Between the Ascension and Bode Verde Fracture Zones, an ∼200 km length of the axis of the Mid-Atlantic Ridge has a large positive residual depth anomaly and an axial high morphology that suggest a plume influence. New wide-angle seismic and gravity data from this area show that (1) the crustal thickness increases from 6 km at the propagating rift marking the northern limit of the anomalous zone to 10 km at the center of the zone and (2) the residual depth anomalies may be largely explained by Airy isostatic support for these crustal-thickness variations. These observations, combined with the lack of long-wavelength gravity and residual depth anomalies associated with the proposed Ascension plume, suggest either the presence of a weak and intermittent plume, or melting of a series of small mantle heterogeneities.

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