Oligocene–Miocene Kailas Basin, Southwestern Tibet: Record of Postcollisional Upper-Plate Extension in the Indus-Yarlung Suture Zone

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. Detrital contraction and extension, respectively, in coarse- to fi ne-grained clastic strata more than and igneous zircon U-Pb ages indicate deposi- 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 elevated 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.

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 graben (Garzione et al., 2000a). B AYS Z Fig. 5A Rectangle 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 adjacent (6714 m) T portion of Himalayan 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 Boundary 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 volcanic 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. 55 and 50 Ma, at least in the Kailas thrust system (Yin et al., 1999) and con- a U-Pb zircon age of 55.0 ± 0.8 Ma (Fig. 4; northwestern syntaxial region (Besse et al., 1984; sists of several thrusts that carry Tethyan sedi- sample 61052, Table DR1 [see footnote 1]) Patriat and Achache, 1984; Garzanti et al., 1987; mentary and low-grade metasedimentary rocks and a hornblende 40Ar/39Ar isochron age of ca. Leech et al., 2005; Green et al., 2008). As pointed as well as rocks that are considered to be part 45 Ma (Yin et al., 1999). Together, the volcanic out by Rowley (1998), Ding et al. (2005), and of the Gangdese forearc basin and accretionary and granitoid rocks are referred to as the Kailas Aitchison et al. (2007), however, the timing of wedge (Yin et al., 1999; Murphy and Yin, 2003). magmatic complex (Honegger et al., 1982) and initial collision in the central and eastern parts These rocks are juxtaposed directly against the constitute the local manifestation of the Gang- of the orogenic system remains weakly con- Kailas Formation along the southern fringe of dese magmatic arc. strained, and, in any case, it would be surprising its outcrop (Fig. 2), and the southernmost part West of the Mount Kailas region, the Indus- if the collision happened simultaneously along of the Kailas Formation is locally overturned in Yarlung suture zone is offset by the dextral the entire Himalayan orogenic belt. the footwall of the fault system (Gansser, 1964; Karakoram fault. Along the southwestern side The geology of southwestern Tibet includes Murphy and Yin, 2003). of the fault, the Ayi Shan (Fig. 1B; also referred four major tectonic features: the Gangdese mag- Yin et al. (1994, 1999) reported a second major to as the Ayilari Shan) consists of ca. 50 Ma matic arc, Indus-Yarlung suture, Karakoram fault, thrust fault, which they referred to as the Gang- granitic orthogneiss that experienced meta- and Tethyan Himalayan thrust belt (Fig. 1). The dese thrust, exposed in the Xigaze and Zedong morphic zircon growth at upper-amphibolite Gangdese arc is a complex of calc-alkaline batho- areas along the eastern part of the suture zone. conditions between 41 Ma and 31 Ma, and sub- liths and related volcanic and volcaniclastic rocks There, the fault dips northward beneath, and sequent partial melting and rapid exhumation that formed as a Cordilleran-style magmatic arc helped to structurally elevate, Gangdese batholith in the footwall of the north-dipping Ayi Shan along the southern fl ank of the Lhasa terrane in rocks prior to slip on the Great Counter thrust. detachment fault at 26–18 Ma (Fig. 5A; Zhang response to subduction of Tethyan oceanic litho- Although the Gangdese thrust is not exposed in et al., 2010). The Great Counter thrust and a sphere from Cretaceous to Eocene time (Allégre the Mount Kailas region, Yin et al. (1999) and thin erosional remnant of the Kailas Forma- et al., 1984; Pan, 1993; Kapp et al., 2007; Pullen Murphy and Yin (2003) inferred it to be present tion in its footwall are also preserved along the et al., 2008); igneous activity within the arc in the subsurface and responsible for uplift of the southwestern fl ank of the Ayi Shan (Fig. 1B; continued during the early stages of the Indo- Kailas magmatic complex during deposi tion of Murphy et al., 2000). As in the Kailas Range, Eurasian collision. The feature that separates the the Kailas Formation. Aitchison et al. (2003) dis- the Kailas Formation here rests unconformably Gangdese arc and related forearc rocks from the puted the existence of the Gangdese thrust along upon rocks of the Gangdese magmatic arc. Himalayan thrust belt is the Indus-Yarlung su- the entire suture zone; our surface observations Miocene slip on the Karakoram fault offset the ture. Within the suture zone, ophiolitic slivers and in the Kailas region are consistent with their in- rocks of the Ayi Shan ~65 km to the northwest sedimentary- and serpentinite-matrix mélanges terpretation, but also do not rule out the possible relative to the Kailas Range (Murphy et al., structurally overlie Tethyan Himalayan strata presence of the Gangdese thrust in the subsurface 2000; Valli et al., 2007). Thus, during late in the south and Gangdese forearc strata in the (Murphy and Yin, 2003). Oligocene–Miocene time, the rocks exposed north (Fig. 1B; Gansser, 1980; Tapponnier et al., The Kailas Formation (Cheng and Xu, 1986) today in the Ayi Shan were probably located 1981; Burg and Chen, 1984; Girardeau et al., is named for its type section on Mount Kailas directly beneath the western part of the Kailas 1984; Ratschbacher et al., 1994; Yin et al., 1994, (6714 m), which is also referred to by its Tibetan Range and were probably experiencing rapid 1999; Murphy and Yin, 2003; Ding et al., 2005). name of Gangrinboche. Based on original work exhumation in the footwall of the Ayi Shan de- Where dated, ophiolitic fragments are Jurassic– reported by Heim and Gansser (1939), Gansser tachment fault (Fig. 5B; Zhang et al., 2010). Cretaceous (Gopel et al., 1984; Zhou et al., 2002; (1964) named this unit the Kailas conglomer- Malpas et al., 2003; Miller et al., 2003; Ziabrev ate; however, conglomerate forms only a fraction STRUCTURAL SETTING OF THE et al., 2003). These ophiolitic rocks were ob- of the unit, so we use the more inclusive Kailas KAILAS FORMATION ducted onto the northern Indian margin during Formation. Adherents to the Hindu, Buddhist, Late Cretaceous–Paleocene (Burg and Chen, Jain, and Bön faiths regard Mount Kailas to be The Kailas Range trends parallel to the South 1984; Girardeau et al., 1984; Searle et al., 1987; sacred and offi cially off limits. We therefore fo- Kailas thrust system and the regional trend of Malpas et al., 2003; Ding et al., 2005) and possi- cused on outcrops in deep canyons 20–40 km the Gangdese magmatic arc (Fig. 2). Kailas bly Eocene (Davis et al., 2002) time. The Tethyan west of the mountain. Searle et al. (1990) cor- strata are mostly only slightly tilted toward Himalaya is composed of generally southward- related sparsely dated lithologically similar parts the south, but in the proximal footwall of the verging thrust sheets of unmetamorphosed Paleo- of the Indus Group in the northwestern Himalaya South Kailas thrust system, they are folded into zoic and Mesozoic sedimentary rocks (Burg with the Kailas Formation. Aitchison et al. (2002) a northward-verging fold pair that parallels the et al., 1987; Ratschbacher et al., 1994; Murphy correlated the Kailas Formation with Upper fault (Fig. 5B; Gansser, 1964). The northern part and Yin, 2003), locally disrupted by large domal Oligocene–Lower Miocene clastic rocks that crop structures involving mylonitic orthogneiss and out along nearly the entire length of the Indus- 1 paragneiss that were exhumed from midcrustal Yarlung suture (e.g., the Qiuwu, Dazhuqu, and GSA Data Repository item 2011057, Table DR1: U-Pb age data from igneous rocks and Table DR2: depths during middle to late Miocene time (Lee Luobusa Formations; see also Yin et al., 1999). U-Pb age data from detrital zircons, is available at et al., 2000, 2004, 2006; Murphy et al., 2002; Lee At Mount Kailas, the Kailas Formation rests http://www.geosociety.org/pubs/ft2011.htm or by re- and Whitehouse, 2007). The northward-verging unconformably upon the Gangdese batholith quest to [email protected].

Geological Society of America Bulletin, July/August 2011 1339 DeCelles et al.

PgrPgr 81°E KvKv 8181°15’E15’E KvKv KvKv

5KR5KR 1818 2525 A’ TuffsTuffs 24.624.6 MaMa 24.224.2 MaMa KvKv PgrPgr 3131°15’N15’N 10101515 3KR3KR 2929 1313 8 OMkOMk 6767 5555 MaMa 8080 8KR8KR 7KR7KR 6767 MaMa Q 3333 1KR1KR 2626 7171 4646 4141 4848 4KR4KR 4747 4949 KvKv 1212 PgrPgr 2KR2KR

Q OMkOMk KvKv A PgrPgr Ice/snowIce/snow mlml OMkOMk Q QuaternaryQuaternary ddepositseposits mlml N-QN-Q Neogene-QuaternaryNeogene-Quaternary ddepositseposits N-QN-Q OMkOMk Oligocene-MioceneOligocene-Miocene KKailasailas FFormationormation SouthSouth KKailasailas OMkOMk ThrustThrust ((GCT)GCT) PgrP313g1r°05'N0PaleogeneP5a'Nleogene ggraniteranite KvKv CretaceousCretaceous vvolcanicolcanic rrocksocks Mt.Mt. KailasKailas mlml MélangeMélange 8181°E 8181°15’E15’E (Gangrinboche)(Gangrinboche) 67146714 m StrikeStrike aandnd ddipip ooff beddingbedding 2626 0 2 4 6 8 1010 kmkm 7171 StrikeStrike aandnd ddipip ofof foliation,foliation, withwith pplungelunge ooff llineationineation TraceTrace ooff ssynclineyncline aaxisxis ThrustThrust fault,fault, barbsbarbs inin hanginghanging wwallall N MinorMinor sstrike-sliptrike-slip ffaultault TraceTrace ooff anticlineanticline aaxisxis 4KR4KR TraceTrace ofof mmeasuredeasured sectionsection 5555 MaMa GeochronologyGeochronology ssampleample llocationocation aandnd aagege

Figure 2. Geological map of the Mount Kailas area, showing locations of measured sections (e.g., 1KR, 2KR, etc.) and locations from which samples were obtained for U-Pb zircon dating. Cross section along line A-A′ is shown in Figure 5B.

of the Kailas Formation onlaps volcanic rocks in ~2500 m of measured stratigraphic sec- laterally down-depositional dip (southwest- of the Kailas magmatic complex, as discussed tions at eight localities (Figs. 6–8). Sections ward) into the fl uvial sandstone member. Both previously. These relationships are consistent were measured using a Jacob staff and tape of these units are capped by a major fl ooding with the geology directly east near Mount Kai- measure at the centimeter scale. Paleocurrent surface, which marks the base of the lacustrine las (Gansser, 1964; Yin et al., 1999; Murphy and data were collected by measuring 10 imbri- member (Fig. 9). The uppermost part of the Yin, 2003) and 65 km to the northwest in the cated clasts per station, or, in some cases, Kailas Formation consists of fl uvial deposits of Ayi Shan (Fig. 5A; Zhang et al., 2010), as well the limbs of trough cross-strata (method I the red-bed member. In the following sections, as elsewhere along the suture zone farther east of DeCelles et al., 1983). Samples for paly- we describe the sedimentological features of (Aitchison et al., 2002). nology, geochronology, and sedimentary these units, and document their mutual strati- petrol ogy were collected in the context of the graphic relationships. Because most of the STRATIGRAPHY AND measured sections. lithofacies in the Kailas Formation have been SEDIMENTOLOGY The Kailas Formation is divisible into four widely documented in the sedimentological informal members based on lithofacies assem- literature, we provide only brief descriptions Depositional environments in the Kailas blages and lithological characteristics (Fig. 9). and include a summary table in which standard Formation are reconstructed on the basis The basal unconformity is overlain by the interpretations are listed alongside abridged of sedimentological observations archived lower conglomeratic member, which grades descriptions (Table 1).

1340 Geological Society of America Bulletin, July/August 2011 Oligocene–Miocene Kailas basin, southwestern Tibet

24.1 ± 0.41 Ma Sample 7KR465 (1.7%) Kailas tuff bed Relative probability A

0 20 40 60 80 Ma 35 Sample PD4 Kailas tuff bed 33

25 Age (Ma) 23

19 Mean = 24.17 ± 0.51 Ma [2.1%], n = 25, MSWD = 1.5, Error bars are 2σ 17

B 40 Sample PD3 36 Kailas tuff bed

Age (Ma) 24

20 Mean = 24.60 ± 0.52 Ma [2.1%] n = 24, MSWD = 2.3, Error bars are 2σ 16

68 Sample 61052 64 Kailas magmatic complex, Granite 60

Age (Ma) 52 Figure 3. (A) Mount Kailas, viewed from the south. Subhorizontally stratiﬁ ed rocks holding up the peak and surrounding buttresses are 48 Mean = 54.98 ± 0.77 Ma [1.4%], n = 28, MSWD = 1.06, Error bars are 2σ the Kailas Formation; dark rocks in right foreground are in the 44 hanging wall of the South Kailas (or Great Counter) thrust; and 86 Sample 527052 82 Kailas magmatic complex, lighter-colored rocks in background at head of the major canyon are Andesite 78 granitoid rocks of the Kailas magmatic complex. (B) View toward 74 the southwest of the buttress unconformity at the base of section 70

1KR (see Fig. 2 for location). Ellipse indicates person for scale. 66

Age (Ma) 62

58 ± 54 Mean = 66.6 1.2 Ma [1.8%], n = 21, MSWD = 0.53, Error bars are 2σ 50

Figure 4. U-Pb zircon ages from samples of Kailas Formation tuffs (7KR465, PD3, PD4) and from the Kailas magmatic complex. See text for discussion and Table DR 1 for data. Note that all ages include both internal and external errors. MSWD—mean square of weighted deviates.

Geological Society of America Bulletin, July/August 2011 1341 DeCelles et al.

succession of conglomerate. In the uppermost A part of the lower conglomerate member, thick NE-SW cross section in the southeastern covered intervals are underlain by sandy con- SW Ayi Shan, from Zhang et al., 2010. NE glomerate and sandstone. AYSD 6 GCT KF Paleocurrent data from imbricated clasts

eiss a d schist (~270 measurements at 27 stations) in the lower gn n all 4 THS tw conglomerate member indicate southward foo tic AYSD oni paleoﬂ ow directions. These data are consistent Myl with provenance data (discussed later herein) 2 ca. 50 Ma granitic orthogneiss

Elevation (km) rapidly exhumed 26–18 Ma that demonstrate derivation from the Kailas magmatic complex. 0 No vertical exaggeration The association of lithofacies in the lower conglomerate member is consistent with depo- NE-SW cross section in the Kailas Range sition in alluvial fans that built southward off of (see Fig. 2 for location) ′ B A A high, rugged topography underlain by the vol- Summit elev. Mt. Kailas 55 Ma canic and granitoid rocks of the Kailas magmatic 67 Ma complex. The relatively scarce occurrence of 6 GCT sediment-gravity-fl ow facies (Gmm and disorganized Gcm) and the abundance of imbricated 4 THS AYSD and stratifi ed beds of conglomerate indicate that most deposition took place in stream fl ows, per- Elevation (km) 2 haps during fl ash fl oods. The lateral transition into amalgamated fl uvial sandstones suggests 0 that these were stream-dominated alluvial fans (Ori, 1982; Nemec and Steel, 1984; Ridgway Kailas Fm. lacustrine member and DeCelles, 1993; Wilford et al., 2005). In section 5KR, coarse boulder beds are abundant Kailas Fm. lower conglomerate, fluvial sandstone and red-bed members and indicate that sediment gravity fl ows were Tethyan Himalayan sequence more important in this part of the Kailas For- Granitic gneiss (in Ayi Shan), granite and andesite (Kailas Range) mation. It is plausible that sections 5KR and 1KR were deposited on different alluvial fans Figure 5. (A) Structural cross section across the southeastern end of the Ayi Shan (see that were dominated by debris fl ows and stream Fig. 1B for location), showing the Ayi Shan detachment fault (AYSD), Karakoram fault fl ows, respectively. (KF), Great Counter thrust (GCT), and the Tethyan Himalayan sequence (THS) modifi ed after Zhang et al. (2010). (B) Structural cross-section A-A′ (see Fig. 2 for location) across Fluvial Sandstone Member the Kailas Range in the study area. Squares indicate locations approximately on line of section from which samples of the Kailas magmatic complex were collected and dated by The fl uvial sandstone member is at least U-Pb on zircon. The topographic surface is from the line of section, and the dashed topo- 310 m thick; its base is not exposed in the graphic surface shows maximum elevations within ~2 km of the plane of the cross section. study area, but its top is marked by an abrupt Also shown is the projected elevation of the summit of Mt. Kailas. Projection of the Zhang transgressive surface. This member consists of et al. (2010) cross section onto plane of this cross section is referenced to the location of stacked and laterally overlapping, broadly len- the Great Counter thrust on both cross sections. Structure in hanging wall of the Great ticular, 0.5–2-m-thick beds of medium- to very Counter thrust is schematic. coarse-grained sandstone. Basal contacts of individual beds are erosional (Fig. 10D), and upward-fi ning sequences are common (Fig. 6, Lower Conglomerate Member 10B). These deposits are arranged in 1–6-m- section 2KR). The uppermost parts of many of thick beds, typically with slightly erosional basal these upward-fi ning sequences consist of dark- The lower conglomerate member is 613 m surfaces and fi ner-grained upper parts. Maxi- gray siltstone, and fragments of woody plant thick in section 1KR (Fig. 6) and at least 381 m mum clast size averages 40–60 cm in the lower material and fossil logs are abundant. Sedi- thick in section 5KR (Fig. 7), which is located part of section 1KR, and decreases to 30–40 cm mentary structures include planar horizontal 12 km northwest of section 1KR (Fig. 2). This in its upper part. In section 5KR, several-meter- lamination, trough cross-stratifi cation, and local member consists of an overall upward-fi ning thick beds of clast- and matrix-supported, disor- ripples. Many beds appear to be massive, and sequence of boulder to pebble conglomerate ganized boulder conglomerate (Gcm and Gmm; exposures form steep, high cliffs. with minor sandstone interbeds. Volumetrically Fig. 10C) are abundant; some of these beds have The broadly lenticular geometry and upward- dominant lithofacies include clast-supported, average maximum clast sizes greater than 1 m, fi ning grain size trends of sandstone beds in the moderately well-organized boulder-cobble with individual clasts >7 m in long dimension. fl uvial sandstone unit suggest deposition in shal- conglomerate that is massive (Gcm), crudely Lenticular beds of granular to coarse-grained low, laterally unstable channels. Absence of typi- horizontally stratifi ed (Gch), and commonly im- sandstone characterized by planar horizontal cal fi ne-grained levee and overbank facies implies bricated with long-axes transverse to paleofl ow lamination and trough cross-stratifi cation are lo- that the fl uvial system was dominated by shallow, direction (Gcmi, Gchi) (Figs. 6, 7, 10A, and cally intercalated within the otherwise unbroken laterally mobile, low-sinuosity stream channels.

1342 Geological Society of America Bulletin, July/August 2011 Oligocene–Miocene Kailas basin, southwestern Tibet

250 500 250 Gcmi Gcm LEGEND St

1KR240 Sh Petrographic sample Sr 240 Gcmi 1KR486 St,Sh 240 St U-Pb detrital zircon age St 10 Gcm Gchi 2KR232 Sm 480 Paleocurrent direction n = number of measurements Sm

220 220 St,Sh Fossil shell fragments St

460 Bioturbation Sm Gcmi,Gcm Vertebrate fossils Sm Sm 200 Leaf fossils 200 Sr 2KR 440 Fossil wood fragments N31°11.201’ Gcmi Sm 2KR184 E81°01.085’ Covered interval elev. 5474 m 180 180 Sr Grain size key: Sm 2KR420 Gcmi si fs cs cg Sm DZ 420 10 SiltstoneFine sandstoneCoarseConglomerat sandstone Fsl 420 1KR165 Gchi Sm St DZ Sp Gcmi Fsl Gchi,Gcm 160 Gcmi,Gcm 10 e 160 Sm St Gcmi Sp Fsl Gcmi,Gcm Sm St Sp 400 1KR Fsl 400 1KR148 Gcmi N31°11.731’ Sm Sm Gcmi E81°01.767’ Fsl 140 elev. 5334 m 140 Sm St TS Sm St 1KR630 Sr St Gcmi St 380 Gcmi 1KR629 Sm 380 1KR624 St DZ TS Sm Gcmi 10 Sm 120 620 TS 120 TS St 1KR615 TS Gcmi Gcmi St St Sm 360 360 Sm Gcmi 1KR353 Gcm 2KR107 Sm 10 Gcmi DZ DZ Sm Sp 600 100 St 100 Gct 10 Gcmi 10 St Gct,Gch 5 Gcmi St Gcmi TS Gcmi Fsl St 340 St 340 Sm 1KR86 Gcp Gcmi Fsl St TS Gcmi 10 2KR330 80 Gcm 580 80 Sr Gcmi Sr Gcm,Gch St 1KR73 St TS Gcmi St 1KR568 Gcmi Fsl 320 320 Sp Sm Gcmi Gcm,Gct 10 10 St Sm St Gcmi Sm 60 St,Sh 560 60 Gcmi Fsl Gcmi Sm St Gchi 10 300 Gcm 300 Gcmi Sm St Gcmi 10 2KR40 Sm St 10 Fsm 540 Gcm 40 40 Sm St Gcmi St Sm Gcm Gcm Sm Fsl Sm Gcm Gcmi Sm 280 St Gcm Fsl 280 Sm Gcmi Gcm Sm Gcmi Sm Sm St Sm Fsm Gct,St St 20 Gcmi 1KR267 520 Gcm 20 Sm Gct,St Sm St 10 A,B St St Gct,Gcm Sm Gcm Gcmi St St Fsl 260 Gcmi St Sm 260 Sm Gcmi Gcm Gcmi St 10 Sm 1KR255 St 2KR1 St Gcmi Gcmi Sm 0 250 10 500 0 250 si fs cs cg 10 si fs cs cg si fs cs cg si fs cs cg si fs cs cg N31°11.525’ 10 E81°02.171’ elev. 4934 m Figure 6. Logs of measured sections 1KR and 2KR. See Table 1 for lithofacies codes. TS—transgressive surface; DZ—detrital zircon sample.

Geological Society of America Bulletin, July/August 2011 1343 DeCelles et al.

3KR LEGEND ° 250 250 TS N31 14.011’ 250 E80°56.752’ St Petrographic sample Sr elev. 5147 m Sr Sm,St 240 Gcm U-Pb detrital zircon age 240 240 Sr TS St Sr 4KR Gcm Paleocurrent direction Fsl n = number of measurements 3KR227P SrFsl 5KR226 Sm N31°11.606’ Gcmi 3KR226 TS Sm,Sr Gcmi E81°00.881’ Gcm Fossil shell fragments Sm,St 220 Sm 220 220 Gcm elev. 5816 m Gcm 10 Gcm Sm 4KR Bioturbation 461 Fsl Sr,Sh Sm,St Sr TS Vertebrate fossils 4KR210 460

200 Leaf fossils 200 200 Gcmi 5KR195 Gcmi Fossil wood fragments

Gcm 10 440 Covered interval 180 Grain size key: 180 180 Gcm si fs cs cg Sm SiltstoneFine sandstoneCoarseConglomerate sandstone Sm Gcmi St 420 Gcm Sm Sm Fsl 4KR 160 160 160 Sr St 408DZ Gcm Sr Sm St Gcmi 4KR400 TS Sm Gcm 3KR147 400 St Sr,Sh 5KR145 10 5KR Gcmi Sh,St Gcm N31°17.151’ Sh,St Sr 140 140 Sm,St 140 Sh,St E80°56.449’ 4KR388 Basalt 10 elev. 5203 m 4KR139 Fsm Fsl 380 Gcm 380 Gcm Sr,Fsl Gcm Gcm Gcm 120 120 Fsl 120

4KR363 Tuff 360 Fsl 360 Gcm St Sm Gcm 3KR100 TS 4KR355A 100 Gcm 100 100 St,Sh Gcm Sm Gcm Gcm 340 St,Sh Sh St 340 3KR88 HCS St,Sh Gcm 80 80 80 Gcmi Sm,Sr TS 5KR325 Sh 3KR72P Gcm Fsl Gcm 320 St 320 Gcm TS St Gcm 3KR65 Sm Gcm Sm Gcm Sp 60 60 HCS 60 Sm Gcm Sh Gcm Sm Srw Sm 4KR301 Sr/Fsl Gcm 300 3KR47P 300 Sm Gmm Sr Gcm Gcm Gcm TS TS Gmm Gcm Sh Fsl Sr Gcm Gcm Sh 40 Gcm 40 St,Sh 40 Gmm 3KR39 Fsl Gcm Sm Gcm Sr,Fsl Gmm 10 Gcm Sr Gct TS Gcm 280 Gcmi Gcm 280 Gcm Sm,St Gmm Gcmi Sr TS 4KR25 Gcm Gmm Sm St Gcm Sm Gmm Sr Gcm 20 20 St 20 St Gcm Gcm 10 Sh Sh Sr Gcm 3KR14 Sm Gchi St Sh Gcm St Gcm 260 3KR10 Sm TS 260 Gcmi Sr Sr Gcmi 10 Srw, Gcm HCS 0 Gcmi 250 Gcm 0 Srw 0 250 St si fs cs cg si fs cs cg si fs cs cg si fs cs cg si fs cs cg N31°17.110’ 10 N31°14.079’ N31°11.183’ E81°56.827’ E80°56.862 E81°00.972’ elev. 5078 m elev. 4959 m elev. 5469 m

Figure 7. Logs of measured sections 3KR, 4KR, and 5KR. See Table 1 for lithofacies codes. TS—transgressive surface; DZ—detrital zircon sample.

1344 Geological Society of America Bulletin, July/August 2011 Oligocene–Miocene Kailas basin, southwestern Tibet

250 St 500 750 Sm Sm TS 7KR246 Sm LEGEND St St Sh,Sr,Sm Petrographic sample 240 Sm,Sh, 740 Sr 7KR483 Sm U-Pb detrital zircon age Sm,Sh,Sr Fsr,Sr 480 Sm Sm,Sh,St Fsl Paleocurrent direction Sm Fsl n = number of measurements Sm 220 7KR465 720 Sm DZ Tuff Sm Sm Fossil shell fragments Fsl

460 Sr Sr Fsm Bioturbation Sm Fsm Sm Vertebrate fossils 200 Sm 700 Sm Sm Sh Sm,Sh Sm Leaf fossils

440 Sm,Sh St,Sm Fossil wood fragments

Sm Sm 180 Sm 680 Covered interval Sm 7KR177 St Paleosol carbonate Fsl 420 Grain size key: Sm M Fsm si fs cs cg 7KR408 M SiltstoneFine sCo C 160 660 o Sm ar ng Sm se sa and lomerat Sr,Sm TS ston n Sh,Sm dstone 400 Fsl Sm e St,Sh Sh e Sr Sm 12 Sm Sh St Fsl 140 Sm 640 Sm 8KR 7KR129 Sm DZ Sh Sm Sm ° 380 Fsl N31 11.857’ Tuff E81°54.736’ Gcm elev. 4943 m Sm Sm 120 620 120 Sm Sh,St M M Fsl Gcm Sm 7KR M M M 360 Sh,Sr Sm Gcm N31°11.603’ Fsl Gcm Sh,Sr Fsm 8KR104 St Sm Sm E81°55.759’ M Sm elev. 4948 m DZ Gcm 100 Sm 600 Sm 100 M Gcm Sm Fsl Sh,Sm Fsm Sm Sm St Sh 7KR847 8KR96 Fsm M Gcmi Sm Fsl 340 M M Sm 840 Sm,Sh Fsl M 10 7KR337 Sm,St Sm M 80 Sm 580 St 8KR 80 M Sm 76A,B M Sm M St Sm M St 8KR 320 Sm 20 Sr Fsl 820 67DZ 20 Sh,St St St Sr 8KR65 Fsl Sm 60 560 Sm 60 Sm Sr Sr Sm Sm Fsm Sm Sm Fsl Sm St,Sm Fsm PS Sm 300 800 TS TS Fsm Sr St,Sh St St,Sh Fsm 40 St,Sh 540 Sm 40 Fsl Sm,Sh Sr Sm Fsl Fsl Sm St Sr,Sh 280 780 St Sm Sm,Sh Sm St Gcm Fsl Sr 22 Sm,Sh Sm Fsm Sm Sm Sm 20 St 520 20 Sr Sr St 8KR17 St St Fsm St Sm,St Sm Sr Fsl Sm Sm Sr Sm 260 760 Fsm Sh,Sr Sm Sh 22 Sh,St 8KR8 Sr Gcm 7KR252 Sm Fsm St Sm Sr,Fsm 0 Sm 250 500 750 0 si fs cs cg si fs cs cg si fs cs cg si fs cs cg si fs cs cg N31°11.974’ N31°11.983’ E81°56.625’ E80°54.726’ elev. 4877 m elev. 4946 m Figure 8. Logs of measured sections 7KR and 8KR. See Table 1 for lithofacies codes. TS—transgressive surface; DZ—detrital zircon sample.

Geological Society of America Bulletin, July/August 2011 1345 DeCelles et al.

NNE 8KR SSW

Red-bed member 24.4 Ma

Lacustrine member 7KR

4KR 500 6KR 3KR 24.2 Ma 24.6 Ma 24.1 Ma

si fs cs cg

Vertical scale in m Vertical 2KR

si fs cs cg 1KR 24.7 Ma 0 26.6 Ma 25.1 Ma 5KR Lacustrine transgressive surface

25.0 Ma 26.1 Ma Fluvial sandstone member

si fs cs cg

Lower BU conglomerate TT RE SS member Kailas Magmatic Complex U-Pb zircon age from tuff UNC Minimum age from detrital zircon U-Pb ONFORMITY Generalized paleoslope direction

Figure 9. Tentative correlation diagram for measured sections in the Kailas Formation. See Figure 2 for locations of sections. No horizontal scale is implied. Correlations were established by tracing key beds in the ﬁ eld and on satellite images. Locations of samples for geochronological analyses and generalized paleoslope directions are also shown.

Unfortunately, the massive, cliffy aspect of and 11B). Individual parasequences range large-scale low-angle progradational clino- the outcrops precluded measurement of paleo- in thickness from a few meters to >75 m forms are present in the upper parts of some current indicators. However, it is possible to (Fig. 7). A typical parasequence consists of: parasequences (Fig. 12). trace beds of the fl uvial sandstone member in (1) a basal transgressive surface overlain by Molluscan debris, fragmented fi sh fos- section 2KR directly northward into the lower laminated black shale (Figs. 11B and 11D); sils, and coaly plant material are common conglomerate member in section 1KR (Fig. 9), (2) a series of beds that become thicker and (Fig. 11E). However, bioturbation is rare and indicating that the former is simply the more coarser up section, from centimeter-scale consists of small, irregular tubular burrows. distal equivalent of the latter, perhaps as a fring- layers to >1 m thick; (3) sedimentary struc- Transgressive surfaces are typically marked by ing braid-plain down depositional dip from the tures that increase in scale upward within the rusty, calcite-cemented horizons, often littered alluvial-fan system. sandy part of the parasequence, from small with pebbles, fi sh and turtle fossil fragments, ripples (both symmetrical and asymmetrical) and carbonaceous plant material (Figs. 11E Lacustrine Member and plane beds in the lowest sandstone beds and 11F). A few beds of biotite-bearing tuff to hummocky cross-strata and trough cross- and phlogopite-bearing trachyandesite/basalt The lacustrine member is composed of strata in the upper parts of the parasequence; (388 m level of section 4KR, Fig. 7) are present black shale and sandstone beds arranged in and (4) uppermost pebbly beds in some within the lacustrine member. upward-coarsening parasequences that are parasequences that contain low-angle planar Sets of parasequences are stacked vertically separated by sharp, fl at transgressive surfaces cross-strata. Channelized lenticular pebbly in upward-thickening packages (Fig. 7, section (Fig. 7, sections 3KR and 4KR; Figs. 11A sandstone beds with trough cross-strata, and 3KR). The lacustrine member is at least 700 m

1346 Geological Society of America Bulletin, July/August 2011 Oligocene–Miocene Kailas basin, southwestern Tibet

TABLE 1. LITHOFACIES (AND THEIR CODES) USED IN LOGS OF MEASURED SECTIONS, AND INTERPRETATIONS IN THIS STUDY, MODIFIED AFTER MIALL (1978) AND DECELLES ET AL. (1991) Lithofacies code Description Process interpretation Fsl Laminated black or gray siltstone Suspension-settling in ponds and lakes Fcl Laminated gray claystone Suspension-settling in ponds and profundal lakes Fsm Massive, bioturbated, mottled siltstone, usually red; carbonate Paleosols, usually calcic or vertic nodules common Sm Massive medium- to fi ne-grained sandstone; bioturbated Bioturbated sand, penecontemporaneous deformation Sr Fine- to medium-grained sandstone with small, asymmetric, 2-D and Migration of small 2-D and 3-D ripples under weak (~20–40 cm/s), 3-D current ripples unidirectional fl ows in shallow channels St Medium- to very coarse-grained sandstone with trough cross- Migration of large 3-D ripples (dunes) under moderately powerful stratifi cation (40–100 cm/s), unidirectional fl ows in large channels Sp Medium- to very coarse-grained sandstone with planar cross- Migration of large 2-D ripples under moderately powerful (~40–60 cm/s), stratifi cation unidirectional channelized fl ows; migration of sandy transverse bars Sh Fine- to medium-grained sandstone with plane-parallel lamination Upper plane bed conditions under unidirectional fl ows, either strong (>100 cm/s) or very shallow Srw Fine- to medium-grained sandstone with symmetrical small ripples Deposition of oscillatory current (orbital) ripples in shallow lakes and ponds Gcm Pebble to boulder conglomerate, poorly sorted, clast-supported, Deposition from sheetfl oods and clast-rich debris fl ows unstratifi ed, poorly organized Gcmi Pebble to cobble conglomerate, moderately sorted, clast-supported, Deposition by traction currents in unsteady fl uvial fl ows unstratifi ed, imbricated (long-axis transverse to paleofl ow) Gch Pebble to cobble conglomerate, well sorted, clast-supported, Deposition from shallow traction currents in longitudinal bars and gravel horizontally stratifi ed sheets Gchi Pebble to cobble conglomerate, well sorted, clast-supported, Deposition from shallow traction currents in longitudinal bars and gravel horizontally stratifi ed, imbricated (long-axis transverse to paleofl ow) sheets Gct Pebble conglomerate, well sorted, clast-supported, trough cross- Deposition by large gravelly ripples under traction fl ows in relatively deep, stratifi ed stable fl uvial channels Gcp Pebble to cobble conglomerate, well sorted, clast-supported, planar Deposition by large straight-crested gravelly ripples under traction fl ows in cross-stratifi ed shallow fl uvial channels, gravel bars, and gravelly Gilbert deltas Gmm Massive, matrix-supported pebble to boulder conglomerate, poorly Deposition by cohesive mud-matrix debris fl ows sorted, disorganized, unstratifi ed HCS Hummocky cross-stratifi ed fi ne- to medium-grained sandstone Deposition by combined unidirectional and oscillatory fl ows during storms on the lower shoreface M Micritic massive gray and yellow marl Lacustrine carbonate mud

A B

Figure 10. Photographs of typical lithofacies in the lower conglomerate and ﬂ uvial sandstone members. (A) Boulder- cobble conglomerates (mainly Gcm and Gcmi) in the lower part of the lower conglomerate member, section 1KR. (B) Imbricated pebble conglomerate (Gcmi) in middle part of the lower conglomerate member, section 1KR. (C) Bed of matrix-supported boulder conglomerate (Gmm) in section C D 5KR. Hammer is 41 cm long. (D) Lithofacies Sm, St, and Sh in lenticular sandstone bodies of the ﬂ uvial sandstone member. Note irregular erosional basal scour surface with ~1 m of relief. Vertical dimension of view is ~8 m.

Geological Society of America Bulletin, July/August 2011 1347 DeCelles et al.

A B

Figure 11. (A) Measured section 3KR, showing stack of six lacustrine parasequences. Thickness of visible portion of the section is 150 m. (B) Upward-coarsening lacustrine parasequence capped by transgressive surface D (arrow at right). Person high- C D lighted by arrow in lower left indicates scale. (C) View toward east from lower part of section 4KR of rhythmically bedded lacustrine member and under- lying massively bedded lower conglomerate member. (D) Thin sandstone beds in lacustrine profundal shale, probably deposited by turbidity currents. Hammer is 41 cm long. (E) Bed- ding surface view of transgres- E F sive lag composed of fossil ﬁ sh vertebrae and mandibles and small chert pebbles, 497 m level of section 7KR. (F) Fossil turtle scute in transgressive lag, section 7KR.

thick, and the regular alternation of fi ne-grained cross-stratifi ed sandstone units at the tops of and nodular limestone beds become abundant and sandy lithofacies imparts a rhythmic char- many parasequences represent fl uvial/deltaic higher in the section. Unlike the arkosic con- acter to the outcrop (Fig. 11C). distributaries. Large-scale clinoforms (Fig. 12) glomerates lower in the Kailas Formation, We interpret these parasequences as the rec- in some of these units probably accumulated in those in the red-bed member contain clasts ords of progradation of sandy nearshore and prograding distributary mouth bars. of chert, limestone, and quartzite. Paleocur- deltaic systems into an offshore muddy lacus- rent data from trough cross-strata indicate trine environment. The laminated black shales Red-Bed Member southward and northward paleofl ow directions are interpreted as offshore profundal deposits. (Fig. 8, section 8KR). Their dark color results from abundant fi ne- In the uppermost part of the Kailas Forma- We interpret the lenticular sandstone units as grained amorphous kerogen (see palynologi- tion, the assemblage of lithofacies changes fl uvial channel deposits; the mottled and nodu- cal information in the following). If maximum abruptly to bright red siltstone with nodular lar siltstones as calcic paleosols (Mack et al., parasequence thickness may be taken as a carbonate zones, pebbly lenticular sandstone 1993); and the nodular and laminated limestone crude estimate of minimum water depth, then units, and laminated marl layers (Fig. 8, sec- beds as carbonate lacustrine deposits. This as- these shales accumulated in water greater than tion 8KR; Fig. 13). Sandstone beds contain semblage of lithofacies suggests deposition in a 80 m deep; this fi gure would increase substan- small ripples and horizontal laminations. mixed fl uviolacustrine system that experienced tially (>30%) upon decompaction. Hummocky Lenticular coarse-grained sandstone units are strongly seasonal, semiarid climate. The ab- cross-stratifi cation and small oscillatory cur- dominated by trough cross-stratifi cation. The sence of dark, organic-rich profundal lacustrine rent ripples in the lower parts of the sandy red siltstones are typically massive, mottled, facies indicates that, although lakes persisted portions of parasequences indicate lower shore- and exhibit nodular weathering. Nodular gray during deposition of the red-bed member, they face storm deposits. The channelized, trough silty carbonate accumulations are common, were not deep and may have been ephemeral.

1348 Geological Society of America Bulletin, July/August 2011 Oligocene–Miocene Kailas basin, southwestern Tibet

Figure 12. Large-scale (~7 m thick) clinoform bedding in deltaic sandstone at top of a lacustrine parasequence. The sandy unit is capped by a transgressive surface, which is in turn overlain by profundal lacustrine black shale.

Similarly, the presence of paleosol carbonate, in marked contrast to the bulk of the Kailas For- A mation, suggests semiarid climate (Retallack, 1990; Mack et al., 1993).

CHRONOSTRATIGRAPHIC CONTROL

The age of the Kailas Formation was determined from U-Pb zircon ages from samples of tuffs within the stratigraphic sections we measured , and from maximum age constraints imposed by the youngest populations of detrital zircon ages. Detrital grain ages also provide valuable provenance information. The U-Pb ages of detrital and fi rst-cycle volcanic zircon grains were determined by multicollector–laser ablation–inductively coupled plasma–mass spectrometry (MC-LA-ICP-MS) B at the University of Arizona LaserChron Center. Spots on individual zircon grains were ablated with a New Wave DUV193 Excimer laser (operating at a wavelength of 193 nm) using a spot diam eter of 35 μm. The ablated material is carried in He gas into the plasma source of a Micromass Isoprobe, which is equipped with a fl ight tube of suffi cient width that U, Th, and Pb isotopes are measured simultaneously. All measurements are made in static mode, using Faraday detectors for 238U, 232Th, 208–206Pb, and an ion-counting channel for 204Pb. Ion yields are ~1 mv per ppm. Each analysis consists of one 20 s integration on peaks with the laser off (for backgrounds), twenty 1 s integrations with the laser fi ring, and a 30 s delay to purge the pre- vious sample and prepare for the next analysis. Figure 13. (A) Abrupt change from dark-colored shales and fi ne- The ablation pit is ~20 μm deep. Common Pb grained sandstones in the upper part of the lacustrine member correction was made by using the measured 204Pb to the red-bed member in section 8KR. Icy peak in background and assuming an initial Pb composition from is Gurla Mandatha. (B) Nodular paleosol carbonate in red-bed Stacey and Kramers (1975) (with uncertainties member, section 8KR.

Geological Society of America Bulletin, July/August 2011 1349 DeCelles et al. of 1.0 for 206Pb/204Pb and 0.3 for 207Pb/204Pb). ters of 24.7–26.6 Ma (Fig. 14, samples 1KR353, and microlitic. Vitric grains include microcrys- Our measurement of 204Pb is unaffected by the 2KR107), indicating that the age of the sand- talline (devitrifi ed) to glassy types; occasional presence of 204Hg because backgrounds were stones can be no older than that age range. Three bubble-wall shard glasses; K-feldspar–rich measured on peaks (thereby subtracting any detrital zircon samples from the lower part of glass; and welded, fl ow-banded glassy tuffs. background 204Hg and 204Pb), and because very the lacustrine member produced minimum age Mafi c grains consist of coarse-grained hypabys- little Hg was present in the argon gas. clusters of 24.7–26.6 Ma (Fig. 14, samples sal varieties composed of epidote ± pyroxene ± Interelement fractionation of Pb/U is gener- 1KR624, 2KR420, 7KR129). The abundance plagioclase. Felsic grains are typically strongly ally ~15%, whereas fractionation of Pb isotopes and tight clustering of grains in the 24–26 Ma altered sericite + quartz ± feldspar aggregates. is generally <2%. In-run analysis of fragments of range in these sandstone samples suggest that Trace minerals include zoisite/clinozoisite, a large zircon crystal (every sixth measurement) the zircon ages are good approximations of epidote, chlorite, biotite, muscovite, clino- and with known age of 563.5 ± 3.2 Ma (2σ error) the depositional ages. Approximately 500 m orthopyroxenes, zircon, sphene, monazite, and (Gehrels et al., 2008) was used to correct for this above the base of the lacustrine member, two opaque grains. fractionation. Fractionation also increases with tuff beds produced mean zircon ages of 24.6 ± depth into the laser pit by up to 5%. This depth- 0.5 and 24.2 ± 0.5 Ma (Fig. 4, samples PD3 and Conglomerate Clast Counts related fractionation was accounted for by moni- PD4). A third tuff bed, slightly contaminated toring the fractionation observed in the standards. with detrital grains, at approximately the same At least 100 clasts were counted at fi ve sta- Analyses that displayed >10% change in ratio stratigraphic level (Fig. 4, sample 7KR465) pro- tions representing the entire stratigraphic thick- during the 20 s measurement are interpreted to duced a U-Pb zircon mean age of 24.1 ± 0.4 Ma. ness of the Kailas Formation (Fig. 14). The lower be variable in age (or perhaps compromised by Finally, near the top of the section, a sandstone part of the Kailas Formation is dominated by fractures or inclusions), and were excluded from sample in the red-bed member (Fig. 14, sample volcanic, granitoid, and hypabyssal clasts, with further consideration. Also excluded were analy- 8KR67) produced a youngest cluster of detrital minor amounts of chert. Conglomerates in the ses that yielded >15% uncertainties in 206Pb/238U zircon ages of 24 Ma, consistent with a deposi- red-bed member contain abundant sedimentary ages or were >5% reverse discordant. tional age younger than this, but providing no clasts, including limestone and chert. Conglom- The measured isotopic ratios and ages are re- further constraint on the minimum age of this erates in section 5KR, which we include in the ported in Tables DR1 and DR2 (see footnote 1). interval. Together, the geochronological data in- lower conglomerate member, contain numerous Errors that propagate from the measurement of dicate that the bulk of the Kailas Formation in clasts of reworked pebbly conglomerate. 206Pb/238U, 206Pb/207Pb, and 206Pb/204Pb are re- this region was deposited rapidly over a period ported at the 1σ level. Additional errors that af- of only 2 to 3 m.y. during the latest Oligocene Detrital Zircon Ages fect all ages include uncertainties from U decay and early Miocene. Our U-Pb ages are consis- constants, the composition of common Pb, and tent with, but slightly older than, biotite and Detrital zircon age spectra from all of the calibration correction; these systematic errors plagioclase 40Ar/39Ar intercept ages reported by seven sandstone samples from the Kailas For- are 1%–2% for most samples. Aitchison et al. (2009) between 22.3 ± 0.7 Ma mation are dominated by ages that are younger An additional factor that complicates analyses and 16.9 ± 0.2 Ma from felsic tuffs interbedded than 100 Ma; fewer than 20% of the dated grains in the 500 to ca. 2000 Ma age range is the change within the Kailas Formation east of Mount Kai- are older than 100 Ma (Fig. 14). Six of the seven in precision of 206Pb/238U and 206Pb/207Pb ages— las and within the Gangrinboche conglomerate samples exhibit predominant age clusters with 206Pb/238U ages are generally more precise for near Dazhuqu ~700 km to the east. peaks at ca. 24–26 Ma, and lesser peaks in the younger ages, whereas 206Pb/207Pb ages are more 80–55 Ma range (Fig. 14). Sample 8KR104, precise for older ages. Therefore, in samples that PROVENANCE near the top of the Kailas Formation, produced contain a cluster of analyses with concordant to a range of ages between ca. 45 Ma and 60 Ma slightly discordant ages, we rely on 206Pb/238U Modal Sandstone Petrology with a sharp unimodal peak at ca. 49.5 Ma. ages up to 1000 Ma and 206Pb/207Pb ages if the One sample (8KR67) also produced several age 206Pb/238U ages are older than 1000 Ma. Ad- The modal framework-grain composition clusters with peaks of ca. 30 Ma, 40 Ma, and ditional details about analytical procedures are of 41 sandstone samples from the Kailas For- 50 Ma. Zircons with ca. 75–80 Ma ages are pres- described by Gehrels et al. (2008). mation was documented by identifi cation of ent in several samples (Fig. 14). The older than Analyses that yielded isotopic data of accept- 450 grains per slide according to the Gazzi- 100 Ma ages exhibit peaks in the Late Archean, able discordance, in-run fractionation, and pre- Dickinson method (Ingersoll et al., 1984). In or- Mesoproterozoic, Neoproterozoic, and early to cision are shown in Tables DR1 and DR2 (see der to aid the identifi cation of feldspars, one half middle Paleozoic. Although some aspects of the footnote 1). In total, 650 zircon ages are reported of each standard petrographic thin section was older than 100 Ma grain populations resemble in this paper. Detrital zircon analyses are plot- stained for potassium- and calcium-feldspars. documented detrital zircon ages of Lhasa terrane ted on relative age-probability diagrams (Fig. Grain types are listed in Table 2, and recalcu- rocks (Leier et al., 2007), statistically rigorous 14), which represent the sum of the probability lated modal values are given in Table 3. comparison is not possible because of the small distributions of all analyses from a sample nor- Quartzose grains include monocrystalline numbers of grains in our older than 100 Ma data malized so that the areas beneath the probability (Qm), polycrystalline (Qp), foliated polycrystal- sets. It is also noteworthy that none of the Kai- curves are equal for all samples. Age peaks on line (Qpt), and quartzite/siltstone (Qmss/Qms) las detrital zircon age distributions bears much these diagrams are considered robust if defi ned grains. Feldspars include the potassium varieties resemblance to detrital zircon age spectra ob- by several analyses, whereas less signifi cance is (K; orthoclase, perthite, microcline) and plagio- tained from the Himalayan thrust belt (Parrish attributed to peaks defi ned by single analyses. clase (P, occasionally with myrmekitic textures). and Hodges, 1996; DeCelles et al., 2000, 2004; Two samples of sandstone from the lower Lithic grains include limestone, mica schist, ser- Gehrels et al., 2003, 2006; Amidon et al., 2005; conglomerate and fl uvial sandstone members pentine schist, and several varieties of volcanic Martin et al., 2005) or from Cenozoic strata of yielded detrital zircons with minimum age clus- grains, including lathwork, vitric, felsic, mafi c, the central Lhasa terrane (DeCelles et al., 2007a).

1350 Geological Society of America Bulletin, July/August 2011 Oligocene–Miocene Kailas basin, southwestern Tibet

Provenance Interpretation Age (Ma) n Modal sandstone compositions in terms of = 402 n = 98 standard ternary diagrams are illustrated in 80.5% 19.5% Figure 15. Overall, Kailas Formation sand- Ages >100 Ma stones exhibit compositions typical of sand- Ages <100 Ma stones derived from magmatic arcs (Dickinson 8KR104 n = 60 et al., 1983; Garzanti et al., 2007), with large amounts of plagioclase feldspar and felsic to intermediate volcanic lithic grains. Conglom- erate compositions are consistent with this interpretation, as they are dominated by volcanic and granitoid clasts. Recycled conglomerate clasts of unknown provenance are common 8KR67 n = 86 n = 21 in the lower conglomerate member in section 5KR; all we can be sure of is that these clasts were derived from the north, based on robust paleocurrent data (Fig. 7, section 5KR). We observed no signiﬁ cant changes in composition throughout most of the Kailas Formation, until the red-bed member. The red-bed member con- 7KR129 n = 76 n = 21 tains abundant sandstone/quartzite, limestone, and chert clasts. The provenance data are consistent with paleo current data that indicate the principal source terrane was the Kailas magmatic complex in the Gangdese magmatic arc to the north of the Kailas basin. Additional volcanogenic n = 28 n = 5 material may have been derived from erup- 2KR420 tive centers located in the western Lhasa terrane and across the Karakoram fault toward the west (Miller et al., 1999; Mahéo et al., 2002; Williams et al., 2004), which were contemporaneously active and possibly produced the air- fall tuffs that we sampled. During deposition of n = 53 n the red-bed member, Tethyan source terranes 2KR107 = 12 became more important, supplying limestone, sandstone/quartzite, and chert clasts from the Paleozoic-Mesozoic sedimentary succession in the hanging wall of the South Kailas thrust system. The detrital zircon age spectra are dominated by age clusters that are typical of the Gangdese arc, with peaks in the ca. 25 Ma, 1KR624 n = 52 n = 20 49–50 Ma, and 70–80 Ma ranges (Fig. 14). Sur- prisingly, even the uppermost samples in section

Figure 14. Age probability diagrams of detrital zircon ages from sandstones of the 1KR353 n = 47 n = 19 Kailas Formation. Diagrams in left-hand column show the younger than 100 Ma populations; those in right-hand column show the older than 100 Ma populations. Letter “n” indicates number of grains in each population. See Table DR2 for data (see text footnote 1). Ages of Kailas magmatic com- 0 204060801000 1000 2000 3000 plex rocks (heavy dashed lines; this work) Ayi Shan Gangdese magmatic and intrusions in the Ayi Shan (gray band; intrusions complex Zhang et al., 2010) are shown for reference.

Geological Society of America Bulletin, July/August 2011 1351 DeCelles et al.

TABLE 2. MODAL PETROGRAPHIC POINT-COUNTING PARAMETERS eral key caveats that must be considered in us- Symbol Description ing stable isotopes to reconstruct paleoelevation Qm Monocrystalline quartz Qp Polycrystalline quartz are discussed in Quade et al. (2007) and Saylor Qpt Foliated polycrystalline quartz et al. (2009). Qms Monocrystalline quartz in sandstone or quartzite lithic grain We collected samples of paleosol carbonate C Chert from the red-bed member of the Kailas For- S Siltstone mation for stable isotope paleoaltimetry. Un- Qt Total quartzose grains (Qm + Qp + Qpt + Qms + C + S) fortunately, no paleosol carbonate was found K Potassium feldspar in the main body of the Kailas Formation. We (including perthite, myrmekite, microcline) also analyzed samples of modern and Quater- P Plagioclase feldspar (including Na and Ca varieties) nary soil carbonate from the Kailas region and F Total feldspar grains (K + P) from the Nima area in central Tibet (DeCelles et al., 2007b) in order to calibrate a modern Lvm Mafi c volcanic grains (epidote ± pyx ± plag) Lvf Felsic volcanic grains (sericite + qtz ± feldspar) baseline relationship between elevation and Lvv Vitric volcanic grains isotopic values. Lvx Microlitic volcanic grains All carbonate samples were heated at 150 °C Lvl Lathwork volcanic grains Lv Total volcanic lithic grains (Lvm + Lvf + Lvv + Lvx + Lvl) for 3 h in vacuo and processed using an auto- mated sample preparation device (Kiel III) at- Lsh Mudstone Lph Phyllite tached directly to a Finnigan MAT 252 mass Lsm Schist (mica schist) spectrometer at the University of Arizona. Lc Carbonate lithic grains The δ18O and δ13C values were normalized to Lm Total metamorphic lithic grains (Lph + Lsm + Qpt) Ls Total sedimentary lithic grains (Lsh + Lc + C + S + Qms) NBS-19 based on internal laboratory standards. Lt Total lithic grains (Ls + Lv + Lm + Qp) Precision of repeated standards was ±0.1‰ for L Total nonquartzose lithic grains (Lv + Ls + Lph + Lsm + Lc) δ18O and ±0.06‰ for δ13C (1σ). Accessory minerals Paleosol carbonate from the uppermost part Epidote/zoisite of the Kailas Formation yielded δ18O values Chlorite cc Muscovite ranging mostly between –15‰ and –18‰ (but Biotite 13 with two values around –22‰), and δ Ccc val- Zircon Sphene ues between –4.5‰ and –6.0‰ (Table 4; Fig. Clinopyroxene 16). Modern soil carbonate in the Kailas area Orthopyroxene and in the Nima area of central Tibet produced Monazite 18 Magnetite δ Occ values ranging between about –10‰ and 13 –14.5‰, and δ Ccc values scattered between +2.3‰ and –5.0‰ (Table 4; Fig. 16). The modern samples were collected >50 cm below 8KR, which contains clasts of clear-cut Tethyan Kailas basin deposits, whereas these taxa are the ground surface over an elevation range of provenance, do not show the strong pre–100 Ma abundant in coeval high-elevation deposits ~4500–4700 m. 18 age clusters typical of Tethyan strata throughout of the Nima Basin in central Tibet (DeCelles The lower δ Occ values in the Kailas Forma- the Himalayan thrust belt (DeCelles et al., 2000; et al., 2007a, 2007b). tion paleosol carbonates compared to those in Gehrels et al., 2003, 2006; Amidon et al., 2005; Vertebrate fossils are abundant in the Kailas local modern soils could at fi rst glance be taken Martin et al., 2005). Formation, particularly in lag concentrations to indicate higher paleoelevations in this region along transgressive surfaces (Figs. 11E and during the early Miocene. However, the most PALYNOLOGY AND VERTEBRATE 11F). These include fi sh fossils (teeth, man- negative modern values of –13‰ to –14.5‰ PALEONTOLOGY dibles, and vertebrae) and turtles (bones and are actually comparable to ancient values in scutes). Woody plant material is also abundant the –15‰ to –18‰ range after accounting Palynological analysis of samples from throughout the Kailas Formation, except in the for the modest isotopic effects (~2‰–3‰) of dark-colored organic-rich shale from the Kai- red-bed member in the uppermost part of warmer, and largely ice-free conditions in the las Formation produced generally poor yields. the formation. early Miocene (Shackleton and Kennett, 1975; The dark colors of Kailas shales result from Zachos et al., 1994; Bowen and Wilkinson, 18 high amorphous kerogen content. The few STABLE ISOTOPE DATA 2002). The higher and more dispersed δ Occ taxa that were recovered include Milfordia/ values in modern soils probably also refl ect the Monoporites annulatus (common in circum- Oxygen isotopic values from sedimentary arid modern climate (greater soil evaporation) 18 equatorial early and middle Cenozoic fl oras), carbonates (expressed as δ Occ in ‰) and the compared to a more humid early Miocene cli- 18 Ulmoidiepites (Elm), and Cyatheacidites an- waters from which they precipitated (δ Omw) mate, as suggested by the abundance of organic nulatus (an equatorial fern). None of these taxa are potentially useful paleoaltimeters because material in the main body of the Kailas Forma- is chronologically or environmentally diagnos- they decrease with increasing elevation (e.g., tion. This difference in climate is fi rmly sup- 13 tic. However, it is signifi cant that pollen from Garzione et al., 2000a, 2000b; Dettman and ported by the carbon isotope results. The δ Ccc temperate and boreal/high-elevation species Lohmann, 2000; Poage and Chamberlain, 2001; values in the Kailas paleosols are much more such as oak, alder, maple, basswood, willow, Mulch et al., 2004; Blisniuk and Stern, 2005; negative and less dispersed than those of mod- fi r, pine, hemlock, and spruce is absent from Currie et al., 2005; Saylor et al., 2009). The sev- ern soils in the area, probably refl ecting higher

1352 Geological Society of America Bulletin, July/August 2011 Oligocene–Miocene Kailas basin, southwestern Tibet

TABLE 3. RECALCULATED MODAL PETROGRAPHIC DATA Sample Qm% F% Lt% Qt% F% L% Qm% P% K% Lm% Lv% Ls% Lv/Lt F/F+Qm K:K+P 1KR73 0.15 0.48 0.38 0.18 0.47 0.35 0.23 0.59 0.17 0.02 0.97 0.01 0.90 76.58 0.23 1KR86 0.11 0.20 0.69 0.17 0.20 0.63 0.35 0.65 0.00 0.00 0.97 0.03 0.91 65.47 0.00 1KR148 0.12 0.33 0.55 0.21 0.33 0.46 0.26 0.58 0.15 0.02 0.95 0.02 0.82 73.63 0.21 1KR165 0.17 0.42 0.41 0.21 0.42 0.37 0.28 0.49 0.23 0.02 0.93 0.04 0.86 71.65 0.32 1KR240 0.10 0.51 0.39 0.10 0.51 0.39 0.16 0.58 0.27 0.13 0.87 0.00 0.86 84.13 0.32 1KR255 0.11 0.55 0.34 0.13 0.55 0.31 0.16 0.61 0.23 0.14 0.79 0.07 0.73 83.90 0.28 1KR486 0.12 0.48 0.40 0.15 0.48 0.37 0.20 0.58 0.23 0.01 0.96 0.02 0.92 80.30 0.28 1KR568 0.13 0.59 0.28 0.14 0.59 0.28 0.18 0.51 0.31 0.18 0.78 0.04 0.76 81.96 0.37 1KR630 0.17 0.34 0.49 0.20 0.34 0.45 0.33 0.56 0.11 0.08 0.89 0.03 0.84 67.11 0.17 2KR1 0.16 0.40 0.44 0.18 0.40 0.42 0.29 0.51 0.20 0.09 0.91 0.01 0.88 70.92 0.28 2KR40 0.15 0.44 0.41 0.17 0.44 0.39 0.26 0.54 0.21 0.11 0.87 0.01 0.83 74.14 0.28 2KR184 0.12 0.46 0.42 0.14 0.46 0.40 0.21 0.55 0.24 0.05 0.95 0.00 0.90 78.99 0.31 2KR232 0.12 0.42 0.45 0.15 0.42 0.43 0.23 0.66 0.11 0.06 0.87 0.07 0.82 77.18 0.14 2KR330 0.08 0.63 0.29 0.09 0.63 0.28 0.12 0.49 0.39 0.02 0.98 0.01 0.95 88.05 0.44 2KR420 0.14 0.43 0.43 0.16 0.43 0.41 0.25 0.50 0.25 0.01 0.97 0.03 0.94 75.10 0.34 3KR14 0.10 0.54 0.36 0.12 0.54 0.34 0.15 0.49 0.36 0.00 0.99 0.01 0.93 85.11 0.43 3KR39 0.11 0.48 0.41 0.13 0.48 0.39 0.18 0.54 0.28 0.01 0.99 0.01 0.95 81.82 0.34 3KR65 0.13 0.35 0.51 0.15 0.35 0.50 0.28 0.47 0.25 0.02 0.93 0.04 0.91 72.48 0.35 3KR100 0.12 0.34 0.54 0.13 0.34 0.53 0.27 0.53 0.20 0.00 1.00 0.00 0.98 73.08 0.27 3KR147 0.13 0.44 0.43 0.14 0.44 0.42 0.23 0.52 0.25 0.06 0.90 0.04 0.89 77.17 0.33 4KR25 0.10 0.61 0.29 0.14 0.61 0.25 0.15 0.60 0.25 0.01 0.96 0.04 0.85 85.44 0.29 4KR139 0.13 0.43 0.44 0.17 0.43 0.40 0.24 0.54 0.23 0.04 0.91 0.05 0.87 76.49 0.30 4KR210 0.10 0.57 0.33 0.13 0.57 0.30 0.15 0.59 0.26 0.01 0.98 0.01 0.90 84.56 0.30 4KR301 0.16 0.46 0.38 0.18 0.46 0.36 0.26 0.56 0.18 0.01 0.99 0.01 0.92 74.28 0.24 4KR400 0.13 0.59 0.28 0.16 0.59 0.25 0.17 0.54 0.28 0.02 0.94 0.04 0.86 82.59 0.09 4KR461 0.16 0.33 0.51 0.20 0.33 0.48 0.33 0.56 0.11 0.03 0.85 0.12 0.82 66.52 0.16 5KR18 0.16 0.31 0.53 0.18 0.31 0.51 0.34 0.60 0.06 0.00 1.00 0.00 0.97 65.71 0.09 5KR195 0.12 0.27 0.61 0.18 0.27 0.55 0.30 0.62 0.08 0.02 0.97 0.01 0.89 69.94 0.11 5KR226 0.11 0.17 0.72 0.17 0.17 0.67 0.40 0.59 0.01 0.01 0.98 0.01 0.92 60.00 0.01 5KR325 0.13 0.25 0.62 0.21 0.25 0.54 0.34 0.62 0.05 0.02 0.94 0.05 0.84 66.27 0.07 7KR177 0.09 0.45 0.46 0.12 0.45 0.43 0.17 0.67 0.16 0.01 0.98 0.02 0.94 82.85 0.19 7KR246 0.10 0.50 0.41 0.11 0.50 0.39 0.16 0.54 0.30 0.02 0.93 0.04 0.90 83.86 0.36 7KR252 0.22 0.50 0.28 0.25 0.50 0.25 0.31 0.43 0.26 0.03 0.93 0.04 0.85 69.38 0.37 7KR337 0.13 0.61 0.26 0.14 0.61 0.25 0.17 0.73 0.10 0.07 0.87 0.05 0.84 82.72 0.12 7KR483 0.22 0.53 0.26 0.28 0.52 0.20 0.29 0.50 0.21 0.04 0.70 0.26 0.60 71.00 0.30 7KR542 0.25 0.54 0.20 0.30 0.54 0.15 0.32 0.37 0.31 0.01 0.93 0.06 0.75 68.24 0.46 7KR847 0.18 0.36 0.46 0.23 0.36 0.41 0.33 0.38 0.29 0.06 0.88 0.06 0.80 66.94 0.43 8KR8 0.11 0.02 0.87 0.20 0.02 0.78 0.85 0.15 0.00 0.53 0.15 0.32 0.15 15.25 0.00 8KR17 0.29 0.44 0.27 0.35 0.44 0.22 0.39 0.31 0.30 0.08 0.82 0.10 0.68 60.75 0.49 8KR65 0.28 0.32 0.40 0.35 0.32 0.33 0.47 0.28 0.25 0.03 0.89 0.07 0.77 53.23 0.48 8KR96 0.07 0.03 0.90 0.14 0.03 0.83 0.66 0.32 0.02 0.56 0.30 0.14 0.29 34.09 0.07 Note: Parameter deﬁ nitions are provided in Table 2.

respiration rates and more plant cover during BASIN ARCHITECTURE AND coarse-grained interval overlain by fi ne-grained the early Miocene. SUBSIDENCE MECHANISM lacustrine deposits, which are in turn overlain Thus, the oxygen isotope data suggest that by a return to relatively coarse-grained fl uvial the paleoelevation of the Kailas Formation The measured sections and additional fi eld deposits (Lambiase, 1990; Schlische, 1992). during deposition of the red-bed member was observations demonstrate that the Kailas Forma- Aitchison et al. (2002) reported that the Kailas essentially the same as the modern elevation tion coarsens northward toward its basal contact Formation at Mount Kailas is divisible into three (>4500 m). We emphasize, however, that the with the Kailas magmatic complex. Figure 9 distinct members that are broadly similar to what data we present are not defi nitive without a depicts a tentative, composite lithostratigraphic we have found, with the exception that all units more rigorous evaluation of the potential ef- correlation based on our measured sections. seem to be somewhat coarser-grained in out- fects of diagenesis. We were not able to run an This part of the Kailas basin was fi lled by three crops located on Mount Kailas (Gansser, 1964). isotopic “conglomerate test” (DeCelles et al., distinct depositional systems: a lower alluvial- A second key feature of the Kailas Forma- 2007b) or other tests (Cyr et al., 2005) to de- fl uvial system that was derived from the deeply tion is that it rests in buttress unconformity di- termine whether the nodules have been reset eroded Gangdese magmatic arc to the north; a rectly upon its principal source terrane, without with respect to δ18O. Barring such tests, several medial lacustrine system into which marginal fault contact or structural disruption (Figs. 3B considerations provide some confi dence in the deltas and nearshore systems prograded, again and 5B). The contact dips uniformly and gently results: The paleosol carbonate nodules we ana- mainly from the north; and an upper fl uvial southward. This onlapping relationship with lyzed are uniformly micritic, burial depth in the system that was mainly derived from Tethyan no evidence for growth structures is not typi- Kailas red-bed member was probably modest rocks exposed in the northern Himalayan thrust cal of thrust-generated wedge-top or foredeep (e.g., less than 2 km), and both our carbon and belt located directly to the south in the hang- sediments, which are characteristic of con- oxygen isotope results are quite similar to those ing wall of the Great Counter thrust. The basin tractional tectonic settings. On the other hand, from early Miocene paleosols at Nima in cen- fi ll exhibits a nearly continuous upward-fi ning the uppermost part of the Kailas Formation is tral Tibet, from which we also concluded that grain-size trend, until the uppermost part of undoubtedly incorporated into contractional paleoelevations stood at >4500 m at that time the section, and, in most respects, it resembles structures associated with the tip of the north- (DeCelles et al., 2007b). a classic “lacustrine sandwich,” with a lower verging South Kailas thrust system (Gansser,

Geological Society of America Bulletin, July/August 2011 1353 DeCelles et al.

8KR96

Qm Qt

TETHYAN 1KR466 SEDIMENTARY Dickinson et al. (1983) magmatic arc provenance fields

RECYCLED 5KR178 CONGLOMERATE & F Lt F L SANDSTONE

Lm Qm

Outliers are from uppermost 1KR82 part of section, with Tethyan component

HYPABYSSAL 1KR2 GRANITOID Lv Ls P K CHERT

VOLCANIC

Figure 15. Ternary diagrams showing modal sandstone petrographic data (left) and pie charts showing conglomerate clast count data (right). See Table 2 for deﬁ nitions of petrographic parameters, and Table 3 for recalculated modal petrographic data.

1964), and the provenance data support deriva- beneath the Kailas basin during its develop- DISCUSSION tion of sediment from its hanging wall. Simi- ment (Fig. 5B). The average rate of sediment lar structural relationships were reported by accumulation during this relatively brief inter- The origin of the Kailas basin has remained Searle et al. (1990) in the Indus Group and val was ~0.5 mm/yr, which is not diagnostic enigmatic ever since Gansser’s observations by Aitchison et al. (2002) in the Gangrinboche of any particular tectonic setting, but is high were fi rst published (Heim and Gansser, 1939; conglomerates. enough to be typical of rapidly subsiding rift Gansser, 1964). Based on recent reconnais- We propose that the bulk of the Kailas For- basins. Transtensional and strike-slip basins sance work, Yin et al. (1999), Aitchison et al. mation was deposited in a basin produced by commonly have sediment accumulation rates (2002, 2009), and Davis et al. (2004) concluded extension or transtension. This would account that exceed 1.0 mm/yr (Allen and Allen, 2005; that the Kailas Formation was deposited in re- for the typical lacustrine stratigraphic sand- Xie and Heller, 2009). Only during deposition sponse to regional uplift of the southern Lhasa wich, the overall upward-fi ning sequence, and of the red-bed member was the Kailas Forma- terrane in an overall contractional tectonic set- the absence of contractional growth structures. tion infl uenced directly by shortening; the red- ting associated with the onset of slip on the An overall extensional setting is also consis- bed member signals the onset of shortening Great Counter thrust to the south and local tent with the presence of phlogopitic trachy- along the South Kailas (or Greater Counter) uplift along the Gangdese thrust system to the andesites/basalts in the section. The timing thrust. The direction of extension in the Kailas north (Yin et al., 1999). Aitchison et al. (2002, of extension would be no older than late Oli- basin is unknown, but data from the Ayi Shan 2009) attributed the Kailas and other units of gocene (ca. 26 Ma) and no younger than slip suggest oblique dextral transtension (Zhang the Gangrinboche conglomerates to braided on the Great Counter thrust, which was com- et al., 2010). Kailas basin extension predates stream deposition in an axial system that fl owed pleted by ca. 15 Ma (Yin et al., 1994). Late by at least 10 m.y. the better-known east-west parallel to the suture zone. Searle et al. (1990) Oligocene–early Miocene extension in the extension that formed north-striking graben reported that units in the Indus Group “mo- Kailas basin is consistent with evidence for in Tibet beginning in late Miocene time (e.g., lasse” (which are likely correlatives with the rapid exhumation in the footwall of the Ayi Harrison et al., 1995; Murphy et al., 2002; Kailas Formation) are composed of lacustrine Shan detachment at that time (Zhang et al., Garzione et al., 2003; Kapp and Guynn, 2004; and alluvial deposits, derived from both sides 2010), which would have been located directly Saylor et al., 2009). of the suture zone, and deposited on top of the

1354 Geological Society of America Bulletin, July/August 2011 Oligocene–Miocene Kailas basin, southwestern Tibet

TABLE 4. LIGHT STABLE ISOTOPE RESULTS FROM SOILS AND PALEOSOLS Kailas Formation suggests a thermal pulse that Sample ID δ13C VPDB δ18O VPDB Soil depth (cm) Elevation (m) led to melting of Asian lithospheric mantle and Modern Kailas soils garnet-bearing lower crust, and is consistent with KAILAS-1 A –14.42 –4.93 95 4414 KAILAS-1 E –11.91 –2.92 95 4414 (though not diagnostic of) eruption in an exten- KAILAS-1 B –12.55 –1.70 95 4414 sional tectonic environment—perhaps associ- KAILAS-1 D –12.57 –1.60 95 4414 ated with slab break-off (Mahéo et al., 2002; KAILAS-24 B –11.08 –0.15 100 4809 KAILAS-24 A –12.06 1.45 100 4809 Chung et al., 2009). KAILAS-24 C –13.06 –1.55 100 4809 The bulk of the Kailas Formation is charac- Modern Nima soils terized by lithofacies assemblages that are con- Nima 1 B –11.59 –0.47 50 4495 sistent with deposition under a warm, tropical Nima 1 D –13.38 –3.22 50 4495 Nima 1 F –11.66 0.09 50 4495 climate. Searle et al. (1990) also reported thick Nima 1 H –13.03 –2.50 50 4495 lacustrine deposits containing abundant plant Nima 4 A –13.83 –2.72 120 4500 remains in the Indus Group, and suggested a Nima 4 D –10.07 2.33 120 4500 Nima 4 E –13.04 –2.01 120 4500 warm temperate climate. The overall pattern of Nima 4 F –13.47 –3.39 120 4500 sedimentation changes abruptly in the red-bed Nima 4 G –12.98 –1.84 120 4500 Nima 4 H –13.60 –2.47 120 4500 member of the Kailas Formation, which contains clasts derived from the hanging wall of the Kailas Formation paleosols from sections 7KR and 8KR 8KR103PS-C –5.14 –16.02 South Kailas thrust system, exhibits northward 8KR76A-A –5.14 –15.78 paleocurrent directions (Fig. 8), and contrasts 8KR76A-B –4.76 –15.41 sharply with the rest of the Kailas Formation in 8KR76A-C –6.02 –15.63 8KR76B-A –5.59 –16.73 containing abundant paleosol carbonate, marl, 8KR76B-B –5.56 –16.78 and highly oxidized fi ne-grained facies (Fig. 13). 8KR76B-C –5.56 –16.64 Lithofacies in the red-bed member are similar 8KR52PS-A –4.53 –15.69 8KR52PS-B –4.88 –16.07 to those in the roughly contemporaneous Nima 8KR52PS-C –4.83 –16.24 Basin of central Tibet, which is documented to 8KR51PS-A –5.37 –16.30 8KR51PS-B –5.15 –16.26 have been at high paleoelevation by late Oligo- 8KR51PS-C –4.84 –16.15 cene time (DeCelles et al., 2007a, 2007b). The 7KR408PS-B –6.91 –21.48 abrupt change from an essentially noncalcare- 7KR408PS-C –6.44 –21.44 ous to a highly calcareous depositional environ- Note: VPDB—Vienna Peedee belemnite. ment in the red-bed member is unlikely to have been simply a matter of changing provenance, because carbonate clasts and calcite cement Ladakh batholith (the western continuation of to the south is remarkable because the Tethyan are also present in the lower part of the Kailas the Gangdese arc). An inherent notion in all thrust belt was certainly active during the Formation. Rather, we suggest that the bulk of published explanations of the Kailas Formation Eocene–Oligocene (Ratschbacher et al., 1994; the Kailas Formation accumulated in a rapidly (and its equivalents) is that it was deposited in a Hodges, 2000; Murphy and Yin, 2003; DeCelles subsiding, warm tropical basin at relatively contractional tectonic regime. et al., 2004; Aikman et al., 2008). (2) We found low elevation, whereas the red-bed member Although the uppermost part of the Kailas no evidence for progressive tilting of bedding accumulated at relatively high elevation under Formation does contain clasts that were de- characterizing contractional growth structures an arid or semiarid climate, more analogous rived from the south (in the hanging wall of the (e.g., Anadòn et al., 1986) in the Kailas Forma- to (but still wetter than, based on the δ13C evi- Great Counter thrust), the bulk of the unit was tion, neither in proximity to the South Kailas dence) the modern setting of the Indus-Yarlung derived from the north, as originally indicated thrust system, nor along its northern outcrop suture. Although the oxygen and carbon isotope by Gansser (1964). Paleocurrent data from the border where it rests unconformably on top of data that we present are limited in number and Kailas Formation provide no evidence for sig- the Kailas magmatic complex. The outcrop evi- stratigraphic coverage, they are consistent with nifi cant axial paleofl ow along the suture zone dence indicates that most shortening related to the interpretation that the uppermost part of the (e.g., Searle et al., 1990; Aitchison et al., 2002). the South Kailas thrust system took place after Kailas Formation was deposited at high eleva- Our results also are not consistent with deposi- depo si tion of the Kailas Formation. (3) The over- tion. More thorough paleoaltimetry studies in- tion of the bulk of the Kailas Formation in a con- all textural and lithofacies pattern in the Kailas dicate that the Zhada Basin to the west (Saylor tractional setting, but instead suggest deposition Formation is not typical of contractional wedge- et al., 2009), Thakkhola graben to the southeast in a rift or transtensional strike-slip basin. The top settings or proximal foredeep settings (see, (Garzione et al., 2000a), and the Oiyug Basin key observations that do not support deposi- e.g., DeCelles and Giles, 1996; Lawton et al., to the east (Currie et al., 2005) were all at high tion in a contractional setting include: (1) The 1999; Heermance et al., 2007), both of which elevation (>4000 m) by middle to late Miocene Kailas Formation sits on local Gangdese arc usually result in upward-coarsening packages time. We infer that the red-bed member of the “basement,” and was derived directly from this of growth strata. Instead, the Kailas Formation Kailas Formation was deposited when the Great basement. This is not typical of thrust-generated generally fi nes upward and consists of a classic Counter thrust became active and began to sup- proximal facies, which are derived from the rift-basin lacus trine sandwich, which could have ply sediment from elevated highlands to the hanging walls of bounding thrust faults and developed in any narrow, deep extensional envi- south, and the Kailas basin began to structur- deposited upon rocks that are structurally below ronment (including oblique strike-slip). (4) The ally invert and gain elevation, culminating in the associated thrust sheet. The general paucity presence of phlogopite-bearing basaltic andesites its present very high elevation (~4.7–6.7 km). of clasts derived from the Tethyan Himalaya and adakitic tuffs (Aitchison et al., 2009) in the Unfortunately, the absence of paleosol carbon-

Geological Society of America Bulletin, July/August 2011 1355 DeCelles et al.

3 along the length of the suture zone. Deﬁ nitive assessment of the regional signiﬁ cance of mid- 2 modern Kailas and Nima soils Cenozoic extension in the Indus-Yarlung suture Kailas paleosols zone requires more detailed basin analyses in Nima paleosols the Gangrinboche conglomerates. 1 GEODYNAMIC IMPLICATIONS 0 Independent data from sediment provenance, –1 depositional facies, paleontology, biostratig- decreasing soil raphy, geochronology, and petrology suggest respiration that the Indo-Eurasia collision occurred during –2 late Paleocene to early Eocene time (e.g., Gar- zanti et al., 1987; Searle et al., 1987; Rowley,

C VPDB –3 1998; di Sigoyer et al., 2000; DeCelles et al., 13

δ 2004; Najman et al., 2005; Zhu et al., 2005; –4 Leech et al., 2005; Chung et al., 2005; Green et al., 2008) and that crustal shortening in the Himalayan thrust belt propagated southward –5 from the suture zone beginning at that time. Alternatively, Aitchison et al. (2002, 2007) sug- –6 gested that the true collision did not commence until ca. 34 Ma, in part based on the interpretation that the Gangrinboche conglomerates were –7 increasing evaporation produced by crustal shortening associated with decreasing temperature the initial Indo-Eurasia collision. Our results –8 from the type area of the Gangrinboche facies –25 –20 –15 –10 –5 do not support the notion that these deposits record initial shortening owing to the collision of δ18O VPDB (‰) India and Eurasia. It is plausible that the Kai- las Formation records local tectonic processes Figure 16. δ18O vs. δ13C values of modern soil carbonates and paleosol carbonate from the in southwestern Tibet, and that the remainder upper Kailas Formation. Modern soil samples all come from >50 cm soil depth (see Table 4). of the Gangrinboche conglomerates along the VPDB—Vienna Peedee belemnite. suture zone were indeed deposited in a contractional setting as suggested by Aitchison et al. (2002). However, we suggest that a resolution to this debate may lie in considering mechanisms ate and marl in the bulk of the Kailas Forma- by Valli et al. (2007), this would suggest that for extension in the Indus-Yarlung suture zone tion precludes documentation of a systematic exhumation of the Ayi Shan took place in a within an overall convergent tectonic setting. increase in paleoelevation through time. transtensional tectonic setting, perhaps feeding Crustal shortening and extension are not mu- Extension of Asian lithosphere during late slip eastward from the southeastern end of the tually incompatible in intercontinental collision Oligocene–early Miocene time in the Kailas re- Karakoram fault into either the Indus-Yarlung zones. For example, extension and shortening gion is supported by recent work by Zhang et al. suture zone (Valli et al., 2007) or southeastward operate side-by-side throughout the Mediterra- (2010), who documented evidence for coeval across the northern Himalayan thrust belt via nean region, where foundering and retrograde ductile extension, anatexis, and exhumation of the Gurla Mandatha detachment (Murphy et al., “rollback” of generally northward-subducting high-grade orthogneisses of Gangdese arc afﬁ n- 2002). Either of these structural-kinematic inter- remnants of Neotethyan oceanic lithosphere, ity from midcrustal depths in the footwall of the pretations would suggest that the Kailas basin stranded between African continental prom- Ayi Shan detachment (Fig. 1B). Restoration of should be localized to the southeastern end of ontories, causes upper-plate extension of the ~65 km of slip on the dextral strike-slip Kara- the Karakoram fault. In fact, the Kailas Forma- European mainland behind major thrust belts koram fault (Murphy et al., 2000) places the tion outcrop belt extends continuously >50 km that propagate from Europe toward Africa Ayi Shan directly west along the outcrop belt east of the end of the Karakoram fault and the (Malinverno and Ryan, 1986; Doglioni et al., that we have studied in the Kailas Range. Thus, Gurla Mandatha dome, and observations by 1997; Cavinato and DeCelles, 1999; Jolivet and the Ayi Shan detachment fault projects into the Searle et al. (1990), Yin et al. (1999), and Aitchi- Faccenna, 2000; Spakman and Wortel, 2004; subsurface beneath the Kailas Range and pro- son et al. (2002) indicate that mid-Cenozoic Cavazza et al., 2004; Faccenna et al., 2004). vides a ready explanation for the presence of coarse clastic rocks correlative with the Kailas Such coupled extensional-contractional systems extensional basin deposits in the Kailas Forma- Formation crop out along at least 1300 km of are characteristic of convergent plate bound- tion (Figs. 1B and 5B). Zhang et al. (2010) also the suture zone. This argues against the local- aries in which the rate of subduction exceeds the reported kinematic indicators that document ized extension/transtension explanation for the rate of plate convergence (e.g., Royden, 1993; top-southeastward (~120°E) ductile shear on Kailas Formation, and suggests instead that the Jolivet and Faccenna, 2000). Although we view the Ayi Shan detachment. Coupled with work mechanism for Kailas basin formation operated it as highly unlikely that subduction of oceanic

1356 Geological Society of America Bulletin, July/August 2011 Oligocene–Miocene Kailas basin, southwestern Tibet lithosphere was occurring along the Indus-Yar- slab broke off and a pulse of high-K and ada- de Sigoyer et al., 2000; Guillot et al., 2003; lung suture ~30 m.y. after the youngest known kitic magmatism occurred all along the south- Leech et al., 2005). After oceanic slab break-off, marine sedimentary rocks were deposited (Gar- ern Lhasa terrane (Fig. 17) (Coulon et al., 1986; all convergence velocity contributed directly zanti et al., 1987; Willems et al., 1996; Rowley, Miller et al., 2000; Mahéo et al., 2002; Ding et to crustal shortening (Chemenda et al., 2000; 1998; Zhu et al., 2005; Green et al., 2008), al., 2003; Chung et al., 2003; Williams et al., Guillot et al., 2003). Our explanation of middle Mediterranean-style, convergent-margin tecton- 2004; Gao et al., 2007; Aitchison et al., 2009; Ceno zoic tectonics along the Indus-Yarlung su- ics controlled by relative rates of convergence Chung et al., 2009). The timing of southward ture suggests that a second signifi cant episode and subduction provide a useful framework for migration of Tibetan magmatism and the hypo- of slab break-off occurred during early Mio- understanding Oligocene–Miocene extension in thetical slab break-off event bracket deposition cene time. Unlike the Eocene break-off event, the southern part of the Eurasian plate. of the Kailas Formation and the other Gangrin- the Miocene event involved continental Indian If Indian continental lithosphere, several boche conglomerates. The abundance of tuffs lithosphere. It is possible that the Indo-Eurasian hundred kilometers of which could have been and detrital zircons with ages of 24–26 Ma re- collisional orogeny operated in two differ- underthrust beneath the Lhasa terrane after the fl ects this magmatic activity. ent modes, depending on whether subducted initial Eocene hard collision and Neotethyan After Indian continental slab break-off (ca. Tethyan/Indian lithosphere was foundering by slab break-off event (e.g., DeCelles et al., 2002; 20 Ma), the system would have reverted to a rollback (soft collision), or underthrusting at Kohn and Parkinson, 2002; Guillot et al., 2003; “hard collisional” mode, with all of the con- low angle beneath Tibet (hard collision; Fig. Ding et al., 2005; Kapp et al., 2007; Lee et al., vergence velocity being accommodated by 17). During episodes of rollback and slab foun- 2009), were to begin foundering into the mantle underthrusting of Indian lithosphere and crustal dering (pre–45 Ma and ca. 32–20 Ma), the rate such that a rolling hinge line migrated south- shortening (Fig. 17). Early to middle Miocene of subduction exceeded the convergence rate, ward relative to Indian lithosphere (Ding et al., activation of several major thrust systems in resulting in tectonic neutrality or upper-plate 2003), then the subduction rate would equal the the Himalaya, including the Main Central, extension. During episodes of underthrusting sum of the absolute values of true Indo-Eurasia Ramgarh, and Great Counter thrusts (which to- (ca. 45–32 Ma and ca. 20–0 Ma), all of the con- convergence and the rate of hinge-line rollback. gether accommodate at least 300 km of shorten- vergence velocity was thrown into shortening of Subduction rate would thus exceed the rate of ing), refl ects this new mode of hard collision. the Indian and Eurasian plates. This idea could convergence, and the upper (Eurasian) plate This explanation for the kinematics of the be tested with a careful assessment of the timing could be thrown into extension near the plate Kailas basin is advantageous because it does of north-south crustal shortening and extension boundary (Fig. 17). This process is analogous to not require post-Eocene oceanic subduction out- episodes in the Himalaya and Lhasa terrane. the mechanism driving upper-plate extension in board of an irregular northern Indian continental the Mediterranean, with the important distinc- margin; it can be reconciled with regional petro- CONCLUSIONS tion that no oceanic lithosphere was involved in logic, metamorphic, structural, and sedimen- the Tibetan case. tological data sets; and a drastic revision in the The Kailas Formation is late Oligocene to Available data suggest that such a situation timing of initial Indo-Eurasia collision (Aitchi- early Miocene in age, and was deposited in existed in the Indus-Yarlung suture zone during son et al., 2007) is unnecessary. Moreover, the alluvial-fan, low-sinuosity fl uvial, and deep the middle Cenozoic. Molnar and Stock (2009) proposed mechanism is consistent with recent lacustrine depositional systems. Kailas For- calculated a middle Cenozoic (ca. 45–20 Ma) tomographic studies of the upper mantle beneath mation lakes were narrow, deep, and localized convergence rate of ~59 mm/yr between north- Tibet, where large, fast P-wave anomalies have along the Indus-Yarlung suture zone. The bulk western India and Eurasia. The rate of south- been detected and interpreted as fragments of of the Kailas Formation was derived from the ward rollback of the Indian plate hinge line can foundered Indian lithosphere (van der Voo et al., Gangdese arc to the north, upon which it was be estimated at ~50 mm/yr from a regional, 1999; Hafkenscheid et al., 2006; Li et al., 2008; deposited; southerly sediment sources in the southward sweep of magmatism over a north to Replumaz et al., 2010a, 2010b). Replumaz et al. hanging wall of the Great Counter thrust and south distance of ~400 km from the Qiangtang (2010a) noted that the distribution of fast anoma- the Tethyan Himalaya became available only terrane to the southern Lhasa terrane between lies beneath western Tibet is consistent with two during the last stages of deposition. Abundant ca. 32 Ma and 25 Ma (Fig. 18) (Ding et al., slab break-off events, and their reconstruction organic material (plant fragments, disseminated 2003; Chung et al., 2005; Kapp, et al., 2007). suggests that these events were coeval with the kerogen, fi sh and amphibian fossils) and sparse For ~8 m.y. during latest Eocene–Oligocene events we postulate (Eocene and late Oligocene– palynological data suggest that Kailas lakes time, the subduction rate would have been early Miocene) based on surface data. Similar were relatively warm and tropical. In contrast, ~84% greater than the rate of convergence. timing of slab break-off and/or lithosphere re- the uppermost part of the stratigraphic section Capitanio et al. (2010) suggested a likely cause moval events is derived from petrological studies contains abundant paleosol carbonate and red- of continental slab rollback and subduction: of Eocene–Miocene igneous rocks in the Lhasa bed paleosols, similar to previously documented Previously thinned continental lower crust and terrane (e.g., Miller et al., 1999; Chung et al., Oligocene–Miocene lithofacies in the central lithosphere that have been stripped of upper- 2009; Lee et al., 2009). part of the Tibetan Plateau from which we have crustal rocks by the development of a thrust Numerous workers have suggested that the obtained carbon and oxygen isotope data that belt would be dense enough to founder into the development and subsequent exhumation to are consistent with very high paleoelevation by mantle. In this case, upper-crustal material was midcrustal depths of eclogites in the northern late Oligocene time. We tentatively suggest that being actively scraped off of Indian lower crust Himalayan thrust belt during Eocene time re- the Kailas basin developed at comparatively low to form the Tethyan Himalayan thrust belt, leav- quired initial subduction by slab-pull forces, elevations until deposition of the uppermost part ing behind a dense, possibly eclogitized, lower followed by buoyant rise when the Neotethyan of the Kailas Formation. crust and mantle lithosphere. Southward roll- oceanic lithospheric slab broke off and foun- The Kailas basin developed along the Indus- back of the Indian plate evidently terminated dered into the mantle (Chemenda et al., 2000; Yarlung suture zone contemporaneously with at ca. 25–20 Ma, when the subducting Indian O’Brien et al., 2001; Kohn and Parkinson, 2002; extension in the Ayi Shan and possibly oblique

Geological Society of America Bulletin, July/August 2011 1357 DeCelles et al.

GCT KB MCT LT QT

20–15 Ma 4400 mm/yrmm/yr Hard

Return to northward collision underthrusting mode

KB TH LT QT 25–20 Ma 6060 mm/yrmm/yr

Greater Indian slab break-off Figure 17. Schematic north- south cross sections showing development of upper-plate

extension in southern Tibet in Soft response to Tethyan and In- ~50 mm/yr collision dian slab rollback and break- KB TH off events, as discussed in text. LT QT The Kailas basin (KB) formed 32–25 Ma 6060 mm/yrmm/yr during the 32–25 Ma and 25– Greater Indian slab rollback 20 Ma frames. Abbreviations and upper-plate extension as follows: GMA—Gangdese magmatic arc; MCT—Main Central thrust; GCT—Great Counter thrust; IYS—Indus- ~30 mm/yr Yarlung suture zone; BS—Ban- IYS gong suture zone; LT—Lhasa TH terrane; QT—Qiangtang ter- 45–32 Ma 6600 mm/yrmm/yr rane; TH—Tethyan Himalaya.

Approximate times of hard and Northward underthrusting of Greater Hard soft collision are indicated by Indian lithosphere beneath Tibet collision solid and dashed bars at right, respectively. Overturned con- figuration of the subducted LT QT Greater Indian litho sphere in 55–45 Ma ~60~60 mm/yrmm/yr the top two frames is after Replumaz et al. (2010a). Soft Tethyan slab rollback, followed by collision break-off and foundering

IYS BS LT QT 55 Ma 110000 mm/yrmm/yr E Hard collision

GMA

60 Ma 100100 mm/yrmm/yr Precollision

1358 Geological Society of America Bulletin, July/August 2011 Oligocene–Miocene Kailas basin, southwestern Tibet

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