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

Abstract 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–Nüjiang zone. In Early–Middle Jurassic time, Songpan–Ganzi had become part of the Eurasian continent, abyssal flysch sediments stretched throughout the Bangonghu–Nüjiang 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–Nüjiang 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–Nüjiang 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.