Nineteen uranium-lead zircon ages of lower crustal gabbros from Atlantis Bank, Southwest Indian Ridge, constrain the growth and construction of oceanic crust at this slow-spreading midocean ridge. Approximately 75% of the gabbros accreted within error of the predicted seafloor magnetic age, whereas È25% are significantly older. These anomalously old samples suggest either spatially varying stochastic intrusion at the ridge axis or, more likely, crystallization of older gabbros at depths of È5 to 18 kilometers below the base of crust in the cold, axial lithosphere, which were uplifted and intruded by shallow-level magmas during the creation of Atlantis Bank.Slow-and ultraslow-spreading ridges with spreading rates of G55 mm/year (1) constitute nearly 60% of the total length of midocean ridges. Results from a variety of studies (2-7) indicate that slow-and ultraslow-spreading oceanic crust is dominantly created by the emplacement of small magma bodies of up to 500 m in thickness (7) into zones of par-

[1] Microstructural observations and mineral thermometry from in situ samples collected from the Atlantis Bank oceanic core complex (SW Indian Ridge) indicate that detachment faulting was initiated under hypersolidus conditions in the ductile regime and continued through subgreenschist temperatures through the ductile, semibrittle, and brittle regimes as strain localized along the exposed, now subhorizontal fault surface. Ductile, semibrittle, and brittle fabrics are developed within dominantly gabbroic rocks. Footwall rocks exhibit crystal plastic fabrics distributed over a structural thickness up to 400 m below the denuded fault surface exposed at the seafloor, whereas semibrittle and brittle fabrics are concentrated in the 80 and 30 m immediately below the principal slip surface of the detachment fault, respectively. Sample fabrics suggest that strain localization was achieved by dynamic recrystallization of plagioclase at temperatures between 910°C and 650°C, by amphibole-accommodated dissolution-precipitation creep at temperatures $750°C-450°C, by chlorite-accommodated reaction softening at temperatures $450°C-300°C, and by brittle fracturing and cataclasis at temperatures <300°C.

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