WHEN DID UPLIFT IN THE WESTERN MARGIN OF THE AFAR DEPRESSION BEGIN?

Precambrian basement and Jurassic marine limestone blocks bordering the western margin of the Afar Depression in northern Ethiopia are found at an elevation of ~3 km. Apart from the well-known Cenozoic doming and rift flank uplift, prior exhumation and uplift histories were not thoroughly investigated. Here, we report evidence for possible Cretaceous uplift of the Mekelle Intra CONtinetal Sag (ICONS). To elucidate the paleo-datum relationship between the Precambrian basement and Mesozoic strata within the Mekelle ICONS, we used Digital Elevation Model (DEM) and optical and RADAR remote sensing data. Three sets of major normal faults are identified: (1) N-trending faults parallel to the major western border faults of the Afar Depression and these have dominantly reactivated Precambrian basement fabric; (2) WNW-trending faults that cut through Jurassic sedimentary units but are sealed by Cretaceous sandstone and these are related to Cretaceous rifting of Western Gondwana and (3) Less pronounced NE-trending faults possibly related to migration of the Afar Depression border faults into the Northwestern Ethiopian plateau. Along the northern rim of the Mekelle ICONS, a Jurassic marine limestone is uplifted along a WNW-trending fault reaching 2.6 km in elevation. An erosional and angular unconformity between Jurassic and Cretaceous sedimentary rocks in the Mekelle ICONS and similar WNW-ESE striking basins in the region suggest major tectonic activity. The presence of unconformable surfaces at different present-day elevations indicates that the area may have risen and subsided multiple times with the last major uplift prior to Cenozoic rifting being due to fragmentation of Western Gondwana during the Cretaceous. Uplifted limestone blocks preserved in juxtaposition with the Precambrian basement, are possible remnants of Cretaceous paleo-topography. This indicates the Mekelle ICONS may have been uplifted to similar datum. Today, the Precambrian rocks surrounding the Mekelle ICONS represent the highest elevated Precambrian basement in northern Ethiopia. Rift flank uplift mechanisms hardly explain such a rise. Therefore, we are tempted to conclude that the Precambrian basement blocks in the western flank of the Afar Depression may have been already at significant elevation prior to the Cenozoic rifting.