On the heat flux related to stretching in the NW-Mediterranean continental margins

Abstract

The surface thermal flux of the continental margins of the northwestern Mediterranean Sea is interpreted on the basis of a 1-D instantaneous pure shear stretching model of the lithosphere in terms of three components: the background heat flowing out from the asthenosphere (38 mW m−2), the transient contribution depending on the rift age and extension amount (35 mW m−2 at the most), and the contribution due to the radiogenic elements of the lithosphere. The radiogenic component is estimated at the continental margins of the Ligurian-Provencal basin and Valencia trough, and in the surrounding mainland areas by means of available data of surface heat generation from Variscan Corsica, Maures-Esterel and the Central Massif along with a geophysical-petrological relationship between heat production and seismic velocity. The lithosphere radiogenic heat contribution ql decreases with the thinning factor β according to the exponential law: ql(β) = a exp(-bβ), in which factor b is greater for that part of the lithosphere below the uppermost 10 km. Considering also the heat generated by radioactive isotopes in sediments, the stable Variscan lithosphere produces an average thermal flux of 30 mW m−2 which decreases by about one half where the lithosphere is thinned by one third. Although the surface heat generation is 2·1 − 3·3 µW m−3 in the Maures-Esterel massif — excepting small outcrops of dioritic rocks with lower heat production — and 1·8 µW m−3 for most of Corsica, the radiogenic heating within the lithosphere for such areas is nearly the same and does not explain the higher heat flux of the Corsica margin. This asymmetric thermal pattern with surface heat flux which is 10 − 15 mW m−2 higher than predictions is probably of upper mantle origin, or can be ascribed to penetrative magmatism.