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

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... h 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 thi [...]