Hannu Seebeck

Structure and kinematics of the Taupo Rift, New Zealand

The structure and kinematics of the continental intra-arc Taupo Rift have been constrained by fault-trace mapping, a large catalogue of focal mechanisms (N = 202) and fault slip striations. The mean extension direction of ~137° is approximately orthogonal to the regional trend of the rift and arc front (α = 84° and 79°, respectively) and to the strike of the underlying subducting Pacific Plate. Bending and rollback of the subduction hinge strongly influence the location, orientation, and extension direction of intra-arc rifting in the North Island. In detail, orthogonal rifting (α = 85–90°) transitions northward to oblique rifting (α = 69–71°) across a paleovertical-axis rotation boundary where rift faults, extension directions, and basement fabric rotate by ~20–25°. Toward the south, extension is orthogonal to normal faults which are parallel to, and reactivate, steeply dipping basement fabric. Basement reactivation facilitates strain partitioning with a portion of margin-parallel motion in the overriding plate mainly accommodated east of the rift by strike-slip faults in the North Island Fault System (NIFS). Toward the north where the rift and NIFS intersect, ~4 mm/yr strike slip is transferred into the rift with net oblique extension accommodating a component of margin-parallel motion. The trend and kinematics of the Taupo Rift are comparable to late Miocene-Pliocene intra-arc rifting in the Taranaki Basin, indicating that the northeast strike of the subducting plate and the southeast extension direction have been uniform since at least 4 Ma.

Geometry of the subducting Pacific plate since 20 Ma, Hikurangi margin, New Zealand

Evolution of slab geometry beneath the North Island, New Zealand, has been investigated using a combination of published arc-type volcanic ages and earthquake locations in the subducting Pacific plate. Arc-front volcanoes migrated SE by 150 km in the last 8 myr subparallel to the present active arc. Migration of the arc is interpreted to mainly reflect slab rollback along the Tonga–Kermadec subduction system changing to fixed hinge slab steepening beneath the central North Island. The strike of the Pacific plate beneath the North Island, imaged by Benioff zone seismicity (50–200 km) and positive mantle velocity anomalies (200–600 km), is parallel to the NE–SW trend of arc-front volcanism. Arc parallelism since 16 Ma indicates that the strike of the subducting plate beneath the North Island was constant over this time interval, in contrast to clockwise vertical-axis rotations of ≥50° of the overriding plate over the same period along the eastern and southern Hikurangi margin. Acceleration of arc-front migration rates (from c. 4 to c. 18 mm a−1), eruption of high-Mg# andesites, increasing eruption frequency and size, and uplift of the overriding plate indicate an increase in the hydration, temperature and size of the mantle wedge beneath the central North Island from c. 7 Ma.

Investigation of the spatio-temporal relationship between normal faulting and arc volcanism on million-year time scales

The spatio-temporal evolution of normal faulting and submarine volcanism during the Mid-Miocene to Recent (<16 Ma) in the Taranaki Basin, New Zealand, provides insights into the processes driving rifting and volcanism. In the Taranaki Basin high sedimentation rates have led to the blanketing and preservation of mainly submarine volcanic edifices and normal faults. Volcanic activity gradually migrated southward along the basin and contrasts with the punctuated migration of normal faulting in the same direction. Gradual southward migration of volcanism since c. 16 Ma has been attributed to progressive steepening and SE rollback of the subducting Pacific Plate. Similarly, the location and NE–SW strike of Late Miocene and younger normal faults mainly west of the North Island appear to have been controlled by the location and NE–SW strike of the underlying subducting plate. Stepwise changes in the locus of faulting at c. 8, 4 and 2 Ma could have been triggered by increases in the rates of vertical-axis rotation of the North Island associated with changes in plate convergence rates and southward migration of the rotation pole. The disparate spatio-temporal migration histories of subduction-related faulting and volcanism indicate that, over time scales of millions of years and distances of tens of kilometres, neither process controls the timing, location and rates of activity of the other.

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.

Managing potential interactions of subsurface resources

Subsurface resources include oil, gas, coal, groundwater, saline aquifer minerals, and heat (for geothermal use). Pore space itself should also be considered as a resource as it can be used for injection of waste fluids, produced water, storage of natural gas, compressed air, and supercritical CO2. Use of subsurface resources can overlap in space, and pressure changes at one site can remotely influence resource use at other sites. Resource use can also vary in time, such as the use of depleted oil or gas fields for natural gas or CO2 storage. Before allocation of a subsurface resource it is therefore useful to understand the potentially wide range of resources available in an area, how they might be developed successively, and how they could affect each other if used concurrently. While these issues are primarily geological, they have critical significance for legal, environmental, and economic considerations.