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

Woods Hole Oceanographic Institution, Woods Hole MA

Investigators

Abstract

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.