Deformation across the active Hikurangi subduction margin, New Zealand, including shortening, extension, vertical‐axis rotations, and strike‐slip faulting in the upper plate, has been estimated for the last ∼24 Myr using margin‐normal seismic reflection lines and cross sections, strike‐slip fault displacements, paleomagnetic declinations, bending of Mesozoic terranes, and seafloor spreading information. Post‐Oligocene shortening in the upper plate increased southward, reaching a maximum rate of 3–8 mm/year in the southern North Island. Upper plate shortening is a small proportion of the rate of plate convergence, most of which (>80%) accrued on the subduction thrust. The uniformity of these shortening rates is consistent with the near‐constant rate of displacement transfer (averaged over ≥5 Myr) from the subduction thrust into the upper plate. In contrast, the rates of clockwise vertical‐axis rotations of the eastern Hikurangi Margin were temporally variable, with ∼3°/Myr since 10 Ma and ∼0°–1°/Myr prior to 10 Ma. Post 10 Ma, the rates of rotation decreased westward from the subduction thrust, which resulted in the bending of the North Island about an axis at the southern termination of subduction. With rotation of the margin and southward migration of the Pacific Plate Euler poles, the component of the margin‐parallel relative plate motion increased to the present. Plate convergence dominated the Hikurangi Margin before ca. 15 Ma, with the rate of margin‐parallel motion increasing markedly since 10 Ma. Vertical‐axis rotations could accommodate all margin‐parallel motion before 1–2 Ma, eliminating the requirement for large strike‐slip displacements (for example, >50 km) in the upper plate since the Oligocene.
Mesoscopic structures from 11 sites close to major thrusts and/or in zones of high shear strain were analysed to determine thrust transport directions in the Northland Allochthon. The sites span 250 km along strike and are at all structural levels: within Tangihua Complex ophiolites (uppermost allochthonous unit), between Tangihua and Mangakahia Complexes, within Mangakahia and Motatau Complexes, and in the autochthon immediately beneath the sole thrust. The structures analysed were shear bands, minor folds, minor thrust and shear zones, conjugate shear fractures, and groups of minor faults. Asymmetric structures (13 suites at 8 sites) indicate that the sense of transport was towards the southwest. From data at all the sites, the overall transport direction of the Northland Allochthon is estimated to have been towards 220 +10°. The consistency of transport directions at all structural levels indicates consistency throughout the thrusting history.The mean transport direction is approximately perpendicular to the thrust front, to folds affecting the allochthon and its basement, to belts of early Miocene andesitic volcanoes, and to the Vening Meinesz Fracture Zone. This strongly suggests that the allochthon was driven by early Miocene subduction beneath Northland, being obducted as a "flake" peeled from the downgoing slab. It further suggests that the Silverdale serpentinites are part of the Tangihua Complex rather than derived from the Dun Mountain Ophiolite Belt (DMOB): were they derived from the DMOB, their position northeast of it would require eastward or northeastward transport, evidence for which has not been found.
Paleomagnetic studies of Neogene marine sediments have documented large clockwise rotations of the Hikurangi margin (East Coast, North Island) during the Neogene, with the exception of the Raukumara Peninsula, which is unrotated with respect to the Australian plate. Immediately south of the Raukumara Peninsula, the Wairoa region has been rotated clockwise by 50-60°; the boundary between these domains is associated with a change in regional structural trends. However, a declination of 70 ± 14° reported from Otaian (19-22 Ma) sediments in the Rakauroa area is located to the north of this change. Characterisation of how differential rotations have been accommodated along the Hikurangi margin has been frustrated by this apparent mismatch between paleomagnetic and structural data. Paleomagnetic analysis of two new Rakauroa localities has yielded declinations of 16 ± 7° and 19 ± 9°, consistent with expected values for the Australian plate. This region is therefore not part of the Wairoa Domain. A strong viscous magnetic overprint was observed in many samples, the incomplete removal of which resulted in the misidentification of a large declination anomaly in the previous study. The paleomagnetically defined boundary between the Raukumara and Wairoa Domains now coincides with the area where regional structural trends alter. Reassignment of the Rakauroa area to the Raukumara Domain also results in a revised rotation history for the Wairoa Domain, suggesting rotation rates of 4-5°/m.y. since the late Miocene (5-10 Ma), and potentially no earlier rotation. No reliable record of early and middle Miocene vertical axis rotation on the Hikurangi margin now exists north of Marlborough; further studies are required to properly constrain the rotation history for this time interval.
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