Ophiolites, seismic velocities and oceanic crustal structure

Abstract

Seismic velocities have been measured to pressures of 10 kbars for 96 cores of 36 rocks collected from ophiolite complexes. Average densities (ρ), compressional wave velocities (Vp), shear wave velocities (Vs) and Poisson's ratios (σ) at 1 kbar for major rock types are as follows: AA1RockρVpVsσ(g/cm3)(km/sec)(km/sec)Serpentinite2.535.052.450.35Trondhjemite2.696.333.670.25Spilite2.725.803.180.28Metagabbro (greenschist facies)2.816.523.620.28Metagabbro (amphibolite facies)2.996.953.730.29Gabbro3.007.203.800.31Pyroxenite3.247.824.250.29 Velocities in metagabbro and gabbro, the probable constituents of the lower oceanic crust, are highly dependent on mineralogy and mineral orientation. Both can be significantly anisotropic, the anisotropies originating from preferred hornblende orientation in metagabbro and olivine orientation in cumulate gabbro. Velocities in serpentinites depend upon amounts of relic olivine and pyroxene and serpentine mineralogy. Of significance, antigorite is faster than lizardite and chrysotile. Due primarily to a high quartz content, trondhjemite has a low Poisson's ratio. Velocity profiles constructed from reported petrology for the Vourinos, Troodos, Semail, Papua and Bay of Islands Massifs have much in common with seismic profiles of the oceanic crust. Major seismic discontinuities result from (1) increasing metamorphic grade with depth (the layer 2–3 boundary) and (2) a transition from gabbro, anorthosite and troctolite to dunite and peridotite (the Mohorovic̆ic̆ discontinuity).