Uplift at lithospheric swells – II: is the Cape Verde mid-plate swell supported by a lithosphere of varying mechanical strength?

Abstract

The Cape Verde mid-plate swell is the largest amplitude oceanic mid-plate swell on Earth at ∼1800 km in diameter, with a crest ∼2.2 km high, and long-wavelength 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 moving-to-stationary African Plate, which concentrates the volcanism and associated geophysical anomalies within a relatively small areal extent, makes it an ideal location to test various proposed mechanisms for swell support. Wide-angle seismic refraction data have been acquired along a ∼474 km profile extending north–south from the swell crest. In this paper, the 2-D velocity–depth crustal model derived from forward modelling of phase traveltime picks is tested using two independent inversion approaches. The final crustal velocity–depth model derived from the combined modelling, shows no evidence for widespread thickened crust or for lower crustal velocities exceeding 7.3 km s−1 that are indicative of undercrustal magmatic material. Using the final velocity–depth model to constrain the crust for 3-D ‘whole plate’ lithospheric flexure modelling of island loading alone, we show that the lithosphere of the Cape Verde region appears stronger than expected for its age. Regional-scale modelling suggests that the majority of the swell height is supported by dynamic upwelling within the asthenosphere coupled with, but to a lesser degree, the effect of a region of low density in the deeper lithosphere, originating most likely from conductive reheating of the overlying plate due to its slow-to-stationary motion. When this regional upward-acting buoyancy force is considered in the context of the shorter wavelength flexure associated with island loading, modelling suggests that the apparent high plate strength is a consequence of, in effect, a regional unbending of a lithosphere that has a long-term strength typical for its age.