140 
Fault, as observed from surface scarps, appears 
to be vertical. 
The ratio of the vertical to horizontal throw 
of the fault since the last glaciation is 1:10 
(Table 4), and the rate of horizontal movement 
(including steady and sudden movement) is 
0.36 inches per year. This rate is similar to the 
rate of 0.5 inches per year of dextral transcur- 
rent creep recorded on the San Andreas Fault 
in California (Whitten et al., I960). 
TABLE 4 
Mean Values for Vertical and Horizontal 
Throw of the Wairau Fault 
LOCALITY 
VERTICAL 
THROW (ft) 
HORIZONTAL 
THROW (FT) 
L. Rotoiti 
Peninsula 
40 
300 
East shore 
L. Rotoiti 
20 
330 
North flank 
St. Arnaud Range 
50 
250 
Blenheim- 
West Coast 
Highway 
30 
300 
Mean Values 
35 
300 
GRAVITY 
In the Tophouse district, intrusive and sedi- 
mentary rocks of the Maitai series, the Te Anau 
series, and the Brook Street volcanics are faulted 
against the homogenous Alpine greywacke by 
the Wairau Fault. In order to provide a simple 
geologic model for the gravity studies, the den- 
sity of each formation has been assumed to be 
uniform throughout and in depth. Lithologic 
boundaries have been of prime importance in 
the interpretation of the gravity data. 
The Worden gravity meter (W283) used 
in the survey has a scale range of 800 divisions, 
each division having a value of 0.0971 mgal. 
The base map has a scale of y 2 m ^ e t° the 
inch and was constructed by standard methods 
from air photographs. Heights were read at 
each gravity station with three altimeters and 
frequent checks were made to points of known 
height. Also, daily variations in air pressure 
were determined with a barograph, located at 
the base station. Drift of the gravity meter 
was corrected by beginning and ending a set of 
readings for one day at the same station. 
PACIFIC SCIENCE, Vol. XXI, January 1967 
Because of the practical difficulty of making 
terrain corrections in areas of irregular topogra- 
phy, station sites were chosen within areas of 
smooth topography. 
Gravimetric Corrections 
All Bouguer gravity values are given in 
terms of Glenhope-Christchurch, i.e., the New 
Zealand Provisional System (Robertson and 
Reilly, I960). The following corrections were 
made to the "observed” values: 
(1) Latitude correction: The 1930 Interna- 
tional Formula was used for determining lati- 
tude corrections. The increase in gravity to the 
south of the Tophouse district is 1.493 mgal 
per minute of latitude, or 1.298 mgal per mile. 
(2) Elevation corrections: 
(a) Free-air correction: This correction 
is constant and is 0.09406 mgal per foot of 
elevation. 
(b) Bouguer correction: The attrac- 
tion of the rock (taken as an infinite sheet 
in all directions) per foot of elevation be- 
tween sea level and the gravity station is 
0.01276 times the density of the rock. The 
density is assumed to be 2.67 gm/cc (the 
standard rock density), and variations from 
the assumed density have been allowed for 
in the interpretation of the gravity profiles. 
(c) The combined correction: The 
Bouguer and elevation corrections are com- 
bined and a correction of 0.05999 mgal per 
foot of elevation was used for all the gravity 
stations. 
(3) Terrain correction: Hammer’s tables 
(Hammer, 1939) and a graticule were used to 
allow for irregular topography, topographic 
data being obtained from direct observations 
out to 558 ft from the gravity station and from 
N.Z.M.S. 1 contoured topographic maps (S26, 
S27, S33, S34) from 558 ft to 14 miles. 
Errors in the Gravity Values 
The position of the gravity stations is 
known to about 100 yards. Latitude errors do 
not exceed 0.05 mgal and hence are negligible. 
Heights are accurate to about 10 ft and produce 
errors of about 0.6 mgal. Errors arising from 
the correction of topography (judging from 
local irregularities) in the observed values do 
not exceed 0.2 mgal. Errors caused by the drift 
