马尼拉俯冲带北段增生楔前缘构造变形和精细结构

摘要

马尼拉俯冲带是南海的东部边界,记录了南海形成演化的关键信息,同时也是地震和海啸多发区域.本文利用过马尼拉俯冲带北段的高分辨率多道地震剖面,分析了研究区内海盆和海沟的沉积特征,精细刻画了区内增生楔前缘的构造变形、结构以及岩浆活动特征.研究区内增生楔下陆坡部分由盲冲断层、构造楔和叠瓦逆冲断层构成,逆冲断层归并于一条位于下中新统的滑脱面上,滑脱面向海方向的展布明显受到增生楔之下埋藏海山和基底隆起的影响;上陆坡的反射特征则因变形强烈和岩浆作用而难以识别;岩浆活动开始于晚中新世末期并持续至第四纪.马尼拉俯冲带北段增生楔的形成时间早于16.5 Ma,并通过前展式逆冲向南海方向扩展;马尼拉俯冲带的初始形成时间可能在晚渐新世,而此时南海海盆扩张仍在持续.南海东北缘19°N-21°N区域为南海北部陆坡向海盆的延伸,高度减薄的陆壳的俯冲造成马尼拉海沟北段几何形态明显地向东凹进.%The Manila subduction zone is the eastern boundary of the South China Sea(SCS),which records key information on the evolution of the SCS,also is an area with active earthquakes and tsunamis.Using high resolution multi-channel seismic data across the northern Manila subduction zone,this paper characterizes the tectonic deformation and structure of the frontal accretionary wedge as well as magmatism and sedimentary features of the oceanic basin and trench in the subduction zone.Results show that the lower slope of accretionary wedge consists mainly of imbricated thrusts,structural wedges and blind thrusts that act as frontal faults,whereas the internal architecture of the upper slope is difficult to identify because of intense deformation and magmatism.All the thrust faults merged into a lower Miocene decollement which cuts off the middle Miocene strata and can be traced upward into the Pliocene strata,indicating that the fault is influenced by local buried seamounts and basement uplift below the accretionary wedge.Magmatic activities lasted from late Miocene to Quaternary in the oceanic basin,trench and accretionary wedge in the study area.We suggest that the accretionary wedge of the northern Manila subduction zone begun to thrust before 16.5 Ma and is still propagating to the SCS through piggyback thrusting at present.The Manila subduction zone probably initiated in the late Oligocene when the oceanic basin of the SCS continued to spread.The region of hyper-extended continental crust extends from the continental slope to the oceanic basin between 19°N and 21°N in the northeastern SCS.The subduction of this hyper-extended continental crust resulted in a significant variation in the geometry of the northern Manila trench,which shows a concave eastward pattern to the Luzon Arc.