GEOPHYSICAL INVESTIGATION OF A MODERN CONTINENT-CONTINENT COLLISIONAL OROGEN: THE SOUTHERN ALPS, NEW ZEALAND

The mechanisms for how transform faults penetrate through the crust and mantle lithosphere at continental plate boundaries remain outstanding problems in plate boundary dynamics. Seismicity indicates localized and usually near-vertical faulting within seismogenic depths at continent-continent strike-slip faults. These localized zones may continue as narrow weak zones which cut the entire lithosphere or become distributed into broadening zones of ductile deformation within the lower crust and mantle lithosphere. An added component of plate convergence will produce transpressive behavior and associated rapid mountain building.

The Pacific-Australian plate boundary in South Island, New Zealand, represents an excellent locality to study lithospheric-scale continent-continent transpressional collision with rapid transcurrent movement and high uplift rates. The NSF Continental Dynamics program and the N.Z. Science Foundation sponsored an international collaboration to study the Alpine plate boundary fault and associated Southern Alps orogen. Over 25 direct participants and an equal number of outer-circle scientists and students from both countries conducted research in this study. The US-NZ geophysical investigation used methods including active source seismic imaging (SIGHT), regional and teleseismic passive seismology (SAPSE), seismic reflection (CDP98), magnetotellurics, laboratory petrophysics, gravity, regional geology, and rheological analyses. The Pacific-Australian plate boundary is characterized by a distributed zone of mantle shear subparallel to the Alpine fault. In the crust the Alpine fault is steep (>60 degrees) to 25-30 km depths, lacks seismicity, has low seismic velocities and low resistivities, indicative of a weak fault zone (with the presence of fluids). The Southern Alps are exhuming rapidly but has an excessively deep root enhanced by mantle pulldown. Deformation in the Pacific plate crust and mantle lithosphere are thus dynamically coupled at this continent-continent transpressional boundary.