Northwest Otago Research Articles

Potassium-argon age of some New Zealand lamprophyre dikes near the Alpine Fault

Abstract Lamprophyre dikes in north-west Otago and north Westland yield whole rock potassium-argon ages mainly lying between 136 and 107 m.y. These ages correspond roughly to the ages inferred by others for the wall rock, suggesting that the ages give the latest major cooling event and that the dikes were possibly intruded earlier. In the dikes, the mica and amphibole ages are younger than their host whole rock age, while the plagioclase ages are older. The mineral ages are best explained as due to loss of argon from micas and amphiboles, and absorption by plagioclase and possibly other tectosilicates, the whole rock remaining approximately a closed system. The redistribution of argon is thought to have occurred at slightly lower temperatures than that causing outgassing of the whole rock. The whole rock dates support an earlier suggestion that the dikes were intruded as a single belt that has since been separated into two parts by a 120 km displacement along the Alpine Fault. The Alpine Fault displacement, which is dextral, was probably in two major periods: one in the Early Cretaceous, and the other in the Late Cenozoic.

Structure of the Otago schists

Abstract The development of previous interpretations of the structure of the Otago schists is outlined. Structural terms are defined, metamorphic structures described, and their interrelations discussed. Mesoscopic folds and lineations in the central zone of schists trend north-west in East Otago, northerly in Central Otago, and easterly in North-west Otago. They plunge subparallel to the dip of the schistosity in the marginal steeply dipping schists. Three main classes of folds designated F1, F2, and F3 are recognised, and are subdivided into a number of characteristic styles. The space and time relations of the mesoscopic folds suggest a sequence of phases of para-metamorphic deformation that may not have affected the terrain synchronously throughout, or everywhere to the same intensity. The regional structure appears to be a stack of nappe-like folds “overthrust” or “facing” eastward and north-eastward, flanked to the south-west and probably to the north-east by reclining and steeply plunging isoclinal folds.

An Upper Jurassic outlier in the Pyke Valley, North-West Otago, and a note on Upper Jurassic belemnites in the South Island

Abstract Upper Jurassic sediments are preserved in a narrow infaulted outlier extending north-eastwards from the Barrier River, and to the west of the Livingstone Fault. Belemnopsis of the aucklandica (Hochstetter) group, probably of Puaroan age, is recorded from these sediments exposed in the Barrier River. The distribution of Upper Jurassic belemnites in the South Island is discussed, and four new occurrences from North Canterbury are recorded: Belemnopsis sp. indet. and Hibolithes aff brownei (Marshall) from the Hurunui River; Belemnopsis aucklandica from the Okuku River; and Belemnopsis ex gr. aucklandica from Gorge Stream, a tributary of the Waiau River.