Structural interpretation of Southern Main Ethiopian Rift basin using constrained full tensor gravity inversion of the basement morphology

Abstract

The Southern Main Ethiopian Rift is part of the great East African Rift system situated within the limit of 37o–38.5o east and 5.5o–7o north. Unlike the Central and Northern Main Ethiopian Rift where numerous geophysical studies have been conducted, the subsurface geology of Southern Main Ethiopian Rift is poorly constrained. Geological field work on Amaro Horst shows the basement outcrops are dominated by High grade gneisses and Intrusive with an average density of 6 gm/cm3 overlain by Mesozoic sedimentary rocks possibly extend into the studied sub-basins. In addition, field structural measurements and extracted lineaments from digital elevation model show NE–SW, NW–SE and N–S directions in agreement with fault block geometries identified by Invariance Tensor analysis. The Precambrian basement morphology of the area is delineated using constrained Tensor Gravity Inversion by applying Parker–Oldenburg algorisms. The basement morphology depicts the sub-basins; Northern Abaya, Southern Abaya, Chamo and Gelana basins where these areas are characterized by low Bouguer anomaly below -175mGal and lower Invariance Tensor anomaly may show the sediment infill of the sub-basins. The deepest depression exceeds 2500m below sea level in northern Abaya sub-basin where the maximum sediment thickness is more than 3700m. The result agrees with Spectral and Euler source depths solutions that show the limits of the crystalline basement vary between 2500m and 3500m. Amaro Horst, Chencha and Agere Selam are the basement structural high with average sediment thickness of 500m. The basement morphology revealed the N–S orientation of Gelana basin and other sub-basins aligns with NE–SW Tertiary Rifting. This result agrees with the structural analysis as well as previous regional studies on directions of prominent geological structures. These structurally controlled sub-basins having thick sedimentary sections are favorable zones for follow-up hydrocarbon exploration.