Seismic anisotropy in the oceanic upper mantle: Evidence from the Bay of Islands Ophiolite Complex

Abstract

Olivine fabrics in 17 field‐oriented ultramafics and mafics from three widely‐spaced traverses in the Bay of Islands Ophiolite Complex, Newfoundland, display a remarkably uniform symmetry in which the olivine a crystallographic axes are aligned subprependicular to the sheeted dikes and the b and c axes lie within the plane of the sheeted dikes. The ultramafics studied consist entirely of tectonites; any olivine formed at the ridge crest by cumulus processes has since been re‐oriented by translation gliding and/or syntectonic recrystallization. Deformation has extended from the ultramafics into the overlying gabbro, which suggests that in many oceanic regions the deepest levels of layer 3 consist of gabbroic tectonites. Compressional wave velocities computed from these petrofabrics display 5–6% anisotropy in the plane of the Mohorovičić discontinuity, with Vp fast parallel to the direction of spreading inferred from dike orientations. Since this pattern is identical to that observed for the oceanic upper mantle, it is concluded that the Bay of Islands Complex is a segment of oceanic crust and upper mantle. Shear wave velocity contours calculated from the same fabrics indicate that the upper mantle is nearly isotropic in terms of the maximum shear wave velocity, , but that the difference in velocity, ΔVs, between shear waves of orthogonal polarization traveling in the same direction may be sufficiently large parallel to the intersection of the Mohorovičić discontinuity and the sheeted dikes to allow detection of two distinct shear wave arrivals.