Velocity structure from forward modeling of the eastern ridge-transform intersection area of the Clipperton Fracture Zone, East Pacific Rise

In the spring of 1994, we undertook an extensive geophysical study of the Clipperton Fracture Zone (FZ) on the fast spreading East Pacific Rise. The Clipperton Area Seismic Study to Investigate Compensation experiment (CLASSIC) included surveys to examine the deep structures associated with the fracture zone and adjacent northern ridge segment. In this paper, we report the results from five seismic profiles acquired over the eastern ridge-transform intersection (RTI), including profiles over the RTI high, the northern ridge segment, and the eastern transform region. The travel time data for crustal phases, Moho reflections, and mantle phases were modeled using two-dimensional ray tracing. Seismic profiles reveal that the crust is similar in thickness north and south of the Clipperton FZ, despite differences in axial topography that have previously been interpreted in terms of differences in magma supply. When compared to older crust, the northern ridge axis is characterized by lower seismic velocities and higher attenuation. In our model, a low-velocity zone exists beneath the ridge axis, probably associated with a zone of partial melt and/or very high temperatures. Within the transform zone, we find that the southeastern trough is underlain by nearly normal crustal structure. The crust is slightly thinner than the adjacent aseismic extension but not enough to compensate for the depths of the trough. Toward the RTI, the trough is replaced by an intersection high which appears underlain by a thickened crust, and a thicker upper crustal section. Both characteristics indicate that the intersection high is a volcanic feature produced by excess volcanism at the intersection. The volcanism acts to ''fill in'' the transform trough, creating the thicker crust that extends under the eastern aseismic extension of the transform. Our results show that the northern ridge segment, often identified as magma-starved, displays the crustal thickness and apparent signal-attenuation characteristic of a plentiful, but perhaps episodic, magma supply.