HR: 09:30h
AN: T51F-07    [Abstracts]
TI: Styles of Detachment Faulting at the Kane Oceanic Core Complex, 23°N Mid-Atlantic Ridge
AU: * Hansen, L N
AF: University of Wyoming, Department of Geology and Geophysics, Laramie, WY 82071, United States
AU: Cheadle, M J
AF: University of Wyoming, Department of Geology and Geophysics, Laramie, WY 82071, United States
AU: John, B E
AF: University of Wyoming, Department of Geology and Geophysics, Laramie, WY 82071, United States
AU: Swapp, S M
AF: University of Wyoming, Department of Geology and Geophysics, Laramie, WY 82071, United States
AU: Dick, H J
AF: Woods Hole Oceanographic Institution, Department of Geology and Geophysics, Woods Hole, MA 02543, United States
AU: Tucholke, B E
AF: Woods Hole Oceanographic Institution, Department of Geology and Geophysics, Woods Hole, MA 02543, United States
AU: Tivey, M A
AF: Woods Hole Oceanographic Institution, Department of Geology and Geophysics, Woods Hole, MA 02543, United States
AB: In 2004, R/V Knorr Cruise 180-2 used ROV Jason II, the autonomous vehicle ABE, and dredges to collect samples and geophysical data from the Kane Oceanic Core Complex (OCC) on the Mid-Atlantic Ridge at 23°N. Examination of the deformed samples by hand-sample analysis, petrography, electron backscatter diffraction, and geothermometry in conjunction with the interpreted bathymetry suggests that the Kane OCC is bound by a detachment fault system that initiated at a moderate to high angle (45-60°), and rooted below the brittle-plastic transition. Constraints on the initial dip of the detachment fault come from the slopes of the ridge forming the breakaway (>23° to the west and >22° to the east). Assuming this ridge formed by flexural uplift, these slopes suggest the detachment fault formed with a dip >45°. Fault rocks, including peridotite mylonites and gabbro ultramylonites, reveal a history of deformation from granulite and amphibolite through sub-greenschist facies including brittle cataclasis. We present two cross sections through the detachment fault and footwall based on samples collected from secondary, high-angle normal faults that cut the detachment. One section, through Cain Dome in the central OCC, is dominated by peridotite and shows a ~450-m thick zone of discrete ductile shear zones with the uppermost portion overprinted by a 200-m zone of semi-brittle and brittle deformation. These are maximum shear zone thicknesses due to the possibility of down-scarp slumping/displacement. The other section, through Adam Dome on the southwest part of the OCC, is dominated by gabbroic rocks and shows little deformation. This section lies <4 km from the breakaway, and is therefore inferred to have undergone only brittle deformation in the shallow crust. A rheologic analysis, using LPO-deduced deformation mechanisms and geothermometry to construct deformation mechanism maps, suggests strain rates for the amphibole-bearing gabbros, the gabbronorites, and the peridotites of 10^{- 10}s^-1, 10^-12s^-1, and 10^-13s^-1 respectively. These strain rates reflect differing amounts of strain localization during the evolution of the detachment fault. The presence of deformed and undeformed diabase dikes, peridotite mylonites that have been intruded by gabbroic melts, and Fe-Ti oxide microstructures indicating deformation with melt present all suggest that detachment faulting was coeval with magmatism. In summary, the bathymetry of the Kane OCC, the initial moderate-steep dip of the fault, and the depth constraints provided by the fault rocks are most consistent with a rolling-hinge detachment fault model.
DE: 3035 Midocean ridge processes
DE: 3625 Petrography, microstructures, and textures
DE: 8004 Dynamics and mechanics of faulting (8118)
DE: 8011 Kinematics of crustal and mantle deformation
DE: 8031 Rheology: crust and lithosphere (8159)
SC: Tectonophysics [T]
MN: 2007 Fall Meeting