Generation of Seismic Anisotropy in the Upper Mantle along the Mid-Oceanic Ridges

Abstract

HESS¹ suggested that the uppermost mantle beneath the oceans might be seismically anisotropic as a result of a preferential orientation of its constituent olivine crystals. In support of his hypothesis he cited seismic refraction measurements by Raitt² in the region of the Mendocino fracture zone and by Shor³ in an area north of the Hawaiian Islands adjacent to the Molokai fracture zone. The measurements showed low seismic (P_n) velocities perpendicular to the fracture zones and high velocities parallel to them. This, Hess claimed, is what one would expect, for with flow in the upper mantle the glide planes of olivine crystals would tend to line up parallel to the major shear zones. Fig. 1, largely from Hess's paper, summarizes the optical and seismic characteristics and the position of the glide plane for magnesian olivine. The seismic velocities shown here are laboratory measurements by Verma⁴. A statistical orientation of olivine crystals so that the b crystallographic axes tend to lie perpendicular to the fracture zones could explain the observed anisotropy.