青藏高原东南部地壳上地幔结构和地球动力学

The overall goal of the proposal is to examine various modes of deformation in the crust and lithospheric mantle in response to plate driving forces and topographic loading and the extent to which there is mechanical coupling between depth and the surface. A great variety of models have been proposed to explain the uplifting, formation, and deformation of the Tibetan Plateau (TP). A major obstacle is the limited resolution of seismic imaging of the sub-surface 3D structure, making it difficult to relate seismic parameters to geological structures and processes, while temporary deployments of seismic stations have been restricted to certain linear profiles or small areas. Here we propose to use joint-inversion methods involving multiple datasets to improve resolution of both P and S structures of the crust and upper mantle from collision front in southern TP to eastern and southeastern margins. We select our study region based on the need for sufficient data coverage and on our desire to study a sufficient large area to avoid possible bias from local heterogeneity and to compare the convergence regime (south and.central Tibet) with the extrusion regime (eastern Tibet and eastern southeastern margins). We propose to jointly interpret P travel times, teleseismic relative travel times, receiver functions, and surface-wave dispersion measurements from both ambient noise correlation and traditional earthquake-based method to derive 3D models of P and S velocities and anisotropies. In seismic imaging, model parameters often trade off with each other. To improve resolution and to resolve the ambiguity,a combination of different data sets that have sensitivities to.different parameters is required. We propose to deploy a dense 2D seismic array in southeastern TP. The combination of the proposed array, the recent deployment of ChinArray-I, as well as existing permanent stations and prior deployments provide abundant data for our joint inversions, which we believe will produce unprecedented resolution of the subsurface structure of the study region. The detailed 3D crustal and upper mantle P and S structures will provide critical means to test key hypotheses on Tibetan plateau formation and deformation. The.results have broad implications for foundamental questions about the mechanisms and processes of orogen uplifting, plateau formation, and continental deformation. Our research also has important implications for seismic hazards in this region, which are severe as we have witnessed the devastation from the recent Wenchuan earthquake. it also thrusts us into full international collaboration to increase our contributions to this major research area, and trains our young generations vital for continuing developments.

本项目科学目标是喜马拉雅和青藏高原如何对板块驱动力及地形荷载响应 的各种地壳和岩石圈地幔变形模式及浅表和深部变形的耦合程度研究。拟在青藏高原东南部布设30个左右宽频地震台站进行2-3年观测，整合利用多种地震数据，包括利用中国台阵（一期）及已有的固定台和前人布设的台阵的数据，使用联合反演，包括地震层析成像、S波分裂、接收函数环境噪声等方法，提高从青藏高原南部碰撞带到其内部和东部区域地壳上地幔 P 波和 S波结构的分辨率，得到研究区域精细的三维地壳和上地幔P、S 波结构、各向异性特征。这些结果将为验证青藏高原的形成和形变各种假说提供关键的依据，对山脉隆升的机制和过程、高原的形成和大陆的形变等基本问题具有广泛的指示作用。同时，研究对该区域及邻区的地震灾害研究有指导作用，也将促进国际合作和交流，增加我国在这一重大研究领域的贡献，为我国培养重要的地学后续人才。

本项目科学目标是喜马拉雅和青藏高原如何对板块驱动力及地形荷载响应 的各种地壳和岩石圈地幔变形模式及浅表和深部变形的耦合程度研究。经过4年的研究，结合野外地质考察、宽频地震观测、地震数据分析和反演，获得如下主要成果: ①依据青藏高原东南部的38个宽频带流动台站记录，采用接收函数共转换点叠加和H-κ叠加两种方法获得了研究区域详细的地壳厚度图像和泊松比值。结果显示：青藏高原东南部地壳厚度存在明显的东西差异和南北差异；喜马拉雅构造区内莫霍面深度变化较大，介于65~80km之间；拉萨地体内莫霍面深度介于72~80km之间；雅鲁藏布缝合带两侧地壳厚度突变，缝合带北侧和南侧地壳厚度相差约8km。研究区域平均泊松比值较小，为0.24，中下地壳广泛存在强转换界面，可能对应中下地壳高速层的上界面，埋深40~70km，表明壳内发生深熔或部分熔融作用，导致壳内发生重力分异，在中下地壳形成了高速薄层。②用接收函数方法研究青藏的地幔转换带结构，并采用接收函数共转换点叠加方法进行成像分析。结果显示，在拉萨地块下方MTZ增厚约20 km, MTZ内高速异常明显；羌塘和松潘-甘孜块体MTZ明显减薄约20 km；地球化学研究证实岩浆岩中有亏损的大洋溢流玄武岩成分。分析认为：印度板块至少已经俯冲至MTZ，从而进一步诱发了青藏高原下方的地幔物质上涌和火山作用。③根据青藏东南缘的300多个宽频流动地震观测波形数据，采用3-D速度模型，消除速 度异常的影响，获得了上地幔过渡带结构，发现上地幔410km和660km界面明显下沉，上 地幔过渡带厚度260-280km， 610km界面下沉10-30km，410km界面下沉10-20km。认为上地幔过渡带底部存在印度板块俯冲板片，腾冲火山起源于上地幔过渡带，与印度板块俯冲至上地幔过渡带有关。本项目已正式发表学术论文15篇，其中国际SCI论文13篇，研究生5名。上述成果对青藏高原的形成和形变各种假说提供关键的依据，对山脉隆升的机制和过程、高原的形成 和大陆的形变等基本问题具有重要参考价值。