A Cenozoic extensional angular unconformity on the northern margin of West Qinling Mountain and its geological significance

Based upon detailed field observations and study for the angular unconformity between the Oligocene-Miocene salt-bearing red bed strata and its underlying orogenic strata in Zhangxian area of the northern margin of West Qinling Mountain and the characteristics of the conglomerates, stratigraphic sequence and sedimentary cycle of strata which cover the unconformity, the authors hold that this angular unconformity is an extensional angular unconformity developed under the crustal tensile tectonic setting. This extensional angular unconformity could provide the constraint on the tectonic attributes of the Oligocene-Miocene salt-bearing redbed basins in Zhangxian area of the northern margin of the West Qinling Mountain, that is, Zhangxian Oligocene-Miocene saltbearing red-bed basins is an extensional rift basin, which implies that the northeastern margin of the Tibetan plateau (at least the northern margin of West Qinling) might have been in an extensional tectonic environment in the Oligocene-Miocene period during the development and filling of the Zhangxian salt-bearing red-bed basin. This extensional tectonic environment was inconsistent with the crustal compression shortening and uplifting tectonic environment caused by spreading and extending of Cenozoic IndianEurasian collision to the northeast margin. It is therefore inferred that the northeastern margin of the Tibetan Plateau should not be involved in the present Tibetan Plateau tectonic system during Oligocene-Miocene.