HR: 0800h
AN: T21B-0523    [Abstracts]
TI: Deep Seismic Discontinuity Structure Beneath New Zealand
AU: * Boyd, O S
EM: oliverb@lithos.colorado.edu
AF: Dept. of Geological Sciences University of Colorado at Boulder, 2200 Colorado Ave., Boulder, CO 80309 United States
AU: Savage, M K
EM: Martha.Savage@vuw.ac.nz
AF: Institute of Geophysics School of Earth Sciences Victoria University of Wellington, P.O. Box 600, Wellington, 00000 New Zealand
AU: Sheehan, A F
EM: afs@mantle.colorado.edu
AF: Dept. of Geological Sciences University of Colorado at Boulder, 2200 Colorado Ave., Boulder, CO 80309 United States
AU: Jones, C H
EM: cjones@terra.colorado.edu
AF: Dept. of Geological Sciences University of Colorado at Boulder, 2200 Colorado Ave., Boulder, CO 80309 United States
AB: We have created receiver functions using broadband seismic stations on either side of Cook Strait, New Zealand. These waveforms have been stacked in common conversion point (CCP) bins and reveal lateral variations in seismic structure above 700 km depth. The tectonic relationships beneath New Zealand vary from westward subduction in the north through transform along the Southern Alps to eastward subduction in the very south. The precise geometry of plate interactions at depth is not well known and is only loosely constrained by seismicity, shear wave splitting, seismic tomography, gravity studies, and plate reconstructions. Several outstanding questions include whether the northern subducting plate continues to subduct south of recorded seismicity in the Nelson region and over what lateral extent does the subducting plate penetrate the mantle transition zone. We image the top of the subducting Pacific Plate as well as several mid mantle seismic discontinuities including the 410 km and 660 km upper mantle transition zone discontinuities. Synthetics are generated for our ray geometry and compared with the CCP stacks and suggest that the subducting plate dips steeply under the southern extent of our stations, the southwestern edge of the Marlborough Fault System. The upper mantle transition zone discontinuities are consistent with the penetration of a cold slab southwest beyond recorded seismicity, i.e. the transition zone is thickened in the region of the cold subducted slab. We interpret the significant northeast deepening of the 410 km seismic discontinuity along the subducted plate to be due to differences in the downward component of subduction rate and the kinetics of the olivine phase transition. We observe splitting of the P to S-wave conversion from the 250 km and deeper discontinuities which indicates seismic anisotropy above 250 km depth oriented approximately north-northeast. The orientation reflects transform relative plate motion and/or mechanisms producing a trench parallel fast axis of anisotropy.
DE: 9355 Pacific Ocean
DE: 7203 Body wave propagation
DE: 7218 Lithosphere and upper mantle
DE: 3672 Planetary mineralogy and petrology (5410)
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting