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 Ethiopian
Rift (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 occurrence
of two distinct groups of volcanic rocks: 1) Pliocene subalkaline basalts, observed only in the Dilo VF, forming
a lava basement faulted during a significant rifting phase; 2)Quaternary alkaline basalts, occurring in the twovolcanic
fields as pyroclastic products and lava flows issued frommonogenetic edifices and covering the rift-related
faults. 40Ar/39Ar dating constrains the emplacement time of the large basal lava plateau to ~3.7 Ma, whereas the
youngest volcanic activity characterising the twoareas 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 direct
relations are observed between the Pliocene, roughly N-S-trending major boundary faults of the Ririba rift and
the NE-SW-oriented structural trend characteristic of the Quaternary volcanic activity. We speculate that this
change in structural trend may be the expression of (1) inherited crustal structures affecting the distribution of
the recent volcanic vents, and (2) a local stress field controlled by differences in crustal thickness, following a
major episode of reorganization of extensional structures in the region due to rift propagation and abandonment