Middle-lower continental crust exhumed at the distal edges of volcanic passive margins

Abstract

AbstractThe extent of continents in oceans is an important scientific, economic and political issue. The crustal types seaward of the necking zones of volcanic passive margins are investigated. From a set of high-quality seismic data from Namibia we discuss the nature of this crust, oceanic or continental. Here we show that over ~100 km, this crust shares few of the characteristics of oceanic crust born from accretion at a slow-spreading oceanic ridge. We alternatively suggest that the middle-lower continental crust could be exhumed and made brittle at the most distal end of volcanic passive margins. We consecutively distinguish the mechanical break-up of the continental lithosphere at volcanic passive margins, i.e., the drop in mechanical strength, from a stage of steady-state syn-magmatic extension, most probably subaerial, predating the onset of the purely magmatic oceanic crust. These findings suggest that the extent of continental material in oceans could be more significant than isolated continental blocks and microcontinents.