Oligocene–Miocene Kailas basin, southwestern Tibet: Record of postcollisional upper-plate extension in the Indus-Yarlung suture zone P.G. DeCelles†, P. Kapp, J. Quade, and G.E. Gehrels Department of Geosciences, University of Arizona, Tucson, Arizona 85721, USA “…a sunlit temple of rock and ice. Its remarkable structure, and the peculiar harmony of its shape, justify my speaking of Kailas as the most sacred mountain in the world.” —A. Gansser, 1939 ABSTRACT Indus-Yarlung suture zone, suggesting that grained deposits, some authors (Aitchison the basin-forming mechanism recorded by the et al., 2002, 2007) have proposed that the Indo- The Kailas basin developed during late Kailas Formation was of regional signifi cance Eurasian collision did not begin until middle Oligocene–early Miocene time along the and not exclusively related to local kinematics Cenozoic time. Indus-Yarlung suture zone in southwestern near the southeastern end of the Karakoram In this paper, we report the results of an in- Tibet. The >2.5-km-thick basin-fi lling Kailas fault. We propose that extension of the south- vestigation of the Oligocene–Miocene Kailas Formation consists of a lower coarse-grained ern edge of the Eurasian plate was caused Formation (Cheng and Xu, 1986) along the proximal conglomerate and more distal fl u- by southward rollback of underthrusting Indus-Yarlung suture in southwestern Tibet (Fig. vial sandstone member, a lacustrine shale Indian continental lithosphere, followed by 1A); this unit is the type example of the Gan- and sandstone member, and an upper red- slab break-off. Alternating episodes of hard grinboche conglomerates. The Kailas Formation bed clastic member. Felsic tuffs and trachy- and soft collision, asso ciated with regional is of interest because it consists of a complex of andesite layers are locally present. De trital contraction and extension, respectively, in coarse- to fi ne-grained clastic strata more than and igneous zircon U-Pb ages indicate depo si- the Tibetan-Himalayan orogenic system may 4 km thick (Heim and Gansser, 1939; Gansser, tion of most of the Kailas Formation between have been related to changing dynamics of 1964), representing a basin of unknown tectonic ca. 26 and 24 Ma. The Kailas Formation was the subducting/underthrusting Indian plate. affi nity that developed ~30 m.y. after the putative deposited by alluvial-fan, low-sinuosity fl u- onset of Indo-Eurasian intercontinental collision vial, and deep lacustrine depositional systems INTRODUCTION (Garzanti et al., 1987), in a region that other- in buttress unconformity upon andesitic vol- wise would be expected to have been structur- canic (ca. 67 Ma) and granitoid (ca. 55 Ma) The Indus-Yarlung suture zone formed when ally ele vated and deeply eroded as the collision rocks of the Gangdese magmatic arc. Abun- the Indian continental landmass collided with the continued. The enigma of the Kailas Formation dant organic material, fi sh and amphibian southern fl ank of Eurasia during late Paleocene– is heightened by the fact that a major portion of fossils, and sparse palynomorphs suggest early Eocene time (Besse et al., 1984; Garzanti it consists of deep-water lacustrine deposits. We that Kailas lakes developed in a warm tropi- et al., 1987; Leech et al., 2005; Green et al., address the tectonic setting and paleogeography cal climate, quite different from coeval ba- 2008). Along much of the length of the suture of the Kailas basin, and its implications for the sins in central Tibet, which formed at high zone, rocks of the northern (Tethyan) Himalayan postcollisional tectonics of the Indus-Yarlung elevation in a dry climate. Provenance and thrust belt and ophiolitic mélange were juxta- suture and southern Tibet. The data we present paleocurrent data indicate that the bulk of posed by the south-dipping Great Counter thrust indicate that the Kailas Formation accumulated the Kailas Formation was derived from the against rocks of the Gangdese magmatic arc in an extensional basin, raising the prospect that northerly Gangdese magmatic arc (Kailas complex (Heim and Gansser, 1939; Burg et al., upper-plate extension was associated with south- magmatic complex). Only during the latest 1987; Yin et al., 1999; Murphy and Yin, 2003). ward rollback of the underthrusting Indian plate stages of basin fi lling was abundant sediment Resting unconformably upon the Gangdese arc and/or transtension along the early Karakoram derived from the southerly Tethyan Hima- rocks in the footwall of the Great Counter thrust fault. In either case, these results present new layan thrust belt in the hanging wall of the is a several-kilometer-thick middle Cenozoic details about the postcollisional history of this Great Counter thrust. Kailas basin stratig- alluvial-fl uvial-lacustrine deposit (Heim and archetypal suture zone that are not explained by raphy resembles a classic lacustrine sandwich Gansser, 1939) referred to by several different existing geodynamic models. and is most consistent with deposition in an names locally along the suture zone. Aitchison extensional or transtensional rift that devel- et al. (2002) proposed the umbrella stratigraphic GEOLOGICAL SETTING oped along the suture zone some 30 m.y. after term “Gangrinboche conglomerates” to include the onset of Indo-Eurasian intercontinental all of these sparsely dated units along ~1300 km The Tibetan Plateau and its southward-fl anking collision. Correlative coarse-grained syn- of the suture zone. The tectonic signifi cance Himalayan rampart have developed in the context tectonic strata similar to the Kailas Forma- of the Gangrinboche conglomerates remains a of northward subduction of Indian Neotethyan tion crop out along a >1300 km length of the fundamental problem in understanding the post- lithosphere beneath the Eurasian plate, climax- collisional history of the suture zone. Indeed, ing with the early Cenozoic collision between †E-mail: [email protected] based partly on interpretation of these coarse- the two continental landmasses (Argand , 1924; GSA Bulletin; July/August 2011; v. 123; no. 7/8; p. 1337–1362; doi: 10.1130/B30258.1; 18 fi gures; 4 tables; Data Repository item 2011057. For permission to copy, contact [email protected] 1337 © 2011 Geological Society of America DeCelles et al. 76°E A 80° 84° 88° 92° 96° 100° 104° 0 100 200 km Tarim Basin North t Qaidam Basin ul Qili China fa an S h han 36° Tag Altyn Kunlun Shan 36° Songpan-Ganzi te rrane JSZ KF Qiangtan Xiangshuihe fault Q g iang IY tan te 32° g anticl rra S inorium ne 32° 76° Z BSZ Figure 1. (A) Tectonic map of Lhasa terrane the Tibetan Plateau and Hima- N Ji South Oi al i f China layan thrust belt, after Yin Legend Lhasa au H lt Folds im T and Harrison (2000). Labeled a IYS 28°N Thrust faults 80° lay solid circles indicate loca tions an Suture zones th of other middle to late Ceno- rust belt Tertiary graben Fig.1B zoic basins in which paleo- Strike-slip faults MFT altimetry studies have been Low-angle normal faults India ≥ conducted: Z—Zhada Basin Elevation 4.5 km 84° 88° 92° 96° 100° (Saylor et al., 2009); Oi—Oiyug Elevation ≥ 3 km Abbreviations Elevation < 3 km IYS: Indus-Yarlung suture zone BSZ: Bangong suture zone Basin (Currie et al., 2005); N— JSZ: Jinsha suture zone KF: Karakoram fault Gangdese magmatic belt Nima Basin (DeCelles et al., MFT: Main Frontal thrust 2007b); T—Thakkhola gra- ben (Garzione et al., 2000a). B AYS Z Fig. 5A Rec tangle indicates location of h a T ° d ibeta 32 N a Karakoram n map shown in part B. (B) Gen- b (Geo Pla a fault logy te s no au eral geological map of south- i t s n how Mt. Kailas n) western Tibet and adja cent (6714 m) T portion of Hima layan thrust et hy Fig. 2 belt, from Murphy and Yin STD an (2003). Major faults in the Hima- layan thrust belt include the GM GCT Great Counter thrust (GCT), MCT Main Central thrust (MCT), 30°N Dadeld hura thrust (DT), Jajarkhot thrust (JT), Main H DT ima Bound ary thrust (MBT), Main laya Frontal thrust (MFT), and the STD South Tibetan detachment D (STD). Other abbreviations: JT GM—Gurla Mandatha; D— H MCT im Daulaghiri. Line across Ayi ala MBT ya MFT Shan (AYS) in northwestern n f ore suture zone is location of cross land 28°N basin section shown in Figure 5A. 0 120 240 km 80°E 82°E 84°E Miocene-Pleistocene basin fill Paleozoic-Eocene Tethyan Sequence Miocene-Pliocene Siwalik Group Neoproterozoic-Cambrian Greater Himalayan Sequence Oligocene-Miocene Kailas Fm. Paleoproterozoic-Neoproterozoic Lesser Himalayan Sequence Cretaceous-Eocene Gangdese arc and magmatic complex Strike-slip fault Detachment fault Jurassic-Cretaceous ophiolitic rocks in Indus Yarlung suture zone Normal fault Thrust fault 1338 Geological Society of America Bulletin, July/August 2011 Oligocene–Miocene Kailas basin, southwestern Tibet Powell and Conaghan, 1973; Molnar and Tap- Great Counter thrust is the northernmost major (Gansser, 1964). In the region of our study, ponnier, 1975; Tapponnier and Molnar, 1977; thrust system in the Tethyan Himalaya (Heim the Kailas Formation rests in buttress uncon- Allégre et al., 1984; Garzanti et al., 1987; Dewey and Gansser, 1939; Burg et al., 1987; Ratsch- formity upon andesitic volcanic rocks dated at et al., 1988). The timing of initial collision re- bacher et al., 1994; Yin et al., 1999; Murphy and 66.6 ± 1.26 Ma by U-Pb zircon (Figs. 3 and 4; mains a topic of debate (for a discussion, see Yin, 2003). In the Kailas Range (Fig. 1B), the sample 527052, Table DR11). These vol canic Aitchison et al., 2007), but most workers place Great Counter thrust is referred to as the South rocks are intruded by granite that yielded the event between ca.