R. H. Herzer

Cretaceous‐Tertiary tectonic history of the Fiordland margin, New Zealand

Abstract Integration of regional seismic reflection data with swath bathymetry and imagery data and onshore geology has revealed the complex sequence of tectonic events that have affected the development of the offshore Fiordland margin. There are no core or dredge samples to constrain the timing of these events, but comparison with recent work on the Tertiary evolution of the Australia‐Pacific plate margin provided a temporal framework for the subsurface interpretation. The integrated dataset allows the prediction of the age and likely lithology of the seismic sequences, the timing of tectonic events, and details of how structures along the Fiordland plate margin developed in response to changes in relative plate motion direction. We conclude that Cretaceous rifting in the Tasman Sea formed half‐grabens along Caswell High and beneath the Fiordland Basin. Up to several kilometres of Cretaceous to Early Eocene sediments, probably terrestrial and marginal marine sediments derived from the Campbell Plateau, are preserved in these grabens. A second phase of rifting in the Eocene, associated with the formation of the Southeast Tasman Sea, resulted in the separation of the Caswell High and Fiordland Basin block from the Campbell Plateau and the start of its journey northwest, northeast, and finally east to its present position adjacent to Fiordland. The rift sediments are overlain by a relatively uniform sequence, probably Late Eocene to Mid Miocene deep‐water carbonates. Strike‐slip faults developed along the continental shelf and slope in the Miocene, and increasing margin‐normal shortening in the Pliocene‐Pleistocene, led to the development of thrust faults along the margins of sediment lobes in the Fiordland and Milford Basins. The youngest sediments are up to several kilometres thick, probably Late Miocene‐Quaternary elastics derived from Westland and Fiordland.

Ultra‐fast early Miocene exhumation of Cavalli Seamount, Northland Plateau, Southwest Pacific Ocean

Abstract We present new photographic, petrological, geochronological, and isotopic data for gneissic and granitic rocks obtained from six sample stations on Cavalli Seamount during two cruises in 2002. These data lead to revision of earlier conclusions based on two dredges of schist in 1999. Based on c. 100 Ma ages of zircon cores, and whole rock petrochemistry and tracer isotopes, we interpret the protoliths of paragneisses and orthogneisses to probably have been sedimentary and plutonic correlatives of the Late Cretaceous Houhora Complex. U‐Pb dating of low Th/U zircon rims confirms an earliest Miocene high‐grade metamorphic episode. A cooling history based on Ar‐Ar K‐feldspar dating indicates ultra‐rapid cooling (c. 2000°C/m.y.) and vertical exhumation (c. 100 mm/yr) of the rocks at 19.9 Ma. Our preferred tectonic model relates the amphibolite facies metamorphism to Northland Allochthon emplacement and the rapid exhumation to dextral transtension along the Vening Meinesz Fracture Zone system and/or a rapidly retreating Pacific trench.