UPPER MANTLE SEISMIC ANISOTROPY IN EAST AFRICA: PLUMES, PLATES, AND MAGMA

Lateral heterogeneities in lithospheric structure influence the distribution of strain and magmatism in continental rift zones. Topographic relief at the lithosphere-asthenosphere boundary may deviate and enhance mantle flow, particularly at the edges of thick, buoyant Archaean cratons. The Turkana Depression and N Tanzania Divergence sectors of the East African rift system, both of which host mantle xenolith fields, provide examples of active rifting in a region with large lateral heterogeneities of lithospheric structure. Yet, their differences in terms of amount of stretching, degree of magmatic modification, and pre-rift lithospheric enable evaluation of the contributions of magma intrusion, metasomatism, pre-existing strain fabrics, and mantle flow on the direction and magnitude of seismic anisotropy in rift settings. Complementary data sets provide a strong contextual framework. We analyse SKS shear wave splitting at the 39-station CRAFTI-CoLiBREA array that straddles an Archaean craton edge, at 34 stations in the Turkana Depression between broad Ethiopian and East African plateaux, and several smaller temporary arrays, affording new opportunities to compare and contrast to evaluate contributions to mantle anisotropy. Our results and those of earlier studies reveal rotations in the fast direction over short distances at craton edges. The short length scale over which the azimuth of anisotropy changes from ENE to rift parallel indicates a relatively shallow depth range for the anisotropic zone. Increased vertical flow at craton edges may explain the smaller amounts of splitting and the larger percentage of null observations. Larger amounts of splitting (> 1s) occur in areas affected by flood magmatism since ~45 Ma, but without a clear increase in areas of lithospheric thinning, arguing against channel flow at the lithosphere-asthenosphere boundary as a major contributor to the observed patterns. The pervasive NNE to NE direction of splitting across cratons and mobile belts is oblique to crustal and lithospheric fabrics, suggesting that patterns are most influenced by mantle dynamics and oriented magma bodies.