歧口凹陷及周缘新生代构造的成因和演化

FORMATION AND EVOLUTION OF THE CENOZOIC TECTONICS WITHIN AND SURROUNDING THE QIKOU SAG

歧口凹陷及周缘构造带发育不同方向的新生代断层 ,主要包括NE、NNE、NEE、近EW和NW向等 ,从运动学平衡角度推测这些断层均应不同程度地表现为具走滑分量的正断层或上盘斜落的走滑断层。本文提出一个双动力过程模式来解释歧口凹陷及周缘构造带的形成和演化。始新世时主要发生NWW—SEE向区域裂陷伸展 ,形成NE—NNE向正断层和NEE—近EW向传递断层 ;渐新世时 ,受纵贯研究区的NNE向深断裂右旋走滑的影响 ,叠加了SN向的局部伸展 ,形成大量NEE—近EW向盖层正断层。晚第三纪时NNE向区域性伸展作用基本停止 ,深断裂仍继续右旋走滑活动 ,引起盆地区断层进一步活动。

The dominate fault structures during the Cenozoic within and surrounding the Qikou Sag were NE , NNE , NEE , approximately EW and NW striking. The faults should be strike slipping normal ones or upper wall oblique dropping strike slip ones deducing from the kinematic balance even if all of the faults showed an appearance of normal ones. A model of two dynamic processes was represented for interpreting the formation and evolution of Cenozoic structures in the Qikou area. The Qikou Sag was a taphrogen created in the hanging wall of the Cangdong fault, a crust scaled detachment fault, where a burried NNE striking deep fracture developed along the west coastline of the Bohai Bay. The appearance of normal faults with varied trending were caused by the local S N extension resulted from the right lateral strike slipping of NNE trending deep fractures which were superposed the rifting related regional NWW SEE extension. The extensional epochs of two dynamic processes varied in time and space, so that the strike varied fault zones had different kinematic evolution. The regional NWW SEE extension dominated the structural kinematics during Eocene, therefore, the NNE NE trending faults were extensional ones and the NEE EW trending faults transferable or transtensional ones. The bi extension dominated the structural kinematics during Oligocene, accordingly, all of the Cenozoic faults of varied trending had an appearance of normal faulting. The local S N extension dominated the structural kinematics in Neogene, and many NEE EW trending normal faults developed within the Qikou Sag.