HR: 17:00h
AN: T44A-05    [Abstracts]
TI: Rapid Changes in Transform Fault Length and the Dynamic Evolution of the Southwest Indian Ridge Since 26Ma.
AU: * Baines, G
EM: gbaines@uwyo.edu
AF: Department of Geology and Geophysics, University of Wyoming, Laramie, Wy 82071 United States
AU: Cheadle, M
EM: cheadle@uwyo.edu
AF: Department of Geology and Geophysics, University of Wyoming, Laramie, Wy 82071 United States
AU: Hosford Scheirer, A
EM: allegra@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025 United States
AU: Kusznir, N
EM: n.kusznir@liverpool.ac.uk
AF: Department of Earth Sciences, University of Liverpool, Liverpool, L69 3Bx United Kingdom
AU: John, B
EM: bjohn@uwyo.edu
AF: Department of Geology and Geophysics, University of Wyoming, Laramie, Wy 82071 United States
AU: Dick, H
EM: hdick@whoi.edu
AF: Woods Hole Oceanographic Institution, Woods Hole Road, Woods Hole, MA 02543 United States
AU: Matsumoto, T
EM: tak@sci.u-ryukyu.ac.jp
AF: Department of Physics and Earth Sciences, University of Ryukyus, Okinawa, 903-0213 Japan
AB: The dynamic evolution of transform faults and non-transform discontinuities (NTDs) over the past 26Ma along the Southwest Indian Ridge (SWIR) between the Gallieni and Melville Transforms (52\deg E to 46\deg E) provides fundamental insight into the growth and development of transform faults along the SWIR and other oceanic ridges in general. Between 54\deg 45'E and 58\deg 45'E along the SWIR and up to 30Ma (280km) from the ridge axis, bathymetric and magnetic data reveal constant full-spreading rates within the area, but the average half-spreading rates since 26Ma vary from highly ($\sim$2:1) asymmetric to symmetric. This extreme asymmetric spreading led to major changes in the offset lengths of NTDs and transform faults over time. In particular, since 26Ma a $>$50km long transform fault at 56\deg 30'E shrank to become an 8km offset NTD today, while at the same time the Atlantis II transform grew by over 70km to reach its current length of 199km. The data suggest that the longest transforms (the Gallieni, Atlantis II and Melville transforms) in the region are lengthening, while the intervening spreading-parallel offsets are shrinking. Between these $\sim$450km spaced large offset transform faults, the SWIR is therefore rotating to become increasingly orthogonal to plate-spreading direction. We infer that these changes in plate boundary geometry are in response to a $\sim$$40\deg$ CW change in plate-spreading direction at 40Ma and a lesser $10\deg$ CCW change at 19.5Ma.
DE: 9340 Indian Ocean
DE: 8155 Plate motions--general
DE: 3005 Geomagnetism (1550)
DE: 3035 Midocean ridge processes
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting