A Proto-Volcanic Margin Along the Asal Rift. the Makarassou Fault System, Djibouti

Abstract

The Makarassou fault system (MFS) is a 40 x 70 km submeridian structure connecting the Asal and Manda Inakir active-recent rift axes, in Eastern Afar. Its previous interpretation as a transform fault structure is challanged on the basis of new structural field data and interpreted satellite images. The MFS involves 3-1 Ma-old basalts of the Stratoid series which onlapped Dalha basalts in the Asa Gayla plateau to the E, ie. on the southern part of the Danakil range. The overall structure of the MFS is dominated by a westerly-dipping monoclinal flexure, downflexed toward the Asal rift axis. It is further disssected by a dense network of extensional faults, dipping consistently outwards, to the E. Lava dips, as steep as 30°, locally occur in individual highly-rotated fault blocks. The causal mechanims of extensional flexuring along the Makarassou are assigned to isostatic loading caused by the >5 km-thick wedge of Stratoid and Dalha basalts that accummulated since 6 Ma beneath the Asal rift. The inward accummulation of mafic material, together with extensional and flexural strain, both resemble the pattern of Seaward Dipping Reflector Sequences that typically develop along volcanic rifted margins, worldwide. The structural interpretation of the MFS as outlining a nascent volcanic rifted margin along the eastern flank of the Asal-Manda Inakir rifted zone is consistent with available geochemical and geophysical records of the mafic crust flooring the Afar depression.