Structure and evolution of the seismically active Ostler Fault Zone (New Zealand) based on interpretations of multiple high resolution seismic reflection profiles

To improve our understanding of active faulting away from the main plate boundary on New Zealand's South Island, we have acquired high resolution seismic data across the Ostler Fault Zone Twelve 1.2 km long lines perpendicular to fault strike and a 1.6 km long crossline were collected in a region of the MacKenzie Basin where surface mapping delineates significant complexity in the form of two non-overlapping reverse fault strands separated by a transfer zone characterised by multiple smaller strands and increased folding. Interpretation of the resultant images includes a 45–55° west-dipping principal fault and two 25–30° west-dipping subsidiary faults, one in the hanging wall and one in the footwall of the principal fault. The geologically mapped complexities are shown to be caused by shallow variations in the structure of the principal fault, which breaks the surface in the north and south but not within the transfer zone, where it forms a triangle zone with associated backthrusting and minor faulting. These complexities only extend to ~ 300 m depth. Structures below this level are markedly simpler and much more 2D in nature, with the principal fault strand extending over a much longer distance than the individual strands observed at the surface. Since longer faults are susceptible to larger earthquakes than shorter ones, seismic hazard at the study site may be higher than previously thought. Multiple surface fault strands that give way to a single more major stand at relatively shallow depths may be a common feature of segmented fault systems.The deepest layered reflections at our site are consistent with the presence of a Late Cretaceous (?)–Tertiary basin underlying the present-day MacKenzie Basin. Structural restoration of the seismic images back to the base of Quaternary fluvioglacial terraces and back to the top of a Late Pliocene–Pleistocene fluviolacustrine unit indicate that compression was initiated prior to the Late Pliocene and that it has continued at a comparatively steady rate of about a millimetre per year to the present day. The fluviolacustrine unit has experienced 440–800 m of along-fault vertical offset and 870–1080 m of horizontal shortening since that time. Our study demonstrates that structural reconstructions based on high resolution seismic reflection data provide critical displacement information that can be used to estimate slip rates.

Research Highlights
► 13 high resolution seismic reflection lines across the Ostler Fault Zone reveal complex geometry varying along-fault strike.
► Complex geometry related to fault segmentation is a surficial feature only, with a single continuous fault strand imaged at depths > 300 m.
► An older sedimentary basin fill that does not outcrop anywhere in the basin is revealed in the hanging wall of the fault, consistent with the interpretation of the Ostler Fault as a reactivated basement normal fault.
► Reconstructions of the interpreted sections show relatively constant deformation rates of ~ 0.3–1.4 mm/year since the Late Pliocene–Pleistocene.