Thesis

Seismic anisotropy beneath African plate

The African continental plate is composed of a series of cratons and mobile belts with activation ages ranging from the present to the Archean. Although previous studies show the assembly of Africa has left behind a complex crust and lithospheric fabric, the style and timing of deformation is not well known. To study the degree and style of deformation of continental lithosphere during rifting and collisional events, I use seismic anisotropy to measure mineral alignment and strain accommodated by these tectonic events. Here I study seismic anisotropy beneath the African continent using instruments within the Africa Array and permanent Global Seismic Network stations located throughout Africa. I use SKS phases and shear wave splitting techniques for 28 teleseismic earthquake events. Stations located in the Ethiopian rift zone yield a NE-SW fast direction with the largest delay times of 1.5 s. One station (KOWA) located in the West African craton and three other stations located on the West African coast display a NW-SE fast direction and average delay times of 0.8 s. A group of stations located on Archean crust in central Africa skirt the Congo craton and display consistent NNE fast directions and delay times of 1.0 s. Two stations, LSZ and TEZI are located in the Damara suture belt between the Congo and Kaapvaal cratons display a NE-SW fast direction parallel to the suture axis. New stations available south of the Kaapvaal craton reveal a NE fast direction with delay times that vary from 0.5 to 1.3 s. These results indicate that seismic anisotropy across the African continent is not uniform as might be suggested from absolute plate motion. Our results, however, do show consistent variations between cratonic regions and mobile belts. The delay times averaged over the cratonic regions is 0.67 s +/- 0.14 s and are significantly smaller than delay times observed in mobile belts of 1.4 s +/- 0.07 s. Comparison of the fast direction of anisotropy within cratons with current plate motion suggests that the majority of the splitting observations within cratons are due to remnant fossil fabric from previous collisional events and has very little contribution from sublithospheric mantle flow today. We suggest that repeated convergent and rifting events causes depletion and dehydration within a craton creating stronger, and stable lithosphere that is resistant to deformation during later collisional events. By contrast, mobile belts that are younger in age and more fertile are more readily deformed under large tectonic stresses. Remnant or ancient anisotropic fabric may thus be preserved in cratonic lithosphere where it is stored from previous collisions while mobile belts reflect the most recent tectonic events.

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