Seismic structure across the rift valley of the Mid-Atlantic Ridge at 23°20′ (MARK area): Implications for crustal accretion processes at slow spreading ridges

International audienc

Focused volcanism and growth of a slow spreading segment (Mid-Atlantic Ridge, 35°N)

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Seismic structure across the rift valley of the Mid-Atlantic Ridge at 23°20′ (MARK area): Implications for crustal accretion processes at slow spreading ridges

International audienc

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

We thank the officers and crew of the R/V L'Atalantef or their assistanced uringt he Suda9oresC ruise, and the rest of the scientific party for data gathering, processing and discussions. We also thank J.P. Canales, J. Dafiobeitia, E. Gfftcia, and G. Ito for discussions of various aspects of this work and comments on the manuscript. Reviews by G. Kent and of R. S. White, and reccomendationbsy the AssociateE ditor E. Klein, contributedt o the final manuscript. M. Muller assisted on the thermal calculations . The GMT software [Wesseland Smittl 1991] was extensively used in this studyInternational audienceV-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 SudAqores 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 •37.5øN) 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