Crustal structure of the Cape Verde Swell:
Insights into the flexural response of the lithosphere to loading

Abstract

Throughout the ocean basins many broad regions of anomalously shallow topography exist that do not fit the widely accepted model of conductive plate cooling and subsidence as a function of lithospheric age. These ‘swells’ often coincide with positive geoid, Free-air gravity and heat flow anomalies as well as groups of ocean islands and seamounts. Various mechanisms have been proposed to explain how this anomalous topography is isostatically supported at depth, including increased crustal buoyancy and dynamic asthenospheric support.
The Cape Verde Swell is the largest oceanic mid-plate swell on Earth at ~1800 km in diameter, with a crest ~2.2 km high, and positive geoid, gravity and heat flow anomalies of 8 m, 30 mGal and 10-15 mW m-2, respectively. These characteristics and its location on the slow-to-stationary African Plate, which concentrates the volcanism and associated geophysical anomalies within a relatively small areal extent, makes the Cape Verde Swell an ideal location to test the various proposed mechanisms for swell support.
Wide-angle seismic refraction data along an ~474 km profile, extending from the Cape Verde Swell crest, is analysed and modelled to produce a 2-D velocity-depth model of the crustal structure. The resulting model reveals no widespread thickening of the lower oceanic basement, despite evidence for localised thickening beneath the islands from other studies. Subsequent 3-D ‘whole plate’ lithospheric flexure modelling reveals that, on a regional scale, the plate is stronger than expected based on its age, with some evidence for localised weakening around the islands.
Overall, the results of this study suggest that the anomalously shallow topography of the Cape Verde Swell is primarily maintained by a dynamic upwelling of hot, low density material impinging on the base of the lithosphere. Over time, conduction from this hot column has thermally rejuvenated the lithosphere on a local scale, leading to additional uplift, melting and volcanism associated with the islands.