EXTENSIONAL TECTONISM AND MAGMATISM DURING THE MIDDLE CRETACEOUS TO PALEOCENE, NORTH WESTLAND, NEW-ZEALAND

Open-file seismic reflection data, from the offshore area west of Greymouth, show a large segmented half-graben structure, over 40 km long, up to 15 km wide, and trending WNW-ESE. Although the structure is not drilled, regional seismic interpretation suggests it is of Cretaceous age. The geometry of the structure indicates extension of about 20%. Strata of similar age onshore are thought to have been deposited in half-grabens with a similar orientation. In contrast, structures containing Late Cretaceous to Paleocene deposits onshore trend NNE-SSW, almost at right angles to the earlier trend. Cretaceous dike orientation data show a bimodal distribution, with the main trends being WNW-ESE and NNE-SSW. Together with published radiometric and fission-track ages from basement and Cretaceous igneous rocks, these data show that there were at least three distinct periods of tectonic/magmatic activity: (1) NNE-SSW directed extension, uplift, plutonism, and minor volcanism in the interval 125-90 Ma; (2) multiple dike intrusion and decreased NNE-SSW directed extension and uplift in the interval 90-80 Ma; and (3) breakup unconformity, volcanism, dike intrusion, waning subsidence in WNW-ESE trending basins, initiation of NNE-SSW transtensional basins, and uplift in the interval 80-60 Ma.