青藏高原东缘地壳变形方式及其动力学的数值模拟

The western Sichuan plateau and surrounding regions are not only one of the most active earthquake regions in China, but also located on the critical position of the eastward expansion of Tibetan plateau. In spite of lots work on the deformation style of the crust as well as its dynamics beneath this region, up to now, controversies about the crustal deformation style have persisted. The high-resolution 3D S-wave velocity structure of the crust and upper mantle beneath the eastern Tibetan plateau was published recently from the waveform data recorded by the Western Sichuan seismic array. Although this result offers a new database for the deformation of the crustal interior, it merely provides a static picture of the crustal deformation. In this study, we are going to investigate the crustal deformation as well as the interrelationships of deformation between different layers and between different subregions under the current condition of the crustal structure by using a newly developed numerical modeling technique, so-called “triple damage model for lithospheric dynamics”, through combining other geological and geophysical information. Our numerical modeling will simultaneously consider the elasto-visco-plastic deformation, partial melt and mechanical damage of shallow structures in order to build the dynamic image of diverse deformation of the crust beneath the eastern Tibetan plateau. The results from this study will provide further proofs not only for the crustal ductile deformation, migration style of the crustal material and its dynamics, but also for the seismogenesis in the east margin of the Tibetan plateau.

川西高原及其邻区是我国地震活动最为强烈的地区之一，它处于青藏高原东向逃逸的关键部位。虽然前人对该地区地壳变形方式及其动力学进行了大量工作，但迄今为止，人们对这个问题的认知依然莫衷一是。最近发表的由川西台阵地震波形数据得到的青藏高原东缘高分辨率地壳三维S波速度模型虽然为台阵下方地壳内部变形及其动力学研究提供了新的观测依据，但它仅提供了一种静态图像。本项目拟在此基础上，综合其它地质、地球物理资料，利用新近发展的、能够综合深部非线性流变、部分熔融及浅部脆性破坏等不同变形方式的数值模拟技术，研究现今地壳结构状态下，中下地壳流变的方式及不同层位、不同区域变形方式的相互影响和相互作用，从而构建川西高原及其邻区地壳差异变形的动态图像。其结果将为研究青藏高原东缘中下地壳是否发生流动，地壳物质的运移方式，川西地区的地震形成机理及其动力学成因提供新的科学依据。

针对青藏高原东缘地壳变形方式及其动力学问题，根据项目设定的研究目标，开展了以下几方面工作：1）建立深部速度结构和浅部活动断裂相结合的三维地质模型；2）根据研究区大地热流数据，建立了研究区地温场；3）在收集研究区岩石化学成分资料的基础上，计算了花岗岩部分熔融的具体物理化学条件；4）分别对三条典型二维剖面和全区三维模型进行了不同物理模型（具体包括纯黏性、黏弹塑性、黏弹塑性+损伤模型、黏弹塑性+损伤+部分熔融四种）和不同边界条件的模拟计算。获得了以下基本结论：1）青藏高原明显偏高的地温场导致研究区岩石粘滞系数大幅降低，是造成研究区高原和盆地差异变形的最主要原因；2）高原内中地壳深度上普遍发育的低速体极大可能存在部分熔融，这加速了高原地壳的软化和变形增强；3）高原与盆地交界处是最容易发生损伤破坏的地区，并且青藏高原的推挤向东传播，可以造成四川盆地内部发生损伤破坏，首先发生破坏的位置与龙泉山断裂带部位对应。