Abstract
AbstractThe 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 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.
Highlights
The development and interaction of faults dominate deformation in the early stages of continental rifting and contribute to the eventual breakup of continental lithosphere (Cowie et al 2005; Ebinger and Scholz, 2012)
There are two possible explanations for the roughly equal distribution of deformation between the intra-rift faults and the rift border faults that we observe in the Zomba Graben (Figure 10): 1) the current distribution represents a period of time prior to the localisation of strain across the rift border faults; or 2) deformation has migrated from the border faults onto intra-rift structures
We show that the active fault scarps occur at the 628 base of the rift border faults and on the rift floor between them
Summary
We report 5 subparallel, previously unrecognised, active faults in the Zomba Graben, Malawi with 10-50 km long, 10-35 m high fault scarps. Border faults are thought to dominate in incipient rifts, but we find intra-rift fault scarps accommodate 55 ± 24 % of extensional strain
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