An explosive seismic refraction experiment using four ocean bottom hydrophone receivers was carried out along a 120-km-long section of the Mid-Atlantic Ridge (MAR) median valley at latitude 23 ¿N immediately south of its intersection with the Kane Fracture Zone. The data are interpreted in terms of laterally homogeneous horizontally layered structures using travel time inversion and synthetic seismogram modeling techniques. These solutions are combined using two-dimensional ray tracing to produce a single model defining the major along-axis structural changes occurring beneath this section of the median valley. With the exception of slightly low layer 3 velocities, the crust beneath several tens of kilometers of the MAR median valley has all the characteristics of simple, mature ocean crust including a well-defined Moho transition zone, ~8 km/s upper mantle velocities, and a total crustal thickness of 6--7 km.

No evidence is found for the presence of a steady state axial magma chamber in the crust or upper mantle, and the excellent propagation characteristics and orderly amplitude versus range relationships imply the existence of a considerable degree of lateral homogeneity beneath the median valley over distances of several tens of kilometers along-axis. We infer from these observations that the basic seismic characteristics of the ocean crust are ''frozen-in'' in a time period less than that separating the major volcanic injection events (10,000--50,000 years) by hydrothermal circulation penetrating to the base of the crust. Along the 100-km-long ridge segment studied here, two major along-axis structural changes occur. The first is a 10- to 15-km-wide (along-axis) zone of lowered velocities in the lower crust centered beneath a major along-axis topographic high. This is interpreted to be the remnants of the most recent phase of injection that has temporarily left behind it a region of elevated temperatures and pervasive cracking and thus reduced velocities. The second occurs abruptly at latitude 23¿15'N and is accompanied by both a major change in rift valley and crestal mountain morphology and the apparent noncoincidence of the median valley and central magnetic anomaly. North of this boundary the ~30-km-long ridge segment that abuts the Kane Fracture Zone has a crustal thickness of 4--5 km and no distinctive layering to produce the characteristic amplitude patterns that typify mature oceanic crust. Interpretation of the cause of this structural change is uncertain but may be related to local tectonic events, perhaps a recent 10- to 20-km ridge jump to the east.