Gravity and Geological Studies of an Ultramafic Mass rn New Zealand!

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 ultram afic 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 W airau 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 grri/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 W airau 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 W airau ultramafic mass was emplaced as a vertical dike when the surrounding rocks were horizont al 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-Iong transcurrent displacement between the two areas along the Alpine Fault system of N ew 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. THE ORIGIN of emplacement of ultramafic rocks has always been a prime geologic problem in world geology. Th ere are two ultramafic belts in New Zealand and these are separated by a 300-mile-Iong displacement along the Alpine Fault system of New Zealand. The New Zealand ultramafic belts have an added interest because of this prominent fault movement. They provide an accessible source for geophysical and geological investigation, in the country where dunite was first described. ! Hawaii Institu te of Geophysics Contribution N o. 159. Manuscript received October 1, 1965. 129 Of particular interest to the geophysicist and geologist alike are the two areas where the ultr amafic rocks abut against the fault planes of the Alpine Fault. In these regions, a genuine physical cross section is obtained across the ultramafic 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