HR: 0830h
AN: V21D-0555    [PDF]
TI: Oceanic Core Complex Development Within the Ductile and Brittle Regimes, Atlantis Bank, Southwest Indian Ridge
AU: * Miranda, E A
EM: emiranda@uwyo.edu
AF: University of Wyoming Dept. of Geology and Geophysics, P.O. Box 3006, Laramie, WY 82071 United States
AU: John, B E
EM: bjohn@uwyo.edu
AF: University of Wyoming Dept. of Geology and Geophysics, P.O. Box 3006, Laramie, WY 82071 United States
AU: Frost, B R
EM: rfrost@uwyo.edu
AF: University of Wyoming Dept. of Geology and Geophysics, P.O. Box 3006, Laramie, WY 82071 United States
AB: In slow-spreading mid-ocean ridge environments, both magmatic and tectonic processes accommodate spreading, where the latter can be manifested as detachment fault systems. Atlantis Bank, an oceanic core complex formed $\sim$12 Ma at the intersection of the ultra-slow-spreading Southwest Indian Ridge and the Atlantis II transform, is dissected by transform-parallel normal faults that provide unique cross-sections through all levels of the detachment fault system. Gabbro collected in ODP Hole 735B and with the manned submersible {\it Shinkai 6500} from the footwall, hanging wall, and detachment fault surface provide details of the extensional fault system exposed over 36 km normal to the ridge axis, ($>$3.5 m.y. of crustal accretion). Together the sample suite provides an ideal basis for interpreting the process of strain localization and fabric development associated with formation of long-lived oceanic detachment faults. Continuous core from ODP Hole 735B demonstrates that the main shear zone associated with denudation of the core complex is 200 m thick. The core samples exhibit magmatic fabrics and metamorphic fabrics of submagmatic, granulite ($650\deg$ - $900\deg$C), amphibolite ($450\deg$ - $650\deg$C), greenschist ($300\deg$ - $450\deg$C) and subgreenschist grades. Analysis of the {\it in situ} dive samples indicates that the structurally deepest samples collected from the footwall are characterized by a zone of granulite-grade crystal-plastic deformation with varying fabric intensity. Structurally above the granulites lies a zone of amphibolite-grade protomylonitic to mylonitic fabrics overprinted by semi-brittle to brittle greenschist-grade deformation. The amphibolites contain 20 - 30 mm-thick zones of high-strain mylonite that increase in number and decrease in thickness upward toward the detachment fault surface. Samples from the localized slip plane or detachment fault surface are highly altered and deformed in both the ductile and brittle regimes. Thermometry and microstructural analysis of {\it in situ} dive samples demonstrates that ductile deformation initiated at $950\deg$C and continued through $600\deg$C. Folded pyroxene porphyroclasts show that ductile deformation began after exsolution. Two-pyroxene thermometry based on mineral composition data from orthopyroxene and clinopyroxene porphyroclast hosts and their exsolution lamellae in gabbro mylonite yield temperatures of $950\deg$C. Equivalent analysis of adjacent recrystallized ortho- and clinopyroxene neoblasts yield temperatures $850\deg$ - $950\deg$C, demonstrating that ductile deformation continued through $850\deg$C. Amphibole-plagioclase thermometry based on mineral composition data from recrystallized amphibole and the associated plagioclase porphyroclasts and neoblasts in oxide gabbro mylonite indicates that ductile deformation began around $890\deg$C and continued through $600\deg$C. The greenschist-grade overprint of granulite gabbro mylonite, manifested by the foliation-parallel growth of green amphibole, chlorite and serpentine around plagioclase neoblasts, preserves a subsequent lower-temperature history. The microstructures of the granulite-grade footwall rocks and those of the detachment fault surface breccias suggest that deformation initiated at high temperatures in the ductile regime, and continued to lower temperatures through the semi-brittle and brittle regimes as fault rocks were denuded along the detachment fault system. Models for the development of oceanic core complexes developed purely within the brittle regime clearly do not apply to the well-exposed and sampled Atlantis Bank region of the Southwest Indian Ridge.
DE: 3035 Midocean ridge processes
DE: 3660 Metamorphic petrology
DE: 8030 Microstructures
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting