基于流体地球物理表征的新几内亚-所罗门弧俯冲起始动力学机制

What is the driving force for the plate subduction initiation? How does the subduction dynamic process evolve? These core issues of classical plate tectonics theories still remain unresolved. Metasomatic dehydration caused by the subduction subsidence of Water-bearing oceanic crust can initiated fluid vermiculation. The interaction between the migrated fluid and the mantle triggers the upwelling of the island arc magma. However, how does the migration of fluid (water) in the plate subduction system control the initiation and movement of subduction? In order to answer these questions, we intend to carry out the stereoscopic combination geophysical exploration of the subduction system in the New Guinea-Solomon Arc, as well as to find out the deep structure and plate subduction morphology. The parameters inversion of fluid enrichment in each layer by geophysical information is explored. The fluid distribution characteristics of subduction system are studied. The subduction dynamic process is constrained by numerical simulation to reproduce the subduction tectonic activity and fluid variation in time and space. Based on analyzing the dynamic environment of intraplate subduction, comparing the northern Indian plate continent-continent subduction to the eastern ocean-ocean subduction, and integrating the knowledge of geochemistry, magmatic petrology and paleomagnetism, we try to reveal the dynamic mechanism of the subduction initiation of the New Guinean-Solomon arc. Our study area is located in the west side of the Western Pacific Warm Pool, where is an active area of material and energy exchange between the Western Pacific Ocean and the Indian Ocean. This study also provides basic data for the core scientific issues (I) and (II) of the Western Pacific major research project.

板块启动俯冲的驱动力是什么？俯冲动力过程是如何演化的？这些经典板块构造的核心问题一直悬而未决。富水洋壳板片俯冲下沉出现交代脱水引发流体蠕散，迁移的流体与地幔相互作用触发了岛弧岩浆上涌，但流体（水）在板块俯冲体系的运移是如何制约俯冲起始和运动的呢？为了回答这些问题，我们拟在新几内亚-所罗门弧开展对俯冲体系的立体组合地球物理探测，搞清其深部结构和板片俯冲形态；探索用地球物理信息表征流体在各圈层富集的参数反演，研究俯冲体系流体展布特征；对俯冲动力过程约束数值模拟，再现俯冲构造活动和流体时空变化；通过剖析当下板内俯冲动力环境、对比印度板块北部陆-陆俯冲和东部洋-洋俯冲，综合地球化学、岩浆岩石学、古地磁学等多学科认识，揭示新几内亚-所罗门弧俯冲起始的动力学机制。研究区位于西太平洋暖池西侧，是西太平洋-印度洋物质能量交换的活跃区，本研究也为“西太”重大研究计划的核心科学问题（一）和（二）提供基础资料。

本项目以西太平洋巨型构造体系中双向俯冲、多期反转的新几内亚-所罗门（PN-SL）俯冲带为研究对象，通过搭载共享航次采集处理了1300km的深水深层反射地震、8台主动源海底地震仪（OBS）和7台海洋大地电磁仪（OBEM）的地球物理立体组合观测数据，结合收集处理反演的480km深水反射地震数据，获得了PN-SL 俯冲体系深部壳幔结构和流体展布特征。据反射地震、二维热力学模型模拟分析，可知翁通爪哇海台与所罗门洋盆汇聚触发年轻洋盆发生俯冲，提出水平挤压力驱动了年轻洋盆热板块发生起始俯冲，而高密度古老海洋板块则在含流体（含量≥0.2wt%）的构造薄弱带诱发起始俯冲；以Ma为窗口发现地球围绕太阳公转轨道偏心率增大周期、太阳系穿越银道面周期与“西太”伊豆-博宁俯冲带、马里亚纳俯冲带、汤加俯冲带的起始俯冲时间和青藏高原快速隆升时间同步，则提出了太阳系尺度轨道变化导致的板块起始的科学猜想。大地电磁和深水反射地震观测到沿PN-SL俯冲带大量流体进入岩石圈超过50km，引发水岩作用、岩浆活动；所罗门洋盆扩张过程形成伸展构造成为流体向深部运移的极佳通道，反射地震成像显示海水沿大量成盆断裂可向下渗入至3~5km，流体的加入改变了俯冲板的流变强度、使岩石圈有效弹性厚度减小1/3，进而影响板块俯冲动力过程。反射地震和OBS台站观测到PN-SL俯冲体系中全球最大的火成岩省翁通爪哇海台洋壳约18~20km，由120Ma和90Ma两期喷发的层状展布玄武岩构成，两期玄武岩体积各占3/4和1/4，翁通爪哇海台后方没有出现新的俯冲带；伴随着PN-SL俯冲极性反转，其弧前、弧后依岛弧出现断陷-箕状对称盆地的构造样式。