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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 degrees C and 650 degrees C, by amphibole-accommodated dissolution-precipitation creep at temperatures similar to 750 degrees C-450 degrees C, by chlorite-accommodated reaction softening at temperatures similar to 450 degrees C-300 degrees C, and by brittle fracturing and cataclasis at temperatures <300 degrees C.

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Geochemistry Geophysics Geosystems




An edited version of this paper was published by AGU. Copyright 2010 American Geophysical Union.

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