Crustal structure, magma chamber, and faulting beneath the lucky strike hydrothermal vent field

The Lucky Strike hydrothermal field sits at the summit of the Lucky Strike segment central volcano, at 37°17'N on the Mid-Atlantic Ridge. The Seismic Studies for the Monitoring of the Mid-Atlantic Ridge (MOMAR) program (SISMOMAR) experiment constrained crustal structure over a range of scales from the central volcano to the entire segment. The experiment reveals an axial magma chamber (AMC) reflector beneath the hydrothermal field, an underlying low-velocity zone (LVZ), and faults that may facilitate fluid flow and heat transfer between the AMC and the seafloor. The AMC reflector lies 3.0-3.8 km beneath the seafloor and is up to 2 km wide by at least 5 km long. The underlying LVZ spans the lower crust and has a maximum anomaly of at least 0.5 km s^-1. The axial valley's Eastern Boundary Fault penetrates to within ̃1 km of the eastern edge of the AMC reflector, and the lower crustal LVZ stops abruptly beneath the bottom of this fault. Faults penetrating from the west flank of the volcano arrive within <1 km of the top of the AMC reflector. Upper crustal velocities are ̃0.5 km s^-1 slower and the layer 2A event 0.2 s later (two-way travel time) inside the axial valley than outside. Within the axial valley, the upper crust is nearly uniform over the central two thirds of the segment, but the layer 2A reflector shallows, and the 4.5 km s^-1 isovelocity contour deepens at the segment ends, suggesting that the spreading regime changes from predominately magmatic at the segment center to predominantly tectonic at the segment ends.