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

Published geological data on Late Quaternary offsets on the Alpine Fault, N ew Zealand. have been assembled into a common format and analysed with resp ect to uncertainties. Uncertainties arise mainly from measurement of offset features, relating apparent offsets to actual fault slip, and dating the o ffset features. Despite the considerable uncertainties. the data form a coh erent 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-clip rates. on the other han d. show considerable variation along strike, rising to a maximum of more th an 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 o f the Alpine Fault occurs in the central section, consistent with predictio ns from critical wedge models. The partitioning of all the fault-normal com ponent 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 pr obably, however. the fault-normal displacement is mainly accommodated by un derthrusting of the Australian plate offshore clue to a change in the natur e of the crust from continental to oceanic. (C) 2001 Elsevier Science Ltd. All rights reserved.