Collaborative Research:Multidisciplinary Investigations of Structure and Deformation Beneath Southern Africa

这项研究提供了对南部非洲太古代古陆内部和附近的结构和变形的分布、几何形状和规模的限制。 地震和数值模拟相结合的方法是用来实现这些目标。 研究结果提供了有关稳定地幔区域的增生和演化、地幔变形的深度分布以及构造板块与岩石圈下地幔耦合程度的重要信息。 具体而言，正在研究以下问题：1）南部非洲太古代克拉通地区地壳和上地幔的三维地震结构，重点是岩石圈的深度范围以及地壳和上地幔速度与地质界定的构造边界的关系; 2）研究区上地幔各向异性的三维分布，以及它对岩石圈变形和岩石圈下地幔流动的意义。一种新的双平面波技术，宽频带地震阵列上的相位和振幅限制了三维（3-D）地壳和上地幔地震结构。 这项分析的数据来自82个台站的南部非洲地震实验，这是大规模多学科Kaapvaal项目的一部分。 与传统的区域面波层析成像，其中平面波前和大圆源-接收器射线路径假设，目前的方法的一个显着的优势是，它占波场扰动由于横向异质性，散射，或沿沿着射线路径的多路径。 利用SASE台站记录的瑞利波和洛夫波确定各向同性和各向异性地震结构，并通过用该地区现有的横波分裂结果反演面波数据，以确定南部非洲地下地震速度和各向异性的最佳拟合三维模型。三维地幔流模型，并确定这些模型与地震各向异性和地幔岩石圈变形观测的关系。 一个适合南部非洲的岩石圈龙骨形态构造通过整合的面波分析结果与以前的体波结果。 这种几何形状，是用来检查模型的地幔流周围的龙骨，其中流动是由水平板块运动或垂直地幔上涌驱动。 预测的体波和面波的各向异性，使用南部非洲地幔结核的弹性参数和其他变形研究的结果进行计算，以评估岩石圈下地幔变形的潜在程度。 由于地幔流动而预测的地震各向异性的结果将与观测到的地震各向异性合并，以帮助限制南部非洲地幔变形的分布。

This study is providing constraints on the distribution, geometry, and magnitude of structure and deformation within and near the Archean cratons of southern Africa. A combined seismic and numerical modeling approach is used to accomplish these goals. Results from this work are providing key information regarding the accretion and subsequent evolution of stable cratonic regions, the depth distribution of mantle deformation, and the degree of coupling between tectonic plates and sublithospheric mantle. Specifically, the following issues are being addressed: 1) the 3-D seismic structure of the crust and upper mantle in Archean craton regions of southern Africa, with a focus on the depth extent of the lithosphere and on the relationship of crust and upper mantle velocities to geologically-defined tectonic boundaries; and 2) the 3-D distribution of upper mantle anisotropy in this area, and its implications for deformation in the lithosphere and flow in the sublithospheric mantle.A new two plane wave technique that inverts variations in surface wave phase and amplitude across a broadband seismic array constrains three-dimensional (3-D) crust and upper mantle seismic structure. Data for this analysis come from the 82 station Southern Africa Seismic Experiment, part of the larger-scale multidisciplinary Kaapvaal Project. Unlike traditional regional surface wave tomography where planar wavefronts and great circle source-receiver raypaths are assumed, a significant advantage of the current method is that it accounts for wavefield perturbations due to lateral heterogeneity, scattering, or multipathing along ray paths. Both isotropic and anisotropic seismic structure are determined using Rayleigh and Love waves recorded at the SASE stations, and by inverting the surface wave data with existing shear wave splitting results in this area to determine best-fitting 3-D models of seismic velocity and anisotropy beneath southern Africa.The numerical modeling component involves calculating 3-D models of mantle flow and determining the relationship of these models to observations of seismic anisotropy and deformation of the mantle lithosphere. A lithospheric keel morphology appropriate for southern Africa is constructed through integration of the results of the surface wave analyses with previous body wave results. This geometry, is used to examine models of mantle flow around the keel in which flow is driven either by horizontal plate motion or vertical mantle upwelling. Predicted anisotropy for both body and surface waves using the elastic parameters of southern Africa mantle nodules and the results of other deformation studies are calculated to evaluate the potential extent of sublithospheric mantle deformation. The results of predicted seismic anisotropy due to mantle flow will be merged with the observed seismic anisotropy to help constrain the distribution of mantle deformation beneath southern Africa.