利用大地电磁和地震数据联合反演羌塘地块深部结构

Most of geophysicists think that the low-velocity and high-conductivity layer (LVCL) is ubiquitous within the crust and mantle of Qiangtang terrane. Hence, the distribution and physical attributes of the LVCL are vital to recognize the deep structures and the corresponding geodynamic process of Qiangtang terrane. However, it is difficult to estimate the resistivity and thickness of the high conductivity layer due to the well-known ‘principle of equivalence’. Therefore, many studies were conducted based on conductance, and the values of thickness and resistivity for the high conductivity layer in the crust need to be set in advance. Based on well-developed magnetotelluric (MT) and seismic traveltime forward and inversion algorithms, a joint inversion algorithm with cross-gradient as structural constraint has been achieved in this study. Then, the joint inversion algorithm was applied to MT and seismic data to reconfiguration the resistivity and velocity model of the crust and mantle of Qiangtang terrane. Reliable estimate the parameters and the distribution and connectivity of the high conductivity layer in mid-to-lower crust of Qiangtang terrane, will provide more evidence for the distribution of crust flow in the central Tibet Plateau. In addition, the characters of the LVCL will be analyzed to explore the elastic-plastic model and thermal structure model of the crust and mantle, and provide better references for deep geodynamics’ study.

羌塘地块壳幔广泛发育低速高导体已经成为多数地球物理学家的共识。准确认识壳幔低速高导体的分布形态和物性参数对于研究羌塘地块深部结构和动力学过程至关重要。然而，由于电磁响应的等值性效应，许多针对青藏高原壳内高导层动力学意义的探讨都是基于纵向电导，并在讨论过程中对高导层的厚度或电阻率值进行了假设。本课题以大地电磁及地震走时正反演算法为基础，通过交叉梯度函数耦合电阻率和速度模型，实现大地电磁与地震走时数据的交叉梯度二维和三维联合反演算法。在此基础上，对羌塘地块已有的大地电磁和地震数据进行联合反演，重构羌塘地块壳幔电阻率及速度模型，增进对岩石圈深部结构的认识；有效反演羌塘地块壳内高导层的电阻率及厚度参数，分析羌塘地块壳内高导层的分布形态及连通情况，探讨高原中部地壳流分布特征。通过分析壳幔低速高导体的分布特征，探索构建羌塘地块深部弹塑性模型及热结构模型，为高原中部深部动力学研究提供依据。

羌塘地块地处青藏高原腹地，其壳幔呈现出重力异常低值、弱磁性、P波速度低、Sn波缺失、泊松比高及Q值低、SKS波各向异性特征强烈、高导异常及地表出露大规模的新生代火山岩等特殊的地球物理特征，正确认识羌塘地块的深部精细结构对理解其特殊的地球物理特征成因和动力学过程至关重要。本项目通过大地电磁数据和地震数据交叉梯度联合反演，精确刻画羌塘地块的深部壳幔结构，有效获取壳幔岩石圈中高导层的电阻率和厚度参数，为进一步探讨羌塘地块深部动力学过程提供依据。反演结果显示，羌塘地块壳内高导层具有南北两分的特征，南、北两个相互对冲的高导层在羌中隆起附近形成一背斜形态的高导异常隆起。结合已有的地质和地球物理资料，推测羌塘地块壳内高导层与三叠纪时期沿金沙江缝合带向南俯冲的松潘甘孜混杂岩的古俯冲轨迹有关。向南俯冲的混杂岩破环了大部分羌塘地块的结晶基底，使得幔源岩浆沿构造薄弱带上涌，导致了壳内部分熔融的发生；根据Archie定律，估算羌塘地块中下地壳部分熔融百分含量，认为羌塘地块壳内富水混杂岩的存在能为班公湖-怒江缝合带两侧明显的地球物理差异特征提供合理的解释。.此外，本项目基于电磁场数值模拟的总场算法和非结构化四面体网格，采用时域有限单元法，实现了复杂形态回线源瞬变电磁法的三维正演算法。利用该算法研究了不同发射电流波形各参数（稳定时间、开启时间、关断时间和波形重复）对回线源瞬变电磁场的影响。