Dominic P. Strogen

Paleogeography of the Taranaki Basin region during the latest Eocene–Early Miocene and implications for the ‘total drowning’ of Zealandia

Latest Eocene–earliest Miocene strata in the subsurface of the Taranaki Basin provide important new regional paleogeographic and tectonic constraints not available from outcrop. Six paleogeographic maps of the Taranaki Basin region have been produced utilising extensive well, seismic reflection and outcrop data. These record three broad periods of sedimentation characterised by (1) variable transgression and initial deformation (c. 40–30 Ma); (2) maximum transgression with moderate deformation (c. 30–21 Ma); and (3) regression with accelerated deformation (< 21 Ma). Local sedimentation patterns were influenced by reverse faulting, producing depocentres and topographic highs adjacent to the Taranaki Fault System. Reverse faulting commenced as early as c. 40 Ma and may signify the onset of subduction beneath the North Island. In common with other parts of New Zealand, the region reached maximum marine inundation in the Waitakian (c. 23 Ma). However, the deposition of thick clastic sediments in eastern parts of Taranaki Basin, coupled with ongoing tectonism, suggests the presence of land throughout the Oligocene and Early Miocene, and is inconsistent with total Oligocene drowning of Zealandia.

Two-phase Cretaceous–Paleocene rifting in the Taranaki Basin region, New Zealand; implications for Gondwana break-up

The break-up of Gondwana resulted in extension of New Zealand continental crust during the Cretaceous–Paleocene. Offshore the geometry and rift history are well imaged by new regional mapping of a large seismic reflection dataset, tied to wells, used here to document the Cretaceous–Paleocene (c. 105 – 55 Ma) evolution of the greater Taranaki Basin region. Two temporally distinct phases of rifting have been recognized in the region, and record Gondwana break-up. The first (Zealandia rift phase) produced half-grabens trending NW to WNW during the mid-Cretaceous (c. 105 – 83 Ma). These rift basins predate, and are parallel to, Tasman Sea spreading centres. They record distributed stretching of northern Zealandia prior to the onset of seafloor spreading in the Tasman Sea. A short period (c. 83 – 80 Ma) of uplift and erosion followed, possibly representing a break-up unconformity, with erosion in southern Taranaki Basin and deposition of the ‘Taranaki Delta’ sequence in Deepwater Taranaki. The second, West Coast–Taranaki rift phase produced north- to NE-trending extensional half-grabens in the shelfal Taranaki Basin during the latest Cretaceous–Paleocene (c. 80 – 55 Ma). This rift was narrow (<150 km wide), orthogonal to Zealandia phase rifting, affected mainly western Zealandia and did not progress to full break-up. Supplementary material: A full set of eight palaeogeographical maps as well as expanded versions of the seismic figures, with both uninterpreted and interpreted versions, are available at https://doi.org/10.6084/m9.figshare.c.3772175

Cretaceous to present-day tectonic reconstructions of Zealandia

Reconstructions of the past relative positions of northern and southern Zealandia provide important constraints on the orientation and amount of strain accumulated between rigid plates within the Australia–Pacific plate tectonic circuit. This configuration of plates ultimately determines how, where and when sedimentary basins formed during and since continental breakup along the eastern margin of Gondwana. Although the first-order geometry of Zealandia is well established, uncertainty remains regarding plate motions through the latest Cretaceous to Eocene. Recent reconstructions are, in some cases, inconsistent with geological observations at key time intervals, highlighting uncertainties inherent in plate reconstructions for the south-west Pacific. Building on previous tectonic reconstructions and incorporating published seafloor magnetic interpretations, paleomagnetic observations and geological constraints (e.g. terrane geometry and distribution), we developed a tectonic framework to reconstruct Zealandia back through to the latest Cretaceous. Using GPlates, we use a simple double-hinge slat concept to describe Neogene deformation within the New Zealand plate boundary zone, while the geometry of northern and southern Zealandia during the Eocene is modified from recently published models based on geologic considerations. This study ultimately highlights the need for integrated studies of the Zealandia plate circuit.