Geologic setting, gravity collapse and hazard assessment of the Kongahu Fault Zone, Westport.

The Buller Coalfield comprises the northern end of the Paparoa Trough, an elongate basin that began to subside in the mid Cretaceous. Subsidence occurred in response to mid Cretaceous extension, leading to crustal thinning, and culminating in opening of the Tasman Sea. The area underwent asymmetric subsidence between the mid-late Eocene and the late Oligocene, controlled by normal faulting, inferred to occupy the same position as the present day Kongahu Fault Zone. Inversion of the Paparoa Trough commenced in the late Oligocene, by reactivation of normal faults as reverse/thrust faults through a regional change from extension to shortening and establishment of the Alpine Fault as a new plate boundary. Positive inversion progressed through a number of phases over the last 25Ma producing the present day Buller Coalfield. Four sets of Quaternary coastal marine terraces are recognised within the Westport region. Preservation and tilting of this terrace sequence is indicative of progressive regional uplift continuing through to the present day. Large scale gravitational collapse structures (forming the Kongahu Fault Zone Failure Complex, with a surface area of approximately 18km² have formed over several hundred thousand years, along the escarpment separating Tertiary units on Denniston Plateau from Quaternary deposits along the coastal plain. Landslide materials of the Kongahu Fault Zone failure are very complex and have been subdivided into four zones based on slide geology and surface morphology. Six separate deformation phases have been identified based on interpretations of geomorphic evolution of the failure complex. Initial failure is inferred to have taken place along unfavourably oriented rockmass defects, such as bedding planes, joint sets and faults, with destabilisation initiated through head loading (caused by tectonic uplift) and removal of toe support through erosion of Late Tertiary units. Preservation of Caledonian Formation marine terraces and associated deposits, on the landslide complex, places this event beyond 334 000 years BP. Later phases of reactivation of the landslide complex are related to interglacial high stands in sea-level and ongoing fluviatile erosion, removing toe area support. The initial rupture surface is inferred to have propagated along bedding planes within the Brunner Coal Measures, but as the failure complex evolved the rupture surface propagated into sheared granitic basement, forming large-scale, deep-seated III collapse. Preservation of coastal marine terraces in the toe area of the main failure complex, fonned during the last interglacial period are indicative of stability of the main failure complex since approximately 58-72 000 years BP. However, interpretation of geomorphic features upon the coastal plain indicate extremely slow deep-seated activity within the central "Mt Rochfort Failure". A seismic hazard assessment of the main Kongahu Fault Zone failure complex indicates that it is inherently stable and unlikely to undergo large scale reactivation through high intensity ground shaking. Seismically triggered local rock falls , rock avalanches and rapid soil flows form the dominant hazard associated with earthquake triggered failure. Only one section of the failure complex, the "Mt Rochfort failure", is considered to still be active although inferred to be failing as extremely slow, deep creep. Localised recent failures are primarily related to antecedent pore water conditions and triggered by intense or prolonged rainfall and seismic events. These create a low level hazard due to lack of human interaction in areas where the failures occur. Reactivation of debris within fluvial channels leading to avulsion onto fan surfaces along the coastal plain forms the dominant hazard. Lake Rochfort is a landslide formed lake (approximately 320 000m ³), 460m above the coastal plain, located within the active Mt Rochfort Failure. By comparison with the (1981) Ram Creek Dam burst, it has been concluded that catastrophic failure of Lake Rochfort would destroy property and services with the potential for causing serious injury and loss of life on the coastal plain