TOPOGRAPHIC AND CRUSTAL THICKNESS VARIATIONS ALONG THE WESTERN ESCARPMENTS OF THE AFAR DEPRESSION

We used the analysis of a combination of digital elevation model (DEM) and satellite gravity data to characterize the topographic and crustal thickness variation in the Northwestern Ethiopian Plateau, and western escarpments of the Afar Depression. This region provides a unique setting to examine the relationship between rift development, magmatism, plateau uplift, and landscape evolution. We used the Bouguer gravity anomalies obtained from the World Gravity Model (WGM 2012) data to generate crustal thickness (Moho depth) estimates by means of the forward and inverse gravity modelling approach. We also generated swath profile patterns of elevation, and steepness index in the Northwestern Ethiopian Plateau and western escarpment of the Afar Depression.

Our crustal thickness estimates for the Northwestern Ethiopian Plateau show anomalously deep Moho that has considerable relief both across and along the escarpment of the Afar Depression. Crustal thickness ranges from ~33 km in the northern part of the escarpment to ~48 km near the southern edge of the escarpment. These Moho depth estimates are deeper compared to CRUST1.0 seismic model, and suggestive of possible magmatic underplating particularly to the south of the Northwestern Ethiopian plateau. Within the Afar Depression, the depth to the Moho ranges from 16 to 27 km.

The western escarpments of the Afar Depression are characterized by a relatively narrow region of extreme relief, steep slopes, and series of small graben controlled by large marginal bounding faults. The Northwestern Ethiopian Plateau itself has a relatively flat topography with an average elevation of 2500 m above sea level (asl) dotted with shield volcanoes and a series of deep gorges carved by the Blue Nile River and the Tekeze River. On the other hand much of the elevation of the Afar Depression bordering the western escarpment ranges between 500 and 0 m asl. Along-strike variation in channel steepness index estimates shows spatial relationship with crustal thickness, and hence, with tectono-magmatic processes within the Afar Depression. This observation indicates possible along-strike topographic segmentation controlled by large scale crustal thickness variation.