Extrusive thickness variability at the East Pacific rise, 9 degrees-10 degrees N: Constraints from seismic techniques

We calculate synthetic shot gathers and their corresponding common depth point (CDP) profiles over plausible East Pacific Rise (EPR) shallow velocity structures, based on the structures obtained from high-resolution on-bottom seismic refraction experiments. We then use these results to analyze the variability in layer 2A thickness at the EPR 9 degrees-10 degrees N region, as measured by CDP, wide-aperture profile (WAP), on-bottom seismic refraction experiments, and conventional air gun refraction data. The synthetics indicate that the accuracy of correlating the prominent shallow reflector observed in CDP and wide-angle data with the layer 2A/2B boundary is strongly dependent on the structure within layer 2A. If layer 2A consists of a surficial low-velocity layer overlying a steep velocity gradient (our gradient model), then there is an excellent correspondence between the two-way travel times to the shallow reflector and the base of layer 2A. However, the shallow reflector may originate from a gradient within layer 2A if the upper crust contains more than one high-gradient region (our step model). This implies that independent estimates of layer 2A velocity structure are needed to properly interpret CDP and wide-angle data. We also determine that the travel time to the layer 2A reflector, for identical velocity structure, can vary by as much as 50 ms (about 125 m) for differing experimental geometries. This can explain the discrepancy in two-way travel time to the layer 2A reflector imaged on zero-age CDP and WAP lines. The depths to a shallow reflector calculated from CDP and wide-angle data in the 9 degrees-10 degrees N region of the EPR generally correlate with estimated layer 2A thicknesses from on-bottom refraction profiles and conventional air gun refraction lines, which suggests that the upper crustal structure in this area is similar to the gradient model. WAP and conventional air gun refraction data indicate that there is a 100-200 m decrease in off-axis layer 2A thickness at 9 degrees 35'N on the EPR, the present-day location of a deviation in axial linearity (deval). There is no bathymetric expression of the 50% decrease in layer 2A thickness. Layer 2A can be interpreted to consist of the extrusive section and transition zone, with the layer 2A/2B boundary corresponding to the top of the sheeted dikes. We suggest that buoyancy forces associated with the axial magma chamber (AMC) are supporting the extrusive layer and sheeted dikes at the neovolcanic zone. With distance from the rise axis, the AMC solidifies, the crust cools, the buoyancy forces are reduced, and the sheeted dike complex subsides. Concurrently, the extrusive layer thickens resulting in significantly less subsidence of the seafloor. We speculate that the 50% decrease in dike subsidence and extrusive thickness at the 9 degrees 35'N deval is due to a local reduction in magma supply within the axial magma chamber. The off-axis pattern of layer 2A thickness suggests that the 9 degrees 35'N deval has persisted for 175,000-275,000 years.