Plate tectonics successfully describes the surface of Earth as a mosaic of moving lithospheric plates. But it is not clear what happens at the base of the plates, the lithosphere-asthenosphere boundary (LAB). The LAB has been well imaged with converted teleseismic waves, whose 10-40-kilometre wavelength controls the structural resolution. Here we use explosion-generated seismic waves (of about 0.5-kilometre wavelength) to form a high-resolution image for the base of an oceanic plate that is subducting beneath North Island, New Zealand. Our 80-kilometre-wide image is based on P-wave reflections and shows an approximately 15° dipping, abrupt, seismic wave-speed transition (less than 1 kilometre thick) at a depth of about 100 kilometres. The boundary is parallel to the top of the plate and seismic attributes indicate a P-wave speed decrease of at least 8 ± 3 per cent across it. A parallel reflection event approximately 10 kilometres deeper shows that the decrease in P-wave speed is confined to a channel at the base of the plate, which we interpret as a sheared zone of ponded partial melts or volatiles. This is independent, high-resolution evidence for a low-viscosity channel at the LAB that decouples plates from mantle flow beneath, and allows plate tectonics to work.

New Zealand's Southern Alps lie adjacent to the continent-scale dextral strike-slip Alpine Fault, on the boundary between the Pacific and Australian plates. We show with a simple 2-D model of crustal balancing that the observed crustal root and erosion (expressed as equivalent crustal shortening) is up to twice that predicted by the orthogonal plate convergence since 11 Ma, and even since 23 Ma when the Alpine Fault formed. We consider two explanations for this, involving a strong component of motion along the length of the plate-boundary zone. Geophysical data indicate that the Alpine Fault has a listric geometry, flattening at mid crustal levels, and has accommodated sideways underthrusting of Australian plate crust beneath Pacific plate crust. The geometry of the crustal root, together with plate reconstructions, requires the underthrust crust to be the hyperextended part of an asymmetric rift system which formed over 500 km farther south during the Eocene-the narrow remnant part today forms the western margin of the Campbell Plateau. At 10 Ma, the hyperextended margin underwent shallow subduction in the Puysegur subduction zone, and then was dragged over 300 km along the length of the Southern Alps beneath a low-angle (<208) section of the Alpine Fault. We speculate that prior to 10 Ma, more distributed lower crustal shortening and thickening occurred beneath the Southern Alps, accommodating southward extrusion of continental crust in the northern part of the plate boundary zone, providing a mechanism for clockwise rotation of the Hikurangi margin.

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The widely accepted 450 km Cenozoic dextral strike-slip displacement on New Zealand's Alpine Fault is large for continental strike-slip faults, but it is still less than 60% of the Cenozoic relative plate motion between the Australian and Pacific plates through Zealandia, with the remaining motion assumed to be taken up by rotation and displacement on other faults in a zone up to 300 km wide. We show here that the 450 km total displacement across the Alpine Fault is an artifact of assumptions about the geometry of New Zealand's basement terranes in the Eocene, and the actual Cenozoic dextral displacement across the active trace is greater than 665 km, with more than 700 km (and <785 km since 25 Ma) occurring in a narrow zone less than 10 km wide. This way, the Alpine Fault has accommodated almost all (>94%) of the relative plate motion in the last 25 Ma at an average rate in excess of 28 mm/yr. It reverses more than 225 km (and <300 km) of sinistral shear through Zealandia in the Late Cretaceous, when Zealandia lay on the margin of Gondwana, providing a direct constraint on the kinematics of extension between East and West Antarctica at this time.

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