THE TRANSITION FROM SUBDUCTION TO CONTINENTAL COLLISION - CRUSTAL STRUCTURE IN THE NORTH CANTERBURY REGION, NEW-ZEALAND

The North Canterbury region marks the transition from Pacific plate su bduction to continental collision in the South Island of New Zealand. Details of the seismicity, structure and tectonics of this region have been revealed by an 11-week microearthquake survey using 24 portable digital seismographs. Arrival time data from a well-recorded subset of microearthquakes have been combined with those from three explosions at the corners of the microearthquake network in a simultaneous invers ion for both hypocentres and velocity structure. The velocity structur e is consistent with the crust in North Canterbury being an extension of the converging Chatham Rise. The crust is about 27 km thick, and co nsists of an 11 km thick seismic upper crust and 7 km thick seismic lo wer crust, with the middle part of the crust being relatively aseismic . Seismic velocities are consistent with the upper and middle crust be ing composed of greywacke and schist respectively, while several lines of evidence suggest that the lower crust is the lower part of the old oceanic crust on which the overlying rocks were originally deposited. The distribution of relocated earthquakes deeper than 15 km indicates that the seismic lower crust changes dip markedly near 43-degrees-S. To the south-west it is subhorizontal, while to the north-east it dips north-west at about 10-degrees. Fault-plane solutions for these earth quakes also change near 43-degrees-S. For events to the south, P-axes trend approximately normal to the plate boundary (reflecting continent al collision), while for events to the north, T-axes are aligned down the dip of the subducted plate (reflecting slab pull). While lithosphe ric subduction is continuous across the transition, it is not clear wh ether the lower crust near 43-degrees-S is flexed or torn.