THE MEKELE SEDIMENTARY BASIN, ETHIOPIA; THE FORGOTTEN ICONS

For the first time, we interpret the Mekele Basin in northern Ethiopia as an Intra-CONtinental Sag (ICONS). Because of its location on the uplifted western flank of the Afar Depression, the Mekele Basin might be the only exhumed ICONS in the world. Other ICONS such as those in the United States (such as Illinois, Michigan and Williston) have remained buried since the time of their formation. The Mekele ICONS is a ~100 km across structure dominated by concentric outcrops of Paleozoic glaciogenic sedimentary rocks at the basin’s outer rim followed inward by Mesozoic sandstones, limestones and shales at the center of the basin. These sedimentary rocks rest unconformably on the Precambrian crystalline rocks of the Arabian-Nubian Shield (ANS, pre-sag) with prominent NNE-trending regional fabric resulting from the accretion of arc systems. Both the Precambrian and the Paleozoic-Mesozoic rocks are unconformably overlain by Cretaceous (~150 Ma) sandstones (post-sag). The Mekele Basin share features common to other known ICONS that contrast them from rift and forearc basins. It is semi-circular in map-view, saucer-shaped in cross-section, lack border normal faults separating it from pre-sag formations and is characterized by an elliptically-shaped gravity minimum. Some models proposed for the evolution of ICONS advocate “vertical tectonics” associated with negative buoyancy within the sub-continental lithospheric mantle (SCLM). This is because of low stretching factor observed in ICONS and their low subsidence rate. Among these models is the development of ICONS due to subsidence associated with thermal cooling of juvenile SCLM following crustal formation through arc systems accretion. We propose a working hypothesis that the Mekele ICONS was formed as a result of subsidence which accompanied gradual cooling of the juvenile ANS SCLM starting in the early Paleozoic. Cooling between ~500 and ~150 Ma allowed for the growth of the ANS SCLM from a thickness of ~25 km to ~100 km producing enough negative buoyancy to form the Mekele Basin as an ICONS. Our working hypothesis can be tested through numerical modeling guided by geological and geophysical observations. Also, because it is almost completely exposed, the Mekele Basin provides an excellent opportunity for establishing detailed depositional architecture of ICONS.