New Zealand Journal of Geology and Geophysics abstracts

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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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