New Zealand Journal of Geology and Geophysics abstracts
Geomorphic implications of fault zone weakening: slope instability along
the Alpine Fault, South Westland to Fiordland
Oliver Korup
School of Earth Sciences
Victoria University of Wellington
P.O. Box 600
Wellington, New Zealand*
email: korup@slf.ch
*Present address: WSL Swiss Federal Institute for Snow and Avalanche
Research SLF,
Flüelastr. 11, CH-7260, Davos, Switzerland.
Abstract Tectonic weakening of bedrock along the Alpine
Fault Zone (AFZ) in South Westland and northern Fiordland, New Zealand, has
favoured erosion of schist-derived mylonite and cataclasite by a variety
of slope failures. This paper describes the basic geomorphic characteristics
and effects of chronic and instantaneous slope instability affecting c. 28
km2 in the AFZ, conditioned by groundwater infiltration, geochemical
alteration, coseismic hanging-wall rock fracturing, and gravitational stress
in thrust nappes. Mass movement involves 30 complex gully/slip systems and
10 long-runout landslide deposits, which were identified up-to-date, and
mainly triggered by fluvial undercutting, increased pore-water pressures,
and possibly, coseismic shaking. Chronic slope instability in gully/slip
systems causes rapid delivery of excessive debris to alluvial fans, producing
high sediment yields (c. 104 t.km–2.yr–1),
aggradation, and large-scale avulsion. The principal hazards from catastrophic
landslides at the Westland mountain-range front are direct physical impact
during runout onto the foreland and short-lived blockage of major rivers.
Several settlements and infrastructure are located along the range front
and are at risk from these geomorphic on-site and off-site effects of fault-related
slope instability.
Keywords fault zone weakening; landslide; gully erosion;
Alpine Fault; aggradation; geomorphic hazard
G03007; Received 20 March 2003; accepted 1 December 2003; Online publication
date 20 May 2004
New Zealand Journal of Geology & Geophysics, 2004, Vol. 47: 257–267
0028–8306/04/4702–0257 © The Royal Society of New Zealand 2004
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