Partitioning of Extension at the Propagating Tips of Continental Rifts: Insights from the Central and East African Rift Systems

Abstract:

The leading tips of continental rift systems commonly terminate within the host continental crust or extend into passive margins and/or the adjacent oceanic crust. However, there is limited understanding of the crustal structure and controls on strain distribution at the propagating tips of continental rift systems that terminate within the host continental crust. We explore this problem in the intracontinental rift termination zones in eastern and central Africa, with special focus on northern tip of the Mesozoic Sudan Rift Zone (SRZ), and the NW (Rhino Graben, RG) and SW (Okavango Rift Zone, ORZ) terminations of the currently active East African Rift System. We utilize joint gravity inversion of seismic and satellite gravity to estimate the variation of crustal thickness across the rift termination zones and integrate our results with surface and/or subsurface expression of extensional faulting and seismicity where observable. In the three study areas, the rift terminations zones occur at their intersection with crustal-scale Proterozoic shear zones given by the termination of the SRZ at the Central African Shear Zone (CASZ), the Rhino Graben at the Aswa Shear Zone (ASZ), and the Okavango Rift at the Sekaka Shear Zone (SSZ). Our analysis show that there exists a zone of minor/diffused extensional faulting ahead, but near-field of the rift termination, which is underlain by a moderately thinned (SRZ & RG, 31 - 34 km, Beta factor = ~1.1) or thick crust (ORZ, up to 46 km). We interpret that the common occurrence of observable rift-related deformation (upper crustal and/or crustal-scale) ahead of the main rift segment and the abutting shear zone represents the localization of extensional stresses ahead of the propagating rift tip and partitioning of the strain across the inherited shear zone. Further, we propose a model of episodic lengthening of early-stage continental rift systems whereby the propagating rift tip initially terminates at a pre-existing crustal structure (acting as mechanical barrier) accompanied by the development of a diffused zone of extension ahead of the rift tip. With progressive crustal extension, this near field zone of diffused extension could develop into major rift segments that facilitates the continued lengthening of the rift system and its propagation into previously unrifted regions.