Pulsed growth of the West Qinling at ∼30 Ma in northeastern Tibet: Evidence from Lanzhou Basin magnetostratigraphy and provenance

The development of Cenozoic basins in the northeast margin of the Tibetan Plateau is central to understanding the dynamics of plateau growth. Here we present a magnetostratigraphy from the Lanzhou Basin, dating the terrestrial deposits from the Eocene (similar to 47 Ma) to the middle Miocene (similar to 15 Ma). The stratigraphic observation, palocurrent, and sediment provenance analysis suggest that the Lanzhou Basin (subbasin of the Longzhong Basin) probably initiated as a topographically enclosed depression during Eocene to early Oligocene (similar to 47-30 Ma). We suspect that right-lateral transtensional deformation inherited from the Cretaceous may result in formation of the Lanzhou Basin at the Eocene. Subsequently, changes in paleocurrent, sandstone and conglomerate compositions and detrital zircon provenance reflect the pulsed growth of the West Qinling at similar to 30 Ma, which triggered not only the formation of new flexural subsidence to the north of the West Qinling, but also renewed subsidence of Lanzhou Basin into the broad foreland basin system. We compare this growth history with major NE Tibet deformation and suggest that it may result from northeastward extrusion of the Tibetan Plateau due to the onset of Altyn Tagh Fault activity at Oligocene.