The complex rift-to-drift transition: surprises and lessons from the Afar region

Since the earliest debates on continental drift theory, the African-Arabian rift system and the Afar region have been used as typical examples for extensional processes and the rift-to-drift transition. New findings suggest that the classical evolution model proposing a linear evolution from continental rifting in the Main Ethiopian Rift to advanced rifting in the Afar and oceanic spreading in the Red Sea is an oversimplification. Instead, the style of rifting seems to have a more important control on the structure and composition of each region than the magnitude or the age of the extension. In particular, the Central Afar region shows important extension, but it is far from showing normal, Penrose-like oceanic spreading. As such, it is considered as a precursor of some types of oceanic plateaus, such as the Greenland-Iceland-Faroe Ridge. This suggests that some features found far offshore and usually considered as purely oceanic might represent an extreme type of passive margin, hyperextended and magma ultra-rich. Conversely, the Danakil Depression, adjacent to Central Afar, shows a typical magma-rich structure on seismic data with well-defined Seaward Dipping Reflector (SDR) packages. However, outcrop data shows that they chiefly consist of sediments with only a small volume of magmatic products. This questions the composition of other margins worldwide that were often assumed to be made of magmatic material solely based on the recognition of SDR.

These new findings suggest that the position, composition, and structure of the continent-ocean transitions might be more complex and diverse than previously assumed.