We present the results of nine on-bottom seismic refraction experiments carried out over young East Pacific Rise crust. The experiments are unusual in that both the source and receiver are located within a few meters of the seafloor, allowing high-resolution determinations of shallow crustal structure. Three experiments were located within the axial summit caldera (ASC) over ''zero-age'' crust. The seismic structure at these three locations is fundamentally the same, with a thin (<60 m) surficial low-velocity (<2.5 km/s) layer, a 100 to 150-m-thick transition zone with velocities increasing by ~2.5 km/s, and a layer with velocities of ~5 km/s at a depth beneath the seafloor of ~130--190 m. The surficial low-velocity layer and transition zone are defined as seismic layer 2A, and the ~5 km/s layer is defined as the top of layer 2B. Both the surficial low-velocity layer and the transition zone double in thickness within ~1 km of the rise axis. We model layer 2A as the extrusive sequence and transition zone and the 2A/2B boundary as the top of the sheeted dikes. The primary implication of this interpretation is that the depth to the top of the sheeted dikes deepens from ~150 m to ~300 m within 1 km of the ASC.

The thickening of the extrusive layer is interpreted to be due to lava that either overflows the ASC walls, is emplaced through eruptions outside of the ASC, or travels laterally from the ASC through conduits. The most probable cause for the thickening of the transition zone is sill emplacement outside of the ASC, either from magma that does not reach the surface in an off-axis eruption or magma that is transported laterally during the drainage process creating the ASC. We suggest that the mechanism controlling the magnitude and rate of the dike subsidence is the mechanism that determines the thickness of the extrusive section and the total thickness of layer 2A. ¿ American Geophysical Union 1994