马尼拉海沟北段俯冲带输入板块的不均一性

Incoming plate variation along the northern Manila Trench

本文整合了横跨马尼拉海沟北段的21条多道地震层位信息、海底地形以及天然地震数据,分析了研究区内的输入板块性质差异及其对增生楔变形和地震活动性的影响.研究发现,沿马尼拉海沟北段的输入板块在地壳性质、基底起伏和沉积物厚度上存在明显的自北向南的差异:(1)最北段基底埋深大,上覆沉积物厚,地壳厚度较薄,地壳性质可能为初始南海洋壳或者圈闭的菲律宾海洋壳;(2)中段基底埋深浅,上覆沉积物薄,地壳厚度大,地壳属性表现为过渡壳性质,受到岩浆活动的影响,初始的地壳性质可能为华南陆块张裂分离出的微小陆壳块体,或者是南海洋壳;(3)南段基底埋深和沉积物厚度介于中间,存在明显的地磁条带,地壳性质为正常的南海洋壳.这一输入板块性质的不均一性可解释该区的特殊增生楔变形现象,如恒春弱变形带的出现,向海方向内凹的海沟形态以及上陆坡海底的大幅抬升等,同时也影响了研究区内的板片俯冲形态和发震构造的地震活动性.研究结果证实了沿马尼拉海沟北段存在南北向的地球物理性质的差异,但对于地壳属性的最终厘定还需要更多的地质与地球化学证据.

According to the differences of the sediment thickness, basement topography, crustal thickness, water content and volcanic activity of the incoming crust, which are revealed by the newly integrated bathymetric, previous seismic data and the reprocessed seismic line 973, we suggest that the incoming crust along northern Manila Trench can be divided into three areas from north to south： （1） the trapped oceanic crust with thick sediment and relatively thin crust; （2） transitional crust （？） with thin sediment, thick crust and widespread magmatism; and （3） normal SCS crust with relatively thick sediment. This incoming plate variation results in a special style of deformation along the accretionary wedge, including the formation of a weak deformation belt, the reentrant trench geometry and the huge uplift of the upper slope. The middle part may also give rise to profound change in the geometry of the subducted slab and the seismicity of seismogenic structures.