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Authors requiring further information regarding Elsevier’s archiving and manuscript policies are encouraged to visit: http://www.elsevier.com/copyright Author's personal copy Tectonophysics 501 (2011) 28–40 Contents lists available at ScienceDirect Tectonophysics journal homepage: www.elsevier.com/locate/tecto Cenozoic anatexis and exhumation of Tethyan Sequence rocks in the Xiao Gurla Range, Southwest Tibet Alex Pullen a,⁎, Paul Kapp a, Peter G. DeCelles a, George E. Gehrels a, Lin Ding b a Department of Geosciences, University of Arizona, Tucson, AZ 85721, USA b Institute of Tibetan Plateau Research, Chinese Academy of Sciences, Beijing 100029, China article info abstract Article history: In order to advance our understanding of the suturing process between continental landmasses, a geologic Received 5 March 2010 and geochronologic investigation was undertaken just south of the India–Asia suture in southwestern Tibet. Received in revised form 30 December 2010 The focus of this study, the Xiao Gurla Range, is located near the southeastern terminus of the active, right- Accepted 5 January 2011 lateral strike-slip Karakoram fault in southwestern Tibet. The range exposes metasandstone, phyllite, schist Available online 13 January 2011 (locally of sillimanite facies), calc-gneiss and marble, paragneiss (± pyroxene), quartzite, metagranite, and variably deformed leucogranite. These metamorphic rocks are exposed in the footwall of a domal, top-to-the- Keywords: – fi Tethyan Himalayan sequence west low-angle normal (detachment) fault, structurally beneath Neogene Quaternary basin ll and Gurla Mandhata serpentinized ultramafic rocks of the Kiogar-Jungbwa ophiolite. The detachment is interpreted to be the Leucogranite northeastern continuation of the Gurla Mandhata detachment fault system that bounds metamorphic rocks of India–Asia suture the Gurla Mandhata Range ~60 km to the southwest. U–Pb geochronology on five detrital zircon samples of U–Pb zircon schist, phyllite, and quartzite yielded maximum depositional ages that range from 644–363 Ma and age probability distributions that are more similar to Tethyan sequence rocks than Lesser Himalayan sequence rocks. A felsic gneiss yielded a metamorphic zircon age of 35.3±0.8 Ma with a significant population of early Paleozoic xenocrystic core ages. The gneiss is interpreted to be the metamorphosed equivalent of the Cambro- Ordovician gneiss that is exposed near the top of the Greater Himalayan sequence. Leucogranitic bodies intruding metasedimentary footwall rocks yielded two distinct U–Pb zircon ages of ~23 Ma and ~15 Ma. Locally, rocks exposed in the hanging wall of this fault and of the southward-dipping, northward-verging Great Counter thrust to the north consist of serpentinite-bearing mélange and conglomerate of inferred Paleogene age dominated by carbonate clasts. The mélange is intruded by a 44 Ma granite and the stratigraphically highest conglomerate unit yielded detrital zircon U–Pb ages similar to Tethyan sequence rocks. We attribute the middle Eocene magmatism south of the suture to break-off of the Neo-Tethyan oceanic slab. In addition, our observations are consistent with the late Eocene shortening and crustal thickening within the Tethyan Himalayan sequence, early-middle Miocene leucogranite emplacement being related to underthrusting and melting of the Greater and possibly Lesser Himalayan sequences, and late Miocene arc- parallel extension in the hinterland of the southward propagating Himalayan thrust belt. © 2011 Elsevier B.V. All rights reserved. 1. Introduction metamorphism, and anatexis of Tethyan Himalayan rocks that comprise the early evolutionary history of the Himalayan fold-thrust A fundamental issue in tectonics concerns the behavior of the belt during Eo-Oligocene time. Earth's lithosphere during intercontinental collisional orogenesis. The The Tethyan Himalayan sequence composes the structurally Himalayan orogen is the manifestation of the ongoing continent– highest tectonic unit of the Himalayan fold and thrust belt and were continent collision between India and Asia and provides an ideal the first Indian-affinity rocks to be deformed immediately after final natural laboratory to investigate tectonic processes involved in such subduction of Neo-Tethys oceanic lithosphere. The timing of onset of collisions. However, the full understanding and exportability of the India–Asia collision is commonly taken to be ~55 Ma (Garzanti concepts learned from this orogen requires knowledge of the et al., 1987; Leech et al., 2005; Searle et al., 1987); however an geological evolution of the India–Asia suture zone and deformation, uncertainty of ±15 Ma (Aitchison et al., 2007; Yin and Harrison, 2000) highlights the need for improvements in our understanding of the initial collisional orogenesis between India and Asia. We conducted a geological investigation of the Tethyan Himalaya ⁎ Corresponding author. Present address: Department of Earth and Environmental – Sciences, University of Rochester, Rochester, New York 14627, USA. Tel.: +1 585 275 and the India Asia suture zone in southwestern Tibet. In this paper, we 5713. present a new geologic map and U–Pb ages for igneous, metamorphic, 0040-1951/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.tecto.2011.01.008 Author's personal copy A. Pullen et al. / Tectonophysics 501 (2011) 28–40 29 and detrital zircon samples. The results provide a better understanding sandstone of the Tethyan Himalayan sequence compose the hang- of the history of crustal deformation, metamorphism, and anatexis ing-wall of the South Tibetan detachment (Brookfield, 1993; Cheng within the hinterland of the Himalayan fold-thrust belt during middle and Xu, 1987; Gansser, 1964; Garzanti, 1999; Gaetani and Garzanti, Eocene–late Miocene time. 1991 and Heim and Gansser, 1939). The youngest, well-documented Tethyan Himalayan sequence strata deposited before final consump- 2. Geologic setting tion of Neo-Tethys oceanic lithosphere in Tibet are marine and Paleocene to Eocene in age (Willems et al., 1996). The Eocene strata The central Himalaya fold-thrust belt has accommodated N600 km are interpreted to record the transition from oceanic subduction to of upper-crustal shortening between India and Asia (DeCelles et al., India–Asia continental collision (Ding et al., 2005, Zhu et al., 2005), 2001, 2002; Murphy and Yin, 2003; Robinson et al., 2006; Srivastava although the possibility of an Eocene collision between India and an and Mitra, 1994)(Fig. 1). The Indian-affinity rocks deformed within intraoceanic arc has also been raised (Aitchison et al., 2007). Motion the Himalayan fold-thrust belt are exposed in four tectonostrati- on the South Tibetan detachment system is thought to have initiated graphic zones that are bounded by major faults zones (Gansser, 1964; prior to 22 Ma and ceased by ~19 Ma in most areas (Dézes et al., 1999; LeFort, 1986; Upreti, 1996). The structurally lowest unit, the Searle, 1999), however some argue for slip as early as 35 Ma (Lee and Subhimalayan unit, consists of Miocene–Pliocene foreland basin Whitehouse, 2007) and as late as 12 Ma (e.g. Murphy and Copeland, deposits that have been incorporated into several major thrust sheets 2005). in the frontal part of the range (DeCelles et al., 1998; Mugnier et al., Metamorphic domes are widespread within the Tethyan Himala- 1993). These rocks are truncated by the Main Boundary thrust, which yan physiographic zone, spanning most of the N2400 km arc-length carries Paleoproterozoic to Mesoproterozoic metasedimentary, sedi- between the eastern and western syntaxes (Fig. 1). The domes are in mentary, volcanic, and plutonic rocks of the Lesser Himalayan places cored by high-grade metamorphic rocks (Burg et al., 1984) and sequence in its hanging wall. The Lesser Himalayan sequence lies were exhumed by orogen-perpendicular (Tso Morari, Kangmar, and structurally beneath amphibolite-grade metasedimentary rocks of Mabja domes) (Berthelsen, 1953; de Sigoyer et al., 2000; Lee et al., Late Proterozoic–Cambrian age intruded by Cambrian–Ordovician 2004) or orogen-parallel extension (Leo Pargil, Gurla Mandhata, and plutons of the Greater Himalayan sequence along the north-dipping Ama Drime; Cottle et al., 2007; Jessup et al., 2008; Murphy et al., 2002; Main Central thrust, which was active during early Miocene time Thiede et al., 2006). (DeCelles et al., 2000, 2004; Hodges et al., 1992, 1994; Hubbard and Rocks exposed to the north of the India–Asia suture are Cretaceous– Harrison, 1989; Pêcher, 1989; Parrish and Hodges, 1996; Vannay and Cenozoic granitoids of the Gangdese batholith, Cretaceous to Eocene Hodges, 1996; Figs. 1 and 2). The northern boundary of the Greater marine strata of the Gangdese forearc (and forearc successor) basin, Himalayan sequence is marked by the generally northward dipping and Oligo-Miocene nonmarine strata of the Kailas (Gangrinboche) South Tibetan detachment system (Burchfiel et al., 1992; Hodges et Formation (Aitchison