 85 67 70 83 cht 40 Ml 60 Ml 70 JKt 75 Qal 15 80 JKt 49 cht 26 35 34 68 35 45 Ml cht 60 50 62 75 JKt Ml 44 50 29°12′ N 32 80 36

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Figure 2 (continued).

granitic intrusive rock in the serpentinite-matrix the Kailas Formation is dominated by granite, Counter thrust system might reflect a shallower mélange, collected within the study area near volcanic, gneiss, limestone, and lithic sandstone depth of exposure in the Lazi region than in the the town of Jiwa (Fig. 2A), indicate a younger cobbles. In the northeast corner of the map area, Mount Kailas region, as Great Counter thrust crystallization age of ca. 111 Ma (Orme and an ~120 km2 leucogranitic pluton crosscuts system splays have previously been interpreted Laskowski, 2016). Xigaze forearc basin strata, Xigaze forearc, Kailas Formation, and Gang- to merge into a single fault at depth (Yin et al., exposed to the north of Jiwa, were observed to dese batholith rocks. 1994, 1999; Laskowski et al., 2017). be in depositional contact with the serpentinite- Three distinct splays of the Great Counter matrix mélange (Fig. 2A). The onset of forearc Fault Systems thrust system were mapped in the Lazi region basin deposition was constrained to ca. 110 Ma (Fig. 2A). The southernmost splay juxtaposes based on the U-Pb age of a tuffaceous sand- The term “Great Counterthrust” was origi- the Tethyan Himalayan sequence against ser- stone directly above the basal unconformity, nally used to describe the south-dipping reverse pentinite-matrix mélange, the Liuqu Formation, persisting until ca. 86 Ma based on detrital fault that placed Tethyan Himalayan sequence and Pomunong mélange from east to west. This zircon maximum depositional ages (Orme and rocks on the Kailas Formation near Mount Kai- fault is poorly exposed but was inferred based Laskowski, 2016). North of the Xigaze forearc, las, ~650 km along strike to the west, where in- on juxtaposition of rock units, and it dips to the a narrow (1–2 km north-south width) but rela- tervening sedimentary- and serpentinite-matrix south based on its relationship with topography. tively thick (~1 km), east-west–trending belt of mélange and Xigaze forearc strata are absent To the north, a second splay juxtaposes hang- boulder-to-pebble conglomerate and sandstone (Heim and Gansser, 1939). Here, we expand ing-wall serpentinite-matrix mélange against of the Kailas (Gangrinboche) Formation (e.g., this nomenclature to include a system of mod- Xigaze forearc strata to the east, transitioning to Aitchison et al., 2002; DeCelles et al., 2011) is erately to steeply south-dipping reverse faults a zone of anastomosing faults that juxtapose the exposed in buttress unconformity on Gangdese that carry Tethyan Himalayan rocks in the struc- Liuqu Formation, Pomunong mélange, serpen- batholith rocks (Leary et al., 2016b). The thick- turally highest position over Kailas Formation tinite-matrix mélange, and Tangga mélange to est accumulations of the Kailas Formation are and Gangdese batholith rocks in the structur- the west (Fig. 2A). Xigaze forearc strata in the up to 4 km thick, near the type locality at Mount ally lowest position, with other splays juxta- footwall of this fault zone are steeply dipping Kailas (Heim and Gansser, 1939; Gansser, posing the intermediary units (Fig. 2A). The and locally overturned (Fig. 2A), and the fault 1964). Along the northern Yarlung River valley, presence of multiple fault splays of the Great contact between serpentinite-matrix mélange

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87°47′ E 87°49′ E Qal JPg 4500 C JPg JPg

Qal

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JPg 61012AL1 JPg 170±4 Ma

OMk 45 40 61012AL2 43 OMk recumbent fold 43 protomylonitic Kailas Fm. 4500 (Fig. 4C) 61012AL3 49 cataclasite breccia

77 50 Qal Figure 2 (continued). cataclasite breccia Ei Ei 70 45

68 61012AL7 Kx 66 60 44±1 Ma 73 60 55

57 Qal

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Qal 29°20′ N

1 0.5 0 KILOMETERS 1 2 N

and Xigaze forearc strata in the Yarlung River cally overturned exposures in the anastomosing in intrusive contact with Xigaze forearc basin valley (Fig. 2A) is characterized by an ~15-m- zone along the Yarlung River valley (Fig. 2B). strata. This contact is sill-like in geometry, and thick zone of fault gouge and brecciated cata- Splays of the Great Counter thrust bound forearc strata structurally above the fine-grained clasite. Similarly, a nearby fault contact between both the north and south sides of the Xigaze intrusive rock display a strong cleavage oriented serpentinite-matrix mélange and a chert block forearc basin, which is ~15 km wide in the ~100/60 SW. The intrusive rock appears to be of the Tangga mélange exhibits brecciation Lazi region (Fig. 2A). Xigaze forearc strata are the product of protracted hypabyssal intrusive and chlorite alteration across an ~20-m-thick folded across a broad syncline in-between these activity, which was later subjected to shearing, fault zone. Some chert blocks are entirely en- zones, producing a roughly fault-parallel aver- as foliation-parallel, fine-grained dikes of simi- cased in serpentinite-matrix mélange and were age fold axial plane (Fig. 2A). At smaller scale, lar composition to the host rock were observed. likely emplaced as tectonic slivers along the the Xigaze forearc strata are folded across open- The northernmost splay of the Great Coun- intra-ophiolitic splay fault (Fig. 2A). Preserva- to-tight, symmetrical folds with amplitudes on ter thrust system (Fig. 2A) was exhumed from tion of the depositional contact between Xigaze the hundred-meter scale, which likely formed as greater depth than the southern splays, because forearc strata and serpentinite-matrix mélange parasitic folds during north-south contraction. the Kailas Formation in the footwall displays near the town of Jiwa is likely the result of In the east, the ~50° southeast-dipping, north- protomylonitic fabrics that were not observed the anastomosing character of this fault zone ern Great Counter thrust splay (Fig. 2A) juxta- elsewhere. Along the northern Yarlung River (Fig. 2D). The faults dip 20°–30° to the south in poses hanging-wall Xigaze forearc basin strata valley (Fig. 2C), the hanging wall consists of the eastern map area, 54° to the south near Jiwa against the Kailas Formation. Along the Yarlung fine-grained, felsic igneous rocks (Figs. 2A and (Fig. 2D), 83° to the south ~4 km west of Jiwa, River valley to the west, the hanging wall of this 2C), characterized by cataclasis and brecciation and between 35° and 75° to the south with lo- splay consists of a fine-grained granitoid that is within ~100 m of the fault. Structurally down

1360 Geological Society of America Bulletin, v. 130, no. 7/8

87°52′ E 87°54′ E 87°56′ E Kx 5000 Kx 4500 D Qal 29°14′ N Qal 4500 50 0 0

Kx Kx Kx

Qal 57

70 60 Qal 74 oph 82 depositional 70 65 80 Kx 77 Kx oph 50 Qal oph 54 50 oph 65 oph 83 80 35 35 29°12′ N Ml 32 70 65 25 72 oph 54 80 Ml 4500 Ml Ml Qal 65 4500 Qal 74 4500 72 Ml Ml 5000 THS 51 THS 7712AL2 THS

20 Qal Qal 25 50 THS 29°10′ N THS N THS KM 0 1 0.5 0 1 2 500

Figure 2 (continued).

section, brittle fabrics give way to a 20-m-thick of ~4 km in the Mount Kailas region (Heim Great Counter thrust system suggests that it is a zone of top-to-the-north, protomylonitic fabrics and Gansser, 1939; Gansser, 1964). Therefore, crustal-scale structure. that are developed in the granitic clasts and sand- the maximum burial that can be attributed to Within the detailed map area near the town of stone matrix of the Kailas Formation (Figs. 4A– subsidence alone is 4 km. Assuming a geother- Jiwa (Figs. 2A and 2D), an ~35° north-dipping 4B). We interpret these features to indicate that mal gradient of 25 °C/km, the mylonitic Kai- reverse fault juxtaposes hanging-wall sedimen- these rocks were tectonically buried in the foot- las Formation rocks were likely tectonically tary-matrix mélange against footwall Liuqu wall of the Great Counter thrust to brittle-ductile buried to depths of at least 10 km to achieve Formation strata. Although this fault is not well conditions (250–400 °C) and were later accreted brittle-ductile conditions (250–400 °C) prior exposed east of Jiwa, we interpret that it extends to the hanging wall to accommodate their exhu- to exhumation in the fault zone hanging wall. along strike, except where it is cut out by a Great mation. Structurally below the mylonite zone, Considering the measured shear fabric orien- Counter thrust splay with the Tethyan Himala- north-vergent recumbent folds were observed tation (45°–50° to the south), our interpreted yan sequence and Liuqu Formation in the hang- within the Kailas Formation, similarly indicat- minimum burial depth (10 km), and assuming ing wall (Fig. 2A). Within the footwall of this ing top-to-the-north shearing (Figs. 2C and 4C). a steady geothermal gradient and the absence fault, bedding in the Liuqu Formation fans up A weak fault-parallel foliation was observed of convective heating by igneous or hydrother- section from locally overturned (steeply north- structurally below the Kailas Formation, in mal activity, displacement across this splay of dipping) to moderately south dipping (Fig. 4E), Gangdese batholith rocks ~1 km north of the the Great Counter thrust system totaled at least consistent with syntectonic sedimentation. We mylonite zone (Fig. 2C). 7–13 km, with 7 km reflecting the maximum interpret these features to indicate that this fault The mylonitic fabrics observed along the 4 km of burial due to subsidence. Mylonitic is an antithetic splay of the Great Counter thrust Great Counter thrust in the Lazi region al- fabrics that are developed in the proximal foot- system (Fig. 3). low us to estimate the depth of tectonic burial wall of the northernmost Great Counter thrust Although fault dips and rock unit juxta- of Kailas Formation strata and the magnitude splay have also been reported near the town of positions are highly variable along different of slip along the northern splay of the Great Langxian, located ~500 km along strike to the splays of the Great Counter thrust system, Counter thrust system. The Kailas Formation is east in southeastern Tibet (Wang et al., 2015). interpreted cross sections through the Lazi re- ~1.2 km thick in the study area (Leary et al., The occurrence of mylonites at both localities gion map area suggest that the regional struc- 2016b), and it achieves a maximum thickness and the >1000 km along-strike continuity of the tural geometry is dominated by a set of three

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bedding trace bedding A (SW) mylonite foliation A′ (NE) parasitic folding 5 THS Qal cht Ei OMk Qal Ml Qal Qal Qal 4 JKp 3 oph Figure 3. Cross sections through (km) Kx

v. JKt JPg 2 the Lazi region map area along

Ele Kx profiles A-A and B-B (Fig. 2A) 1 ′ ′ with no vertical exaggeration 0 oph no vertical exaggeration (1:1 vertical to horizontal scale). Map units and colors are keyed B (SW) B′ (NE) to Figure 2. Structural data from nearby measurements 5 Ml oph KEx Qal QalQal OMk 4 THS JPg were projected into the cross- oph 3 Kx section plane. (km) v. 2 JKp Ng

Ele Kx 1 Ng oph 0 no vertical exaggeration

imbricate, south-dipping thrust sheets (Fig. 3). a large aliquot of grains (usually >1000) was be accessed at the Arizona LaserChron Center The steeply dipping faults are interpreted to mounted in epoxy together with crystal stan- Web site (Laserchron.org). Detrital zircon U-Pb sole into moderately southwest-dipping (~30°) dards. For igneous samples, 20–50 selected analytical data are reported in Supplementary master faults (Fig. 3), especially in the anasto- zircon grains were mounted alongside crystal Table DR1, and igneous zircon U-Pb analytical mosing fault zone south of Xigaze forearc basin standards. Mounts were sanded to a depth of data are reported in Supplementary Table DR2.1 strata (Fig. 2A). Foliation measurements from ~20 μm to expose zircon cores, polished, im- Uncertainties in these tables are at the 1σ level the Pomunong and Tangga mélange, bedding aged with backscattered electron or cathodo and include only measurement errors. Analyses measurements in the Liuqu Formation and the luminescence techniques for navigation pur- that were >20% discordant or >5% reverse dis- Kailas Formation near the northernmost Great poses, and then cleaned prior to analysis. cordant were rejected. Counter thrust splay, and foliation in both U-Pb geochronology was conducted by laser- Crystallization ages for zircons were isolated Gangdese batholith and Kailas Formation rocks ablation–inductively coupled plasma–mass from inherited ages in igneous U-Pb samples surrounding this fault are consistent with this spectrometry (LA-ICP-MS) at the Arizona using the Arizona LaserChron Center (www interpretation (Fig. 3). In contrast, the Linzi- LaserChron center following the techniques .laserchron.org) in-house program AgePick, zong Formation volcanic rocks, exposed on the outlined in Gehrels et al. (2006, 2008) and which provides tools for identifying inheritance, north side of the Gangdese Mountains, display Gehrels and Pecha (2014). Pb loss, and/or overgrowth and recrystalliza- a regional northward dip (Fig. 1), suggesting All samples were ablated using a Photon Ma- tion of metamorphic zircon. Zircons interpreted that the Gangdese batholith may be exposed chines Analyte G2 excimer laser equipped with as inherited in igneous samples are displayed in the core of a broad anticline oriented paral- a HelEx ablation cell, with a spot size ranging as probability distribution functions alongside lel to the Great Counter thrust system. There from 10 to 30 μm, depending on the size of the weighted mean age determinations incorpo- is no exposed north-dipping fault contact be- target crystal zone. Typically, ablation pits were rating analytical and systematic error (Fig. 5). tween Gangdese batholith rocks and the Xigaze ~15 μm in depth. One sample (61012AL3) was Detrital zircon data from this study and refer- forearc basin, or between Gangdese batholith analyzed using an Element2 high-resolution ences from the literature are also displayed as rocks and the Tethyan Himalayan sequence, ICP-MS, which sequences rapidly through U, probability distribution functions (Fig. 6), all of that might correlate to the Gangdese thrust, Th, and Pb isotopes. Signal intensities were which were created using the DZstats program which has been documented ~400 km along measured with a secondary electron multiplier (Saylor and Sundell, 2016). strike to the east (Yin et al., 1994). detector that operates in pulse-counting mode for signals less than 50,000 counts per second Igneous Zircon U-Pb GEOCHRONOLOGY AND (cps), in both pulse-counting and analog mode Geochronology Results THERMOCHRONOLOGY for signals between 50,000 and 5,000,000 cps, and in analog mode above 5,000,000 cps. The Igneous U-Pb samples were collected from U-Pb Geochronology Methods remaining samples (n = 9) were analyzed us- the Gangdese batholith (sample 61012AL1) ing a Nu high-resolution ICP-MS, which was in the northwestern portion of the map area Five igneous and five detrital zircon U-Pb equipped with a flight tube of sufficient width (Figs. 2A and 2C), a fine-grained felsic geochronology samples were collected from that U, Th, and Pb isotopes could be measured intrusive rock (61012AL7) in the hanging map units across the study area to confirm map simultaneously. All Nu measurements were wall of the northernmost Great Counter thrust unit identities, constrain sediment provenance, made in static mode using Faraday detectors and determine the age of igneous intrusions. with 3 × 1011 ohm resistors for 238U, 232Th, 1GSA Data Repository item 2018068, containing 208 206 U-Pb geochronologic data and compiled thermo Zircons were extracted from ~2 kg samples by and Pb- Pb, and discrete dynode ion coun- chronologic data, is available at http://www .geosociety 204 202 crushing, followed by density and magnetic ters for Pb and Hg. Further details of the .org/datarepository/2018 or by request to editing@ separation techniques. For detrital samples, Element2 and Nu analytical procedures can geosociety.org.

1362 Geological Society of America Bulletin, v. 130, no. 7/8

A B mylonitic fabric in Kailas Formation liation fo

sheared granite clast

south north south north

C Great Counter thrust system D

Liuqu Fm. folded Kailas Fm.

ormity angular unconf

Radiolarian chert block in Tangga mélange

south north

E hanging wall Antithetic spla serpentinite-matrix mélange Liuqu Formation y of Great Counter thrust system growth strata

beddin

south north

Figure 4. Photographs from the Lazi region. (A) Outcrop exposure of protomylonitic Kailas Formation conglomerate along the northernmost splay of the Great Counter thrust, displaying top-to-the-north sense of shear. (B) Protomylonitic fabrics in both Kailas Formation matrix and cobbles along the exposure in photograph A. (C) North-dipping recumbent fold and synthetic fault in the Kailas Formation beneath the Great Counter thrust system shear zone shown in A and B. (D) Liuqu Formation pebble conglomerate deposited in angular unconformity atop a bedded, radiolarian chert block within the Tangga mélange. The Liuqu Formation at this locality coarsens upward to cobble and boulder conglomerate and is dominated by chert clasts. (E) Fanning beds in the Liuqu Formation that transition from overturned (to the north) to moderately south-dipping (to the south) in the footwall of a north-dipping fault that we interpret as an antithetic splay of the Great Counter thrust system.

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15 61312AL1 61012AL7 14 xenocrystic zircon 13 n = 7 final age = 9.9 ± 0.3 2σ 12 MSWD = 0.8, systematic error = 0.8% n = 10, youngest population of 17 total 11 Age (Ma) 10

8 0.0 1.0 2.0 0 500 1000 1500 2000 U/Th Ma 60 58 61012AL7 61012AL7 56 xenocrystic zircon 54 n = 14 52 final age = 43.7 ± 0.8 2σ MSWD = 0.5, systematic error = 0.8% 50 n = 2, youngest population of 16 total 48 Age (Ma) 46 44 42 40 0.0 1.0 2.0 3.0 4.0 0 500 1000 1500 2000 Ma U/Th 15 6912AL1 6912AL1 extends off chart 13 xenocrystic zircon n = 5 11

9 Age (Ma) final age = 9.9 ± 0.3 2σ 7 MSWD = 1.0, systematic error = 0.8% n = 12, youngest population of 17 total 5 0.0 2.0 4.0 6.0 8.0 10 0 500 1000 1500 2000 U/Th Ma 15 6912AL2 6912AL2 13 xenocrystic zircon n = 17 11

9 Age (Ma) final age = 9.7 ± 0.6 2σ 7 MSWD = 0.1, systematic error = 0.8% n = 4, youngest population of 21 total 5 0.0 2.0 4.0 6.0 8.0 10 0 500 1000 1500 2000 U/Th Ma 190 61012AL1 185 Figure 5. Igneous U-Pb results shown as both weighted 180 mean age determinations with 1σ error (left), plotted 175 as a function of U/Th ratio, and probability distribution functions (PDFs) of U-Pb ages interpreted to re- 170 flect inheritance (right). Zircon dates excluded from 165 Age (Ma) the weighted mean age determination are indicated by 160 final age = 170.1 ± 3.8 2σ the light weighted symbols. PDFs were generated using MSWD = 0.1, systematic error = 0.7% 155 n = 16,16 total DZStats (Saylor and Sundell, 2016). MSWD—mean 150 square of weighted deviates. 0.0 1.0 2.0 U/Th

1364 Geological Society of America Bulletin, v. 130, no. 7/8

87 n = 107 Kailas Formation: 61012AL3 50

90 177 n = 113 Kailas Formation: 61012AL2 n = 2 516 221 549 n = 6 180 Tethyan: 62211PK3 n = 70 515

546 Tethyan: 7712AL2 n = 87

509

543 Tethyan: 62211PK5 n = 89 554

0 50 100 150 200 250 250 500 750 1000 1250 1500 1750 2000 2250 2500 2750 3000

Asia Lhasa terrane igneous (Orme et al., 2015) Lhasa terrane (Gehrels et al., 2011) n = 263 n = 733

Lazi area forearc (Orme and Laskowski, 2016) n = 2163

India Tr iassic Tethyan (Aikman et al., 2008; Cai et al., 2012; Li et al., 2015) n = 1413

Tethyan & Upper LHS (Gehrels et al., 2011) n = 3912

0 50 100 150 200 250 250 500 750 1000 1250 1500 1750 2000 2250 2500 2750 3000 U-Pb Age (Ma) U-Pb Age (Ma) Figure 6. Detrital zircon U-Pb results presented in this study (top) alongside reference curves from literature (bottom). The color-coding indicates detrital-zircon–based provenance correlation, discussed further in the text. Probability distribution functions were generated using DZStats (Saylor and Sundell, 2016). The solid gray bar indicates the range of ages most common in Kailas Formation samples, allow- ing for evaluation of our provenance interpretation. Peak ages, calculated using the AgePick program (available from www.laserchron.org), are labeled on the plots. LHS—Lesser Himalayan sequence.

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splay directly below the sill-like intrusive con- tance. This pluton provides a minimum age of been missed during analysis via LA-ICP-MS) tact with Xigaze forearc strata (Figs. 2A and ca. 10 Ma for the Great Counter thrust system, prevented reliable maximum depositional age 2C), and from fine-grained dikes (6912AL1, which it crosscuts in the northeastern portion of determination in this case. 6912AL2) and a large, detached boulder of two- the study area (Fig. 2A). mica leucogranite (61312AL1) in close prox- Compilation of Thermochronological Data imity to a large (~100 km2) leucogranite pluton Detrital Zircon U-Pb (Fig. 2A). Sample 61012AL1, a hornblende- Geochronology Results Published medium- to low-temperature ther plagioclase-biotite-quartz granodiorite, was mochronologic data were compiled for the located ~1.3 km north of an outcrop exposure Detrital zircon samples were collected from Gangdese batholith, Kailas Formation, and of the northernmost splay of the Great Coun- Tethyan Himalayan sequence rocks (three sam- Liuqu Formation along a 1000-km-long swath ter thrust, beneath the unconformable contact ples) and the Kailas Formation (two samples) to encompassing the Gangdese Mountains and with the Kailas Formation (Fig. 2C). The out- confirm field identifications and refine deposi- Yarlung suture zone, between Mount Kai- crop from which this sample was collected dis- tional ages. Two samples were collected from las and the Lhasa region (Fig. 7). In addition, plays a weak foliation striking approximately Triassic (62211PK3) and Jurassic (62211PK5) thermochronologic data from an Early Creta- east-west (87°) and dipping 60° to the south, Tethyan Himalayan sequence quartz arenites ceous plutonic belt in the northern Lhasa ter- similar to the orientation of shear fabrics along exposed 15–25 km southwest of the city of rane were compiled for comparison with the the Great Counter thrust (Fig. 2C). The sample Lazi outside of the detailed study area (Fig. 1) southern Lhasa terrane data. The combined data yielded a U-Pb age of 170.1 ± 3.8 Ma (Fig. 5). to examine previously documented provenance set included 40Ar/39Ar biotite (Copeland et al., This was the only sample of the five samples differences between the Triassic interval of 1987; Sanchez et al., 2013), zircon fission-track that lacked evidence for complexly zoned zir- the Tethyan Himalayan sequence and the rest (ZFT; Wang et al., 2015; Ge et al., 2016), zircon cons with xenocrystic cores. Sample 61012AL7 of the Tethyan Himalayan sequence (e.g., Cai (U-Th)/He (Hetzel et al., 2011; Dai et al., 2013; yielded a poorly constrained crystallization age et al., 2016). Sample 62211PK3 produced a Haider et al., 2013; Li et al., 2016; Laskowski of 43.7 ± 0.8 Ma based on the weighted mean detrital zircon age spectrum characterized by et al., 2017), apatite fission-track (AFT; Cope- of the two youngest dates. Four other zircons a broad 1230–180 Ma age-probability peak land et al., 1995; Hetzel et al., 2011; Rohrmann from this sample yielded dates between 48 with a few older Paleoproterozoic and Archean et al., 2012; Haider et al., 2013; Carrapa et al., and 52 Ma, whereas the remaining 10 analy- ages (Fig. 6). In contrast, sample 62211PK5 2014; Wang et al., 2015; Ge et al., 2016; Li et al., ses yielded Paleozoic and Proterozoic dates produced an age spectrum characterized by a 2016), and apatite (U-Th)/He (Hetzel et al., 2011; (Fig. 5). We interpreted the dates not included broad distribution of ages between 1250 and Rohrmann et al., 2012; Haider et al., 2013; Dai in the weighted mean age determination as in- 490 Ma, alongside a few older Proterozoic and et al., 2013; Ge et al., 2016) data for Gangdese herited based on the presence of zircon cores Archean ages, with a dominant age-probability batholith and northern Lhasa terrane plutonic identified using high-resolution backscattered peak at ca. 509 Ma (Fig. 6). Sample 7712AL2 rocks, zircon (U-Th)/He and AFT data (Carrapa electron and cathodoluminescence imagery; was collected from a Jurassic quartz arenite et al., 2014) for the overlying Kailas Formation, however, it is also possible that the true crystal- exposed on a mountaintop ~1.5 km south of a and apatite (U-Th)/He data for the Liuqu Forma- lization age is younger than the poorly defined Great Counter thrust splay (Figs. 2A and 2D), tion (Li et al., 2015). Closure temperatures for youngest age population. and it is characterized by a similar age spectrum these systems can vary based on radiation dam- Samples 6912AL1 and 6912AL2 were col- to that of 62211PK5, with the addition of four age, crystal chemistry, grain size, zonation, and lected along an approximately north-south– 450–320 Ma ages (Fig. 2D). other factors, but the approximate closure tem- oriented valley south of Zha Xilincun and the Two samples were collected from sheared peratures are 350–300 °C for 40Ar/39Ar biotite Phuntsoling Monastery, west of the leucogran- sandstone beds within the conglomerate-domi (McDougall and Harrison, 1999), 250–230 °C ite pluton (Fig. 2A). Both samples were col- nated Kailas Formation, exposed in the foot- for ZFT (e.g., Zuan and Wagner, 1985), 180– lected from fine-grained dikes that intruded wall of the northernmost Great Counter thrust 160 °C for zircon (U-Th)/He (e.g., Guenthner subparallel to east-west–striking bedding of splay (Figs. 2A and 2C). Both samples revealed et al., 2013), 120–60 °C for AFT (e.g., Green Xigaze forearc sandstone and shale, which a dominance of 100–40 Ma ages, largely dis- et al., 1986), and 80–60 °C for apatite (U-Th)/He dips steeply to the north (strike ~350°, dip tributed between 100–80 Ma and ca. 50 Ma (e.g., Farley, 2000). Therefore, the data in this 70°–85°). Sample 6912AL1 yielded an age of age-probability peaks (Fig. 6). Although these compilation record when samples were exhumed 9.9 ± 0.3 Ma (Fig. 5). Five 600–300 Ma dates data suggest an Eocene maximum deposi- through paleodepths of ~17 km (40Ar/39Ar bio- from this sample are interpreted to reflect in- tional age for the Kailas Formation, another tite) to ~2 km (apatite U-Th/He), assuming a heritance (Fig. 5). Sample 6912AL2 yielded detrital zircon sample from the same locality 20–30 °C/km, steady-state geotherm and the ab- an age of 9.7 ± 0.6 Ma (Fig. 5). The 17 older presented in a separate study provided a well- sence of significant heat convection by plutons dates from this sample included 300–600 Ma constrained Miocene maximum depositional and/or hydrothermal fluids. and older, Proterozoic-age populations that are age of 22.8 ± 0.3 Ma (Leary et al., 2016b). In The proximity of thermochronology samples also interpreted to reflect inheritance. Sample contrast, three other samples from the same to the Yarlung suture was calculated by measur- 61312AL1, a phaneritic two-mica leucogran- study yielded older, Eocene maximum depo- ing the shortest straight-line distance from each ite, was collected due east of the dike samples sitional ages. Therefore, five of the six detri- sample to the nearest exposure of ophiolitic on the east side of the pluton (Fig. 2A) from tal zircon samples from this locality (Figs. 2A rocks, and it was plotted against thermochrono a boulder in a large terminal moraine below a and 2C) overestimate the depositional age of logical age to reveal orogen-perpendicular cirque composed entirely of leucogranite. This the Kailas Formation by ~20 m.y., suggesting (generally north-south) trends (Fig. 7). Thermo sample yielded an age of 9.9 ± 0.3 Ma (Fig. 5). that zircon availability, extreme local sourc- chronologi cal data for the Kailas Formation, Two 15–14 Ma dates and five others between ing, and/or complex zonation (possibly with Gangdese batholith, and northern Lhasa terrane 800 and 30 Ma are interpreted to reflect inheri- inherited cores and young rims that might have reveal an increase in cooling age with distance

1366 Geological Society of America Bulletin, v. 130, no. 7/8

82ºE 88ºE Apatite-He AFT TIBETAN PLATEAU drainage divide Zircon-He ZFT 18 19 18 Biotite Ar-Ar 19 17 Mt. Kaillas 20 7 17 14 17 Ga 16 ng Yarlung Rivedr es e Mountains 30ºN 17 30ºN 17 G C 16 T 21 14 G T

study area

Tibetan t < 15 n South Deta me c 28ºN 15–20 h 20–23 Mt. Everest 23–27

Age (Ma) 27–38 > 38 400 km 88ºE error bars are 1 110

100

70 A-He AFT 60 Ar-Ar

Age (Ma) Z-He 50 ZFT 40

0 0 50 100 150 200 250 300 350 Distance North from Yarlung Suture Ophiolite (km) Figure 7. Top: Compiled thermochronological data for Gangdese batholith and Kailas Formation rocks in the southern Lhasa terrane, and an Early Cretaceous pluton in the northern Lhasa terrane, with thermochronological system indicated by the symbol shape and age indicated by color. Zircon (U-Th)/He and apatite fission track (AFT) ages for the Kailas Formation are indicated by the black dots and are labeled with thermochronologic age (zircon [U-Th]/He in bold). Major geologic structures, including the Gangdese thrust (GT), Great Counter thrust system (GCT), South Tibetan detachment, and active N-S rifts (in red) are plotted for reference (modified after Orme et al., 2015). The base map is a digital elevation model from the Global Multi-Resolution Topography Synthesis (Ryan et al., 2009) annotated with geographical features discussed in the text (top). Bottom: Gangdese batholith and Lhasa terrane data are plotted as a function of distance from the Yarlung suture zone ophiolites and ophiolitic mélange. Compiled thermochronological data are available in Supplementary Table DR3 (see text footnote 1), which also contains references for data sources. ZFT—zircon fission track age.

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from the Yarlung suture, punctuated by an ap- rapid cooling from ~300 °C (biotite 40Ar/39Ar) to ca. 26 and 12 Ma (Figs. 7 and 8). Therefore, we parently sudden transition across the Gangdese ~80 °C (apatite U-Th/He). conclude that if the Gangdese thrust is a major Mountains from mainly Oligocene–Miocene Probability distribution functions created structure, it was likely active during this period, ages in the southern Lhasa terrane to mainly Eo- from Yarlung suture zone data, separated by which also overlaps in time with the permit- cene and older ages in the northern Lhasa terrane thermochronological system and geologic unit ted age range of the Great Counter thrust. Age (Fig. 7; Rohrmann et al., 2012). A plot of age (Fig. 8), reveal a dominance of late Oligocene– spectra from the Gangdese batholith are domi- versus distance from the Yarlung suture ophio middle Miocene cooling, except for apatite nated by age-probability peaks at ca. 22 Ma and litic rocks reveals a dominance of 25–7 Ma ages (U-Th)/He data from the Gangdese batholith and ca. 17 Ma, accompanied by a ca. 25 Ma peak in within ~20 km, and a broader 60–6 Ma range Liuqu Formation, which record late Miocene the biotite 40Ar/39Ar data and a younger ca. 9 Ma between 20 and 125 km (Fig. 7). The oldest ages cooling (Figs. 7 and 8). Age-probability peaks peak in apatite (U-Th)/He data, which may re- in the southern Lhasa terrane are from the zircon for Gangdese batholith and Kailas Formation flect differences in closure temperature (Fig. 8). (U-Th)/He and ZFT systems, whereas the young- samples display a slight dependence on closure Age-probability peaks for the Kailas Formation est ages are mainly from the apatite (U-Th)/He temperature (decreasing age with decreasing clo- are between 20 and 12 Ma, i.e., slightly younger and AFT systems (Fig. 7), likely reflecting the sure temperature). Aside from a few older ages than those of the Gangdese batholith, consistent higher and lower closure temperatures, respec- from the Gangdese batholith in the Lhasa region, with a southward progression of exhumation. tively. In the Lhasa and Lopu Range regions, which may reflect igneous activity or conduc- Thermochrono logic data from the Liuqu For- biotite 40Ar/39Ar ages are between 27 and 17 Ma, tive cooling after emplacement, the majority of mation indicate that it cooled through apatite overlapping in age with zircon (U-Th)/He, AFT, cooling through 40Ar/39Ar biotite to AFT closure (U-Th)/He closure between 10 and 4 Ma, likely and apatite (U-Th)/He data, and likely indicating temperatures (~350–60 °C) took place between recording Yarlung River incision (e.g., Carrapa

Magmatism Sedimentation Deformation Metamorphism Thermochronology 0 Gangdese ArcKailas Fm. Liuqu Fm.

10 E-W Extension Himalayan Himalayan Thrusting Liuqu Fm. 20 GT GC T STD S

Ma Kailas Fm. KF

30 North-HImalayan Leucogranites

40 Eo-Himalayan Tethyan Thrust Belt High-Himalayan Leucogranites Gangdese Arc 50 ZFT (n = 6) Z He (n = 5) A He (n = 5) AFT (n = 62) AFT (n = 11) Z He (n = 15) A He (n = 14) Ar/Ar Bio (n = 6) Normalized Probability Figure 8. Summary of tectonic events discussed in the text, including magmatism, sedimentation, deformation, and metamorphism. In addition, probability distribution functions (PDFs) of compiled thermochronological data are shown, separated by system and rock unit (Supplementary Table DR3 [see text footnote 1]), from the Yarlung suture zone across southern Tibet. Timing of South Tibetan detachment system (STDS) is interpreted from data summarized in Webb et al. (2017). Sample locations and ages are provided in Figure 7. PDFs were generated using DZStats (Saylor and Sundell, 2016). KF—Kailas Formation; GT—Gangdese thrust; GCT—Great Counter thrust; ZFT—zircon fission track age; AFT—apatite fission track; ZHe—zircon (U-Th)/He; AHe—apatite (U-Th)/He.

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et al., 2014) and possibly influenced by Miocene 61012AL2 (Fig. 6). Comparison with a prob- was unlikely to have initiated until after 23 Ma (ca. 16 Ma) to recent orogen-parallel extension ability distribution function representative of in the Lazi region. If the Kailas Basin is contrac- (Fig. 8; e.g., Sundell et al., 2013). Gangdese arc activity generated from a compi- tional (e.g., Yin et al., 1999; Wang et al., 2015), lation of igneous samples (Orme et al., 2015) in- then the Gangdese thrust might have been active DISCUSSION dicates that samples 61012AL2 and 61012AL3 as early as 27 Ma. The Great Counter thrust sys- were likely derived almost exclusively from tem cuts the Kailas Formation (Figs. 1, 2, and 3), Detrital Zircon Provenance Analysis Gangdese magmatic arc rocks (Fig. 6). Similar and provenance shifts and clast composition in detrital zircon age spectra characterize the Kai- the upper Kailas Formation indicate deposition Detrital zircon samples from the Yarlung las Formation at other locations along the Yar- synchronous with contraction. Therefore, we suture zone in the vicinity of the Lazi region lung suture zone (DeCelles et al., 2011, 2016; interpret that the Great Counter thrust system track sediment provenance prior to and during Leary et al., 2016b) and are indicative of local initiated ca. 23 Ma, possibly at the same time as India-Asia collision. Samples 62211PK5 and derivation, as no Pan-African or older zircons the Gangdese thrust (or equivalent structures). 7712AL2 (Figs. 1 and 2A), which we correlated from either the Lhasa terrane or India were The younger limit of Great Counter thrust with the Tethyan Himalayan sequence, are domi- identified (Fig. 6). Interestingly, the granodio- and Gangdese thrust activity is constrained by nated by Pan-African (500–600 Ma) detrital zir- rite immediately below the Kailas Formation, the timing of the transition from north-south cons that are typical of both the Lhasa terrane, close to the sample locality, is Middle Jurassic contraction to east-west extension in southern which rifted from Gondwana, and the Tethyan in age (sample 61012AL1; Figs. 2A and 3), Tibet, as north-south–oriented normal faults Himalayan sequence (Fig. 6). However, the ab- while only a small population of zircons with crosscut the Great Counter thrust system in sence of younger detrital zircons that might have similar ages from sample 61012AL2 was identi- multiple locations (Fig. 7). A compilation of been derived from the Late Triassic–Paleogene fied. Therefore, the Middle Jurassic Gangdese extension onset and acceleration ages (Sundell Gangdese magmatic arc is indicative of deriva- batholith rocks were likely exposed over a much et al., 2013) indicates that the majority of ex- tion from Indian sources alone, as is typical of smaller area compared to those of Late Creta- tensional structures initiated between 15 and the majority of the Tethyan Himalayan sequence ceous–Paleogene age at the time of deposition. 11 Ma. Some structures may have initiated (DeCelles et al., 2000; Gehrels et al., 2011). The youngest detrital zircons in both samples earlier, such as the ≥16 Ma Lopukangri fault, Similarities between age spectra from these two overlap in age with the fine-grained granitic ig- which crosscuts the Great Counter thrust system samples and a composite reference curve for the neous rocks (sample 61012AL7) located in the ~250 km to the west (Laskowski et al., 2017). Tethyan Himalayan sequence (Fig. 6; Gehrels hanging wall of the northernmost Great Counter An absolute minimum age for the Great Coun- et al., 2011) support our field correlations. thrust splay to the south (Fig. 2A). Therefore, it ter thrust system is provided by the crosscutting Sample 62211PK3, located northeast of sample is possible that these rocks were exposed to the relationship with the ca. 10 Ma pluton in the 62211PK5 and southwest of sample 7712AL2 south of the Kailas Basin during deposition. northeast corner of the study area (Figs. 1, 2A, (Fig. 1), is characterized by Pan-African and and 3B). The timing of slip on the South Tibetan older ages alongside younger, Paleozoic–Early Timing Constraints for Major Structures detachment system is largely interpreted from a Jurassic (~ca. 180 Ma, n = 2) ages. The detrital in Southern Tibet compilation of tectonic data provided by Webb zircon age spectrum for this sample is similar et al. (2017), who indicated a 26–16 Ma range to that of other Triassic–Early Jurassic Tethyan A synthesis of crosscutting relationships and at the longitude of the study area, with signifi- Himalayan rocks in southern Tibet (Fig. 6; Aik- geochronological results from the Lazi region cant along-strike variation. If the South Tibetan man et al., 2008; Cai et al., 2012; Webb et al., (Fig. 1) with regional tectonic data provides detachment system is kinematically linked with 2013; Li et al., 2015; Cai et al., 2016). Since the some new information on the timing of slip the Great Counter thrust system, then it may not Lhasa terrane had rifted from Gondwana prior to across major structures in southern Tibet. From have been activated until ca. 23 Ma, if the con- deposition of the Triassic–Early Jurassic Tethyan north to south, these include the north-dipping straints from the extensional interpretation of Himalayan sequence strata (e.g., Li et al., 2016), Gangdese thrust (or equivalent north-dipping the Kailas Basin apply. and there is no known Indian source of Permian– structures beneath the Gangdese Mountains; Early Jurassic detrital zircons, we interpret that Fig. 1), the south-dipping Great Counter thrust Observations that Guided Structural zircons in sample 7712AL2 were derived from system, the inferred north-dipping listric nor- Model Development crustal fragments along the northwestern margin mal fault that provided accommodation space of Australia (modern-day West Papua), trans- for deposition of the Kailas Formation (Kailas Mapping, structural geology, geochronol- ported westward onto the northern margin of fault; e.g., DeCelles et al., 2011), and the South ogy, and thermochronology data presented in India, and incorporated into the Tethyan Hima Tibetan detachment system (Fig. 1). The Gang- this study, alongside constraints from previous layan sequence, in accordance with previous dese thrust was originally interpreted to have studies, guided development of our structural interpretations of age-equivalent Tethyan Hima- been active between 27 and 23 Ma, based on model for Oligocene–Miocene Yarlung suture layan rocks in southern Tibet (Cai et al., 2016). 40Ar/39Ar thermochronology data (Harrison zone evolution. At the regional scale (Fig. 1), Two detrital zircon samples from conglomer- et al., 1992; Yin et al., 1994; Copeland et al., Gangdese batholith rocks are exposed in a belt atic strata of the Kailas Formation (61012AL2, 1995). However, these constraints overlap with north of the Xigaze forearc that coincides with 61012AL3) yielded markedly different age the timing of deposition of the lower Kailas the highest-elevation portion of the ~1600-km- spectra than those of the Tethyan Himalayan se- Formation, which was deposited between 25 long Gangdese Mountains, with summit eleva- quence (Fig. 6). Detrital zircons in these samples and 23 Ma (Lazi Region detrital zircon data— tions that commonly exceed 5500 m north of are mainly distributed between two populations along-strike variation up to ~3 m.y.; Leary et al., Lazi (Fig. 1). In contrast, the Xigaze forearc with age-probability peaks at ca. 90 Ma and 2016b). If the extensional nature of the Kailas basin and Yarlung suture zone mélanges to the ca. 50 Ma. A few additional Middle Jurassic and Basin is correct (DeCelles et al., 2011, 2016; south are exposed in a relatively low-lying re- Permian detrital zircons were present in sample Leary et al., 2016b), then the Gangdese thrust gion, with peaks between 4500 and 5000 m

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surrounding the ~3900 m Yarlung River valley. antiform, as north-dipping Linzizong volcanic region surrounding Lhasa, and in the northern Together, thermochronologic data (Fig. 7) and rocks (the Paleocene–Eocene Gangdese mag- Lhasa terrane (Fig. 7). map relationships suggest that the Gangdese matic arc volcanic carapace) and the variably Structural model development was guided by batholith experienced higher exhumation mag- (~35°–60°) south-dipping Kailas Formation multichannel seismic reflection data obtained nitudes in the southern Gangdese Mountains are exposed along the northern and southern during the International Deep Profiling ofTibet than to the north, where the volcanic carapace is flanks of the Gangdese Mountains, respectively and the Himalaya (INDEPTH) experiment preserved (Fig. 1). The thermochronologic data (Fig. 1). This geometry can also be explained (Brown et al., 1996; Nelson et al., 1996; Alsdorf (Figs. 7 and 8) suggest that the Gangdese batho- by a ramp along the north-dipping Gang- et al., 1998; Hauck et al., 1998), collected along lith experienced some cooling during 25–23 Ma dese thrust (Yin et al., 1994). The majority of a transect from the High Himalaya into the deposition of the Kailas Formation (Leary et al., thermochrono logic ages older than ca. 25 Ma in Lhasa terrane (Fig. 9). We chose to favor these 2016b). The Gangdese batholith appears to be our data set are from the samples farthest north data over the more recent Hi-CLIMB receiver exposed in the core of an east-west–trending in the Gangdese Mountains, particularly in the functions experiment (e.g., Nabelek et al., 2009)

SW CCRegional Cross Section (Fig. 10) ′ NE A 15 YDR YDR YDR BTS YDR 30 MHT YZR MHT 45 Depth (km) 60 GDR km 25 50 Moho 75 INDEPTH multichannel common midpoint migrated seismic reflection profile

B km 50 100 s file ro p n o ti Lhasa Terrane c 30ºN le C′ f e r

c wide-angle i

m segment

Yarlung Suture Zone H T map area P E D N I

Tethyan Himalaya

C 28ºN Greater Himalaya 87ºE 91ºE

Figure 9. (A) International Deep Profiling of Tibet and the Himalaya (INDEPTH) common-midpoint projected seismic reflection profile from Alsdorf et al. (1998), including labels for prominent seismic reflectors. These include the Moho, the Main Himalayan thrust (MHT), the back-thrust system (BTS), the Yarlung-Zangbo reflector (YZR), the Yamdrok-Damxung reflection (YDR), and the Gangdese deep reflector (GDR). The simplified regional cross section presented in Figure 10 is shown atop these data to illustrate where our interpretations are supported by projected seismic reflection data. (B) Generalized tectonic map of southern Tibet, show- ing the location of the regional-scale cross-section C-C′ through the Lazi region map area (labeled with the box), and the location of INDEPTH seismic reflection profiles used to create the profile in A. Active structures (adapted from Taylor and Yin, 2009) are shown to highlight the coincidence of the INDEPTH profiles and rift hanging walls.

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due to the comparatively high mid- to upper- an ophiolitic slab separating Indian- and Asian- mation along-strike to the west (Fig. 8; Carrapa crustal resolution of the INDEPTH experiment. affinity rocks (Makovsky et al., 1999). Farther et al., 2014). We integrated the orientation, depth, and previ- south, series of south-dipping reflections (re- Any tectonic model of Yarlung suture zone ously published interpretations of major reflec- ferred to as the “back-thrust system”) are visible evolution during Oligocene–Miocene time tors by projecting a composite profile generated in the INDEPTH data that project beneath the must explain the geological, geophysical, and from migrated rift-parallel seismic lines (Fig. 9; North Himalayan domes (BTS in Fig. 9), in- thermochronological constraints summarized Alsdorf et al., 1998) ~250 km to the west into cluding the Mabja dome south of Lazi (Fig. 1). herein and, in particular, provide a mechanism a roughly parallel, regional-scale cross section The back-thrust system (BTS) reflection has for Gangdese arc exhumation both during and through the Lazi region (Fig. 9). previously been interpreted as the downdip after deposition of the Kailas Formation, syn- Several prominent reflectors are visible in the projection of the Great Counter thrust system, chronous with deposition of the upper Kailas INDEPTH data (Fig. 9); their interpretation is requiring that the Great Counter thrust system Formation. If a north-dipping fault like the critical to understanding the subsurface geol- cut Greater Himalayan rocks that are currently Gangdese thrust were not active at the same ogy of the Yarlung suture zone. The northern- exposed in the North Himalayan domes (Fig. 1; time as the Great Counter thrust system, then most reflector is the Yamdrok-Damxung reflec- Hauck et al., 1998). we would expect that that Gangdese batholith tion band (YDR in Fig. 9; Brown et al., 1996), The Kailas and Liuqu Formations provide a rocks would have experienced burial beneath consisting of a band of reflectors with varying rich record of sediment transport, suture zone the Kailas Formation and additional tectonic north and south dips between 12 km and 18 km basin evolution, and thermal evolution during burial in the Great Counter thrust footwall dur- depth that was interpreted either as a midcrustal Oligocene–Miocene time. The basal uncon ing this period. Furthermore, between 25 and partial melt layer (Nelson et al., 1996) or as a formity between the Kailas Formation and 23 Ma, the Kailas Basin was in a position of preexisting structural or lithologic boundary Gangdese arc plutonic rocks, which are Middle low relative elevation and characterized by a that was subsequently deformed, likely af- Jurassic to Eocene in age in the Lazi region warm and wet climate (DeCelles et al., 2011, ter the shutdown of Gangdese arc magmatism (Fig. 2A), implies that the Gangdese batholith 2016), whereas today, the Kailas Formation (Alsdorf et al., 1998). Results from the subse- underwent significant exhumation prior to the is exposed at high elevation (4800—6700 m), quent Hi‑CLIMB receiver function experiment onset of Kailas Formation deposition, which draped along the southern margin of the Gang- indicate that the Yamdrok-Damxung reflec- occurred between 25 and 23 Ma in the Lazi re- dese Mountains, with an arid- to semiarid tion (Fig. 9) is likely constrained to rift valleys gion (Leary et al., 2016b). Growth strata in the climate and mean annual temperatures of ap- (Fig. 9) and is largely discontinuous (Nabelek Kailas Formation (Wang et al., 2015) indicate proximately –5 °C at 5000 m elevation (Quade et al., 2009; Hetényi et al., 2011). Therefore, progressive southward steepening of the basal et al., 2011). A possible alternative explanation we did not consider the Yamdrok-Damxung re- unconformity during deposition, while detrital involves erosion of the Great Counter thrust flection in development of the structural model. zircon data, conglomerate clast counts, sand- system hanging wall(s) at a rate greater than Three north-dipping reflectors between ~20 and stone modal petrography, and paleocurrent indi- or equal to the rate of rock uplift, preventing 30 km depth are present beneath the southern cators indicate initial Gangdese arc provenance tectonic burial of the Gangdese arc rocks in the extent of the Yamdrok-Damxung reflection (re- from the north in the lower Kailas Formation, southern Gangdese Mountains (Fig. 1). We in- flection “4” in Fig. 9), displaying decreasing followed by provenance from the south in the terpret that this scenario is unlikely given that northward apparent dip with increasing depth. upper Kailas Formation (DeCelles et al., 2011, it would require the Yarlung River to remain The uppermost of these reflections projects into 2016; Leary et al., 2016b). Leary et al. (2016a) relatively stationary atop a zone of enhanced the Yamdrok-Damxung reflection. These were interpreted growth strata in the Liuqu Formation rock uplift throughout the period in which the interpreted as a hinterland-dipping duplex above within the footwall of a Great Counter thrust Great Counter thrust system was active (con- a footwall ramp along a north-dipping fault that system splay (Fig. 4E; Leary et al., 2016a) to strained to 23–16 Ma). This scenario is also dif- projects to the surface within the Yarlung suture indicate syntectonic sedimentation. ficult to evaluate, as evidence for the existence zone, possibly equivalent to the Gangdese thrust Thermochronologic data from Gangdese of the Yarlung River during early Miocene time (Alsdorf et al., 1998; Makovsky et al., 1999). batholith rocks just to the east of the Lazi re- is limited to detrital zircon geochronology data Deep imbrication structures are also apparent gion study area (Fig. 7) indicate that the Gang- from fluvial deposits in northeast India (e.g., in Hi-CLIMB data at depths of 50–60 km, im- dese arc was being exhumed immediately prior Lang and Huntington, 2014). mediately above the Moho, extending north- to, during, and shortly after Kailas Formation Despite the obscurity of north-dipping struc- ward of the Yarlung suture zone for ~30 km deposition, with maximum age probability be- tures such as the Gangdese thrust, and the poorly (Nabelek et al., 2009). Due to the great depth tween 20 and 15 Ma (Figs. 5 and 7). The most constrained magnitude of the Great Counter of the anomalies and comparatively low reso- proximal Gangdese batholith thermochrono- thrust due to a lack of hanging-wall cutoffs, the lution at mid- to upper-crustal depths, it is un- logic data, collected along an approximately Yarlung suture zone in southern Tibet appears clear whether these structures can be linked to north-south transect ~5 km to the northeast of to have experienced large-magnitude shortening “reflection 4” (Fig. 9), or any faults that breach the Lazi region geologic map (Figs. 2A and 8), during Oligocene–Miocene time. Indeed, the the surface near the Yarlung suture zone. To the include three AFT ages between 25 and 23 Ma >1000 km along-strike continuity of the Great south of the Yamdrok-Damxung reflection, a alongside one older age at ca. 28 Ma and two Counter thrust system indicates that it accom- near-horizontal reflection band, referred to as ages at ca. 9 Ma (Ge et al., 2016) and seven modated significant shortening, while differ- the Yarlung-Zangbo reflector (YZR in Fig. 9), zircon (U-Th)/He ages between 23 and 17 Ma ential exhumation of Gangdese batholith rocks is visible at ~25 km depth (Fig. 9), possibly (Dai et al., 2013; Ge et al., 2016). Also of note relative to Xigaze forearc (Fig. 1) or Tethyan representing a low-angle structural discontinu- is the short duration between Kailas Formation Himalayan strata (primarily east of Lhasa) is ity below the surface exposure of the Yarlung deposition (25–23 Ma) and subsequent exhuma- consistent with the existence of a fault equiva- suture (Alsdorf et al., 1998), a hydrothermal or tion, which began by 17 ± 1 Ma, based on zircon lent to the Gangdese thrust (Searle et al., 1987; magmatic boundary (Alsdorf et al., 1998), or (U-Th)/He and AFT data from the Kailas For- Yin et al., 1994). It is possible that the Gangdese

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thrust reactivated the original, north-dipping Liuqu Formation and (4) growth structures in During early Miocene time, we interpret that megathrust—essentially the India-Asia suture the Liuqu Formation (Fig. 4E; Leary et al., the inferred normal fault that possibly accom- projected downdip—that accommodated con- 2016a), which indicate increasing southward modated Kailas Basin deposition was reacti- vergence between India and Asia. dip of the basal unconformity throughout depo vated as a top-to-the-north thrust system that sition, and (5) thermochronology data that in- roots into a hinterland-dipping duplex beneath Structural Model dicate significant exhumation of the Gangdese the Gangdese batholith (Fig. 10). South-directed batholith and Yarlung suture zone during Oligo- thrusting fed slip into a system of top-to-the- Passive roof duplexes, which are character- cene–Miocene time. north thrusts (the Great Counter thrust system), ized by imbricated, foreland-dipping thrust Our preferred model (Fig. 10) invokes an which together comprise a foreland-dipping, sheets in the absence of an emergent, hinter- initial, late Oligocene (ca. 26–23 Ma) phase passive roof duplex. In this model, splays of the land-dipping fault, have been recognized in the of extension along a top-to-the-north normal hinterland-dipping duplex, not exposed but pos- field and in seismic data since the 1980s (Banks fault, similar in orientation to the India-Asia su- sibly evidenced by INDEPTH seismic reflection and Warburton, 1986). Physical experiments ture projected downdip, to accommodate lower data (Fig. 9), can be considered equivalent to the predict that passive roof duplexes are more Kailas Formation deposition in an extensional Gangdese thrust of Yin et al. (1994). The lack common in zones of efficient surface erosion basin. The rationale for late Oligocene exten- of surface exposure of the Gangdese thrust in (Mora et al., 2014, and references therein), sion is based on the work of DeCelles et al. southern-central Tibet likely reflects the shal- consistent with critical taper wedge mechanics (2011, 2016) and Leary et al. (2016b), who lower depth of exhumation relative to south- of thin-skinned thrust belts (e.g., Davis et al., documented characteristic extensional basin eastern Tibet, where Gangdese batholith rocks 1983). Since their first recognition, it has been architecture, growth strata, and paleoenviron- are juxtaposed against Tethyan Himalayan se- postulated that efficient sediment storage in the mental indicators. These authors interpreted that quence rocks and the Yarlung suture zone as- adjacent monocline along the deformation front rollback (DeCelles et al., 2011) or peeling-back semblages are absent. is critical to the formation of passive roof du- (Leary et al., 2016b) of the subducted Greater The structural model (Fig. 10) initiates with plexes (Mora et al., 2014). In the Lazi region, Indian slab led to a phase of upper-crustal exten- deposition of the Kailas Formation between 26 evidence that might support the existence of a sion along the Yarlung suture zone during late and 23 Ma in a fault-bounded, extensional ba- passive roof duplex includes: (1) the presence Oligocene time. This phase of deformation is sin associated with the inferred, north-dipping of imbricate, foreland-dipping (south-dipping) included in the structural model for the sake of Kailas normal fault (DeCelles et al., 2011, 2016; thrust sheets, (2) the absence of an emergent, completeness, though the nature of the subse- Leary et al., 2016b). We favor this interpretation hinterland-dipping fault, (3) early Miocene quently formed contractional structures does not because it is explains the fanning southward- syntectonic sedimentation in a contractional depend on prior development of an extensional dipping growth strata in both the lower and setting in the upper Kailas Formation and the basin along the Yarlung suture zone. upper Kailas Formation (Wang et al., 2015),

Figure 10. Restored (top) and 25–23 Ma (Prior to Duplexing) modern (bottom) cross sections from the Himalayas, through C (South) Kailas Fault C′ (North) 10 sediment transport the Lazi region study area, and JKp oph OMk into the southern Lhasa ter- JKt Kx KPlv 0 JPg rane (A-A′; Fig. 1) based on the geologic map in Figure 1, km THS JPg International Deep Profil- 10 GHS ing of Tibet and the Himalaya 20 LHS (INDEPTH) seismic reflection data (Fig. 9), and our mapping results (Figs. 2A–2D). No Modern modern topography and exposure vertical exaggeration. The syn- Gangdese tectonic Liuqu Formation was Mabja Dome Mountains

10 Ml deposited 20–19 Ma (Li et al., STDS THS JKp oph Kx KPlv 2015; Leary et al., 2016a), so it a GHS THS OMk

0 t does not appear in the restored JK JKt Gangdese th JPg km ru cross section. Color-coding and st

10 JPg JPg unit names are keyed to Fig- BTS GHS GHS 4 ures 1 and 2A. Structural data LHS Duplex? 4 20 LHS 4 are indicated by the ball and YZR tick mark symbols, whereas the circled numbers indicate constraints from previous studies. Constraint “YZR” is the subhorizontal orientation of structural fabric referred to by previous workers as the Yarlung-Zangbo reflector (Fig. 9), constraint “BTS” refers to the south-dipping reflectors interpreted as the downdip projection of the Great Counter thrust (Fig. 9), and constraint “4” is the imbricate north-dipping reflectors beneath the Yamdrok- Damxung reflection (Fig. 9), previously interpreted as a hinterland-dipping duplex (Alsdorf et al., 1998). Constraint “a” is the estimated thickness of the Greater Himalayan sequence beneath the Tethyan Himalaya based on exposures in the footwall of the Gurla Mandata metamorphic core complex, ~600 km to the west of the study area (Murphy, 2007). LHS—Lesser Himalayan sequence.

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the narrow-but-deep, “lacustrine sandwich” tonic model of Webb et al. (2017), in which et al., 2017). Its exhumation might be related to basin architecture that is typical of the Kailas rollback and breakoff of the Greater Indian slab diffuse normal faulting–related, orogen-parallel Formation (DeCelles et al., 2016), and organic following Kailas Formation deposition resulted extension, or efficient fluvial incision along the geochemical data consistent with warm-water in a dynamic steepening of the basal décolle- Yarlung River valley. lacustrine deposition, which are interpreted ment, driving subcritical taper conditions after to reflect deposition at lower elevation than at ca. 20 Ma. Lithospheric-Scale Tectonics and the present (DeCelles et al., 2011, 2016). The posi- Our structural model provides an explanation Gangdese Culmination Model tion of the Kailas Basin adjacent to the Gang- for Miocene cooling ages in the Kailas Forma- dese magmatic arc, in the hinterland of the tion and Gangdese batholith in southern Tibet Documentation of the timing and nature of Himalayan thrust belt, implies that the Yarlung (Fig. 7), which in a simple structural model Yarlung suture zone fault systems, and their suture was likely at high elevation during the should have been experiencing tectonic burial relationship to contemporaneous sedimentary Oligocene, following >20 m.y. of collisional rather than exhumation during Great Counter basins and igneous activity, enables investi- orogenesis (e.g., Najman et al., 2010; Hu et al., thrust shortening. As deformation progressed, gation into the ways in which the suture zone 2016). Therefore, the Kailas Basin might have three splays of the Great Counter thrust sys- geology might relate to important structures transitioned to lower elevation between 26 and tem initiated, possibly propagating from north at lithospheric scale. The suture zone geology 23 Ma as the result of localized crustal exten- to south based on the depositional ages of the (Figs. 2 and 3) was integrated with the geology sion (DeCelles et al., 2011). The 25–23 Ma AFT upper Kailas Formation and the Liuqu Forma- of the Himalaya (specifically, the South Tibetan ages from Gangdese arc rocks proximal to the tion and trends in the Gangdese batholith and detachment system) using constraints from geo- Lazi region study area (Fig. 7; Ge et al., 2016) Kailas Formation thermochronologic data physical data (Fig. 9). Aside from implementa- might reflect erosional exhumation of Gang- (Figs. 7 and 8). tion of the Gangdese culmination model, devel- dese arc rocks during deposition of the Kailas Orogen-parallel extension mainly initiated oped herein, the key interpretation in Figure 10 Formation. during Miocene time (16–12 Ma) along the su- is linkage of the South Tibetan detachment During deposition of the upper Kailas For- ture zone in southern Tibet, providing a mini- system and the basal décollement of the Great mation, a transition to southerly provenance mum age constraint for the Great Counter thrust Counter thrust system. Although this relation- coincided with, or shortly preceded, a return to system (Sanchez et al., 2013; Sundell et al., ship was not observed in the field, their similar contraction along the Yarlung suture. Regional 2013; Laskowski et al., 2017). In addition, the timing (Fig. 8), spatial convergence (Figs. 1 and exhumation of the Gangdese batholith and Kai- ca. 10 Ma pluton exposed in the east part of 10), and consistent top-to-the-north kinematics las Formation occurred mainly between 27 and the study area (Figs. 2A and 5) provides an un- suggest that linkage was possible, if not prob- 15 Ma, based on prominent 40Ar/39Ar biotite, equivocal minimum age for the northern splay able (Yin et al., 1994, 1999, 2010; Yin, 2006; ZFT and AFT, and zircon and apatite (U-Th)/He of the Great Counter thrust system. Detrital zir- Webb et al., 2007, 2017; He et al., 2016). The age-probability peaks (Figs. 7 and 8). Biotite con geochronology on samples from foreland South Tibetan detachment system forms the Ar-Ar data (Copeland et al., 1987) provide the basin deposits in India (Lang and Huntington, roof fault of the North Himalayan domes (e.g., strongest evidence for cooling of Gangdese arc 2014), the termination of north-south sediment Larson et al., 2010), exposed just to the south of rocks between 26 and 23 Ma (Fig. 8). It is un- transport associated with the Kailas and Liuqu the Yarlung suture zone (Fig. 1), consistent with clear whether these data can be explained by Formations (Leary et al., 2016a), and previous the interpretation in Figure 10. Kailas Basin extension (e.g., DeCelles et al., interpretations of Yarlung suture zone thermo- The geology of the Lazi region (this study), 2011), as the sample locations are in the hang- chronology (Carrapa et al., 2014) suggest that and possibly the Yarlung suture zone as a whole ing wall of the interpreted north-dipping normal the Yarlung-Siang-Brahmaputra River system (e.g., Zhang et al., 2011; Laskowski et al., fault (Fig. 7). Alternatively, these data could be also initiated during early Miocene time. It is 2017), is characterized by alternating episodes explained by an earlier switch from extension to possible that the Yarlung River was established of extension and contraction. The timing of tec- contraction east of the field area, as evidenced through a combination of topographic inversion tonic mode switches appears to be consistent by recent summaries of the depositional history (i.e., uplift of the Himalayas to higher eleva- with changes in the behavior of the subducting of the Kailas Basin (Leary et al., 2016b) and tion than the Yarlung suture zone)—thought to Great Indian slab, characterized by contraction synthesis of regional tectonic data in the Hima- have taken place during Miocene time based during episodes of shallow underthrusting, ex- laya (Webb et al., 2017). We interpret that sub- on paleoelevation records and structural inter- tension during rollback, and duplexing during sequent 23–15 Ma exhumation was primarily pretations (Murphy et al., 2009)—and relief slab breakoff and return to northward under- driven by growth of the hinterland-dipping du- generation along the Gangdese Mountains re- thrusting (e.g., DeCelles et al., 2011; Laskowski plex beneath the Gangdese Mountains, resulting lated to duplexing. Middle to late Miocene AFT et al., 2016, 2017; Webb et al., 2017). The geol- in structurally higher fault-bend folding to tilt and apatite (U-Th)/He ages from the Gangdese ogy of the Yarlung suture zone in the Lazi region the Kailas Formation southward, drive erosional batholith, Kailas Formation, and Liuqu For- integrates well with these “subduction control” exhumation in the southern Gangdese Moun- mation (Figs. 5 and 9) likely reflect efficient models. It is plausible that the Kailas Basin tains, and generate the regional northward dip of erosional exhumation along the Yarlung River formed due to north-south extension associated Linzizong Formation, in the northern Gangdese valley following establishment of this continent- with rollback of the Greater Indian slab between Mountains (Fig. 10). Duplexing would be ex- scale drainage system (Carrapa et al., 2014). 25 and 23 Ma, with timing variation of up to pected in the hinterland of the Himalayan thrust The ca. 10 Ma leucogranite pluton that cuts 3 m.y. along strike (Leary et al., 2016b). Sub- belt, beneath the Yarlung suture zone, to regain across the Yarlung suture zone in the eastern sequently, the slab broke off, initially resulting crustal thickness and build taper following the portion of the study area (Fig. 2A) was possibly in dynamic steepening of the Himalayan basal extension episode associated with deposition of generated by isothermal decompression driven décollement (and subcritical taper conditions; the Kailas Formation. Similarly, duplexing would by orogen-parallel extension, or possibly by Webb et al., 2017), a plausible driver of the also be expected based on the lithospheric tec- decompression related to slab breakoff (Webb switch from north-south extension (the Kailas

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Cai, F., Ding, L., Laskowski, A.K., Kapp, P., Wang, H., Xu, ACKNOWLEDGMENTS Structures and rocks units exposed in the Q., and Zhang, L., 2016, Late Triassic paleogeographic Lazi region record a pronounced episode of reconstruction along the Neo-Tethyan Ocean margins, late Oligocene–early Miocene contractional We acknowledge Devon Orme and Kathryn Metcalf southern Tibet: Earth and Planetary Science Letters, for field collaboration and informative conversations, v. 435, no. C, p. 105–114, https://doi.org/10.1016/j deformation that drove exhumation of the Yar- the Arizona LaserChron Center for analytical support, .epsl.2015.12.027. lung suture zone and southern Lhasa terrane. and Ding Lin for field and laboratory collaboration. Carrapa, B., Orme, D.A., DeCelles, P.G., Kapp, P., Cosca, Our mapping reveals the presence of south- This research was supported by grants from the U.S. M.A., and Waldrip, R., 2014, Miocene burial and exhumation of the India-Asia collision zone in southern National Science Foundation Continental Dynamics dipping imbricate splays of the Great Counter Tibet: Response to slab dynamics and erosion: Geol- thrust system that juxtapose Tethyan Himalayan Program (EAR-1008527), the U.S. National Science ogy, v. 42, no. 5, p. 443–446, https://doi.org/10.1130 Foundation Instrumentation and Facilities program sequence strata, sedimentary- and serpentinite- /G35350.1. (EAR-1338583) to the Arizona LaserChron Center, Carrapa, B., Faiz bin Hassim, M., Kapp, P., DeCelles, P., and matrix suture zone mélange, Xigaze forearc ba- the China National Science Foundation (41490610), Gehrels, G., 2017, Tectonic and erosional history of sin strata, and the Kailas Formation, from south and the Geological Society of America (student re- southern Tibet recorded by detrital chronological sig- to north. Detrital zircon provenance analysis of search grant). 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