RIDGE-PLUME INTERACTIONS OR MANTLE HETEROGENEITY NEAR ASCENSION ISLAND

Between the Ascension and Bode Verde Fracture Zones, an similar to 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 plu me influence, New wide-angle seismic and gravity data from this area s how that (1) the crustal thickness increases from 6 km at the propagat ing 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 lar gely explained by Airy isostatic support for these crustal-thickness v ariations. These observations, combined with the lack of long-waveleng th gravity and residual depth anomalies associated with the proposed A scension plume, suggest either the presence of a weak and intermittent plume, or melting of a series of small mantle heterogeneities.