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Australia and New Zealand share in part a history in the Gondwana supercontinent. Australia has a long and complex tectonic history with the last major accretion in the early Paleozoic, whereas New Zealand is still undergoing major plate boundary processe

Lithospheres of different thicknesses are often juxtaposed by movement on a continental-transform boundary. Such a boundary with a step change in densities may trigger a gravitational instability as lateral pressure gradients are created where normal mantle lithosphere terminates against less dense asthenospheric mantle. Here we show, for plausible values of the lithospheric viscosity, a mechanism by which the thicker mantle lithosphere will drip off into the lower density asthenosphere. As the mantle deforms it also progressively thickens and then thins the overlying crust, creating a topographic wave that migrates in concert with the removal of mantle lithosphere. Within western North Island, New Zealand, geophysical data defi ne a sharp lithospheric step across the Taranaki-Ruapehu line, and geological observations provide a history of uplift and subsidence that has propagated southward in the past 12 m.y. The rate of observed north to south migration of the wave (~30 mm/yr), its wavelength (~250 km), and amplitude (~±1 km) are compatible with it being caused by progressive removal of mantle lithosphere, if the viscosity of the uppermost lithospheric mantle is ~5 × 1020 Pa·s, providing one of the clearest examples yet of this fundamental geological process.

Receiver function analysis reveals a 7 km step-like change in crustal thickness across the Taranaki-Ruapehu Line (TR-line) of North Island, New Zealand. The TR-line runs east-west between the active andesite volcanoes of Mt Taranaki and Ruapehu and marks

Two geophysical features characterise the anomalous crust-mantle structure beneath central North Island of New Zealand: the apparent thinning of the felsic crust by a factor of at least 50%, and upper mantle P-wave velocities (Pn phase) about 10% less than normal (∼ 7.3-7.4 km/s). Pn velocities increase slowly to a maximum of ∼ 7.8 km/s at a depth of about 80-100 km. Above a depth of 15 km seismic P-wave velocities (Vp) are 6 km/s or less, representing what is likely to be a mix of the original greywacke crust that has been stretched, along with an unknown volume of intruded igneous rocks that are in various stages of cooling. Defining the Moho is problematic in this dynamic region. Below a depth of 15 km there is a continuous increase in Vp such that velocities ∼ 7 km/s are reached at a depth as shallow as 20 km on a east west line and ∼7.4 km/s on a north-south line. We interpret these wave-speeds as a transition zone into low Pn velocities, rather than high lower-crustal values of Vp. Pn wave-speeds as low as 7.4 km/s are detected by tomographic methods to depths of at least 65 km and laterally over an area of at least 10,000 km2. A 1-4% partial melt in the mantle wedge beneath the central North Island is estimated, based on Qp, seismic anisotropy, low values of Pn and amplitudes of deep reflections. These upper mantle features are more indicative of asthenosphere than lithosphere and, coupled with the geological evidence for rapid uplift here at 5 Ma, are consistent with convective removal of the mantle lithosphere from western and central North Island in late Miocene times.

The central part of the South Island of New Zealand is a product of the transpressive continental collision of the Pacific and Australian plates during the past 5 million years, prior to which the plate boundary was largely transcurrent for over 10 My. Subduction occurs at the north (west dipping) and south (east dipping) of South Island. The deformation is largely accommodated by the ramping up of the Pacific plate over the Australian plate and near-symmetric mantle shortening. The initial asymmetric crustal deformation may be the result of an initial difference in lithospheric strength or an inherited suture resulting from earlier plate motions. Delamination of the Pacific plate occurs resulting in the uplift and exposure of mid-crustal rocks at the plate boundary fault (Alpine fault) to form a foreland mountain chain. In addition, an asymmetric crustal root (additional 8 - 17 km) is formed, with an underlying mantle downwarp. The crustal root, which thickens southwards, comprises the delaminated lower crust and a thickened overlying middle crust. Lower crust is variable in thickness along the orogen, which may arise from convergence in and lower lithosphere extrusion along the orogen. Low velocity zones in the crust occur adjacent to the plate boundary (Alpine fault) in the Australian and Pacific plates, where they are attributed to fracturing of the upper crust as a result of flexural bending for the Australian plate and to high pressure fluids in the crust derived from prograde metamorphism of the crustal rocks for the Pacific plate.

Isostatic considerations exhibit differences between the northern, central and southern parts of the Pacific–Australian plate collision in South Island, New Zealand. In the northern part mean elevations are moderate and the gravity low is small; the central part contains the highest elevations, and gravity and elevations correspond to each other relatively well; and in the southern part the gravity low is strongest whereas the mean elevations are moderate again. These differences indicate changes in the character of the isostatic compensation and are explained by increased thickening and widening of the crustal root from north to south, and also by the long wavelength gravity response to a mantle density anomaly that increases towards the south. A simple 3‐D gravity model is derived that includes the detailed crustal structures from the South Island GeopHysical Transect (SIGHT) experiment as well as a high‐density anomaly in the mantle inferred from teleseismic data. The model indicates that cold and, therefore, dense upper mantle material penetrates the asthenosphere to a greater extent in the south, similar to the behaviour of an apparently highly ductile lower crust. As plate reconstruction suggests more lithospheric shortening in the north, our model corresponds to lithospheric material escaping laterally to the south, almost perpendicular to the compression caused by lithospheric shortening of the mantle. Therefore, in addition to the prevailing mantle shear in New Zealand, there may also be a component of extrusional mantle creep beneath the Southern Alps orogen, which could have caused some of the observed large seismic anisotropy in this region. We may have also found evidence for submerged Eocene–Miocene oceanic lithosphere beneath the southeastern part of South Island that has been unaccounted for after plate reconstruction.