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Gravity and Geological Studies of an Ultramafic Mass in New Zealand

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Title: Gravity and Geological Studies of an Ultramafic Mass in New Zealand
Authors: Malahoff, Alexander
Issue Date: Jan 1967
Publisher: University of Hawai'i Press
Citation: Malahoff A. 1967. Gravity and geological studies of an ultramafic mass in New Zealand. Pac Sci 21(1): 129-149.
Abstract: A gravity and geologic survey was carried out over a port ion of the
Nelson ultramafic belt of the South Island . In this region, the ultramafic rocks outcrop
over a 5-mile-wide belt and abut against the Alpine greywacke along the right
lateral transcurrent Alpine Fault. The dunite and peridotite of the ultramafic belt
as well as the overlying geosynclinal sediments strike north . At their southern extremity,
these rocks are faulted by the northeast-southwest striking Alpine Fault
against the massive Alpine greywackes to the south of the fault. There is a complete
discordance of the stratigraphic elements between the two sides of the fault.
The basal Permian ultramafic belt (Wairau ultramafic mass) to the north of the
fault is horizontally layered and shows inch-scale layering comparable to that observed
by Hess in the Stillwater complex of Montana. Stratigraphically above the
Wairau ultramafic mass and also on the northern side of the fault lies a vertically
dipping, 31,000-ft-thick sequence of serpentinite, spilite, grey slate, red and green
slate, and tuffaceous sandstone. The density of the rocks surrounding the Wairau
ultramafic mass varies between 2.65 gm/cc and 2.75 gm/cc, while that of the
peridotite and dunite varies between 3.2 gm/cc and 3.3 gm/ cc. A total thickness
of 7,000 ft for the Wairau ultramafic mass was computed, using the average density
contrast of 0.5 gm/cc between the ultramafics and the country rock. Gravity
analysis also shows that the Alpine Fault dips 67° southeast along the contact
between the ultramafics and the Alpine greywacke.
It is thought that the Wairau ultramafic mass was emplaced as a vertical dike
when the surrounding rocks were horizontal and that the dike and the surrounding
rocks have been rotated by 90° so that the dike is now horizontal and the beds are
vertical. Comparisons between the stratigraphic sequence studied here and an
almost identical sequence on the southern side of the Alpine Fault in Otago province
supports the previously postulated 300-mile-long transcurrent displacement between
the two areas along the Alpine Fault system of New Zealand. Studies of displacement
of post-glacial river terraces along the Alpine Fault in Nelson show an average
right lateral movement of 0.36 inches per year along the fault since the last glaciation.
ISSN: 0030-8870
Appears in Collections:Pacific Science Volume 21, Number 1, 1967

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