Off-axis volcano-tectonic activity during continental rifting: Insights from the transversal Goba-Bonga lineament, Main Ethiopian Rift (East Africa)

Abstract

The Main Ethiopian Rift, East Africa, is characterized by the presence of major, enigmatic structures which strike approximately orthogonal to the trend of the rift valley. These structures are marked by important deformation and magmatic activity in an o ff -axis position in the plateaus surrounding the rift. In this study, we present new structural data based on a remote and fi eld analysis, complemented with analogue modelling experiments, and new geochemical analysis of volcanic rocks sampled in di ff erent portions of one of these transversal structures: the Goba-Bonga volcano-tectonic lineament (GBVL). This integrated analysis shows that the GBVL is associated with roughly E – W-trending prominent volcano-tectonic activity a ff ecting the western plateau. Within the rift fl oor, the approximately E – W alignment of Awasa and Corbetti calderas likely represent expressions of the GBVL. Conversely, no tectonic or volcanic features of similar (E – W) orientation have been recognized on the eastern plateau. Analogue modelling suggests that the volcano-tectonic features of the GBVL have probably been con- trolled by the presence of a roughly E – W striking pre-existing discontinuity beneath the western plateau, which did not extend beneath the eastern plateau. Geochemical analysis supports this interpretation and indicates that, although magmas have the same sub-lithospheric mantle source, limited di ff erences in magma evolution dis- played by products found along the GBVL may be ascribed to the di ff erent tectonic framework to the west, to the east, and in the axial zone of the rift. These results support the importance of the heterogeneous nature of the lithosphere and the spatial variations of its structure in controlling the architecture of continental rifts and the distribution of the related volcano-tectonic activity