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PACIFIC SCIENCE, VoL XXI, January 1967 
in 1961-1962 over the Tophouse district in 
order to map the geologic boundaries of the 
ultramafic belt and all visible fault traces of 
the Alpine Fault system. Another purpose of 
the geological survey was to compare the lithol- 
ogy, structure, and thickness of the geologic 
formations in the Tophouse district immediately 
north of the Wairau Fault with similar forma- 
tions south of the Alpine Fault in Otago, 300 
miles southwest of the Tophouse district. 
A gravity survey was carried out simultane- 
ously with the geological survey over the Top- 
house district in order to map the maximum 
thickness of the ultramafic rocks, and their at- 
titude as they abut against the Wairau Fault, 
and to investigate whether there is any sub- 
surface extension of the ultramafic belt south 
of the fault in the Tophouse district. Gravity 
surveying appears to be one of the best geo- 
physical methods to use in the study of ultra- 
mafic belts because of the high density contrast 
usually measured between the peridotite and the 
country rock into which the peridotite has been 
intruded. 
It is hoped that this paper will serve as a use- 
ful contribution to the gravimetric study of 
ultramafic rocks throughout the world. 
GEOLOGY 
Outline of Stratigraphy 
The rocks of the Tophouse district (Fig. 1 
and Table 1) consist of two classes, pre-Ter- 
tiary rocks and extensive post-glacial deposits. 
The pre-Tertiary rocks are divided into three 
fault blocks (Fig. 2) : (1) the Brook Street vol- 
canics west of the Waimea Fault and probably 
underlain by rocks of the Rotoroa igneous com- 
plex; (2) the Maitai and Te Anau series, east of 
the Waimea Fault and north of the Wairau 
Fault; and (3) the Alpine greywacke, south of 
the Wairau Fault. 
The relationship between the rocks of the 
Tophouse district and those of the ultramafic 
belt in Otago south of the Alpine Fault is 
shown in Table 2. 
Outline of Structure 
The Wairau Fault, the major one of the dis- 
trict, is a right lateral fault downthrown to the 
north. It is probably a branch of the Alpine 
Fault, together with which it forms a 300- 
mile-long transcurrent fault system which sepa- j 
rates the Maitai and Te Anau rocks of the Top- 
house district from those of Otago (Wellman, j 
1956:25). 
The strike of the Brook Street volcanics has 
changed as a result of stresses associated with 
movement along the Wairau Fault. At Top- 
house, the Brook Street volcanics strike at 360°, j 
1. e., parallel to the Waimea Fault. At Lake 
Rotoiti, these volcanics have been regionally j 
bent to strike at 60° and fault swarms have jj 
developed at intermediate angles between the j 
strike of the Waimea and Wairau faults. The 
Waimea Fault has not been active during the 
post-glacial period. 
The Maitai and Te Anau formations have 1 
undergone strike-faulting, but the Wairau ul- 1 
tramafic mass shows little sign of structural de- j 
formation. Detailed geology of the Tophouse 
district is shown on the geological map, Figure { 
2, and the stratigraphic units are presented in '■ 
Table 1. 
Brook Street Volcanics 
The Brook Street volcanics of the Te Anau 
series are part of the southern end of a sequence j 
of Upper Paleozoic volcanics which extend 
from d’Urville Island to Tophouse. The con- 
tinuous Waimea Fault between the Brook Street 
volcanics and the Maitai series makes strati- 
graphic relationship at Tophouse uncertain. No j! 
fossils were found in the Brook Street volcanics j 
in the Tophouse district; however, they are jj 
considered Upper Paleozoic here, the age as- !j 
signed by Bruce (1962:166) to the Brook !j 
Street volcanics of Nelson. 
The bulk of the rocks are massive metasoma- 
tized spilites and green-grey keratophyres. Along § 
the Waimea Fault, there is an outcrop of a j 
green volcanic conglomerate. The spilites are 
dense, hard and nonvesicular. The dip of the j 
volcanics has been determined from alignment 1 
of xenoliths and mineral grains. The spilite ex- 
posed in the Motupiko Valley is equigranular jj 
and fine grained, with 20% subcalcic augite I 
(2V = 20°) and with 60% plagioclase feld- 
spar. The augite crystals are corroded and set 
in a highly altered groundmass of epidote and 
chlorite. The green color of the Brook Street 
volcanics results from the alteration of mafic 
minerals to chlorite. 
