HR: 10:40h
AN: V22A-02 INVITED     [Abstracts]
TI: Paleomagnetic constrains to the evolution of the mid-Atlantic Ridge from 14§ to 16§ N.
AU: * Garces, M
EM: mgarces@ub.edu
AF: Group of Geodynamics and Basin Analysis, University of Barcelona, Facultat de Geologia, Campus de Pedralbes, Barcelona, ESP 08028
AU: Gee, J S
EM: jsgee@ucsd.edu
AF: Spripps Institution of Oeanography, University of California, San Diego, Geosciences Research Division, La Jolla, CA 92093-0220 United States
AU: Kikawa, E
EM: kikawa@jamstec.go.jp
AF: JAMSTEC, Deep Sea Research Dept., 2-15 Natsushima-cho, Yokosuka, 237-0061 Japan
AB: Mantle rocks outcrop extensively at the sea floor near the Mid-Atlantic Ridge between 14§ and 16§ N. Tectonic extension occurring at the ends of ridge segments, where igneous crustal accretion is unable to balance sea floor spreading, causes uplift and exhumation of lower crustal and upper mantle materials at the footwall of normal faults. Models of extension along normal faults dipping towards the rift valley require a certain amount of block tilting, in a predictable sense top away from the ridge. Paleomagnetic data from cores can provide information on the amount of tectonic block rotation, provided that orientation of the rotation axis can be constrained. Paleomagnetic results from several sites of Leg 209 yield mean inclinations which are significantly lower that expected for the geomagnetic axial dipole at the site. Assuming that the magnetic remanence has averaged secular variation and that there is negligible error due to magnetic anisotropy, these results imply that syn-extensional block rotations have affected the rocks after they were magnetised at temperatures below 580§C. At sites 1268 and 1270, where structural trends at the sea floor are consistently parallel to the rift, the rotation axis can be considered approximately horizontal and parallel to the spreading axis. For these sites, as much as 90§ of tectonic rotation may be required to match observed and expected paleomagnetic inclinations of gabbroic samples. At site 1268, talc-altered serpentinized peridotites yield remanences which are significantly steeper than the gabbros. This suggests, first, a different magnetization timing between the gabbros and altered peridotites and, second, that important tectonic rotations occurred at site 1268 after cooling of the gabbros and prior to peridotite alteration.
DE: 3035 Midocean ridge processes
DE: 1525 Paleomagnetism applied to tectonics (regional, global)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting