Contrast in crustal structure across the Clipperton transform fault from travel time tomography

A three-dimensional (3-D) seismic refraction study of the Clipperton transf orm fault, northern East Pacific Rise, reveals anomalously low compressiona l velocities from the seafloor to the Moho, We attribute this low-velocity anomaly to intensive brittle deformation, caused by transpression across th is active strike-slip plate boundary. The seismic velocity structure south of the Clipperton transform appears unaffected by these tectonic forces, bu t to the north, seismic velocities are reduced over 10 km outside the zone of sheared seafloor. This contrast in seismic velocity structure correspond s well with the differences in mid-ocean ridge morphology across the Clippe rton transform. We conclude that the amount of fracturing of the upper crus t, which largely controls seismic velocity variations, is strongly dependen t on the shallow temperature structure at the ridge axis. Intermittent supp ly of magma to the shallow crust north of the Clipperton transform allows s eawater to penetrate deeper, and the cooler crust is brittle to a greater d epth than south of the transform, where a steady state magma lens is known to exist. The crustal thickness averages 5.7 km, only slightly thinner than normal for oceanic crust, and variations in Moho depth in excess of simila r to0.3 km are not required by our data. The absence of large crustal thick ness variations and the general similarity in seismic structure imply that a steady state magma lens is not required to form normal East Pacific Rise type crust. Perhaps a significant portion of the lower crust is accreted in situ from a patchwork of short-lived gabbro sills or from ductile flow fro m a basal magma chamber as has been postulated in some recent ophiolite stu dies.