VOL. 13, NO. I REVIEWS OF GEOPHYSICS AND SPACE PHYSICS FEBRUARY 1975 Structure and Constitution of the Lower Oceanic Crust

Any petrologic model of the lower oceanic crust must be consistent with three sets of data: (I) the seismic structure of the oceanic crust, (2) the petrology of oceanic dredge samples, and (3) laboratory measurements of seismic velocity through such samples. A review of these data indicates that within the framework of earlier three-layer models of oceanic seismic structure the crust is internally complex and varies markedly with age, azimuth, and tectonic province. Mantle compressional wave velocities Vp are anomalously low under the ridge (7.2-7.7 km/s) but increase to ~.O-I:U km/s beyond 15 m.y.; layer 3 thickens by 2 km within 40 m.y. of formation and decreases in Vp from 6.g to 6.5 km/s within gO m.y.; both the mantle and layer 3 are statistically anisotropic. Dredge lithologies consist predominantly of serpentinized ultramafics and mafic igneous rocks ranging from basalt to gabbro, the gabbro often show-ing evidence of fractionation. Metamorphism of mafic rocks from zeolite to amphibolite facies grade is common. Velocities in oceanic serpentinites and basalts are generally lower than layer 3 refraction velocities. Unaltered gabbros have compressional wave velocities of approximately 7.0 km/s, which is high for layer 3, together with shear wave velocities V, of 3.g km/s and values of Poisson's ratio Uof 0.30. Metabasites containing hornblende and plagioclase have values of Vp 6.g km/s, V