Mesozoic-Paleogene Sedimentary Facies And

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GEOLOGICAL JOURNAL Geol. J. 37: 217–246 (2002) Published online 15 July 2002 in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/gj.911

Mesozoic–Paleogene sedimentary facies and paleogeography of Tibet, western China: tectonic implications

KAI-JUN ZHANG1*, BANGDONG XIA1and XIWEN LIANG2 1Department of Earth Sciences, Nanjing University, Nanjing 210093, People’s Republic of China 2Institute of Petroleum Geology, Jianghan Petroleum Company, Jianghan, Hubei 433124, People’s Republic of China

In Early–Middle Triassic time, an abyssal sea covered most of the Songpan–Ganzi area, whereas a Central Tibetan Landmass, up to 400 km wide, may have stretched across the Lhasa and Western Qiangtang terrains. In Late Triassic time, the Songpan– Ganzi sea closed, the Central Tibetan Landmass receded westwards away from southern Western Qiangtang, a littoral environment dominated Eastern Qiangtang, middle Western Qiangtang, and southeastern Lhasa, a shelf environment existed only in northern and southeastern Western Qiangtang and northwestern Eastern Qiangtang, and abyssal flysch was spread along the eastern Bangonghu–Nu¨jiang zone. In Early–Middle Jurassic time, Songpan–Ganzi had become part of the Eurasian continent, abyssal flysch sediments stretched throughout the Bangonghu–Nu¨jiang zone, the Central Tibetan Landmass was only locally present in southwestern Lhasa, and the Tethyan epicontinental sea nearly covered all Tibet southwest of the Jinsajiang suture. In Late Jurassic time, oceanic flysch deposition existed only along the westernmost Bangonghu–Nu¨jiang zone, nearly all of Tibet was covered by coastal deposits, and shelf deposits existed only in northern Western Qiangtang and westernmost Lhasa. In the early stage of Early Cretaceous time, the majority of Qiangtang had become dry land, and a supralittoral environment dominated across the entire Lhasa terrain. However, during the late stage of the Early Cretaceous time, platform–shelf carbonates prevailed on southern Western Qiangtang and northern Lhasa. In Late Cretaceous time, the majority of Qiangtang had become emergent land, and a supratidal environment dominated Lhasa, the western rim of Western Qiangtang, and Tarim. In Paleogene time, the majority of Tibet became emergent land, and a supratidal environment existed only on the southern and western rims. The dominance of Upper Triassic–Jurassic shelf carbonates on the northwestern Eastern Qiangtang corner and the northern Western Qiangtang rim suggests a diachronous closing of the Jinsajiang paleo-Tethys ocean, first during latest Triassic time when the Eastern Qiangtang terrain collided with Asia and finally in Jurassic time when the Western Qiangtang terrain was amalgamated to Asia. Rich picotites in Upper Triassic sandstones of middle Qiangtang suggest that the Shuanghu suture could have extended along the middle of Qiangtang, and stable shelf sedimentation during Late Triassic–Middle Jurassic time in the Western Qiangtang terrain shows that the suture probably could not have formed until Middle Jurassic time. The opening time of the Bangonghu– Nu¨jiang mid-Tethys ocean could be Late Triassic time due to the existence of the Central Tibetan Landmass across Western Qiangtang and Lhasa during Early–Middle Triassic time. However, its opening was diachronous, at Late Triassic time in the east and at Early–Middle Jurassic time in the west. Furthermore, its closing was also diachronous, first in the east at the beginning of Late Jurassic time and later in the west in latest Jurassic to earliest Cretaceous time. Widespread upper Lower Cretaceous limestone up to 5 km thick over the northern half of Lhasa indicates that southern Tibet could have undergone an extensive backarc subsidence during late Early Cretaceous time. Continuous shallow marine sedimentation through the entire Cretaceous time over much of southern Tibet indicates that southern Tibet was intensely elevated only after the end of Paleogene time, its high topography being the product of the Indo-Asian collision. Copyright # 2002 John Wiley & Sons, Ltd.

Received 20 March 2001; revised version received 6 November 2001; accepted 16 November 2001

KEY WORDS Tibet; Tethys; Mesozoic; Paleogene; sedimentary facies; paleogeography; tectonic reconstruction; Jinsajiang suture; Bangonghu–Nu¨jiang suture; Shuanghu suture

* Correspondence to: Dr K.-J. Zhang, Department of Earth Sciences, Nanjing University, Nanjing 210093, People’s Republic of China. E-mail: [email protected]

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218 k.-j. zhang, b. xia and x. liang

Figure 1. Sketch tectonic map of eastern Asia, revised after Sengo¨r (1990) and Zhang (1997, 2000, in press). The dotted line indicates the outline of the Tibetan plateau. Terrains: EQT, Eastern Qiangtang terrain; WQT, Western Qiangtang terrain. Main sutures or faults: A, Altyan fault zone; B, Bangonghu–Nu¨jiang suture; J, Jinsajiang suture; K, Kunlun suture; L, Longmenshan fault zone; Qi, Qilian suture; Qn, Qinling suture; S, Shuanghu suture; T, Tanlu fault zone; Ts, Tianshan suture; Y, Yarlung Zangbo suture; Ys, Yinshan suture. The eastern extension of the Shuanghu suture (S) to 89E is highly speculative.

1. INTRODUCTION

Tibet in western China has long been interpreted as a locus of continental collision and accretion since early Mesozoic time (Figure 1; e.g. Allegre et al. 1984; Wang and Sun 1985; Chang et al. 1986; Liu et al. 1990, 1992; Burchfiel and Royden 1991). However, reactivation of intraplate orogens during and following interplate collisions has profoundly complicated original structures and even obliterated some tectonic units marking sutures between formerly distinct tectonic terrains. This has severely restricted our understanding of the tectonic history of this largest plateau on Earth and has led to many controversies, such as the boundaries of accreted terrains, their subduction polarity, and the origin and timing of the development of the Tibetan plateau. An alternative source of data lies in the sedimentary sequences preserved in the several terrains, and within the sutures between these terrains, that underlie Tibet. These deposits hold the singular advantage that they contain a vertical stacked and relatively undeformed record of erosion and sedimentation and, therefore, obviate many of the difficulties inherent in unraveling complex structural overprints. They often provide a more continuous and precise record than data available from deformed belts, and reveal some of the principal attributes of the geology (Carroll et al. 1995). Therefore, their study forms a vital approach to understanding the tectonic evolution of Tibet (Zhang 1997, 1999, 2000; Zhang et al. 1998). Since the pioneering paleogeographic studies by Wang and Sun (1985), much sedimentary and stratigraphic data have accumulated. In particular, during the summers of 1993–98, hundreds of Chinese workers conducted field mapping on the Tibetan plateau, approximately at a scale of 1:200 000, sponsored by the China Petroleum Corporation. This work has brought us voluminous useful sedimentary and stratigraphic information. Therefore,

paleogeographic-tectonic evolution of tibet 219

it is possible to refine significantly the paleogeographic maps of Tibet. The present study focuses on the Chinese areas of the Tibetan plateau, north of the Yarlung Zangbo suture, and provides new field data on Mesozoic outcrop exposures of sedimentary rocks of the southern Tarim, Songpan–Ganzi, Qiangtang, and Lhasa terrains (Figure 2A– G). The sedimentary data from the Qiangtang and northern Lhasa terrains are mainly based on our own investigations during 1993–2001. In order to provide an overview of the Mesozoic and Paleogene sedimentary facies and paleogeography of the Tibetan plateau, we also incorporate voluminous sedimentary data from several other terrains by Chinese investigations during the last few years. Although some data cited here are not unequivocal in the determination of facies, they are helpful on the delineation of the overall paleogeography of Tibet. Because of the poorly documented stratigraphy on the plateau with extremely difficult topography, we have paid great attention to the biostratigraphic ages of the sedimentary rocks and incorporated radiometric data for rocks with volcanic inter- beds, which constitutes a sound foundation for the facies and paleogeographic studies. In the localities where marine Mesozoic strata are well exposed, detailed studies were conducted, and these are listed below. Long-range extrapolation is necessary to believe the facies reconstruction and the consequent tectonic reconstruction. The pet- rographic studies are based on field observations and more than 500 standard thin-section samples. Mineralogical

Figure 2. Sketch map of Mesozoic and Paleogene facies distribution and paleogeography in Tibet, China. T, Tarim. Terrain Sutures: BNS, Bangonghu–Nu¨jiang suture; JSS, Jinsajiang suture; SHS, Shuanghu suture; YZS, Yarlung Zangbo suture. The eastern extension of the Shuanghu suture (SHS) beyond 89E is highly speculative. The area south to the Yarlung Zangbo suture is not included in this study. The arrows show the sediment transport directions.

220 k.-j. zhang, b. xia and x. liang

Figure 2. Continued

paleogeographic-tectonic evolution of tibet 221

Figure 2. Continued

composition was determined by X-ray diffraction (XRD), carried out at the X-ray laboratory, Nanjing University. Although preliminary in nature, the data presented here place signiﬁcant constraints on evaluating and evolving alternative hypotheses for the evolution of Tibet and provide a starting point for more comprehensive future studies.

2. TECTONIC FRAMEWORK OF TIBET

The Tibetan plateau is underlain by a major part of the Tethyan orogenic collage (Sengo¨r 1990). It consists of four or perhaps more terrains, separated by three main Mesozoic suture zones (Chang et al. 1986; Dewey et al. 1988;

222 k.-j. zhang, b. xia and x. liang

Sengo¨r 1990; Liu et al. 1990, 1992) (Figure 1). The Yarlung Zangbo suture marks the boundary between the Indian subcontinent to the south and the Lhasa terrain to the north (Figure 1). It is generally believed that the Lhasa terrain was separated from the Gondwanan supercontinent around the boundary between Triassic and Jurassic time (e.g. Allegre et al. 1984; Searle et al. 1987; Dewey et al. 1988). During Jurassic and Cretaceous time, a relatively wide passive continental margin existed along the northern rim of the Indian Plate (Liu and Einsele 1994). However, Carboniferous calc-alkaline volcanic rocks near Lhasa (Pearce and Mei 1988; XZBGM 1993) indicate that the neo-Tethys ocean could have existed possibly much earlier than is generally believed. During latest Cretaceous and earliest Tertiary time, the Indian subcontinent collided with an amalgamated Eurasian collage. After the collision, northward indentation of India since about 40 Ma caused about 2000 km of crustal shortening, giving rise to the largest plateau on Earth (e.g. Allegre et al. 1984; Chang et al. 1986; Dewey et al. 1988; Burchfiel and Royden 1991; Liu and Einsele 1994). The Lhasa terrain, bounded to the north by the Bangonghu–Nu¨jiang suture and the Qiangtang terrain (Figure 1), belonged to the Gondwanan supercontinent during Paleozoic time (Liu and Einsele 1994; Zhang 1998, 2000), as evidenced by voluminous late Paleozoic tillites and glacio-marine faunas and Glossopteris floras (CIGMR–SCGR 1992; Zhang 1998). The mid-Tethys oceanic branch between the Lhasa and Qiangtang terrains was open by about Late Triassic time (Allegre et al. 1984; Dewey et al. 1988) and was closed along the Bangonghu–Nu¨jiang suture during Late Jurassic time (Girardeau et al. 1984; Chang et al. 1986). However, the evidence for subduction polarity is ambiguous (cf. Dewey et al. 1988; Yu and Wang 1990; Liu et al. 1990, 1992). The Qiangtang terrain, which is divided by the Shuanghu suture into an eastern and a western terrain (Figures 1 and 2) (e.g. Li et al. 1995; Zhang 2001a, and references cited therein), could be a composite continent (e.g. Sengo¨r 1990). The Western Qiangtang terrain has long been documented by rich late Paleozoic tillites and glacio-marine faunas (e.g. Li et al. 1995; Zhang 2001a, and references cited therein), whereas the Eastern Qiang- tang terrain is believed to be covered by late Paleozoic warm-water sediments and faunas and floras (e.g. Yin et al. 1988; Zhang 2001a, and references cited therein). Therefore, the Shuanghu suture zone could define the divide between the Gondwanan and Cathaysian realms. In the central Qiangtang area (from about 84 to 89E), the Shuanghu suture is well exposed and is clearly marked by ophiolitic me´lange and high-pressure blueschists (Li et al. 1995). However, in the other areas of the Qiangtang terrain, the suture is covered by Mesozoic sediments, and so its location is highly speculative. The timing of the closing of the Shuanghu suture is possibly Mesozoic, but is subject to debate (Li et al. 1995). The Eastern and Western Qiangtang terrains are separated from the Songpan–Ganzi complex to the north by the Jinsajiang suture and the Jiangda arc (Figures 1, 2A and B). The paleo-Tethys ocean, represented by the present Jinsajiang suture, opened about Early Carboniferous time (Liu et al. 1992) and closed during latest Triassic time (Chang et al. 1986; Dewey et al. 1988; Burchfiel et al. 1989). Two different opinions also exist about its subduction polarity (e.g. Allegre et al. 1984; Dewey et al. 1988; Liu et al. 1992). The main body of the Songpan–Ganzi area is entirely composed of Triassic flysch, but its northeastern part is distinctly covered by coastal sediments (QHBGM 1991; SCBGM 1991), both units being in tectonic contact (Zhang 2001b, in press). Recent geophysical and geological studies indicate that the eastern part of the Songpan–Ganzi complex is underthrust by Precambrian continental basement (Cui et al. 1996), and that the huge quantity of Triassic flysch within the complex has been explained as allochthonous tectonic flakes (Zhang 2001b, in press).

3. MESOZOIC–PALEOGENE SEDIMENTARY FACIES AND PALEOGEOGRAPHY OF TIBET

3.1. Early–Middle Triassic time Lower–Middle Triassic strata are only present on the southern edge of the Tarim terrain, within the Songpan–Ganzi area, along the western edge of the Eastern Qiangtang terrain, within the middle of the Western Qiangtang terrain, and at the southeastern corner of the Lhasa terrain (Figure 2A). Detailed observations about Early–Middle Triassic stratigraphy and facies have been compiled mainly from ten locations and are brieﬂy summarized in Figure 2A and Table 1.

Copyright # 2002 John Wiley & Sons, Ltd. Geol. J. 37: 217–246 (2002) 中国科技论文在线 Copyright Table 1. Brief descriptions of marine Lower–Middle Triassic deposits in Tibet; location numbers as for Figure 2Aa Location Lithology Main fossils Environment Ref.

# 1. 35240N Middle Triassic. Upper: massive-bedded argillaceous Daonella, Posidonia wengesis Shallow AB 02Jh ie os Ltd. Sons, & Wiley John 2002 81050E (T) limestone and limestone; Lower: purple conglomerate supratidal– and sandstone, intercalated with limestone, with intertidal plant fragments. >1809 m 2. 34280N Monotonous interbedded siltstone and shale, often Halobia convexa, Slope– CD 99050E (SG) Bouma sequences and sometimes olistostromes with H. substyriaca oceanic basin EF variable size observed. >7750 m 3. 32500N Siltstone and shale, intercalated with andesite Entolium sichuanensis, Japonites sp., Carbonate G 0 96 48 E (SG) and limestone. >625 m Paracrochodiceras sp., Plagiostoma beyrichii platform–tidal flat tibet of evolution paleogeographic-tectonic 4. 31300N Interbedded purple conglomerate, sandstone, Eumorphotis sp., Japonites sp., Natiria aff. Supratidal– CG 98100E (SG) and limestone, intercalated with andesite. >2000 m costata Munster, Neocalamites sp., tidal flat Owenites sp., Paraceratites sp., Paranannites sp., Posidonia sp., Sageceras sp., Trigonodus sp. 5. 30500N Interbedded purple conglomerate, sandstone, Japonites sp., Natiria aff. costata Munster, Supratidal– G 98100E (SG) and limestone, intercalated with andesite. >2000 m Owenites sp., Paraceratites sp., Posidonia sp., tidal flat Sageceras sp., Trigonodus sp. 6. 29350N Interbedded purple conglomerate, sandstone, Eumorphotis sp., Neocalamites sp., Supratidal– G 98500E (SG) and limestone, intercalated with andesite. >2000 m Owenites sp., Paranannites sp., tidal flat Posidonia sp., Sageceras sp. 7. 33450N Bioclastic calcareous sandstone and sandy Claraia concentrica, Tidal flat– AB 80200E (Q) limestone, medium- to massive-bedded dolomite Eumorphotis inaequicostata lagoon and dolomitic limestone. >100 m 8. 34100N Middle Triassic. Thin-bedded limestone, Acrochordiceras cf. carolinae, Arthaberites Beach B 83100E (Q) sandstone, and siltstone, intercalated with alexandrea, Daonella bulongensis bifurcatea, pebbly sandstone. >2666 m Haydenites hatscheki, Lepismatina cf. hsuei, Mentzelia cf. subspherica, Nudirostralina griesbachi 9. 33400N Upper: oolitic limestone and calcirudite, Bakevellia exporrecta, Claraia aurita, Entolium, Supratidal– 87000E (Q) intercalated or interbedded with mudstone Eumorphotis inaequicostata, Myophoria lagoon http://www.paper.edu.cn sandstone, and siltstone; Lower: interbedded laevigata elongata, Promyalina putiatensis, sandstone and purple conglomerate, with coal seams Unionites canalensis el J. Geol. at the base. Cross-bedding, parallel beddings with parting lineations, wavy bedding, ripple marks, and mud cracks often observed. 2181–2566 m (Figure 3) 0 37 10. 29 36 N Upper: andesite; Middle: limestone, bioclastic Acromytilus sp., Judicarites sp., Myophoricardium sp., Carbonate B

1–4 (2002) 217–246 : 90570E (L) limestone, sandstone; Lower: limestone, intercalated Mytilus sp., Parallerodon beyrichii, Promathilda platform– with andesite. >1196 m sp., Straparollus sp., Trypanostylus baueri Geibel tidal ﬂat

aTerrains: T, Tarim; SG, Songpan-Ganzi; Q, Qiangtang; L, Lhasa. References: A, Guo et al. (1991); B, XZBGM (1993); C, Rao et al. (1987); D, Hou et al. (1991); E, QHBGM (1991); F, Liu et al. (1992); G, CIGMR–SCGR (1992). 223 中国科技论文在线 http://www.paper.edu.cn

224 k.-j. zhang, b. xia and x. liang

On the southern corner of the Tarim basin, Lower–Middle Triassic sediments were formed in a supratidal– intertidal environment (Location 1). In the main body of the Songpan–Ganzi area Lower–Middle Triassic rocks are characterized by a ﬂysch association representing a slope–oceanic basin environment (Location 2; e.g. Rao et al. 1987). In contrast, on the western rim of this triangular area the succession is composed mainly of interbedded shallow marine limestone, littoral–continental conglomerate and sandstone, and andesitic volcanics, with a thickness of more than 2 km (Locations 3–6; Rao et al. 1987; QHBGM 1991). In the middle of the Qiangtang area, Lower–Middle Triassic strata contain mainly coarse-grained clastic rocks, and oolitic and dolomitic limestone, deposited in supratidal–lagoonal environments (Locations 7 and 9; Figure 3). Northwards, these strata

Figure 3. Representative Triassic sequences in central Tibet. Numbered as in Figure 2. See Figure 2A for the location for Early–Middle Triassic (T1–2) sequences and Figure 2B for Late Triassic time (T3) sequences. Patterns: 1, conglomerate/sandy conglomerate; 2, sandstone/siltstone; 3, mudstone/shale; 4, coal/limestone; 5, muddy limestone/breccia; 6, oolitic limestone/dolomitic limestone; 7, gypsum/andesite; 8, horizontial/ parallel bedding; 9, cross/wavy bedding; 10, graded/lenticular bedding; 11, hummocky/vein bedding; 12, water escape/laminated structure; 13, birds-eye/herringbone structure; 14, marl/sand/mud lens; 15, marl/siliceous concretion; 16, desiccation crack/scour mold; 17, ripple mark/ stylolite; 18, plant fossil/broken animal fossil.

paleogeographic-tectonic evolution of tibet 225

change into thin-bedded limestone, sandstone, and siltstone, possibly reﬂecting a shelf–beach setting (Location 8). In the southeastern Lhasa terrain, rocks of Early to Middle Triassic age consist of limestone and calc-alkine volcanics more than 1196 m thick (Location 10; XZBGM 1993). Therefore, in Early–Middle Triassic time, the main body of the Songpan–Ganzi area is interpreted as an abyssal sea or oceanic basin (Figure 2A). Simultaneously, a landmass, named the Central Tibetan Landmass, up to 400 km wide, could have stretched across the southern Western Qiangtang and the majority of the Lhasa terrain, in view of the stratigraphic hiatus there (Figure 2A). Sediment sources, which were determined by paleocurrent measure- ments (e.g. Figure 3), were located in the southern part of the Western Qiangtang terrain and the Lhasa terrain. This means that the Western Qiangtang and Lhasa terrains were probably connected together then. In contrast, a volcanic arc (Jiangda arc) existed at the western end of the Songpan–Ganzi complex along the Jinsajiang suture (Figure 2A).

3.2. Late Triassic time Upper Triassic sequences are present over much of Tibet (Figure 2B). Detailed observations of Late Triassic stratigraphy and facies were conducted at 22 locations, and they are briefly summarized in Figure 2B and Table 2. Similar to Lower–Middle Triassic strata, on the southern corner of the Tarim basin, Upper Triassic strata largely consist of argillaceous limestone and limestone up to 1370 m thick, possibly reflecting deposition in a tidal flat environment (Location 1). In the main body of the Songpan–Ganzi area, lower–middle Upper Triassic rocks are characterized by a monotonous shale and siltstone turbiditic association, over 5 km thick (Locations 2–4), whereas its southwestern rim is dominated by interbedded shallow-marine limestone, littoral–continental coarse-grained clastic rocks, and volcanics (Locations 5 and 6; Rao et al. 1987; QHBGM 1991). However, upper- most Triassic rocks over the entire Songpan–Ganzi area consist largely of purple conglomerate, sandstone, and coal-bearing shale, reflecting a supralittoral–continental environment (Location 4; Rao et al. 1987; Hou et al. 1991). Marine Upper Triassic rocks are preserved over much of the Eastern Qiangtang terrain and the majority of the Western Qiangtang terrain (Figure 2B). Upper Triassic strata in the Qiangtang region can be divided into three distinct areas of different sedimentary environments. On the northern (Location 8) and southeastern (Loca- tion 13) rims of the Western Qiangtang terrain and the northern rim of the Eastern Qiangtang terrain (Location 12), Upper Triassic rocks are composed mainly of thin-bedded limestone, shale, and sandstone, which could have been deposited in a shelf–slope environment. However, on the middle of the Western Qiangtang terrain and the majority of the Eastern Qiangtang terrain (Locations 7, 9–11, 14–18), Upper Triassic rocks largely consist of carbonate and continental purple conglomerate, sandstone, including coal-bearing shale in some locations (e.g. Location 11 in Figure 3), reflecting a supratidal–tidal flat environment. An Upper Triassic flysch-like association east of 87E along the present Bangonghu–Nu¨jiang suture largely consists of shale and siltstone (Location 19, Figure 3; Loca- tions 13, 19, 21, 22. Figure 2). In the Lhasa terrain, Upper Triassic rocks are localized in its southeastern corner (Location 20), and are composed of interbedded carbonate and clastic rocks, representing deposition in a shallow- shelf environment (Rao et al. 1987; Hou et al. 1991; QHBGM 1991) (Figure 2B; Table 2). These data can be used to delineate roughly the Late Triassic paleogeography of Tibet. In Late Triassic time, the main body of the Songpan–Ganzi area maintained a bathyal or abyssal environment (Figure 2B). However, this so- called triangular sea (Wang and Sun 1985) could have closed and become dry land in latest Triassic time, because of the widespread deposition of continental conglomerate (Location 4; Rao et al. 1987; QHBGM 1991). Littoral environments dominated much of both the Eastern and Western Qiangtang terrains and the southeastern corner of the Lhasa terrain, and shelf environments may have existed only in the northern corners of both the Eastern and Western Qiangtang terrains and the southeastern rim of the Western Qiangtang terrain. The Bangonghu–Nu¨jiang mid-Tethys ocean, now represented by the present Bangonghu–Nu¨jiang suture, may have opened along its eastern segment. The Qiangtang and Lhasa terrains could have begun to split along the eastern segment of the Bangonghu– Nu¨jiang suture, with attendant reduction in the Central Tibetan Landmass westwards from the southeastern rim of the Western Qiangtang terrain (Figure 2B).

Copyright # 2002 John Wiley & Sons, Ltd. Geol. J. 37: 217–246 (2002) 中国科技论文在线 226 Copyright Table 2. Brief descriptions of marine Upper Triassic deposits in Tibet; location numbers as for Figure 2Ba Location Lithology Main fossils Environment Ref.

# 1. 35240N81050E (T) Argillaceous limestone, limestone and Caucasorhynchia baliana Tidal flat AB 02Jh ie os Ltd. Sons, & Wiley John 2002 shale. 1370 m 2. 35550N88300E (SG) Monotonous siltstone and shale. >5000 m Myriophyllum sp., Thecosmilia cf. clathria Oceanic basin C 3. 3456N 92450E (SG) Monotonous siltstone and shale. >3500 m Thecosmilia cf. clathria Oceanic basin C 4. 33400N99000E (SG) Upper: purple conglomerate; Lower: Myriophyllum sp., Thecosmilia cf. clathria Upper: continental; D siltstone and shale. >7750 m Lower: oceanic basin 5. 31320N98250E (SG) Upper: andesite and sandstone; Middle: Protrachyceras sp., Supratidal–tidal flat E argillaceous limestone; Lower: Trachyceras sp. conglomerate and sandstone. 2587 m 6. 30540N98200E (SG) Interbedded sandstone, siltstone, and shale, Clathropteris meniscioides, Drepanozamites Supratidal–tidal flat AE intercalated with coal. 1562–m nilssani, Neocalamites cf. carrerei, Pterophyllum cf. aequale, Taeniopteris sp., Trigonodus cf. keuperinus liang x. and xia b. zhang, k.-j. 7. 33550N80260E (Q) Algal reef, coral reef, bioclastic limestone, Neomegalodon ladakhensis, Diderocadium sp. Supratidal lagoon B and dolomitic limestone. >1000 m 8. 34100N83100E (Q) Limestone, argillaceous limestone, Arietoceltites cf. arietitoides, Burmesia sp., Shelf or platform and shale. 2670 m Discotropites sp., Eoseptaliphoria tulungensis, interior Indopecten sp., Qilianoconcha curvifrons 9. 34000N85100E (Q) Upper: thick bedded limestone; Middle: Amphiclina intermedia, A. taurica, Restricted siltstone, sandstone, and shale; Lower: Indopecten sp., Plagiostoma nuitoense, platform–lagoon dark massive bedded bioclastic sparite, Rhaetinopsis pentagonalis argillaceous limestone, intercalated with chert nodules and beds. 3580 m 10. 32350N86200E (Q) Medium-thick bedded oncolite and Montlivaltia norica, Palaeocardita Tidal–lagoon oolitic limestone, shale, intercalated langnongensis, Thecosmilia fenestrata with sandstone. >4700 m 11. 33030N87450E (Q) Upper: coarse- and fine-grained Entolium cf. quotidianum, Palaeocardita Lagoon–tidal flat sandstone, shale, and argillaceous langnongensis, Thecosmilia sp. limestone; Lower: dark thick-bedded http://www.paper.edu.cn limestone. >1478 m (Figure 3) 12. 34400N89120E (Q) Upper: sandstone, shale, and argillaceous Palaeocardita langnongensis, Shelf–platform el J. Geol. limestone; Lower: Limestone. >2900 m Thecosmilia sp. interior 13. 32350N90000E (Q) Thin-bedded limestone. >1550 m Thecosmilia sp. Shelf–slope 14. 33220N91100E (Q) Interbedded sandstone, siltstone, shale, Clathropteris meniscioides, Drepanozamites Tidal flat

37 intercalated with coal. >1500 m nilssani, Keuperinus, Pterophyllum cf.

1–4 (2002) 217–246 : aequale, Taeniopteris sp. 15. 34100N92000E (Q) Interbedded sandstone, siltstone, and Drepanozamites nilssani, Tidal ﬂat shale, intercalated with andesite Pterophyllum cf. aequale and coal. >476 m 16. 33420N92200E (Q) Interbedded sandstone, siltstone, and Clathropteris meniscioides, Continental–supratidal shale, intercalated with coal. >2805 m Drepanozamites nilssani, Keuperinus, Taeniopteris sp. 中国科技论文在线 Copyright 17. 31130N96350E (Q) Upper: purple conglomerate, and sandstone, Ectolcites qabdoensis, Rhaetinopsis ovata, Continental–supratidal intercalated with shale; Lower: Robinsonella sp., Sanquiaothyris aff. elliptica, interbedded limestone, shale, and sandstone, Septaliphoria sp.

# with basal conglomerate. >500 m 0 0

02Jh ie os Ltd. Sons, & Wiley John 2002 18. 30 40 N9730 E (Q) Shale, sandy shale, intercalated Halobia cf. superbescens, Tylotrochus sp. Supratidal–beach E with coal streaks. >395 m 19. 31580N87550E (L) Interbedded sandstone, siltstone, silty Elegantinia elegans Slope C shale, pelitic siltstone, and shale. The beds thinner than 1 cm usually. An incomplete Bouma sequence observed and only planktons such as ammonites and planktonic bivalves discovered. >810 m (Figure 3) tibet of evolution paleogeographic-tectonic 20. 30150N91220E (L) Interbedded sandstone, shale, and Epigondolella, Procyclolitidae Tidal ﬂat (?) C thin-bedded limestone. 1651 m 21. 31500N93050E (L) Monotonous siltstone. 1800 m Elegantinia sp. Slope 22. 31200N95400E (L) Monotonous sandstone and shale, Prorotrigonia sp. Slope–oceanic basin intercalated with thin-bedded limestone containing chert bands. 2661 m

aTerrain: T, Tarim; SG, Songpan-Ganzi; Q, Qiangtang; L, Lhasa. References: A, Rao et al. (1987); B, Guo et al. (1991); C, XZBGM (1993); D, QHBGM (1991); E, CIGMR–SCGS (1992). http://www.paper.edu.cn el J. Geol. 37 1–4 (2002) 217–246 : 227 中国科技论文在线 228 Copyright Table 3. Brief descriptions of marine Lower–Middle Jurassic deposits in Tibet; location numbers as for Figure 2Ca Location Lithology Main fossils Environment Ref.

# 1. 35300N77500E (T) Limestone, sandy mudstone, sandstone, Monticlarella, Phacellastrea Beach–supratidal AB 02Jh ie os Ltd. Sons, & Wiley John 2002 and conglomerate. >579 m rotogensis, Thecosmilia tibetensis 2. 33400N99000E (S) Lower Jurassic. Sandstone, sandy Cladophlebis nebbensis, Hausmannia Alluvial C conglomerate, and conglomerate. >600 m ussuriensis, Neocalamites sp. 3. 33450N80200E (Q) Middle Jurassic. Shale, sandstone, reefal Montlivaltia sp., Stylina cf. kutchensis Coastal A limestone, and calcirudite. >750 m. 4. 34350N80150E (Q) Limestone, intercalated with calcareous Burmirhynchia tenuiplicata Reed, Shelf B sandstone. 2400 m Holcothyris tanggularica Ching, Protocardia sp., Trocholina cf. umbo 5. 35000N83050E (Q) Shale, intercalated with thin-bedded Chlamys tipperi Cox, Corniceras Shelf–platform limestone, mudstone, calcisiltite, and sp., Pholadomya socialia Morris interior argillaceous limestone. Ripple marks, and Lycett, Protocardia wavy bedding, and locally chert nodules qinghaiensis Wen liang x. and xia b. zhang, k.-j. observed. >1932 m (Figure 4). 6. 34100N87500E (Q) Thin-bedded limestone, intercalated with Avonothyris distorta, Burmirhynchia, Shelf–platform shale. 1660 m Holcothyris, Liostrea birmanica interior 7. 33400N88000E (Q) Middle Jurassic: medium-massive bedded Top: Corbula sp., Lamprotula Supratidal–restricted D sandstone and bioclastic boundstone, (Eolamprotula) cf. guangyanensis, platform intercalated with conglomerate (>120 m) Myopholas multicostata, Undulata and 13 layers of anhydrite (>74 m), massive cf. tangulaensis. bedded dolomite and dolomitic limestone. Lower: Burmirhynchia, Herringbone bedding. >2900 m (Figure 4). Holcothyris, Liostrea birmanica, Lower Jurassic: interbedded shale, Lopha asella, Pholadomya socialis, sandstone and andesite. >300 m Pseudolimea cf. duplicata 8. 32300N89500E (Q) Shale, intercalated with siltstone, Camptonectes lamintus, Dorsetensia, Slope–shelf, argillaceous limestone. Only ammonites sp., Parvamussium cf. pumilum, platform discovered. Horizontal and wavy bedding, Witchellia sp. interior ripple mark, and hummocky cross-bedding. 2121 m. In the middle there is a 292 m http://www.paper.edu.cn thick micritic limestone and calcarenite segment and at the top there are subaerial el J. Geol. exposure structures (Figure 4) 9. 33550N91100E (Q) Middle Jurassic. Mudstone, siltstone, Astarte elegans, Corbula sinensis, Tidal sandstone, conglomerate, calcarenite, and Protocardia truncata

37 bioclastic and oolitic limestone. 3200 m

1–4 (2002) 217–246 : 10. 31150N97150E (Q) Purple sandstone and conglomerate. 3287 m Damalasaurus magnus, Lufengosaurus Continental– E changduensis, Hybodus changduensis, tidal Plesiosaurus changduensis, Scelidosaurus sp., Steneosuchus microobtusidens 11. 29100N98050E (Q) Sandstone, limestone, and siltstone. 648 m. Astarte sp., Burmirhynchia shanensis, Beach–tidal E Holcothyris sp., Modiolus sp., Plagiostoma sp., Pleuromya sp. 中国科技论文在线 Copyright 12. 33200N79550E (L) Middle Jurassic. Monotonous siltstone. Archicapsa sp., Cyrtocalpis sp. Slope–oceanic 5400 m basin 13. 32200N84100E (L) Monotonous siltstone. 14 700 m Cenosphraera sp., Dictyomititra sp. Slope–oceanic basin B 0 0 # 14. 30 50 N8520 E (L) Middle Jurassic unconformably on the Praeexogyra acuminata Continental– B

02Jh ie os Ltd. Sons, & Wiley John 2002 Paleozoic. Conglomerate, sandstone and supratidal pebbly sandstone, intercalated with bioclastic limestone in the base. >630 m 15. 32000N90500E (L) Shale, sandstone, and siltstone. 14 796 m Dioloptyxis sp., Nerinea sp., Nerinellacea, Slope–oceanic B Nododelphinula sp., Obornella sp., Parepismilia basin amdoensis, Pleurotomaria sp., Trochoptigmatis sp. 16. 31180N90350E (L) Middle Jurassic. Rhythmic intercalations Gutnicella (Lucasella) cayeuxi, Slope BF of dark shales, slates and yellow sandstones Haurania sp., Nauticulina sp. with scarce bio-detritic limestones forming tibet of evolution paleogeographic-tectonic lenses several meters thick. >1000 m. 17. 30200N91320E (L) Limestone, shale, and sandstone, Coniopteris sp., Equisetum cf. sarrani, Tidal intercalated with coal, with 30 m Neocalamites sp., Rhactipollis germanicus, thick basal conglomerate. 630 m. Todisporites minor 18. 30400N92300E (L) Middle Jurassic unconformable on the Inoperna sp., Plicatostylus sp., Beach–tidal– B Paleozoic. Upper: dark limestone, intercalated Ptygmatis sp. supratidal with bioclastic limestone; Middle: gray dolomitic limestone, intercalated with purple sandstone, conglomerate, and shale, with plant fragments; Lower: interbedded gray sandstone, siltstone, and slate, with basal conglomerate. Typical lenticular tidal-bedding observed in the lower (Figure 4). >3628 m 19. 31300N93380E (L) Middle Jurassic. Interbedded shale, Alligaticeras sp., Macrocephalites sp., Shelf slope siltstone, and thin-bedded sandstone, Reineikenia intercalated with andesite. Mainly ammonites. >800 m 20. 30550N95450E (L) Shale and siltstone. 8223 m Sethocyrtis sp. Slope 0 0

21. 30 40 N9605 E (L) Middle Jurassic. Dark shale, Alligaticeras sp., Kinkeliniceras sp., http://www.paper.edu.cn intercalated with lenticular siltstone. Macrocephalites sp., Shelf–slope >349 m. Mainly ammonites Reineikenia, Subkossmatia el J. Geol. aTerrains: T, Tarim; S, Songpan–Ganzi; Q, Qiangtang; L, Lhasa. References: A, Guo et al. (1991); B, XZBGM (1993); C, QHBGM (1991); D, Yu and Wang (1990); E, CIGMR–SCGR (1992); F, Girardeau et al. (1984). 37 1–4 (2002) 217–246 : 229 中国科技论文在线 http://www.paper.edu.cn

230 k.-j. zhang, b. xia and x. liang

3.3. Early–Middle Jurassic time Lower–Middle Jurassic sequences also cover most of Tibet. Detailed observations about Early–Middle Jurassic stratigraphy and facies were conducted at 21 locations, and these are briefly summarized in Figure 2C and Table 3. On the southern edge of the Tarim terrain, Lower–Middle Jurassic strata contain carbonate, medium- to coarse- grained sandstone, and conglomerate (Location 1), reflecting a supratidal–beach setting. However, within the Songpan–Ganzi area, only sporadic Lower Jurassic alluvial conglomerate was found (Location 2). In the Qiangtang area there are three main sedimentary environments present. On the southern and western parts of the Western Qiangtang terrain (Locations 4, 5, 8) and on the northwestern corner of the Eastern Qiangtang terrain (Location 6), Lower–Middle Jurassic rocks consist of mainly thin-bedded limestone intercalated with fine-grained clastic rocks such as shale and sandstone (Figure 4), which possibly reflects a shelf setting. However, on the eastern side of the Eastern Qiangtang terrain, an 3287 m of molasse (Leeder et al. 1988; Yin et al. 1988) is present and consists of purple conglomerate, sandstone, and coal-bearing shale, deposited in a continental to supratidal setting

Figure 4. Representative Early–Middle Jurassic sequences (Locations 5, 7, 8) and sedimentary structures (Location 18) in central Tibet. Numbered as in Figure 2C and patterned as in Figure 3. J1–2, Early–Middle Jurassic; J2, Middle Jurassic.

paleogeographic-tectonic evolution of tibet 231

(Locations 9, 10). In the middle of both the Eastern and Western Qiangtang terrains, carbonate-clastics representing deposition in a coastal facies predominated (Locations 3 and 7). In Location 7, the top part of the sandstone is purple and contains a rich assemblage of plant fragments and freshwater bivalves (Figure 4; Table 3), indicating the adjacent area could have been elevated slightly in latest Middle Jurassic time. In the Lhasa terrain (except on its southwestern rim), Lower–Middle Jurassic strata have been discovered in two main different settings. On the northeastern rim, ﬁne-grained and thin-bedded clastic rocks formed in a shelf–slope environment (Locations 16, 19–21), and in the middle and on the southeastern rim a carbonate to conglomerate assemblage was deposited in a littoral setting (Locations 17, 18). Over the entire Bangonghu–Nu¨jiang zone, 15 km of Lower–Middle Jurassic ﬂysch was deposited, consisting of rhythmic siltstone and slate (XZBGM 1993). Consequently, in Early–Middle Jurassic time, the Songpan–Ganzi area maintained approximately the same paleogeography as in latest Triassic time, and had become a mountainous part of the Eurasian continent. The Eastern and Western Qiangtang terrains may have completely split from the Lhasa terrain, and a bathyal or abyssal sea (ocean) represented by the Bangonghu–Nu¨jiang suture could have existed between these two terrains. The Central Tibetan Landmass could have been reduced within the Qiangtang and Lhasa terrains, only locally traceable on the southwestern rim of the Lhasa terrain. A Tethyan epicontinental sea covered most of Tibet southwest of the Jinsajiang suture (Figure 2C).

3.4. Late Jurassic time Marine Upper Jurassic strata cover most of the southern taper of the Tarim terrain, the Eastern and Western Qiangtang terrains, and the Lhasa terrain. Detailed observations of Late Jurassic stratigraphy and facies were conducted at 18 locations, and these are briefly summarized in Figure 2D and Table 4. Upper Jurassic strata were predominantly formed in a supratidal–beach environment throughout Tibet (Locations 1, 3, 6–10, 12–18), as shown in a detailed log of the upper part of the section in Location 7 (Figure 5). However, on the northern half of the Western Qiangtang terrain (Locations 4, 5), Upper Jurassic rocks still com- prise a thick interval of thin-bedded carbonate and fine-grained clastic rocks, possibly reflecting deposition in a continental shelf environment (Figure 5). Coeval flysch assemblages, representing an oceanic setting, also exist in the westernmost segment of the Bangonghu–Nu¨jiang suture (Location 2) (Wang and Yang 1991). In summary, in Late Jurassic time, the bathyal or abyssal sea (ocean), represented by the Bangonghu–Nu¨jiang suture, between the Qiangtang and Lhasa terrains could have closed east of 83E, but flysch continued to be deposited in the westernmost Bangonghu–Nu¨jiang suture. The Songpan–Ganzi area remained elevated, and a small- scale landmass may have existed across the middle of the Eastern and Western Qiangtang terrains and the southwestern rim of the Lhasa terrain. The rest of Tibet was covered by littoral deposits, with the exception of neritic deposits on the northern corner of the Western Qiangtang terrain (Figure 2D).

3.5. Early Cretaceous time Lower Cretaceous marine rocks are spread only over the southern rim of the Western Qiangtang terrain and on the Lhasa terrain. Neritic deposits exist in these two regions and oceanic sediments occur in the westernmost end of the Bangonghu–Nu¨jiang suture (Figure 2E). Detailed observations of Early Cretaceous stratigraphy and facies were conducted at 18 locations, and these are briefly summarized in Figure 2E and Table 5. The majority of the Eastern Qiangtang terrain was elevated into dry land (Zhang, 2000). On the western rim of the Western Qiangtang terrain, however, Lower Cretaceous strata conformably overlie Upper Jurassic rocks (Guo et al. 1991) and they are composed of more than 500 m of mainly fine-grained clastic rocks, including mudstone and sandstone (Location 3). On the southeastern rim of the Western Qiangtang terrain, only transgressive upper Lower Cretaceous strata exist, unconformably overlying Middle Jurassic rocks (Location 4). Lower Cretaceous rocks on the Lhasa terrain are varied, although we show only the coastal facies in Figure 2E. On the northern half of the Lhasa terrain, Lower Cretaceous strata can be divided into two distinct parts: a lower part consisting of clastic rocks that reflect deposition in a littoral environment, and an upper part consisting of monotonous carbonate strata, up to 5 km thick, possibly representing a platform interior–shelf setting. The

Copyright # 2002 John Wiley & Sons, Ltd. Geol. J. 37: 217–246 (2002) 中国科技论文在线 232 Copyright Table 4. Brief descriptions of marine Upper Jurassic deposits in Tibet; location numbers as for Figure 2Da Location Lithology Main fossils Environment Ref. # 1. 3530’N 77500E (T) Dark limestone, rich in bioclasts of benthos Astrohizopora sp. Phacellastrea rotogensis Carbonate platform AB 02Jh ie os Ltd. Sons, & Wiley John 2002 such as bivalves, brachiopods, corals and Liao, Monticlarella cf. czenstochaviensis, stromatoporas. 1000 m Rutorhynchia 2. 33250N79400E (Q) Monotonous interbedded shale and Acanthocircus variabilis, Angulobracchia Slope–oceanic C siltstone, only radiolarian fossils purisimaensis, Emiluvia hopsoni Pessagno, basin discovered. >4500 m Hsuum maxwelli Pessagno, Mirifusus guadalupensis Pessagno, Paronaella bandyi Pessagno, Perispyridium ordinarium, Ristola procera, Tripocyclia jonesi Pessagno 3. 33450N80200E (Q) Dark medium-, thick-, and massive-bedded Cladophyllia dichotoma, Carbonate platform D limestone, reefal limestone, breccia, Epistreptophyllum giganteum

and calcirudite. 3000 m liang x. and xia b. zhang, k.-j. 4. 34350N80150E (Q) Thin-bedded limestone, intercalated with Burmirhynchia tenuiplicata Reed, Shelf A calcareous sandstone. 2400 m Holcothyris tanggularica Ching, Protocardia sp., Trocholina cf. umbo 5. 35000N83050E (Q) Thin-bedded limestone, intercalated with Chlamys tipperi Cox, Corniceras sp., Shelf–carbonate sandy limestone. Hummocky Pholadomya socialia Morris and Lycett, platform cross-bedding. >855 m Protocardia qinghaiensis Wen

6. 34300N86100E (Q) Thick-bedded dolomitic, oolitic, and Corniceras sp., Protocardia Lagoon–supratidal bioclastic limestones, and medium-bedded qinghaiensis Wen mudstone with pancake calcareous concretion, intercalated with sandy limestone. >2915 m 7. 34100N87500E (Q) Upper: mudstone, siltstone, intercalated Avonothyris distorta, Upper: restricted with sandstone, argillaceous limestone, Burmirhynchia, Holcothyris, platform; Lower: and anhydrite (Figure 5). Massive beds of Liostrea birmanica tidal flat–carbonate the latter are up to 50 cm thick and platform http://www.paper.edu.cn argillaceous, dark and thin partings define bedding of all these deposits. Lower: medium- to thin-bedded limestone, el J. Geol. intercalated with shale. 1660 m 8. 34050N91250E (Q) Upper: sandstone, intercalated with Camptonectes sp., Delta plain–tidal flat siltstone; Lower: interbedded mudstone, Ceratomya sp., 37 siltstone, sandstone, and limestone. Holcothyris sp. 1–4 (2002) 217–246 : Parallel bedding and cross-bedding are common. 2925 m (Figure 5) 9. 33350N92020E (Q) Conglomerate, sandstone, shale, Burmirhyris sp., Camptonectes sp., Tidal–delta plain EFG and limestone. 2900 m Ceratomya sp., Holcothyris sp. 10. 30300N97550E (Q) Purple mudstone and sandstone, Saurischia indet. Supratidal intercalated with conglomerate. 1437 m 中国科技论文在线 Copyright 11. 32300N82250E (L) Medium–thick-bedded sandstone, and Buchia sp., Epistreptophyllum sp., Continental–intertidal conglomerate, intercalated with bioclastic Heliocoenia cf. orbignyi, limestone. >350 m H. meriani, Latiastrea sp., Ostrea sp.,

# Perisphinctinae, Stylina sp. 0 0

02Jh ie os Ltd. Sons, & Wiley John 2002 12. 31 00 N8505 E (L) Conglomerate, sandstone, and sandy Camptonectes punctatus, Continental–tidal B shale, intercalated with coal. >2010 m. Nerinea, Praeexogyra acuminata Typical tidal sedimentary sequence in the middle bears lenticular, herringbone, vein, and parallel bedding (Figure 5) 13. 31200N90000E (L) Shale, sandstone, limestone, and Cladophyllia turbinata, Continental–tidal conglomerate, intercalated with Collignoastraea jumarense, andesite. 2622 m Grammatodon virgatus, Kobyastrea sp., Pseudocoenia tibet of evolution paleogeographic-tectonic hexaphyllia, Stylosmilia michelini, Thamnoseri cf. blauensis, Virgatosphinctes sp. 14. 30000N90000E (L) Limestone, sandstone, and shale, Cladophyllia sp., Cossmanea sp., Continental–tidal intercalated with coal. 800 m Lochmaeosmilia sp., Nerinea sp., Polyptyxis sp., Ptygmatis sp. 15. 32000N90500E (L) Upper: limestone; Middle: siltstone, Cladocoropsis mirabilis, C. nanoxi, B carbonaceous shale, and coal streaks; Dermoseris delogudoi, Heliocoenia Upper: tidal; Lower: sandy conglomerate and cf. orbignyi, H. meriani, Mid–lower: sandstone, bearing ultrabasic and Latiastrea, Parastromatopora continental chromite pebbles and picotite. 100 m compacta, Ptyllophylum sp., Stylina, Thecosmilia magna 16. 31500N92500E (L) Siltstone and sandstone, intercalated Aspidoceratinae, Astarte sp., Beach–tidal with limestone, with basal purple Berriasella sp., Homomya sp., conglomerate and andesite. 2310 m Protetragonites sp., Rasenioides sp., Virgatosphinctes sp. 17. 29290N92310E (L) Limestone, shale, and sandstone, Cossmanea sp., Haplophragnium, Beach–tidal B with basal conglomerate. 800 m Nerinea sp., Pseudocyclammina

cf. lituns, Ptygmatis sp. http://www.paper.edu.cn 18. 30400N96050E (L) Shale and sandstone, Collignoastraea jumarense, Beach–tidal containing plant fragments. >300 m Pseudocoenia hexaphyllia, Stylosmilia michelini, Thamnoseri el J. Geol. cf. blauensis, Virgatosphinctes sp.

aTerrain: T, Tarim; Q, Qiangtang; L, Lhasa. References: A, Sun and Xu (1991); B, XZBGM (1993); C, Wang and Yang (1991); D, Guo et al. (1991); E, Leeder et al. (1988); F, Yin et al.

37 (1988); G, QHBGM (1991). 1–4 (2002) 217–246 : 233 中国科技论文在线 http://www.paper.edu.cn

234 k.-j. zhang, b. xia and x. liang

Figure 5. Representative Late Jurassic sequences in central Tibet. Numbered as in Figure 2D and patterned as in Figure 3.

carbonate rocks involve the southern rim of the Western Qiangtang terrain and cover an apparently larger area than the clastic rocks. These carbonate strata could have extended into the Cenomanian stage of Late Cretaceous time, as indicated by fossils (mostly foraminifera) (Pan 1985; Yin et al. 1988; Lin et al. 1989; Zhang 2000). On the southern rim of the Lhasa terrain, Lower Cretaceous strata are mainly composed of coarse clastic rocks that were deposited in a littoral environment. On its western rim, however, they consist of limestone, shale, and fine-grained sandstone that reflect an inner shelf environment (Figure 2E). The Lower Cretaceous sequence around Location 10 could be representative of the strata on the northern part of the Lhasa terrain and are shown in Figure 6. This sequence can be subdivided into two distinctive parts. An upper part is characterized by a thick interval of carbonates, in general composed of massive limestone, with single layers 50–200 cm thick, and rich in peloids and broken bioclasts of foraminifera, algae, brachiopods, crinoids and corals. Chert nodules and beds replace carbonate layers and bioclastics (10–70%) (Figure 6C). The strata and fossils indicate a platform interior–marginal bank environment. However, in the lower part of the carbonate rocks, some limestone layers are about 3–17 cm thick, and contain only about 3% fossils, which are intact. In these layers, hummocky cross-bedding is present (Figure 6C). These layers could represent a shelf environment. At the top of the carbonate rocks, massive dolomitic limestones with bird’s eye and brecciform structures prevail (Figure 6D), which we interpret as restricted platform–supratidal flat facies. Similar facies may extend to Late Cretaceous time (Yin et al. 1988). The lower part of the sequence is characterized by sandstone, carbonaceous

Copyright # 2002 John Wiley & Sons, Ltd. Geol. J. 37: 217–246 (2002) 中国科技论文在线 Copyright Table 5. Brief descriptions of marine Lower Cretaceous deposits in Tibet; location numbers as for Figure 2Ea Location Lithology Main fossil Environment Ref.

# 1. 3955’N 77050E (T) Interbedded sandstone and conglomerate. Nodosaria sp., Saccammina globasa Supratidal A 02Jh ie os Ltd. Sons, & Wiley John 2002 1000–1300 m 2. 33250N79400E (Q) Siltstone, silicalite, and basalt. >1000 m Cenellipsis sp., Lithamitra sp. Slope–oceanic basin B 3. 33450N80200E (Q) Purple massive-bedded sandstone Astarte, Nesinea sp., Pteria sp. Beach C and mudstone, with single layer 50 cm thick. >500 m 4. 32500N90120E (Q) Upper: gray medium–thick-bedded breccia, Hedberqella siqali, Orbitolina Upper: platform and micritic limestone, intercalated with birmanica sahni, Orbitolina interior; Lower:

bioclastic and oolitic limestones; trochus, Textularia sp. tidal–continental tibet of evolution paleogeographic-tectonic Lower: purple conglomerate. 1209 m 5. 33210N79180E (L) Upper: thin-bedded limestone; Lower: Cladophlebis browniana, Mesorbitolina Shelf thin-bedded sandstone and shale. 5400 m birmanica, Praeorbitolina corrnyi 6. 32300N80570E (L) Upper: thin-bedded limestone; Lower: Adiozoptyxis sp., Mesorbitolina Shelf sandstone and shale, intercalated birmanica, Palorbitolina with andesite. >5000 m lenticularis, Praeorbitolina corrnyi 7. 31250N81050E (L) Interbedded thin-bedded limestone Ampullina xainzaensis, Glauconia trorreri, Shelf and andesite. 6071 m Nerinea cf. pauli, Nerinella dayi, Plesioptyxis aff. langshanensis 8. 31460N84510E (L) Upper: massive limestone, with single Adiozoptyxis sp., Axosmilia sp., Upper: platform layer 55–210 cm thick, and rich in peloids Cladophlebis browniana, interior; Lower: and bioclasts of foraminifera, algae, Montlivaltia sp., Stylinaparvistella supratidal–tidal brachiopods, crinoids and corals. Chert ﬂat nodules and beds are found replacing carbonate layers and bioclasts; Lower: sandstone, carbonaceous shale (coal), intercalated with conglomerate. >617 m 9. 31420N85170E (L) Upper: massive limestone, with single Mesorbitolina birmanica, Upper: platform layer 64–240 cm thick, and rich in peloids Palorbitolina lenticularis, interior; Lower: and bioclasts of foraminifera, algae, Praeorbitolina corrnyi supratidal–tidal http://www.paper.edu.cn brachiopods, crinoids and corals. ﬂat Chert nodules and beds are found el J. Geol. replacing carbonate layers and bioclasts; Lower: sandstone, carbonaceous shale, intercalated with conglomerate. >620 m 0 0 37 10. 31 33 N8652 E (L) Upper: limestone; Lower: sandstone, Euthymiceras, Neocomites, Upper: tidal

1–4 (2002) 217–246 : carbonaceous shale, mudstone, intercalated Orbitolina lenticularis, ﬂat–shelf; Lower: with massive micritic limestone, O. tibetica, Triporopollenites alluvial fan–tidal rich in plant fragments. ﬂat >3100 m (Figure 6)

Continues 235 中国科技论文在线 236 Copyright Table 5. Continued Location Lithology Main fossil Environment Ref.

# 11. 29250N87100E (L) Shale and sandstone, intercalated Orbitolina textena, Supratidal–beach D

02Jh ie os Ltd. Sons, & Wiley John 2002 with coal seam and Palorbitolina lenticularis conglomerate. >623 m 12. 30570N88550E (L) Pebbly sandstone, sandstone, and shale, Cyprina teolluensis, Cyprimeria Supratidal–beach intercalated with andesite. 905 m cf. quadrata, Veneridea meritix 13. 31100N89250E (L) Upper: massive limestone, with Mesorbitolina birmanica, Upper: subtidal; single layer 45–235 cm thick, and rich Orbitolina sp., Palorbitolina Lower: supratidal in peloids and bioclasts of foraminifera, lenticularis, Praeorbitolina algae, brachiopods, crinoids and corals. corrnyi, Thamnasteria sp. Chert nodules and beds are found replacing carbonate layers and bioclasts; Lower: sandstone, carbonaceous shale, intercalated with andesite. 1300 m liang x. and xia b. zhang, k.-j. 14. 30320N90200E (L) Upper: massive limestone, with single layer Adiozoptyxis sp., Palorbitolina Upper: subtidal; 60–210 cm thick, and rich in peloids and lenticularis, Praeorbitolina corrnyi Lower: supratidal bioclasts of foraminifera, algae, brachiopods, crinoids and corals. Chert nodules and beds are found replacing carbonate layers and bioclasts; Lower: sandstone and shale, intercalated with conglomerate. >5000 m 15. 30150N91220E (L) Siltstone, mudstone, and limestone, Hemiaster sp., Meyeria magna, Supratidal D with basal conglomerate intercalated Notopocorystes xizangensis, with coal seams. >573 m Orbitolina prisca, O. tibetica 16. 30400N92300E (L) Massive limestone, with single layer Orbitolina lenticularis, O. tibetica Platform interior 45–200 cm thick, and rich in peloids and bioclasts of foraminifera, algae, brachiopods, crinoids and corals. Chert nodules and beds are found

replacing carbonate layers and http://www.paper.edu.cn bioclasts. >3100 m 17. 30400N96050E (L) Sandstone, siltstone, and shale, Cladophlebis sp., Klukia sp., Tidal–beach E

el J. Geol. intercalated with coal, chert, and Weichselia reticullate, limestone. >3140 m Zamiophylium buchianum 18. 30000N97060E (L) Purple conglomerate, sandstone, siltstone, Baxoitrigonia vhligi, Continental–supratidal E

37 and shale, intercalated with coal, chert, Cladophlebis sp., Klukia sp.,

1–4 (2002) 217–246 : and limestone. >3140 m Weichselia reticullate, Zamiophylium buchianum

aTerrain: T, Tarim; Q, Qiangtang; L, Lhasa. References: A, Guo (1995); B, Guo et al. (1991); C, Sun and Xu (1991); D, XZBGM (1993); E, CIGMR–SCGR (1992). 中国科技论文在线 http://www.paper.edu.cn

paleogeographic-tectonic evolution of tibet 237

Figure 6. Representative Early Cretaceous sequences in central Tibet. Numbered as in Figure 2E and patterned as in Figure 3.

238 k.-j. zhang, b. xia and x. liang

shale, and mudstone intercalated with massive micritic limestone rich in plant fragments. The fossils are com- monly incomplete. Herringbone tidal bedding, cross-bedding, lenticular bedding, parallel bedding, and scour molds are common (Figure 6B). These strata could represent a tidal flat environment. The base of the sequence is dominated by massive conglomerate, sandstone, and mudstone in which bedding is indistinct and plant fragments and parallel bedding were found locally. Therefore, these basal strata could have formed in an alluvial fan–delta plain environment. To summarize, Lower Cretaceous successions contains a transgressive sequence, with the lower part deposited in an alluvial fan–tidal flat environment and the upper part deposited in a possible tidal flat–shelf environment (Figure 6). The existence of the oceanic sediments in the westernmost Bangonghu–Nu¨jiang suture (Location 2) indicates that the oceanic realm represented by the suture had not closed completely by Early Cretaceous time (Guo et al. 1991). Significantly, during the late stage of Early Cretaceous time, and possibly extending into the Cenomanian stage (Yin et al. 1988; Lin et al. 1989; Zhang 2000), an extensive marine transgression occurred in southern Tibet, involving both the southern rim of the Western Qiangtang terrain and the northern rim of the Lhasa terrain.

3.6. Late Cretaceous time Coarse-grained clastic rocks characterize Upper Cretaceous strata in Tibet and its adjacent areas (Zhang 2000). Upper Cretaceous marine rocks are present only in the Tarim basin, the western rim of the Western Qiangtang terrain, and the Lhasa terrain (Figures 2F, 7; Table 6). During Late Cretaceous time, marine sedimentation began over the southern rim of the Tarim terrain (Tang et al. 1992; Zhang et al. 1998). However, the entire Qiangtang and Lhasa terrains underwent an extensive marine regression in comparison with the late stage of Early Cretaceous time. Except for its western rim, the majority of Western Qiangtang terrain and the area studied of the Eastern Qiangtang terrain was elevated to form dry land (Figure 2F). A littoral environment predominated along the southern rim of the Tarim terrain (Location 1), the western rim of the Western Qiangtang terrain (Locations 2, 3, Figure 2F), and the Lhasa terrain (Locations 2–16, Figure 2F) (Han et al. 1983; Liang and Xia 1983; Wang 1983; Li 1985; Pan 1985; XGS 1986; Guo et al. 1991; Li and Wu 1991; Tang et al. 1992; HNGS 1993; XZBGM 1993). In Figure 2F, we speculate that the southern corners of both the Eastern Qiangtang terrain and the Songpan–Ganzi complex were covered by littoral sediments, because of the occurrence of Late Cretaceous marine sediments in the western part of the Sichuan basin (SCBGM 1991; Zhang 2000), and Paleogene marine sediments on the southern fringe of the Songpan–Ganzi area (Li et al. 1987), as discussed below.

3.7. Paleogene time In Tibet and its adjacent areas, Paleogene time is also characterized by the deposition of coarse-grained clastic rocks (Zhang 2000). Paleogene marine sequences are only present in the Tarim basin, the western rim of the Western Qiangtang terrain, and the western half of the Lhasa terrain (Figure 2G). We only discuss Paleogene marine deposits (Figures 2G, 7; Table 7). In Paleogene time, all of Tibet was elevated. The Songpan–Ganzi area and most of the Qiangtang and Lhasa terrains became dry land (Figure 2G). However, we believe that the topographic relief was low, possibly a pene- plain, because the littoral (coastal) marine sediments occur in the Tarim basin (Location 1; Tang et al. 1992), on the western rim of the Western Qiangtang terrain (Location 2; Guo et al. 1991), in the western half of the Lhasa terrain (Locations 3–9; Zhang and Mo 1979; Wan 1987; Li and Xu 1988; Lin et al. 1989; Pan et al. 1990; Guo et al. 1991; XZBGM 1993), and along the southern fringe of the Songpan–Ganzi area (to the east of Figure 2G; Li et al. 1987; Zhang 2000).

4. IMPLICATIONS FOR MESOZOIC–PALEOGENE TECTONIC EVOLUTION OF TIBET

According to our present understanding, it is possible to delineate roughly Mesozoic–Paleogene stratigraphy and eustasy, and Mesozoic reconstructions, of Tibet, as brieﬂy summarized in Figures 7 and 8 respectively. We can

# Table 6. Brief descriptions of marine Late Cretaceous deposits in Tibet; location numbers as for Figure 2Fa 02Jh ie os Ltd. Sons, & Wiley John 2002 Location Index sediment Key fossils Interbed./Ref.

1. 35400N81050E (T) Reef limestone Biradiolites boldjuanensis, Braarudosphaera bigelowii A 2. 34560N81140E (Q) Bioclastic limestone Biradiolites lumbricoides, Gorjanovicia acuticostata, 1B Pycondonte cf. costei, Radiolites crassus, R. angeiodes 3. 33450N80200E (Q) Reef limestone Bournonian sp., Trigonioides (Diverstr.) bangongwenesis Gu, Trigonioides (Diverstr.) xizangensis Gu aegorpi-etnceouino tibet of evolution paleogeographic-tectonic 4. 32010N81050E (L) Bioclastic limestone Nerinea parahicoriensis, Orbitolina concava BC 5. 32000N83350E (L) Reef limestone Bournonia sp., Neoptyxis sp., Plesioptyxis huzitai, D Plisiotygmatis cf. pupoides, Praeradiolites sp. 6. 29500N84100E (L) Bioclastic limestone Lepidorbitoides gangdisicus, L. minor, L. zhongbaensis, C Libycoceras, Pseudorbitoides yini, Sphenodiscus 7. 31580N86070E (L) Reef limestone Orbitolina concava Lamarck, Trigoniodes sinensis 2 8. 31480N86160E (L) Reef limestone Orbitolina concava Lamarck, Trigoniodes sinensis 9. 32010N87050E (L) Bioclastic limestone Plicatula placunen, Plicatula cf. inflata, Orbitolina concava CE 10. 29250N87050E (L) Reef limestone Lepidorbitoides gangdisicus, Libycoceras, Manambolites, C Pseudorbitoides yini, Sphenodiscus 11. 31550N9000E (L) Bioclastic limestone Bournonia sp., Neithea sexcostatus, Orbitolina concava, CEFG Plicatula placunen, Plicatula cf. inflata 12. 31150N90200E (L) Bioclastic limestone Cymopolia sp., Fusus shumardi Hall and Meek, Fusus sp., 3 CGHI Natica, Orbitolina concava, Turritella magnoliana Stephenson, T. cf. whitei Stanton, T. xizangensis Pan, Y. T. (sp. nov.), T. maerbolongbaensis Pan Y. T., T. bangeensis Pan, Y. T. (sp. nov.), Tritonium kanabense Stanton, T. xizangense Pan, Y. T. (sp. nov.), T. angulatum Pan, Y. T. (sp. nov.) 0 0 13. 30 50 N9005 E (L) Reef limestone Bournonia sp., Neithea sexcostatus, Orbitolina concava, CEG http://www.paper.edu.cn Plicatula placumen, Plicatula cf. inflata 14. 30150N91220E (L) Bioclastic limestone Aetostreon zhongshanensis, Altanicypris aff. bispinofero, C

el J. Geol. Amphidonte ostracina, Cypridopsis aff. bugintsavicus, Pycnodonte vesiculosa, 15. 3055N 94150E (L) Bioclastic limestone Nonion cf. sichuanensis Li JK 16. 31200N95400E (L) Bioclastic limestone Nonion cf. sichuanensis Li, Nonion sp. K 37

1–4 (2002) 217–246 : aTerrains: T, Tarim; Q, Qiangtang; L, Lhasa. 1, interbedded with andesites dated at 77.8 Ma (K–Ar); 2, interbedded with andesites dated at 69.2 Ma (K–Ar); 3, interbedded with andesites dated at 77 Ma, 83 Ma (K–Ar). References: A, Tang et al. (1992); B, Guo et al. (1991); C, XZBGM (1993); D, Li and Wu (1991); E, XGS (1986); F, Han et al. (1983); G, Wang (1983); H, Liang and Xia (1983); I, Pan (1985); J, Li (1985); K, HNGS (1993). 239 中国科技论文在线 http://www.paper.edu.cn

240 k.-j. zhang, b. xia and x. liang

Table 7. Brief descriptions of marine Paleogene deposits in Tibet; location numbers as for Figure 2Ga Location Index lithology Key fossils Ref.

1. 35400N, 81050E (T) Bioclastic limestone, Cibicidina sp., Flemingostrea, A reef limestone Miliola sp., Sokolovia 2. 33450N, 80200E (Q) Reef limestone Astrocoenia gibbosa, Dendrophyllia, B Deplhelia papillosa, Oculina alabaminsis, Stephanocoenia microtuberculata, Stylocoenia 3. 32300N, 79400E (L) Bioclastic limestone, Nummulites wadiai, Ranikothalia C reef limestone 4. 32010N, 81050E (L) Radiolarian chert, Collonia B reef limestone 5. 29500N, 84100E (L) Bioclastic limestone, Assilina levis, Lockhartia, Nummulites D reef limestone wadiai, Ranikothalia 6. 29500N, 84500E (L) Bioclastic limestone, Fasciolites gambaensis, Lockhartia, DE reef limestone Nummulites wadiai, Ranikothalia 7. 32010N, 87050E (L) Bioclastic limestone Nummulites rotularius, Deshayes F 8. 31150N, 90200E (?) (L) Limestone Acicularia americana, A. antiqua, Cymopolia G tibetica, Trinocladus megacladus 9. 30150N, 91220E (L) Bioclastic limestone Distichoplax biserialis, Furcoporella diplopora, DEH Keramosphaera tergestina, Ovulites morelleti

aTerrains: T, Tarim; Q, Qiangtang; L, Lhasa. References: A, Tang et al. (1992); B, Guo et al. (1991); C, Li and Xu (1988); D, XZBGM (1993); E, Wan (1987); F, Pan et al. (1990); G, Lin et al. (1989: 219); H, Zhang and Mo (1979).

therefore draw several main preliminary conclusions about the Mesozoic–Paleogene paleogeographic–tectonic evolution of Tibet.

4.1. The Jinsajiang paleo-Tethys ocean The present Jinsajiang suture represents the closed paleo-Tethyan oceanic realm in Tibet. Generally, it is believed that the Jinsajiang paleo-Tethys ocean closed in latest Triassic time (Chang et al. 1986; Dewey et al. 1988; Burchfiel et al. 1989). A main conclusion from the 1985 British–Chinese Tibet Geotraverse Lhasa to Golmud, was that Jurassic sediments in the Eastern Qiangtang terrain belong to a foreland clastic molasse related to the collision along the Jinsajiang suture (Chang et al. 1986; Leeder et al. 1988; Yin et al. 1988). However, in contrast, Yu and Wang (1990) and Liu et al. (1992) pointed out the entire Qiangtang terrain contains passive continental margin strata that range in age from the Paleozoic through to the Jurassic. Our investigation shows that the sedimentation in the eastern part of the Eastern Qiangtang terrain indeed occurred in a contractional foreland basin, which was filled with thick coarse clastics, since latest Triassic time (Figure 2B). However, clastic sedimentation was mainly localized on the eastern part of the Eastern Qiangtang terrain near the Jinsajiang suture and migrated gradually westwards. In the northwestern corner of the Eastern Qiangtang terrain and on the northern rim of the Western Qiangtang terrain, shelf carbonates predominated in Late Triassic–Jurassic time (Figures 2A-D) (Rao et al. 1987; XZBGM 1993), suggesting a diachronous closing of the Jinsajiang paleo-Tethys ocean, first in the eastern side due to the latest Triassic suturing between the Eastern Qiangtang terrain and Asia (Figure 8B) and clearly later in the western side due to the Jurassic collision of the Western Qiangtang terrain and Asia (Figures 8C and D).

4.2. The Shuanghu suture between the Eastern and Western Qiangtang terrains Our study provides useful constraints on the debate and speculations about extension and timing of the suturing between the Eastern and Western Qiangtang terrains (Li et al. 1995). Through preliminary analysis of four artiﬁ- cially panned sandstone samples, we found rich picotites (up to 16–95 g/T, Table 2) in Upper Triassic sandstones at two locations in the assumed eastern extention of the Shuanghu suture (Locations 11 and 14, Figure 2B). The

Copyright # 2002 John Wiley & Sons, Ltd. Geol. J. 37: 217–246 (2002) 中国科技论文在线 Copyright # 02Jh ie os Ltd. Sons, & Wiley John 2002 aegorpi-etnceouino tibet of evolution paleogeographic-tectonic http://www.paper.edu.cn el J. Geol. 37 1–4 (2002) 217–246 : Figure 7. Schematic curves showing the transgressive changes of four main terrains during Triassic–Paleogene periods. Facies: 1, continental; 2, supralittoral and intertidal; 3, subtidal and neritic; 4, bathyal–abyssal. Patterned as in Figure 3. 241 中国科技论文在线 http://www.paper.edu.cn

242 k.-j. zhang, b. xia and x. liang

Figure 8. Schematic reconstructions of Tibet during the main stages of Mesozoic time. Not drawn to scale. The teeth on the thick lines indicate the directions of subduction.

picotites, readily decomposed heavy minerals, should have come from nearby ultrabasic rocks, most likely ophiolitic complexes. These picotites could not have been related either to the Bangonghu–Nu¨jiang suture (because the ocean represented by the suture began to close only after Early Jurassic time (Yin et al. 1988)), or to the Jinsajiang suture (because we have not found any picotites in Upper Triassic sandstones in the area north of Locations 11 and 14 in Figure 2B). Therefore, it is possible that the Shuanghu suture could be present near the positions of these two locations. On the other hand, the stable shelf sedimentation during Late Triassic–Middle Jurassic time in the

paleogeographic-tectonic evolution of tibet 243

Western Qiangtang terrain (Figure 2B and C) suggests that the suture may not have formed until Middle Jurassic time (Figure 8C).

4.3. The Central Tibetan Landmass and the Bangonghu–Nu¨jiang mid-Tethys ocean A Lower–Middle Triassic hiatus (Figures 2A, 7) and data from sediment transport directions (left, Figure 3) in both the Qiangtang and Lhasa terrains indicate that a landmass, termed here the Central Tibetan Landmass, could have existed in central Tibet during Early–Middle Triassic time (Figures 2A, 8A). Beginning in Early–Middle Triassic time, this landmass was at least 400 km wide; by Late Triassic time it had narrowed by a gradual transgression from the east (Figures 2B, 8B). Finally, in Early–Middle Jurassic time, it gave way to the Bangonghu–Nu¨jiang mid- Tethys ocean completely, and a remnant of this landmass existed only on the southwestern rim of the Lhasa terrain (Figures 2C, 8C). We infer that the mid-Tethys ocean could have begun to open during Late Triassic time (Figure 8C), a popular view shared by many other authors (e.g. Chang et al. 1986; Dewey et al. 1988). However, Late Triassic abyssal or bathyal sediments were found only in the eastern segment of the Bangonghu–Nu¨jiang suture east of 87E (Figure 2B), and the earliest known oceanic sediments in the western segment are Middle Jurassic in age (Guo et al. 1991; Location 12, Figure 2C). Opening of the mid-Tethys ocean was therefore diachronous, occurring in Late Triassic time along its eastern segment and in Middle Jurassic time along its western segment. Bilateral lithofacies changes across and along the Bangonghu–Nu¨jiang suture (Figures 2B and C, 8B and C) are consistent with this preliminary conclusion. Figures 2D and E and 8C and D show that the suture began to close at the beginning of Late Jurassic time along its eastern segment, which was marked by the simultaneous deposition of coarse clastic rocks of molasse type on the northern rim of the Lhasa terrain and the termination of oceanic sedimentation. How- ever, Upper Jurassic (Location 2, Figure 2D) and Lower Cretaceous (Location 2, Figure 2E) ﬂysch rocks were still being deposited in the western segment of the Bangonghu–Nu¨jiang suture. This illustrates that the closing of the Bangonghu–Nu¨jiang mid-Tethyan oceanic realm (i.e. collision between the Lhasa and the Qiangtang terrains) was also diachronous (Figure 8D) and ﬁnally closed along its western segment only in latest Jurassic to earliest Cretac- eous time. Lowest Cretaceous coarse molasse on the northwestern half of the Lhasa terrain probably provides further evidence of this collision. However, upper Lower Cretaceous, and possibly lowest Upper Cretaceous (e.g. Yin et al. 1988), platform–shelf limestones, up to 5 km thick, stretched over the southern rim of the Western Qiangtang terrain and the northern half of the Lhasa terrain, and apparently covered a larger area than the lower continental– littoral sediments (Figure 8E; e.g. Lin et al. 1989; XZBGM 1993). This cannot be attributed only to a marine transgression event, because the rest of Tibet experienced an intense coeval marine regression (Figure 7; see Zhang (2000) for details). We propose that southern Tibet could have undergone subsidence during the late stage of Early Cretaceous time. This subsidence could be due to the extension behind the Gangdese arc. Upper Cretaceous coarse clastic rocks indicate the onset of another stage of compressional deformation. However, rich marine faunas and reef limestone in Upper Cretaceous and Paleogene strata (Figures 2G, 7) imply low topographic relief (Zhang et al. 1998; Zhang 2000).

4.4. The Tibetan plateau England and Searle (1986) believed that the Lhasa terrain was elevated before its collision with India. Furthermore, mainly on the basis of a traverse through the central Lhasa terrain in southern Tibet, Murphy et al. (1997) suggested that the southern Tibetan plateau had attained an elevation of 3–4 km by ca.99–Ma and maintained signi- ﬁcant topography until the onset of the Indo-Asian collision. They speculated that this elevation was due to the collision between the Lhasa and Qiangtang terrains during Early Cretaceous time. This study of the Mesozoic–Paleogene paleogeography provides a signiﬁcant test for previous conclusions, at least on whether southern Tibet had been elevated to 3–4 km prior to the Indo-Asian collision. Our investigations (Zhang et al 1998; Zhang 2000; and this study), as well as those by other Chinese colleagues in this region (e.g.

244 k.-j. zhang, b. xia and x. liang

Han et al. 1983; Liang and Xia 1983; Wang 1983; Li 1985; Pan 1985; XGS 1986; Guo et al. 1991; Liu et al. 1992; Tang et al. 1992; XZBGM 1993), show that over much of southern Tibet shallow marine sedimentation was not terminated at the end of Late Cretaceous time (Figures 2F, 7, 8F) and possibly continued until Eocene time (Pan et al. 1990; Guo et al. 1991; Liu et al. 1992; Zhang et al. 1998) (Figures 2G, 7). This sedimentation is marked by reef limestone, radiolarian chert, etc., and its timing is well constrained by rich fossil assemblages and radiometric dating of the interbedded volcanic rocks (Figure 2F and G). In addition, the backarc extension in southern Tibet during the late stage of Early Cretaceous time (possibly extending into earliest Late Cretaceous time; Figure 7), marked by local transgression on the southern rim of the Western Qiangtang terrain and the northern half of the Lhasa terrain, is in accord with this conclusion. These observations indicate that the collision between the Qiangtang and Lhasa terrains could have been completed at the end of early Early Cretaceous time. Therefore, the collision could not be the mechanism to create the elevation of southern Tibet to 3–4 km, for which no paleogeographic and lithofacies evidence is found by the end of Early Cretaceous time, as suggested by Murphy et al. (1997). Marine Paleogene sedimentation seems to have covered most of western Tibet, including the southwestern Tarim basin (Tang et al. 1992) and both sides of the Bangonghu–Nu¨jiang (Pan et al. 1990) and Yarlung Zangbo sutures (Wan 1987) (Figure 2G). Therefore, we believe that southern Tibet, in particular its western segment, was only slightly elevated during Late Cretaceous time in contrast to Early Cretaceous time, but not up to 3–4 km. Tibet was intensively elevated only during Late Tertiary time (Zhang et al. 1998), its high topography being only the product of the Indo-Asian collision (e.g. Xu 1981).

ACKNOWLEDGEMENTS

This work was supported by the National Science Foundation of China (40072075). Hundreds of Chinese geolo- gists took part in the ﬁeldwork in Tibet, China. We are grateful to Professor D. C. Bi at the Nanjing Institute of Geology and Mineral Resources, Professor H. F. Ye and Professor Y. T. Li at the PetroChina Company Limited for their logistical help in preparation of the paper. Professor G. T. Pan at the Chengdu Institute of Geology and Mineral Resources provided useful references. Professor B. C. Burchﬁel, Dr Alan Carroll, and Dr I. D. Somerville are greatly thanked for considerate and constructive review comments and careful editorial help.

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