Origin of small-scale segmentation and transpressional thrusting along the Alpine fault, New Zealand

Abstract

The Alpine fault is the major structural feature of the Australian-Pacific plate boundary in the South Island of New Zealand. Geologic evidence suggests that half to three-quarters of the plate boundary displacement is accommodated by movement on the fault. Detailed investigation of the central section of the Alpine fault has revealed that it consists of oblique thrust sections striking 020°–050° that are linked by subvertical right-lateral faults striking between 065° and 090°. The segmentation is on a scale of 1–10 km. Similarly oriented right-lateral faults are abundant southeast of the Alpine fault and are consistent with stresses induced in an elastic layer by an oblique-slip ductile fault zone below. Propagation of the fault to the surface is predicted to result in an en echelon arrangement of strike-slip and thrust segments. It is suggested that the spatial distribution of segments is affected by the existence of deeply incised valleys in the hanging wall that disturb the stress field to depths of 1–4 km. The segmentation is near surface and does not appear to act as a barrier to the propagation of large earthquakes.