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PACIFIC SCIENCE, Vol. XXI, January 1967 
Fig. 7. Gravity profile E-F across the Brook 
densities fits the observed curve to within 2 
mgal in all places. The most surprising feature 
of the profile is the 90° angle between the dips 
of the Maitai series-Te Anau volcanics and the 
attitude of the calculated base of the Wairau 
ultramafic mass. 
2. DISCUSSION OF GEOLOGICAL STRUCTURE 
AS INTERPRETED FROM GRAVITY RESULTS: The 
Brook Street volcanics lie between the Waimea 
Fault to the east and the Rotoroa igneous com- 
plex to the west. The Waimea Fault has been 
observed to dip steeply west and the Brook 
Street volcanics have been observed to dip be- 
tween 50° and 60° east. Interpretation of the 
gravity profiles shows that the Waimea Fault 
dips 70° west and that the maximum strati- 
graphic thickness of the Brook Street volcanics 
in the Tophouse district is about 13,000 ft, 
reaching a maximum vertical thickness of 12,- 
000 ft near the Waimea Fault. The Brook Street 
volcanics are probably underlain by rocks of the 
Rotoroa igneous complex, for which a density 
of 2.7 gm/cc has been assumed. The nature of 
this contact is unknown. A series of faults west 
of the Waimea Fault have been observed from 
air photographs and appear to dip steeply west. 
Rocks of the Maitai series range in density 
from 2.70 gm/cc (Rangitoto formation) to 
2.92 gm/cc (Stevens formation sandstone). 
However, no significant gravity anomalies were 
observed over these rocks, and it is inferred 
Street volcanics to the Wairau ultramafic mass. 
that the denser rock members do not extend to 
depth as expected from surface observations and 
are probably underlain by Maitai rocks with a 
mean density of 2.7 gm/cc at depth. 
Te Anau spilite, dolerite, and red and green 
volcanic breccia, with a mean density of 2.9 
gm/cc, lie east of the Maitai series. Gravity 
results suggest that these relatively dense rocks 
are underlain at 12,000 ft below sea level by 
rocks of standard density. The nature of the 
contact is uncertain. 
Geological interpretation of the positive 
gravity anomaly associated with the Wairau 
ultramafic mass has been based on two profiles 
(Figs. 5 and 7) constructed from average values 
over a width of 2 miles. Gravity values indicate 
that the peridotite is about 7,000 ft thick, with 
a maximum thickness below the Plateau (Fig. 
7). South of the Plateau, the peridotite is 
faulted by the Wairau Fault against Alpine 
greywacke. At the fault contact the average dip 
from the surface down to the base of the 
peridotite appears to be 67° southeast with an 
error of about ±15°. The Wairau ultramafic 
mass does not extend southward beyond the 
Wairau Fault. 
A subsurface western extension of the Wairau 
ultramafic mass below the Te Anau volcanics 
is necessary in order to explain the large posi- 
tive gravity values observed over the volcanics. 
The peridotite apparently intrudes the upper 
