HR: 13:40h
AN: T33G-01    [Abstracts]
TI: Structure of Kane Megamullion
AU: * Tucholke, B E
EM: btucholke@whoi.edu
AF: Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, MA 02543 United States
AU: Dick, H J
EM: hdick@whoi.edu
AF: Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, MA 02543 United States
AU: Tivey, M A
EM: mtivey@whoi.edu
AF: Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, MA 02543 United States
AU: Cheadle, M J
EM: Cheadle@uwyo.edu
AF: Department of Geology and Geophysics, University of Wyoming, Laramie, WY 82071 United States
AB: Kane megamullion is an oceanic core complex that was formed by exceptionally long-lived slip on a detachment fault at the western wall of the Mid-Atlantic rift valley immediately south of Kane Fracture Zone. The fault initiated at 3.3 Ma and continued to slip for ca. 1.2 m.y., with footwall rollover creating a domed edifice that exhibits prominent mullion structures with amplitudes up to 600 m. During a recent cruise (Knorr 180-2, Nov.-Dec. 2004), we studied Kane megamullion in detail using the autonomous vehicle ABE, the ROV Jason II, and dredges. Near-bottom studies concentrated on two high-angle, west-facing normal faults that offset the detachment surface by up to 500 m and that probably formed in response to bending stresses during footwall exhumation, but they also covered parts of the detachment surface unaffected by faulting. The western normal fault, about 3-4 km from the detachment breakaway, cuts through a section of primarily gabbros and diabase dikes in its southern part and through serpentinized peridotites in its central section. The eastern fault, 8-9 km from the breakaway, exposes mostly serpentinized peridotites with lesser gabbros, suggesting exhumation of deeper lithosphere. High-resolution ABE multibeam bathymetry documents large mass failures of these weak, serpentinized rocks. Cross-sections of the detachment footwall exposed by these faults indicate initial high-temperature deformation followed by strain localization in a semi-brittle to brittle shear zone focused at the detachment surface. The northern part of the eastern fault is completely covered by intact pillow basalts, and it is interpreted as a volcanic growth fault. Survey across the mullions away from the normal faults shows that these corrugations are original features of the fault surface and are not produced by flowline-parallel normal faulting. Diabase dikelets are observed within serpentinite mylonites at the detachment surface, and in one location a 0.4 m-thick diabase sill was intruded along the shear zone. In addition, the detachment surface is also peppered in places with small (multi-meter scale) stacks of iron-rich claystones and associated pillow basalts, which may indicate localized, past venting of melt and hydrothermal fluids. These features document a small but potentially important role of melt in evolution of the detachment fault.
DE: 3035 Midocean ridge processes
DE: 3045 Seafloor morphology, geology, and geophysics
DE: 3075 Submarine tectonics and volcanism
DE: 8010 Fractures and faults
DE: 8178 Tectonics and magmatism
SC: Tectonophysics [T]
MN: Fall Meeting 2005