Anomalous heat flow and geoid across the Cape Verde Rise: evidence for dynamic support from a thermal plume in the mantle

Abstract

Summary. We describe results from 200 heat flow measurements across the Cape Verde Rise in the North Atlantic. They show that the heat flow through normal, 125 Myr-old crust is 45.5 ± 3.4mWm-2, close to the prediction for a lithospheric plate model. Heat flow increases towards the centre of the Rise, reaching a maximum of 16 ± 4 mW m-2 above the normal oceanic value. The concommitant geoid anomaly at the centre of the Rise is 7.6 ± 0.3 m and the depth anomaly is 1900 ± 200 m. The anomalous heat flow, geoid and bathymetric values are used to constrain a variety of theoretical models of hot spot mechanisms. Lithospheric reheating models alone, whether involving transient or sustained thinning of the lithosphere, fail to model the broad heat flow anomaly and the long duration of uplift and volcanism observed on the Cape Verde Rise. Axisymmetric convection models provide a better fit to the observed data, and suggest that the swell is a consequence primarily of dynamic uplift generated by an ascending thermal plume in the underlying mantle. Uplift caused by expansion of the solid lithosphere is of smaller importance. The observed geoid, bathymetry and heat flow anomalies are reproduced well using convection models with upper mantle viscosities of 2-4 times 1020 Pa s and a coefficient of thermal expansion in the range 2.0 - 2.5 times 10-5 K-1. The effect of melting in the ascending plume is to buffer the heat flow anomalies. In the case of the Cape Verde Rise, partial melting is minimal, but its effect on other hotspots will be to limit observed heat flow anomalies at the surface to less than 20-25 mW m-2.