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

Abstract

V-shaped ridges propagating along the Mid-Atlantic Ridge axis south of the Azores and Iceland hot spots indicate that ridge-hot spot interactions produce temporal and spatial variations in melt supply to the ridge axis. Estimates of relative crustal thickness variations associated with the ridges south of the Azores hot spots, based on gravity and bathymetry data collected during the SudAçores cruise (1998), provide constraints on the rate of propagation of these melt anomalies and on the variations in melt production along the axis and in time. The maximum apparent crustal thickness along the Azores V ridge is ∼14 km near the Azores, decreasing to normal crustal thickness of ∼6 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 ∼10 Myr ago at the Azores hot spot. The temperature anomaly decreased as it propagated southward, reaching ambient mantle temperatures at the present time at its predicted location under the axis. The excess melt was emplaced on axis forming discrete, shallow (<1000 m) oceanic plateaus (∼100 km in diameter at ∼37.5°N) that are isostatically compensated. The numerous seamounts, lack of normal faults, and smooth basement at the summit of these plateaus suggest high effusion rates that persisted for ∼5 Myr or less, with little or no tectonic strain. As the melt anomaly propagated along axis, the magmatic activity at the plateaus ceased, resulting 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 fluctuations are larger for the Azores than for the Iceland hot spot. Copyright 2001 by the American Geophysical Union.