HR: 1340h
AN: T53B-1304    [Abstracts]
TI: Shallow seismic structure of the Kane core complex, Mid-Atlantic Ridge 23°30'N
AU: * Xu, M
EM: minxu@mit.edu
AF: MIT/WHOI Joint Program, Department of Geology and Geophysics, MS 24, Woods Hole, MA 02543, United States
AU: Canales, J
AF: Woods Hole Oceanographic Institution, Department of Geology and Geophysics, Woods Hole, MA 02543, United States
AU: Tucholke, B
AF: Woods Hole Oceanographic Institution, Department of Geology and Geophysics, Woods Hole, MA 02543, United States
AU: Dubois, D
AF: Woods Hole Oceanographic Institution, Department of Geology and Geophysics, Woods Hole, MA 02543, United States
AB: We present high-resolution travel-time seismic tomography models obtained along and across the Kane core complex (KCC), a proposed IODP drilling target located off-axis on the Mid-Atlantic Ridge (MAR) near 23°30'N south of the Kane Fracture Zone. Together with existing geological studies, our results characterize the lateral variability in structure and composition of this well developed oceanic core complex. The KCC is a large (~23 km by ~35 km in the dip and strike directions, respectively) megamullion formed by a long-lived detachment fault between ~3.3 and 2.1 Ma. The detachment is cut along-strike by a high- angle, west-dipping normal fault (East fault). Extensive ROV and dredge sampling indicates that the northern part of East fault is covered by in situ pillow basalts, but at the central dome the fault exposes the underlying lithosphere, which is dominated by highly altered harzburgites. In contrast, the northern dome to the east of East fault appears to consist largely of gabbros. We derived the two-dimensional, shallow P-wave velocity structure of the KCC along six ~20- to 40-km-long profiles, including three strike lines and three dip lines. The seismic data were acquired in 2001 using the 6-km- long hydrophone streamer and air-gun array of R/V Ewing (cruise EW0102). The dense sampling (shots spaced every 37.5 m, with receivers spaced every 12.5 m) and the shallow seafloor of the area (<3000 m) allows us to image lateral variations in velocity structure at scales of 1 km or less within the upper ~0.5-1.5 km of the lithosphere. Our results show significant lateral variations in velocity structure in both strike and dip directions, and these variations to first order correlate with sampled lithologies. The lowest observed velocities (~3.3 km/s at seafloor increasing to ~5.1 km/s at ~1 km depth) correlate with the zone of volcanics found along the northern part of East fault. Low velocities also occur beneath the volcanic terrain of the remnant hanging wall to the east of the detachment termination. The northern dome near Kane Fracture Zone, and the eastern, younger part of the central dome are characterized by relatively high velocities and high vertical velocity gradients near the seafloor. Seafloor velocities there are ~4.1-4.4 km/s and rapidly increase to ~6.0 km/s within the upper ~450 meters. In contrast, the southern dome and the central and western (older) part of the central dome show more moderate near-seafloor velocities and vertical gradients (~3.7 km/s at the seafloor, increasing to ~5.0 km/s at ~580 meters below the seafloor). We interpret the relatively high velocities of the northern dome and the eastern part of the central dome as representing primarily gabbroic rocks. This is based on available rock samples but also on a striking similarity in velocity structure between these parts of the KCC and the Atlantis core complex on the MAR at 30°N [Canales et al., this meeting], where drilling at IODP Hole U1309D recovered a ~1.5-km-thick section of gabbros. Geological samples at the KCC also suggest that the more moderate seismic velocities beneath the southern dome and the western and central part of the central dome probably represent highly serpentinized mantle peridotites. In summary, our results show that the shallow sub-seafloor velocity structure of the KCC is highly heterogeneous, indicating that lower crustal gabbros are dominant beneath some parts of the detachment surface while serpentinized mantle peridotites predominate in other areas.
DE: 1032 Mid-oceanic ridge processes (3614, 8416)
DE: 3035 Midocean ridge processes
DE: 8150 Plate boundary: general (3040)
DE: 8416 Mid-oceanic ridge processes (1032, 3614)
DE: 8424 Hydrothermal systems (0450, 1034, 3017, 3616, 4832, 8135)
SC: Tectonophysics [T]
MN: 2007 Fall Meeting