Structure and constitution of the lower oceanic crust

Abstract

Any petrologic model of the lower oceanic crust must be consistent with three sets of data: (1) 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 V p are anomalously low under the ridge (7.2–7.7 km/s) but increase to 8.0–8.3 km/s beyond 15 m.y.; layer 3 thickens by 2 km within 40 m.y. of formation and decreases in V p from 6.8 to 6.5 km/s within 80 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 showing 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 s of 3.8 km/s and values of Poisson's ratio σ of 0.30. Metabasites containing hornblende and plagioclase have values of V p = 6.8 km/s, V s = 3.8 km/s, and σ = 0.28, in good agreement with those of layer 3. On the basis of petrology and velocity it is suggested that layer 3 is composed of hornblende metagabbro underlain by normal gabbro. In a model consistent with geophysical observations of heat flow, seismicity, gravity, and seismic structure at the ridge it is proposed that layer 2 and the upper levels of layer 3 form near the median valley but that deeper levels of layer 3 thicken for 40 m.y. by intermittent offridge intrusion fed from the underlying anomalous mantle. Ophiolites in such a model represent segments of thin immature ridge crest obducted onto continental margins during subduction of a spreading ridge.