Heterogeneity of the upper crust.

Abstract

Recent research confirms that the uppermost several hundred meters of the young oceanic crust is a distinct unit with low compressional velocity (<2.5 km/s) and a shear velocity lower than that of the acoustic velocity of the overlying water column. The acoustic contrast at the bottom of this waveguide is larger than that at the seafloor. Seismic data from ocean spreading centers can also address the question of whether upper crustal heterogeneity is intrinsic to the crustal accretion process or is imprinted subsequently by hydrothermal and tectonic processes. Seismic lines collected along axis at the fast spreading, northern East Pacific Rise indicate that, within the narrow zone of emplacement, the crustal structure is remarkably homogeneous in terms of layer thickness and velocity. However, as the crust moves from the axis and is buried by subsequent lava flows, the integrated effect of differences in magmatic supply rates gives rise to a wide variation in the total thickness of the waveguide. The waveguide is approximately 150 m on axis, but can vary between 200 and 700 m off axis.