渤海中部郯庐断裂带的近期活动与渤海新近纪新生断裂

渤海中部的郯庐断裂带在平面上表现为不连续的几条北北东走向断层,地震反射剖面和钻探资料显示断层两侧沉积厚度的巨大差异,表明新近纪以来它们是在沉降运动背景下活动的具有大幅度倾滑位移的正断层,构成渤海盆地内凹陷与凸起的边界。这些断层在剖面上有一定弯曲和倾斜,向下延伸深度不超过10~12km。在此深度以上的地壳浅部,没有水平方向位移以及其它直接变形证据表明郯庐断裂带有走滑运动分量。从地震机制解得到的走滑断层运动反映渤海地区地壳深部的变形及相应的构造应力状态,与浅部残留的伸展构造应力同时存在。根据断裂力学分析,认为中新世末以来渤海浅层新近系内出现的大量近东西向小断裂可能是现代构造应力场作用的结果,与郯庐断裂带或其它基底老断裂没有继承性或派生的成因关系。

RECENT ACTIVITIES OF THE TAN-LU FAULT ZONE IN THE CENTRAL BOHAI SEA AND NEWLY GENERATED FAULTS DURING NEOGENE IN BOHAI

When the NNE trending Tan-Lu fault zone passes through the central Bohai Sea, it manifests as several discontinuous faults in the plane view. Data of seismic reflection and drilling revealed great differences of sediment thickness on the both sides of each fault, indicating normal faulting with large amplitudes in a subsiding tectonic setting since the Neogene. These faults constituted boundaries between depressions and uplifts in the central Bohai Sea. In the vertical profiles, these faults had flexure and tilting of some degrees, extending downward to 10~12 km depth. In the shallow portion of the crust above this depth, there was no direct evidence such as horizontal displacements and characteristic deformation showing strike-slip of the Tan-Lu fault zone as previous studies suggested. The strike-slip faulting inferred from focal mechanisms of earthquakes represented the deformation and stress state at depth, which existed together with the residual extensional stress in the shallow crust. Based on analysis of fracture mechanics, it was suggested that the large number of small fractures, which were roughly EW striking in the upper Neogene System of the Bohai Sea, were likely generated by the recent tectonic stress field. They might not be related with the Tan-Lu fault zone or other old faults in the basement in an inheritance or derivative manner.