Late Cenozoic Tectonic Evolution In The West Qinling Areas

The Tianshui basin is located in the northeastern margin of the Tibetan Plateau, and the junction of the N-S-striking Helanshan-Liupanshan structural belt and the Qinling-Dabie Orogen, which is cut through by the north frontal fault of the Western Qinling. This basin was filled with the Late Cenozoic sediments that recorded the deformation process of the area and bore important implications for the northeastern expanding of the Tibet Plateau. On the basis of field structural measurement and analysis, coupling with previous chronologic and sedimentologic studies, we suggest tentatively a deformation process and tectonic stress field sequences in this basin, and further, rebuild the Late Cenozoic tectonic evolution history, and further discuss the sequence, pattern and the characteristics of coupled relationship between deep and shallow structures.1. A three-stage tectonic evolution sequences are proposed as follows:The first episode (the Early Miocene to Late Miocene) presented by the formation of sedimentary basin due to NW-SE extension, which was accompanied by the eruption of alkaline ultramafic volcanics. The subsequent NE-SW compression, which is likely to be related to the northeastward growth of the Tibetan Plateau, led to the basin inversion in the Late Miocene-Early or middle Pleistocene. Finally, a transpressional regime(nearly N-S extension and E-W compression)became dominated in this region since the Late Pleistocene, which resulted in the limited eastwards extrusion with clockwise rotation intrigued by the rotation of regional shortening direction.2. The latest ruption age of Lixian volcanic rocks is accurately restricted by optical luminescence dating (OLD) age of loess which located at the above and under the volcanic ash, indicating the strong deformation and the concrete response of material flowing of northeastward expansion of Tibetan Plateau in West Qinling.3. The West Qinling region begun to be affected by the northeastern expanding of the Tibet Plateau, which led to the first eruption of Lixian volcanics and the rise of East Asian monsoon; the material diffusion of Tibet Plateau started to significantly impact the West Qinling areas at ～10Ma, resulting in the Tianshui basin inversion; Since the late Pleistocene, these faults occuered differertly slip, resulted in the limited eastwards extrusion with clockwise rotation intrigued by the significant growth of Tibet Plateau again.