Active fault scarps in southern Malawi and their implications for the distribution of strain in incipient continental rifts

The distribution of deformation during the early stages of continental rifting is an important constraint on our understanding of continental breakup. Incipient rifting in East Africa has been considered to be dominated by slip along rift border faults, with a subsequent transition to focussed extension on axial segments in thinned crust and/or with active magmatism. Here, we study high-resolution satellite data of the Zomba Graben in southern Malawi, an amagmatic rift whose topography is dominated by the west-dipping Zomba fault. We document evidence for five sub-parallel fault scarps between 13 and 51 km long spaced ~10-15 km apart. The scarps consist of up to five segments between 4-18 km long, separated by minima in scarp height and river knickpoints. The maximum height of each fault scarp ranges from 9.5 ± 4.2 m to 35.3 ± 14.6 m, with the highest scarp measured on the intrabasin Chingale Step fault. We estimate that the scarps were formed by multiple earthquakes of up to Mw7.1, and represent a previously unrecognized seismic hazard. Our calculations show that 55 ± 24 % of extensional strain is accommodated across intrabasin faults within the ~50 km wide rift. This demonstrates that a significant proportion of displacement can occur on intrabasin faults during early stage rifting, even in thick continental lithosphere with no evidence for magmatic fluids.