Seismic velocity structure of the ophiolite at Point Sal, southern California, determined from laboratory measurements

Abstract

The Jurassic ophiolite at Point Sal, California, contains all the rock types typically found in other ophiolites. The total thickness is just over 3 km, much thinner than seismically determined ocean crust thicknesses of 5-6 km. We measured the seismic velocities of 34 representative water-saturated samples in the laboratory under pressure and at room temperature. The velocity profile is a function of both metamorphic grade and the concentration of mafic minerals. Oceanic layer 2A compressional velocities are not found in the ophiolite. Layer 2B velocities of 4.9-5.4 km s−1 are found for greenschist facies metabasalts. A velocity of 6.0 km s−1 represents high grade greenschist facies mineral assemblages and amphibole in a dyke and sill complex. Late stage differentiates in the upper plutonic section have layer 3A velocities over 6.0 km s−1. Velocities increase with depth to about 7.1 km s−1due to the introduction of cumulus olivine and other mafic phases. Olivine—clinopyroxene gabbros and troctolites have layer 3B velocities of about 7.4 km s−1. Beneath this, clinopyroxenites and cumulate ultramafics have slightly higher velocities. Serpentinized peridotite (dunite and harzburgite) occurs at the base of the ophiolite and has compressional velocities around 5.5 km s−1. The velocity profile with depth shows prominent gradients in the basaltic and gabbroic layers, but marked discontinuities are seen between the dyke and sill complex and the plutonic section and also between the upper plutonic section and the lower cumulate gabbros and ultramafic rocks. A pronounced velocity inversion, produced by serpentinized dunite underlying the gabbros, has been seismically modelled as a possible in situ characteristic of ocean crust and mantle structure. If a basal low velocity serpentinized layer underlies the ocean crust in some areas, then the Moho is within the mantle and marks the top of unaltered peridotite. Model travel-time curves and seismograms for such a structure show that it would be erroneously interpreted as a thicker ocean crust and that it would yield prominent late reverberations on seismic records.