Structural styles and kinematics of deformation on the edge of the New Zealand plate boundary zone, mid-Waipara region, North Canterbury

Abstract

In North Canterbury, on the SE edge of the New Zealand plate boundary zone, deformation in late Cretaceous and younger cover rocks is complex. Detailed mapping of the cover sequence (which is about 1 km. thick), has revealed a pattern of synchronous faulting and folding about orthogonal E-ESE and N-NNE orientations younger than early Pleistocene. Complex and irregular basin and dome fold interference patterns are defined by distinctive limestone marker horizons, and partial star, triangular and corrugated hose fold surface geometries (defined by structure contours) are common. These geometries are often associated with folds that vary in shape along their hinge line and secondary folds developed oblique to the main interfering fold sets. Fold interference geometries are also characterised by composite conical geometries, composed of several distinct cone segments. Folding in the cover sequence has developed in response to: (1) fault propagation and displacement in the basement; and, (2) shortening within the fault bounded blocks. The folds related to faulting are asymmetric and parallel the major faults. Fold amplitudes of 0.5-1.6 km. are proportional to vertical fault displacements, and wavelengths of 5-20 km. are approximately equal to the strike-normal distances between the major faults. Fold interference patterns are indicative of the styles and levels of activity of the orthogonal faulting. Analysis of fault and slickenside striation geometries suggests that faulting is dominated by oblique-reverse faults and thrusts, which verge north, south, east and west. Interspersed with the regional contractional faulting are, local areas of E-ESE oblique-normal faults, inferred to be reactivated late Cretaceous structures. Computed stress tensors derived from minor fault motion data suggest that the principal stress axes most commonly plunge at shallow to moderate angles. Locally they have variable orientations but regionally these data imply a predominant NW-SE compression, comparable to local geodetic shortening and focal mechanism compression directions. Geometric and spatial analysis of large numbers of sets of small scale, mainly brittle structures, documents the local history of deformation. Multiple joint, pressure solution seam and stylolite, macrofracture and mesofault sets record only two periods of deformation since the mid-Cenozoic. The first was a weak regional mid-late Oligocene compression; the second involving a NW-SE compression since the Pliocene. Post Pliocene deformation is characterised by initial NW-SE shortening, followed by approximate N-S and E-W shortening associated with folding. The sequence of structural development during the late Pliocene and Pleistocene implies a progressive increase in the intensity of deformation and a rapid widening of the plate boundary zone during the last 2-3 ma. The mainly contractional deformation in North Canterbury, commonly associated with thrusting to the NW, is distinct from the major right-lateral and eastward directed thrust tectonics in Marlborough. This not only reflects a difference between inner and outer plate boundary deformation, but also the diminishing influence of subduction related tectonics.