HR: 1340h
AN: T53B-1305    [Abstracts]
TI: Geometry of a Polarity Reversal Boundary in Lower Crust and Upper Mantle at Kane Megamullion
AU: Williams, C M
EM: clare@whoi.edu
AF: MIT/WHOI Joint Program in Oceanography, Dept. of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, United States
AU: * Tivey, M A
EM: mtivey@whoi.edu
AF: Dept. of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, United States
AU: Behn, M D
EM: mbehn@whoi.edu
AF: Dept. of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, United States
AU: Dick, H J
EM: hdick@whoi.edu
AF: Dept. of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, United States
AU: Tucholke, B E
EM: btucholke@whoi.edu
AF: Dept. of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, United States
AB: Magnetic polarity reversal (C2r/C2An,~2.581 Ma) is coherently recorded in lower crustal gabbro and serpentinized upper mantle peridotites exposed on the seafloor of the northern and central domes, respectively, of Kane megamullion, which formed by detachment faulting at ~3.3-2.1 Ma. Rock magnetic results suggest that both lithologies contribute to the magnetic signal. Serpentinized peridotites have higher mean natural remanent magnetization (NRM) (4.7±5.8 A/m) than the gabbros (1.5±2.5 A/m), but gabbro NRM is more stable and has higher median destructive field and Koenigsberger ratio values. Paleomagnetic data identify both normal and reverse polarity samples, with most of the reverse polarity samples located in polarity transition zones. Polarity boundary geometry is estimated from two near-bottom magnetic profiles acquired across the northern and central domes. The analytic signal approach is used to calculate a range of dip angle solutions for varying rotations of the magnetic source layer, assuming an initial remanence direction parallel to the geocentric axial dipole. The amount of footwall rotation is estimated to be at least ~35° away from the ridge axis, as interpreted from the shape of the anomaly across a basalt ridge in the northern region. Based on this rotation, the dip of the polarity boundary is 46°W±14° (away from the ridge axis) in the northern region and 41°E±17° (towards the ridge axis) in the central region. The opposing dip angles in the two regions correspond to observed differences in seafloor sample lithology. We interpret the polarity boundary in the northern region to be a rotated cooling isotherm in a predominantly gabbroic layer. Numerical modeling of core complex thermal structure indicates that ≤45° outward rotation of a 580°C cooling isotherm with an initial dip of ~15°W at the ridge axis is possible. In contrast, the polarity boundary in the central serpentinized peridotite region likely reflects a rotated alteration front that was initially close to vertical. The geometry of this boundary may result from perturbed isotherms that were steeply dipping towards the ridge axis in the detachment footwall due to seawater penetration. The continuity of the polarity boundary across Kane megamullion implies that both gabbros and serpentinites acquired their magnetization close to the ridge axis and at approximately the same time.
DE: 1527 Paleomagnetism applied to geologic processes
DE: 3005 Marine magnetics and paleomagnetics (1550)
DE: 3045 Seafloor morphology, geology, and geophysics
DE: 9325 Atlantic Ocean
SC: Tectonophysics [T]
MN: 2007 Fall Meeting