Cenozoic Tectonic Deformation And Evolution Of The Northeastern Margin Of The Tibetan Plateau: A Case Study Of Longxi-Wushan Region

The continental collision between India and Eurasia and the successive indentation of the Indian continent into the northern area during the Cenozoic era not only created the Tibetan Plateau, but also caused the widespread tectonic deformation over several thousands of kilometers in western China and middle Asia. The questions like how the tectonic stress from the plate boundary, which is several thousands of kilometers away, is transferred, and how the deformation of the Tibetan Plateau evolves in spatial and temporal domain remained to be answered, which are relevant to continental dynamics and intra-continental tectonic deformation. The uplift of mountains, the formation and evolution of basins in and around the Tibetan Plateau in the Cenozoic time are predominately controlled by the Cenozoic continental collision. If the style and time of deformation and the tectonic evolution history in the Cenozoic era in the surrounding regions of the Tibetan plateau can be determined, the dynamics and the tectonic stress transfer mechanism can be inferred, and most important constraints can be provided to the study of the spatial and temporal evolution of the Tibetan plateau.An ideal area for relevant studies is the northeastern rim of the Tibetan plateau which is the latest and growing part and characterized by strong extensive tectonic deformation in late Cenozoic. A wide intracontinental basin developed there since Paleogene, and thick terrestrial deposition formed until late Cenozoic time when it was subjected to compressional deformation and uplifted. No only did the developing history of the basin reflect the forming and expanding process, but its deposition would record the relevant tectonic and depositional situation due to the uplift of the Tibetan plateau. These deformational and depositional records will help us restore the tectonic evolution history of this area and provide important information about the deformation process of the Tibetan plateau.The West Qinling Northern Front fault (WQNF) is situated at the frontier of the northeastern part of the Tibetan plateau. The development of the Cenozoic Tange basin (upper basin), Wushan-Zhanxian basin (middle basin) and Longxi basin(lower basin) along and on both sides of WQNF is closely related to the evolution of this fault belt. These three basins are chosen as the research areas. By detailed studies on the deposition and deformation, the attribute, evolution history, and mutual relationship among these basins are determined, and the Cenozoic tectonic history of WQNF and its tectonic status as part of the northeastern boundary of the Tibetan plateau are analyzed. A conspicuous Eocene angular unconformity is discovered in the Tange basin, which, combined with other evidences, shows that a strong tectonic movement characterized by nearly NS contractional deformation occurred at the middle Eocene in the NE part of the Tibetan plateau.The spatial-temporal evolution of the tectonic movement and the uplift of the Tibetan plateau since the early Cenozoic era and some relevant dynamic mechanism are discussed on the basis of the data acquired in this work and previous studies. The main conclusions are as follows:1) Tange basin-deposition, stratigraphy and tectonic evolutionThere is no Neogene system in this basin, and all the Cenozoic strata are paleogene. including three sequences from bottom to top. Sequence I is composed of conglomerate, sandstone and clay deposited in an alluvial-fluvial-shallow lacustrine system during Paleocene and early to middle Eocene epoch. Sequence II is mainly consisted of conglomerate and sandstone deposited in a braided river system during middle to late Eocene epoch, of which the typical rock is the red Mali sandstone exposed near the small town Mali. Sequence III is a series of alluvial conglomerate deposited during Oligocene.The above mentioned three sequences are separated by two unconformities. The lower one between sequence I and II is an obvious angular unconformity. The differences in the rocks, depositional environments and the deform...