a geophysical study of nelson — cook strait region, new zealand

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A geophysical study of Nelson— Cook Strait region, NewZealandT. Hatherton aa Geophysical Survey, Geophysics Division,Department of Scientific and Industrial Research ,WellingtonPublished online: 05 Jan 2012.

To cite this article: T. Hatherton (1967) A geophysical study of Nelson — CookStrait region, New Zealand, New Zealand Journal of Geology and Geophysics, 10:6,1330-1347, DOI: 10.1080/00288306.1967.10423219

To link to this article: http://dx.doi.org/10.1080/00288306.1967.10423219

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1330 VOL. 10

A GEOPHYSICAL STUDY OF NELSON - COOK STRAIT REGION, NEW ZEALAND

T. HATHERTON

Geophysical Survey, Geophysics Division, Department of Scientific and Industrial Research, Wellington

(Received for publicatioll 18 luly 1966)

ABSTRACT

Recent magnetic and gravity surveys in the Nelson region permit a fuller discussion of the nature of the long major magnetic anomaly associated with the trend of the Nelson Syncline. The previous view that the anomaly is due to buried Upper Paleozoic igneous rocks such as the Brook Street Volcanics and Rotoroa Igneous Complex is held to be incompatible in some ways with the relationships between gravity and magnetic force values and with the physical properties of these rocks. It is suggested that a buried serpentinised ultramafic system might be the most appropriate geological model to explain the anomalies; and that the Dun Mountain Ultramafics may either have been emplaced directly from this system, or may be the surface expression of these anomalies displaced to the east as part of a sheet-type movement.

INTRODUCTION

The presence of a major linear magnetic anomaly in the Tasman Bay-Northern Cook Strait region has been known for several years as a result of the airborne total magnetic force surveys of Gerard and Lawrie (1955). Wellman (1959) examined the relationship of this anomaly to the underlying geology and concluded that the anomaly is caused by Upper Paleozoic igneous rocks, which are exposed in the Tasman Bay region and which are considered by Wellman to extend northwards across Cook Strait and along the west coast of the North Island. Recently, vertical force magnetic observations and gravity measurements have been made over the Upper Paleozoic igneous complex in Nelson Province, and a preliminary total-force contour map of Cook Strait has been compiled from airborne and seaborne profiles. These, together with measurements of the physical properties of the exposed rocks, allow a more detailed examination of the nature and cause of the major magnetic anomaly. The anomaly appears to be related to the New Zealand Marginal Syncline (Wellman, 1952) and the three constituent parts of this syncline, namely the Southland Syncline, the Nelson Syncline, and the Kawhia Syncline, will be referred to frequently in this paper, in accordance with their definition by Grindley, Harrington, and Wood (1959).

MAGNETIC MAPS

Vertical Force Anomaly Map of the Lake Rotom(l- Tasman Bay Region Fig. 1 shows the vertical force anomalies over the sediments of the

Moutere Depression and the flanking basement rocks. These anomalies have

N.Z. II Geol. Geophys. 10 : 1330-47

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HATHERTO, - COOK SIR.IIT REGio, Ceol. Geoph)s.10(Panjlc,I, 1967

'5, .....

\, L 0 0

FIG. I-Vertical magnetic force anomaly map, Lake Rotoroa. Tasman Bay.

LEGEND ,-----~~

!1ft 11t"ZeroAMmaly(oMour : Negative Values on • hatched side 01 line '-100 _Posu1veanomaly

(on(Ours

® Anomailes In (hiS lone greater than +SOO gammas

Anomalies In this lone , greaterthantlOOO

gammas

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I~

\I

7' 500 ,.

IlZ'30' Ill-DO' o Creu.ceous, Tertiary and Recent

o Triassic

o Lower Mesozoic

o Maitai Group

HATHERTON - COOK STRAIT REGION

D D D 0

x

D'URVILLE ISLAND o ~

k:

x

·0 WELLINGTON

'39' 3~'

40"30'

41 00

LEGEND 41°30' 1f( 71'i: 11\ Zero ~nomaly Contour

Negative Values on hatched side of line

_ 100 _ Positive anomaly contours

® •

Anomalies in this zone greater than + 500 gammas

Anomalies in this zone greater than +! 000 gammas

17r3O' 174°00' 174°30' '42'00

175'00' Brook Street V~lcanjc" and D Rotoroa Igneous Complex

Tasman IntrUSives

Dun Mountain Ultram.afics and 0 Separation Point Granite (roisilles Volcanics

Rai Formation and Pelorus Group Sediments·

Marlborough Schist

D x

lower Paleozoic Sedimentaty MetamorphiC and Igneous Rocks

Location of Gravity Stations at Sea

FiG, 2-Composite magnetic anomaly map, Nelson - Cook Strait. Values in Cook Strait and Taranaki are total force anomalies; values in Nelson - Marlborough are vertical force anomalies,

N;:', JI Geol, Geophys. lO(Pa"jic), 1967

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1967

I,

" I,

C STRAIT REGION HATHERTON - OOK

_L'

' omaly map, F 3-Isostatic gravity an IG, Lake Rotoroa· Tasman Bay,

l

lEGE"N.D__ : Zero AnomalyConloUfi

rtr. m""'Neg3tivevalues~n ! hatched side of line . Positive anomaly

-100- contours ., . i

Anomalre5in~i;~one: Sgreater!han ! S gammas :

Anomalie5Inth'lsl~~e: , greaterthant i

gammas J

Lo"'~'

8roo\ Street 'l0:(3[11($

, .1r.d T3sman intiliSIVes

Dun Mounta!fI Uitramafics

.1nd Croll:ib Volcanics

I R1!Fol{1',21:IOllarC

P~:crus Gro\lp Sedlnlellts

~o:oroa I?neo~s Complex

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No.6 HATHERTON - CoOK STRAIT REGION 1331

been obtained by observing the differences in vertical force between the stations and a base at which the vertical force anomaly has been defined by Cullington (1954). The anomaly is the algebraic sum of the base anomaly, the measured base-station difference, and the difference in normal field between base and station as defined by Cullington.

A simple primary pattern emerges from the map. A broad region of positive values lies between the Wangapeka-Motueka Rivers system in the west and the Pelorus River in the east, and is flanked on the east by small negative anomalies and on the west by variable, often large, negative anomalies. Within the broad positive belt occur a number of highs. On the eastern side of the Moutere Depression there are highs (reading from north to south) centred on the north end of Nelson Haven, Richmond, west of Wakefield, Golden Downs, and Nestor Peak. With the exception of the small high west of Wakefield this group lies along a straight line. On the western side of the depression there are several small highs east of the Wangapeka-Motueka Rivers system. The only other disturbances in the broad positive belt are the high at Ruby Bay and the erratic and highly variable values over the ultramafic belt (Dun Mountain Intrusives). The latter have been ignored in drawing Fig. 1 as it is impossible to contour the few ground observations over the exposed ultramafic rocks.

Total Force Anomaly Map of the Northern Cook Strait Region

Since Gerard and Lawrie (1955) flew their airborne magnetometer traverses across Cook Strait there have been several journeys across the Strait by ships towing seaborne magnetometers. Fig. 2 is a compilation of the results of such journeys as a total force anomaly map, "normal" total force values having been provided by A. L. Cullington, Superintendent, Magnetic Survey, Christchurch. To integrate the seaborne results with the airborne results the anomalies at sea level have been extrapolated to 5,000 ft, the flight level, and Fig. 2 represents the total force anomalies at 5,000 ft above sea level. The method used for this upward continuation is that of Hender-son and Zietz (1949) for anomalies which can be regarded as essentially two-dimensional (loc. cit., p. 532). The major features of the vertical force anomalies on land have been added to the figure.

The major magnetic anomaly falls into two parts, as indeed Wellman (1959) indicated from a study of the airborne profiles. The southern part, more intense and probably shallower, has several peaks. The northern part of the anomaly is of smaller amplitude and appears to be deeper seated probably as a result of the sinking of the Rangitikei Basin. The northern and southern parts of the anomaly are separated by a saddle in Lat. 40 0 35' S, Long. 1740 E, and a fault trending ESE in this vicinity, with vertical throw, could well separate the less intense northern anomaly from the larger southern anomaly.

GRAVITY MAP

The isostatic gravity anomalies of the Tasman Bay - Lake Rotoroa region are shown in Fig. 3. In this figure the Bouguer values have been corrected

Geology-2

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1332 N.Z. JOURNAL OF GEOLOGY AND GEOPHYSICS VOL. 10

for topographic and isostatic attraction on the Airy-Heiskanen system (T = 30 km, density of topography = 2'67 g/cm3, density of compensation 0'6 g/cm3 ). The map shows three major positive anomalies and several minor ones. Except for the regional negative anomaly on the eastern Bank there are only minor negative anomalies of which the most important are at Stoke and Motueka. It is inferred from geological evidence that at least 7,000 ft of Tertiary sediments underlie the Stoke area (Bruce, 1962). With a mean density contrast with basement of about 0-30 g/cm3 (Hatherton and Leopard, 1964), these principally Middle to Lower Tertiary sediments would normally give a gravity deficiency of about 25 mgal. Thus the isostatic correction appears to leave a regional anomaly of about 18 mgal, a dis-crepancy very similar to that in Southland where a flat secondary regional of + 20 mgal has been introduced to match the isostatic anomalies to the local geology (Hatherton, 1966b) _ Further, the depth of the gravity "low" at Stoke in section AN (Fig. 4) is about 25 mgal below the gravity values on either side and this may be a reasonable gravity expression of the Tertiary Syncline, confirming Bruce's estimate of its thickness.

Isostatic anomalies are close to + 20 mgal throughout the Moutere Depression and if the regional anomaly can be taken to be a Bat + 20 mgal, a considerable thickness of Tertiary sediments (,-- 5,000 ft) or Moutere Gravels (,-- 3,000 ft) under Motueka is indicated. Also, the maximum values of the three major positive anomalies would be reduced by 20 mgal. Thus the anomaly due to the Red Hills ultramafic body would be about +45 mgal, the Nestor - Golden Downs anomaly would become +25 mgal and the Wangapeka River anomaly about +20 mgal. The values of the minor anomalies at Ruby Bay and in the Pelorus Group near Mt Richmond in the Richmond Range would be about + 10 mgal.

PHYSICAL PROPERTIES OF THE ROCKS

As a basis for the interpretation of the gravity and magnetic maps the physical properties of the principal rock groups in the area have been measured. The mean rock properties are given in Table 1 below, the sequence being from east to west.

THE GEOPHYSICAL ANOMALIES AND THE UPPER PALEOZOIC IGNEOUS ROCKS OF NELSON PROVINCE

Gerard and Lawrie (1955) and Wellman (1959) suggested that the major magnetic anomaly in Tasman Bay, Northern Cook Strait, and the west coast of the North Island arises from magnetic Upper Paleozoic igneous rocks specifically outcropping as the Brook Street Volcanics and the Rotoroa Igneous Complex in Nelson Province.

The Brook Street Volcanics are the correlatives of the Eglinton Volcanics of the Southland Syncline and it may be instructive here to consider the geophysical anomalies over the Southland Syncline (se'e Hatherton, 1966 a, b). Briefly, gravity and magnetic anomalies occur over the Banking rocks

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No.6 HATHERTON - CooK STRAIT REGION 1333

on the north-eastern side of the Southland Syncline (Red Mountain Ultra-mafics and Livingstone Volcanics?) and on the south-western side of the syncline (Longwoods-Bluff and Takitimu groups). Within the Southland Syncline are broad magnetic anomalies unaccompanied by gravity anomalies. Access problems prohibit detailed geophysical surveys in the Eglinton Valley area where the syncline is very narrow.

TABLE 1-Physical Properties of Rock Groups in Nelson Region

Density g/cm3 Magnetic Formation or Rock Type I No. of I Susc<ftibility

Group Samples Wet __ Particle X 10- cgs units -----

Pelorus greywacke, argillite 5 2'80 2'82 70

Matai argillite 45 2'75 2'78 50

Dun Mountain serpentinite, dunite See Fig. 9

Moutere Gravels weathered 4 2'20* 2·61t 30 conglomerate

Brook Street basalt, andesite, 31 2'81 2'87 280 Volcanics tuff

Rotoroa Igneous gneissic quartz-hornblende

33 2'79 2'83 1600

diorite

r biotite granite 28 2'59 2·64 340 Separation Pt I

Granite ~ granodiorite 6 2·68 2'75 330

l diorite 13 2'70 2'79 200

*Mean of four bulk sa/IlJPles. tTypical greywacke pebbles.

In Nelson it might be anticipated that geophysical anomalies would be found on the eastern side of the Nelson Syncline from the ultramafic system and on the western side of the syncline from the continuation, as the Brook Street Volcanics, of the Longwoods-Takitimu-Eglinton volcanic and intrusive system. However, the comparison of the geology of the western flank of the Southland Syncline and that of the western flank of the Nelson Syncline on the basis of the geophysical anomalies is by no means simple, as the follow-ing discussion of the igneous rocks shows.

Brook Street Volcanics

When Wellman (1959) discussed the relationship of the igneous rocks of Nelson to the aeromagnetic anomalies there were no measurements of rock

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properties available to assist him in his interpretation. In the last few years many such measurements have been made. According to the laboratDry measurements the Brook Street Volcanics are almost non-magnetic and their lack of magnetism is confirmed by the profiles (Figs. 4, 5, 6) which pass over Brook Street Volcanics or their presumed extension. Hence the magnetic anomalies in the Moutere Depression, and presumably alsO' the anomalies in Cook Strait, are unlikely to be caused by these rocks.

The Brook Street Volcanics are intruded between Nelson City and Pepin Island by the Tasman Intrusives. The Tasman Intrusives are not only mag-netic but also dense (--,2'9 g/cm3 ), The magnetic anomaly north of Nelson City, in the vicinity of the Tasman Intrusives is not accompanied by any similarly shaped gravity anomaly and, indeed, magnetic values become greater to the west as'the gravity values diminish. This lack Df correlation suggests that these granodiorites are not responsible for the anomaly north of Nelson.

.... E ..

l' ____________________ _ ' " - - ---- ------~ 0 --------------

=r ",

-250rrl~ 'P :7 4<: ~F/E----Y --¥-- E • Ultr.m.f,c,

-500

-7501

-1000

-12'

-1500

-17WI

-20001

-uso

-2500

SECTION AA'

FIG. 4-Section AA'---Gravity and magnetic anomalies. ri = Riwaka metavo1canics, sp Separation Point Granite, Wn = Moutere

Gravels, G = Triassic sediments, Y = Permian sediments, E = Carboniferous sediments.

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No.6

,L

1500'

1250

HATHERTON - CooK STRAIT REGION 1335

NVV\

'='==t=. " -"" ,. ~

FIG. 5-Section BB'-Gravity and magnetic anomalies. Q = Ordovician sediments and volcanics, sp = Separation Point Granite,

Wn = Moutere Gravels, Y = Permian sediments, E = Carboniferous { ?) sediments. Serrated magnetic anomaly pattern over ultramafics denotes highly variable anomalies.

Rotoroa Igneous Complex Rocks of the Rotoroa Igneous Complex have widely varying magnetic

susceptibilities reflecting the range in composition of the samples from acidic to basic (Fig. 7). The more basic of these rocks could certainly provide sufficient field to account for the anomalies of the Moutere Depression. But, as will now be shown, the morphology of the magnetic anomalies is not entirely consistent with the Rotoroa Igneous Complex as the source of the anomalies.

Over the outcropping Rotoroa complex, vertical force anomalies are variable and usually negative (see the magnetic anomaly map, Fig. 1, and section CO, Fig. 6). Often "false" negatives occur when observations are taken, because of topography, virtually inside a magnetic body. The possi-bility has to be considered that the Nestor high, which develops as the section (profile CO) passes from outcropping Rotoroa complex to (presum-ably) Rotoroa complex covered by Moutere Gravels, might be due to measurements being made above the body. This thesis is not sustained, however, by an examination of the magnetic anomaly values along the road east of Glenhope, where for some 4 miles inferred Rotoroa complex is over-lain by Moutere Gravels, and magnetic anomalies though positive are not high.

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1336

"" ~ l 40

1 ::i )0

.~ .,

1250

1000

-;, 750 E ~ ~ 500

.2

.~ 250

N.Z. JOURNAL OF GEOLOGY AND GEOPHYSICS

/0_

............ 0 ....... ° \ o

VOL. 10

/

/O_O/' "''''''-0 /"'. / ~,

"o-~ 0

if E

l,y~---\/\ / 3\x_F \J/

=" . /

,--- 'P --..... SECTION ee' '8 ~t! bs ...,,<: to

FIG. 6-Section CC'---Gravity and magnetic anomalies. sp = Separation Point Granite, rt = Rotoroa Igneous Complex, sg = Spear-

grass Formation, Upper Pleistocene, bs = Brook Street Volcanics, to = Tophouse Formation, overlying Permian sediments (Y).

Two other factOors could increase the magnetic intensities and gravity anomalies due to the ROotoroa Igneous Complex. These are an increase in basicity, and thus in magnetic intensity and density, to the east; and an increase in thickness to the east. Fig. 8 shows the densities and magnetic susceptibilities of the Rotoroa Igneous Complex samples plotted against their distances east of the Separation Point Granite - Rotoroa Igneous contact. There is no evidence of a significant change of either density Oor magnetic susceptibility with distance from ,the contact. Thus considerable increase in thickness remains the only manner by which the Rotoroa Igneous Complex could be responsible fOor the large magnetic and gravity anomalies running north-east from Lake Rotoroa. A gravity profile along the length of Lake Rotoroa and across the Alpine Fault shows little variation and the Rotoroa Igneous Complex does not appear to be very thick in that region. But the thickness of the ROotoroa Igneous under the gravels to the north is, of course, unknown.

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No.6 HATHERTON - CoOK STRAIT REGION

16 I i 279

I I

12 I I I

8 I'

4

01 I 2 6 - --

RotOroa Igneous

<100 SOD

: 1.600

I I I I I

-1000 -5000 100 1000 >5000

-500 -3000

~ 20 c '" " <J ~ u.

2-81 I I 280

16

12 Brook St Volcanics

8

4

01 I I I I I I I 262728293031 <100

1337

Density (wet) g, em) Susceptibility x 10 -', cgs units

FIG. 7-Density and magnetic susceptibility histDgrams Df all samples Df RDtDroa Igneous (upper pair) and Brook Street Volcanics (lower pair). Dashed lines represent average values.

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3·01r------r----,-----.----,----,----.,.------,

• •

2'91- • max

• • • ~E • ~ 2·S/-

• ••• • •

I I I I I

gravity

t-.;;;

" " o • • 2'71- •

•

• •

•

2·61 1 I ••• o 2 3 4

Miles Distance from Sep. Pt. granite ~

•

• •

•

6

-

-

-

7

FIG. !'lA-Variation of density of Rotoroa Igneous Complex, east of Separation Point Granite.

Geological sections across the Moutere Depression (see, for example, Bowen, 1964) generally show unlimited extension of Rotoroa Igneous Complex and Brook Street Volcanics. Only at one point has it been possible to test the section. At Ruby Bay, from the bottom (,-0930 ft depth) of a stratigraphic hole drilled by Tasman Petroleum Ltd., was recovered a 6 ft core of diorite. A petrological report by W. A. Watters of N.Z. Geological Survey, quoted by permission of Mr T. J. McKee, describes this rock as a "tonalite possibly transitional to granodiorite. It is probably to be placed within the Rotoroa complex though correlation with some of the more basic rocks within the Separation Point Granite should not be completely ruled out". The physical properties of two samples taken from the ends of the core were: wet densities 2'78 and 2'81, magnetic susceptibilities 140 and 280 X 10-6 cgs units. These measurements, also, do not lead to a definite classification of the rock (see Table 1).

The role of the Ruby Bay magnetic and gravity highs is not easy to assess. It may be part of the major anomaly system which runs from Lake Rotoroa through Cook Strait and Taranaki (Fig. 2) or it may be marginal to it.

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",,000, r -~----- I -- -r- T ~ r

• ~ • ~ 3,000 • )(

:l max. 'c , " ;:. • I ~ +ye I -ve ~ , :a .[ 2,000 • I .. magnetic :.: ;;: • • u • "t c .. .. I: •

I,OOOt- • • • • • • • • • 01

., ... t· 0 I 2 '" 5 6 7

Miles Distance from Sep. Pt. granite ~

FIG. SB-Variation of magnetic susceptibility of Rotoroa Igneous Complex, east of Separation Point Granite.

Ultramafic Rocks

In general, as igneous rocks become more basic, their densities and susceptibilities increase. Values of the physical properties of New Zealand rocks are given in Table 2 to illustrate this phenomenon.

On the other hand, the very dense rock, dunite, is weakly magnetic, whereas its lower density alteration product, serpentine, is often strongly magnetic. Fig. 9 illustrates the increase in the upper limit of magnetic susceptibility with decrease in density for samples taken from the exposed ultramafic belt. This inverse density-susceptibility relationship can provide a wide variety of combinations of gravity and magnetic anomalies, depending on the degree of serpentinisation of the olivine, and the nature of the host rock. The types of anomaly that one would expect from serpentine, from dunite, and from a mixture of the two are shown in Table 3.

The Dun Mountain Ultramafics form the "mineral belt" which passes from Red Hill through Dun Mountain to d'Urville Island. This classical complex of serpentine, peridotite, gabbro, and dunite lies between 8 miles

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(at the northern end) and 15 miles (at the southern end) east of the linear anomaly system which forms the subject of this paper. Though there are sharp local magnetic anomalies over outcropping serpentine, volcanics, and gabbro, the mineral belt has a gravitational expression only in the vicinity of Red Hill, where the anomaly due to these rocks reaches a maximum value of +45 mgal. This anomaly would indicate a thickness of several thousands of feet even if the rock were principally dunite. Reduction of the mean density by the presence of substantial amounts of lower density alteration products would increase the thickness of the ultramafic rocks there.

The existence of ultramafic bodies without exposed roots or dikes in the Lee River Group on which the ultramafics once rested has always been considered a problem, and Grindley et al. (1959, p. 9) suggest that the ultramafics "are therefore believed to have been emplaced from the side, probably from the region of volcanism to the west". Thus it is not impossible that ultramafic rocks exist below the Moutere Gravels. At this stage it may be profitable to examine the anomalies and rocks to the north.

TABLE 2-Physical Properties of New Zealand Igneous Rocks (Means of All Samples Measured by Geophysics Division)

Rock Type Wet Density g/cms

Magnetic Susceptibility

X 10-6 cgs units

Granite 2·61 250 Intrusive Diorite 2'76 3500

Gabbro 2'93 7200 Rhyolite 2·18 280

Extrusive Andesite 2·64 2200 Basalt 2'76 3500

TABLE 3-Gravity and Magnetic Anomalies Associated with Ultramafic Complexes

Host Rock

Upper Tertiary Seds. Lower Tertiary Seds. Indurated Seds. Paleozoic Igneous

Serpentine

Gravity Magnetic

+ ++ o to + ++ - to 0 ++

o to +

Dunite

Gravity Magnetic

+++ 0 ++ 0 ++ 0

+

50/50 Mixture Dunite, Serpentine

Gravity Magnetic

++ + + to ++ +

+ + 0 o to +

+ + + indicates very large +ve anomaly, + + indicates large +ve anomaly, + indicates +ve anomaly, - indicates -ve anomaly, 0 indicates no appreciable anomaly.

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10.0001 I. f- ~ • • • : . • ... . • ••

~ 1 OOO~ • • :l • • • 'c ::0

:. • ~

~ • :0 'w • I •• "-~ • ;;: u • " 100 • • • " c: .. • E

•

101 I I 2· 3 2·5 2·7 2·9 3· 1 3·3

Serpentine .. <E-------~---------- Dunlte Density g/cm J

FIG. 9-Density and magnetic susceptibilities of dunite and serpentinised dunite.

THE GEOPHYSICAL ANOMALIES IN COOK STRAIT

Fig. 2 shows the generalised total magnetic force anomalies north of Nelson Province. The only gravity measurements at sea in Cook Strait are those determined in 1956 by Lamont Geological Observatory using pen-dulums aboard HMS T elemachus. The locations of the four stations are shown in Fig. 2; the isostatic gravity anomaly values are shown in Fig. 10 (b) together with the magnetic anomaly profile along the line connecting the gravity stations. Even these few gravity stations provide evidence of a positive anomaly of at least 50 mgal associated with the NNE-trending rock system. This anomaly distorts the -160 mgal Rangitikei negative anomaly considerably (Robertson and Reilly, 1958; Reilly, 1965).

Gravity anomaly values along the coast of South Taranaki from Hawera to Waitotara and beyond are shown in Fig. 10 (a), again accompanied by the total magnetic force profile. The gravity anomaly profile represents residual anomalies obtained by removing, from the Bouguer anomalies, a regional gradient derived from observations on pre-Cretaceous basement rocks in the North Island. This residual anomaly is similar in character and "total mass" to the anomaly produced by the Telemachus observations, north of

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~ .;- ~ I'f ; 3 ... > .-

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FIG. lO-Gravity and magnetic anomalies (a) south Taranaki coast, and (b) Cook Strait (see Fig 2). The profiles are at right angles to the strike of the principal anomaly belt.

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d'Urville Island (Fig. 10 (b)). (The regional gravity gradient along the Telemachus line of observations appears to be almost zero from extrapolation of observations on land).

Both pairs of curves in Fig. 10 resemble the gravity and vertical magnetic intensity of a long horizontal cylinder (Fig. 11) except that there is excess mass, but not magnetism on the eastern side of the curves. A cylinder of radius 28,000 ft and depth to centre of 40,000 ft, with density contrast of 0'2 g/cm3 and effective susceptibility contrast of 1,000 X 10-6 cgs units would produce anomalies of the shape and amplitudes similar to those measured in Cook Strait and Taranaki (Fig. 10). These values of density and effective susceptibility contrasts are not far removed from those of the Rotoroa Igneous Complex against indurated sediments. Alternatively a mix-ture of serpentine and peridotite (see Fig. 9) would provide similar con-trasts with sediments. A cylinder of smaller radius would require larger values of density and susceptibility, putting the rock into a gabbroic type. A larger cylinder would require density/susceptibility relationships not often found.

THE MAGNETIC ANOMALIES IN THE NORTH ISLAND

Several magnetic anomaly profiles over the Marginal Syncline in the North Island are given in Fig. 12. The five northern profiles show a positive anomaly of about 300 gammas (at 5,000 ft) or 500 gammas (at ground level) on the eastern flank of the syncline, coincident with the junction of the Hokonui (marginal) facies and the Alpine (geosynclinal) facies. Peridotite and ser-pentinised peridotite are exposed at Wairere (Fleming, 1947), near the crest of this anomaly, which is also coincident with the Waipa Fault. Samples of the peridotite have densities of 3'1 g/cm3 and magnetic susceptibilities less than 100 X 10-6 cgs units, whereas the local serpentinites have wet den-sities about 2'55 g/cm3 and magnetic susceptibilities varying between 1,000 and 5,500 X 10-6 cgs units. The existence of gravity and magnetic anomalies coincident with the Macpherson Thrust Fault and the junction of the Hoko-nui and Alpine facies sediments in Southland has been demonstrated by

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FIG. l1--Gravity and vertical magnetic force due to an infinitely long horizontal cylinder normal to the figure.

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1344 N.Z. JOURNAL OF GEOLOGY AND GEOPHYSICS

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FIG. 12-Magnetic anomaly profiles, Taranaki and South Auckland. Profiles 1, 2, 3, and 7 are total force anomalies at 5,000 ft altitude. Profiles 4, 5, and 6 are vertical force profiles at ground level.

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No.6 HATHERTON - COOK STRAIT REGION 1345

Hatherton (1966b). There is also evidence of a less prominent magnetic high on the western flank of the Marginal (Kawhia) Syncline in the northern five profiles of Fig. 12. Again, substantial magnetic anomalies have been shown to exist over the Hokonui rocks of the south-western flank of the Southland Syncline. Wellman (1959) has joined the northern part of the Cook Strait anomaly to the eastern magnetic anomaly of the Kawhia Syncline, but examination of Figs. 2 and 12 shows that it is not really clear at present whether the Cook Strait anomaly joins the eastern or western flank of the Kawhia Syncline. An unpublished gravity anomaly map of Taranaki (in Geophysics Division files) shows a definite bifurcation of the Patea gravity high with the western arm running up the west coast. The northward con-tinuation of the eastern arm is not well defined by the results available.

DISCUSSION

In the North Island the Marginal Syncline is everywhere marked by a broad belt of positive magnetic anomalies. This belt of positive anomalies extends southwards through Cook Strait to Tasman Bay (possibly crossing a major normal fault in Lat. 40° 35' S), and thence through the Moutere Depression to the Alpine Fault. Previous publications (Gerard and Lawrie, 1955; Wellman, 1959) have suggested that the Upper Paleozoic Brook Street Volcanics and Rotoroa Igneous Complex are the rocks responsible for this anomaly belt. It has been shown above that the Brook Street Volcanics are almost non-magnetic. Though the Rotoroa Igneous Complex does have physical properties which could be responsible for some of the anomalies, there are several morphological features which lead to reluctance to ascribe the anomaly system to this rock suite; among these features are:

(1) the lack of any major magnetic anomalies over the exposed Rotoroa Igneous Complex, and

(2) the occurrence of positive magnetic anomalies without corresponding positive gravity anomalies (Richmond, north of Nelson City).

Any series of gravity and magnetic anomalies may be caused by a large number of rock types with differing physical properties, and this may be true in the present case. On the other hand, an explanation may be preferred' which considers a long, linear anomaly in terms of a single, simple rock system. It is suggested that the third rock group discussed above, the ultra-mafics, may underlie the Moutere Depression, Cook Strait, and the western side of the North Island.

One other piece of evidence adds weight to the suggestion that these anomalies are due to rocks containing a high proportion of serpentine. In February 1965 Mr R. A. Garrick carried out a short seismic survey over the Golden Downs anomaly (Fig. 1). Shooting into a 2,500 ft spread from both ends and at offsets of 4,250 ft at one end and 5,300 ft at the other Mr Garrick obtained, from both reflection and refraction data, the following section:

0-1,500 ft Va

1,500-5,070 ft Vi

10,500 ft/sec. This is the upper part of the range of Moutere Gravel velocities.

11,400 ft/sec.

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From outcrops in the Nelson area Garrick (1963) has obtained (among others) the following velocities: Rotoroa Igneous 17,500 ft/see, Brook Street Volcanics 11,750 ft/see, and serpentine 11,200 ft/see. Thus in the Golden Downs area the rock immediately below the Moutere Gravels is not Rotoroa Igneous. It may be either Brook Street Volcanics or serpentine, and in view of the intensive magnetic anomalies is probably the latter. A reflecting horizon exists at 5,070 ft deep but no information is available about the rock below this horizon.

If an ultramafic system does underlie the eastern half of the Moutere Depression the problem arises of its relationship to the exposed Dun Moun-tain Ultramafics. The view of Grindley et aI. (1959) that the Dun Mountain Ultramafics have been emplaced from the west does not conflict with the above discussion. However, from the points adduced above an alternative suggestion may be made. These points are:

(a) that a major gravity and magnetic anomaly system runs from Lake Rotoroa north-north-east through Nelson, Cook Strait, and the North Island;

(b) that the anomaly system, which is associated in the North Island and Southland with the Marginal Syncline, lies to the west of the syncline in the Nelson region;

(c) that the anomaly system persists in its NNE-SSW strike leading towards the Permian sediments and Dun Mountain Ultramafics in the vicinity of the Matakitaki River, whereas the Nelson Syncline adopts a southerly strike;

(d) that a serpentinised ultramafic rock system would allow the observed gravity and magnetic anomalies to be explained although the exposed basement rocks would not.

Therefore it is postulated that the Dun Mountain Ultramafics of the Nelson Mineral Belt could be the surface exposures of the major anomaly belt which have been moved eastwards by 10-15 miles as part of a sheet, leaving the Moutere Depression.

ACKNOWLEDGMENTS

The author thanks Mr R. A. Garrick, who collected many of the' rock specimens, the physical properties of which were used in this paper, and who provided the seismic information from Golden Downs:

REFERENCES

BOWEN, F. E. 1964: Sheet 15-Buller. "Geological Map of New Zealand 1 : 250,000." N.z. Dep. Sci. Industr. Res., Wellington.

BRUCE, J. G. 1962: The geology of the Nelson City area. Trans. Roy. Soc. NZ., Geology,l (11): 157-81.

CULLINGTON, A. L. 1954: The geomagnetic field in New Zealand at epoch 1950'5. N.Z. Dep. Sci. Industr. Res .. Geophys. Mem. 2.

FLEMING, C. A. 1947: Serpentinite at Wairere, Totaro Survey District, King Country. NZ. J. Sci. Technol. 29B: 100-15.

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GARRICK, R. A. 1963: A geophysical reconnaissance of the Nelson Basin. Unpublished Report. Geophysics Division, Dep. Sci. Industr. Res., Wellington.

GERARD, V. B.; LAWRIE, J. A. 1955: Aeromagnetic surveys in New Zealand 1949-1952. N.Z. Dep. Sci. Industt". Res. Geophys. Mem. 3.

GRINDLEY, G. W.; HARRINGTON, H. J.; WOOD, B. L. 1959: The geological map of New Zealand. N.Z. Dep. Sci. Industt". Geol. Bull. (n.s.) 66.

HATHERTON, T. 1966a: A comparison of total magnetic force profiles across Northern California and Southern New Zealand. Nature, 209: 466-8.

HATHERTON, T. 1966b: A geophysical study of the Southland Syncline. NZ. Dep. Sci. Indust. Res. Bull. 168.

HATHERTON, T.; LEOPARD, A. E. 1964: The densities of New Zealand rocks. N.Z. ,. Geol. Geophys. 7: 605-14.

HENDERSON, R. G.; ZIETZ, 1. 1949: The upward continuation of anomalies in total magnetic intensity fields. Geophyshs, 14: 517-34.

REILLY, W. I. 1965: "Gravity Map of New Zealand 1: 4,000,000 : Isostatic Anomalies." N.Z. Dep. Sci. Industr. Res., Wellington.

ROBERTSON, E. I.; REILLY, W. I. 1958: Bouguer anomaly map of New Zealand. N.Z. J. Geol. Geophys. 1: 560-64.

WELLMAN, H. W. 1952: The Permian-Jurassic stratified rocks, New Zealand. Sym-posium on Gondwana Series. Pt"oc. 19th into Geol. Congt".: 13-24.

WELLMAN, H. W. 1959: Geological interpretation of airborne magnetometer observa-tions from Nelson to Waikato River, New Zealand. Geol. Mag. 96: 118-24.

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a geophysical study of nelson — cook strait region, new zealand

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