Late Quaternary slip rates and slip partitioning on the Alpine Fault, New Zealand

Published geological data on Late Quaternary offsets on the Alpine Fault, New Zealand, have been assembled into a common format and analysed with respect to uncertainties. Uncertainties arise mainly from measurement of offset features, relating apparent offsets to actual fault slip, and dating the offset features. Despite the considerable uncertainties, the data form a coherent set consistent with a relatively constant rate of strike-slip of 27±5 mm/year between Milford Sound and Hokitika. This rate represents 70–75% of the fault-parallel interplate motion. North of the confluence with the Hope Fault, the rate drops substantially. Dip-slip rates, on the other hand, show considerable variation along strike, rising to a maximum of more than 10 mm/year in the central section and decreasing to zero at the southern end. Partitioning of c. 25% of the interplate slip on to structures east of the Alpine Fault occurs in the central section, consistent with predictions from critical wedge models. The partitioning of all the fault-normal component of displacement on to other structures in the south may be related, in part, to a doubling in width of the deforming wedge to the east. Most probably, however, the fault-normal displacement is mainly accommodated by underthrusting of the Australian plate offshore, due to a change in the nature of the crust from continental to oceanic.