The Bogota Peninsula shear zone has been interpreted as a paleotransform fault in the mantle section of the New Caledonia ophiolite. New, detailed fi eld 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 fi eld fabrics, with increased alignment and fl attening 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 fi eld fabrics but provides less consistent information across the shear zone than other fabric methods. This suite of fi eld 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.

No abstract

No abstract

No abstract

The Algoman suite of granites and granodiorites, located within the Rainy Lake region of the Superior Province, intruded the Wabigoon–Quetico subprovince boundary about 2686 Ma. Deformation occurred during the late Archean collision between the Wabigoon and Wawa volcanic arcs. The Algoman plutons are the youngest rocks in the region. Their apparent lack of macroscopic deformation fabrics has led previous researchers to use their age as a constraint for the cessation of regional deformation. However, contrasting competence between the relatively rigid granites and surrounding volcanic and metasedimentary rocks suggests that obvious deformation fabrics may not be observed in the plutons even if they were present during deformation. A field, anisotropy of magnetic susceptibility (AMS), gravity inversion model, and microstructural study were undertaken on two neighboring Algoman plutons to determine their emplacement history relative to the regional deformation. There is a steep macroscopic foliation in the adjacent rocks and in parts of the plutons. The magnetic foliation is very strong and roughly coincident with the field foliations. The gravity inversion models indicate that the plutons are relatively shallow (1 km and 4.5 km at their deepest points), and their shapes are consistent with emplacement during dextral transpression. Quartz microstructures, such as undulose extinction and subgrain formation, indicate that there was some solid-state deformation accommodated by dislocation creep mechanisms. The field, AMS, gravity, and microstructural evidence indicate that the plutons were most likely emplaced syntectonically. Thus, the pluton age does not provide a conclusive constraint to the cessation of regional deformation.