Lithospheric and Preexisting Upper Crustal Structures of the Broadly Rifted Zone

Abstract:

The Broadly Rifted Zone (BRZ) is a diffused rift in southern Ethiopia that connects the Ethiopian Rift with the Eastern Branch of the Eastern African Rift System (EARS). The BRZ assumes the Basin and Range type topography. However, unlike the Basin and Range, it has no exposed low-angle detachment faults. The fundamental mechanism through which the BRZ has evolved into its current width of 300 km, from a narrow 48-64 km wide rift, has not been well studied. A few studies hypothesized that the upwelling of the mantle plume beneath the BRZ has resulted in a diffused rift setting in which strain is localized across a broad zone. Conversely, pure crustal scale kinematic shifts in geologic time of a strongly deformed and volcanically weakened lithosphere might have resulted in similar topography. In order to reconstruct the structural history of the BRZ, we have attempted to characterize its subcontinental lithosphere and the upper crustal structures by adopting four fundamental geophysical methods: 1) 2D power spectral analysis of WGM 2012 satellite gravity data to estimate depths to the Moho, 2) remote sensing techniques for structural mapping, 3) directional filtering of gravity to characterize the upper crustal structures, and 4) 2D forward modelling to estimate the basins thickness. The results of the study suggest that there is significantly elevated Moho beneath the BRZ in which the shallowest depth is 27 km. The preexisting structures that trend NW-SE have been imaged though various directional filters applied in wavenumber domain onto the WGM 2012 satellite gravity data. Results show that the NW-SE trending structures moderately modified the BRZ’s rift architecture as some pronounced NW-SE escarpments are plainly delineated from the satellite gravity data filters. We conclude that the BRZ is the result of mantle related processes in which the upwelling of asthenosphere resulted in significant uplifting and considerable arching of the region to an extent that extensional collapse is initiated due to the formation of unstable topography. Also, the preexisting structures served as weakness zones along which the collapse had taken place. The BRZ is one of few examples in the world that shows the role of dynamic topography instability in the formation of wide rift zones.