Jan Grobys

Quantitative tectonic reconstructions of Zealandia based on crustal thickness estimates

Zealandia is a key piece in the plate reconstruction of Gondwana. The positions of its submarine plateaus are major constraints on the best fit and breakup involving New Zealand, Australia, Antarctica, and associated microplates. As the submarine plateaus surrounding New Zealand consist of extended and highly extended continental crust, classic plate tectonic reconstructions assuming rigid plates and narrow plate boundaries fail to reconstruct these areas correctly. However, if the early breakup history shall be reconstructed, it is crucial to consider crustal stretching in a plate-tectonic reconstruction. We present a reconstruction of the basins around New Zealand (Great South Basin, Bounty Trough, and New Caledonia Basin) based on crustal balancing, an approach that takes into account the rifting and thinning processes affecting continental crust. In a first step, we computed a crustal thickness map of Zealandia using seismic, seismological, and gravity data. The crustal thickness map shows the submarine plateaus to have a uniform crustal thickness of 20–24 km and the basins to have a thickness of 12–16 km. We assumed that a reconstruction of Zealandia should close the basins and lead to a most uniform crustal thickness. We used the standard deviation of the reconstructed crustal thickness as a measure of uniformity. The reconstruction of the Campbell Plateau area shows that the amount of extension in the Bounty Trough and the Great South Basin is far smaller than previously thought. Our results indicate that the extension of the Bounty Trough and Great South Basin occurred simultaneously.

Neogene sediment structures in Bounty Trough, eastern New Zealand: Influence of magmatic and oceanic current activity

New seismic reflection profiles have been interpreted to shed more light on the Neogene deposition in Bounty Trough, an aborted rift characterizing the eastern New Zealand continental margin. Two major processes influencing the deposition were identified. Magmatic activity led to the formation of basement highs, which deform the sedimentary sequences up to early and middle Miocene. The origin of the magmatism unfortunately cannot be solved with the new data presented here. It is difficult to say whether the magmatic activity is an on-going process. Drape structures in the youngest sedimentary unit argue against this. The Oligocene–Miocene represents a period of major change. Bottom current activity then took over as the most important depositional process. Strong cooling events in the late Miocene resulted in modification in the oceanographic regime east of New Zealand. This led to the formation of channels, sediment drifts, and sediment waves. At least since the Miocene, bottom current activity has been the dominating depositional process.