Abstract

Sedimentary strata in southern Tibet indicate that upper crustal deformation occurred throughout the region during Early Cretaceous time, suggesting that construction of the Tibetan plateau commenced tens of millions of years before the Late Cretaceous–early Tertiary Indo-Asian collision. Lower Cretaceous strata in the northern portion of the Lhasa terrane are characterized by lithic-rich conglomerate beds deposited in shallow marine and meandering-river fluvial environments. Sediments in these units were derived from two primary sources: volcanic rocks associated with Early Cretaceous intrusions, and sedimentary strata eroded from the northern Lhasa and southern Qiangtang terranes. The majority of detrital zircons from Lower Cretaceous fluvial conglomerate beds in northern Lhasa have U–Pb ages between 125 and 140 Ma and provide a maximum depositional age for these units of 125 ± 2 Ma. Lower Cretaceous strata in the southern portion of the Lhasa terrane consist of mudstone, quartzose sandstone, and subordinate quartzite-pebble conglomerate beds that were deposited in shallow marine and fluvial environments. Populations of detrital zircons in Lower Cretaceous conglomerate beds in southern Lhasa have U–Pb ages between 140 and 150 Ma, 500 and 600 Ma, and 850 and 950 Ma, and provide a maximum depositional age for these units of 143 ± 2 Ma. Both the modal composition and detrital-zircon U–Pb ages of the Lower Cretaceous conglomerate exposed in northern and southern Lhasa suggest different source areas, diachronous deposition, and possibly distinct genetic histories. Throughout most of the Lhasa terrane, the Lower Cretaceous clastic strata are overlain by a widespread limestone of Aptian–Albian age that was deposited in a shallow carbonate sea containing rudist patch reefs and muddy inter-reef zones. With respect to the tectono-sedimentary setting of the Lhasa terrane during Early Cretaceous time, the sedimentological and stratigraphic data are most consistent with a peripheral foreland basin model, which is interpreted to have resulted from the collision between the northern margin of the Lhasa terrane and the southern margin of Asia (the Qiangtang terrane). Several characteristics of the Aptian–Albian succession can be attributed to a peripheral foreland basin setting, although deposition within the region may have been influenced by a combination of mechanisms. Sedimentary characteristics of Lower Cretaceous rocks in the Lhasa terrane are consistent with recent ideas suggesting that portions of southern and central Tibet were deformed and above sea level before the Indo-Asian collision.