Depleted swell root beneath the Cape Verde Islands

Abstract

The hotspot swell—an area of uplifted bathymetry or topography surrounding regional volcanism—is a defining hotspot characteristic, yet its origin is poorly understood. To test current ideas about swell formation, we studied the crust and shallow mantle structure of the Cape Verdes in a passive seismic experiment. The Cape Verde Islands are ∼450 km west of Senegal in the tropical Atlantic Ocean and are on the southwest flank of the Cape Verde Rise, the largest bathymetric anomaly in the oceans, rising ∼2 km above the surrounding seafloor ([Crough, 1982][1]). The archipelago occupies a unique position, an approximately stationary one in the hotspot frame of reference ([Gripp and Gordon, 2002][2]) and therefore with respect to the melting source believed to have produced it. Here we present an analysis of compressional to shear (P to S) converted seismic phases, recorded on a temporary network of seismograph stations on the Cape Verde Islands, that indicate a crust thickened to 22 km is underlain by a high-velocity, low-density layer, which overlies a zone of low shear-wave velocity starting at ∼80 km depth. We also measured shear-wave splitting delay times for teleseismic SKS phases, which are ∼0.81 s, compatible with an origin in this same layer. We interpret these observations as effects of hotspot melting, which produces a thickened crust and a depleted swell root that buoys the ocean floor and spreads laterally as it grows over time. [1]: #ref-5 [2]: #ref-11