南海海盆俯冲消亡机制:马尼拉海沟俯冲起始的启示

Closure mechanism of the South China Sea:Insights from subduction initiation along the Manila Trench

马尼拉俯冲带作为南海东部唯一的汇聚边界,其构造特征对于探索南海的俯冲起始机制及深化理解南海海盆从陆缘张裂、海底扩张及俯冲消亡的演化全过程等科学问题具有重要意义。最新的层析成像数据表明,南海海盆的原始范围从现今的马尼拉海沟向东仍可延伸400~500km,但前人对南海初始俯冲时间存在争议,并对其俯冲起始机制的研究相对较少。基于横跨马尼拉俯冲带地震剖面资料、火成岩的年龄及平衡剖面结果,本文对其分段特征及初始俯冲时间进行了系统分析和限定。研究发现,分布于菲律宾岛的锆石年龄数据主要集中在早中新世-中中新世时期,且平衡剖面分析结果表明南海海盆在中中新世就已经开始俯冲消减。因此,马尼拉俯冲带的初始俯冲时间为早中新世-中中新世,且由南向北变新。结合南海周缘新生代板块重建结果,马尼拉俯冲带新生代的演化可划分为三个阶段:(1)古新世-早中新世,马尼拉俯冲带作为一条大型走滑断裂带或俯冲带边界存在;(2)早中新世-中中新世,在多个板块汇聚背景下,受澳大利亚板块的快速北向运动、太平洋板块的NWW向俯冲及菲律宾海板块NWW向漂移与北巴拉望微陆块(民都洛)发生碰撞的共同作用下,导致了菲律宾海盆西缘大型走滑边界开始触发马尼拉俯冲带的形成,南海开始收缩消亡;(3)晚中新世-早更新世,随着菲律宾海板块持续向北西运动,马尼拉俯冲带的北端进入碰撞造山阶段,而南部则仍处于洋壳俯冲状态。这个过程最终形成了现今的海沟-增生楔-弧前盆地(北吕宋海槽和西吕宋海槽)-火山弧(吕宋火山弧)的地貌组合。总之,南海消亡是被动消亡,因为马尼拉俯冲带属于太平洋板块动力系统,是诱发式被动俯冲所致;与苏门答腊-班达俯冲带属于印度-澳大利亚板块动力系统的主动俯冲不同,但两者形态上构成了一个东南亚环形俯冲系统。

As the only convergent boundary in the eastern South China Sea(SCS),the Manila subduction zone is a key to understand the subduction mechanism of the SCS and to deepen the tectonic evolution of the SCS from marginal breakup,via spreading to closure.The latest tomographic images show that the original range of the SCS extends from the present Manila Trench eastward to 400~500km.However,the initial time of the SCS subduction is controversial and the subduction mechanism is poorly understood.Based on the seismic profiles across the Manila subduction zone,the ages of igneous rocks,and the results of balanced cross-sections,this paper systematically analyzes and defines the segmentation and initial subduction time.This study found that the zircon age data distributed in the Philippine Islands are mainly in the Early Miocene-Middle Miocene period,and the balanced profile analysis shows that the SCS subduction has begun in the Middle Miocene.Therefore,the initial subduction time is proposed to be from Early Miocene to Middle Miocene,and the subduction migrated from south to north.Combined with the results of the Cenozoic plate reconstruction around the SCS,the evolution of the Manila subduction zone can be subdivided into three stages:(1)During the Paleocene-Early Miocene,the Manila subduction zone existed as a large-scale strike-slip fault zone or subduction zone boundary.(2)In the Early Miocene-Middle Miocene,under the background of multiple plate convergence,due to the joint effect of the rapid northward movement of the Australian Plate,the WNW-directed subduction of the Pacific Plate,and the WNW-directed drift of the Philippine Sea Plate which collided with the North Palawan Microplate(Mindoro),the large-scale strike-slip boundary in the western Philippine began to form as the Manila subduction zone,and the SCS began its subduction.(3)During the Late Miocene-Early Pleistocene,as the Philippine Sea Plate continued to move northwestward,the northern segment of the Manila subduction zone entered a collisional orogenic stage,while the southern segment was still in a state of oceanic crust subduction.This process eventually formed the present-day trench-accretionary wedge-forearc basins(North Luzon Trough and West Luzon Trough)-volcanic arc(Luzon Volcanic Arc)geomorphological combination.In summary,the extinction of the SCS is passive,because the Manila subduction zone belongs to the dynamic system of the Pacific Plate and is caused by induced passive subduction,different from the Sumatra-Banda subduction zone triggered by active subduction of the Indian-Australia Plate dynamic system.However,both subduction zones shape the circular Southeast Asian subduction system together.