Abstract

The period following Late Jurassic continental collision but preceding Late Cretaceous–Cenozoic arc magmatism represents a significant, yet poorly understood tectonic transition in central Tibet, western China. The Asa basin (31°00′–32°40′, 84°30′–87°05′) in central Tibet, which crosses the southern margin of the Qiangtang block, the Bangong-Nujiang suture zone, and the northern half of the Lhasa block, is characterized by up to 5000 m of continental-marine clastic-carbonate sediments and is ideally positioned to provide constraints on this transition. Sedimentary and volcanic data from recent field studies suggest that contraction controlled Berriasian-Valanginian tectonics, whereas back-arc extension may have controlled Hauterivian–Early Cenomanian basin evolution.

Berriasian-Valanginian strata occur over the northern half of the Asa basin (approximately north of 31°40′) and unconformably overlie a varied basement. Sediment transport and fining directions are generally toward the south. The sandstones (Q78F11L11, Qm72F11Lt17, Qp38Lvm8Lsm54) in the central Bangong-Nujiang suture zone are dominated by monocrystalline quartz grains with undulose extinction but also contain abundant chert grains and some ultrabasic lithic fragments and radiolarian fragments. Hauterivian–Lower Barremian strata are characterized by siliciclastic rocks deposited in high-energy tidal environments, and barrier-bank skeletal bioclastic grainstone and wackestone. Intraformational limestone breccia or conglomerate and bimodal volcanic rocks are in places found within the sedimentary sequences in the southern margin of the basin (approximately south of 31°35′). Sandstones (Q54F20L26, Qm45F20Lt35, Qp20Lvm59Lsm21) are characterized by complex derivations from both recycled orogen and volcanic arc provenances. Widespread thick carbonate is a characteristic feature of Upper Barremian–Lower Cenomanian lithofacies, possibly formed in both restricted platform and platform interior environments. Lithic fragments in the sandstones (Q34F24L42, interbedded Qm33F24Lt43, Qp3Lvm87Lsm10) with the carbonates predominantly reflect volcanic source areas (65%).

The Berriasian-Valanginian sequence is interpreted to represent molasse sediments from a northern orogen and indicates that the collision between the Qiangtang and Lhasa blocks could have extended into the Valanginian. The change from orogenic to magmatic arc source areas for sandstones during the beginning of the Hauterivian, along with a coeval extensive marine transgression in central-southern Tibet, normal faulting, and rich intercalations of bimodal volcanic rocks, are interpreted to be related to back-arc extension of the Gangdese arc during the Middle Cretaceous (ca. 120–95 Ma). This may have occurred in response to a rollback of the Tethyan oceanic slab. During the Middle Cretaceous, the entire southern Eurasian margin could have been characterized by a series of back-arc rift basins (including those in Kohistan and Ladakh). Some basins may have been floored by oceanic crust whereas others remained underlain by continental or transitional basement, much like the present western Pacific margin.

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