Pliocene Uplift of the Northern Tibetan Plateau

Downloaded from geology.gsapubs.org on June 14, 2015 Pliocene uplift of the northern Tibetan Plateau Hongbo Zheng State Key Laboratory of Loess and Quaternary Geology, Chinese Academy of Sciences, Xian 710054, China, and Tectonics Special Research Centre, Department of Geology and Geophysics, University of Western Australia, Nedlands, Western Australia 6907, Australia Christopher McAulay Powell Tectonics Special Research Centre, Department of Geology and Geophysics, University of Western Australia, Nedlands, Western Australia 6907, Australia Zhisheng An State Key Laboratory of Loess and Quaternary Geology, Chinese Academy of Sciences, Xian 710054, China Jie Zhou Guangrong Dong Institute of Desert Research, Chinese Academy of Sciences, Lanzhou 730000, China ABSTRACT INTRODUCTION Neogene redbeds passing upward into upward-coarsening conglomerate and debris-flow Cenozoic uplift of the Tibetan Plateau has been deposits at the foot of the Kunlun Mountains record the change in paleoslope related to uplift of hypothesized to have exerted a profound effect the surface of the northern Tibetan Plateau. Detailed magnetostratigraphy of a 4.5 km section upon regional and global climate, and could have near Yecheng in the western Kunlun Mountains shows that the change from deposition on contributed to the inception of the Indian monsoon distal alluvial plains to proximal alluvial fans occurred during the Gilbert reversed chron and Northern Hemisphere glaciation (e.g., Ruddi- (4.5–3.5 Ma). The change in depositional facies was accompanied by an increase in sedimenta- man and Raymo, 1988; Raymo and Ruddiman, tion rate from an average ~0.15 mm/yr between the earliest Oligocene and the earliest Pliocene 1992; Quade et al., 1989; Harrison et al., 1993; to 1.4 mm/yr in the Gauss normal chron (3.6–2.6 Ma). We interpret the change in depositional Molnar et al., 1993). However, the mechanism and facies and increase in sedimentation rate as indicating that the main uplift of the northwestern timing of uplift of the Tibetan Plateau have been Tibetan Plateau began ca. 4.5 Ma. controversial for almost three decades (e.g., Powell and Conaghan, 1973; Dewey and Burke, 1973; Keywords: Tibet, Kunlun, magnetostratigraphy, Pliocene, uplift. Zhao and Morgan, 1985; Dewey et al., 1986; Field- ing, 1996; Meyer et al., 1998). One view is that Tibet was uplifted mostly during the Oligocene and Miocene (Tapponnier et al., 1986; Copeland et al., 1987), and that the plateau reached its maximum height by 8 Ma, and was lowered thereafter by extensional faulting (Molnar et al., 1993; Harrison et al., 1992). An alternative view is that most of the uplift occurred in the late Pliocene and Pleistocene (Hsü, 1976). Evidence cited to support the young uplift model includes the presence of Hipparion fossils now preserved at elevations of 4.5 km (Ji et al., 1980), geomorphology (Zhang et al., 1981), neotectonics (Li et al., 1981), and lacustrine and fluvial deposition (Li, 1995) in and around the plateau. In this paper we present sedimentological evidence that bears on timing of the surface uplift of the northwestern Tibetan Plateau (now ≥5 km) relative to the adjacent Tarim basin (now 1–1.5 km). The results bring new evidence to bear on the debate as to whether this part of the Tibetan Plateau rose to its current 5 km elevation by the mid- Miocene (Meyer et al., 1998), or whether most of the surface uplift relative to the Tarim basin was Pliocene-Pleistocene (e.g., Li, 1996). GEOLOGICAL SETTING Three broad tectonic domains can be distin- guished from the surface geology of the southern Tarim basin and Tibet (Molnar et al., 1987; Chang et al., 1986; Chang, 1994). The northern domain, comprising the Tarim basin and Kunlun Moun- tains, represents the southern margin of Paleozoic Asia and is separated from the Paleo-Tethyan Figure 1. Geological map of western Kunlun and adjacent areas (after Chang, 1994). KBT— domain by the Mazar-Kangxiwa fault (Fig. 1). The Kunlun boundary fault; KS—Kunlun suture; JRS—Jinsa River suture; MKF—Mazar- northern domain contains continental crust that is Kangxiwa fault; LLS—Longmucuo–Lancang River suture; SGT—Songpan-Ganzi terrane; NQT—Northern Qiangtang terrane; SQT—Southern Qiangtang terrane. 1—Southern up- 1500 m.y. old or older and includes remnants of lifted belt of Tarim basin; 2—middle Kunlun oceanic belt; 3—central Kunlun belt. Dark bar former late Proterozoic to early Paleozoic oceanic indicates line of section in Figure 2. crust (Chang, 1994; Hsü et al., 1995). The central Geology; August 2000; v. 28; no. 8; p. 715–718; 4 figures. 715 Downloaded from geology.gsapubs.org on June 14, 2015 domain comprises deposits of the Paleo-Tethys and final early Oligocene westward regression of MEASURED SECTION AT YECHENG includes the Songpan-Ganze and northern Qiang- the Peri-Tethys sea (Li et al., 1996; Sobel and The Yecheng section (Figs. 2 and 3) at the foot tang terranes (Fig. 1). The southern domain com- Dumitru, 1997). Early Neogene sediments are of the western Kunlun Mountains contains more prises the central and southern Pamir, the Karako- characterized by red mudstone, siltstone, sand- than 4.5 km of late Neogene sediment. The lower ram, and the southern Qiangtang and Lhasa terranes, stone, and evaporitic deposits (redbeds), deposited part of the section is alternating red mudstone and includes continental fragments that were sepa- in terrestrial playa and distal alluvial fan to flood- and fine-grained pale yellow sandstone. The rated from Gondwana in the Paleozoic and early plain environments. The fine grain size indicates a sandstone was deposited in shallow streams and Mesozoic (Chang, 1994; Hsü et al., 1995). relatively low paleodrainage gradient. Our recon- as sheetflood deposits; the intercalated red mud- The Tarim basin is a vast flat-lying basin naissance paleocurrent measurements in the early stone represents overbank deposits. The inter- bounded by the Tien Shan along its northern and Neogene redbed succession indicate a paleoslope preted depositional environment is distal alluvial northwestern flanks and by the Kunlun Mountains toward the west. Coarse clastic sediments in the fan and/or flood plain, and our reconnaissance along its southern margin. The surface of the southern Tarim basin form a 2–8-km-thick clastic paleocurrent information suggests that distal Tarim basin is between 1000 and 1500 m, and is wedge along the northern flank of the western alluvial fans sloped northward, and shallow trunk covered by Quaternary deposits of the Taklimakan Kunlun. These sediments, shed off the area of the streams drained westward. On the basis of re- Desert. The Tarim basin has remained relatively Tibetan Plateau, preserve a continuous record of gional stratigraphic correlation, the lower part of undeformed during the Cenozoic convergence of the Tertiary tectonic and paleoenvironmental evo- the Yecheng section is equivalent to the Miocene India with Asia. Stratigraphic studies of the sub- lution of the region, from which we can infer the Wuqia Group. surface geology suggest that the area became a relative change in paleoslope between the northern The middle part of the section (Fig. 3) consists unified subsiding basin in the Miocene after the Tibetan Plateau and adjacent Tarim basin. of medium-grained sandstone and sandy mudstone with thin layers of fine gravel, known as the Artux Formation in the southern Tarim basin. The gravel beds are poorly sorted; there is little evidence of reworking by running water. The fine- grained sandstones are well sorted and internally structureless; they could be eolian deposits. The uppermost part of the section is domi- nated by pebble to boulder conglomerate typical of alluvial fan debris-flow deposits. The conglomerate is very poorly sorted; clasts range from granules to boulders ≥2 m in diameter. There is some channeling at the base of individual layers, and reverse grading is common. There is no evidence of fluvial reworking of the conglomerate. The debris flows are intercalated with lenses of pale gray, fine-grained to very fine grained sandstone, which we interpret as coeval eolian deposits. This conglomerate, the Xiyu Formation, is widely distributed along the margins of the Tarim basin, and has been assigned either Pliocene (Zhou and Chen, 1990) or early Pleistocene (Li, Figure 2. Schematic geological section from Yecheng to Maza (from Chang, 1994). Box 1984) ages. The Xiyu Formation overlies the shows location of Yecheng section. Artux Formation conformably in the studied section, but unconformably in other places (Sobel and Dumitru, 1997). Paleomagnetic Sampling We collected ~2500 paleomagnetic samples through the Yecheng section with a sampling interval of about 1 m in the Artux Formation. For the Xiyu Formation, sampling was possible only from the fine sandstone bands. All samples were thermally demagnetized progressively and their magnetizations were measured using a 2G- cryogenic magnetometer at the University of Western Australia. The observed magnetostratigraphy can be cor- related with Cande and Kent’s (1995) polarity time scale (Fig. 4). The Artux Formation is within the Gilbert reversed chron, and ranges from 4.6 to 3.5 Ma; the Xiyu Formation has an age between 3.5 and 1.8 Ma. The ~1 km of redbeds below the Figure 3. Measured geological section through upper Neogene at Yecheng. Dips are consistent Artux Formation can be dated paleomagnetically from Miocene into base of Artux Formation, but shallow upward thereafter. to about 8 Ma. The Yecheng section is incomplete 716 GEOLOGY,August 2000 Downloaded from geology.gsapubs.org on June 14, 2015 at its base, but elsewhere (e.g., along the Yarkand River) there is a 4.5-km-thick redbed section that provides a near-continuous record of fine-grained deposition back to the Eocene-Oligocene shallow- water limestone of the Bashibulake Formation (Sobel and Dumitru, 1997). Structure and Sedimentology Tectonic movement during and after the deposition of the Xiyu Formation has tilted the entire Miocene to early Pleistocene section, resulting in all the strata in the Yecheng section dipping northward toward the Tarim basin.

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