Structure and kinematics of late Cenozoic deformation along the western margin of the Culverden Basin, North Canterbury, New Zealand.

Abstract

Along the western margin of the Culverden Basin, North Canterbury, on the SE edge of the New Zealand plate boundary zone, deformation in Late Cretaceous and younger cover rocks has produced synchronous faulting and folding about orthogonal NNE-NE and WNW-NW orientations that are younger than early Pleistocene. The folds in the cover sequence are asymmetric, steeply inclined, gently plunging structures that are parallel to the major faults. Non-classical irregular basin and dome interference patterns are defined by prominent marker horizons, and triangular, corrugated hose and irregular T-shaped fold surface geometries (defined by structure contours) are common. The main folds, which vary in shape along their hinge line, have secondary folds developed at various angles to their axial trace. The major NNE-NE trending folds are characterised by composite conical geometries and are defined by several distinct cone segments. Folding in the cover rocks has developed in response to both fault propagation and displacement in the basement, and to shortening within the fault bounded blocks. The axial surfaces of the folds that parallel the faults are truncated by the propagating faults. Fault-related folds have half wavelengths of 1.5-5km and amplitudes of 0.5-2.4km whilst folding within the fault bounded blocks have dimensions of 0.5-1.5km and 0.2-0.6km respectively. Meso scale faults, joints, calcite veins, fractures and pressure solution seams indicate two phases of post Late Cretaceous deformation: 1) a weak late Oligocene event related to the beginning of plate boundary inception, and 2) a post Pliocene phase related to widening of the plate boundary deformation during the last 2-3ma. Analysis of fault and slickenside striation data suggests faulting is dominated by oblique-reverse faults and thrusts, which verge NW, SE and SW. Locally the stress and strain directions are variable but generally imply a predominant NW-SE compression comparable to local geodetic shortening and focal mechanism compression directions. Principal incremental shortening axes and stress tensors suggest the shortening and compression directions plunge gently to horizontally NW-SE but can switch with the intermediate stress to NE-SW. Movement planes tend to be less conclusive, often showing girdle distributions of M-plane intersections. Interspersed with the regional contractional faulting are local areas of oblique-normal NNE and normal faults indicating strain partitioning. Late Holocene terraces incised into older Pleistocene aggradation gravels are dated by weathering rinds and document deformation. Two seismically triggered landslides and a ground rupture event of the Balmoral Fault appear to have occurred during the same seismic event, approximately 1700 years B.P. A magnitude 6.5+ event necessary to produce ground rupture and the landslides would cause both local, and possibly regional, destruction. The main contractional deformation along the western margin of the Culverden Basin, commonly associated with thrusting to the southeast is atypical of North Canterbury structure, is distinct from the right-lateral tectonics of Marlborough and marks a transition between the inner and outer plate boundary zones and subduction related tectonics.