Active folding of Pleistocene unconformities on the edge of the Australian‐Pacific plate boundary zone, offshore North Canterbury, New Zealand

Abstract

In North Canterbury, New Zealand, the southeastern edge of an active fold and thrust belt lies 20 km offshore beneath the continental shelf. High‐resolution marine seismic reflection profiles and a detailed Quaternary sequence stratigraphy are integrated to map and characterise the seismic potential of actively growing folds. Eleven large‐scale folds occur within the upper few hundred metres of a Pliocene to Recent succession of silty mudstone, representing the upper part of a sedimentary cover sequence up to 2 km thick. The folds are gentle, NE‐SW trending, overlapping, asymmetric structures approximately 10–32 km in length, which verge consistently to the northwest in accordance with major thrust faults and folds exposed in nearby coastal hills. The folds are inferred to overlie a system of blind, southeast dipping thrust faults that are accommodating a small component of regional NW‐SE crustal shortening in North Canterbury and to develop by coseismic uplift during thrust fault earthquakes. The critical interactions between the rates of folding, regional tilting, coastal uplift, outer shelf subsidence, and the local sedimentation response to high‐amplitude Quaternary sea‐level cycles have led to the development and preservation of a stack of folded sedimentary sequences, an essentially smooth, gently sloping seafloor throughout most of the off‐shore deformation zone, and net uplift close to zero at the deformation front. Eleven unconformity‐bounded sedimentary units of middle Pleistocene to Recent age (< 0.75 Ma) constrain the rates and timing of folding and enable comparisons with onshore deformation rates, examination of the coastal tectonic gradient, and speculation about the rates of upper crustal deformation beneath the shelf. Deposition, folding, and coastal uplift have occurred contemporaneously throughout the last 0.75 m.y., and all measurable deformation can be accommodated within approximately 0.8 m.y. Fold amplitude growth rates of 0.02 m/kyr to 0.14 m/kyr (typically 0.05–0.09 m/kyr) near the deformation front are low and up to 25 times lower than some actively growing folds exposing basement rocks onshore. There is a significant decline in strain rate across the coastal zone and inner shelf, toward the off‐shore deformation front. It is inferred that blind thrust faults beneath the offshore folds have slip rates typically of the order of 0.1–0.9 mm/yr and that the probable recurrence interval of moderately large magnitude (M 6.2–6.8 ± 0.3) thrust earthquakes beneath individual folds is of the order of several tens of thousands of years.