LARGE-SCALE DUPLEXES WITHIN THE NEW BRITAIN ACCRETIONARY WEDGE - A POSSIBLE EXAMPLE OF ACCRETED OPHIOLITIC SLIVERS

A multichannel seismic line acquired in the western Solomon Sea images reflectors within the New Britain accretionary wedge which we interpret as accreted duplexes of the downgoing oceanic plate. The seaward edge of the largest duplex is located just 6 km from the toe of the accretionary wedge suggesting recent incorporation into the wedge. These data therefore provide an excellent opportunity to determine the dynamics of oceanic basement accretion. The oceanic basement is represented by a high-amplitude, low-frequency reflector that can be traced up to 40 km arcward of the toe of the accretionary wedge on several seismic lines in this survey. Another prominent reflector, similar in character to the basement reflector, lies approximately one second higher in at least one seismic section. This reflector is likely to be the top of a basement duplex. Structures imaged above the basement duplex define a preduplex accretionary wedge, whereas those at the toe indicate a new episode of postduplex accretion. We have tested this interpretation using both magnetic modeling and interval velocity analysis. Magnetic models of the wedge that include basement duplexes provide a good match to the observed magnetic data and provide limits on the size and interval velocities of the slivers. Interval velocities between the two strong reflectors, calculated using stacking velocities and constrained through magnetic modeling, exceed 5000 m/s and are consistent with our interpretation of ophiolitic slivers within the accretionary wedge.