PALINSPASTIC RECONSTRUCTION OF THE EAST COAST BASIN, NEW ZEALAND: CONTROLLING TECTONIC EVOLUTION OF THE ANCIENT GONDWANA MARGIN

A phase of extension and thermal subsidence followed the mid-Cretaceous Rangitata Orogeny on the New Zealand sector of the Gondwana margin, resulting in the development of several marine basins around New Zealand. One of the largest of these, the East Coast Basin, remains poorly understood because of complex facies relationships and Neogene deformation associated with the development of the modern Hikurangi subduction margin. Understanding the sedimentary, tectonic and paleoenvironmental development of the basin is important for the constraining the evolution of the proto-New Zealand subcontinent and for resource exploration. Substantial variability currently exists between paleogeographic reconstructions, which typically either omit the East Coast Basin or use the region to accommodate large and unconstrained deformation.

To facilitate reconstruction we subdivide the basin along major basement-penetrating faults to produce a micro-plate model detailed enough so that aspects of brittle deformation—particularly strike-slip motions—are not obscured, but not so detailed that plastic deformation across structural blocks is lost. Lineations in basement terranes provide piercing points for the base reconstruction, which are utilised in conjunction with paleomagnetic constraints on the rotation and movement of individual structural blocks in order to remove Neogene deformation.

This structural base map, in combination with sequence- and biostratigraphic data, results in a reconstruction that explains spatial, temporal and depositional relationships for the Late Cretaceous to Paleogene succession of the East Coast Basin. Provenance information obtained from detrital zircon ages and heavy mineral assemblages provides insight into sediment sources and hinterland tectonics, and contributes to the geodynamic model of basin development, through the transition from basement compression and Mesozoic subduction, through mid-Cretaceous extension, to latest Cretaceous and Paleogene passive margin subsidence.