Abstract:
Oceanic gabbro and peridotite recovered during ODP Leg 153 are exposed in the dip-slope of a major east-dipping brittle-ductile normal fault, which makes up the western wall of the median valley of the Mid-Atlantic Ridge, approximately 23 degrees N latitude. Gabbroic rocks record a continuous spectrum of deformational fabrics that range from magmatic to crystal plastic to cataclastic. Crystal-plastic fabrics show a wide range of degrees of dynamic recrystallization and neoblast sizes. Based on individual microstructures and patterns of dynamic recrystallization, clinopyroxene is significantly more competent in this ductile regime than plagioclase. Olivine shows a relative competence intermediate between clinopyroxene and plagioclase. The degree of plagioclase recrystallization increases with decreasing modal abundance of olivine and increasing abundance of orthopyroxene such that troctolitic gabbro is relatively less recrystallized than are gabbronorite and ferrogabbro. We interpret this strong partitioning of crystal-plastic deformation into more evolved gabbros to reflect differences in hydrolytic weakening and fractionation of volatiles into the more evolved gabbroic rocks. Therefore, magmatic fractionation may strongly control the strength characteristics of juvenile oceanic crust. Crystal plastic, semibrittle, and cataclastic fabrics all show normal-sense shear and likely developed in a kinematically coordinated brittle-ductile shear zone similar to those found in Cordilleran metamorphic core complexes. Cross-cutting relationships show that neoblast size progressively diminished and deformational style evolved from ductile to brittle. Microfabrics, compositional zoning of minerals, and synkinematic mineral assemblages demonstrate that the microfabrics developed under decreasing temperature and increasing pore-fluid pressure, which suggests that the lower crust and mantle were emplaced on the seafloor by tectonic unroofing in the footwall of a major normal fault.