CRUSTAL SEISMIC OBSERVATIONS ACROSS THE CONVERGENT PLATE BOUNDARY, NORTH-ISLAND, NEW-ZEALAND

Seismic reflection profiles recorded across southern North Island, New Zealand, define the structural details of a convergent plate boundary. The eastern profile delineates the thrust wedges and back-tilted basins of the accretionary prism which overlies a detachment zone marking the top of the subducted Pacific plate. Most of the younger part of this wedge is isolated from the landward part of the sequence by an oceanward dipping detachment zone. The western profile defines a broad crustal downwarp in the "back-arc" region of the plate boundary overlying the 20-50 km deep subduction zone. The crustal section farther to the west comprises a generally transparent middle crustal layer and a reflective lower crust. Dipping reflective zones in the middle crust may delineate low-angle normal faulting associated with a Cretaceous extensional episode. Detailed Seismological data demonstrate the close association of the top of a zone of high Seismicity, inferred to mark the subducted Pacific plate, with the base of the reflective sequence identified as the base of the overriding Australian plate. © 1990.