The three-dimensional Vp and Vp/Vs structure of a region where subduction transitions to oblique transform faulting has been determined using arrival times from 579 local earthquakes recorded during a temporary deployment, and 3146 earthquakes have been relocated. Between 40 km and 100 km depth, the subducted plate is imaged as a relatively low-velocity feature in the uppermost mantle, reflecting the continental nature of the subducted crust in this region. An increase in amplitude of this low-velocity feature from northeast to southwest can be related to 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 between the plates which extends well beyond the plate interface and is consistent with other geophysical and geological evidence that the plate interface beneath Marlborough is currently not accommodating much active subduction. In the overlying plate, the Awatere fault is a major structural feature, associated with a low-velocity zone extending to 23 km depth. There is a marked change in structure near this fault, with 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 for this is elevated pore pressures caused by fluids derived from dehydration of the continental subducted crust. The low-velocity region in the overlying plate coincides with the region of most intense active deformation, suggesting it is relatively weak.¿ 1997 American Geophysical Union