Deformation monitoring of the ostler fault zone, South Island, New Zealand

The Ostler Fault Zone is a zone of Quaternary reverse faulting along the western margin of the Mackenzie Basin, South Island, New Zealand. During investigations for and construction of the Ohau-A Power Project, a number of survey patterns were designed and installed to monitor ground deformation in the area of the fault zone. A triangulation network, two long and two short levelling routes, and six tilt levelling patterns have been installed at various times since 1964. Analysis of the levelling data shows that vertical deformation has occurred within the ault zone at a near constant rate for the last 20 years. Deformation consists of buckling about fault traces at a maximum rate of uplift of approximately 1 mm/yr. The vertical deformation preserved over 16,000–25,000 yrs has a similar rate although it is dominated by displacement across the fault zone as a whole. Analysis of triangulation data spanning up to 100 years indicates that significant horizontal strain is occurring, with the axis of relative contraction approximately perpendicular to the trend of the fault.