Late Quaternary displacement rate, paleoseismicity, and geomorphic evolution of the Alpine Fault: evidence from Hokuri Creek, South Westland, New Zealand

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New Zealand Journal of Geology and Geophysics abstracts

Late Quaternary displacement rate, paleoseismicity, and geomorphic evolution of the Alpine Fault: evidence from Hokuri Creek, South Westland, New Zealand

RUPERT SUTHERLAND*
R. J. NORRIS

Geology Deptartment
University of Otago
P.O. Box 56
Dunedin, New Zealand

*Present address: Institute of Geological & Nuclear Sciences Ltd, P.O. Box 30368, Lower Hutt, New Zealand.

Abstract A 400 +/- 100 m offset of Lake McKerrow, South Westland, New Zealand, combined with dated (15.6 ka) glacial lake silts, requires an Alpine Fault displacement rate of 26 +/- 7 mm/yr. Moraines associated with Hokuri Creek (assumed to be 17 +/- 2 ka) are offset by 440 +/- 40 m and require a displacement rate on the Alpine Fault of 26 +/- 6 mm/yr. Slickensides, fault exposure, and offset topography are consistent with an almost pure dextral sense of movement on a vertical or subvertical fault. Locally, a small vertical component of up-to-the-west movement is observed.

Folding in late Quaternary sediments indicates active tilting of sediments at up to 0.4deg./ka and variations in local uplift/subsidence rates of up to 4 mm/yr. At one locality c. 1 km northwest of the Alpine Fault and near the core of an anticline, uplifted shells require an uplift rate of 1.4 +/- 0.5 mm/yr relative to sea level.

Displaced river channels provide estimates of the last two coseismic displacements on the fault of 9 m (penultimate) and 8 m. This suggests characteristic earthquake behaviour with a recurrence interval of 330 +/- 90 yr and probable Mw > 7.5. Radiocarbon dating suggests the last coseismic displacement occurred just after 370 +/- 150 cal yr B.P.

Keywords Alpine Fault; displacement rate; seismicity; earthquakes; geomorphology; Quaternary; glaciation; radiocarbon; neotectonics; paleoseismicity

New Zealand Journal of Geology and Geophysics, 1995, Vol. 38: 419-430

0028-8306/95/3804-0419 $2.50/0 (c) The Royal Society of New Zealand 1995

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