Crust at slow-spreading ridges is formed by a combination of magmatic and tectonic processes, with magmatic accretion possibly involving short-lived crustal magma chambers. The reflections of seismic waves from crustal magma chambers have been observed beneath intermediate and fast-spreading centres, but it has been difficult to image such magma chambers beneath slow-spreading centres, owing to rough seafloor topography and associated seafloor scattering. In the absence of any images of magma chambers or of subsurface near-axis faults, it has been difficult to characterize the interplay of magmatic and tectonic processes in crustal accretion and hydrothermal circulation at slow-spreading ridges. Here we report the presence of a crustal magma chamber beneath the slow-spreading Lucky Strike segment of the Mid-Atlantic Ridge. The reflection from the top of the magma chamber, centred beneath the Lucky Strike volcano and hydrothermal field, is approximately 3 km beneath the sea floor, 3-4 km wide and extends up to 7 km along-axis. We suggest that this magma chamber provides the heat for the active hydrothermal vent field above it. We also observe axial valley bounding faults that seem to penetrate down to the magma chamber depth as well as a set of inward-dipping faults cutting through the volcanic edifice, suggesting continuous interactions between tectonic and magmatic processes.

[1] We estimate the seismic structure of the slow spreading Lucky Strike segment of the Mid-Atlantic Ridge, located approximately 300 km south of the Azores platform, using seismic reflection and seismic refraction data acquired in June 2005 as a part of the Seismic Study for Monitoring of the Mid-Atlantic Ridge (SISMOMAR) survey. The threedimensional velocity model shows an upper crustal low-velocity anomaly running parallel to the ridge axis, which is limited by the median valley bounding faults. The velocity models also show a low-velocity anomaly underlying the axial melt lens reflector located at the segment center below the Lucky Strike volcano. This lower crustal low-velocity region can be explained by elevated temperatures and possibly small amounts of melt. The lower crustal low-velocity anomaly and the axial melt lens reflector constrain the geometry of the magma chamber responsible for the construction of the Lucky Strike volcano. The presence of this magma chamber and thick crust at the segment center are consistent with a focused melt supply to the segment center.

We present results from three‐dimensional (3‐D) processing of seismic reflection data, acquired in June 2005 over the Lucky Strike volcano on the Mid‐Atlantic Ridge as a part of the Seismic Study for Monitoring of the Mid‐Atlantic Ridge survey. We use a 3‐D tomographic velocity model derived from a coincident ocean bottom seismometer experiment to depth convert the poststack time‐migrated seismic volume and provide 3‐D geometry of the axial magma chamber roof, fault reflectors, and layer 2A gradient marker. We also generate a high‐resolution bathymetric map using the seismic reflection data. The magma chamber roof is imaged at 3.4 ± 0.4 km depth beneath the volcano, and major faults are imaged with dips ranging between 33° and 50°. The magma chamber roof geometry is consistent with a focused melt supply at the segment center and steep across‐axis thermal gradients as indicated by the proximity between the magma chamber and nearby faults. Fault scarps on the seafloor and fault dip at depth indicate that tectonic extension accounts for at least 10% of the total plate separation. Shallow dipping reflectors imaged in the upper crust beneath the volcano flanks are interpreted as buried lava flow surfaces.

[1] 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.