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Geomorphic implications of fault zone weakening: Slope instability along the Alpine Fault, South Westland to Fiordland

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 shortlived 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.