Active Deformation at the Transition from Subduction to Oblique Collision in the Culverden Basin, Aotearoa New Zealand

The termination of a subduction zone along a single plate boundary and the transfer of plate motion to a continental transform results in a transition zone of characteristically distributed deformation. In central Aotearoa New Zealand, where the Pacific plate moves ~41 mm/yr SW relative to the Australian plate, the termination of the Hikurangi subduction zone results in a transition between oblique subduction and oblique continental collision. The right-lateral Marlborough fault system absorbs more than 75% of the relative plate motion at the northern South Island. Farther south, the North Canterbury region accommodates oblique contraction in the tectonic transition zone on an array of faults and folds with three principal orientations. Despite the well-studied 2016 Mw 7.8 Kaikōura earthquake, several active faults in the epicentral region in North Canterbury remain poorly characterized over ~103-105 yr timescales. This study uses recent lidar elevation data and field observations to map active faults and Quaternary landforms in the western Culverden Basin of North Canterbury. Geologic mapping and geomorphic analysis identify several generations of terraces in the study area cut by south-striking reverse faults, SW- and NE-striking thrust faults, and NW- and SSE-striking left-oblique faults. Scarp profiling, reconstruction of strike-slip offsets, and in-progress luminescence (IRSL/OSL) age dating of sediments from late Pleistocene to Holocene fluvial terraces resolve slip magnitudes and rates on six active fault zones. At the range-bounding Hurunui Peak fault zone (HPFZ), up to ~15 m of vertical separation and dip slip of ~23 m on latest-Pleistocene surfaces indicate multiple large surface ruptures and a ~0.9-1.9 mm/yr dip-slip rate since the late Pleistocene. Other south-striking reverse faults record dip-slip rates as high as ~0.6 mm/yr and accommodate the WNW-ESE maximum compressive stress orientation in North Canterbury. NW- and SSE-striking faults in the mapping area slip left-laterally in response to this regional compression. The intersection and interaction of multiple fault orientations and kinematics in the western Culverden Basin reflect the wide distribution of oblique plate convergence around the plate boundary transition in a manner that is likely transient on ~106 yr timescales as the plate boundary evolves.