Gondwana Research 22 (2012) 434–446

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The vast proto-Tibetan Plateau: New constraints from Paleogene Basin

Jingen Dai a, Xixi Zhao b, Chengshan Wang a,⁎, Lidong Zhu c, Yalin Li a, David Finn b a State Key Laboratory of Biogeology and Environmental Geology, School of Earth Sciences and Resources, Research Center for Geology, China University of Geosciences, Beijing 100083, China b Department of Earth and Planetary Sciences and Institute of Geophysics and Planetary Physics, University of California, Santa Cruz, CA 95064, USA c Chengdu University of Technology, Chengdu 610059, China article info abstract

Article history: The surface uplift of the Tibetan Plateau is the key boundary condition in many Cenozoic geological events Received 19 May 2011 ranging from global cooling to changes of Asian environments during Cenozoic. However, poorly constrained Received in revised form 18 July 2011 timing for the uplift of Tibetan Plateau makes these interpretations highly debatable. Here we report results Accepted 16 August 2011 from sedimentology, detrital zircon U–Pb and Lu–Hf isotopic compositions, and paleomagnetic signatures Available online 17 October 2011 from both the eastern and western Hoh Xil basins of north-central Tibetan Plateau. Sedimentary lithofacies and facies associations analyzed in the western Hoh Xil basin indicate they were deposited in a braided fluvial Keywords: Hoh Xil Basin system and alluvial fan, similar with the Fenghuoshan Group, eastern Hoh Xil basin. Provenance analyses Tibetan Plateau from conglomerate clast compositions, paleocurrent orientations, and detrital zircon U–Pb and Lu–Hf isotopic Detrital zircon compositions document sediments in both western and eastern basins were derived from the Qiangtang and Paleomagnetism Lhasa blocks. These observations, in combination with comparative paleomagnetic results, imply that Hoh Xil Cenozoic global cooling basin was a single, wide basin during Paleogene. The period of Hoh Xil basin deposition was coeval with sig- nificant period of the early Cenozoic uplift and erosion of the Qiangtang and Lhasa blocks. These observations not only reinforce the suggestion that the Qiangtang and Lhasa blocks were uplifted during Eocene to form a proto-Tibetan Plateau, but also imply that the proto-Tibet Plateau is vast in areal extent. The large dimension and high elevation of the proto-Tibetan Plateau probably contributed to the global cooling during the early Eocene. © 2011 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.

1. Introduction Qiangtang blocks occurred prior to India–Asia collision (England and Housemann, 1986; Murphy et al., 1997; Kapp et al., 2003, 2007). These Known as the roof of the world, the Tibetan Plateau is the most ex- observations imply that the central Tibetan Plateau might have already tensive region of elevated topography in the last 100 million years of elevated during Cretaceous and early Cenozoic. Paleoaltimetry studies, the Earth's history. How such high topography, which should have an providing direct information about the paleoelevation, reveal that the effect on climate and ocean chemistry (Raymo and Ruddiman, 1992; central Tibetan Plateau has obtained its current elevation since Eocene Richter et al., 1992; An et al., 2001; Dupont-Nivet et al., 2007, 2008; (e.g. Rowley and Currie, 2006; Polissar et al., 2009). Recently, based on Garzione, 2008), has developed through geologic time remains a topic studies from the eastern Hoh Xil basin (EHXB) (Fig. 1), some of us pro- of major disagreement. The reason might be derived from poorly con- posed that uplift of the Tibetan Plateau was progressive, and the central strained timing and mechanism for the uplift of the Tibetan Plateau. Tibetan Plateau (the Qiangtang and Lhasa blocks) was elevated by Models of the Tibetan Plateau surface growth vary in time, from Eocene 40 Ma ago (termed as the proto-Tibetan Plateau) (Wang et al., 2008a). to Pliocene (Coleman and Hodges, 1995; An et al., 2001; Blisniuk et al., The areal extent of the proto-Tibetan Plateau, however, remains ambig- 2001; Spicer et al., 2003; Currie et al., 2005; Rowley and Currie, 2006; uous because of limited geological knowledge of the western Hoh Xil DeCelles et al., 2007b), and range in magnitude, from wholesale uplift to basin (WHXB). The boundary between the EHXB and WHXB is along progressive growth (Molnar et al., 1993; Chung et al., 1998; Tapponnier about 90°E (Fig. 1; Pan et al., 2004). et al., 2001). Here we present new results from sedimentology, detrital zircon Previous studies including numerical modeling and structural U–Pb and Lu–Hf isotopic compositions, and paleomagnetic signatures geology have documented that crustal shortening in the Lhasa and from both the EHXB and WHXB of north-central Tibetan Plateau. These results indicate that the existence of a single, wide Hoh Xil basin during Paleogene as response to uplift and erosion of the

⁎ Corresponding author. Tel./fax: +86 10 82322171. proto-Tibetan Plateau (the Qiangtang and Lhasa blocks), thus provid- E-mail addresses: [email protected] (J.G Dai), [email protected] ing new constrains on the areal extent of proto-Tibetan Plateau and (C.S Wang), [email protected] (X.X Zhao). new possible insight into the global cooling during Eocene.

1342-937X/$ – see front matter © 2011 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.gr.2011.08.019 J. Dai et al. / Gondwana Research 22 (2012) 434–446 435

N B 0 200 Cenozoic Strata W E km S Late Triassic Volcanic rocks

Altyn Tagh Fault Detrital zircon in this study Eastern Kunlun Shan Kunlun Fault AKSZ Detrital zircon in literature Western HXB Fig. 2 EHX09Y06 P01-19W Paleoaltimetry study 35°N Tanggula Eastern HXB Shan JSSZ 45-38 Ma Adakitic rocks Thrust Systems EHX0905 47-38 Ma north-south trending dikes 06AQ60 06AQ66 Qiangtang N 06AQ197 E 06AQ47 W 0 400 S Altyn Tagh Fault 06AQ187 Eastern Kunlun Shan km Lunpola Basin Hoh Xil Basin AKSZ Legend Nima Basin Tanggula Thrust Systems 35°N <1000 m BNSZ JSSZ 1000-2000 m LNPLA Qiangtang 2000-3000 m 7-11-05-02 BNSZ 3000-4000 m Lhasa Lhasa 4000-5000 m 30°N 5000-6000 m YZSZ 6000-7000 m 30°N Himalaya 7000-8000 m YZSZ Main Front Thrust 8000-9000 m A 85°E90°E 80°E 85°E 90°E 95°E 100°E

Fig. 1. A, Digital elevation model of Tibetan Plateau showing major sutures (modified from Yin and Harrison, 2000): AKSZ, A'nemaqin-Kunlun suture zone; JSSZ, Jinshajiang suture zone; BNSZ, Bangong-Nujiang suture zone; YZSZ, Yurlung Zangbo suture zone. B, Cenozoic sedimentary basins of central-northern Tibetan Plateau showing the sample locations, the compared sample location, the distribution of the Late Triassic volcanic rocks (Fu et al., 2010) and Eocene adakitic rocks (Wang et al., 2008b). The published pre-Cenozoic samples are outlined in gray pentagram: sample 06AQ47, 06AQ60, 06AQ66, 06AQ187, 06AQ197 from Pullen et al. (2008); sample 7-11-05-02 from Kapp et al. (2007); sample LNPLA from Leier et al. (2007). The red triangles represent the paleoaltimetry study: Nima basin (DeCelles et al., 2007b), Lunpola basin (Rowley and Currie, 2006; Polissar et al., 2009), and Eastern Hoh Xil basin (Cyr et al., 2005; Polissar et al., 2009). The blue rectangle represents the study area of Eocene north–south trending dikes (Wang et al., 2010).

2. Geological setting exposure of Fenghuoshan Group and Yaxicuo Group may exist in the WHXB (Li et al., 2002, 2006a Pan et al., 2004; Yue et al., 2006). This ob- The Hoh Xil Basin, situated in the north-central Tibetan Plateau, is servation leads to some important questions: 1) whether or not the one of the largest Cenozoic sedimentary basins in the hinterland of WHXB has the similar pattern of sedimentation with the EHXB; 2) the Tibetan Plateau (Fig. 1; Liu and Wang, 2001). Our previous work fo- whether or not WHXB and EHXB formed a single, wide Paleogene cused on the EHXB, particularly in the Fenghuoshan area near the basin. To address these questions, we conducted systematic investiga- Lhasa–Golmud highway (Fig. 1; Liu and Wang, 2001; Liu et al., 2003). tions with sedimentology and paleomagnetism in the Dead Valley These studies revealed that relatively thick Cenozoic terrestrial strata area of WHXB (Fig. 2). We also performed detrital zircon measurements are exposed in the EHXB. These strata can be divided into three lithos- on samples from both WHXB and EHXB. tratigraphic units. The bottom unit Fenghuoshan Group consists of cob- ble–pebble conglomerate, red sandstone, and bioclastic limestone of 3. Stratigraphy and sedimentology fluvial, fan-delta, and lacustrine origin. The overlying Yaxicuo Group comprises sandstone, mudstone, marl, and gypsum deposited in fluvial We measured ~1200 m of the Cenozoic stratigraphic section in the and playa environments. The uppermost Wudaoliang Group consists of western Hoh Xil basin (Figs. 2 and 3). This section is located in a val- lacustrine marl and minor amounts of black shale. Fenghuoshan Group ley with plenty of dead . We termed it as the Dead Yak section. and Yaxicuo Group together are >5000-m-thick and deformed by over- Depositional environments of these strata are reconstructed based turned folds and numerous south-directed thrusts, whereas the 100- to on sedimentological observations. As detailed in the following, the 200-m thick Wudaoliang Group is only gently tilted (Liu and Wang, Dead Yak section is divisible into three units on the basis of lithofacies 2001; Wang et al., 2008a; Wu et al., 2008). Magnetostratigraphic stud- assemblages and litholotical characteristics representative of braided ies suggest that Fenghuoshan Group was deposited ~52.0–31.3 Ma ago, fluvial and alluvial-fan environments. The detailed descriptions of and Yaxicuo Group was deposited 31.3–23.8 Ma ago (Liu et al., 2003), lithofacies are presented in Table 1. Because lithofacies which we while the base of Wudaoliang Group was biostratigraphically dated at documented here have been widely reported in the literature on flu- ~22Ma (Wang et al., 2008a). Structural geological investigations vial environment (Miall, 1977, 1985; Spalletti and Colombo Piñol, show that a large-scale south-dipping Cenozoic thrust system occurred 2005; Uba et al., 2005; DeCelles et al., 2007a, 2011), the following in the Tanggula Shan (=mountain), south of the EHXB. This thrust sys- text just focuses on standard interpretations of depositional processes tem was termed as the Tanggula thrust system (TTS), stretching for a and environments. Paleocurrent data were collected by limbs of distance of more than 320 km in NW–SE direction (Fig. 1; Li et al., trough cross-beddings, asymmetry ripples, flute and parting linea- 2006b). The EHXB was considered to be a foreland basin system coeval tion. Samples for geochronology, paleomagnetism and sedimentary with the surface uplift of central Tibetan Plateau (Li et al., 2006b; Wang petrology were collected in the measured section. et al., 2008a). The lower unit of the Dead Yak section is ~799 m thick (0–799 m). However, the geology of the WHXB remains unclear because of its This unit consists of medium- to coarse-grained sandstones interbedded remoteness and inhabitable condition. Existing geologic maps of the with pebbles to cobbles (Fig. 4A). Individual sandstone beds vary in WHXB show sparse outcrops of Paleogene strata, suggesting that thickness from medium to very thickly bedded (~0.1–1.5 m) and extend 436 J. Dai et al. / Gondwana Research 22 (2012) 434–446

88°00´N 88°30´N 0 km 5 N W E S Dead Yak Section Triassic clastic rocks

Jurassic limestones

Xuehuan lake Paleogene sediments

35°00´N 35°00´N

Quaternary sediments

Lower Jurassic andesites

Cretaceous granites

Lakes

Thrust fault

88°00´N 88°30´N

Fig. 2. Geological map of study area showing the location of the Dead Yak section.

Fenghuoshan Section

Dead Yak Valley Section 30.0 Ma This study Mudstone 5000 /shale m C12n

1080 YG n=52 Sandstone m 31.3 Ma FG EHX09Y06 Conglomerate C12r-6 960 Mar/ 4000 C12r-7 limestone C13n 840 Gypsum C15n n=8 n=83 Paleocurrent directions C16n.1n n=30 n= data number 720 P01-19W 3000 C16n.2n Grain size n=20 C17n.1n M S C 600 MudstoneSandstoneConglomerate C17n.2n C17n.3n 480 C18n.1n 2000 n=8 C18n.2n EHX0905 360 C19n

n=4 C20n 240 1000 C21n 120 n=6 C22n

n=22 0 C23n.1n M S C 0 51.0 Ma M S C

Fig. 3. Comparisons of lithologic sections between the Dead Yak Valley area, western Hoh Xil basin (WHXB) and the Fenghuoshan area, eastern Hoh Xil basin (EHXB) (Liu and Wang, 2001; Liu et al., 2003). The pentagrams indicate the sample location in their sections. J. Dai et al. / Gondwana Research 22 (2012) 434–446 437

Table 1 consists primarily of poorly sorted, massive, disorganized conglomer- Description and interpretation of sedimentary lithofacies. ates interbedded with massive to stratified pebbly sandstones After Miall, 1985 and Uba et al., 2005. (Fig. 4E). Individual conglomerate beds are clast- or matrix-supported Lithofacies Description Interpretation (Fig. 4F), and are laterally extensive for tens of meters (Gcm, Gmm, code and Gh). They have lenticular to sheet-like geometries. Sandstone Ss Fine to coarse grained sandstone Scour fills beds are thinly bedded and typically contain very fine grained sand- and conglomerates; scour surface. to pebble-rich lenses that are laterally continuous for tens of meters St Medium- to very coarse grained Migration of large 3D ripples (Sm, Sh). This facies association represents deposition by high-energy sandstone with trough cross- (dunes) under moderately fl stratification powerful, unidirectional flows in sediment and gravity ows in the alluvial fan. Disorganized, clast- and large channels matrix-supported conglomerates are interpreted as the products of Sh Fine- to medium-grained, trace Planar bed flow, upper flow clast-rich and plastic debris flows, respectively. The more organized, pebbles and cobbles; horizontally regime clast-supported conglomerate facies represents traction bedload sedi- stratified, moderately to well mentation during waning sheet floods in poorly confined channels. sorted Sp Medium- to very coarse-grained, 2D dunes, lower flow regime Based on the observed sedimentary lithofacies and facies associa- may be pebbly; planar cross- tions, the Cenozoic strata in the western Hoh Xil basin is believed to stratification represent deposition in a braided fluvial system and alluvial fan. Sr Fine-to medium-grained sand- 2D and 3D current ripples, upper Paleocurrents measured in the section are dominated by north- stone with small asymmetric flow regime ripples direction, indicating that these sediments were derived from the Sm Very fine to coarse-grained, pebbly; Rapid deposition, sediment basin southern side-the Qiangtang and Lhasa blocks (Fig. 3). The massive, moderately to well sorted gravity flow compositions of conglomerates are primarily composed of limestone, Gh Clast-supported, crudely Longitudinal bars, lag deposits, sandstone, granite, and volcanic rocks (including andesite and ba- fi horizontally strati ed or massive sieve deposits salt), consistent with the regional distribution of these rocks in the conglomerates; granules to cobble with imbrications; Qiangtang block (Figs. 1 and 2; Fu et al., 2010; Pan et al., 2004; Pullen moderately sorted, rounded to et al., 2011). subrounded Regionally, the Cenozoic strata of the Dead Yak section were fi Gt Clast-supported trough cross- Channel ll, transverse bar termed as the Kangtog Formation (e.g. Li et al., 2002, 2006a; Yue stratified conglomerates. Granules to cobbles, normal grading with et al., 2006). The age of the Kangtog Formation was determined imbrications from radiometric and paleontological investigations. The Paleogene Gcm Pebble to boulder conglomerate, Deposition from sheetfloods and charophytes Obusochara sp., O. lanpingensis and Gyrogona qinajiangica poorly sorted, clast-supported, clast-rich debris flows were reported from the Kangtog Formation (Yue et al., 2006). The fi unstrati ed, poorly organized K–Ar ages of andesite from the base of the Kangtog Formation vary Gmm Massive, matrix-supported pebble Deposition by cohesive mud-matrix 40 –39 to boulder conglomerate, poorly debris flows from 65.1 to 65.5 Ma, while the Ar Ar ages of the overlying sorted, disorganized, unstratified volcanic rocks range from 27.8 to 30.5 Ma (Li et al., 2002, 2006a). Fl Five to medium sandstone, and Deposits of waning floods, These observations suggest that the age of Kangtog Formation mud with bioturbation overbank deposits should be Paleocene to Eocene (Yue et al., 2006). This conclusion is supported by the similar lithologies and depositional environments between our measured strata and the Fenghuoshan Group in the EHXB which was deposited ~52.0–31.3 Ma ago (Liu et al., 2003). laterally for tens to hundred meters. Predominant lithofacies of sand- stones are St, Sh, Ss, Sp (Table 1). Interbedded with sandstones are 4. Analytical methodology clast-supported, subrounded to rounded pebble to cobble conglomer- ates. Individual conglomerate beds are medium to very thickly bedded 4.1. Detrital zircon U–Pb and Lu–Hf isotopic composition (~0.2 to >2 m) and persist laterally for tens of meters. Conglomerates are dominated by lithofacies Gh and Gt with minor Gcm (Table 1). Shal- Zircon concentrates were separated from sample P01-19W (WHXB) low erosional scours are common (Fig. 4A and B). Deposition of this fa- and EHX0905, EHX09Y06 (EHXB) using heavy liquids and magnetic cies association is interpreted to have occurred in braided river deposits, separation techniques. One andesite cobble (sample P01-31A) was including longitudinal bar and bar-flank depositions and channel de- also analyzed. Individual crystals were handpicked and were mounted posits. The clast-supported conglomerates generally exhibit lenticular in epoxy resin. Cathodoluminescence images were used to check the geometries and erosive basal contacts consistent with deposition in internal structures of individual zircon grains and to select positions shallow channels of low sinuosity. for analyses. U–Pb dating of zircon was performed by laser ablation- The middle unit of the Dead Yak section is ~213 m thick inductively coupled plasma-mass spectrometry (LA-ICP-MS) at the (799–1012 m). It is primarily composed of fine- to medium-grained State Key Laboratory of Mineral Deposit Research, Nanjing University, sandstones, mudstones and minor granules to cobble conglomerates. China, following the method described by Jackson et al. (2004).The Sandstones are interbedded with mudstones (Fig. 4C), while con- laser-ablation spot diameter was 25 μm. The common-Pb correction glomerates occur as lenses a few tens of meters. Individual sandstone followed the method described by Andersen (2002).The206Pb/238U beds are thinly to thickly thick and extend laterally for tens of meters. ages are used for the grains with ages younger than 1000 Ma, while Their lithofacies are mainly Sr, Sh, Sp (Table 1). Interbedded mud- the 206Pb/207Pb ages are used for the grains with ages older than stone layers are thickly laminated to very thinly bedded. The lithofa- 1000 Ma. For statistical purposes, zircons>1000 Ma with dis- cies are dominated by Fl (Table 1). This facies association represents cordanceb10% and b1000 Ma with discordance b20% were considered the overbank environment in a braided river. The bioturbated mud- as usable. The detrital zircon U–Pb dating results are summarized in stone (Fig. 4D), in combination with the rippled sandstone (Fig. 4C), Table S1. is consistent with overbank deposits. In situ zircon Hf isotopic analyses were conducted by a Nu Plasma The upper unit of the Dead Yak section is ~188 m thick HR MC-ICP-MS equipped with a 193 nm laser at the State Key Labora- (1012–1200 m). Even we did not measure the top of this unit, we tory of Continental Dynamics, Northwest University, China. Lu–Hf estimated this unit should be more than 1000 m thick as indicated isotopic analyses were obtained on the same zircon grains that by the fact that plenty of similar lithofacies within our study area. It had been analyzed for U–Pb dating. During analyses, spot sizes of 438 J. Dai et al. / Gondwana Research 22 (2012) 434–446 AB

CD

EF

Fig. 4. Photographs of representative lithofacies in the Dead Yak section. A. Interbedded conglomerates (Gh, Gt and Gcm) and sandstones (Ss and St) in the lower unit. The cobble to pebble conglomerates displaying channel-fill geometry and erosive base (arrows), representing deposition in braided fluvial environment. B. Horizontally stratified sandstone (Sh) overlain by granule to cobble conglomerates with scour surface (arrow) in the lower unit. C. Thinly- to medium-bedded sandstones (Sr and Sm) interbedded with mudstones (Fl). The sandstones show asymmetric ripples (arrow) in the middle unit. D. Mudstones with bioturbations (Fl) in the middle unit. E. Clast-supported cobble to pebble conglomerates (minor boulders; Gcm) interbedded with massive sandstones (Sm) in the upper unit, representing deposition in alluvial fan. F. Matrix-supported disorganized conglomerate with subrounded to subangular, pebble to boulder (Gmm) in the upper unit.

44 μm and the laser repletion rate of 8 Hz were used. Isobaric interfer- 4.2. Paleomagnetic sampling, experimental procedures and data analysis ence of 176Lu on Hf was corrected by measuring the intensity of an interference-free 175Lu isotope and also recommended 176Lu/175Lu Sampling followed standard paleomagnetic practice with in situ ratio of 0.02669 to calculate 176Lu/177Hf. The interference of 176Yb drilling by a portable gasoline-powered core drill. Orientation was on 176Hf was corrected by measuring an interference-free 172Yb iso- done by using both a magnetic compass mounted on an orienting tope and using a 176Lu/172Yb ratio of 0.5886 (Chuetal.,2002). device and a sun compass. The mean difference between the two Time-dependent drifts of Lu–Hf isotopic ratios were corrected using compass readings is ±1° in excellent agreement with the local geo- a linear interpolation (with time) according to the variations of magnetic field declination predicted from the 1995 International 91500. During the analysis, 176Hf/177Hf ratios of the 91500 standard Geomagnetic Reference Field (IGRF) for the region (IAGA Division V, zircon were 0.282302±0.000032 (n=30, 2σ), consistent with the 1995), indicating that local magnetic anomalies are moderate and recommended ratio (0.282307±0.000031, 2σ)(Wu et al., 2006). averaged out in the mean. Samples were trimmed into 2.2 cm

The εHf(t) values and TDM were calculated following Griffinetal. long cylinders for subsequent paleomagnetic analysis. (2000) using the 176Lu decay constant given in Blichert-Toft and All the experimental work was undertaken in a magnetically Albarède (1997). The decay constant of 176Lu applied in this paper is shielded room. The samples were subjected to progressive thermal 1.867×10−11 per year (Söderlund et al., 2004). The zircon Hf isotopic (mainly) and alternating field (AF) demagnetization and measured data are listed in Table S2. at each step of treatment by 2 G cryogenic magnetometer at the J. Dai et al. / Gondwana Research 22 (2012) 434–446 439

and site mean directions of all demagnetized data were derived by P01-19W (n=96) (94 usable) EHX0905 (n=84) giving unit weight to each mean sample direction. The analysis of (78 usable) 3000 tectonic motion employed the techniques of Coe et al. (1985). 2600 EHX09Y06 (n=84) (76 usable) U 3000 2200 2600 5. Results 238

1800 2200 2600 Pb/ 5.1. Zircon U–Pb and Lu–Hf isotopic results 206 1400 1800 2200

1000 1400 1800 Zircons from sample P01-19W (medium- to coarse-grained sand- 1000 1400 stone) are mainly rounded to subrounded and have a length ranging

1000 from ~50 μm to ~150 μm with an average of ~100 μm. Oscillatory zonings are observed in CL images. A total of ninety-six zircon crystals were analyzed and yielded ninety-four concordant U–Pb ages with the range of 151–2731 Ma (Table S1; Figs. 5 and 6). The young zircons 207 235 Pb/ U cluster at the range of 151–282 Ma (n=13), and most of them show fi Fig. 5. Compound U–Pb Concordia diagrams of samples collected from both the western oscillatory zoning, indicating their magmatic origins. Ninety- ve zir- and eastern Hoh Xil basin. U–Pb ages are in Ma and ellipses show 1σ errors. Number cons were also analyzed for Lu–Hf isotopic compositions. They dis- fi 176 177 of analyses and number of usable ages with suf cient concordance are noted. Analyses play various Hf/ Hf isotopic ratios and εHf(t) values ranging in gray are highly discordant, and are excluded in our discussion. from 12.4 to -24 (Table S2 and Fig. 7). Sample EHX0905 (medium- to coarse-grained sandstone) were paleomagnetic laboratory of the University of California, Santa Cruz collected from the Fenghuoshan Group in the EHXB (Figs. 1 and 3). (UCSC). A few representative samples were also selected for a set of Zircon crystals are mainly rounded to subrounded and have a length rock magnetic measurements to examine their mineralogical charac- ranging from ~40 μm to ~120 μm with an average of ~80 μm. teristics. These rock magnetic measurements included Curie tempera- Seventy-eight usable zircon U–Pb ages were obtained from eighty- ture determinations by measurement of low-field magnetic four analyses (Fig. 5). Their ages show a wide range from 120± susceptibility (using the Kappabridge susceptometer at the University 2 Ma to 2935±10 Ma. The youngest group is clustered at the range of California Santa Cruz), and hysteresis loops and the associated pa- from 120 ±2 Ma to 272 ±4 Ma (n= 18). Fifty-two zircon grains rameters using alternating gradient magnetometers (AGFM; Prince- were determined for Lu–Hf isotopic compositions. They also show 176 177 ton Measurements Corporation) at UCSC. various Hf/ Hf isotopic ratios and εHf(t) values ranging from Magnetization directions were determined by principal compo- 14.4 to -23.5 (Table S2 and Fig. 7). nent analysis (Kirschvink, 1980). The distributions of paleomagnetic Sample EHX09Y06 (medium-grained sandstone) were collected directions at each site were calculated using Fisher (1953) statistics, from Yaxicuo Group in the EHXB (Figs. 1 and 3). Zircon crystals are

Age of Qiangtang volcanic rocks

12

10 EHX09Y06 (Eastern HXB: YxG) 8 (n=76) 6 4 2 0 12 10 8 EHX0905 (Eastern HXB: FhG) 6 (n=78) 4 2 0 10 Number 8 P01-19W (Western HXB) 6 Relative Probability (n=94) 4

2

0 60 Qiangtang and Lhasa 50 (n=610) 40 30 20 10 0 0 500 1000 1500 2000 2500 3000 3500 Zircon U-Pb Age (Ma)

Fig. 6. U–Pb age probability plots for the Cenozoic sediments from the Hoh Xil Basin and the pre-Cenozoic sediments from the Qiangtang and Lhasa block (Kapp et al., 2007; Leier et al., 2007; Pullen et al., 2008). They display similar ranges of U–Pb ages, indicating the sediments in both western Hoh Xil basin (WHXB) and eastern Hoh Xil basin (EHXB) derived from the Central Tibetan Plateau. FhG: Fenghuoshan Group; YxG: Yaxicuo Group. 440 J. Dai et al. / Gondwana Research 22 (2012) 434–446

20 of the Fenghuoshan Group in the EHXB which is also interpreted to Depleted Mantle deposit in fluvial and alluvial fan (Fig. 3; Liu and Wang, 2001). The 10 conglomerate compositions of both basins imply that they might be derived from the Qiangtang block (Fig. 2). One andesite cobble (sam- 0 ple P01-31A) in the upper unit yielded a weighted mean U–Pb zircon Chondrite crystallization age of 191.8±1.2 Ma (Fig. 11 and Table S4). The age Hf(t) ε -10 fits the early Jurassic andesite in the Qiangtang block (Fig. 2; Pan et al., 2004). The paleocurrent measurements in both basins are mainly -20 EHX0905 directed to the north (Fig. 3; Liu and Wang, 2001). The similarities be- P01-19W tween lithologies and depositional environment in both basins imply that the Hoh Xil basin might be a single, wide basin during Paleogene. -30 0 500 1000 1500 2000 2500 3000 3500 The U–Pb and Lu–Hf isotopic composition of zircon grains can fin- Zircon U-Pb Ages (Ma) gerprint their source area, especially when distinctive U–Pb age sig- natures in source rocks have been well-defined by previous work – Fig. 7. Lu Hf isotopic compositions of sample P01-19W, western Hoh Xil basin (WHXB) (e.g. Elliot and Fanning, 2008; Veevers and Saeed, 2009; Wu et al., and EHX0905, eastern Hoh Xil basin (EHXB). 2010; Cai et al., 2011; Drost et al., 2011; Ren et al., 2011; Liu et al., 2011). The analyzed samples display similar ranges of U–Pb ages, with significant zircon peaks at 2600–2400 Ma, 2000–1800 Ma, – – – mainly rounded to subrounded and have a length ranging from 900 700 Ma, 600 400 Ma, and 300 150 Ma, respectively (Fig. 6; ~40 μm to ~100 μm with an average of ~60 μm. Seventy-six usable Table S1). The similar Hf isotopic compositions between sample zircon U–Pb ages were obtained from eighty-four analyses(Fig. 5). EHX0905 from the EHXB and sample P01-19W from the WHXB indi- Their ages show a wide range from 132±2 Ma to 2949±10 Ma. cate that they have the same origin (Fig. 7; Table S2). Compared with The youngest group is clustered at the range from 130±2 Ma to the published detrital zircon ages from the Qiangtang and Lhasa 298±4 Ma (n=7; Table S1 and Fig. 6). blocks (Kapp et al., 2007; Leier et al., 2007; Pullen et al., 2008), they All zircons from these samples are rounded to subrounded, prob- display quite similar characteristic features, indicating that they ably implying that they either underwent a long transport distance were all derived from the central Tibetan Plateau, in agreement from their source area or they were derived from a recycled orogenic with the paleocurrent and conglomerate composition analyses men- provenance. All samples display similar ranges of U–Pb ages, with sig- tioned above. Thus, these geological observations strongly suggest nificant zircon populations at 2600–2400 Ma, 2000–1800 Ma, that the WHXB was integrated with the EHXB during Paleogene, as 900–700 Ma, 600–400 Ma, and 300–150 Ma, respectively (Fig. 6). the northern boundary of the proto-Tibetan Plateau. The U–Pb and Hf data of sample EHX0905 all fall into the field of Additional support comes from comparative paleomagnetic stud- sample P01-19W, suggesting that they have the same origin ies. The paleomagnetic data from the WHXB (demagnetization be- (Fig. 7;TableS2). havior, magnetic mineralogy, paleomagnetic pole, declination, inclination, paleolatitude, and tectonic parameters) are very similar to those of the EHXB (Figs. 8–10 and Table 2 and Table S3). These pa- 5.2. Paleomagnetism results leomagnetic data are consistent with the contention that the strata in both the WHXB and EHXB are contemporaneous and record same an- We collected a total of 248 individual oriented paleomagnetic cient geomagnetic field when they were deposited. It is obvious from – samples from the same stratigraphic section (Figs. 8 10). We also the data in Fig. 10 and Table S3 that sections in both the WXHB and sampled a local fold to constrain the age of magnetization. Progres- EHXB show no relative rotations as demonstrated by the fact that sive thermal demagnetization to 700 °C revealed a high unblocking the observed declinations are not significantly different from those temperature component with both normal and reversed polarities expected to be inferred from the contemporaneous Eurasian poles. that are antiparallel (Fig. 8). The high temperature component is On the other hand, the Tertiary paleomagnetic pole positions of the interpreted as the characteristic remanent magnetization (ChRM) EHXB and WHXB imply a significant northward convergence with re- on the basis of linear trajectories of demagnetization towards the or- spect to Eurasia since Eocene times (50–40 Ma). The differences be- igin and a similar direction from sample to sample. The rock- tween the observed paleolatitudes from this study (and those from magnetic analyses on representative samples suggest that the ChRM EHXB) and those expected from paleomagnetic reference poles for is carried by haematite for most of our samples. As shown in Fig. 9A, Eurasia are much larger than geological estimates of crustal shorten- the hysteresis loop from sample 09X117B displays constricted loop ing (Table S3). Our results are consistent with the pattern of disturb- (wasp-waisted) behavior, which is typical for the presence of low- ingly low paleolatitudes derived from a large number of high-quality coercivity magnetite and high-coercivity haematite (Tauxe et al., paleomagnetic studies of the Tertiary rocks in central Asia. However, fi 1996). The fold test results are positive at 95% con dence level and the paleolatitude results from both the WHXB and EHXB are very con- no synfolding signature was detected, leading us to believe that the sistent with those derived from the 40 Ma paleomagnetic poles for ChRM is primary magnetization. The average site-mean direction be- Mongolia to the north (Table S3; Hankard et al., 2007) and the Qiang- α fore tilt correction is D=4.8°, I=10.5°, 95=19.1, k=8.2, n=9 tang block to the south (Lippert et al., 2011), demonstrating that the α sites; and D=4.0°, I=44.3°, 95=16.5, k=10.7, n=9 sites after existing Tertiary reference poles for Eurasia may be in error (Cogné et tilt correction (Fig. 10). Paleomagnetism results are listed in Table 2 al., 2010). Regardless, we emphasize the strike similarity exhibited by and Table S3. the paleomagnetic data from both the WHXB and EHXB, implying certain generic connection between them. 6. Discussion 6.2. Hoh Xil basin response to uplift and erosion of the proto-Tibetan 6.1. A single, wide Hoh Xil basin during Paleogene Plateau

The Cenozoic strata section in the Dead Yak Valley of the WHXB Just as mentioned above, crustal thickening which might cause the consists of purple-red cobble–pebble conglomerate, fine to coarse uplifting of the Tibetan Plateau (Murphy et al., 1997; Kapp et al., sandstone, and minor mudstone, closely resembling the middle part 2003, 2007) occurred prior to India–Asia collision. Based on the J. Dai et al. / Gondwana Research 22 (2012) 434–446 441 AB W, UP N, UP Sample 09X009A from WHXB Sample 09X170B from WHXB

450350300 625 610 550 225 575 N 660 400 600 550 640580 500 150°C 525 500 680 450 400 NRM E 350 2.49E-3 A/m 300

225

150

75°C 1.37E-2 A/m NRM CD N, UP N, UP Sample 09X004A from WHXB Sample 09X172C from WHXB

NRM NRM 225 150°C 300 350 150°C 400 500 550 450 350 600 450 225 575525 300 E 580 400 E 625 1.19E-2 A/m 660 610 500 1.16E-3 A/m 680 550 640 670

EF W, UP W, UP Sample 98x2301A from EHXB Sample 98x4002A from EHXB 600 550 650 700 400 625 670 680 N 3.71 E-3 A/m 200°C 680 670 690 660 650 700 400 N 5.13 E-3 A/m 200°C NRM

NRM

Fig. 8. Vector end-point diagrams of thermal demagnetization for representative samples from the WHXB (A–D) and EHXB (E, F). Directions are plotted in geographic coordinates. Samples 09X009A (A) and 98X2301A (E) show normal polarity of magnetization, and samples 09X170B (B), 09X004A (C), 09X172C (D), and 98x4002A (F) show reversed polarity of magnetization. Squares and triangles represent the projection of the magnetization vector end points on the horizontal and vertical planes, respectively. NRM: natural remanent magnetization. Straight lines indicate ChRM component using principal component analysis (Kirschvink, 1980). Temperature intervals for defining the ChRM component general range from 400 to 680 °C. WHXB: Western Hoh Xil Basin; EHXB: Eastern Hoh Xil Basin.

study in central Lhasa, Murphy et al. (1997) reveal the north–south are consistent with sedimentary records. DeCelles et al. (2007a) re- crust shortening of the Lhasa block occurred during Early Cretaceous port a major hiatus in deposition from Late Cretaceous to early Tertia- and the Lhasa obtained 3–4 km elevation prior to the India–Asia col- ry time in the Nima basin, and they conclude that this event should be lision. Kapp et al. (2007) deduce that the Nima basin experienced associated with regional uplift in the southern and central Lhasa block crust shortening predating India–Asia collision. These observations (Fig. 1B). The growth strata in Nangqian–Yushu basin of east-central 442 J. Dai et al. / Gondwana Research 22 (2012) 434–446

A Sample 09X117B from WHXB Table 2 Paleomagnetic results from the Dead Yak Valley section, Western Hoh Xil Basin. 3E-008

Site N G-Dec G-Inc α95 kg S-Dec S-Inc α 95 ks A 15 6 7.8 27.8 2.9 3.4 37.4 27.8 2.9 2E-008 B 4 17.7 19.6 21.5 3.2 4.8 49.4 21.5 3.2 C 11 194 −6.3 24.4 4.5 200.7 −47.5 24.4 4.5 D 7 204.2 −23.5 26.3 6.2 222.8 −62 26.3 6.2 1E-008 E 13 19 9.1 22.3 4.4 23.9 40.8 22.3 4.4 F 12 341.9 5.1 21.8 4.9 330.8 37.3 21.8 4.9 G1619−2.1 23.3 3.5 31 43.6 23.3 3.5 N. Limb 8 188.6 7.1 11.1 26 190.9 −38.3 11.1 26 0 S. Limb 6 114.2 −23.7 7.5 80.7 135.7 −11.9 7.5 80.7 Overall 9 4.8 10.5 19.1 8.2 4 44.3 16.5 10.7

Explanation: N, number of samples. G-Dec/G-Inc, S-Dec/S-Inc, declination and inclination Magnetic Moment -1E-008 in geographic and stratigraphic coordinates, respectively. α95, radius of the circle of 95 percent confidence concerning the direction in degrees. kg/ks, Fisher (1953) precision parameter for direction before and after tilt correction. -2E-008

-3E-008 -1 -0.5 0 0.5 1 that the Lhasa and Qiangtang blocks had obtained high elevation dur- Applied Field (T) ing Cretaceous and Paleogene. However, all of these evidences are in- direct proxies for the paleoelevation of Tibetan Plateau. B Sample SQ1010 from EHXB Lines of evidence from quantitative paleoaltimetry show that the 20E-005 proto-Tibetan Plateau has formed during Paleogene (Fig.1B). The stable isotope-based paleoelevation studies on the ancient paleosol 15E-005 carbonate in the Nima basin, central Tibetan Plateau, show that central Tibetan Plateau gained its current elevation as early as 26 Ma 10E-005 (DeCelles et al., 2007b). The paleoaltimetry of late Eocene deposits of the Lunpola basin indicates that the surface of this region has been at 5E-005 an elevation of more than 4 km for at least 35 Ma ago (Rowley and Currie, 2006), consistent with the n-alkanes δD-based paleoaltimetry 0 estimates (Polissar et al., 2009). These interpretations are supported by other evidences. The geochronology and geochemistry studies on the Duogecuoren adakitic rocks from the central-western Qiangtang -5E-005

Magnetic Moment block indicate that the uplift of central Tibetan Plateau initiated as early as 45–38 Ma because these rocks were believed to be derived -10E-005 from the melting of eclogitic crust which was thickened by the south- ward Songpan–Ganzi continental subduction (Fig. 1; Wang et al., -15E-005 2008b). Moreover, Wang et al. (2010) argue that the onset of east– west extension (i.e. the regional uplift of central Tibetan Plateau) -20E-005 – – -1 -0.5 0 0.5 1 began at 47 38 Ma based on the north south trending dikes in cen- Applied Field (T) tral Tibetan Plateau (Fig. 1). Hoh Xil basin was located at the northern edge of the high- Fig. 9. Diagrams of room temperature hysteresis loops for representative rock samples elevation proto-Tibetan Plateau. Therefore, it could record uplift and in this study. A, Sample 09X117B from the WHXB; B, sample Sq1010 from the EHXB. exhumation of proto-Tibetan Plateau. The sedimentology, detrital zir- Horizontal axis is applied field up to 1 T. Both plots display constricted loop (wasp- – waisted) behavior, which is typical for the presence of low-coercivity magnetite and con, Lu Hf isotopic, and paleomagnetic signatures all show that the high-coercivity haematite (Tauxe et al., 1996). Data are shown corrected for the sample EHXB and WHXB were integrated as a single, wide basin, implying probe only. WHXB: Western Hoh Xil Basin; EHXB: Eastern Hoh Xil Basin. that uplift and erosion of proto-Tibetan Plateau took place when these basins received their deposition. The deposits of conglomerates Tibetan Plateau indicate sedimentation synchronous with fold-thrust represent the rapid acceleration in sedimentation rates at ~40 Ma ago deformation, which show the crustal shortening occurred during Pa- in both the EHXB and WHXB, which are synchronous with the rapid leocene–Eocene (Horton et al., 2002). All these observations show uplift of proto-Tibetan Plateau and rapid activity on basin-bounding

AB N WHXB Mean N EHXB Mean D=4.8 D=319.8 I=10.5 D=4.0 D=10.1 I=73.6 k=8.2 I=44.3 I=39.8 k=27.6 n=9 k=10.7 k=70.8 n=9 n=3 W n=3 E W E

Fig. 10. Equal-area plot of site-mean direction of the ChRM component for rocks from both the EHXB and WHXB. The data also pass the regional fold test at more than 99% confidence, as indicated by the wide separation of means before tectonic correction (A) and close overlap after the tectonic correction (B). n, number of studied areas; D, declination; I, inclination, k, Fisher (1953) precision parameter for the mean direction. WHXB: Western Hoh Xil Basin; EHXB: Eastern Hoh Xil Basin. J. Dai et al. / Gondwana Research 22 (2012) 434–446 443

210 thrusts such as the Tanggula thrust system (Li et al., 2006b; Wang et Weighted Mean=191.8±1.2 Ma al., 2008a). 206 MSWD=0.91 n=18 Although magnetostratigraphic investigations of the Fenghuoshan Group in the EHXB has been carried out (Liu et al., 2003), its age is 202 still controversial. Some Early Cretaceous fossils have been founded 198 from this group (e.g. Zhang and Zheng, 1994). If so, deposition of the Fenghuoshan group in the whole Hoh Xil basin should be re- 194 sponse to the exhumation of the Qiangtang block as result of the col- lision between the Lhasa and Qiangtang blocks during Early Ages (Ma) 190 Cretaceous (Kidd et al., 1988; Kapp et al., 2007; Leier et al., 2007). And this model predicts that the uplift of Hoh Xil basin occurred be- 186 fore Paleogene because of the sporadic outcrop of Paleogene strata. However, we prefer to interpret that the Fenghuoshan group is Paleo- 182 cene in age for the following reasons: 1) Both magnetostratigraphic P01-31A results from the Cenozoic sediments of the EHXB and WHXB show 178 various reversed and normal polarity ChRM components, which are different from the long normal polarity zone (chron C34, between Fig. 11. Zircon U–Pb weighted average ages from one andesite cobble (sample P01-31A) in the upper unit of Dead Yak section. The age is consistent with early Jurassic crystallization 118 and 83 Ma) during the Cretaceous (Cande and Kent, 1995); 2) age of the andesite in the Qiangtang block. MSWD: mean square of weighted deviates. Chareae and ostracoda fossils found in both the Fenghuoshan Group and the Cenozoic strata of western Hoh Xil basin have been assigned ages from Middle to Late Eocene (e.g. Liu et al., 2003; Yue et al., 2006).

W A Eocene (50-40 Ma)

N

proto-Tibetan Plateau

Himalayan foreland basin Hoh Xil basin

ZGT

TTS

Lhasa+Qiangtang India Hoh Xil-Songpanganzi

B proto-Tibetan Plateau Adakitic rocks S Lunpola basin N 5 5

4 4 3 GST SGAT 3 2 Himalayan foreland 2 Hoh Xil basin 1 basin 1 LZZG 0 0 -1 ZGT TTS -1 -2 -2

Fig. 12. A. Schematic paleogeographic reconstruction of the Tibetan Plateau during Eocene showing the topographic characteristics. B. Schematic paleogeographic cross section of the and Tibetan Plateau during Eocene (modified from Wang et al., 2008a). The estimated paleoelevations of proto-Tibetan Plateau, Hoh Xil basin and Himalayan foreland basin are from Cyr et al., 2005; Polissar et al., 2009; Rowley and Currie, 2006; Wan et al., 2001; Wang et al., 2002, 2011; Yan et al., 2005. The high elevation and vast area extent of proto-Tibetan Plateau not only perturbs the atmosphere circulation, but also accelerates the mechanical erosion rate. This process increases the surface area of

Ca- and Mg-rich silicate rocks for chemical weathering, consuming the atmospheric CO2, and probably responsible for the global cooling since early Cenozoic. TTS: Tanggula Shan Thrust Systems. GST: Galze-Siling Tso Thrust; SGAT: Shiquanhe-Gaize-Amdo Thrust; ZGT: Zhongba-Gyangze Thrust; LZZG: Linzizong Group volcanic rocks. 444 J. Dai et al. / Gondwana Research 22 (2012) 434–446

6.3. Possible implication for early Cenozoic global cooling Paleogene. This interpretation is sustained by the similar paleomag- netic results of both western and eastern Hoh Xil basin. The northern and southern boundaries of the proto-Tibetan The Hoh Xil basin is among the best exposed records of Paleogene Plateau is believed to be the Tanggula Shan and Gangdese, respec- influx from the proto-Tibetan Plateau. The period of the Hoh Xil basin tively (Wang et al., 2008a). The Hoh Xil basin and southern Tibet deposition was coeval with significant period of early Cenozoic uplift foreland basin were located at the northern and southern edges of and erosion of the proto-Tibetan Plateau. About 500-km width of the the proto-Tibetan Plateau (Fig. 12). The paleoelevation estimates Hoh Xil basin implies that the proto-Tibetan Plateau was vast in areal using the oxygen isotopic data from the Fenghuoshan Group carbon- extent. The uplift and erosion of the high and intense proto-Tibetan ates indicate that the hypsometric mean elevation of the Hoh Xil Plateau has probably contributed to the early Cenozoic cooling. Clear- basin was ≤2 km after careful sedimentologic, petrologic, and geo- ly, further work needs to be done on the erosion rates and environ- chemical analyses (Cyr et al., 2005). However, these results are mental indicators to investigate linkages among the extent of the 18 most likely underestimated because the modern δ Omw versus ele- proto-Tibetan Plateau, the amount of erosion along its flanks, and vation relationships used to reconstruct the paleoelevation was the degree to which this influence early Cenozoic global cooling. established in the distant Himalaya, instead of the northern Tibetan Supplementary data (Tables S1–S4) associated with this article Plateau (DeCelles et al., 2007b; Molnar et al., 2010; Quade et al., can be found, in the online version, at doi:10.1016/j.gr.2011.08.019. 2011). The occurrence of Paratethys at southwestern Tarim basin until early Miocene (Ritts et al., 2008), which might provide vapor Acknowledgements sources for northern Tibet similar with modern southern Tibetan Plateau. Thus, the δ18O values during Eocene in the Hoh Xil mw We appreciate two anonymous reviewers for their thorough and basin might be similar with those of modern southern Tibetan Pla- constructive comments. Editor-in-Chief Prof. M. Santosh and Guest teau, rather than the northern Tibetan Plateau. If so, we can confi- Editor Wenjiao Xiao are acknowledged for their editorial handlings. dentially conclude that the paleoelevation of Hoh Xil basin was We thank Licheng Wang, Wenguang Yang, Jun Meng, Hui Huang, below 2 km during Eocene to Oligocene. The paleoelevation of the Jinglong Wu and five drivers for their assistance with the fieldwork. Wudaoliang Group was about 4 km from the n-alkanes δD-based This study was supported by the Fundamental Research Funds for paleoaltimetry estimates (Polissar et al., 2009). These observations the central Universities (Project 2009PY03), National Science Founda- showthatHohXilbasinobtaineditscurrentelevationintheearly tion of China Grant 40974035, and U.S. National Science Foundation Miocene, consistent with the occurrence of vast early Miocene lake Grant EAR-0911331. This article is contribution no. 508 of the Paleo- in this area (Wu et al., 2008). The Himalayan foreland basin was magnetism Laboratory and Center for the Study of Imaging and Dy- still in deepwater setting according to the sedmentological and bio- namics of the Earth (Institute of Geophysics and Planetary Physics, stratigraphic studies (Wan et al., 2001; Wang et al., 2002; Yan et al., University of California, Santa Cruz). 2005; Wang et al., 2011). More than 500 km width of the Hoh Xil basin strongly indicates that the proto-Tibetan Plateau was vast in areal extent. The proto- References Tibetan Plateau also had high elevation (>4 km) (e.g. Rowley and Currie, 2006). 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