Recognition of active reverse faults and folds in North Canterbury, New Zealand, using structural mapping and geomorphic analysis

In areas of limited geological exposure and discontinuous marker beds, many faults or parts of faults are difficult to detect and their displacement and strike length, for example, hard to quantify. Here, we use structure contouring of a partly exhumed basement unconformity surface together with stream gradient and sinuosity indices to help resolve moderate sized faults (<100 m throws) and folds that were either not detected or only poorly delineated by either structural or geomorphic mapping. Data are from active reverse faults and associated folds in North Canterbury, at the outer edge of the Australia-Pacific plate boundary collision zone in New Zealand. Structural mapping defines three major, parallel, ENE-striking reverse fault systems, with associated folds and vertical displacements of 100-950 m. In addition to showing greater trace lengths and connectivity of the main faults, structure contours and stream analyses highlight six faults with throws of <100 m, which were previously unrecognised. These faults accounted for <15% of the total vertical displacement across the region of study. Conventional mapping, therefore, permits the identification of the principal faults, while geomorphic analysis provides a more complete understanding of the locations and displacements of faults in the system. In addition, geomorphic analysis provides a means of identifying low slip-rate structures (e.g., <0.2 mm/yr) which have experienced Quaternary activity.