Physical conditions and deep processes for intensive subsidence of the middle Bohai Sea during Cenozoic time
新生代渤海中部强烈沉降的物理条件和深部过程

Since Cenozoic time the middle Bohai Sea has been experienced intensive subsidence which results from the joint function of many physical conditions, These conditions were generated by deep processes during the rifting and post-rift periods. In the rift period the crust beneath the Bohai Sea uplifted, thinned, and ruptured extensionally, where the hot mantle materials intruded into the crust and the Moho interface rised, increasing gravity potential. In the post-rift period the cooling and contraction of the lithosphere as well as the phase change of the lower crust led to density growth. Added by increasing thickness of sediments, the crust of Bohai Sea was in the state of gravity nonbalance, implying that the downward force was much greater than the upward buoyancy force. Meanwhile east to the Bohai Sea exists the plate boundary of retreat displacement, and the lower crust beneath this area can flow outward laterally. These factors results in subsidence of the middle Bohai Sea with the largest rate and amplitude. In the early stage the subsidence was scattered and local, exhibiting a mosaic pattern of alternating depressions and swells. The late stage of subsidence was featured by a large-scale and whole motion, indicating the isostatic compensation mechanism at depth became dominant. The pre-existing Tanlu fault zone bounded the development of the basin and was reformed strongly by the subsidence of the Bohai Sea. The essential mechanism of the basin evolution is the horizontal tensile rupture and normal faulting-nearly vertical shear fracture within the brittle upper crust. Meanwhile the focal solutions of modern earthquakes and co-seismic ruptures on the ground reflect the strike-slip faulting stress state in the middle and lower crust, which is compatible and co-existing with the extension and descending of the upper crust.