gravity and magnetic measurements in the south taranaki bight, new zealand

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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

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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

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

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

r (gamma)

II ~::::o

~o I =- -500

(km) "- 0

HUNT & WOODWARD - SOUTH TARANAKI BIGHT

A BW CD

103---~ ___ 'L --- --~

"",,'" """ II ~-~- ~

TOTAL FORCE MAGNETIC ANOMALIES

1+ GeomagnetIC Range Indl<:es

CRUSTAL SECTION

FREE AIR GRAVITY

ANOMALIES

-------- / \ /."" ~:~::~

3D 327

200 __ l __ _

]·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

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 .

I I (mgal)

i ro ! ,- 0 I

I (kr.,)

(mgal)

[' (km) o

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 __ _

(' \ "'\\

/ \ /\ , '\~;:~J~~---_ "~ :j~' "-- ~7

Vertical exaggeration

200(km) I

0 c: ::<

e ~ [f

-< 0 r I-'

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.

54 N.Z. JOURNAL OF GEOLOGY AND GEOPHYSICS VOL. 14

w Isostatic Gravity

Anomalies

mgal -20

EI Taranak i Fault

\7 North Taranaki

l="" +70

l +40 ~ mg.1

O~ Mid Taranaki

South Taranaki

South Taranaki Bight (Oceanographer)

Nelson

[~l !J f- 50 km .t

FIG. 4-Isostatic gravity anomaly profiles across the Taranaki Fault, New Zealand. Location of the profile lines shown in Fig. 1.

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.

REFERENCES BECK, A. C. 1964: Sheet 14-Marlborough Sounds. "Geological Map of New Zea

land 1: 250,000." New Zealand Department of Scientific and Industrial Research, Wellington.

COPE, R. N.; REED, ]. ]. 1967: The Cretaceous Paleogeology of the Taranaki - Cook Strait Area. Proc. Australas. Inst. Min. Metall. 222: 63-72.

GARRICK, R. A. 1968: A Reinterpretation of the Wellington Crustal Refraction Profile. N.z. JI Geol. Geophys. 11 (5): 1280-94.

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.

HATIIERTON, T. 1967: A Geophysical Study of Nelson - Cook Strait Region, New Zealand. N.z. JI Geol. Geophys. 10 (6) : 1330-47.

HATIIERTON, T.; LEOPARD, A. E. 1964: The Densities of New Zealand Rocks. N.z. JI Geol. Geophys. 7 (3) : 605-14.

HAY, R. F. 1967: Sheet 7-Taranaki. "Geological Map of New Zealand 1: 250,000." New Zealand Department of Scientific and Industrial Research, Wellington.

HOUTZ, R.; EWING, ].; EWING, M.; LONARDI, A. G. 1967: Seismic Reflection Profiles of the New Zealand Plateau. J. geophys. Res. 72 (18): 4713-29.

LENSEN, G. ]. 1959: Sheet 10-Wanganui. "Geological Map of New Zealand 1 : 250,000," New Zealand Department of Scientific and Industrial Research, Wellington.

MATTHEWS, D. ]. 1939: Tables of the velocity of sound in pure water and sea water for use in echo sounding and echo ranging. Admiralty Hydrographic Department, London. 52 pp.

OFFICER, C. B. 1959: On some offshore seismic refraction profiles in the Cook Strait, Tasman Bay, and Golden Bay areas of New Zealand. N.z. JI Geol. Geophys. 2: 350-4.

REILLY, W. I. 1962: Gravity and Crustal Thickness in New Zealand. N.z. JI Geol. Geophys. 5: 228-33. '

1965: Gravity Map of New Zealand 1: 4,000,000. Bouguer Anomalies, Isostatic Anomalies (1st Ed.). New Zealand Department of Scientific and Industrial Research, Wellington, New Zealand.

ROBERSTON, E. 1.; REILLY, W. 1. 1958: Bouguer anomaly map of New Zealand N.Z. JI Geol. Geophys. 1 : 560-4.

TALWANI, M.; WORZEL, ]. L.; LANDISMAN, M. 1959: Rapid gravity computations for two-dimensional bodies with application to the Mendocino submarine fracture zone. J. geophys. Res. 64 (1): 49-59.

THOMPSON, A. A.; EVISON, F. F. 1962: Thickness of the earth's crust in New Zealand. N.z. JI Geol. Geophys. 5 (1): 29-45.

VAN DER LINDEN, W. ]. M. 1968: Cook Bathymetry. N.z. Oceanographic Institute Chart, Oceanic Series 1 : 1,000,000.

WELLMAN, H. W. 1959: Geological Interpretation of Airborne Magnetometer Observations from Nelson to Waikato River, New Zealand. Geol. Mag. 96: 118-24.

WOODWARD, D. ].; HUNT, T. M. 1970: Crustal Structu(e across the Tasman Sea. N.z. JI Geol. Geophys. 13 (4) (this issue) : 39-45.

WORZEL, ]. L. 1965: Pendulum gravity measurements at sea 1936-1959. Wiley, New York, 422 pp.

gravity and magnetic measurements in the south taranaki bight, new zealand

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