Varying styles of magmatic strain accommodation across the East African Rift

Abstract

Observations of active dike intrusions provide present day snapshots of the magmatic contribution to continental rifting. However, unravelling the contributions of upper crustal dikes over the timescale of continental rift evolution is a significant challenge. To address this issue, we analyzed the morphologies and alignments of >1500 volcanic cones to infer the distribution and trends of upper crustal dikes in various rift basins across the East African Rift (EAR). Cone lineament data reveal along-axis variations in the distribution and geometries of dike intrusions as a result of changing tectonomagmatic conditions. In younger (<10 Ma) basins of the North Tanzanian Divergence, dikes are largely restricted to zones of rift-oblique faulting between major rift segments, referred to here as transfer zones. Cone lineament trends are highly variable, resulting from the interplay between (1) the regional stress field, (2) local magma-induced stress fields, and (3) stress rotations related to mechanical interactions between rift segments. We find similar cone lineament trends in transfer zones in the western branch of the EAR, such as the Virunga Province, Democratic Republic of the Congo. The distributions and orientations of upper crustal dikes in the eastern branch of the EAR vary during continental rift evolution. In early-stage rifts (<10 Ma), upper crustal dikes play a limited role in accommodating extension, as they are confined to areas in and around transfer zones. In evolved rift basins (>10 Ma) in Ethiopia and the Kenya Rift, rift-parallel dikes accommodate upper crustal extension along the full length of the basin.

Lithospheric low-velocity zones associated with a magmatic segment of the Tanzanian Rift, East Africa

“…A sequence of damaging earthquakes, dike intrusion, and eruption at Ol Doinyo Lengai volcano in 2007 documents ongoing activity (Baer et al 2008;Calais et al 2008). The source of the dike that intruded the flank of Gelai volcano in the central Natron basin was assumed to be from an active magma chamber beneath Gelai, but the youngest lava flows here have been dated at ∼1 My (Mana et al 2015;Muirhead et al 2015). Prior to this study, it was unclear whether magma chambers do occur beneath volcanoes.…”

“…1). The western flank of the Manyara basin is cross-cut by NE-trending faults and eruptive centres of the seismically active 'Crater Highlands' area which includes a number of Pliocene-Recent eruptive centres (Dawson 1992;Muirhead et al 2015), at the western extremity of the Ngorongoro-Kilimanjaro transverse volcanic axis Mana et al 2015) (Fig. 1).…”

“…1). This area is the location of a major change both in the structural expression of the rift (Dawson 1992;Foster et al 1997;Le Gall et al 2008) and in magmatic behaviour Muirhead et al 2015;Lee et al 2016). Imaging lithospheric structures is one major step to understand the interactions between mantle rheology, magmatism, tectonics and inherited structures in this rifted area.…”

Historical Volcanism and the State of Stress in the East African Rift System

“…Recent examples in the EARS include the 100-km long Dabbahu dyke intrusion in Afar (Wright et al, 2006) and the 2007 Lake Natron dyke intrusion in Tanzania (Calais et al, 2008;Biggs et al, 2009), which were both aligned perpendicular to the plate motion. However, superimposed upon the large-scale stress regime are local stresses related to topography, seismic and magmatic processes (e.g., Biggs et al, 2013b;Maccaferri et al, 2014;Pagli et al, 2014) and which are also seen to control the orientation of magmatic features, such as the Jebel al Tair eruption in the Red Sea (Xu and Jonsson, 2014) and the orientation of fissures around Oldoinyo Lengai in Tanzania (Muirhead et al, 2015).…”

Extension parallel to the rift zone during segmented fault growth: application to the evolution of the NE Atlantic

“…Notably, evidence for this style of inter-rift system architecture has also been noted in the East African Rift, where, in younger portions of the rift, obliquely oriented dikes accommodate rift-zone-parallel extension in the intervening relay zone between rift segments (scale of tens of kilometres; e.g. Muirhead et al, 2015). Rift-parallel-striking fault sets along both margins represent the first and final structural set.…”