柴达木盆地北部岩石圈电性结构研究

A NE-SW trending magnetotelluric (MT) profile is proposed to be deployed from Dangjin Shankou to Mangwu in northern Qaidam Basin on the northern margin of the Tibetan Plateau. The proposed profile is roughly perpendicular to the thrust fault systems on the western and eastern margins of the Qaidam Basin, as well as the Youshashan and Lenghu anticlinorium systems within the Basin. 20 broadband and 5 long period MT stations are designed along the ~250km long profile with average station spacing of ~10km. Original time series data is processed with regular robust routines. Dimensionality and regional strike direction are determined for the dataset through data analysis. Final model of electrical structures of the study region is obtained on the basis of 2D and 3D inversions. Joint interpretation of the electrical model is made in combination with other geophysical and geological models for the same region. The following scientific issues will be investigated for the Qaidam Basin: pattern of tectonic deformations of the crust and upper mantle beneath the Basin, formation mechanism of the Qaidam Basin, as well as the lithospheric background of tectonic differences between western and eastern Qaidam Basin. This study will provide new evidence and constraint for deep rheology properties and to distinguish between competing tectonic models for the northern Qaidam Basin from the perspective of electrical properties. MT data collected by this project will be further integrated with the MT data from INDEPTH-IV 4000 and 5000 lines to conduct 3D inversion study on a broader scale to investigate the tectonic patterns on a whole for the northern margin of the Tibetan Plateau. 2-3 SCI papers are planned to be published under the auspices of this project.

在青藏高原北缘柴达木盆地北部由当金山口至茫屋布设一条近北东-南西向的大地电磁测深剖面。剖面近垂直穿过盆地西缘及东缘的逆冲断裂系及其内部的油砂山复背斜、冷湖复背斜等构造体系。剖面长约250km，设计20个宽频及5个长周期测点，平均点距10km。对采集的原始数据通过精细处理、维性与构造走向分析、二维及三维反演等一系列技术手段获取研究区可靠的电性结构模型，并结合其他地质与地球物理资料对电性结构模型进行综合解释，探讨柴达木盆地北部地区的壳幔构造变形样式、盆地的成因机制，以及柴达木盆地东西向构造差异的深部背景等重要科学问题，为研究该区域深部流变结构及变形机制以及判别不同的构造模型从电学性质的角度提供有力的约束与佐证。进一步将本项目数据与INDEPTH-IV 4000及5000线MT数据相结合进行整体三维反演研究，对青藏高原北缘整体的构造模式从电性的角度提供新的认识。在此基础上发表2-3篇SCI文章。

柴达木盆地作为青藏高原北缘地区一个独特的地质构造单元，在约束青藏高原北缘的隆升以及控制高原北缘、东北缘的横向生长方面都发挥着至关重要的作用。然而，柴达木盆地地下地质条件十分复杂，后期构造运动改造作用强烈，断裂非常发育。加上地表条件恶劣，勘探程度低，许多基本地质问题都有待深入研究。为了更好地理解研究区内的岩石圈深部结构与变形机制，本研究于柴达木盆地北部完成了一条近北东-南西向的大地电磁测深剖面。剖面北东端起始于当金山口附近，向西南方向延伸至老茫崖附近，以近垂直的走向穿过了柴达木盆地西缘及东缘的逆冲断裂系以及盆地内部的油砂山复背斜、冷湖复背斜等构造体系。剖面全长约250km，共计完成测点25个，包括20个宽频测点及5个宽频-长周期联合测点。经过数据精细处理获取了高质量的张量阻抗数据，利用阻抗张量G-B分解等分析手段得出了研究区内的区域构造走向，在此基础上通过综合对比二维/三维反演结果获取了研究区岩石圈电性结构模型。进一步利用抽样统计分析的方法构建了能够定量反映反演模型可靠性的标准差分布模型，为反演模型可靠性的评估提供了定量的依据。在获取电性结构模型的基础上，结合电导岩石学高温高压实验给出的上地幔矿物电导率-温度依赖关系，并利用Hashin-Shtrikman（HS）边界条件估算了研究区上地幔温度及熔融百分比的分布情况。综合以上研究结果表明：（1）盆地西部壳幔边界存在一层连续的低阻层，其温度介于1200-1300℃之间，熔融百分比介于5-8%左右，标志着壳幔变形的解耦；（2）盆地东部存在一组大规模东倾的高导体，从上地壳一直延伸至上地幔，标志着该地区的壳幔变形机制以垂直连贯变形为主，经估算该高导区域的局部熔融百分比最高可达10%左右，可能调节吸收了盆地边界与祁连地块之间强烈的逆冲变形作用；（3）壳幔解耦的连续变形与垂直连贯变形两种机制在青藏高原北缘同时存在并发挥着各自的调节作用。