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 R ise. The Clipperton Area Seismic Study to Investigate Compensation exp eriment (CLASSIC) included surveys to examine the deep structures asso ciated 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 profile s over the RTI high, the northern ridge segment, and the eastern trans form region. The travel time data for crustal phases, Moho reflections , and mantle phases were modeled using two-dimensional ray tracing. Se ismic profiles reveal that the crust is similar in thickness north and south of the Clipperton FZ, despite differences in axial topography t hat have previously been interpreted in terms of differences in magma supply. When compared to older crust, the northern ridge axis is chara cterized by lower seismic velocities and higher attenuation. In our mo del, a low-velocity zone exists beneath the ridge axis, probably assoc iated with a zone of partial melt and/or very high temperatures. Withi n the transform zone, we find that the southeastern trough is underlai n by nearly normal crustal structure. The crust is slightly thinner th an the adjacent aseismic extension but not enough to compensate for th e depths of the trough. Toward the RTI, the trough is replaced by an i ntersection high which appears underlain by a thickened crust, and a t hicker upper crustal section. Both characteristics indicate that the i ntersection high is a volcanic feature produced by excess volcanism at the intersection. The volcanism acts to ''fill in'' the transform tro ugh, creating the thicker crust that extends under the eastern aseismi c extension of the transform. Our results show that the northern ridge segment, often identified as magma-starved, displays the crustal thic kness and apparent signal-attenuation characteristic of a plentiful, b ut perhaps episodic, magma supply.