青藏高原北缘阿尔金断裂带中段岩石圈三维电性结构及壳幔变形机制研究

The Altyn Tagh Fault (ATF) bounding the northern margin of the Tibetan Plateau plays a significant role in accommodating the collision between India and Eurasia plates. The central segment of the ATF is generally considered to be most representative of the orogenic and tectonic activities on the northern margin of the plateau. However, the deep lithospheric structures and deformation mechanism of this region are still highly debated. In this proposal, a three-dimensional (3D) Magnetotellusic (MT) array is proposed to be deployed in the region of central ATF, which is still not covered by previous studies. The proposed MT array locates between the Mang’ai and Aksay segments of the central ATF, covering the main trace of ATF, the southeastern Tarim Basin, as well as northern Qaidam Basin. The array consists of 60 broadband MT stations and 15 joint stations of both broadband and long period MT data recording, with relatively uniform distribution in the study area. After conducting robust data processing and comprehensive data analysis, a preferred 3D lithospheric electrical structure model of the study region will be obtained on the basis of 3D inversions. The electrical model will be further used to quantitatively constrain distribution of temperature and melt fraction in the upper mantle. Based on these results, the deep lithospheric rheological structure and deformation mechanism will be investigated from the perspective of electrical properties, and a joint interpretation will be made in combination with previous geophysical and geological data to achieve a comprehensive understanding of the deformation mechanism, structural pattern, as well as their geodynamic implications for the northern margin of the Tibetan Plateau.

标志着青藏高原北缘的阿尔金断裂带在调节印度与欧亚板块的碰撞-汇聚运动过程中发挥着至关重要的作用。其中，断裂带中段是高原北缘构造活动与造山作用最为典型的地区。但是关于该地区岩石圈深部结构、壳幔变形机制等重要科学问题长期存有争议。本项目拟在断裂带中段研究空白区内开展三维阵列式大地电磁观测。观测区位于断裂带中段茫崖至阿克塞区间，覆盖了此区间内的主断裂及塔里木盆地东南缘、柴达木盆地北缘等地区。设计15个宽频+长周期联合测点及60个宽频测点，计75个测点，均匀分布于研究区内。通过数据处理、分析及三维反演获取研究区内可靠的岩石圈三维电性结构模型，并实现利用电性结构模型定量/半定量地约束上地幔温度及熔融百分比分布，为研究岩石圈深部流变结构及壳幔变形机制从电学性质的角度提供重要约束条件。进一步结合研究区内其他地质与地球物理资料进行综合解释，探讨青藏高原北缘整体的壳幔变形机制、构造模式及其大陆动力学意义。

位于青藏高原北缘的阿尔金断裂带在控制高原的纵向隆升与横向扩张的过程中发挥着至关重要的作用。其中，阿尔金断裂带中段通常被认为是青藏高原北缘构造运动最为活跃，造山作用最具典型性的地区。但是，长期以来，对于阿尔金断裂带中段的深部结构及其壳幔变形机制还存在着较大的争议。在本项目的资助下（国家自然科学基金面上项目41774087），在位于阿尔金断裂带茫崖段与阿克塞段之间的区域内（90-94.5°E）完成了三维大地电磁测深数据的采集工作。MT数据覆盖了阿尔金主断裂、塔里木盆地东南缘以及柴达木盆地北缘的广大地区。将这些三维数据与已有的INDEPTH国际合作项目MT数据相结合，得到了由178个MT测点组成的三维数据体，其中包括152个宽频测点和26个长周期测点。首先，我们利用常规的远参考及Robust流程进行数据处理；进一步，通过相位张量与感应矢量等手段对数据进行了维性分析；最终，我们利用ModEM代码对MT数据进行了L-BFGS三维反演研究。最终的反演模型表明，研究区内各个盆地的浅部普遍分布有低阻层，柴达木盆地的基底表现为中-高阻，而塔里木盆地的基底则表现得非常高阻。阿尔金断裂带下方普遍发现有小规模低阻体，但是其延伸深度在断裂带走向方向上变化很大，表明具有强烈的横向不均一性。同时，该模型还表明塔里木地块向青藏高原内部俯冲的幅度非常有限，与“倾斜俯冲”的构造模式更为一致。在柴达木盆地东缘的上地幔深度发现存在大规模高导体。这一高导体可能反映了软流圈热物质的上涌和侵蚀作用。我们推测这一作用可能导致了上覆地壳的弱化，从而使得中-中新世以来的构造变形从以刚性块体的走滑运动为主向柴达木/祁连地块之间的韧性逆冲发生转变。