Regional and local deformations at Erta Ale caldera (Afar, Ethiopia)

Acocella V., Barberi F.
Dip. Scienze Geologiche Roma Tre, Italy

Erta Ale volcano is located within the Danakil depression, northern Afar. The NW-SE elongated depression marks the southern propagation of the Red Sea Rift, separating, along a ~NE-SW direction, the Nubia and Arabia plates. Erta Ale consists of a shield volcano with a NW-SE elongated summit caldera, hosting a lava lake, active since at least a century. Field work was carried out at Erta Ale, aiming at defining its deformation pattern. The caldera, collapsed for few tens of metres, is bordered by subvertical scarps. Extensional fractures are found outside the rim, within a distance shorter or similar to the local height of the rim scarp, and are systematically parallel to the rim. Extensional fractures and normal faults cluster N and S of the caldera rim, accompanied by volcanic activity (dikes, hornitos, spatter cones, shield volcanoes). To the N, the N-S trending fractures have a ~N70W extension direction (implying a dextral component of shear), accommodating an extension ~6.6 m. To the south, the NW-SE trending fractures have a ~N65E extension direction (orthogonal extension), accommodating an extension of ~6.3 m. The collected data suggest that the local deformation, caldera induced, does not extend at surface beyond a distance larger than the height of the scarp. Even though the N-S and NW-SE trending fractures may be due to the emplacement of dikes propagating laterally along the volcano slope, they are consistent with the regional structures observed in the area. The geometry and kinematics of the NW-SE rift zone are consistent with those of the regional structures in the Danakil depression. Conversely, the geometry and kinematics of the N-S rift zone are interpreted as the result of the interaction between the Erta Ale and the Bora Le Ale (to the N) tectonic segments, forming a right-stepping offset. This configuration, as shown by available numerical and analogue models, is responsible for the observed local readjustments in the geometry and kinematics of the fractures within the interaction zone.