【摘要】 新生代时期渤海中部的强烈沉降,是多种物理条件的共同作用结果,这些条件是由裂谷期和后裂谷期的深部过程产生成的.裂谷期在异常热地幔背景下的渤海地壳隆起、减薄、张裂,地幔热物质上升侵入地壳,莫霍界面位置升高,积累了重力势能;后裂谷期岩石圈(层)的冷却、收缩及下地壳的相变导致密度增大,加上巨厚沉积物的持续增生,使地壳处于重力不平衡状态,向下的垂直力远大于向上的浮托力,同时还有东部后退位移性板块边界和下地壳侧向流动的支持,使渤海中部成为下沉速率最快、沉降幅度最大的凹陷盆地.后裂谷期的早期的盆地下沉具有分散、局部性特征,表现为多个凹陷和凸起交替组合格局;晚期转变为大范围的整体沉降,显示重力均衡和补偿过程是从浅往深发展的.先存的郯庐断裂带对沉降的空间范围有局部边界控制性作用,其本身可能受到盆地发展的强烈改造影响.盆地的基本变形机制是上地壳的水平向脆性张破裂和垂直向或近垂直向的正断层-剪切破裂,地震震源机制解和大地震时的地表破裂表现的水平错动反映中、下地壳的走滑-平移型应力状态及相应的瞬间水平剪切破裂,它与上地壳残留的伸展、下沉相容并存于三维地壳体内.更多还原

【Abstract】 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 non-balance,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.更多还原