Recent volcano-tectonic activity of the Ririba rift and the evolution of rifting in South Ethiopia

The relationships between volcanic activity and tectonics at the southernmost termination of the Main EthiopianRift (MER), East Africa, still represent a debated problem in the MER evolution. New constraints on the timing,evolution and characteristics of the poorly documented volcanic activity of the Dilo and Mega volcanic fields(VF), near the Kenya-Ethiopia border are here presented and discussed. The new data delineate the occurrenceof two distinct groups of volcanic rocks: 1) Pliocene subalkaline basalts, observed only in the Dilo VF, forminga lava basement faulted during a significant rifting phase; 2) Quaternary alkaline basalts, occurring in the two volcanic fields as pyroclastic products and lava flows issued from monogenetic edifices and covering the rift-relatedfaults. 40Ar/39Ar dating constrains the emplacement time of the large basal lava plateau to ~3.7 Ma, whereas theyoungest volcanic activity characterising the two areas dates back to 134 ka (Dilo VF) to as recent as the Holocene(Mega VF). Volcanic activity developed along tectonic lineaments independent from those of the rift. No directrelations are observed between the Pliocene, roughly N-S-trending major boundary faults of the Ririba rift andthe NE-SW-oriented structural trend characteristic of the Quaternary volcanic activity. We speculate that thischange in structural trend may be the expression of (1) inherited crustal structures affecting the distribution ofthe recent volcanic vents, and (2) a local stress field controlled by differences in crustal thickness, following amajor episode of reorganization of extensional structures in the region due to rift propagation and abandonment.