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 thermo chronologic 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 geo physics 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 . Geological Society of America Bulletin, v. 130, no. 7/8 1355 © 2018 The Authors. Gold Open Access: This paper is published under the terms of the CC-BY license Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/130/7-8/1355/4224640/1355.pdf by guest on 27 September 2021 Laskowski et al. 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 exten- sional 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

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