Crustal evolution of the submarine plateaux of New Zealand and their tectonic reconstruction based on crustal balancing

Abstract

The last supercontinent fell into pieces with the break-up of Gondwana. In this context, the separation of the microcontinent of New Zealand from Antarctica is a jigsaw puzzle of many pieces. Its parts lay at the convergent margin of East Gondwana, which changed into a divergent margin within a geologically short time. That is why the microcontinent of New Zealand experienced different tectonic regimes and phases of the Wilson cycle. Although it is a good object of investigation due to its changing history, remarkably little is known about the submerged parts of the microcontinent. Knowledge of the magmatic-tectonic development of the submarine plateaux such as Campbell Plateau and Chatham Rise will improve the understanding of the processes that led to the late Gondwana break-up, and, in turn, lead to better reconstructions of East Gondwana, as Zealandia is a key piece in plate-kinematic reconstructions of this part of Gondwana.The central part of this thesis deals with the separation process of Zealandia from Antarctica leading to an improved reconstruction of New Zealand with emphasis on the submarine plateaux. Bounty Trough separating Chatham Rise from Campbell Plateau, and the Great South Basin separating Campbell Plateau from the South Island are investigated with seismic refraction and reflection methods. They are interpreted jointly with magnetic and gravity data. The results of crustal thickness modelling based on satellite gravity data are combined with existing information about crustal thickness of Zealandia. With these data, a crustal thickness grid is calculated which creates the basis for a novel technique for plate-kinematic reconstructions in areas of crustal thinning and in the absence of magnetic seafloor anomalies. This reconstruction consists of crustal balancing to compensate for extension within basins and troughs.The seismic refraction and reflection survey across the Bounty Trough shows a strongly thinned crust in the trough. Zones of high P-and S-wave velocities were found in the lower crust shows. Comparison of the P-wave model and a Poisson's ratio model with rock type diagrams leads to a compositional model of the crust. The joint interpretation of all models suggests that extension in the Bounty Trough proceeded until seafloor spreading in the Middle Bounty Trough began. Geophysical data from the Great South Basin show underplating beneath the Central Campbell Plateau and crustal thinning in the basin, to a lesser extent than in Bounty Trough. Comparison of the seismic data with existing magnetic data across the Great South Basin (Stokes Magnetic Anomaly System - SMAS) and the Campbell Plateau (Campbell Magnetic Anomaly System - CMAS) resulted in the conclusion that these anomaly systems have different origins or histories. Contrary to the results of this thesis, previous investigations assumed a common origin of SMAS and CMAS. Plate-kinematic reconstruction on the base of observations and interpretations combined with existing and modelled crustal thickness shows that extension in Bounty Trough and Great South Basin as well as in New Caledonia Basin was significantly less than previously assumed. The novel technique for plate tectonic reconstructions in thinned continental crust presented in this thesis has the potential to improve plate-kinematic reconstructions for early break-up settings and failed rift systems with stretched continental crust worldwide.