KINEMATICS OF THE ASAL RIFT (DJIBOUTI) DETERMINED FROM THE DEFORMATION OF FIEALE VOLCANO

Because of its subaerial exposure the Asal rift segment provides an ex ceptional opportunity to quantify the deformation field of an active r ift and assess the contribution of tectonics and volcanism to rifling processes. The present topography of the Asal rift results from the te ctonic dismemberment during the last 100,000 years of a large central volcanic edifice that formed astride the rift zone 300,000 to 100,000 years ago. Three-dimensional deformation of this volcano has been quan tified from the combined analysis of the topography and geology. The a nalysis indicates that spreading at 17 to 29 millimeters per year in a N40 degrees +/- 5 degrees E direction accounts for most of the separa tion between Arabia and Somalia. The small topographic subsidence rela tive to extension suggests that tectonic thinning of the crust has bee n balanced by injection and underplating of magmatic material of near crustal density. The methodology developed in this study could also be applied to quantify deformation in relatively inaccessible areas wher e the main available information is topography or bathymetry.