OLIGOCENE - PLEISTOCENE SEDIMENTATION IN LINXIA BASIN ON THE NORTHEASTERN EDGE OF THE TIBETAN PLATEAU, GANSU PROVINCE, CHINA

Linxia Basin (current elevation=1,200 m to 2,400 m) is situated adjacent to the topographic front of the northeastern margin of the Tibetan Plateau. In this region, the plateau mainly consists of Devonian to Triassic marine deposits of the Songpan-Ganzi terrane and intermediate to felsic plutonic rocks of inferred Jurassic-Cretaceous age. A 29 million year record of sedimentation in Linxia Basin (Li et al., 1995, 1997; Fang et al., 2000) gives insight into the processes driving subsidence on the margin of the plateau. Siliciclastic sedimentary rocks of Oligocene to Pleistocene age are up to 1600 m thick and are interpreted as fluvial, lacustrine, deltaic, alluvial fan and eolian deposits. The oldest deposits in the basin are fluvial. Widespread lacustrine deposition dominates the basin fill from ~20 Ma to at least 8 Ma. Loess deposition was clearly established by 2.5 Ma, but may have begun as early as 6 Ma to 8 Ma. Analysis of depositional systems, sediment dispersal patterns, sedimentary provenance, and subsidence history suggests that sedimentation occurred in a flexural basin. Growth strata, deposited between ~6 Ma and 2.5 Ma, define the age of thrusting in Linxia Basin and the initiation of wedge-top deposition, associated with lateral growth of the Tibetan Plateau. Conglomerates overlying the growth strata are interpreted as fluvial deposits of transverse alluvial fans. Conglomerate clast compositions indicate unroofing of plutonic rocks, exposed in the fold-thrust belt on the margin of the plateau, by 3.6 Ma. The incision of Linxia basin fill began by ~1.7 Ma.