Fault evolution in the Kenya Dome: an area of highly elevated topography within the East African Rift System

Mantle flow and the resulting surface deformation play a critical role in shaping continental rift systems; sublithospheric flow supports topography and applies tractions to the base of the non-convecting lid.  Surface observations of faulting, and of fault evolution through time, can be compared with predictions of flow from mantle convection models.  However, continental lithosphere is extremely heterogeneous and these heterogeneities apply a fundamental control on the way that the crust responds to stress.  It is therefore important to assess the contribution of factors such as crustal age, structural inheritance and seismogenic thickness to faulting patterns and the kinematics of continental deformation through time.  The Kenya Rift is an area of high topography within the eastern, mainly volcanic, branch of the East African Rift System (EARS).  Within the Kenya Rift, the spatial distribution of fault activity is puzzling.  The rift is bounded by impressive border faults which often exceed 40 km in length, with shorter (typically less than 10 km long), more closely spaced faults in the centre of the rift.  These observations suggest that - as expected - fault activity has migrated towards the rift centre, accompanied by a reduction in seismogenic thickness, with time.  If this is correct, the area should pose a relatively low seismic hazard.  However, our intial remote sensing and field observations, combined with an earlier palaeoseismological study, suggest recent activity along the border faults.  Given the length of the border faults, and their large accumulated offset, widespread continuing activity would have significant implications for seismic hazard assessment in the area.  The potential for large-magnitude earthquakes originating from these major faults warrants a re-evaluation of tectonic activity and associated risks in this rapidly urbanising area. In addition, the border faults are locally oriented obliquely in comparison to the orientation of the faults in the rift centre, and compared to the present-day - largely E-W oriented - extension direction.  This observation has been used to infer a rotation of the stress field through time.  We use remote sensing data to map these structures in order to compare them with inherited structural orientations and with predictions of sub-lithospheric flow through time from mantle convection models.  These observations not only challenge assumptions about fault migration and rift evolution, but also underline the potential for complex interations between mantle flow, surface deformation and seismic hazard in continental rifts.