Thrust transport directions in the Northland Allochthon, New Zealand

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 degrees. 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.