Control of the stress field and rift structures on the distribution and morphology of explosive volcanic craters in the Manyara and Albertine rifts

摘要

Monogenetic volcanoes are widespread in the East African Rift. Their morphologies are diverse with positive (scoria and tuff cones) and negative reliefs (tuff rings and maars). The Manyara Rift (Tanzania) and Albertine Rift (Uganda) are characterised by a singular abundance of negative volcanic reliefs with about 300 and 350 maars and tuff rings, respectively, covering a surface of about 3000 km(2) in each country. Such high density of explosive craters is not observed elsewhere in the East African Rift.We perform spatial distribution and alignment analyses using two different approaches (Two Point Analysis and Point Alignments) to study the link between the distribution of the monogenetic volcanic vents, the rift structures, and the stress field. Both methods give similar results although the Point Alignments perform better for large radius of investigations. Volcanic alignments in the Manyara rift suggest a dominant control of the old basement and rift structures on dyke emplacement and distribution of volcanic vents, under an oblique rifting. However, data further suggest that this influence is reducing, and the rift is entering a more mature phase with orthogonal rifting. On the other hand, volcanic alignments in the Albertine rift imply a dominant control of the stress field on dyke emplacement and on the distribution of volcanic vents under orthogonal lifting. This suggests that the Albertine rift is a mature rift since at least the past 50 000 yrs.We further tested the influence of rift structures and stress field on maar and tuff ring morphologies. Most of the craters are near-circular, or a least not strongly elongated, and show minor complexities in shape which we attribute to lateral migration of explosions. Few maars and tuff rings in Manyara show that lateral migration have followed pre-existing faults. In general, we advise not to fit an ellipse to individual maars and tuff rings to assess the orientation of the feeder intrusions.