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"S22A-0422"

HR: 0830h
AN: S22A-0422
TI: Evidence for distributed lower crustal deformation within a continental strike-slip fault zone: Marlborough Fault System, South Island, New Zealand
AU: * Wilson, C K
EM: wilsonck@cires.colorado.edu
AF: University of Colorado, Boulder Department of Geol. Sci. and CIRES, CB 399, Boulder, CO 80302 United States
AU: Jones, C H
EM: cjones@cires.colorado.edu
AF: University of Colorado, Boulder Department of Geol. Sci. and CIRES, CB 399, Boulder, CO 80302 United States
AU: Molnar, P
EM: molnar@cires.colorado.edu
AF: University of Colorado, Boulder Department of Geol. Sci. and CIRES, CB 399, Boulder, CO 80302 United States
AU: Sheehan, A F
EM: afs@cires.colorado.edu
AF: University of Colorado, Boulder Department of Geol. Sci. and CIRES, CB 399, Boulder, CO 80302 United States
AU: Boyd, O
EM: oliverb@cires.colorado.edu
AF: University of Colorado, Boulder Department of Geol. Sci. and CIRES, CB 399, Boulder, CO 80302 United States
AU: Stern, T
EM: tim.stern@vuw.ac.nz
AF: Victoria University School of Earth Sciences, P.O. Box 600, Wellington, 1111 New Zealand
AU: Savage, M
EM: martha@geo.vuw.ac.nz
AF: Victoria University School of Earth Sciences, P.O. Box 600, Wellington, 1111 New Zealand
AB: Teleseismic converted wave images from a passive seismic imaging experiment (2000-2002) across the Marlborough fault system, South Island, New Zealand show a continuous, unbroken Moho beneath the two northernmost faults of the fault system, suggesting that accommodation of lower crustal deformation occurs through distributed, ductile deformation and not by slip on a narrow vertical fault. Beneath the northernmost fault, the Wairau fault (~450 km offset), the Moho dips between 25 to 30 degrees from a depth of ~25 km northwest of the fault to a depth of 34 km southeast of the Wairau fault. Further to the southeast, the Moho arrival appears with a constant amplitude and depth of 34 km beneath the Awatere fault (~30 km offset). Mid crustal arrivals appear to stretch across the Awatere fault at 10, 17, and 27 km depth but their continuation across the Wairau is not clear, possibly indicating a change in the depth of transition to ductile deformation north of the Wairau. Images derived using common conversion point stacking schemes will lose coherence and resolution in the presence of either large, lateral variations in seismic velocity or interface topography with wavelength similar to the smallest bin size of the stacking algorithm. To test the possibility that our image is actually produced by an offset in the Moho, we construct synthetic converted-wave images from seismograms calculated for models with either a Moho step or dip using the same station-event geometry as the processed data set. The synthetic image produced from the dipping Moho model matches our results, but that with a step does not. Large velocity variations associated with the terrain boundary represented by the Wairau fault could affect the coherence of the Moho conversion across the Wairau. Although, restacking with several different velocity models does not affect the lateral continuity of the Moho. The observation of a continuous but dipping Moho under the Wairau Fault and its 450 km of displacement implies distributed strain over a broader region of weak, ductile lower crust.
DE: 7205 Continental crust (1242)
DE: 8150 Plate boundary--general (3040)
DE: 8159 Rheology--crust and lithosphere
SC: Seismology [S]
MN: 2003 Fall Meeting