Tectonic structures developed in Oligocene limestones: implications for New Zealand plate boundary deformation in North Canterbury

Systematic analysis of brittle and semibrittle tectonic structures developed in two Oligocene limestones provides a good record of the post middle Cenozoic low-strain deformational history in North Canterbury. These structures imply two periods of deformation. (1) Mild mid-late Oligocene shortening along a NNE horizontal axis is inferred from fractures at a low angle to bedding. These are widespread throughout the northeastern South Island and indicate a regional tectonic event associated with the distal effects of major Oligocene extension in western New Zealand. (2) Late Pliocene - Recent shortening was associated with the development of orthogonal fracture sets (many containing stylolites), macrofractures, small-scale faults, and macroscopic folding. The late Cenozoic deformation is characterised by northwest-southeast shortening, which is locally overprinted by east-west and north-south shortening associated with folding and the activation of macroscopic faults in basement. These late Pliocene and younger structures record a progressive increase in the magnitude of deformation and imply a rapid widening of the plate boundary zone during the late Pliocene - early Pleistocene. This phenomenon, often associated with an increase in local uplift rates, is widespread throughout New Zealand and signifies a major change in the distribution and magnitude of strain accommodated across the plate boundary in the last 2 Ma. This may indicate an increase in the compressional component of relative plate motion across the plate boundary. Small-scale structures imply a marked change of 60 degrees in the azimuth of horizontal shortening between the mid-late Oligocene and late Pliocene - early Pleistocene. However, during the last 2-3 Ma, regional shortening in North Canterbury has not changed significantly in orientation.