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

这项研究为南部非洲太古宙克拉通内部和附近的构造和变形的分布、几何形状和大小提供了限制。采用地震和数值模拟相结合的方法来实现这些目标。研究结果为研究稳定克拉通区域的增生和后续演化、地幔形变的深度分布以及构造板块与岩石圈下地幔的耦合程度提供了关键信息。具体而言，主要研究以下问题：1)南部非洲太古宙克拉通地区地壳和上地幔的三维地震结构，重点研究岩石圈的深度范围以及地壳和上地幔速度与地质构造边界的关系；2)该区上地幔各向异性的三维分布及其对岩石圈变形和岩石圈下地幔流动的启示。一种新的双平面波技术，通过宽带地震阵列反演表面波相位和振幅的变化，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，这是更大规模的多学科Kaapvaal项目的一部分。与传统的区域表面波层析成像不同，传统的区域表面波层析成像假设是平面波前和大圆源-接收器射线路径，当前方法的一个显著优势是它考虑了由于横向非均质性、散射或沿着射线路径的多路径引起的波场扰动。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，并通过将该地区现有的横波分裂结果与面波数据进行反演，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。数值模拟部分包括计算地幔流动的三维模型，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。通过将表面波分析结果与先前的体波结果相结合，构建了适合南部非洲的岩石圈龙骨形态。这种几何结构被用来检验龙骨周围地幔流动的模型，其中流动是由水平板块运动或垂直地幔上升流驱动的。利用南部非洲地幔结核的弹性参数和其他变形研究的结果计算了体波和面波的预测各向异性，以评估岩石圈下地幔变形的潜在程度。由于地幔流动引起的地震各向异性预测结果将与观测到的地震各向异性相结合，以帮助约束南部非洲地幔形变的分布。

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.