New Zealand Journal of Geology and Geophysics abstracts
Thrust transport directions in the Northland Allochthon, New Zealand
G. J. RAIT*
Institute of Geological & Nuclear Sciences
P.O. Box 30 368
Lower Hutt, New Zealand
*Present address: School of Earth Sciences, Victoria University of Wellington,
P.O. Box 600, Wellington, New Zealand. Email: geoff.rait@vuw.ac.nz
Abstract 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 +/- 10deg.. 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.
Keywords Northland; allochthon; structure; tectonics; thrust;
transport direction; ophiolite; obduction; Miocene
New Zealand Journal of Geology & Geophysics, 2000, Vol. 43:
271-288
0028-8306/00/4302-0271 $7.00/0 (c) The Royal Society of New Zealand
2000
PDF file of entire paper: medium quality (5221K); (scanned from paper original: notes about this process)
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