Development of structurally inverted basins: a case study from the West Coast, South Island, New Zealand

Abstract

Abstract Cretaceous-Palaeogene basins in the West Coast of South Island, New Zealand, underwent inversion during the Neogene. Initial extension was oriented WNW-ESE and had a typical magnitude of c. 5%, whilst subsequent shortening in the same direction was typically c. 3–4%. This deformation produced a NNE-SSW trending basin and range province. Shortening was commonly partitioned into narrow zones along basin and range margins, leaving unreactivated extensional structures inbetween. Such margins are characterized by subparallel and en echelon Cretaceous — Palaeogene faults, which have been reactivated as predominantly reverse faults during the Neogene. Reverse faults propagated into the syn-inversion sedimentary sequence with increasingly shallow dip. Inversion caused local erosion of the original rift deposits. New back-thrusts, low-angle thrusts and folds developed during the late Neogene. Locally, incompetent Eocene mudstones deformed in a quasi-plastic manner, so that reverse faults at deeper structural levels became decoupled from thrust faults at shallower levels. Computer-aided section restoration has allowed validation of the interpretations, provided estimates of the timing and magnitude of deformation, and given a better understanding of the sequential development of inversion structures. Natural inversion structures show both contrasts and similarities to those produced in sand-box experiments.