大地电磁与地震面波联合反演研究拉萨地块深部壳幔结构

Integrated geophysical study is a necessary way to solve complex geological problems. As the key of the integrated geophysical study, joint inversion can effectively couple and satisfy different geophysical data at the same time using cross-gradient structural constraints. In this study, Magnetotelluric (MT) and Seismic surface wave methods are used to study the deep structures. MT plays an important role in studying the electrical structures and the rheology of the continental lithosphere, and the Seismic method is one of useful tools to constrain crust-mantle structure. The joint inversion of MT and Seismic data can give full play to their respective advantages, reducing inversion ambiguity and obtain accurate and reliable crust-mantle structure model. The Lhasa block, located in the central south of Tibetan plateau, is a giant tectonic-magmatic belt which recorded the whole deep process of the subduction of the oceanic plate and collision of the continental plates. But the tectonic evolution and the dynamic process of the Lhasa block are still unclear and controversial. We invert jointly the collocated MT data and Seismic surface wave to obtain the high resolution crust-mantle structure of the Lhasa block using cross-gradient method. The joint inversion results allow us to comprehensively study the mechanism of the rifts, shallow-deep structure relationship and the dynamic process of the Lhasa block from the perspectives of conductivity, velocity, rheological properties and physical relations.

综合地球物理研究是解决地下复杂地质问题的必要途径，联合反演是综合地球物理研究的关键环节，通过交叉梯度法可有效实现不同地球物理方法的联合反演和综合研究。在地球深部探测中，大地电磁法是研究大陆岩石圈电性结构和推演流变性的重要方法，地震面波则是认识地壳上地幔结构的重要信息来源，两者的联合反演可充分发挥各自方法的优势，减少反演的多解性，获得准确可靠的深部壳幔结构模型。位于青藏高原中南部的拉萨地块，是一条巨型的构造-岩浆岩带，记录了大洋板块俯冲消减和陆陆碰撞汇聚的深部过程。但是拉萨地块的深部构造演化仍存在不同的认识和争议。本项目拟利用位于拉萨地块中部同测线采集的大地电磁数据和天然地震数据，通过大地电磁与地震面波交叉梯度联合反演，获得高分辨率的拉萨地块深部壳幔结构，旨在从电性、地震波速以及流变性、物性关系等多个角度综合研究裂谷形成机制，为拉萨地块的深浅构造关系和动力学过程提供新的证据与约束。

藏南南北向裂谷系作为青藏高原晚新生代以来最显著的伸展构造样式之一，是研究高原生长机制的重要窗口。本项目主要采用大地电磁方法，结合地震等地球物理结果，分别揭示了藏南定结-申扎裂谷和错那-沃卡裂谷等区域深部结构特征，为探索青藏高原伸展构造形成机制提供新证据和约束。.大地电磁和天然地震结果显示，定结-申扎裂谷下方存在显著的高导中下地壳和较高速上地幔结构，表现为弱地壳和强地幔特征。而高原中部的共轭走滑断裂区域则显示较低电导的中下地壳和较低速上地幔结构。同时，错那-沃卡裂谷北部下方也发现明显高导异常层。分析认为这些普遍存在的高导异常主要为地壳部分熔融结果，结合地质资料和数值模拟结果，研究认为弱地壳在藏南裂谷形成过程中扮演了关键角色，即在印度-欧亚大陆持续斜向汇聚作用下，弱地壳通过解耦上地壳和下部岩石圈，促进了高原上地壳变形分解，进而发育了藏南裂谷系。