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Metamorphic zones, terranes, and Cenozoic faults inthe Marlborough Schist, New ZealandNick Mortimer aa Institute of Geological and Nuclear Sciences Ltd , P.O. Box 30 368, Lower Hutt, NewZealandPublished online: 23 Mar 2010.

To cite this article: Nick Mortimer (1993) Metamorphic zones, terranes, and Cenozoic faults in the Marlborough Schist, NewZealand, New Zealand Journal of Geology and Geophysics, 36:3, 357-368, DOI: 10.1080/00288306.1993.9514581

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New Zealand Journal of Geology and Geophysics, 1993, Vol. 36: 357-3680028-8306/93/3603-0357 $2.50/0 © The Royal Society of New Zealand 1993

357

Metamorphic zones, terranes, and Cenozoic faults in the Marlborough Schist,New Zealand

NICK MORTIMER

Institute of Geological and Nuclear Sciences LtdP.O. Box 30 368Lower Hutt, New Zealand

Abstract The northeast-striking Picton Fault Zone is amajor structural and metamorphic break that divides theMarlborough Schist into two separate blocks. The north-western (Kaituna) block shows a regular increase in texturaland metamorphic grade to the southeast from prehnite-pumpellyite to greenschist facies and textural zone I to IV. Thesoutheastern (Arapawa) block shows an irregular increase ingrade to the southeast from prehnite-pumpellyite topumpellyite-actinolite facies and textural zone I to IIB. TheKaituna block is correlated with the Caples and TorlesseTerranes and the Otago Schist of the southern South Island.The Arapawa block is correlated with the Waipapa Terrane(possibly also the Torlesse Terrane) and Kaimanawa Schist ofthe North Island.

Isotects and the Picton Fault Zone are dextrally offset by10-15 km across the ENE-striking Queen Charlotte FaultZone. The latter should be included with the late CenozoicMarlborough faults.

Keywords Pelorus Group; Caples Terrane; TorlesseTerrane; Waipapa Terrane; Haast Schist; Marlborough Schist;Queen Charlotte Fault Zone; metamorphic petrology;structural geology; tectonics

(1985), and Landis & Blake (1987) and are not repeated here.Four whole-rock K-Ar ages of low-grade schist indicateJurassic metamorphism and deformation (Landis & Blake1987).

The main objectives of the present study were to establishthe distribution of metamorphic mineral zones, revise texturalzones according to the scheme of Bishop (1972), and identifyterrane boundaries in the Marlborough Schist in order to betterunderstand the distribution of fundamental tectonic andmetamorphic units of the New Zealand microcontinent (Fig.1). Significant new information on the pattern of Cenozoicfaulting in the Marlborough Schist, based on offset of isotects,was also obtained.

METHODS

Most of this paper is based on data collected during 14 daysfieldwork in the Marlborough Sounds and examination ofabout 450 hand specimens and 400 thin sections from theNational Petrology Reference Collection, Institute of Geo-logical and Nuclear Sciences Ltd, Lower Hutt (to whichsample numbers prefixed "P" refer). Most of these sampleswere collected by C. J. Vitaliano, M. R. Johnston, and theauthor. Rock modes were determined using a Leading EdgePty Chromatic Colour Image Analyser. Chemical analyseswere determined according to procedures oudined in Palmeret al. (1991). Grid references (e.g., 028/459564) were takenfrom NZMS 260 1:50 000 topographic maps and include thosefor geographic features not shown in Fig. 2.

INTRODUCTION AND PURPOSE OF STUDY

The Marlborough Schist (Marlborough Schist Zone ofJohnston 1982) along with the Otago, Alpine, Kaimanawa,and Chatham Schists make up the Haast Schist belt of NewZealand (Grindley et al. 1959; Suggate et al. 1978, pp. 281,756) (Fig. 1). The Pelorus Group, which grades into theMarlborough Schist, is a late Paleozoic - early Mesozoicstratigraphic unit consisting dominantly of lithic and felds-pathic clastic sedimentary rocks (Beck 1964; Johnston 1982,1990). Johnston (1982) has extended Pelorus Group lith-ologies into schist of at least textural zone (t.z.) IIIA (seebelow for textural zone discussion), but as yet no clearmaximum extent of the Pelorus Group in the MarlboroughSchist has been established.

The Marlborough Schist is dominated by psammitic andpelitic greyschist; metachert (quartzite) and metabasite(greenschist) are rare. Detailed petrographic descriptions ofPelorus Group and Marlborough Schist lithologies havepreviously been given by Vitaliano (1968), Watters & Challis

G92049Received 7 October 1992; accepted 22 March 1993

170°E 180°

200km

KaimanawaSchist

MarlboroughSchist

Alpine AFSchist

35°S-

Wellington

ChathamSchist

AREA OF FIG. 2

45°S-

Haast Schist belt

Fig. 1 Location of the Marlborough Schist. AF, Alpine Fault.

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358 New Zealand Journal of Geology and Geophysics, 1993, Vol. 36

Maitai, MurihikuandBrook Street Terranes

Quaternary alluvium

METAMORPHIC ZONES

Prehnite-pumpellyite faciesPelorus Group &

Pumpellyite-actinolite facies Marlborough SchistGreenschlst facies

ContactFault (dashedwhere concealedor inferred)

Cape Jackson

41°00'S

• Prehnite + pumpellyitePumpellyite

o Pumpellyite + actinotiteFoliation and A Epidotefclinozoisitelineation D Pumpellyite + epidotefczAxial trace of • Epidotefcz + actindite

Arapawablock

Kaitunablock V

GoulterSynform -(-Assemblage + stilpnomelane

Fig. 2 Foliation, major faults, distribution of key metagreywacke mineral assemblages, and proposed metamorphic zones in the study area.Contacts in the western part of the area are simplified and slightly modified from mapping of Johnston (1982, 1990, in press, unpubl.) andLandis & Blake (1987); contacts in the rest of the area and all metamorphic boundaries are from the author's observations. Facies offsetsacross faults are drawn to be approximately compatible with the more precisely located isotect offsets in Fig. 3. The main break of the PictonFault Zone has been thickened for emphasis. Abbreviations as follows: H, Havelock; QCS, Queen Charlotte Sound; KS, Kenepuru Sound;MS, Mahau Sound; El, Endeavour Inlet; TB, The Brothers; PFZ, Picton Fault Zone, KF, Kenepuru Fault.

METAMORPHIC MINERAL ZONES

Metamorphic quartz, albite, chlorite, muscovite (includingsericite), and titanite are common to all metagreywacke thinsections. Weathered specimens contain a brown pleochroicbiotite-like mineral that is probably oxychlorite; biotite wasnot identified in any thin section, though possible oxychloritepseudomorphs after biotite porphyroblasts occur in P52850(Wairau Valley at 028/468633). Carbonate is rare. Thecalcium aluminium silicates prehnite, pumpellyite, actinolite,and epidote/clinozoisite occur in minor and variable quantitiesand, along with stilpnomelane, form the basis for establishingmineral zones and facies (Fig. 2). Stilpnomelane and garnetare locally common in metacherts (Vitaliano 1968; Waiters &Challis 1985) but neither metabasite nor metachert lithologieswere examined in this study. A careful search was made forlawsonite, which was reported in four of Landis & Blake's(1987) samples, but none was found.

The regional pattern of high-grade schist flanked to thenorthwest and southeast by lower grade schist and greywacke(Hector 1878; Beck 1964; Vitaliano 1968) remains unchanged(Fig. 2). A major structural and metamorphic break on thesoutheast side of the low-grade schists (Mackay 1879; Morgan1921; Henderson 1935; Branch & Bartrum 1939; Beck 1964;Vitaliano 1968; Nicol & Campbell 1990) is confirmed by thepresent work and is here called the Picton Fault Zone. Forconvenience, rocks northwest and southeast of the PictonFault Zone are referred to in this paper as belonging to theKaituna and Arapawa blocks, respectively (Fig. 2).

The following points can be made about the distribution ofmetamorphic mineral assemblages (Fig. 2):1. Quartz + albite + chlorite + muscovite + titanite +

pumpellyite is the most common assemblage in low-grade(t.z. I-IIB) metagreywacke of both the Kaituna andArapawa blocks. This is not diagnostic of any meta-morphic facies.

2. On the basis of characteristic metagreywacke mineralassemblages, prehnite-out and pumpellyite-out isogradscan be crudely defined. These isograds representboundaries between prehnite-pumpellyite, pumpellyite-actinolite, and greenschist facies assemblages.

3. Metamorphic grade increases to the southeast in both theKaituna and Arapawa blocks. Two one-sided tracts ofschist are thus defined, rather than one two-sided tract,which might have been expected by analogy with theOtago Schist.

4. In general, isograds appear to be subparallel to the maptraces of isotects and foliation (Fig. 2, 3), though three-dimensional relations cannot be satisfactorily determined.

5. Facies-diagnostic assemblages are absent in the Picton -Tory Channel area. Consequently, metamorphic fieldgradients in the Arapawa block may be more complex thanthe unidirectional southeasterly increase shown in Fig. 2.

6. Stilpnomelane is rare and widely scattered. It may possiblybe more abundant in Arapawa than Kaituna blockmetagreywackes, relative to the area of exposure andnumber of samples.

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Mortimer—Marlborough Schist 359

I j Quaternary alluvium

TEXTURAL ZONESt. z. I (Incl. unfollated Pelorus Group)

t. z. HA

Maitai, Murihiku andBrook Street Terranes

Crolsllles '174°OO'E10 km

w<&

!•

41°00'S

TB

ArapawaIsland

ToryChannei

+S' Axial trace of

. Textural gradedata point

Gouler Synform

Contact

Fault (dashed whereconcealed or Inferred)

Fig. 3 Distribution of textural zones in area of Fig. 2, as determined in this study. Textural zone IIIB and IV rocks are grouped togetherbecause satisfactory identification of high-grade psammitic versus pelitic lithologies (needed for t.z. classification) requires chemicalanalyses of schists (Mortimer & Roser 1992); analyses of large numbers of rocks were outside the scope of this study. The position of theWakamarina Quartzite is from Johnston (in press). The main break of the Picton Fault Zone has been thickened for emphasis. See Fig. 2 forabbreviations and data sources.

7. Zeolites are not developed as part of regional metamorphicassemblages but ?detrital stilbite occurs in P52876 (a t.z. IPelorus grey wacke) and laumontite veins are present in thePicton Fault Zone (see discussion below).This work, being of a reconnaissance nature, means that

Fig. 2 shows only the most rudimentary outline of meta-morphic zones.

TEXTURAL ZONES, FOLIATION, AND LINEATION

Textural zone boundaries based on the Chi. 1-4 classificationscheme of Hutton & Turner (1936) were mapped by Lensen(1962) and Beck (1964). The revised distribution of texturalzones according to the six-fold scheme of Bishop (1972) isshown in Fig. 3; major differences between the two inter-pretations are outlined below.

Kaituna block1. Large areas on either side of Pelorus Sound and between

Kenepuru Sound and Cook Strait, shown by Beck (1964)as unfoliated Pelorus Group, are actually foliated t.z. IIAand IIB schists (e.g., Fig. 4A).

2. Unfoliated t.z. I rocks are restricted to a 5-10 km wide stripmainly adjacent to the Dun Mountain Ophiolite Belt(Maitai Terrane), but see (3) below.

3. t.z. IIA rocks occur in a 1-2 km wide strip adjacent to theMaitai Terrane near and north of Croisilles Harbour(Landis & Blake 1987; Fig. 3), similar to the situation inpart of Otago (Mortimer in press).

4.

5.

t.z. IIIA-IV rocks are exposed on the northern side of innerQueen Charlotte Sound (Hector 1878; Mackay 1879;Vitaliano 1968).

t.z. IIIB-IV rocks (e.g., Fig. 4C) are extensively exposedsouthwest of Picton (Watters & Challis 1985; Nicol &Campbell 1990).

6. Apart from local, ductile, high-strain zones south of RaiValley (Johnston in press), and (3) above, textural gradeshows a consistent, progressive increase towards thesoutheast, similar to the one-sided metamorphic faciespattern.Foliation in the Kaituna block develops as a preferred

spatial orientation of micas in argillites in t.z. I, andmetagreywackes in t.z. IIA. At low textural grades it isgenerally moderately southeast dipping (Fig. 2). Withincreasing textural grade (i.e. towards the southeast), foliationbecomes more shallowly dipping in the vicinity of the GoulterSynform (Fig. 2,3 and see below), then changes to a gently tomoderately northwest dip as far southeast as the Picton FaultZone. The Goulter Synform (Johnston 1990, in press) can befollowed from the Wairau Valley to Cook Strait (Fig. 2). It isan enigmatic, probably synmetamorphic structure acrosswhich textural and metamorphic grade increase progressively(Mortimer & Johnston 1990; Fig. 2, 3). The Goulter Synformis correlated with the Taieri - Wakatipu Synform, an identicalmacroscopic structure in the Caples Terrane of the OtagoSchist (Mortimer in press).

Stretching lineations first develop as elongated argilliteclasts in t.z. IIA sandstones and conglomerates, and are

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360 New Zealand Journal of Geology and Geophysics, 1993, Vol. 36

Fig. 4 A, Photomicrograph of sample P52396, t.z. IIA, Kaituna block, Caples Terrene schist containing 7% detrital quartz, formerlymapped as Chi. 1 (Beck 1964). From Clova Bay at P26/966104. Height of photo = 2 mm; section cut parallel to lineation and perpendicularto foliation. B, Photomicrograph of sample P52388, t.z. IIB, Arapawa block, Waipapa (or Torlesse?) Terrane schist containing 24% detritalquartz. From Fighting Bay at Q27/105877. cf. Kaimanawa Schist. Scale and orientation as in A. See Table 1 for chemical analysis. C, Kink-folded, segregated Kaituna block, Torlesse Terrane pelitic schist near the Picton Fault Zone in the Wairau Valley at P28/726685. This is thedominant lithology in area mapped as "t.z. IIIB-IV" in Fig. 3. Compass is 125 mm long. D, North-dipping gouge plane within 300 m of themain (submerged) strand of the Queen Charlotte Fault, on Arapawa Island at Q27/156018. Person is 1.8 m high.

represented by quartz rods and quartz pressure-shadowovergrowths in t.z. III A and greater rocks. They generallyplunge west to northwest and east to southeast (Fig. 5). As inthe Otago Schist (Mortimer 1993, in press), there is acontinuum in lineation azimuth at different textural grades andstructural levels, and foliation dips at shallow to moderateangles (Fig. 2, 3, 5).

Arapawa block1. Much of the area between Port Underwood, Picton, and

Arapawa Island, shown by Beck (1964) as unfoliatedPelorus Group, is actually foliated t.z. IIA-IIB schist.

2. Unfoliated t.z. I rocks are restricted to a small stripsouthwest of Picton and an area straddling Tory Channel.They do not overlie schist (cf. Vitaliano 1968; Beck 1964)but grade laterally into it.

3. Overall, textural grade shows an increase to the southeast(Mackay 1879; Branch & Bartrum 1939) from t.z. I-IIAnear Picton to t.z. IIA-IIB east of Port Underwood (e.g.,Fig. 4B). This is similar to the one-sided metamorphicfacies pattern.

4. Textural grade appears variable on a scale of hundreds ofmetres, and the trend described in (3) above is not as

regular as in the Kaituna block (the greywackes straddlingTory Channel are a kilometre-scale example of thistextural heterogeneity). This rapid, lateral variation intextural grade invites comparison with the KaimanawaSchist of the central North Island (Sporli 1978) and t.z.IIA-B schist bands in the Cape Terawhiti area (Mortimerpers. obs.) near Wellington (Fig. 1).

Unlike the Kaituna block, foliation in the Arapawa blockschist is consistently steep, irrespective of metamorphic andtextural grade (Fig. 2, 3, 5), and no Goulter Synform-likefeature is recognised. Several mesoscopic fold closures inbedding in t.z. IIA schist (e.g., Robin Hood Bay, P27/997825and Karaka Point, P27/993945) were noted but no vergenceinformation, relevant to macroscopic structures, was deter-mined. Many stretching lineations plunge steeply and are lessconsistent in trend than the Kaituna block lineations (Fig. 5).These mesoscopic structural features are also similar to thoseof North Island schists (Mortimer pers. obs).

TERRANES

The Maitai Terrane (including Dun Mountain Ophiolite Belt)clearly defines the northwestern edge of the present study area

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Mortimer—Marlborough Schist 361

Fig. 5 Equal area, lower-hemi-sphere summary stereonets forfoliation and stretching lineationsin the Kaituna and Arapawablocks. Data are mainly from theeastern half of Fig. 2.

KAITUNA BLOCK

• 7t-poles to foliation (n=129)*• Lineation (n=71)

ARAPAWA BLOCK

7t-poles to foliation (n=53)Lineation (n=21)

(Fig. 2). Adjacent to this, the Pelorus Group, which grades intothe Marlborough Schist, has previously been correlated withthe Caples Terrane of Otago on the other side of the AlpineFault (Fig 1; Coombs et al. 1976; Bishop et al. 1985; Johnston1990). Other terranes that may be present in the MarlboroughSchist are the older Torlesse (Rakaia) Terrane (as shownschematically by Bishop et al. 1985) either recrystallised to t.z.IIIB-IV schist as in Otago (Mortimer & Roser 1992), or asgreywacke and t.z. IIA-IIB schist as in the Wellington areaand axial ranges of the North Island (Spbrli 1978; Mortimerpers. obs.). The Waipapa Terrane (terminology of Sporli1978) lies west of the Torlesse Terrane in the northwest NorthIsland, and must also be considered. Waipapa Terraneboundaries are covered by Cenozoic sedimentary rocks and/orseawater, and their natures are unknown.

In the Croisilles Harbour area, Landis & Blake (1987)defined the Rai Terrane as including the rocks between theCroisilles Melange and the Maitai Terrane (Fig. 2). Recentmapping by Johnston (in press) has shown that several PelorusGroup formations can be mapped on both sides of theCroisilles Melange, an interpretation that eliminates the needfor the separate Rai Terrane of Landis & Blake (1987).

The late Paleozoic - early Mesozoic Caples, Torlesse, andWaipapa Terranes are characterised by distinctive ranges ofgreywacke detrital modes and bulk chemical compositions(MacKinnon 1983; Bishop et al. 1985; Roser & Korsch 1986,1988). By comparing the composition of rocks in the studyarea with reference suites, correlations of the Kaituna andArapawa blocks with each other and with various NewZealand terranes can be explored.

Landis & Blake (1987) noted the difficulty in obtainingcomplete detrital modes of Pelorus Group greywackes. Forthis study, partial modes of detrital quartz, pyroxene, andamphibole were obtained for medium- and coarse-grained t.z.I-IIA greywackes. Detrital quartz was also measured in somet.z. IIB psammitic schist, though detrital amphibole andpyroxene have been totally to partially consumed by meta-morphic reactions at this higher grade. Detailed descriptions of

Pelorus Group clastic rocks, including the nature andcomposition of ferromagnesian minerals, have been given byVitaliano (1968) and Landis & Blake (1987).

Kaituna blockKaituna block samples typically have very low detrital quartzcontents (Fig. 4A, 6A),. These are comparable to samples ofKays Creek and Bold Peak Formation (Caples Group;Turnbull 1979), Tuapeka Group (Becker 1973), and lowerquartz recounts of some of Turnbull and Becker's data byMacKinnon (1980) (Fig. 6C, F). A majority of Kaitunasamples have a pyroxene/amphibole ratio >1 (Fig. 7A), againsimilar to Caples Group greywackes (except Momus Sand-stone samples, which constitute the amphibole-rich cluster inFig. 7C). With two exceptions, whole-rock chemical analysesof t.z. I-IIIA Kaituna block psammites (Table 1; Vitaliano1968, tables 2,3, and 4; Landis & Blake 1987, table 3) plot infields characteristic of mafic-intermediate provenance, alsosimilar to Caples psammites (Roser & Korsch 1988) (Fig. 8A-D).

In general, Kaituna block petrofacies compare well withCaples petrofacies, though Momus Sandstone and Upper PeakFormation compositions are apparently absent (Fig. 6A, C).Despite these differences, a direct correlation of t.z. I—IIBKaituna block schist (or low-grade PeloYus Group) with theCaples Terrane, as proposed by Coombs et al. (1976), Bishopet al. (1985), and Johnston (1990), is supported by the presentdata.

The maximum extent of the Pelorus Group in the Kaitunablock Marlborough Schist is determined using field criteria. InOtago, the Caples-Torlesse boundary is marked by a changein lithology from greenish-grey psammitic schist of the CaplesTerrane (cf. Fig. 4A) to grey-black pelitic schist of theTorlesse Terrane (cf. Fig. 4C), and/or a change fromincipiently or weakly segregated t.z. IIIA psammitic schist ofthe Caples Terrane to strongly segregated t.z. IIIB-IVpsammitic schist of the Torlesse Terrane (Mortimer & Roser1992). These field criteria have been used in the MarlboroughSchist to define an approximate position for the Caples-

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362 New Zealand Journal of Geology and Geophysics, 1993, Vol. 36

20

&

d)

I10

A KAITUNA BLOCK (Marlborough)• t.z. I-IIA (n=56)• t.z. IIB(n=12)

„>0 10 20 30 % quartz 40

20

10

II

C CAPLES TERRANE (Otago)H BoldPeakFmn(n=15)Q ] Kays Creek Fmn (n=8)I I Momus Fmn (n=11)• Upper Peak Fmn (n=9)

Turnbull(1979)

n

Becker (1973) • Tuapeka Group (n=32)

FIR,>0 10 20 30 % quartz 40

20

&

If10

E CAPLES TERRANE (Otago)MacKinnon (1980)

^ recounts of 17 ofTurnbull & Beckersamples

>0 10 20 30 % quartz 40

20

Icr

S

10

B ARAPAWA BLOCK (Marlborough)• t.z. I-IIA (n=36)• t.z. IIB(n=13)

.un,™>0 10 20 30 % quartz 40

D WAI PAPA TERRANE• Hunua facies (n=79)• Morrinsville facies (n=45)

>0 20 30 % quartz 40

South IslandH Upper Triassic (n=25)[ D ?lower Upper Triassic (n=27)• Mid Triassic (n=18)• Permo-Carboniferous(n=31)

North Island• I Wellington & Tongariro (n=29)

F TORLESSE TERRANE

>o 10 20 30 % quartz 40

Fig. 6 Histograms showing detrital quartz content of medium-coarse-grained greywacke and low-grade psammitic schist. A, Kaitunablock (data from this study). B, Arapawa block (this study). C, Caples Terrane of Otago (Becker 1973; Turnbull 1979). D, WaipapaTerrane of North Island (Mayer 1965,1969; Elliot 1967; Finlow-Bates 1970; Skinner 1972; Amos 1979; Beetham & Watters 1985; Jennings1987). E, Caples Terrane of Otago (MacKinnon 1980). F, Torlesse Terrane (MacKinnon 1980 and this study). Grouped class interval is 2vol.%, but lowest class is for samples with 0% quartz.

Torlesse boundary in the Kaituna block (Fig. 9). The presenceof several pods of up to 2 x 0.5 m ultramafic schist in theWairau Valley near 028/385562 (N. Mortimer, M. R.Johnston pers. obs.; Fig. 9) supports this interpretation, as theyare in an analogous position to the nine ultramafic pods nearthe Caples-Torlesse Terrane boundary in the Otago Schist(Mortimer & Roser 1992).

Given the uncertainty in interpreting the terrane affiliationof the two t.z. IIIB-IV pelitic samples (Mortimer & Roser1992; Fig. 8A-D), and the recognised presence of minor schistof Caples chemistry in t.z. IIIB-IV Otago Schist (Graham &Mortimer 1992; Mortimer & Roser 1992), the area shown asKaituna block Torlesse Terrane in Fig. 9 should be regarded asa maximum possible extent of that terrane.

Arapawa blockThe more quartzose nature of at least some of the easternMarlborough Schist relative to that in the west was brieflynoted by Wellman (1956, fig. 1), Reed (1957), and Vitaliano

(1968, p. 11). Figures 4A-B, 6A-B, and 9 show that, relativeto Kaituna block samples of similar grain size, and textural andmetamorphic grade, Arapawa block samples indeed have bothhigher values and a wider range of modal quartz. Differencesare also seen in the detrital ferromagnesian mineral popu-lation, which, in the Arapawa block, is dominated byamphibole to the near exclusion of pyroxene (Fig. 7 A, B). TheArapawa block dataset also contains a higher proportion ofpyroxene- and amphibole-free greywackes than the Kaitunablock. These latter Arapawa greywackes are typically quartzrich and have a detrital mineral assemblage dominated byepidote, biotite, and muscovite, a feature that is shared byTorlesse greywackes from the Wellington area some 40 km tothe east (Fig. 1,6F,7D).

Quartz modes for medium-coarse-grained WaipapaTerrane greywackes (Fig. 6D) show a wide range that overlapsthat of the Caples and Torlesse Terranes (Fig. 6C, E, F). Noseparate modes for pyroxene and amphibole have beenreported for medium-coarse-grained Waipapa greywackes,

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Mortimer—Marlborough Schist 363

A KAITUNA BLOCK(Marlborough)

n=56

B ARAPAWA BLOCK(Marlborough)

n=36

| Momus Fmn

C CAPLES TERRANE(Otago)

n=43

D TORLESSE TERRANE(Wellington)

n=18

Fig. 7 Histograms showing detrital pyroxene and amphibole content of medium-coarse-grained greywacke and low-gradepsammitic schist. A, Kaituna block (data from this study). B, Arapawa block (this study). C, Caples Terrane of Otago (Turnbull1979). D, Torlesse Terrane of Wellington area (this study). Note amphibole-rich samples from the Momus Formation in C (cf. Fig. 6C).Grouped class interval is 0.5 vol.%, but separate classes are for 0% and >6%.

but amphibole may dominate over pyroxene (Mayer 1969;Skinner 1972).

Chemical analyses of psammitic samples (Table 1;Vitaliano 1968, tables 2,3, and 4) indicate that Arapawa blocksamples also have consistently higher SiC>2 and Y^OfHa.-than Kaituna block samples (Fig. 8A). The majority ofArapawa analyses fall in the field of Waipapa greywackes, and

grain size - composition vectors between psammite-pelitepairs at Cape Koamaru and Waikawa (Fig. 8A-D) are alsoconsistent with those outlined for the Waipapa Terrane byRoser & Korsch (1986). The samples on Fig. 9 with modalquartz of 22, 26, and 28% (P15227, 15438, and 15142,respectively; Vitaliano 1968) also have SiO2 > 71 wt% (Fig.8 A) and plot in or near the P3 (felsic) field on Fig. 8B. Samples

10 r• K2o/Na2O

x >

X

, .••*•«*!]

X v

ARCTX

IV P52368

PS2375

ACM

—t—FB•>< x —

\

A

-—f

wt% SiO,

0.1 .

0.01

80 r Ti/Zr

60

40

50 60 70

20

80

c

P53

_CK\P53271

CaplesFB

TorlesseLa/Sc

B

0.5Caples

La/Y

0.5 1.5

KAITUNA BLOCKx t.z. I-MIA psammitic* t.z. IIIB-IV pelitic

ARAPAWA BLOCKt.z. I-IIB psammitic. Arrows point to pairedpelitic sample from same location

Fig. 8 Diagrams showing geochemical variation and terrane affinity of Marlborough greywacke and greyschist with fields of Caples,Torlesse, and Waipapa Terranes. A, SiO2 versus K2O/Na2O (Roser & Korsch 1986). ACM, active continental margin field (all Torlesse);ARC, volcanic arc field (most Caples). The shaded area is the field of Waipapa greywackes from Roser & Korsch (1986). B, Discriminantfunction diagram (Roser & Korsch 1988). C, La/Sc versus Ti/Zr (Mortimer & Roser 1992). D, La/Y versus Ce/V (Mortimer & Roser1992). Abbreviations for psammite-pelite pairs are as follows: FB, Fighting Bay; CK, Cape Koamaru; WA, Waikawa. Major element data forA and B are from Table 1, Vitaliano (1968), and Landis & Blake (1987). Trace element data for C and D are from Table 1.

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364 New Zealand Journal of Geology and Geophysics, 1993, Vol. 36

from the southeasternmost part of the Arapawa block (coast 1.from Fighting Bay to Tory Channel) show exclusively highmodal quartz and the strongest Torlesse chemical trends (Fig. T8,9).

In summary, Arapawa block greywackes and low-gradepsammitic schists differ in modal and bulk chemical com-position from those of the adjacent Kaituna block (Fig. 6A-B,7A-B) and cannot necessarily be assumed to be part of the ,same tectonostratigraphic terrane. On the basis of quartzmodes and chemistry, most of the Arapawa block is provi-sionally correlated with the Waipapa Terrane. The south-easternmost Arapawa block schist is speculatively correlatedwith the older Torlesse (Rakaia) Terrane, which could becontinuous with schistose Torlesse rocks in the North Island.As with the Kaituna block, the area shown as ArapawaTorlesse on Fig. 9 must be regarded as a maximum.

Regional implicationsThe nature of the putative Waipapa-Torlesse boundary in the 4.Arapawa block has not been examined in detail, but therecognition of Waipapa Terrane in Marlborough is significantin four respects.

This becomes the only place where any boundaries of theWaipapa Terrane are known to be exposed.

The basic regional terrane geometry in Marlborough (twoone-sided schist belts with the Waipapa Terrane present inthe eastern belt) is quite different to that in the Otago Schist(one two-sided schist belt with the Waipapa Terraneabsent).

The Otago/Kaituna and Kaimanawa/Arapawa schists maybe genetically distinct parts of the Haast Schist belt, notonly in that they comprise different terranes, but also intheir tectonic setting of schist formation. Shallow foli-ations in the Otago Schist are consistent with Jurassicductile underthrusting of the Torlesse beneath the CaplesTerrane (Mortimer 1993). Steep foliations, variablelineations, and heterogeneous textural development in theKaimanawa Schist, Arapawa block, and Cape Terawhitiarea may speculatively indicate Jurassic strike-slipjuxtaposition of the Waipapa and Torlesse Terranes.

The fact that the Waipapa Terrane of the Arapawa block iscut by the Wairau Fault raises the possibility that theWaipapa Terrane may actually be present in the AlpineSchist.

Table 1 New X-ray fluorescence analyses of greyschist from the Marlborough Sounds. Analyst Ken Palmer, Victoria University ofWellington. All major and trace elements have been normalised to 100 wt% anhydrous. LOI = loss on ignition. A photomicrograph of P52388is shown in Fig. 4B.

Sample no.Location

Grid referenceComposition

Terrane correlation

SiO2 (wt%)TiO2

A12O3

Fe2O3TMnOMgOCaONa2OK2OP 2 O 5Total

Original totalOriginal LOI

Cr (ppm)NiVSc

RbSrBaLaCeUThZrYNb

GaPbCuZnAs

P52375Linkwater

P27/869891Psammitic MA

Caples

59.400.90

18.567.550.112.653.545.511.610.17

100.0099.933.09

3710

1511952

52446322402.57.617423

621173588

9

P52368West of Picton

P27/935924Pelitic (IIIB-IV)

Torlesse

64.450.84

16.426.580.092.413.613.252.150.19

100.00100.18

2.66

3916

14011

74452563

19402.37.216120

7

191819842

P52372Waikawa

P27/977927Psammitic IIA

Waipapa

65.030.80

16.296.120.062.133.723.572.110.17

100.00

100.154.35

3916

1471565

372453

21321.76.2

179214

201625837

P52373Waikawa

P27/977927Pelitic

Waipapa

64.800.81

17.886.530.041.651.582.533.970.21

100.00

100.195.02

5220

14614

15131563829413.6

12.91863711

232844

11410

P12201Cape KoamaruQ26/259119

Psammitic IIA

Waipapa

60.480.81

19.056.870.072.712.804.342.680.19

100.00

100.23.52

4321

1521796

311613

17401.06.614122

52017338815

PI 2200Cape Koamaru

Q26/259119Pelitic

Waipapa

62.690.82

18.505.870.062.103.103.273.430.17

100.00100.24

3.32

5622

13815

12854382933703.8

13.2

1933111

232434

11112

P52388Fighting BayQ27/106880

Psammitic IIB

Torlesse?

74.270.45

13.093.290.080.971.124.462.190.08

100.00100.14

1.64

145

534

79337498

21392.59.0

162156

15163

504

P52389Fighting BayQ27/106880

Pelitic

Torlesse?

61.860.70

18.775.990.082.052.693.354.230.29

100.00100.05

3.25

4016

1099

165344771

3373

4.118.0

2464312233528

10213

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Mortimer—Marlborough Schist 365

Maitai, Murihiku andBrook Street Terrenes

Quaternary alluvium

TERRANE CORRELATION „ . • Terrane boundary overprintedby schist fabrics

;TTTTL • Otago Schist| ; I I | y Torlesse | (southern South Island)

Waipapa I Continuation ofI KaimanawaSchist

Torlesse? | ( N o r t h jsland)

Fault (dashed whereconcealed or i n f e r r e d L ^ ^ Nalson

volume %detrital quartz

sample site ofchemical analysis:

• this study (Table 1)o Vitaliano (1968)

Kaitunablock

Arapawablbck

Fig. 9 Proposed provisional distribution of Marlborough Schist terranes, based on criteria discussed in the text. Areas shown as TorlesseTerrane are the maximum likely extent of that terrane. Boxes show locations of t.z. I to low IIB psammitic lithologies for which percentdetrital quartz has been determined. Locations of chemically analysed psammitic and psammite-pelite pairs are also shown (data fromVitaliano 1968 and Table 1). The main break of the Picton Fault Zone has been thickened for emphasis. Abbreviations not in Fig. 2: AI,Allports Island; RB, Resolution Bay; um, ultramafic schist pods in Wairau Valley.

CENOZOIC FAULTS

Picton Fault Zone

The term Picton Fault zone was first used by Morgan (1921) todescribe a 2 km wide zone of northeast-striking faults passingthrough the Picton area that cut greywacke, schist, and smallslivers of Oligocene sedimentary rocks. This terminology hasbeen adapted in the present study to describe a 0.5-10 km widezone of faults that extend from the Wairau Valley to outerQueen Charlotte Sound and that separate t.z. IIIA-IV schist ofthe Kaituna block from t.z. I-IIA schist of the Arapawa block.The main fault break is clearly shown by the abrupt change inmetamorphic and textural grade, and terrane boundaries inFig. 2,3, and 9, but the Picton Fault Zone as defined here alsoincludes nearby faults within each block. The Picton FaultZone is widest adjacent to the Wairau Valley and narrows topossibly a single fault strand on the northern side of QueenCharlotte Sound. In terms of fault development and movementalong its length, the Picton Fault Zone is composite, and has acomplex history.

One well-documented feature of the Picton Fault Zonenear Picton is a sequence of three, stacked, post-Oligocene(probably Miocene), east-verging thrusts (Nicol & Campbell1990). During the present study, a fault plane in the PictonFault Zone 8 km southwest of Picton (P27/873848) wasobserved; it was oriented 066/66NW with northeast-trendingmesoscopic kink fold axes in the pelitic schist of the hangingwall. The direction of overturning of these folds indicatednear-pure thrusting.

Despite the apparent thrust geometry at and southwest ofPicton, the Picton Fault Zone includes a series of prominentlineaments (i.e. probable subvertical faults) extendingsouthwest to the Wairau Valley where strongly kink-foldedpelitic schist (e.g., at P28/782726, P28/726685; Fig. 4C) andsteep faults (e.g., quarry at P27/762701) are exposed. Thepresently known distribution of high and low grade schisteither side of the Picton Fault Zone also apparently requiresthe presence of a curious SSE-striking cross fault NNW ofTuamarina (Fig. 2,3). However, the existence of this structure,its exact relation to other faults, and the significance of east-and southeast-dipping foliation in the Kaituna block to thewest of it have not been established. It may be that thetopographic depression southwest of Picton is some sort ofpull-apart graben.

Textural grade IIB-IIIA schist occurs in at least four placesbetween Picton and the Wairau Valley along the fault contactbetween t.z. IIIB-IV Kaituna block and t.z. I-IIA Arapawablock rocks (Nicol & Campbell 1990; N. Mortimer pers. obs.near Tuamarina at P28/864737 and P27/878835). Thesignificance and correlation of these intermediate-gradeschists remains puzzling. They could represent thrust slicesdragged up during Cenozoic thrusting or, more likely, atextural upgrading of Arapawa block Waipapa Terrane rocksas they were emplaced against Kaituna block Torlesse schistin a mid-crustal kinematic regime in the ?Mesozoic. Thiswould be similar to the aforementioned increase in texturalgrade of Caples Terrane rocks against the Maitai Terrane(Landis & Blake 1987; Mortimer in press; Fig. 3). Laumontite

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366 New Zealand Journal of Geology and Geophysics, 1993, Vol. 36

veins cut Arapawa block greywackes P37758 and P52369 inthe Picton Fault Zone near Picton. These, too, may indicatefault movement in a metamorphic regime intermediatebetween that of peak regional metamorphism and the present-day surface.

Queen Charlotte Fault ZoneFaults in Queen Charlotte and/or Kenepuru Sounds wereproposed by Mackay (1879) and depicted on maps byHenderson (1935), Beck (1964), and Vitaliano (1968). Theyseem to have been postulated solely on the basis of thelinearity of the Sounds, without demonstrated offset of anygeological features. This study and that of Johnston (in press)are the first to collectively show sensible offset of isotects,lithologic horizons, an earlier fault zone, and a macroscopicfold axis by these faults (Fig. 3). Two main fault strands can beidentified, called (following Vitaliano (1968) and Johnston (inpress)) the Queen Charlotte Fault and the Kenepuru Fault. Thearea between these faults is termed the Queen Charlotte FaultZone (Johnston in press). Within the fault zone there areseveral faults (not shown on Fig. 2) subparallel to thebounding faults; schist foliation between Kenepuru and QueenCharlotte Sounds has variable attitude and is folded byabundant kink folds with variably trending axes.

Queen Charlotte Fault

The Queen Charlotte Fault can be traced from Arapawa Islandwhere east-striking gouge zones and fault planes cut schist(Fig. 3D), through central Queen Charlotte Sound acrosswhich there is a pronounced break in textural grade (Fig. 3), tosouth of Dun Mountain (Johnston in press). Allports Island(P27/985960) and the north shore of Queen Charlotte Soundare both composed of t.z. IIIB-IV schist; the main strand of theQueen Charlotte Fault thus passes south of Allports Island(Fig. 3,9).

The Picton Fault Zone is apparently dextrally offset byc. 7 km across the Queen Charlotte Fault, and the Caples-Torlesse boundary may be dextrally offset by up to 4 km (Fig.9). Johnston (in press) has indicated dextral offset of theKaituna block IIA-IIB and IIB-IIIA isotects, axial trace of theGoulter Synform, and the Wakamarina Quartzite (metachertband) by c. 3 km (Fig. 3).

Kenepuru Fault

The Kenepuru Fault is present at least as far east as EndeavourInlet, where there is a major dip discordance (Fig. 2), and aprominent lineament can be traced still further northeasttowards Cape Jackson. Gouge zones and fault planes havebeen observed on the shores of Kenepuru Sound (P27/966013), and Vitaliano collected a sample of cataclasite fromnear Havelock (P14950; P27/808957). Johnston (in press) hasextended the Kenepuru Fault westward up the Pelorus Valleyto within 10 km of Dun Mountain.

The Kenepuru Fault apparently offsets the axial trace ofthe Goulter Synform dextrally by up to 15-20 km (Fig. 2) but,because of the narrow angle of intersection and uncertainty ofextrapolation under Kenepuru Sound, this must be considereda maximum amount. The Kenepuru Fault also apparentlydextrally offsets the Kaituna block IIA-IIB isotect by c. 10 kmand the IIB-IIIA isotect by c. 8 km (Fig. 3). Johnston (in press)has shown an apparent dextral offset of the I-IIA isotect ofc. 3 km and the Wakamarina Quartzite of c. 10 km.

Regional significanceApparent dextral offsets on the northwest-dipping Caples-Torlesse boundary and IIB-IIIA isotect could possibly beproduced by down-to-the-north fault movement. However,the fact that the steeply east-dipping IIA-IIB isotect and thesubvertical I-IIA isotect and Picton Fault Zone also show aconsistent sense of offset means that an interpretation of netdextral-slip movement across the whole Queen CharlotteFault Zone is probably correct, though the estimated 15 km isonly a maximum value, and perhaps 10 km is morereasonable.

Constraints on the timing of movement of the QueenCharlotte Fault Zone are poor. Because the Queen CharlotteFault offsets the Picton Fault Zone, Queen Charlotte FaultZone movement must postdate the ?Miocene Picton FaultZone thrusting documented by Nicol & Campbell (1990). Noactive fault traces in the western part of the Queen CharlotteFault Zone are known (Johnston in press) and, in the easternpart, most of the main strands are under water.

The Wairau (Alpine) Fault has previously been consideredto be the northernmost member of the system of Neogenetranscurrent faults in Marlborough, which also includes theWaihopai, Awatere, Clarence, Hope, and Porters Pass Faults.By virtue of their dextral offsets and parallelism with theWairau Fault (Fig. 3), the Queen Charlotte and KenepuruFaults should also be included with this system of faults,across which late Cenozoic Pacific-Australian plate boundarystrain has been distributed.

CONCLUSIONS

This reconnaissance investigation of the structure andpetrology of the Marlborough Schist, incorporating previouswork, has resulted in significant new regional geologicalinterpretations:

1. Unfoliated greywacke is much less common and schistmore widespread in the Marlborough Sounds thanpreviously recognised.

2. The Marlborough Schist is actually composed of twoseparate one-sided schist belts, coincident with Cenozoicstructural blocks and separated by the Picton Fault Zone.The northwest (Kaituna) block shows a regular increase inmetamorphic and textural grade to the southeast fromprehnite-pumpellyite to greenschist facies and t.z. I to IV.The southeast (Arapawa) block also shows an increase ingrade to the southeast (but less regular than the Kaitunablock) from prehnite-pumpellyite to pumpellyite-actino-lite facies and t.z. I to IIB.

3. The Kaituna block is a piece of the Caples and TorlesseTerranes of the Otago Schist, offset along the Alpine Fault.The Arapawa block is correlated with the WaipapaTerrane (and possibly also the Torlesse Terrane) of theNorth Island, and is the along-strike extension of theKaimanawa Schist into the South Island.

4. Fundamental genetic differences between the Otago/Kaituna and Kaimanawa/Arapawa schist belts are empha-sised by regional differences in heterogeneity of pro-gressive textural zone development, dip of foliation, anddip of stretching lineations.

5. About 10-15 km of post-Oligocene dextral offset can bedemonstrated across the Queen Charlotte Fault Zone

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Mortimer—Marlborough Schist 367

between the Queen Charlotte and Kenepuru Faults. TheQueen Charlotte Fault Zone should be included with theMarlborough fault system.

These interpretations can be tested by more detailedthematic studies. Priorities for further investigation would bestructural, sedimentological, geochemical, and isotopicstudies of Arapawa block greywacke and low-grade schist,kinematic studies of the Picton Fault Zone, and 1:50 000mapping of isotects and lithologic markers in and near theQueen Charlotte Fault Zone.

ACKNOWLEDGMENTS

I thank Mike Johnston for introducing me to Marlborough Schistgeology, Neville Orr for making thin sections, Stewart Bush forcrushing rocks, Ray Soong for X-ray diffraction work, Ken Palmerfor chemical analyses, Jane Forsyth and Jan Hanbury-Sparrow forliterature and database searches, Wayne Kelliher for driving theboat, and Andy Nicol and Roy Grose for supplying specimens ofschist from Picton and The Brothers, respectively. I also thankWayne Jennings, Bernhard Sporli, and Rick Sibson for permissionto use data from unpublished theses, Mike Johnston, GrahamBishop, David Skinner, and Tim Little for useful comments on anearly version of the manuscript, and Chuck Landis and Clark Blakefor helpful manuscript reviews.

Institute of Geological & Nuclear Sciences contribution 90.

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