Sea floor spreading, progressive alteration of layer 2 basalts, and associated changes in seismic velocities

Abstract

Compressional ( V p ) and shear ( V s ) wave velocities and the dependent elastic constants have been determined by the pulse transmission technique to 10 kb for basalts from six Atlantic sites of the Deep Sea Drilling Project. V p and V s are found to vary linearly with density, V p ranging at 0.5 kb from 4.53 km/sec for the lowest density sample (2.40 g/cm 3) to 6.37 km/sec for the highest (2.92 g/cm 3), while V s ranges similarly from 2.09 to 3.38 km/sec. This range of V p is consistent with the wide variability in layer 2 velocities found from refraction studies. Petrographic and X-ray analyses demonstrate that the wide range in densities observed for these basalts is the result of progressive clay alteration and low-grade metamorphism; elastically this can be noted as an increase at 10 kb in d V p /d P from 0.01 to 0.05 km/sec kb and an increase at 0.5 kb of compressibility from 1.38 to 2.58 Mb −3 with decreasing density. Of particular interest, in the ridge traverse of Leg 3 where the basalt ages are well known, density and velocity decrease linearly with age from the ridge crest to the abyssal plain. At least provincially, where uniformity of rock type may be suspected, it may be possible to derive submarine weathering rates and approximate layer 2 ages from careful refraction surveys.