Magmatic expression of lithospheric thinning across continental rifts

Abstract

Abstract Studies of magmatism associated with continental rifting have traditionally focused only on volcanism within the downfaulted axial zone and along its immediate flanks. Teleseismic travel-time delay studies during the last decade have confirmed the results of earlier gravity surveys of rifted areas, showing that thinning at the base of the continental lithosphere occurs throughout a zone up to about 10 times wider than the physiographic expression of the rift. It is, therefore, logical to consider rifting-related magmatism on the same scale. Potential sources of mafic magmas in rift zones are the thinned subcontinental lithospheric mantle (SCLM), the convecting mantle beneath the continental plate and mixtures of the two. Detailed elemental and radiogenic isotope geochemical studies show that, during the initial extension of continental rifts, the associated mafic magmatism tends to be: (1) relatively sodic and from predominantly convecting mantle sources at the rift axis; (2) relatively potassic and from predominantly lithospheric mantle sources at the margins of the thinned-plate zone. This underlying geochemical pattern is obscured in many instances by such processes as crustal contamination and magma mixing within open-system reservoirs. The mafic ultrapotassic component that provides a distinctive input to SCLM-source magmas appears to be largely fusible at temperatures well below the dry solidus of SCLM; so that, in some cases, prolonged magmatism at a site causes removal of most or all of the potassic lithosphere-source melt (as mafic ultrapotassic magmas or as a contribution to mixed-source melts) without destruction of that lithosphere segment as a geophysically defined unit. Such a zone of refractory lithosphere permits subsequent, recognisable, convecting mantle source melts to penetrate it and reach the surface. These principles are illustrated by discussion of the Neogene-Quaternary magmatism of the Rio Grande, East African, Rhine and Baikal rifts, in the context of the most recent published models of their geophysical structures to depths > 200 km. Teleseismic and gravity studies identify lithospheric thinning beneath the Rio Grande, East African and Baikal rifts across zones 700–800 km wide. The failure of the southern Rhine graben to show a similar deep seismic structure may be a result of efficient buoyant migration of low-viscosity mafic alkalic melt out of the underlying mantle during the 7 Ma period since magmatism ceased, causing seismically defined asthenosphere to revert to lithosphere. A 700 km geochemical traverse across the Rio Grande rift at ~ 37°N, focusing on Oligocene-Miocene magmatism minimally affected by post-genesis processes, shows a clear symmetrical pattern of relatively sodic volcanics at the rift axis and mafic, ultrapotassic magmatism on its outer flanks. The geochemistry of these contrasting magma types is consistent with the view that they originated predominantly within the convecting mantle and SCLM, respectively. The same geochemical pattern is detectable in the volcanism within the equatorial segment of the East African rift system but it is complicated in two zones: east of the Gregory rift and southwest of Lake Kivu, by the effects of previous Cretaceous-Palaeogene magmatism. Limited, published, appropriate, geochemical data show that regional compositional variation in the volcanics associated with the Baikal rift appear to fit the pattern proposed here as a general model for rifting-related magmatism.