We present a three-dimensional velocity model of the upper crust around the central volcano of the Lucky Strike Segment, Mid-Atlantic Ridge. The model, constructed from a 3-D array of air gun shots (37.5 m spacing along line and 100 m between lines) to ocean bottom seismometers fired during a 3-D seismic reflection survey, shows an off-axis velocity increase (similar to 1 km/s), a low-velocity region within the median valley, and a low-velocity anomaly underneath the Lucky Strike volcano. Our observations indicate a porosity decrease of 1%-9% (corresponding to a velocity increase of similar to 0.5-1 km/s) over a distance of 8 km from the ridge axis (similar to 0.7 Ma) and a porosity decrease of 4%-11% (corresponding to a velocity increase of similar to 2 km/s) between a depth of 0.5 and 1.75 km below seafloor. A sinusoidal variation in the traveltime residuals indicates the presence of azimuthal anisotropy with cracks aligned approximately along the ridge axis. We favor an interpretation in which upper crustal porosities are created by a combination of magmatic accretion (lava-sheeted dike boundary) and active extension (faults, fractures, and fissures). The porosity variation with depth probably depends on pore space collapse, hydrothermal alteration, and a change of stress accommodation. The off-axis porosities are possibly influenced by both hydrothermal precipitation and the aging of the crust.
Seher, Tim Singh, Satish C. Crawford, Wayne C. Escartin, Javier