Distributed Faulting in the Early Stages of Continental Rifting: New Evidence from the Zomba Graben in Southern Malawi

Rift basins in the East African Rift System (EARS) are bounded by < 100 km long border faults. These faults have throws of several kilometres and are generally assumed to accommodate a large fraction of the tectonic strain during the early stages of rifting. During the later stages of rifting, observed in the northern EARS, strain migrates towards the rift axis and is taken up by dyke intrusions and active magmatism. In contrast, the distribution of strain during early, incipient stages of rifting is poorly constrained by observations.

We present new evidence from the Zomba Graben in Southern Malawi, at the southern end of the topographical expression of the EARS, which represents an example of an early stage rift. Using observations from high-resolution topography, augmented with fieldwork and geological maps, we show evidence that strain is distributed away from the main border fault. We analysed topographic metrics combined with geomorphic analysis of river profiles, to identify active normal faults that are distributed across the width of the graben. We identify fault scarps that indicate that the most recent activity in the Zomba graben is concentrated in the centre of the rift, on faults with surface offsets ≤ 20 m. We also show data that suggest these fault scarps have been formed by multiple events with offsets on the order of 5-10 m per event.

These findings suggest that the early stages of continental rifting may not be dominated by activity on border faults, as had previously been thought, and prompts a re-evaluation of conceptual models of rift formation. Furthermore, the orientation of the fault scarps within the Zomba graben is oblique ( 60° difference in mean strike) to major faults to the north (Lake Malawi) or south (Shire Graben) and hence raises questions regarding the orientation of the stress field and influence of pre-existing fabrics during early-stage rifting.