HR:     11:15h
AN:     T31D-10
TI:     GPS Constraints on Forearc Sliver Motion, Plate Coupling,
        and Strain Partitioning in Northwestern Oregon
AU:     *McCaffrey, R 
EM:     mccafr@rpi.edu
AF:     Dept. of Earth and Environmental Sciences,
        Rensselaer Polytechnic Institute Troy, NY 12180-3590  United States
AU:     Goldfinger, C 
EM:     gold@oce.orst.edu
AF:     College of Oceanic and Atmospheric Sciences,
        Oregon State University
         Corvallis, OR 97331  United States
AU:     Murray, M H
EM:     mmurray@pangea.stanford.edu
AF:     Department of Geophysics,
        Stanford University Stanford, CA 94305-2215  United States
AU:     Zwick, P 
EM:     zwickp@rpi.edu
AF:     Dept. of Earth and Environmental Sciences,
        Rensselaer Polytechnic Institute Troy, NY 12180-3590  United States
AU:     Nabelek, J L
EM:     nabelekj@ucs.orst.edu
AF:     College of Oceanic and Atmospheric Sciences,
        Oregon State University
         Corvallis, OR 97331  United States
AU:     Johnson, C K
EM:     johnsc2@rpi.edu
AF:     Dept. of Earth and Environmental Sciences,
        Rensselaer Polytechnic Institute Troy, NY 12180-3590  United States

AB:     Repeated Gobal Positioning System measurements along a
        forearc-crossing transect in NW Oregon between Eugene and
        Salem ($44\deg$N to $45\deg$N, $121\deg$W to $124\deg$W) are being used to explore
        the motion of the Oregon forearc relative to North America, 
        plate coupling, and strain partitioning associated with
        oblique convergence between the Juan de Fuca plate and North
        America. We are utilizing GPS occupations made by the US
        Geological Survey in 1992 and 1994, by Rensselaer and Oregon
        State in 1996, 1997, and 1998 and by the National Geodetic
        Survey in 1998. (The August 1998 survey is part of a
        statewide reference survey conducted by the NGS and many
        local county and private surveyors.) In addition, we have
        operated 2 continuous sites in this corridor since early 1996
        and a third was installed by Central Washington University in
        July, 1998, all forming part of the Pacific Northwest
        Geodetic Array (PANGA). Preliminary results based on 1994,
        1996, and 1997 data suggest that the Oregon forearc moves
        north relative to North America at several mm/yr possibly
        with localized shear along the Cascades arc. We see
        evidence for distributed margin-parallel shear strain
        across the forearc but uncertainties are still too large to
        allow resolution of spatial variations in it. The
        overall principal contraction direction is rotated from the
        plate convergence direction toward the trench-normal, a
        sign of strain partitioning. These results suggest that
        plate coupling extends farther inland at this latitude than
        is interpreted from uplift data alone. Similar rotation of
        the principal contraction direction occurs at the Nankai
        trough near Shikoku where convergence is oblique. Results
        from Oregon that incorporate the 1992 USGS and 1998 RPI/OSU
        measurements in the velocity determinations will be
        presented at the meeting. We will also discuss the expected
        relationships between plate coupling variations, rotations
        of the principal strains, and the across-forearc
        distribution of margin-parallel shear strain based on
        finite element models. 
DE:     8150  Plate boundary--general (3040)
DE:     8158  Plate motions--present and recent (3040)
DE:     8159  Rheology--crust and lithosphere
DE:     9350  North America
SC:     T
MN:     1998 Fall Meeting