Seismotectonics and active faulting of Usangu basin, East African rift system, with implications for the rift propagation

Abstract

Seismicity and focal mechanisms within the NE-SW extending Usangu basin of the Western branch, north to northeast of the Rungwe Volcanic Province (RVP) southern Tanzania were determined from data recorded on temporary SEGMeNT and permanent AfricaArray seismic stations. The aim of the study has been to investigate stress regime and evaluates the role played by the Usangu basin in accommodating the relative movement between the two sub-basins of the Rukwa rift and the Malawi rift. Results show that most of the seismicity (~70%) concentrates along the Usangu border fault. A lesser amount of seismicity (~30%) is found within the basin and in the southwestern end of the basin towards the RVP. These findings suggest that the Usangu border fault, intrabasinal and transfer faults within the basin are associated with the seismic activities. This implies ongoing active deformation within the basin, which is accommodated by slip on the Usangu border fault system and seismicity associated with intrabasinal faults could indicate the temporal strain migration from the border fault. Although it has previously been suggested that the Eastern branch propagates southeasterly towards the coast, however, given the dense and long deployment of seismic station within the study area the possibility of second propagation of the Eastern branch towards the RVP in the Western branch cannot be ruled out. The seismicity therefore indicates that the Western branch could likely link with the Eastern branch via a wide zone of deformation combining the Mamizi-Kilombero-Ruhuhu fault zone and the zone of Mtera-Ruaha-Usangu rift graben, Focal mechanisms along the Usangu border fault and transfer faults within the basin show a combination of normal faulting and strike slip motion indicating extension with sinistral and dextral components of motion, consistent with palaeostress data and the strike slip deformation within the basin through accommodating the relative movement between the southern-most sub-basin of the Rukwa rift and northern-most sub-basin of the Malawi rift.