gravity and magnetic measurements in the south taranaki bight, new zealand
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This article was downloaded by: [University of Hong Kong Libraries]On: 09 October 2014, At: 22:49Publisher: Taylor & FrancisInforma Ltd Registered in England and Wales Registered Number: 1072954Registered office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH,UK
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Gravity and magneticmeasurements in the SouthTaranaki Bight, New ZealandTrevor M. Hunt a & Derek J. Woodward aa Geophysics Division, Department of Scientific andIndustrial Research , Wellington , New ZealandPublished online: 09 Jan 2012.
To cite this article: Trevor M. Hunt & Derek J. Woodward (1971) Gravityand magnetic measurements in the South Taranaki Bight, New Zealand,New Zealand Journal of Geology and Geophysics, 14:1, 46-55, DOI:10.1080/00288306.1971.10422455
To link to this article: http://dx.doi.org/10.1080/00288306.1971.10422455
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46 VOL. 14
GRAVITY AND MAGNETIC MEASUREMENTS IN THE SOUTH TARANAKI BIGHT, NEW ZEALAND
TREVOR M. HUNT and DEREK J. WOODWARD
Geophysics Division, Department of Scientific and Industrial Research, Wellington
(Receiz,ed for publication 17 September 1969)
ABSTRACT
Gravity measurements suggest that the earth's crust has a thickness of about 20 km beneath the south-eastern margin of the New Caledonia Basin. From a point 50 km within the edge of the New Zealand continental shelf the crust thickens to a maximum value of about 36 km near the Marlborough Sounds. Residual gravity and magnetic anomalies indicate that a broad belt of dense, magnetic rocks underlies the South Taranaki Bight but does not appear to extend beneath Taranaki Province. These rocks may be either Paleozoic intrusives similar to those found in the South Island, Or
Pleistocene volcanics similar to Egmont Andesite.
INTRODUCTION
Geophysical measurements made during the crossing of the Tasman Sea from Sydney, Australia, to WellingtDn, New Zealand, in September 1967 by the r.v, Oceano~raPher were kindly made available to' us by the Pacific Oceanographic Research Laboratory, SeattIe, U.S.A., for interpretation. In this paper those continuous gravity, magnetic, and bathymetric measurements made across the New Zealand continental shelf in the South Taranaki Bight and Cook Strait are discussed. This area (Fig, 1) is of special interest because of the recent offshDre Dil discovery there, and because the ship's track crosses the largest negative gravity anomaly in New Zealand, the Rangitikei-Waiapu Anomaly (Robertson and Reilly, 1958). The area is also Df interest because Df the uncertainty of geological correlations between the North and South Islands,
Most of the published geophysical data from the South Taranaki Bight have been in the form Df profiles of magnetic anomalies which have allowed limited geological correlations to be made (Wellman, 1959; Hatherton, 1967), The only previous gravity measurements at sea in the area were five submarine pendulum observations made by Lamont GeDlDgical Observatory (Worzel, 1965), which have been incorporated in the gravity map of New Zealand (Rieilly, 1965). Much offshore seismic exploration has been done by oil companies, but little Df their work has been published,
GEOPHYSICAL MEASUREMENTS
The speed and course of the Oceanograrpher were recorded every 5 minutes, and the position determined by satellite fix at approximately 2-hourly intervals, except in sight of land when radar and visual navigation were
N.z. Journal of Geology and Geophysics 14 (1); 46-55
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No.1 HUNT & WOODWARD- SOUTH TARANAKI BIGHT 47
used. The position of the ship betwen navigational fixes was laber determined by adjusting the dead-reckoning position with respect to time.
Depths were taken at 5-minute intervals from a precision depth recorder and corrected for variations in the velocity of sound with depth in water using Matthews' (1939) tables.
The free air gravity anomalies and total fowe magnetic anomalies were obtained using the methods described in Woodward and Hunt (1970) and are shown in Fig. 2. The free air gravity anomaly profile has been extrapolated to the point E (Fig. 2) using data from Reilly (1965).
GEOLOGICAL STRUCTURES TRAVERSED
The track of the Oceanographer across the New Caledonia Basin and the New Zealand continental shelf from 38° 00' S, 172° 28' E (point A, Figs. 1, 2) to near Wellington, together with the geology of the adjoining land, is shown in Fig. 1. The continental shelf between the North and South Islands is remarkably fiat and lies at a depth of 100-150 m. Seismic refi,ection profiles taken west of Cape Farewell have shown the outer portions of the shelf to be formed of a monotonously fiat sequence of sediments at least 1 km thick (Houtz et ai., 1967). The track passes 40 km south-west of Moa 1 drillhole (Fig. 1) near the edge of the shelf, where 3'5 km of sediments were found to overlie schistose basement rock similar (Mr G. W. Grindley, pefs. comm.) to that found in the north-west of the South Island.
The track then crosses a deep sedimentary basin, the Taranaki Basin (Cope and Reed, 1967), which contains an almost complete sequence of Cenozoic deposits and which is thought to extend from North Taranaki to the South Taranaki Bight. Seismic measurements show that the sediments are 6-8 km thick beneath the track across the Taranaki Basin (Dr R. A. Couper, pers. comm.), and these are thought to overlie a basement of Lower Paleozoic sedimentary rocks intruded by granite plutons (Cope and Reed, 1967). Seismic reflection profiles (Houtz et at., 1967) have shown that the near surface « 1 km) sediments are disturbed and contain eastwarddipping reflectors. The Taranaki Basin is bounded on the east by a structural basement high (encountered in Taranaki as the Patea High) resulting from a vertical displacement of about 7 km on a major fault zone, the Taranaki Fault. This fault zone is thought to be one of the major structural features of the region, and has been correlated by Cope and Reed (1967) with the Waimea Fault and associated sub-parallel faults (Eighty-eight, Heslington, and Whangamoa Faults) along the eastern margin of the Moutere Depression in the South Island. Intruded into basement rocks on either side of the Waimea Fault in the northern part of the South Island is a belt of dense and magnetic rocks, which are associated with the New Zealand Marginal Syncline, and which have been traced geophysically from the Alpine Fault near Lake Rotoiti to Kawhia in the North Island (Wellman, 1959; Hatherton, 1967). Only on the edges of the belt do these rocks outcrop: on the western ,edge as the Riwaka Complex and on
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48 N.z. JOURNAL OF GEOLOGY AND GEOPHYSICS VOL. 1·j
the eastern edge as Dun Mountain Ultramafics (Beck, 1964). The remainder
are mainly wvered by Upper Tertiary and Quaternary deposits in the
Moutere Depression.
In South Taranaki the basement forming the Patea High hes at depths
of less than 3 km, and dips gently eastward towards a second major sedi
mentary basin, the Wanganui Basin. This sedimentary basin has a maximum
thickness of about 6 km near the mouth of the Wanganui River (Lensen.
1959). The track of the LV. Oceanographer passes south of the Wanganui
Basin close to the Marlborough Sounds, where previous seismic refraction
measurements (Officer, 1959) have indicated that less than 1 km of sedi
ments overlie basement rocks similar to the Paleozoic greywackes and schist
outcropping in the Marlborough Sounds.
INTERPRETATION
The smoothed free air gravity and total force magnetic anomaly profiles
along the track, and a crustal section consistent with the gravity profile, are
shown in Fig. 2.
The crustal section was obtained by a trial and error procedure, starting
from a model suggested by the regional structure shown in Fig. 1, together
with the sediment thicknesses indicated by seismic measurements, and
assuming that the major part of the Rangitikei-Waiapu Anomaly could be
attributed to thickening of the crust. In order to simplify interpretation of
the data, all structures were assumed to be sufficiently long in the direction
of the regional strike (025°) for the "two-dimensional" gravity computing
method of Talwani et al. (1959) to be used. It is estimated that the errors
caused by this assumption will be negligible. The geophysical model was
taken as being made up of homogeneous horizontal prisms representing
sea water, sediment, crust, and mantle, and having mean densities of 1'03.
2'60, 2'80, and 3'27 g/cm3 (Mg/m3) respectively. A greater than usual
sediment density was adopted to account for the effect of gravitational com
paction within the thick sedimentary sequence in the Taranaki Basin. The
adopted value of 2'60 g/cm3 is that found for rocks at a depth of 3 km
in the Midhurst Well, Taranaki (Batherton and Leopard, 1964).
CRUSTAL STRUCTURE
It is possible to determine only relative values of crustal thickness from
the gravity measurements alone. The absolute thickness at some point on
the profile must be determined independently, generally from seismic
measurements, in order to establish a datum. No deep seismic measure
ments have been made at any point on the profile line. The nearest is one
unreversed seismic refraction profile made on basement rocks of the North
Island, east of the track of r.v. Oceanographer. From this Garrick (1961')
obtained a value of 36 km for the crustal thickness. This figure is consistent
with the mean thickness of the crust for New Zealand obtained from surface
wave dispersion measurements (Thompson and Evison, 1962) and inferred
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No.1
r (gamma)
II ~::::o
r
~o I =- -500
(km) "- 0
10
15
HUNT & WOODWARD - SOUTH TARANAKI BIGHT
2' 2'
A BW CD
103---~ ___ 'L --- --~
260
,--
"",,'" """ II ~-~- ~
280
TOTAL FORCE MAGNETIC ANOMALIES
1+ GeomagnetIC Range Indl<:es
x
CRUSTAL SECTION
FREE AIR GRAVITY
ANOMALIES
r---
/
49
l
-------- / \ /."" ~:~::~
20
25
3D 327
35
200 __ l __ _
300 I
]·27 \//11 D,"my (glom])
~ V"""I "'gg""'oo 15x
6~"--___ '~ ,.. J 500 1 __ _
FIG. 2-Total force magnetic anomalies, free air gravity anomalies, and the crustal section along the track of r.v. Oceanographer in the South Taranaki Bight.
Geology-7
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50 N.z. JOURNAL OF GEOLOGY AND GEOPHYSICS VOL. 14
from gravity observations (Reilly, 1962). In order to obtain a reasonable
solution to the gravity data, this value of 36 km has been adopted for the
maximum crustal thickness on the profile line.
A crustal section consistent with the gravity anomalies is given in Fig. 2.
In this section the crust is about 20 km thick beneath the New Caledonia
Basin west of the New Zealand continental shelf, and thickens from a
point 50 km inside the edge of the shelf to the adOlpted maximum thickness
OIf about 36 km off the Marlborough Sounds. South-east of the Marlborough
Sounds the crust thins rapidly, being only about 10 km thick about 50 km
south of Cape Palliser.
MARGINAL SYNCLINE
Along that part OIf the r.v. Oceanographer's track south-least of point B
(Fig. 1) it was difficult to obtain a reasOlnable interpretation of the gravity
anomalies without introducing a more complicated model than is shown
in Fig. 2. This is presumably due to the presence of the broad bdt Df dense,
magnetic intrusive rocks, parts of which are associated with the New Zealand
Marginal Syncline (Hatherton, 1967). Detailed interpretation Df these
gravity anomalies cannot be made until the position of the Taranaki Fault
and the sediment thickness on both sides have been determined. However,
residual gravity anomalies, thought to comprise mainly the gravity effects
of this belt of rocks, were obtained by subtracting the gravity effects Df the
sediments and crust shown in the geDphysical model (Fig. 2) from the
measured gravity anomalies; they are shown in Fig. 3. The residual anomalies
within 20 km of the indicated position of the Taranaki Fault may be
unreliable because of the uncertainty in the position and dip of the fault.
The residual gravity anomalies have amplitudes OIf up to 50 mgal and occur
for a distance of about 100 km north-west, and about 20 km south-east of
the Taranaki Fault. Associated with them is a broad magnetic anomaly,
having an amplitude of about 400 gamma, which has been cOlrrelated by
Hatherton (1967) with magnetic anomalies further south in Tasman Bay
and in the Moutere Depression.
The density and magnetic susceptibility of the rocks exposed in the
northern part of the South Island have been measured and are shown in
Table 1. It can be seen from this table that of these rocks only the Dun
Mountain Ultramafics and the Cobb, Rameka, and Riwaka Intrusives
(Grindley, 1961; Beck, 1964) have the requisite density and magnetic
susceptibility to explain the anomalies. If the residual anomalies are caused
by these rocks, then the size of the source can be judged from the models
shown in Fig. 3, which have gravity effects consistent with the residual
anomalies. The source bodies were assumed to be "two-dimensional", to have
a density contrast of 0'2 g/an3 with the surroundings, and to lie completely
within basement rocks.
FIG. 3 (opposite)-Residual gravity anomalies associated with the New Zealand
Marginal Syncline and geophysical models postulated for the source body. Density
contrast of the source bodies is + 0.2 g/cm3 .
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I I (mgal)
i ro ! ,- 0 I
I (kr.,)
I r 0
I f 5
10
(mgal)
[' (km) o
10
15
Residual Gravity Observed ___ _
Anomalies ~----=\ /2:\, Computed __ _
- ~// \(\j \VV~---7 _~~- 'cJL- - ~/
-, I I
J B W x
Vertical exaggeration J xlO
Residual Gravity _-__ /_\ Observed __ _ Anomalies /I ~---/ .. , \ Computed __ _
(' \ "'\\
o I
/ \ /\ , '\~;:~J~~---_ "~ :j~' "-- ~7
B w C
100 I
D x
Vertical exaggeration
xlO
200(km) I
j
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VI
N
TAB
LE
I-M
easu
red
Wet
Den
sity
and
Mag
neti
c S
usce
ptib
ilit
y of
Roc
ks
in t
he N
ort
her
n P
art
of t
he S
outh
Isl
and.
Geo
logi
cal
Uni
ts a
fter
B
eck
(196
4) a
nd G
rind
ley
(196
1)
Lit
holo
gy
For
mat
ion
or
Gro
up
W
et D
ensi
ty
Mag
neti
c S
usce
ptib
ilit
y Z
(N
o o
f M
ean
(No
. of
M
ean
N
Sam
ples
) (g
/cm
3)
(s.d
.)
Sam
ples
) (X
lO
-'cg
s)
(s.d
.)
'-<
0 c: ::<
I
(a)
Wes
t o
f th
e W
aim
ea F
ault
Z
:>
B
asem
ent
grey
wac
kes
Hau
piri
, M
t A
rthu
r, A
orer
e 75
2·
66
0·1
3
50
50
19
0 t-<
Bas
emen
t sc
hist
s W
aing
aro,
Gol
den
Bay
, 42
0 0
49
2·66
0
·18
42
12
50
"1
One
kaka
, P
ikik
irun
a,
C'l
Wak
amar
ama
~
0
Aci
d in
trus
ives
S
epar
atio
n, K
aram
ea
138
2·64
0
·07
13
4 26
0 54
0 t-<
0 C'J
In
term
edia
te i
ntru
sive
s R
otor
oa I
gneo
us,
Bro
ok S
tree
t 94
2·
79
0·1
4
84
800
1450
><:
:>
Maf
ic a
nd u
ltra
maf
ic
Cob
b, R
amek
a, R
iwak
a 62
2'
91
0·2
3
64
4750
48
80
Z
intr
usiv
es
Intr
usiv
es
t::I
(b)
Eas
t of
the
Wai
mea
Fau
lt
~ 0 B
asem
ent
grey
wac
kes
Mai
tai,
Lee
Riv
er,
Pel
orus
, 21
0 2·
68
0·1
0
174
40
135
'1:l
Tor
less
e ~ [f
J
Bas
emen
t sc
hist
M
arlb
orou
gh S
chis
t 36
2·
68
0·0
9
30
10
30
n [fJ
Aci
d in
trus
ives
T
asm
an I
ntru
sive
s 11
2
'87
0
·21
1
0
470
705
Bas
ic a
nd u
ltra
maf
ic
Du
n M
ount
ain
Ult
ram
afic
s 5
7
2'8
9
0'3
4
43
20()0
26
80
intr
usiv
es
-< 0 r I-'
II>-
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No.1 HUNT & WOODWARD-SOUTH TARANAKI BIGHT 53
Steep residual gravity gradients near points C and D (Fig. 3) could not be explained by lateral density changes within the basement, and are interpreted as ~eing due to near-surface density variations within the upper part of the sedtmentary «wer. An alternative explanation would be that the dense, magnetic rocks which cause the residual anomalies, themselves lie within the sedimentary cover.
The Pleistocene volcanics which outcrop as Egmont Andesite (Hay, 1967), east of Cape Egmont, have a mean density of 2'68 -t- 0'15 g/cm3, and several of the samples collected have densities greater than 2'90 g/cm3 •
However, if Pleistocene volcanics similar to the Egmont Andesite were the cause of the residual gravity anomalies in the South Taranaki Bight, they would need to be at least 2 km thick. Thick Miocene volcanics, similar to the Whareorino and Orangiwhao Andesites (Fig. 1) (mean density 2'56 -t- 0'10 g/cm3
), may be present within the Cretaceous-Recent sedimentary sequence, and could be J.1esponsible for some of the residual gravity anomalies, but they are unlikely to be wholly responsible, since at depth they will have only a small density contrast «0'2 g/cm3 ) with the sediments.
Isostatic gravity anomalies (Airy-Heiskanen System, T = 30 km) across the South Taranaki Bight together with isostatic anomaly profiles across Taranaki and Nelson aJ.1e shown aligned with respect to the Taranaki (Waimea) Fault in Fig. 4. In the southern profiles (Nelson and South Taranaki Bight) there are numerous gravity anomalies with amplitudes of 10 to 50 mgal either side of the Taranaki (Waimea) Fault. The South Taranaki profile is dominated by the large regional gradient of the Rangitikei-Waiapu Anomaly which, along this profile, has only one short wavelength anomaly superimposed upon it, corresponding to the uplifted basement east of the Taranaki Fault. The isostatic gravity profiles to the north similarly have anomalies of small amplitudes and wavelengths. This suggests that the broad belt of dense bodies causing the residual gravity anomalies measur,ed in Nelson and the South Taranaki Bight either do not extend into Taranaki, or are buried to such a depth that their gravitational effects at the surface are small. However, since the magnetic anomalies associated with the residual gravity anomalies in Nelson and in the South Taranaki Bight can be traced north into Taranaki (Wellman, 1959; Hatherton, 1967), it is possible that there is a change in the dominant rock type of the source bodies. A change in rock type from mafic and ultramafic igneous intrusives (such as pyroxenite, gabbro, and dunite) to serpentinite (which has been shown to have low density but high magnetic susceptibility) would account for this.
No detailed interpretation of the total force magnetic anomalies has been attempted as there is insufficient information available about the magnetic properties of rocks outcropping in the northern part of the South Island. From the gravity and magnetic information available, it is not possible to confirm or deny the existence of the Cook Strait Fault proposed by Cope and Reed (1967) to lie between the Nelson and South Taranaki Bight profiles.
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54 N.Z. JOURNAL OF GEOLOGY AND GEOPHYSICS VOL. 14
w Isostatic Gravity
Anomalies
mg. I
~-30
E,"~
mgal -20
-50
-loa
U I
mg. I
EI Taranak i Fault
mg.1
\7 North Taranaki
V R
l="" +70
l +40 ~ mg.1
[
+20
O~ Mid Taranaki
South Taranaki
y ~I
South Taranaki Bight (Oceanographer)
Nelson
I X
[~l !J f- 50 km .t
46 S
FIG. 4-Isostatic gravity anomaly profiles across the Taranaki Fault, New Zealand. Location of the profile lines shown in Fig. 1.
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NO.1 HUNT & WOODWARD - SOUTH TARANAKI BIGHT 55
ACKNOWLEDGMENTS We thank the captain and crew of the r.v. Oceanographer, Dr T. V. Ryan and the
Pacific Oceanographic Research Laboratory of the United States Environmental Science Services Administration for the geophysical data. Weare also indebted to Dr R. A. Couper, of Shell, B.P., and Todd .oil Services, for unpublished offshore seismic data, and Dr C. P. \XTood for samples of Egmont Andesite. Mr G. W. Grindley, Dr T. Hatherton, Dr M. P. Hochstein, and Mr W. 1. Reilly offered helpful suggestions in reviewing this paper.
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GRINDLEY, G. W. 1961: Sheet 13-Golden Bay. "Geological Map of New Zealand 1 : 250,000." New Zealand 'Department of Scientific and Industrial Research, Wellington.
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