Crustal thickness of V-shaped ridges south of the Azores: Interaction of the Mid-Atlantic Ridge (36 degrees-39 degrees N) and the Azores hot spot

V-shaped ridges propagating along the Mid-Atlantic Ridge axis south of the Azores and Iceland hot spots indicate that ridge-hot spot interactions prod uce temporal and spatial variations in melt supply to the ridge axis. Estim ates of relative crustal thickness variations associated with the ridges so uth of the Azores hot spots, based on gravity and bathymetry data collected during the SudAcores cruise (1998), provide constraints on the rate of pro pagation of these melt anomalies and on the variations in melt production a long the axis and in time. The maximum apparent crustal thickness along the Azores V ridge is similar to 14 km near the Azores, decreasing to normal c rustal thickness of similar to6 km toward the south. This crustal thickness variation may be explained by enhanced melt production associated with the propagation of a mantle temperature anomaly that initiated similar to 10 M yr ago at the Azores hot spot. The temperature anomaly decreased as it prop agated southward, reaching ambient mantle temperatures at the present time at its predicted location under the axis. The excess melt was emplaced on a xis forming discrete, shallow (< 1000 m) oceanic plateaus (similar to 100 k m in diameter at similar to 37.5 degreesN) that are isostatically compensat ed. The numerous seamounts, lack of normal faults, and smooth basement at t he summit of these plateaus suggest high effusion rates that persisted for similar to5 Myr or less, with little or no tectonic strain. As the melt ano maly propagated along axis, the magmatic activity at the plateaus ceased, r esulting in rifting of the plateau and onset of normal seafloor spreading. The variations in crustal thickness inferred for the V ridges south of the Azores are at least twice that inferred for the Iceland structures. In both cases the V ridges record temporal variations in temperature and/or mantle flux that affect melt production under the ridge axis, but the fluctuation s are larger for the Azores than for the Iceland hot spot.