华北克拉通壳幔各向异性特征及动力学模拟

Seismic anisotropy is an effective method for understanding the crust and mantle deformation. To study the deformation beneath the North China Craton (NCC), we will determine the Pms and XKS (SKS, SKKS, PKS) fast polarization directions and delay time for 1750 permanent and temporary broadband seismic stations using the transverse-component minimization method, and evaluate the crust anisotropic impact on the XKS shear-wave splitting by comparing between the Pms and XKS splitting observations. Based on the Pms and XKS splitting measurements, the images of crust and mantle anisotropy in the NCC will be acquired. To further study the dynamics patterns of the NCC, we will analyze the extent of crust-mantle coupling in lithosphere and mantle flow in asthenosphere beneath the NCC through the joint analysis of shear-wave splitting and GPS data. 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 on differential velocities between predicted mantle flow and the velocity field at the surface, with orientations with the observed anisotropy.

地震各向异性是研究壳幔变形的有效方法之一。为认识华北克拉通的壳幔变形特征，本项目拟收集该区域内1750个宽频带固定和流动地震台记录的XKS（SKS、SKKS和PKS）和Pms波形资料，开展横波分裂测量，对比XKS和Pms波分裂结果，分析地壳各向异性对XKS波分裂的影响，得到该区域壳幔各向异性结果图像，分析壳幔各向异性和深部变形特征。为进一步认识华北克拉通的动力学特征，我们将利用横波分裂和GPS测量数据联合分析岩石圈和软流圈的变形模式。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导致各向异性的地幔流并分析其动力学含义。

地震各向异性是研究壳幔变形的有效方法之一。为认识华北克拉通的壳幔变形特征，本项目测量了华北克拉通1860个宽频带固定和流动地震台记录的远震XKS（SKS、SKKS和PKS）和Pms波分裂结果，通过对比XKS和Pms波分裂参数结果显示地壳各向异性对XKS波分裂的影响很小，华北克拉通的各向异性主要分布在上地幔。为进一步认识华北克拉通的动力学特征，利用横波分裂和GPS测量数据联合分析了岩石圈和软流圈的变形模式。分析表明，构造稳定的鄂尔多斯块体具有弱各向异性，可能保留了古老克拉通根的“化石”各向异性。阿拉善块体、鄂尔多斯块体西缘和北缘快波方向主要为NW或NNW方向，反映了青藏高原沿NE方向推挤过程中岩石圈沿NW方向和NNW方向发生了伸展变形；夹在两个刚性块体间的秦岭造山带的快波方向为近E-W方向或NWW方向，时间延迟较大，推测岩石圈东向挤出和软流圈东流共同促进了观测的各向异性；在鄂尔多斯块体南部边缘，快波方向自西向东呈逆时针旋转特征，推断该区域可能存在一个绕刚性块体的逆时针软流圈绕流，与上覆岩石圈左旋简单剪切变形产生了观测的各向异性，并一起驱动了鄂尔多斯块体的逆时针旋转。作为东西部过渡带的华北中部的各向异性相对复杂，其东部与克拉通东部的各向异性一致，主要反映了太平洋板块西向俯冲作用引起的地幔流作用；其西北部吕梁山的各向异性主要由岩石圈沿NNW到NW的拉张变形导致，而西南部太行山的各向异性还反映了软流圈绕流作用。鄂尔多斯块体东北缘大同火山区存在一个快波方向顺时针快速旋转且时间延迟较小的区域，可能与火山群下地幔岩浆上涌形成的局部地幔对流相关。紧邻华北北部的中亚造山带中南部快波方向为近E-W方向，其各向异性不仅受到与构造走向一致的岩石圈变形作用，而且也受到太平洋板块西向俯冲引起的地幔流影响。