Elliptic calderas in the Ethiopian Rift: control of pre-existing structures

The Ethiopian Rift is characterized by several Quaternary calderas. Remote sensing and field analyses were used to investigate the regional structural control on three calderas (Fantale, Gariboldi, Gedemsa) in the axial part of the rift. These calderas are located along the Wonji Fault Belt (WFB), a zone of Quaternary NNE-SSW normal faults and extensional fractures. The three calderas show E-W elongation and major E-W vent alignments, oblique with regard to the mean NW-SE extension direction. No significant evidence of E-W tectonic structures has been found near the calderas, the only relevant systems being those of the WFB. Conversely, left-lateral E-W-trending faults are present at the rift borders and on the Nubia and Somalia plateaus, implying a predominant pre-rift activity. The E-W fractures were partly reactivated during rifting, possibly controlling the development of the magma chambers. Thus, the E-W elongation of the calderas would be the surface expression of such a control, rather than the result of regional extension. An evolutionary model on the role of different structures on magmatism at different crustal levels within the rift is proposed.