Title: "Rifting in Africa: Seismological views from Afar"
Abstract: The rifting of continents and eventual formation of ocean basins is a fundamental component of plate tectonics, yet the mechanism for break-up is poorly understood. Rifting of the continents leading to plate rupture occurs by a combination of mechanical deformation and magma intrusion, but the available driving forces have been estimated to be as much as an order of magnitude smaller than those required to rupture thick continental lithosphere. The East Africa Rift system (EARS) is an ideal place to study this; it captures the initiation of a rift in the south through to incipient oceanic spreading in north-eastern Ethiopia - Afar.
In this talk I will describe how seismological investigations can be used to test models of rifting. A series of seismic experiments over the past decade have led to the deployment of nearly 200 seismometers in Ethiopia and surrounding regions, in regions varying from fertile rift valley lakes to the harsh environment of the Afar depression. The resulting data have provided unprecedented images of the deep Earth beneath a rifting environment. I review our latest seismic images of the velocity and discontinuity structure in the region. Models of rifting can be tested because they predict different temporal and spatial patterns of crustal and upper-mantle structure. Furthermore, changes in plate deformation produce strain-enhanced crystal alignment and increased melt production within the upper mantle, both of which can cause seismic anisotropy. The pattern of shear-wave splitting – a tell-tale sign of anisotropy – is best explained by dyke-induced faulting and oriented melt inclusions near volcanic centres.