地震学方法研究青藏高原南北向裂谷深部结构及其动力学机制

Tibetan plateau is produced by the north-south convergence between the Indian and Eurasian continents and the shortening and thickening of lithosphere. The east-west extension, however, has become a prominent tectonic feature and initiated several north-south trending rifts in southern and central plateau since about 15 Ma. The relationship between N-S shortening, uplift, and E-W crustal extension is fundamental to the mechanics of collisional orogeny. Several models have been proposed to explain the opening of N-S trending rifts in southern plateau or equivalently the mechanisms for Tibetan extension, such as gravitational collapse, eastward extrusion, oblique convergence, and intrusion of Indian lower crust. Geochemical studies and geophysical investigations suggest that the upper mantle has been involved in the rifts opening. The scientific geophysical studies on the rifts are absent but will provide the key to evaluate the mechanisms. We will study the deep structure beneath the rifts based on a 900-km-long east-west passive-source linear seismic array which was operated in central Lhasa block and crossing several major N-S trending rifts. Our work include imaging the geometry of interfaces in crust and upper mantle by the receiver function method and the virtual deep seismic sounding, inferring the seismic velocity structure of crust and upper mantle through joint inversion of P-receiver functions and the surface-wave phase velocity dispersion from ambient noise correlation and the teleseismic wave tomography. The geodynamic relationship between the geophysical characters in various depths, such as the fluctuation/offset of interface, the variations of the low wave-speed layer, and the deformation of upper mantle, will be built after comparison their spatial positions. We will discuss the dynamic processes of rifts opening and the relation between it and the subducting Indian lithospheric slab.

青藏高原是由印度与欧亚近南北向的碰撞和持续的岩石圈汇聚、缩短增厚形成的，然而大约15个百万年以来，高原中部、南部发生了广泛的东西向拉张，形成了数条近南北向的裂谷。高原垮塌、东向挤出、斜向汇聚和印度下地壳挤入等多种模型被用来解释南北向裂谷的形成。已有的地球化学研究和地球物理探测表明裂谷的形成与上地幔的动力学过程有关，但目前尚缺乏针对裂谷带开展的系统性的地球物理探测研究。本项申请拟使用已有的一条东西向跨越多个裂谷的宽频带地震台阵，利用接收函数方法和虚拟震源地震测深探明裂谷带下方壳幔几何结构，使用面波与接收函数联合反演以及体波走时成像探测地壳和上地幔速度结构，通过分析裂谷带下方，特别是亚东-谷露裂谷，界面的起伏、错断和壳内低速层横向变化以及上地幔的形变特征的空间位置对应关系，建立不同深度层次地球物理特征之间的动力学关系，进而探讨裂谷的形成的动力学过程，以及裂谷发育与印度岩石圈俯冲板片间的关系。

新生代以来，印度与欧亚大陆的持续汇聚挤压造就了巨厚地壳和高海拔的青藏高原。然而，约15百万年开始高原中南部开始发育大量东西向拉张构造，如南北向裂谷。由于缺乏拉张构造延伸深度以及深部结构的约束，地质学家们提出了多种动力学模型来解释拉张构造的产生机制，如重力垮塌、地壳流、印度下地壳挤入等。 .本项目使用青藏高原南部布设的东西向宽频带地震台阵（TIBET-31N）记录的天然地震数据，利用虚拟震源地震测深方法获取剖面下方的Moho 面深度变化图像。结果显示在亚东谷露裂谷附近Moho面东深西浅，错断可达10公里。综合亚东-谷露裂谷附近的地球物理结果和断层及震源机制解，表明亚东-谷露裂谷切穿地壳，裂谷的发育与深部过程有关。近年来多个研究根据南北向裂谷空间分布特种的动力学分析、火山岩成分、地震波走时和剪切波分裂，推测印度大陆俯冲板片在高原下方撕裂，软流圈物质上涌拉张导致裂谷发育。然而印度俯冲板片撕裂仍然缺乏直接的深部观测证据。为此，本项目使用青藏高原中部布设的两个宽频带流动台阵（SANDWICH和TIBET-31N）记录的天然远震事件的波形记录提取S波接收函数，通过共转换点叠加成像获得了高原中部和南部上地幔速度界面的起伏形态。结果显示：在亚东-谷露裂谷下方，印度俯冲板片底界面都发生了明显的东西向错断，两侧板片深度相差70公里；在裂谷的两侧，青藏高原岩石圈底界面在东西向也存在30公里的深度差异。 . 我们的研究不但为印度俯冲板片撕裂提供了直接证据，同时也建立了深部过程与裂谷发育的动力学关系，即：青藏高原在印度的北向推挤下，岩石圈缩短增厚，在南北挤压力和重力共同作用下，高原中部物质发生东西向的伸展；由于印度大陆俯冲板片发生撕裂，形成板片窗为深部的软流圈物质上升提供了通道；热的软流圈物质侵蚀、破坏上方东西向伸展的高原岩石圈，促使南北向裂谷的发育。