HR: 0800h
AN: T41D-1332    [Abstracts]
TI: The Nature of a Magnetic Polarity Boundary in the Lower Crust and Upper Mantle at Kane Megamullion
AU: * Williams, C M
EM: cwilliams@whoi.edu
AF: Woods Hole Oceanographic Institution, 360 Woods Hole Rd, Woods Hole, MA 02543 United States
AU: Tivey, M A
EM: mtivey@whoi.edu
AF: Woods Hole Oceanographic Institution, 360 Woods Hole Rd, Woods Hole, MA 02543 United States
AU: Tucholke, B E
EM: btucholke@whoi.edu
AF: Woods Hole Oceanographic Institution, 360 Woods Hole Rd, Woods Hole, MA 02543 United States
AU: Dick, H J
EM: hdick@whoi.edu
AF: Woods Hole Oceanographic Institution, 360 Woods Hole Rd, Woods Hole, MA 02543 United States
AB: Kane megamullion is interpreted to be the exhumed footwall of a normal detachment fault in 3.3-2.1 Ma crust. Recent sampling and surveying (Knorr Cruise 180-2, Nov.-Dec. 2004) show that long-term slip (~1.2 m.y.) on the fault exposed lower-crustal and upper-mantle rocks across most of the footwall. Sea-surface magnetic data over the footwall show very well defined magnetic lineations of Chron 2A and the polarity boundary with Chron 2. The development of these lineations implies that gabbros and peridotites have significant stable remanent magnetization and that they contribute to off-axis marine magnetic anomalies. Assuming that the polarity boundary between Chron 2 and 2A is represented by a cooling isotherm in the gabbros and peridotites, we can estimate the dip of the boundary from magnetic profiles acquired at varying altitudes above the seafloor. Sea-surface magnetic data have long wavelengths and sample deep magnetic sources, while profiles closer to the seafloor are dominated by short wavelengths from shallow magnetic sources. No change in the location of the polarity boundary at all altitudes implies a vertical boundary. We acquired two sets of WNW-ESE magnetic profiles using the autonomous vehicle ABE at altitudes of 20 to 60 meters above the seafloor across the megamullion. Analysis of the sea-surface and ABE data suggests that the polarity boundary is dipping towards the west. The magnetic profiles that cross the northern section of the megamullion indicate a dip of ~40§ for the Chron 2/2A boundary, and profiles across the central part of the megamullion indicate a dip of ~75§. These differing dip angles suggest that the cooling isotherm may have a 3-D structure over ridge-segment scales. Future detailed modeling of ridge-axis isotherms may help to constrain rotation of the footwall as it was exhumed.
DE: 3005 Marine magnetics and paleomagnetics (1550)
DE: 3035 Midocean ridge processes
DE: 9325 Atlantic Ocean
SC: Tectonophysics [T]
MN: Fall Meeting 2005