HR: 0830h
AN: T51D-21
TI: Transition From Spreading Ridge To Transform Plate Boundary: Integration Of Marine Geophysical Data And Field Data From Macquarie Island, Southern Ocean
AU: * Wertz, K
EM: karah@mail.utexas.edu
AF: Department of Geosciences, University of Texas at Austin, Austin, TX 78712 United States
AU: Daczko, N R
EM: ndaczko@mail.usyd.edu.au
AF: Division of Geology and Geophysics, School of Geosciences, University of Sydney, Sydney, NSW 2006 Australia
AU: Mosher, S
EM: mosher@mail.utexas.edu
AF: Department of Geosciences, University of Texas at Austin, Austin, TX 78712 United States
AU: Coffin, M F
EM: mikec@utig.ig.utexas.edu
AF: Institute for Geophysics, University of Texas at Austin, Austin, TX 78759-8500 United States

AB: Transition From Spreading Ridge To Transform Plate Boundary: Integration Of Marine Geophysical Data And Field Data From Macquarie Island, Southern Ocean Macquarie Island, adjacent to the Australian-Pacific plate boundary $\sim$1200 km south of New Zealand, records the last stages of oblique spreading along the Macquarie Ridge Complex (MRC) just prior to the cessation of magmatism and the transition to an active transform margin at $\sim$10 Ma. Geophysical data from the surrounding seafloor display a tectonic spreading fabric (faulted abyssal hills) and associated orthogonal fracture zones that curve asymptotically into the plate boundary. The island, located on the eastern slope of the MRC, lies between the main active transform fault zone ($\sim$9 km to the east) and a fracture zone ($<$12 km to the west). Structures related to both tectonic regimes - relict oblique spreading and the active transform faulting - are observed on the island. A major fault zone related to oblique spreading, the Finch-Langdon zone, juxtaposes a lower crustal and upper mantle section of sheeted dikes, gabbro and peridotite with a volcanic section. The zone trends NW, but is composed of intersecting high angle fault segments (0.5- 2.25 km long) that form a complex pattern of nearly orthogonal ($\sim$E and $\sim$N trending) and oblique (NW-trending) faults. Fault motion is primarily strike to oblique slip with both right and left lateral motion observed locally. Hydrothermal minerals including prehnite and epidote, form gouge cement, mineral slickenlines, and associated veins. One fault plane shows sulfide mineralization. Sheeted dikes adjacent to the fault zone are generally sub-parallel to one set of fault segments, in an orientation consistent with that predicted from the seafloor data. The fault geometries within the Finch-Langdon zone, coupled with the extensive hydrothermal alteration along the zone, are consistent with formation near an active ridge-transform intersection prior to the change from dominantly divergent to mainly transform motion along the plate boundary. Most major recent faults related to the active transform plate boundary formed sub-aerially and show pronounced fault scarps. Faults with right lateral strike slip and reverse motion are generally NE trending, sub-parallel to the plate boundary and oblique to faults within the Finch-Langdon fault zone. Associated normal faults and pull-apart basins are common. Direct evidence for reactivation of preexisting submarine faults is rare. Although the seafloor data suggest a smooth transition from oblique spreading to transform faulting, field data from Macquarie Island show that the dominant structures related to the two tectonic regimes are distinct in style and orientation. Preexisting structures related to seafloor spreading do not appear to influence the subsequent transform-related faults. Instead submarine structures are cut by transform related structures, leaving the structural relationships related to seafloor spreading intact.

DE: 8010 Fractures and faults
DE: 9355 Pacific Ocean
DE: 3035 Midocean ridge processes
DE: 3040 Plate tectonics (8150, 8155, 8157, 8158)
DE: 3045 Seafloor morphology and bottom photography
SC: T
MN: Fall Meeting 2000