Plio-Quaternary volcanotectonic activity in the northern sector of the Main Ethiopian Rift: relationships with oblique rifting

Deformation and magmatism within the similar to 90 km wide northern Ethiopi an Rift system is concentrated along a narrow zone - the Wonji Fault Belt. Two key areas (the Nazret-Dera and Asela-Ziway areas), located along the ea stern margin of the north-northeast to northeast trending Main Ethiopian Ri ft, have been investigated in order to reconstruct the recent tectonomagmat ic evolution of the northern branch of the Main Ethiopian Rift. In these ar eas, Early Pleistocene volcanic products (Wonji Group) overlie Pliocene vol canic rocks (Eastern Margin Unit). Detailed stratigraphical reconstructions have revealed the presence of several tectonomagmatic units which can be c orrelated between the two study areas. The stratigraphical and petrological study of these units outlined (1) the bimodal composition (basalts-pantell erites) of the oldest and youngest units and the unimodal character (pantel lerites) of the products erupted during the intervening period; (2) the mai nly fissural origin of the ignimbrites and oldest basalts; and (3) a mafic/ felsic volumetric ratio of 1:5. The geological data suggest that, around the Pliocene-Quaternary boundary, a change in the stress field occurred in this Main Ethiopian Rift sector, p assing from a direction of extension roughly orthogonal to the rift shoulde rs, to oblique rifting related to an east-west trending extension. In this framework the change in the style of volcanism observed in the Nazret-Dera and Asela-Ziway areas can be related to the change of the stress field. A n ew geodynamic model is presented for the Late Pliocene to Recent evolution of this sector of the Main Ethiopian Rift. According to this model, a large volume of rhyolitic products was erupted during an oblique rifting phase, following a previous period of pure extension. The change in the tectonic r egime favoured partial melting of the underplated basalts as a decrease in the pressure and an elevation of isotherms occurred. (C) 2000 Elsevier Scie nce Limited. All rights reserved.