南海东部现时地壳运动、震源机制及晚中新世以来的板块相互作用

Present-day crustal movement and focal mechanism solutions,and plate interaction since late-Miocene in the eastern South China Sea

南海东部的板块汇聚带是了解南海和菲律宾海晚中新世以来构造演化的一个重要窗口.针对这一区域地壳运动的研究,获得了该区内微块体的现时地壳运动特征及其动力机制,在此基础上结合俯冲板片形态和震源机制资料提出了该区晚中新世以来的板块汇聚作用特征.研究发现:晚中新世晚期,菲律宾海板块西缘在南、北部的西向运动均受到限制的情况下,位于中间区域的吕宋岛北部则由于其西侧相对自由而继续往NW方向运动.该过程中,菲律宾大断裂等汇聚带内部的走滑断裂对于协调不同块体之间的地壳运动速度差异有着重要作用.受南、北阻挡的影响,中间部分的西向运动速率呈现出中间大(吕宋岛北部)两端小的特征.因而北吕宋西侧的马尼拉海沟也以相对于南部更快的速率不断向NW迁移.然而俯冲的南海岩石圈受其下方SE向地幔流的影响,未能发生相应的俯冲板片后撤,而是在两板块之间的直接接触面形成强烈推挤并发生反向弯曲.结合这一板块作用特征认为,马尼拉海沟现今的构造形迹是在上述背景下北段多次向NW方向变迁形成的,而双火山弧的形成则主要是由菲律宾海板块在吕宋岛弧南、北部的西向运动速率差异引起的.

The plate convergent belt in the eastern South China Sea （SCS） is an important window for studying the tectonic evolution of the SCS and the Philippine Sea （PhS） since late- Miocene. We studied the plate interactions of this area since late-Miocene based on block kinematics, morphology of the subducted SCS slab and focal mechanism solutions （FMS）. At the late stage of the late-Miocene, the westward motion of the PhS plate was first impeded by its collision with the Palawan micro continental plate in the south, and then by the collision of the Luzon arc with the South China continental margin in the north. Consequently, the middle part between the two collision zones became a passage where westward movement of the PhS plate continued relatively smoothly. Because of these two impediments, the rate of westward motion increased northwards to its maximum in the northern Luzon and then decreased northwards, leading to the westward migration of the Manila trench. The subducted SCS slab, however, did not roll back accordingly due to the SE-directed mantle flow. As a result, the subducted slab suffered strong push and slightly reverse bending of the subducted slab occurred in the contact face between PhS and SCS lithosphere. According to this model, morphology of the Manila trench was shaped by the differential westward motion rates from south to north of the northern Luzon, and the width of the subducted slab played little role. Furthermore, the double arc in the north Luzon arc was also induced by a differential westward moving velocity along the arc. A shear zone whose location coincides with the east arc might have facilitated its formation.