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 over 150 seismometers in Ethiopia, 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.
Overall, our observations support models of magma-assisted rifting, rather than those of simple mechanical stretching. Upwellings, which most probably originate from a deeper and larger super-plume, thermally erode the lithosphere along sites of pre-existing weaknesses or topographic highs. Decompression leads to magmatism and dyke injection that weakens the lithosphere enough for rifting and the strain appears to be localized to plate boundaries, rather than wider zones of deformation. In many ways the scenery in mantle beneath Ethiopia is as spectacular as the scenery that captures the transition from the Ethiopian rift valley lakes to the Danakil depression in Afar.
Mike is Professor of Seismology and Head of the School of Earth Sciences at the University of Bristol. He has a PhD in geophysics from Queen's University in Canada and he was a NSERC postdoctoral fellow at the Scripps Institution of Oceanography in the USA. He has had faculty positions at the University of Toronto and the University of Leeds, and he worked briefly for Chevron Canada Resources in Calgary. He is currently president of the British Geophysical Association and vice-president of the Royal Astronomical Society. He has published over 150 papers in leading journals and in 2011 he was elected fellow of the American Geophysical Union.
Mike's research interests cover pure and applied seismology, with connections to mineralogy, tectonics and engineering. Current research in global geophysics concentrates on the nature of the core-mantle boundary, continental cratons, continental rifting, mid-ocean ridges and subduction zones. He has led seismic field experiments in geologic settings ranging from the Canadian Arctic to remote parts of Ethiopia.
Techniques that he has developed to study wave propagation in the deep Earth have also been applied to his research in exploration seismology. His interests lie in microseismicity and passive seismic monitoring, rock-fracture characterisation, and linked geophysics, geomechanics and fluid-flow modelling. He has managed a number of large industry-funded consortia, including BUMPS (Bristol University microseismicity projects) and SAIL (seismic anisotropy as an indicator of lithology).