HR: 14:55h
AN: T33G-06    [Abstracts]
TI: Mass Transfer and Fluid Flow During Detachment Faulting and Development of an Oceanic Core Complex, Atlantis Massif
AU: * Boschi, C
EM: chiara.boschi@erdw.ethz.ch
AF: ETH-Zurich, Dept. of Earth Sciences, Zurich, CH-8092 Switzerland
AU: Fruh-Green, G L
T33G-06 AF: ETH-Zurich, Dept. of Earth Sciences, Zurich, CH-8092 Switzerland
AU: Karson, J A
T33G-06 AF: Duke University, Division of Earth and Ocean Sciences, Durham, NC 27708 United States
AU: Kelley, D S
T33G-06 AF: University of Washington, School of Oceanography, Seattle, WA 98195 United States
AB: The Atlantis Massif (Mid-Atlantic Ridge, 30°N) is an example of an oceanic core complex (OCC) exposed by a major fault system. Our integrated field and analytical study of mafic and ultramafic rocks exposed on the south wall of the massif provide new insights into how major detachment shear zones evolve during the development of OCCs and demonstrates the complex interplay of fluids, mass transfer, and metamorphism in strain localization associated with this process. Pervasive talc-amphibole-chlorite metasomatism as well as heterogeneous, crystal-plastic and cataclastic deformation characterize a strongly foliated, 100-m-thick zone of detachment faulting. Metasomatic fault rocks are key elements of this OCC and record a deformation and metamorphic history that is distinct from the underlying basement rocks. Talc-rich fault rocks, consisting of fine-grained syntectonic talc, amphibole and chlorite, preserve textural and geochemical characteristics of their ultramafic protoliths. Although primary textures and mineral parageneses are commonly obliterated in rocks dominated by amphibole, bulk rock data point to a mafic protolith. Major and trace elements indicate a complex mutual interaction between mafic and ultramafic rocks during metasomatism and suggest localized circulation of oxidizing, Si-Al-Ca-rich fluids in high strain deformation zones. This type of flow was distinct from the more pervasive circulation that led to strongly serpentinized domains in the south wall. The talc-rich assemblages and microstructures in the fault rocks indicate multiple phases of fluid infiltration and high strain-deformation in limited domains and that pre-existing, high-temperature deformation zones in the lithologically heterogeneous lithosphere served as permeable pathways for subsequent localized fluid flow and greenschist-facies alteration and metasomatism. In contrast, cataclastic microfracturing is associated with a dominantly static metasomatism in less deformed domains, suggesting that a significant amount of metasomatism was controlled by diffuse flow and mass transfer associated with fractures that lack a strong preferred orientation.
DE: 1034 Hydrothermal systems (0450, 3017, 3616, 4832, 8135, 8424)
DE: 3035 Midocean ridge processes
DE: 3653 Fluid flow
DE: 3660 Metamorphic petrology
DE: 8010 Fractures and faults
SC: Tectonophysics [T]
MN: Fall Meeting 2005