Gangdese culmination model: Oligocene–Miocene duplexing along the India-Asia suture zone, Lazi region, southern Tibet Item Type Article Authors Laskowski, Andrew K.; Kapp, Paul; Cai, Fulong Citation Andrew K. Laskowski, Paul Kapp, Fulong Cai; Gangdese culmination model: Oligocene–Miocene duplexing along the India- Asia suture zone, Lazi region, southern Tibet. GSA Bulletin ; 130 (7-8): 1355–1376. doi: https://doi.org/10.1130/B31834.1 DOI 10.1130/B31834.1 Publisher GEOLOGICAL SOC AMER, INC Journal GEOLOGICAL SOCIETY OF AMERICA BULLETIN Rights © 2018 The Authors. Gold Open Access: This paper is published under the terms of the CC-BY license. Download date 09/10/2021 12:19:58 Item License https://creativecommons.org/licenses/by/4.0/ Version Final accepted manuscript Link to Item http://hdl.handle.net/10150/628513 Manuscript Text Click here to download Manuscript Lazi_Manuscript_Laskowski.docx 1 The Gangdese Culmination Model: Oligocene— 2 Miocene Duplexing along the India-Asia Suture Zone, 3 Lazi Region, Southern Tibet 4 Andrew K. Laskowski1*, Paul Kapp1, and Fulong Cai2 5 1Department of Geosciences, University of Arizona, Tucson, Arizona 85721, USA 6 2Key Laboratory of Continental Collision and Plateau Uplift, Institute of Tibetan Plateau 7 Research, Beijing 100101, China 8 *Current address: Department of Earth Sciences, Montana State University, Bozeman, 9 Montana 59717, USA 10 ABSTRACT 11 The mechanisms for crustal thickening and tectonic exhumation along the 12 Yarlung (India-Asia) suture in southern Tibet are debated, as the magnitudes, relative 13 timing, and interaction between the two dominant structures—the Great Counter thrust 14 and Gangdese thrust—are largely unconstrained. In this study, we present new geologic 15 mapping results from the Yarlung suture zone in the Lazi region, located ~350 km west 16 of the city of Lhasa, along with new igneous (6 samples) and detrital (6 samples, 474 17 ages) U-Pb geochronology data to constrain the crystallization ages of Jurassic— 18 Paleocene Gangdese batholith rocks and provenance of Tethyan Himalayan and 19 Oligocene—Miocene Kailas Formation strata. We supplement these data with a 20 compilation of 124 previously published thermochronologic ages from Gangdese 21 batholith, Kailas Formation, and Liuqu Formation rocks, revealing a dominance of 22 Oligocene—Miocene (23-15 Ma) cooling contemporaneous with slip across the Great 23 Counter thrust system. These data are systematically younger than 98 additional compiled 24 thermochronologic ages from the northern Lhasa terrane, recording mainly Eocene 25 cooling. Structural and thermochronologic data were combined with regional geological 26 constraints—including INDEPTH seismic reflection data—to develop a new structural 27 model for the Oligocene—Miocene evolution of the Tethyan Himalaya, Yarlung suture 28 zone, and southern Lhasa terrane. We propose that a hinterland-dipping duplex beneath 29 the Gangdese mountains—of which the Gangdese thrust is a component—is 30 kinematically linked with a foreland-dipping passive roof duplex along the Yarlung 31 suture zone—the Great Counter thrust system. This interpretation, referred to as the 32 Gangdese Culmination model, explains why the Gangdese thrust system is only locally 33 exposed (at relatively deeper structural levels) and provides a structural explanation for 34 early Miocene crustal thickening along the Yarlung suture zone, exhumation of the North 35 Himalayan domes, relief generation along the modern Gangdese Mountains, Early 36 Miocene Yarlung river establishment, and creation of the modern internal drainage 37 boundary along the southern Tibetan Plateau. 38 INTRODUCTION 39 Documenting the structural style and timing of crustal thickening that produced 40 the ~5 km average surface elevation of the Tibetan Plateau is key to understanding the 41 response of continental crust to intercontinental collision and recognizing feedbacks 42 among climate, surface processes, and tectonics (e.g. Quade et al., 2003; Harrison et al., 43 1992; Beaumont et al., 2001; Whipple, 2009). The significance and timing of Cenozoic 44 fault systems along the ~1300-km-long Yarlung (India-Asia) suture in southern Tibet 45 (Fig. 1), however, remain a subject of debate. Juxtaposition of deeply-exhumed magmatic 46 arc rocks of the southern Lhasa terrane against Indian passive margin strata, as well as 47 thermochronologic data and field mapping, led to the discovery of a north-dipping 48 mylonitic shear zone—the Gangdese thrust—that carried magmatic arc rocks southward 49 in its hanging wall (Yin et al., 1994; 1999). Primarily documented southeast of the city of 50 Lhasa, the Gangdese thrust was interpreted as a crustal-scale, Late Oligocene—Early 51 Miocene (27-23 Ma) structure based on 40Ar/39Ar thermochronology data, with a 52 minimum displacement of 46±9 km (Harrison et al., 1992; Yin et al., 1994; Copeland et 53 al., 1995). However, this structure is apparently not exposed along-strike to the west of 54 Lhasa, leading others to call into question its significance and along-strike continuity 55 (Aitchison et al., 2003). The dominant structures along the Yarlung suture west of Lhasa 56 are a system of south-dipping reverse faults called the Great Counter thrust (Heim and 57 Gansser, 1939; Yin et al., 1999; Murphy and Yin, 2003), which typically places Indian 58 passive margin rocks on suture zone mélange, mélange on forearc basin strata, and 59 forearc basin strata on the Oligocene-Miocene conglomerate, from south to north. A lack 60 of hanging wall cutoffs and no clear thermochronological date difference across 61 individual fault splays renders constraints on the timing and magnitude of Great Counter 62 thrust activity tenuous, but most studies agree that it was active by Late Oligocene— 63 Early Miocene time (Quidelleur et al., 1997; Harrison et al., 2000; Yin et al., 1999; Wang 64 et al., 2015), temporally overlapping or closely following the Gangdese thrust. Despite 65 the close spatial and temporal relationship between the Great Counter thrust system and 66 the Gangdese thrust (where the Gangdese thrust is exposed), the crosscutting or 67 branching relationships between them are not known. Whether the Gangdese thrust is an 68 orogen-scale structure that accommodated significant crustal shortening during Cenozoic 69 time, and the nature of its relationship to the more prominently exposed Great Counter 70 thrust system (Fig. 1), are open questions with major implications for Himalayan-Tibetan 71 tectonics. 72 To the north of the Yarlung suture zone is a belt of calc-alkaline plutonic rocks 73 that are dominantly Cretaceous to Paleogene in age (Schärer et al., 1984), referred to as 74 the Gangdese batholith, and related volcanic and volcaniclastic rocks that are dominantly 75 Paleocene to Eocene in age, referred to as the Linzizong Formation (Lee et al., 2009). 76 Collectively, these rocks compose the Gangdese magmatic arc, which developed along 77 the southern Lhasa terrane (Asian) margin during northward subduction of Neo-Tethyan 78 oceanic lithosphere and persisted during India-Asia collision (Kapp et al., 2007). The 79 Gangdese Mountains (Fig. 1) in southern Tibet (also called the Trans-Himalaya) are 80 composed mostly of Gangdese magmatic arc rocks. The Gangdese mountains define the 81 northern boundary of the Yarlung river watershed, in southern Tibet, and the southern 82 boundary of the internally drained portion of the Tibetan plateau. 83 Along the southern flank of the Gangdese Mountains (Fig. 1), an Oligocene- 84 Miocene, conglomerate-rich, continental unit referred to as the Kailas (Gangrinboche) 85 Formation (Gansser, 1964; Aitchison et al., 2002; DeCelles et al., 2011; 2016, Leary et 86 al., 2016b) is exposed in buttress unconformity atop Gangdese arc rocks. Nonmarine 87 strata of similar composition and structural position are continuous, with some variations 88 in sedimentary facies, for over 1300 km along the Yarlung suture zone (Leary et al., 89 2016b). Some workers interpret the Kailas Formation as the product of contractional 90 deformation, associated with a lithospheric flexure during a late stage of India-Asia 91 collision (Aitchison et al., 2007), flexural foreland basin deposition related to the Great 92 Counter thrust system (Wang et al., 2015), or wedge-top sedimentation related to out-of- 93 sequence Great Counter thrust system activity (Yin et al., 1999). However, recent 94 investigations of the Kailas Formation sedimentology, fossil assemblages, and basin 95 architecture indicate that the Kailas Formation was deposited in an extensional basin 96 bounded by a north-dipping normal fault, perhaps related to Oligocene—Miocene 97 rollback of the subducted Great Indian slab (DeCelles et al., 2011; 2016; Wang et al., 98 2013; Leary et al., 2016). 99 The Cretaceous—Paleogene Xigaze forearc basin (Einsele et al., 1994; Dürr, 100 1996, Wang et al., 2012; An et al., 2014; Orme and Laskowski, 2016) is exposed to the 101 south of the Kailas Formation across a splay of the Great Counter thrust system. Xigaze 102 forearc basin strata were deposited atop serpentinite mélange (Orme and Laskowski, 103 2016)—exposed along its southern margins in the Lazi region—suggesting that the 132- 104 122 Ma Yarlung suture zone ophiolites (Hébert et al., 2012; Chan et al., 2015) were in a 105 suprasubduction zone position at the onset of forearc basin deposition ca. 110 Ma (Huang 106 et al., 2015; Orme and Laskowski, 2016). Another conglomerate unit—the Liuqu 107 Formation—is locally exposed to