Understanding the nature of mantle upwelling beneath East Africa

Abstract

Over the last decade several theories have been used to explain hotspot phenomena occurring at Earth’s surface. The most prominent is that of hot upwelling material, so called mantle plumes. To date, our understanding of these structures is limited due to seismic images of the deep upper mantle/lower mantle lacking the resolution to image these features. One of the best locations to address this is the northern East African Rift (EAR), the location of the Afar hotspot, where, due to its presence on land, many broadband seismometers have been deployed over the last 20 years. Despite this, numerous models exist about the nature of mantle upwelling beneath this region. This study uses these data to provide P- and S/SKS-wave velocity models of the structure below EAR extending from the surface to the top of the lower mantle of a higher resolution than have been available to date. Both our P- and S-wave images provide evidence of two clusters of low-velocity structures with diameter of 100-200 km that extend through the transition zone, the first beneath Afar and a second just west of the Main Ethiopian Rift. Considering seismic sensitivity to temperature, we interpret these features as predominantly thermal upwellings with excess temperatures of 100±50K. Dynamic models for realistic Earth parameters can match the geometry and spacing of these features and synthetic tests of these numerical models, accounting for resolution of the tomographic models show that the likely source of these upwellings is from a thermal boundary layer at the top of the lower mantle. Finally, our tests show that due to resolution issues tomographically imaged structures from small-scale upper-mantle plumes may look substantially more complex than the simple vertical cylinders that are often anticipated and that tomographic studies of other hotspot regions should be reanalysed with this in mind.