HR: 10:50h
AN: T32A-03    [Abstracts]
TI: Time-Averaged Rate of Detachment Faulting at Atlantis Bank, Southwest Indian Ridge: Evidence for Highly Asymmetric Spreading Rates During the Formation of Oceanic Core-Complexes
AU: * Baines, A G
EM: gbaines@uwyo.edu
AF: Department of Geology and Geophysics, University of Wyoming, Laramie, WY 82070 United States
AU: Schwartz, J J
EM: schwartz@uwyo.edu
AF: Department of Geology and Geophysics, University of Wyoming, Laramie, WY 82070 United States
AU: Cheadle, M J
EM: cheadle@uwyo.edu
AF: Department of Geology and Geophysics, University of Wyoming, Laramie, WY 82070 United States
AU: John, B E
EM: bjohn@uwyo.edu
AF: Department of Geology and Geophysics, University of Wyoming, Laramie, WY 82070 United States
AB: Determining the rates of oceanic detachment faulting and how they compare to rates of seafloor spreading is of fundamental importance for constraining how these features form and their relationship to tectonic processes, including magmatic accretion, in slow-spreading environments. We use Pb/U ages of igneous zircon from lower crustal gabbros and sea-surface magnetic data from Atlantis Bank to determine half-spreading rates during detachment faulting. We suggest that these rates, which record crustal accretion and cooling of lower crust in the Antarctic plate, also reflect the time-averaged rate of motion on the detachment fault. During the time Atlantis Bank formed, from 11.2-12.5Ma, both techniques yield a half-spreading rate of 14±1km/Ma. The magnetically determined full-spreading rate has been 14km/Ma since 20Ma and magnetic data from neighboring segments show that this full spreading rate did not increase during the period of detachment faulting. Our data also show that the period of highly asymmetric spreading was confined to the segment containing Atlantis Bank, and so was associated with ridge migration and closure of the non-transform discontinuity to the East. Therefore, detachment faulting at Atlantis Bank occurred during a period of highly asymmetric spreading; a result consistent with sea-surface magnetic data from the FUJI dome at 63° 45'E on the SWIR. Our data require a re-evaluation of existing models that predict structural asymmetry but not asymmetric plate spreading rates during the formation of oceanic core-complexes. The half-spreading rate during detachment faulting suggests that for ~1.3Ma the detachment fault acted as the primary plate boundary and that the axial valley was (within resolution) stationary with respect to the hanging wall (African Plate). If volcanism continued within the axial valley, a thick layer of extrusive material would have formed. This has not been identified. A combination of hydrothermal cooling and strain localization on the detachment fault might lead to limited diking and volcanism within the hanging wall, thus restricting crustal accretion to the lower crust and producing a thick plutonic layer in the Antarctic Plate, a prediction consistent with seismic velocities and crustal thickness observed beneath Atlantis Bank.
DE: 1115 Radioisotope geochronology
DE: 3005 Marine magnetics and paleomagnetics (1550)
DE: 3035 Midocean ridge processes
DE: 8157 Plate motions: past (3040)
SC: Tectonophysics [T]
MN: Fall Meeting 2005