Neogene deformation in part of the Buller Coalfield, Westland, South Island, New Zealand

Abstract

Abstract The Buller Coalfield, Westland, is an area of Eocene coal measures and marine mudstones stretching along the western Papahaua Range, from the Buller River in the south to the Mokihinui River in the north. These deposits form a thin cover sequence on Paleozoic‐Mesozoic basement rocks which have been uplifted during the Neogene. In the Stockton Plateau region, basement and cover have been shortened by folding and reverse movement on faults trending NNW‐SSE to NE‐S W, and sinistral movement on faults trending NW‐SE to NNW‐SSE. Subvertical joints of Neogene age in the Eocene coal measures form sets: extensional, shear, and hybrid types are identified. The regional shortening direction is derived from joint orientations to be WNW‐ESE (285°), the same as that derived from a study of historic earthquakes. Comparison of the shortening direction (WNW‐ESE) with fault orientations indicates that the faults and joints were not formed by the same strain. Faults were formed prior to the Neogene and have been r...