GRAVITY AND MORPHO-TECTONIC ANALYSES OF THE BROADLY RIFTED ZONE IN SOUTHERN ETHIOPIA

The Broadly Rifted Zone (BRZ) is a diffused rift in southern Ethiopia between the Main Ethiopian Rift and the Eastern Branch of the East African Rift System (EARS). Rifts width within the EARS ranges from 50 to 65 km with the exception of the 300 km-wide BRZ, which is a wide zone of narrow graben and horst structures that form 10-50 km wide basins. Individual grabens within the BRZ are bounded by steeply-dipping normal faults with no low angle detachments exposed on the surface. The anomalous width of the BRZ is unusual to be developed through extension associated within a narrow rift system. We propose that mantle upwelling caused uplifting and subsequent gravitational collapse resulting in the broadly distributed extensional deformation. To test this hypothesis, we used two-dimensional (2D) power-density spectral analysis of ~ 9 km spatial resolution satellite gravity data and morpho-tectonic analysis of streams extracted from the 90 m spatial resolution Shuttle Radar Topography Mission (SRTM) Digital Elevation Model (DEM) to examine the relationship between the BRZ, Moho depths, and topography. To estimate the Moho depth, 2D power density spectral curves were computed from 1° X 1° windows of the gravity data with 50 % N-S and E-W overlaps. The morpho-tectonic analysis was used for the calculation of normalized steepness indices of streams in the BRZ. The gravity data shows shallow Moho depths under the BRZ with the shallowest depth of ~ 27.5 km in its northern limit. The areas surrounding the BRZ have Moho depths that reach up to 43 km. These results are consistent with previous seismic Moho depth estimations. The morpho-tectonc analysis showed that stream longitudinal profiles with the BRZ have high normalized steepness values following the ~ 1 km and ~ 2 km contour lines, which suggest two uplifting episodes. This is consistent with the presence of exhumed Precambrian basement rocks in the northern part of the BRZ at an elevation as high as 3.4 km. These results suggest that the development of the BRZ is related to uplifting due to the thermal effects of mantle upwelling which was followed by gravitational collapse to form the wide zone of extensional deformation. There are only few examples in the world of wide rifts besides the BRZ, including the Basin and Range of the western US, which also show heterogeneous crustal thinning.