Geophysical exploration and dynamics of the Alpine Fault Zone

Abstract

The Alpine Fault of central South Island New Zealand, can be tracked with seismic reflection methods to depths of ∼35 km as a listric-shaped surface with strong reflectivity. Maximum dips of the surface are ∼60 degrees at 15 km depth and the dip then lessens with depth until the reflectivity is sub-horizontal at ∼35 km. Wide-angle seismic methods are used to show that the P-wave velocities of the rocks are up to 10% less than normal in the zone above the fault surface. In cross-section this low-velocity Alpine Fault Zone is elongate, sits above the fault surface, and has dimensions roughly 45 by 20 km. A magnetotelluric study shows a low-resistivity anomaly that is roughly coincident with the zone of low seismic velocity. A straightforward interpretation is that both the electrical and seismic anomalies are caused by interconnected fluids at lithostatic pressure. The inference of fluids in the lower crust is supported by an attribute analysis of seismic reflections on specific shot gathers where the Alpine Fault reflections can unequivocally be identified. We reference both the amplitude and phase of the fault-zone reflections to the distinctive side-swipe reflections generated at the far shore of Lake Pukaki. High reflection coefficients of ∼0.25 are estimated for the Alpine Fault reflections, which may require both anisotropy and fluid to explain. We interpret the source of water to be metamorphic dewatering of the schist-greywacke rocks that thicken into the orogen. A detachment surface along which the greywacke-schist rocks are obducted is recognised as a zone of strong reflectivity on an 80-km-long, unmigrated seismic reflection section. This zone of strong reflectivity, which apparently merges into the Alpine Fault reflections, does not correlate with depth to the Moho but rather with the boundary between the base of the schist-greywacke rocks (Vp ∼6-6.2 krn/s) and the lower crust (Vp ∼7-7.2 km/s). We interpret the strong reflectivity on this boundary as being due to a shear fabric. Both geological and geophysical observations imply deformation in the lower and mid-crust and mantle that appears to be caused by a combination of ductile and brittle behaviour, with no evidence of lithospheric flexure. We interpret the Alpine Fault Zone as a profoundly hot, wet, and weak region of continental crust.