Strong, eastward dipping (∼40°) reflectors found 23 km below the central South Island were recorded by a joint U.S.-New Zealand geophysical project during austral summer 1995–1996. When projected to the surface, the reflectors define a plane that coincides with the surface trace of the Alpine Fault (Figure 1).
Data processing is still at an early stage, but preliminary results, reported at a workshop held at Victoria University of Wellington February 17–19, 1997, give an overall picture of asymmetric crustal thickening and active processes in the mid to lower crust. A jump in crustal thickness of about 20 km is inferred across the Alpine Fault, and the locus of thickening does not occur beneath the highest mountains, but is imaged at least 20 km farther to the east. Low seismic velocities and high electrical conductivity roughly coincide with the strong eastward-dipping reflections in the mid to lower crust. Excess fluids and high pore pressure probably explain both the low seismic velocities and high electrical conductivity.