Active Fault Scarps in Southern Malawi and Their Implications for the Distribution of Strain in Incipient Continental Rifts

Abstract

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 focused 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 subparallel fault scarps between 13 and 51 km long spaced~10-15 km apart. The scarps consist of up to five segments between 4 and 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 M w 7.1 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.Plain Language Summary When continents begin to stretch, earthquakes occur on faults that incrementally accumulate slip to eventually form a rift valley. To estimate the hazard posed by these earthquakes, it is important to understand where these faults are located and how much stretching they each accommodate. We analyze faults in the Zomba Graben, a young rift in southern Malawi, using high-resolution satellite data. Steep scarps indicate that recent earthquakes have occurred at both the edges and in the middle of the rift valley. Pronounced activity in the rift middle is thought to require a thinned rigid outer layer of the Earth or mechanically unfavorable faults at the rift edge. Neither of these factors have been observed in southern Malawi. We suggest that the distribution of active faults, and hence of earthquakes, is likely controlled by weaknesses in the middle and lower parts of the Earth's crust.

Controls on Rift Faulting in the North Basin of the Malawi (Nyasa) Rift, East Africa

“…The observation that intrarift faulting may contribute to cumulative rift opening in at least some parts of the North Basin is consistent with the formation of these faults early in the rift history (McCartney & Scholz, ; Mortimer et al, ) and their continued activity over the life of the rift (McCartney & Scholz, ). The proportion of deformation accommodated by intrarift faults in the North Basin of the Malawi Rift (~20–25%) is greater than in the Tanganyika Rift, where the intrarift faults account for ~10% of total extension (Muirhead et al, ) but less than in the southern Malawi Rift, where intrarift faults accommodate ~55% of Quaternary extension (Wedmore et al, ). One possible explanation for this regional pattern is progressive localization on the border fault (Cowie et al, ).…”

“…It would also be somewhat surprising because the dimensions of the Livingstone Border Fault are already larger than what is estimated to be mechanically favorable (Accardo et al, ; Olive et al, ; Scholz & Contreras, ), which would predict less activity on the border fault and more activity on intrarift faults in the future. Another possible explanation is that the observed variations could reflect differences in crustal rheology between different parts of the Western Rift, resulting in different styles of faulting (Wedmore et al, ; Williams et al, ). For any of the scenarios, our results suggest that intrarift faults have contributed to rift opening in the northern Malawi Rift.…”

“…These faults can be synthetic or antithetic to the main border fault, and their orientations with respect to the border fault vary. Their evolving role in accommodating extension over the life of the rift is debated (Mortimer et al, ; Muirhead et al, ; Muirhead et al, ; Wedmore et al, ). Intrarift faults in the Western Branch formed early in rift history and were active synchronously with the border fault, at least during the earlier phases of rift evolution (McCartney & Scholz, ; Morley, ; Muirhead et al, ).…”

“…However, seismic reflection data do indicate continued activity on some intrarift faults based on offsets of shallow, young horizons (Mortimer et al, ), consistent with seismicity associated with intrarift faults (Biggs et al, ; Ebinger et al, ; Gaherty et al, ). In the southernmost Malawi Rift, intrarift faults accommodate ~50% of recent extension (Wedmore et al, ).…”

Structural inheritance and border fault reactivation during active early-stage rifting along the Thyolo fault, Malawi

“…We used scarp height as a proxy for displacement (e.g. Morewood and Roberts, 2000;Hodge et al, 2018bHodge et al, , 2019Wedmore et al, 2020) and identified segments based on local minima in the along-strike scarp height profile. We used adapted versions of the SPARTA scarp measuring tools (Hodge et al, 2019) to measure the height of the fault scarp along the Thyolo fault on the 12 m DEM.…”