The protracted development of focused magmatic intrusion during continental rifting

Abstract

The transition from mechanical thinning toward focused magmatic intrusion during continental rifting is poorly constrained; the tectonically active Main Ethiopian Rift (MER) provides an ideal study locale to address this issue. The presence of linear magmatic-tectonic belts in the relatively immature central MER may indicate that the transition from mechanical to magmatic rifting is more spatially distributed and temporally protracted than has previously been assumed. Here we examine lava geochemistry and vent distribution of a Pliocene-Quaternary linear magmatic chain along the western margin of the central MER-the Akaki Magmatic Zone. Our results show limited variability in parental magma that evolve in a complex polybaric fractionation system that has not changed significantly over the past 3 Ma. Our results suggest the following: (1) channeling of plume material and the localization of shear-or topography-induced porosity modulates melt intrusion into the continental lithosphere. (2) Pre-existing lithospheric structures may act as catalysts for intrusion of magmas into the lithospheric mantle. (3) The midcrustal to upper crustal strain regime dictates the surface orientation of volcanic vents. Therefore, although linear magmatic belts like those in the central MER may young progressively toward the rift axis and superficially resemble oceanic style magmatism, they actually represent prebreakup magmatism on continental crust. The oldest linear magmatic belts observed seismically and magnetically at the edge of the ocean basins thus may not, as is often assumed, actually mark the onset of seafloor spreading.

The making of an underplate: Pyroxenites from the Ethiopian lithosphere

“…1) (Abebe et al, 1998;Abebe Adhana, 2014). Reactivation of such zones of inherent lithospheric weakness and the formation of topography along the lithosphere-asthenosphere boundary in the vicinity of these sutures may facilitate the focused flow of melt into the lithosphere thereby fundamentally altering lithospheric structure (King and Anderson, 1998;Keranen and Klemperer, 2008;Rooney et al, 2014a). Consistent with this model, magmatism is observed along the YTVL (12 MaQuaternary), post-dating the Oligocene flood basalt event (Abebe et al, 1998).…”

“…The topography on the lithosphere-asthenosphere boundary has been previously shown to facilitate focused melt intrusion into the continental lithosphere at the intersection of the YTVL and Main Ethiopian Rift (Rooney et al, 2014a). We propose that such melt focusing occurred either: A) recently -associated with the Afar plume or subsequent rift-related magmatism, B)…”

“…Regionally, magmatism has migrated towards the modern rift valleys, in part due to the melt focusing effect of lithospherphic thinning, thereby increasing the volume of melt transiting the lithosphere beneath the rift. We suggest that the focusing of alkaline melts as a result of topography along lithosphere-asthenosphere boundary of the YTVL (Rooney et al, 2014a), facilitated the formation of clinopyroxene-rich pyroxenites beneath Dedessa. Our results, which demonstrate the existence of metasomatic episodes similar to xenoliths suites from the rift, thus show that the lithospheric mantle beneath Dedessa may be more rift-like than previously acknowledged.…”

“…The focusing of melt within rifts due to lithosphere-asthenosphere topography has been previously reported (Shillington et al, 2009;Keir et al, 2015). The YTVL is considered to have undergone shearing during the Cenozoic (Abebe et al, 1998), and as a result has also likely developed significant topography along its lithosphere-asthenosphere boundary, which has resulted in melt focusing (Keranen and Klemperer, 2008;Rooney et al, 2014a). Models describing the infiltration of melt into the lithospheric mantle suggest that shear is an effective mechanisms to allow for efficient transfer of magma from the asthenosphere into the lithosphere (Havlin et al, 2013).…”

“…With the development of continental rifting in Ethiopia ca. 12-11 Ma (Wolfenden et al, 2004), magmatism became largely focused within the rift valley into linear belts of basaltic cinder cones (Mazzarini et al, 2013;Rooney et al, 2014a), and isolated silicic centers (Peccerillo et al, 2003). A notable exception to this is the quaternary…”