A NEW LOOK AT THE NEW-CALEDONIA BASIN, AN EXTENSION OF THE TARANAKI BASIN, OFFSHORE NORTH ISLAND, NEW-ZEALAND

The New Caledonia Basin is a major bathymetric trough extending north-westwards from Taranaki and continuing past New Caledonia. This paper is concerned with the 250 km of the feature closest to New Zealand and adjacent to the Taranaki Shelf. The New Caledonia Basin appears to have formed as the surface expression of a failed rift system. Gravity models, constrained by seismic reflection and refraction data, indicate thinned crust below the basin. The date of inception of rifting is not known, but it must be pre-Cenozoic and may be pre-Cretaceous. New seismic reflection data reveal tensional block faulting within the basin and on the flanks of the Challenger Plateau and the West Norfolk Ridge. Offset gravity anomalies and apparent compressional faulting observed on seismic data provide evidence for transverse, possibly transform, faulting accompanying and facilitating oblique extension. More than 1.5 s (TWT) of sediments were deposited during rifting. This is equivalent to a thickness of up to 3 km. They are likely to consist of Cretaceous clastics and volcaniclastics deposited in fluvial and marginal marine environments. Terrigenous clays and mudstones probably cap the rift sequence. Post-rift sedimentation commenced with thick authigenic limestone, reflecting regional thermal subsidence and remoteness from a clastic source. There is little record of tectonic activity until at least the Late Eocene, and probably not until the Early Miocene when volcanic activity began. Seismic sections show domal uplift at the base of the Challenger Plateau and the formation of several large submarine volcanoes. This activity may have been related to renewed rifting in the Bellona Trough. An average thickness of approximately 0.5 km of Plio-Pleistocene sediments onlap the margins of the basin. Channels and fans are evident on the seismic data and account for the distribution of a significant portion of the sediment eroded from the rising Southern Alps. The fragmentation of this part of the Gondwana margin resembles the evolution of the North-east Atlantic. In particular, the structure of the Rockall Trough closely resembles that of the New Caledonia Basin. Hydrocarbon source rocks may be present in the rift sequence, which has been buried to a depth great enough for thermal maturation. Structural and stratigraphic traps may exist at a number of levels.