Résumé
Lateral heterogeneities in crust and mantle structure influence the distribution of strain and magmatism in continental rift zones, and relief at the lithosphere-asthenosphere boundary may enhance mantle flow. Sutures between Archaean cratons and younger orogenic belts represent some of Earth's largest lateral heterogeneities: > 170 km-thick, buoyant and relatively dry lithosphere juxtaposed to approximately 120 km-thick, more volatile-rich mantle lithosphere. The magma-rich Eastern rift formed near the eastern edge of the Archaean Tanzania Craton in northeastern Tanzania, which has been affected by a deep-seated mantle upwelling, and provides an example of active rifting in a region with large lateral heterogeneities of lithospheric structure. Our objective is to evaluate spatial variations in the direction and magnitude of seismic anisotropy, which is strongly influenced by mantle flow patterns along lithosphere-asthenosphere topography, fluid-filled cracks (e.g., dikes), and pre-existing mantle lithosphere strain fabrics. Complementary tomography, petrological, seismicity, and gas chemistry results provide a strong contextual framework. We use the Split Racer algorithm to analyze waveforms of teleseismic earthquakes recorded on the 39-station CRAFTI-CoLiBREA temporary array in southern Kenya and northern Tanzania. We also analyze longer time series for permanent stations KIBK, KMBO, and LODK in Kenya. Our results and those of earlier studies show a consistent NE splitting direction within the craton and the Pan-African orogenic belt outside the rift that matches the African Plate motion in a no-net rotation framework. Stations within the rift zone are rotated to a rift parallel direction (N0-25E) of splitting, with the largest delay times of approximately 2 s at the margin of the heavily intruded Natron-Magadi Basins. The short length scale over which the azimuth of anisotropy changes from NE to rift parallel indicates a relatively shallow depth range for the anisotropic zone, which matches the seismically imaged, steep edge of cratonic lithosphere. The sharp change to rift-parallel mantle anisotropy at the craton edge is consistent with models of enhanced mantle upwelling at cratonic edges that transfers old carbon from the craton into upwelling, volatile-rich mantle. These in turn enhance melt production and localize magmatism, heating, and subsequent strain to cratonic edges.