Stephanie M. Maes

Fabric development in the mantle section of a paleotransform fault and its effect on ophiolite obduction, New Caledonia

The Bogota Peninsula shear zone has been interpreted as a paleotransform fault in the mantle section of the New Caledonia ophiolite. New, detailed field measurements document the rotation of foliation, lineation, and pyroxenite dikes across a 50-km-wide region. Deformation intensity recorded by folding and boudinage of dikes increases toward a central, 3-km-wide mylonitic zone. We used several additional methods to characterize fabric patterns across the shear zone. The shape-preferred orientation of orthopyroxene grains, computed from outcrop tracings, closely parallels field fabrics, with increased alignment and flattening near the center of the shear zone. The lattice-preferred orientations of olivine are consistent with high-temperature fabrics; the a axes within the mylonitic core were used to constrain the orientation of shear zone boundaries. Seismic anisotropy calculations, based on the lattice-preferred orientation of olivine, indicate 5%–11% shear-wave anisotropies, with increased values in the center of the shear zone. The magnetic silicate fabric in the rocks, determined from anisotropy of magnetic susceptibility techniques, broadly matches field fabrics but provides less consistent information across the shear zone than other fabric methods. This suite of field and laboratory data provides a unique and detailed view of strain and fabric patterns across a shear zone in oceanic mantle lithosphere. Because the primary mantle fabrics seem to be related to the present distribution of ophiolitic rocks in New Caledonia, we propose that ophiolite obduction and Neogene extension may have been controlled by preexisting fabrics and structures in the oceanic lithosphere.