HR: 1330h
AN: B12A-0774    [PDF]
TI: Cross-Section of the Atlantis Massif --- Geologic Framework for the Lost City Hydrothermal Vent Field
AU: * Williams, E A
EM: eaw12@duke.edu
AF: Duke University, Division of Earth & Ocean Sciences, Durham, NC 27708-0230 United States
AU: Karson, J A
EM: jkarson@duke.edu
AF: Duke University, Division of Earth & Ocean Sciences, Durham, NC 27708-0230 United States
AU: Kelley, D S
EM: kelley@ocean.washington.edu
AF: University of Washington, University of Washington, School of Oceanography, Seattle, WA 98195 United States
AU: Fr\"{u}h-Green, G L
EM: gretli@erdw.ethz.ch
AF: ETH-Z\"{u}rich, Dept. of Earth Sciences, Z\"{u}rich, CH-8092 United States
AB: The Atlantis Massif is an oceanic core complex situated on 1.5-Ma-old crust at $30\deg$N on the Mid-Atlantic Ridge. Its domed, corrugated upper surface is interpreted as a major detachment fault. The steep, mass-wasted south wall of the massif provides a window into the architecture of the footwall of this detachment fault and underlying rocks. {\it Alvin} and {\it Argo II} were used to collect outcrop-scale geological data in the context of fine-scale bathymetry from ABE surveys. Variably serpentinized peridotite and lesser gabbroic rock form the bulk of the massif. Outcrops below $\sim$1800 m.b.s.l. are generally massive, grading upward into a region cut by an array of shear zones, faults, and joints. The uppermost $\sim$50 m of basement rock show a pervasive anastomosing foliation dipping gently west and cropping out for about 3 km. Samples from this interval include mylonitic material wrapping around less deformed phacoids of basement rock. The mylonites are unconformably overlain by a thin (1-3 m) cap of breccia, grading upward into pelagic limestone. The breccia is massive to crudely bedded with both clast- and matrix-supported textures. It includes subangular clasts of basalt and serpentinite in a limestone matrix. Small normal faults offset all of these units and the unconformity. The Lost City Hydrothermal Vent Field sits on top of the breccia unit. We interpret the mylonites as a detachment fault responsible for exhumation of the upper mantle material that forms the core of the massif. The overlying sedimentary rocks represent debris shed onto the detachment fault surface when it defined the median valley wall. As the detachment fault surface moved off axis and flattened into its present orientation, the sedimentary material changed progressively from clastic to pelagic and became consolidated. This assemblage may cover the rest of the Atlantis Massif and occur on other oceanic core complexes.
DE: 8010 Fractures and faults
DE: 8135 Hydrothermal systems (8424)
SC: Biogeosciences [B]
MN: 2003 Fall Meeting