基于横波分裂、GPS和断裂第四纪滑动速率数据研究青藏高原东缘岩石圈/软流圈动力学特征

The extent of crust-mantle coupling in lithosphere and mantle flow in asthenosphere beneath the eastern margin of Tibetan Plateau can be determined through the joint analysis of surface and mantle deformation fields. In order to infer the mantle deformation field, we will determine the SKS fast polarization directions and delay time for 1810 permanent and temporary broadband seismic stations where the fast polarization and delay time for each arrival were determined following the method of Silver and Chan (1991) and stacking procedure. A continuous surface deformation field will be determined using continuous spline functions to interpolate between observed surface strain rate data inferred from both GPS observations and Quaternary fault slip rates. According to the magnitude of strain rate and lithosphere thickness, we divide the study area into two parts, one is high strain rates and thick lithosphere, and another is low strain rates and thin lithosphere. In high strain rates and thick lithosphere area, we will measure the deformation pattern (left-lateral simple shear, right-lateral simple shear or pure shear) at each station based on lithosphere deformation model. We can constrain the extent of crust-mantle coupling through comparing the predicted anisotropy orientations with the observed anisotropy. In low strain rates and thin lithosphere area, we can assume the anisotropy is generated by the differential flow between the lithosphere and asthenosphere. The mantle flow is determined in an iterative inversion by comparing the predicted anisotropy, based differential velocities between predicted mantle flow and the velocity field at the surface, with orientations with the observed anisotropy.

青藏高原东缘岩石圈/软流圈的变形模式可以通过联合分析地表变形场和地幔变形场来确定。本项目拟收集位于青藏高原东缘的宽频带固定和流动地震台（共1810个台）记录的SKS波形资料，采用最小切向能量的网格搜索和叠加分析方法求取表征地幔变形场特征的横波分裂参数；并利用发表的GPS和断裂第四纪滑动速率数据，采用连续样条函数方法求取地表连续变形场。根据应变率分布和岩石圈构造特征，按照高应变率和厚岩石圈区域采取岩石圈变形模式分析，定量求取和确定各个观测点的变形类型（左旋简单剪切、右旋简单剪切或纯剪切变形），通过预测的各向异性参数与实测参数的对比来确定岩石圈壳幔力学耦合的程度并讨论其动力学含义；在低应变率和薄岩石圈区域采用简单软流圈变形模式分析，依据预测的各向异性参数与实测参数的对比来确定由迭代反演求取的地幔流并分析其动力学含义。

青藏高原东缘处于中国大陆中部，是青藏高原与华北克拉通和扬子克拉通相互作用的地带，区域内强震活动频繁，集中了我国有史以来一半的8级以上大地震，是开展地球动力学的理想实验室。本项目通过1810个宽频带地震台站记录的远震XKS（SKS, SKKS和PKS）波分裂结果分析了青藏高原东缘的地幔变形场特征。横波测量结果显示了上地幔各向异性特征，由北至南，快波方向分布显示青藏高原东北缘的快波方向主要表现为NW-SE方向，逐渐过渡到川滇西北部的近N-S方向，以~26.5°为界，快波方向突然由北部的近N-S方向转到云南南部的近E-W方向。基于GPS和断裂第四纪滑动速率数据定量分析了地表连续变形场，并预测了岩石圈上地幔各向异性快波方向。通过比较快波方向的横波分裂测量值与由地表变形场预测值，除了鄂尔多斯块体和四川盆地内部二者的差别较大外，青藏高原东缘大部分地二者的差异都比较小，平均值只有11°，表明各向异性主要由岩石圈变形引起，地表变形与地幔变形一致，地壳耦合于地幔，是一种垂直连贯变形模式；然而在秦岭造山带和云南南部较薄的岩石圈不足以产生较大的时间延迟，说明软流圈地幔流也起到重要的作用。简单软流圈模型分析表明秦岭造山带下软流圈被上覆岩石圈板块直接驱动产生了东向的地幔流，与岩石圈左旋剪切变形共同促进了观测的各向异性形成。而云南南部的各向异性主要由软流圈地幔流引起，缅甸和巽达板片的后撤/回转作用产生了指向西南的软流圈地幔流，在岩石圈底部和软流圈之间产生了一个水平差异运动，产生了一个与简单剪切一致的软流圈变形结构，从而产生了观测的各向异性。鄂尔多斯块体和四川盆地深浅变形不一致，具有弱的各向异性、厚的岩石圈和构造稳定的特征，我们认为其各向异性可能保留了古老克拉通的“化石”各向异性。