Accommodation of compressional inversion in north-western South Island (New Zealand): Old faults versus new?

In the NW South Island, New Zealand, high-angle faults inherited from episodes of Late Cretaceous–Paleocene and Eocene extension have, since the early Miocene, undergone compressional inversion in association with right-lateral shearing and transpression on the Alpine Fault. Active reverse faulting and large historical earthquakes occur along N–S to NNE–SSW trending faults which at the surface dip 45–75° to both the east and west. The faults truncate subparallel folds that deform the Tertiary sequence overlying a composite Paleozoic–Mesozoic crystalline basement. However, the deep geometry of these faults, their penetration into the middle-to-lower crust and their relationship to the Alpine Fault are poorly understood. The tectonic architecture of this compressional inversion province is analysed by reconstructing structural contours at the base of the Oligocene carbonate sequence in the north-west of the South Island. Deformation of the Oligocene carbonate sequence, structural analyses in the field and subsurface data indicate a mixed style of inversion with (1) reactivation of some high-angle normal faults and (2) thrusting on new, moderate-dipping cross-cutting faults that detach slivers of basement and cause flexural folding in the sedimentary cover. These faults may remain blind or concealed beneath cover sequences but are likely to control seismic rupturing in the basement at depths of ∼10–15 km.