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Abstract
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.
