Gravity and Geological Studies of an Ultramafic Mass in New Zealand 1 
Alexander Malahoff 
ABSTRACT: A gravity and geologic survey was carried out over a portion of the 
Nelson ultramafic belt of the South Island. In this region, the ultramafic rocks out- 
crop 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 ex- 
tremity, 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 com- 
plete 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 ob- 
served 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 den- 
sity contrast of 0.5 gm/cc between the ultrarnafics and the country rock. Gravity 
analysis also shows that the Alpine Fault dips 67° southeast along the contact 
between the ultrarnafics 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 displace- 
ment 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 glacia- 
tion. 
The origin of emplacement of ultramafic rocks 
has always been a prime geologic problem in 
world geology. There are two ultramafic belts 
in New Zealand and these are separated by a 
300-mile-long displacement along the Alpine 
Fault system of New Zealand. The New Zea- 
land ultramafic belts have an added interest be- 
cause of this prominent fault movement. They 
provide an accessible source for geophysical and 
geological investigation, in the country where 
dunite was first described. 
1 Hawaii Institute of Geophysics Contribution No. 
159. Manuscript received October 1, 1965. 
Of particular interest to the geophysicist 
and geologist alike are the two areas where the 
ultramafic rocks abut against the fault planes of 
the Alpine Fault. In these regions, a genuine 
physical cross section is obtained across the ul- 
tramafic rocks and their associated formations 
where the Alpine Fault system has cut across 
the formations. 
One of these two regions is located in South 
Nelson in the northern part of the South Island 
of New Zealand and has been named, in this 
paper, the Tophouse district. A reconnaissance 
geological survey was carried out by the author 
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