Magmatism during the continent – ocean transition

Abstract

As continents break apart, the dominant mechanism of extension transitions from faulting and lithospheric stretching to magma intrusion and oceanic crust formation in a new ocean basin. A common feature of this evolution preserved at magmatic rifted margins worldwide are voluminous lava flows that erupted close to sea level during the final stages of development of the continent-ocean transition (COT). The mechanisms responsible for the generation of the melts that contribute to these voluminous flows, the so-called seaward dipping reflectors (SDR), and their significance in the context of COT development, are relatively poorly understood; they lie deep below post-rift strata along submarine rifted margins where they cannot be studied directly. Extensive coring of the Afar Stratoid Series - an areally-extensive sequence of Pliocene-aged basalts and intercalated sediments that lie atop the developing COT in the sub-aerial Afar Depression, northern Ethiopia - offers fresh scope to address this issue. We present a numerical model simulating the formation of enriched metasomes within the continental lithospheric mantle by the passage of magmas resembling modern axial basalts. Thermal destabilization of the metasome, caused by plate stretching, initiates melt formation within the metasome. These melts, when mixed with a depleted lithospheric mantle component, closely match the range of compositions of the Afar Stratoid Series lavas in this study. Metasomatic re-enrichment and subsequent melting of the lithospheric mantle during the COT may contribute to further plate thinning. These results demonstrate a novel mechanism by which large-volume flows may be erupted during the COT.