Uplift and heat flow following the injection of magmas into the lithosphere

Abstract

Summary. The thermal effect of a rapid injection of hot magmas into the lower part of the lithosphere is modelled as an increase in heat production through the invaded region. The change in surface heat flow and the uplift resulting from the thermal expansion are determined in three-dimensional axially symmetric geometry: they are expressed as the space time convolutions of a Green’s function with the anomalous heat production. The anomalies with shorter wavelength (compared to the lithospheric thickness) are attenuated. This filtering affects the surface uplift more than the heat flow anomaly; the attenuation effect is larger when only the lower part of the lithosphere is invaded. The uplift time constant is of the same order as the heat conduction time if the lower lithosphere is invaded by magmas at a moderate rate (i.e. the rate of injection does not exceed the equivalent of 0.1 per cent of the lithospheric volume in 106yr). Fifty per cent of the total uplift takes place in about 80x 106yr for a lithosphere lOOkm thick. The uplift is slightly faster when the whole lithosphere is invaded. The heat flow anomaly is delayed when the lower part of the lithosphere is invaded. The spatial extent and the timing of the uplift and heat flow anomalies are critical in determining the mechanism’s feasibility. Magma injections explain rapid uplifts [> 100 m (lo6 yr)-’] only if the magma is supplied at a very high rate (i.e. at least 10 per cent of the lithosphere volume per 106yr). It is a feasible mechanism for uplifts that occur over longer periods of time (- 30 x 106yr) such as those that seem to have occurred when the African plate came to rest with respect to the mantle.