横穿“挑战者深渊”的深部地壳结构及蛇纹石化程度的研究

The Challenge Deep at the southern part of Mariana subduction zone has a typical trench-arc-basin system. It is the deepest trench in the world. Its deep crustal structure not only contains important evolution history of subduction zone, but also is an important area to investigate the interaction between lithospheric composition and dynamics. However, the composition in crust, water content in the incoming plate and response mechanism between plats in the subduction zone are still unclear because of the tremendous depth here. In 2017, we collected one active-sourced Ocean Bottom Seismometers (OBS) survey line (TS2017-2) across the whole trench-arc-basin system at the Challenger Deep. Based on these data, we will acquire P- and S- wave crustal structure along the OBS profile by using forward and inverse modeling, our study focus on: (1) determine the composition of crust at different tectonic units through detailed P- and S-wave velocity structure in the crust and upper mantle of the subduction, (2) Quantitative estimation of the initial water content in the incoming plate through the extent and degree of serpentinized peridotite in the upper mantle. (3) the formation and response mechanism of trench between overriding plates and underlying plates. Through the above work, it is expected to provide a scientific basis for understanding of the process on the transition in lithospheric composition and the formation mechanism of subduction zone at the Challenge Deep.

位于马里亚纳俯冲带南段的“挑战者深渊”，拥有全世界最深的海沟和独特的沟-弧-盆体系，其深部地壳结构不仅蕴含着俯冲带丰富的构造演化历史，也是研究岩石圈物质循环与相互作用的天然实验室。然而受限于海沟的极端水深条件，该区域的地壳物质属性、俯冲板块初始水含量和海沟形成机制仍不清楚。本项目基于2017年采集的横穿“挑战者深渊”沟-弧-盆等构造单元的主动源海底地震仪测线（TS2017-2），我们使用正、反演模拟的方法力求获得沿测线的纵、横波速度结构，重点研究（1）俯冲带地壳和上地幔纵波和横波速度结构，确定不同构造单元内的岩石物质属性；（2）俯冲带地幔蛇纹石化的范围和程度，计算进入俯冲系统的初始水含量；（3）板片之间的构造响应，分析海沟的形成机制。以上工作将为我们进一步认识“挑战者深渊”俯冲带的物质循环过程和海沟形成机制提供科学依据。

马里亚纳俯冲带南段拥有全球最深的海沟“挑战者深渊”，显示出强烈的构造变形和独特的构造单元特征，指示了复杂的俯冲过程和构造响应。本项目基于横穿“挑战者深渊”主动源OBS深地震测线TS2017-2，通过对沿测线18个台站的数据处理，取得了如下成果和认识：（1）发现了OBS仪器内部新型时间误差，提出了针对性方法进行修复，保证了后续结构模拟的准确；（2）拾取7501个震相走时，通过正演和反演结构模拟、振幅合成和对比，获得了研究区精细的地壳和上地幔速度结构，发现马里亚纳俯冲带南段的输入板块的地壳和上地幔发生了破碎和蛇纹岩化，其程度远远高于其他俯冲带，属于典型冷俯冲带特征；基于PnP震相约束了俯冲板块上地幔内的反射界面，提出其形成与地幔橄榄岩的力学性质有关；结合弧前深部结构、基底构造走向和天然地震定位，发现马里亚纳俯冲带南段复杂而有规律的构造变形与板块后撤差异有关，指出这可能是形成全球最深点的重要影响因素；（3）对邻近的TS2016-2测线的数据处理，获得了卡洛琳海底高原附近横跨马里亚纳海沟的地壳结构。发现输入板块的地壳厚度达到13 km，基底断裂方式与洋壳俯冲（TS2017-2测线）完成不同，代表了高原俯冲下岩石圈强度和构造响应的变化。获得的结果反映了板块之间复杂的构造变形，为研究马里亚纳南段俯冲带的动力过程提供重要依据。