SEISMIC STRUCTURE OF THE PERIDOTITE RIDGE , WEST OF THE GALICIA BANK (WEST IBERIA)

Refraction profiles have been carried out along and across the peridot ite ridge at the ocean-continent boundary west of the Galicia margin. Deep sea drilling (ODP Leg 103) shows that the ridge is composed of se rpentinized peridotite. The shallow seismic velocity within the ridge is as low as 3.70 km/s, denoting a high grade of serpentinization. Mod elling the ridge illustrates its heterogeneity and the large extent of the serpentinization from the crest to 6-8 km below sea floor. A uniq ue downward velocity gradient within the ridge is ruled out. At a dept h of 3 km below sea floor, the velocity gradient decreases abruptly th rough a second order discontinuity bisecting the ridge from west to ea st. Within a west-dipping transition zone at a depth of roughly 11 km, velocities increase from 7.40 to 7.65 km/s.