The SISMOMAR experiment carried out seismic measurements of the slow spreading Lucky Strike segment of the Mid-Atlantic Ridge, located approximately 300 km south of the Azores platform. We present results from a segment scale reflection study of seismic layer 2A that is centered on the Lucky Strike volcano and covers the central 54 km of the 70 km long segment and extends 30 km on both sides of the ridge axis. Our study allows new conclusions about the role of tectonic, magmatic and hydrothermal processes in shaping the upper crustal structure of the Lucky Strike segment. The seismic reflection measurements show an almost constant layer 2A two-way time within the median valley and an abrupt layer 2A two-way time decrease off-axis. This two-way time decrease is caused by a layer 2A velocity increase on the order of 1 km/s. The uniform two-way time in the median valley results from a constant thickness layer 2A, which could be caused by the existence of a porosity threshold linked to the lithologic lava/dike boundary. The regularity of the layer 2A/2B interface within the median valley indicates that layer 2A is built entirely inside the median valley. The off-axis velocity increase is consistent with hydrothermal alteration and the aging of the crust.
Seher, Tim Crawford, Wayne C. Singh, Satish C. Cannat, Mathilde