Recent Alpine Fault thrusting at Kaka Creek, central Westland, New Zealand

Abstract

A newly discovered outcrop of the Alpine Fault at Kaka Creek, central Westland, New Zealand, shows successive overthrusting of green mylonite and schist over alluvial and talus/scree gravels. From the southwestern bank, two rare wood fragments from the underlying gravels yield radiocarbon ages of 2600 ± 60 and 3350 ± 100 years B. P., and date two movements of the Alpine Fault. A minimum of 5.0 ± 0.1 m movement perpendicular to the Alpine Fault trend over 750 years corresponds to an average convergence rate of 6.7 ± 1.5 mm/year, the rate determination limited primarily by the small amount of outcrop remaining. On the northeastern bank, similar gravels have been overthrust a distance of 70 m by a 60 m thickness of schist, and the calculated thrusting rate and average convergence rate are likely to be considerably higher. The young ages of the underlying gravels confirm very recent, rapid movement of the Alpine Fault, possibly associated with frequent large seismic events in central Westland.

Keywords:

• C-14
• thrust faults
• Alpine Fault
• seismicity
• Westland