Tectono-thermal History Of Xihu Depression, East China Sea

In light of the theroy of Prof. Zhu Xia on tectonic mobilism and basin analytical procedure of tectonic setting (T) -process (S) -response (M) system (TSM), this paper studies tile basin structure, sequence stratigraphy and erosion thickness in Xihu depression. Based on available seismic, borehole and geochemical data, the buried and thermal history as well as tectonic history of Xihu depression are reconstructed by the application of TSM basin analysis system.. This, in combination with regional onshore of the eastern coast of Fujian-Zhejiang, enabled the establishment of the tectonic setting and evolution of the Xihu depression.As other Tertiary basins in east china, Xihu depression is composed of two structural layers. The lower on the surface (T₃⁴) is fault depression made of Paleocene and mid-lower Eocene, and the upper is downwarping depression made of mid-upper Eocene to Quartenary. Four basin protototypes of Xihu depression can be distiguished: fault depression (E₁/K²-E₂^1-2), early transform plate boundary depression (E₂^2-3-E₂³), late inland depression (E₃-N₁) and back-arc depression (N₂-Q).During Paleocene to early and middle Eocene, there were NNE striking horsts in the middle and grabens on the sides in Xihu depression, the center of depression was controlled by fault but it was not very obvious and continuous along the strike, During middlle to late Eocene, the depocenter was located on the east of Xihu depression, the thickness of Pinghu Formation became thicker from west to east and was not controled by fault. In the end of Eocene the east of Xihu depression was uplifted and about 2000-3000 meters of strata was removed. Since Oligocene the eastern of Xihu depression continuously uplifted and the depocenter gradually migrated northwestward, the thickness of Huagang Formation, Longjin Formation, Yuquan Formation and Liulang Formation also migrated in a similar way. After the deposition of Liulang Formation, basin inversion resulted in the formation of anticlinal structures in the middle of X ihu depression.Thermal modling results suggest that paleo-temperature was very different between fault depression and depression stage, thermal history was related to different tectono-thermal regimes. The paleo-temperature field during the deposition of Pinghu Formation had itself special character, and it varied as the depression evolved and migrated. It was as same as the overlying depression, which belonged to a unified depression paleo-temperature evolution. The paleo-ternperature was controled by depocenter and the buried depth and the scope of equivalent paleo-temperature moved as depression migrated. The inversion of depression delayed the progress of paleo-temperature evolution but not changed the track of evolution. The elements prompted paleo-temperature were superimposed depression and regional subsidence.The tectonic setting of the formation of Xihu Depression was related to the interactions among India plate, Pacific plate and Phillipine plate, during the early stage it belonged to the convergent boundary between Eurasian plate and Pacific plate, then it evolved into transform boundary due to the Philiipine plate movement and rotation, after Philiipine plate subducted under East China Sea it evolved into back-arc extension setting. The fault depression was formed by the lithosphere extension due to mantle upwelling, as the lithosphere cooled the fault depression converted into compensated flexure depression.