CONTINENTAL SUBDUCTION AND 3-DIMENSIONAL CRUSTAL STRUCTURE - THE NORTHERN SOUTH ISLAND, NEW-ZEALAND

The three-dimensional Vp-and Vp/Vs structure of a region where subduct ion transitions to oblique transform faulting has been determined usin g arrival times from 579 local earthquakes recorded during a temporary deployment, and 3146 earthquakes have been relocated. Between 40 km a nd 100 km depth, the subducted plate is imaged as a relatively low-vel ocity feature in the uppermost mantle, reflecting the continental natu re of the subducted crust in this region. An increase in amplitude of this low-velocity feature from northeast to southwest can be related t o an increase in the thickness of the crust of the subducted plate in this direction. Velocity variations within the subducted and overlying plates show some spatial correlation. This suggests an interaction be tween the plates which extends well beyond the plate interface and is consistent with other geophysical and geological evidence that the pla te interface beneath Marlborough is currently not accommodating much a ctive subduction. In the overlying plate, the A watere fault is a majo r structural feature, associated with a low-velocity zone extending to 23 km depth. There is a marked change in structure near this fault, w ith seismic velocities being lower to the southeast. A relatively high level of seismicity occurs in this region of lower seismic velocities , suggesting a relationship between the two. A possible explanation fo r this is elevated pore pressures caused by fluids derived from dehydr ation of the continental subducted crust. The low-velocity region in t he overlying plate coincides with the region of most intense active de formation, suggesting it is relatively weak.