The Early Cretaceous Deposit And Tectonic Evolution Of Liupanshan Basin

With intense tectonic activities and huge environmental effects the Tibetan Plateau is the one of the most important area to test the theory of continental dynamics and the mechanism of global climate change. In order to well understand the formation of Cenozoic tectonic-environmental pattern, there is an urge need to understand the tectonic and environmental background prior to the collision between Indian and Asian Plate. Furthermore, the formation process of continental tectonic-environmental differentiation from east to west and its dynamics gradually has been paid to lots of attention. The Liupanshan region, an unique tectonic location, which located in the northeastern margin of Tibet Plateau and the northern section of the north-south belt of Helan-Hengduan Mountain, is the key position to decipher the tectonic deformation of Tibet Plateau and structure differentiation from East to West in China after the collision of Indian and Asian Plate and further discuss the dynamics in East Asia since Mesozoic. In this study, the tectonic evolution of Liupanshan Basin prior to the collision of Indian and Asian Plate is discussed based on the research of Cretaceous deposition, magnetostratigraphy, paleomagnetic data jointly with the Basin's tectonic style, thus it is of great significance to understand tectonic evolution of the northeastern of Tibet Plateau and the formation process of continental tectonics and environmental differentiation in the China continent.In Liupanshan Basin, Cretaceous has been only exposed the lower Cretaceous Liupanshan Gr., which could further divided into Sanqiao Formation, Heshangpu Formation, Liwaxia Formation, Madongshan Formation and Naijiahe Formation from the bottom to the top, of which the thickness of Sanqiao Formation and Heshangpu Formation is thinning from west to east with high disposition rate, while the thickness of Liwaxia Formation, Madongshan Formation and Naijiahe Formation is also thinning to around with low disposition rate. Based on the magnetostratigraphy analysis of three sections (Huoshizhai, Sikouzi, Shixianzi) in the north of the basin and one section (Liupanshan) in the middle of the basin, we found that the age of the Liupanshan Gr. Spans from 130 to 103.06 Ma. Of which, the age of the Liupanshan Gr. in Huoshizhai, Shixianzi and Sikouzi are 130-107.56 Ma,114-106.7 Ma, 116.24-103.06 Ma, respectively.The Liupanshan Group is composed of the fan-fluvial-lacustrine sediments. There are eleven basic stratigraphic sequences which belong to the proximal alluvial fan and the distal alluvial fan, braided fluvial, delta, lakeside shallow lake and semi-deep lacustrine respectively. By analyzing the deposition thickness change of each Formation and paleocurrent and the composition of gravels in the Sanqiao Formation and Heshangpu Formation, the results show that narrow width of the south basin and wide width of the north, north-south distribution was narrow like band. The basin area was smaller in the initiation than the period of expansion. The sedimentation has a positive cycle of deposition, the lower coarse particle and the upper small particle. Deposition rate is 5-31cm/kyr at Sanqiao Formation and Heshangpu Formation and is 1-4cm/kyr at Liwaxia Formation thorough Naijiahe Formation. Paleocurrent direction is general towards to the center of the basin and early subsidence center is located in the west, as well there are two centers-Huoshizhai and Sanqiao in the north and south of the basin. Later subsidence center continuously moved to the center of the basin. Based on analysis of paleomagnetic declination and tectonic-sedimentary characteristics, we preliminary determine that Liupanshan basin is the strike-slip extensional basin. Liupanshan basin had undergone the early strike-slip extensional subsidence stage spanning 130-125Ma, intermediate strike-slip extensional and expansion stage during 125-109.3Ma, the overall basin took place the counter-clockwise rotating. Later dying stages of lake shrank during 109.3-103. 1Ma.