structural and lithological continuity and discontinuity in the otago schist, central otago, new...

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Structural and lithological continuity anddiscontinuity in the Otago Schist, Central Otago, NewZealandD. J. Mackenzie a & D. Craw aa Geology Department , University of Otago , P.O. Box 56, Dunedin, New ZealandPublished online: 22 Sep 2010.

To cite this article: D. J. Mackenzie & D. Craw (2005) Structural and lithological continuity and discontinuity in theOtago Schist, Central Otago, New Zealand, New Zealand Journal of Geology and Geophysics, 48:2, 279-293, DOI:10.1080/00288306.2005.9515115

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New Zealand Journal of Geology & Geophysics, 2005, Vol. 48: 279-2930028-8306/05/4802-0279 © The Royal Society of New Zealand 2005

279

Structural and lithological continuity and discontinuity in the Otago Schist,Central Otago, New Zealand

D. J. MACKENZIE

D. CRAWGeology DepartmentUniversity of OtagoP.O. Box 56Dunedin, New Zealand

Abstract The Otago Schist in Central Otago has undergonecomplex late metamorphic and post-metamorphic deformationduring uplift and exhumation. Two late metamorphic structuralgenerations can be recognised and these may be geneticallyrelated. The earlier Manorburn Generation has been widelyrecognised and described previously. This generation hasfold axes subparallel to a prominent syn-metamorphicquartz rodding lineation. A later generation, herein namedPoolburn Generation, has folds which superficially resembleManorburn Generation, but has fold axes that are at a highangle to the quartz rodding lineation. Both generations occurin mappable fold zones (kilometre-scale) that are generally notvergence boundaries, and some minor relative displacementmay occur across fold zones. Fold zones occur withinstructurally and lithologically uniform schist domains. Abruptchanges in lithological sequences and orientations of structuralelements such as Manorburn and Poolburn Generation foldaxes and quartz rodding lineations occur at post-metamorphicfaults which separate different schist domains. CentralOtago schist can be subdivided on the regional scale (tens ofkilometres) into at least nine schist domains whose structuraland lithologic continuity is disrupted by fault discontinuities.The domains and bounding discontinuities developed duringLate Jurassic and Early Cretaceous uplift. Syn-metamorphiccompressive ductile deformation evolved to localised foldzones in the early stages of this uplift. Subsequently, regionalextension caused juxtaposition of domains with differenttextural zones, and schists from slightly different structurallevels. The Caples/Torlesse Terrane boundary is a compositefeature, and different segments formed at different stagesthrough the transition from ductile compression to brittleextensional deformation.

Keywords Otago Schist; structure; folds; faults; lineations;lithology; domains

G04013; Online publication date 25 May 2005Received 15 March 2004; accepted 3 November 2004

INTRODUCTION

The Otago Schist is part of the Haast Schist Belt (Coombs et al.1976), which extends across the South Island of New Zealand(Fig. 1, inset). This metamorphic belt is structurally complexat all scales, particularly in the central portion of the OtagoSchist. Structural continuity on a regional scale has commonlybeen assumed, for simplicity, when studying the belt as awhole (Wood 1963, 1978; Suggate et al. 1978; Mortimer1993a,b). However, more recent work has shown that post-metamorphic structural discontinuities were important in theevolution of the geometry of the belt (Craw 1998; Mortimer2000; Deckert et al. 2002; Forster & Lister 2003). In this study,we describe the geometry of the schist belt on a regional scaleand identify key structural discontinuities. These structuraldiscontinuities separate structural blocks, or domains, thatare structurally and lithologically homogeneous. Hence, oneof the principal aims of this study is to subdivide the schistbelt into these domains and to delineate the discontinuitiesbetween the domains.

Construction of cross-sections through the metamorphicbelt is difficult, and most previous attempts have beengeneralised. Previous sections have been either schematic innature (e.g., Wood 1978), or were small-scale compilationsof data from diverse sources accompanying regional maps(Mortimer 1993b, 2003; Turnbull 2000; Forsyth 2002). Mostdetailed sections have been confined to specific boundaryregions, for example, west of Lake Wanaka at the Torlesse/Aspiring lithologic association boundary (Craw 1985,1998)and east of Lake Wakitipu at the Caples/Aspiring lithologicassociation boundary (Cox 1991; Craw 1998). These previouscross-sections were drawn on 2D planes to depict particularsets of structures, and therefore did not display some structuressubparallel to the section lines. Further, most regionaldescriptions of the schist belt emphasise general continuityof increasing metamorphic grade and degree of texturalreconstitution towards the centre of the belt from both thenorth and south (e.g., Wood 1978; Mortimer 1993b). In thisstudy, we have constructed block diagrams of lithological andstructural variations in three dimensions for large areas ofCentral Otago, to depict the various internally homogeneousdomains and the structural discontinuities that separatethem. Combination of these block diagrams constitutes a3D structural cross-section right across the schist belt, astructural depiction that has not been attempted previouslyat this scale.

Our regional mapping at the scale of the above 3D cross-section has defined discrete folded zones that are traceablefor kilometres within structural domains. These fold zonesare distinctive structural features that formed in the latterpart of the metamorphic history of the schist belt. In thisstudy, we correlate these fold zones on a regional scale,and define two generations of folds that have consistentrelationships with each other and with earlier structuralelements in the schist. These fold generations are therefore

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280 New Zealand Journal of Geology and Geophysics, 2005, Vol. 48

/ -«?Aspiring lithologic association- wSS? y

Wanaka lithnlnnin association ^ \ K?7^ \Wanaka lithologic associationUndifferentiated Torlesse Terrane

^ Caples Terrane _ , , „ , „IWiChrystalls Beach complex " —x pauns

I li ata Cretaceous-Cenozoic cover

Fig. 1 Simplified geological mapof the Otago Schist, modified afterMortimer (1993b) and Turnbull(2000), showing the principalsubdivisions based on terraneand lithologic associations. Thelocations of the block diagrams(Fig. 6, 9, and 12) are outlined inblack. Inset: Location of the studyarea in relation to the Haast Schistand South Island.

Fig.2 TZIIIschistwithprominentquartz-albite segregations defininga relatively planar foliation surface,Spen. Some of the segregationsform rootless fold hinges and areisoclinally folded, axial planar-parallel to the segregation layering.Hammer is 365 mm long.

important mappable features in the schist belt, and we describethem in the context of the domains and their boundingstructural discontinuities.

REGIONAL GEOLOGY AND STRUCTURE

Otago Schist consists mainly of metasediments from theCaples and Torlesse Terranes, which have been amalgamatedinto a single metamorphic belt. These rocks were variablydeformed and metamorphosed to pumpellyite-actinolite orgreenschist facies during Mesozoic collisional orogenesis.

The schist was exhumed from metamorphic depths in theJurassic and Cretaceous, accompanied by varying degreesof ductile deformation (Mortimer 1993a; Forster & Lister2003; Gray & Foster 2004). The belt has been deformed inmore brittle manner by several tectonic episodes since thisuplift, and deformation is continuing today associated withthe Australian-Pacific plate boundary.

The rocks in the central and northern parts of the studyarea are predominantly Torlesse Terrane quartzofeldspathicschist of pelitic and psammitic composition (Mortimer1993b). Caples Terrane volcanogenic metasediments lieon the southern side of the study area, but the schistose

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MacKenzie & Craw—Structural discontinuity in Otago Schist 281

Fig. 3 TZ IV schist lookingdown on the prominent foliation(Spen) surface. Quartz roddinglineation (Lq) plunges in thedirection of the pencil. A moresubtle later crenulation associatedwith Poolburn Generation folds(FP) overprints Lq and plunges inthe direction of the arrow. Pencil is140 mm long.

equivalents are difficult to distinguish from Torlesse Terranemetasediments (Mortimer 1993b). Some locally continuouslayers of greenschist up to 100 m thick occur, along with somerare horizons of metachert, marble, and ultramafic lithologies(Turnbull 2000).

Syn-metamorphic structures

Texturally the rocks grade from unfoliated metasediments(greywackes and argillites), through slates and phyllites(semischists), to strongly segregated and laminated schists.The schist is subdivided into textural zones based on thesystem of Turnbull et al. (2001) and rocks within the arearange from TZ I through TZ IV. In general, the schist in thisregion has a well-developed flat-lying penetrative foliation.The foliation in TZ III and IV schist is generally definedby alternating layers of quartz and albite-rich (light) andmicaceous (dark) segregation laminae (Fig. 2). Widespreadpreservation of disrupted and commonly rootless folds(Fig. 2) lying within segregation laminae indicates that thepredominant schistosity within greenschist facies TZ III—IVis at least second generation (Mortimer 1993b). For thepurposes of description in this paper, the dominant penetrativefoliation seen at outcrop is designated Spen (cf. Craw 1985).This penetrative foliation is therefore used as a datum formapping subsequent structures in this paper.

Intrafolial folds that are characterised by tight or isoclinalprofiles with axial surfaces parallel to Spen are recognisablelocally (e.g., Fig. 2). Quartz-rich hinges of these folds ofmetamorphic segregations have produced a well-developedquartz rodding lineation (Fig. 3), herein designated Lq, overlarge areas of the schist belt. Mineral elongation lineations andfoliation intersection lineations are locally discernible and aregenerally subparallel to Lq. Lq developed during progressiveductile deformation and was locally rotated up to 90° by thisprogressive deformation (Craw 1985; Cox 1991).

Late metamorphic folds

The penetrative foliation, Spen, and Lq lineations are locallydeformed by late metamorphic open to tight mesoscopicfolds with rounded hinges and attenuated limbs. Folds of

Fig. 4 TZ IV schist with millimetre to centimetre scale asymmetricManorburn Generation folds (FM) of the penetrative foliation (Spen)and associated segregation layering. FM fold axial surface liesparallel to the pencil at high angle to the overall foliation (Spen)and segregations. Circles show prominent quartz rods (Lq; seenhere in cross-section) that are parallel to FM fold axes. Pencil is80 mm long.

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F i g . 5 A, Highly foldedoutcrop of TZ IV schist withsubvertical foliation, Spen, actingas form surface. The face striking205° (right) contains PoolburnGeneration folds that plunge120° parallel to the pocket knife(indicated by double arrow). Theface striking 125° (left) containsManorburn Generation folds thatplunge 230° parallel to the pencil(indicated by single arrow). B,Close-up of Manorburn Generationfolds located at the top of theoutcrop above (A) on the 125° face.These Manorburn Generation foldsremain intact because PoolburnGeneration folds are only weaklydeveloped in this part of the outcrop(see upper part of 205° face in A).C, Close-up of indicated box in (A),showing Poolburn Generation foldsof quartz rodding lineation, Lq.Poolburn Generation fold hingesrun right to left in images A andC. Pencil is 140 mm long.

these later deformation generations have Spen as form surface,with fold axial surfaces at an angle to the foliation (Fig. 4).These structures were first described in detail for the Otagoregion by Wood (1963) and Means (1963, 1966), and foldsof comparable style occur in many parts of the Otago Schist(Brown 1968; Turnbull 1981; Craw 1985; Mortimer 1993a).These structures are commonly viewed as having formedin a continuum of progressive development of macroscopic

structures (e.g., Means 1966; Brown 1968; Turnbull 1981;Craw 1985; Cox 1991). Consequently, early formed folds inthis continuum are overprinted by later formed structures.The late metamorphic folds commonly have consistent foldvergence over large areas, and these can be mapped (Means1966). In this study, we have used the depiction of vergenceson maps as did Means (1966) to facilitate comparison. Thevergence depiction is described by Bell (1981).

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FaultTZ III-IV boundary

\ Cenozoic cover

I Greyschist

Greenschist

"I Highly folded zone

>•::::: -i--..| Snan foliation surface^•?:--:::"-="--l P e n

Manorburn Generationfold axis - M-foldsfold axis w/ vergence

Poolbum Generationfold axis - M-foldsfold axis w/ vergence

1

. Rough Ridgei/er hemisphere

equal area stereonets

= 276quartz rodding,

mineral elongationlineations

—— * • • • •

' J ; ' ' -

-•--s-— gn = 244

mesoscopic fold axis

Fig. 6 Block diagram of the area north of Blackstone Hill to Dovedale Creek. Inset: Location of blocks in relation to the Otago Schist.

Folds formed in this stage of schist development arecommonly called Manorburn Generation structures (Norris1977; Craw 1985). These folds are characterised by roundedhinges and similar style. Manorburn Generation fold axes(FM) trend generally parallel or subparallel to Lq. Limbsare commonly attenuated, with extensive reorientation ofmicas into subparallelism with the fold axial surface. Micareorientation has resulted in localised development of an

incipient cleavage, and there has been minor recrystallisationof chlorite to accentuate this cleavage. Quartz and albite-bearing leucocratic layers are attenuated but maintaincontinuity across cleavage zones. Manorburn Generationstructures are widespread throughout Central Otago, butthey occur in discrete zones (kilometre-scale) separated byzones of relatively planar foliation with few folds (Turnbull1981; Craw 1985; Mortimer 1993a, 2003). Transition into

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"I Cenozoic cover

"I Greyschist

T Greenschist

Highly folded zone

N

2 km

foliation surface

Foliation strike and dipwith qz rodding lineation


Poolburn Generationfold axis - M-foldsfold axis w/ vergenc

Fig. 7 Block diagram of the Dovedale Creek area in Fig. 8 showing in detail the greenschist layer mapped along the southwest side ofNorth Rough Ridge.

folded zones generally occurs over one or more kilometres,with progressively increasing fold tightness and cleavagedevelopment towards the fold zones. These fold zones aretraceable laterally for several kilometres, and have beenused as mapping features in this study. The fold zones arenot necessarily vergence boundaries (cf. Wood 1963), andgenerally have consistent fold vergence throughout.

Manorburn style folds are locally overprinted by opento tight folds that have similar style but have axes at a highangle to FM axes and Lq (Fig. 5). These later folds can havemore angular hinges and more prominent crenulation cleavageon limbs than FM folds. The cleavage is partly defined byreoriented micas, as in the Manorburn Generation, but also bydiscrete fractures. Some such fractures have minor (millimetreto centimetre-scale) displacement of metamorphic layeringacross them. The later folds can form a prominent set ofcrenulations across Lq and FM on Spen (Fig. 3). We interpretthese folds to have formed in the latter stages of the samelate metamorphic deformation continuum as ManorburnGeneration structures described above, but for convenience

we refer to these later structures as Poolburn Generation(MacKenzie et al. 1999; MacKenzie & Craw 2001). Werefer to the folds as FP, with axes at a high angle to FM andLq(Fig.3).

Poolburn Generation structures occur in discretezones (kilometre-scale) in a similar manner to ManorburnGeneration (above), but the transition from unfolded tofolded zones is more abrupt (tens to hundreds of metres).Limited vertical relief allows only crude estimation of dipof the fold zones, but moderate dip (40-60° is implied). Thebest exposed Poolburn fold zones are on southern RoughRidge (Fig. 1), where two northwest-trending zones deformgreenschist layers (Fig. 6,7). The fold zones strongly disruptmetamorphic layering at the outcrop scale, but do not causeoffset of the main greenschist layer (Fig. 6,7). However, thegreenschist layer is strongly attenuated immediately northof Dovedale Creek, and thins from an apparent width of800-125 m (Fig. 7). This thinning is associated with apparentdextral offset. Northwest-trending Poolburn Generation foldsoverprint northeast-trending Manorburn Generation folds on

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Fig. 8 Geological map of theOtago Schistshowingthe orientationof quartz rodding lineations andthe location of different geologicdomains discussed in text. TGF,Thomsons Gorge Fault; HMSZ,Hyde-Macraes Shear Zone.

m•Aspi r ing lithologic associate|:::::|Wanaka lithologic association| | Torlesse Terrene

omain[MMM] North Dunstan domain

Raggedy-Rough Ridge domain

EastOtagodomain S

Northeast Otago domain ^

E 5 Q Caples Terrene \ after Mortimer 1993 •I IChrystalls Beach complex Terrane boundary \I | Late Cretaceous-Cenozoic cover

the margins of the fold zones (Fig. 5), and FM is obscuredand disrupted by development of FP within the fold zones.Poolburn Generation folds have generally consistent vergenceacross the fold zones, although some of the most intenselydeformed portions have symmetrical folds.

Post-metamorphic faults

The generally low relief and poor outcrop of Central Otagomakes direct observation of faults difficult, and manysuch faults are inferred for most of their lengths, based ontopographic expression and geological contrast across thestructures. In this study, we focus on faults that predate theregional unconformity (mid Tertiary; LeMasurier & Landis1996), so no neotectonic features are considered. We haveidentified several post-metamorphic faults (see Fig. 6,9,12)that juxtapose geologically different schist domains in CentralOtago and contribute to the regional structural discontinuitiesthat are the topic of this paper. There are many more faults inthe schist belt, as seen in areas of clean outcrop such as roadcuts and river gorges. However, we focus on a small numberof faults that represent significant discontinuities in the overallschist structure on the regional scale.

Discontinuous change in the orientation of Lq is animportant feature used to identify significant faults in thisstudy. Lq has generally consistent orientation over largeareas of Otago Schist, with only local deviations. However,the Lq orientation can change abruptly between adjacentareas which are separated by a fault. Domains in which Lq

is relatively consistent are shown in Fig. 8, with separatingfaults. Manorburn Generation fold axes (FM) are commonlyparallel or subparallel to Lq (above), and discontinuities inorientations of FM also occur at faults.

Lithological characteristics are consistent over large

areas of schist, particularly with respect to proportions ofinterlayered greenschist horizons. Differences in proportionsof greenschist and associated metacherts, and proportionsof fissile micaceous schists, have been used to distinguishdistinctive regions of Otago Schist in the regions around LakeWakatipu and Lake Wanaka (Fig. 1) (Craw 1984). Similardistinctions can be made on a smaller scale in Central Otago,and faults defined by changes in structural elements (above)are also boundaries between schists with different lithologicalcharacteristics (see below).

Textural zone boundaries are theoretically transitional onthe margins of the Otago Schist (Bishop 1972). However, latemetamorphic or post-metamorphic faults can juxtapose rocksof different textural zones. Abrupt, rather than transitionalchanges in textural zones across some faults described inthis study are further indications of the significance of thesefaults.

Caples/Torlesse boundary

Caples Terrane schists are all TZ III or lower, whereas TorlesseTerrane schists reach TZ IV (Mortimer 1993b). The boundarybetween the Caples Terrane and Torlesse Terrane in OtagoSchist is difficult to define in the field because the geologicaldifferences across the boundary are subtle. The boundary hasbeen mapped geochemically by Mortimer & Roser (1992)in western and central parts of Otago. The location of theboundary farther east in Otago was poorly constrained becauseof limited outcrop and limited geochemical sampling. In thisstudy and in the preliminary work presented by MacKenzie etal. (1999) we have tried to refine the boundary in the east anddefine the boundary as a mappable feature in the field.

The Caples/Torlesse boundary is now placed from northof Lake Mahinerangi in the the Lammermoor Range to

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^'Wanaka lithologic \'; association x/'YO

UpperManorburnDam

f = 49!S V V-;-'-

/^0^&^^^

Fault /• TZ III-IV boundary

| 1 Cenozoic cover

| | Greyschist

| ' - ^ ~*^"| Highly folded zone

Foliation


Poolburn Generationfold axis - M-foldsfold axis w/ vergence

Geochemical sample sites

lower hemisphereequal area stereonetsI = no. of measured

lineationsf = no. of measured

fold axes

Fig. 9 Block diagram of the Alexandra-Roxburgh region. The location of geochemical sample sites discussed in the text is indicated.Inset: Location of blocks in relation to the Otago Schist.

an area south of Lake Onslow (Fig. 1, 9). The change inlithology across the boundary is subtle but is discernible inthe field. On the north (Torlesse Terrane) side, the schist isgrey with brown weathered surfaces. Coarse-grained micason the foliation surfaces give the rock a characteristic sheen(TZ IV). The schist is dominated by centimetre-scale quartzsegregations and well-developed quartz rods. On the south(Caples Terrane) side of the boundary, the schist is also greyand weathers brown, but it is slightly more greenish andlighter in appearance. Micas on the foliation surfaces arefiner grained (TZ III) and appear duller. Generally the schisthere is more massive and has fewer quartz segregations (withwidths on a millimetre-scale). Observations of the schist onthe south side of the boundary are consistent with a lowertextural grade and abundant chlorite. The boundary lieswithin a zone of subdued topography that extends from theLammermoor Range, north of Lake Mahinerangi (Fig. 1),

to an area east of Roxburgh (Fig. 9), and is notable for itslack of tors. South of Lake Onslow (Fig. 9) the boundary ismarked by distinctive outcrops of massive schist, containingregularly spaced, centimetre-wide, foliation-parallel veinsand/or segregations. To the west, the boundary is offset to thenorth, and its location, as mapped geochemically by Mortimer& Roser (1992), lies within a highly folded zone (describedin a later section) that extends to Alexandra (Fig. 9).

Samples of schist from the Lake Onslow region (Fig. 9)were analysed for major and trace element contents by X-ray fluorescence to quantify the field observations of theterrane boundary. Discrimination diagrams such as thoseused to distinguish Caples and Torlesse Terrane parentage byMortimer & Roser (1992) are presented in Fig. 10. Althoughthere is some overlap, the schist samples taken north of theboundary and mapped as TZ IV generally plot within theTorlesse Terrane field and those samples taken to the south

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45 50

oCM

COz

0

1 :

. 1 -

r j

A

ARC

X

~W* *• ^ ^ *

••

TZ• TZ

X

x°

D \

\\

IV schists n =III schists n =

X

X

ACM

•

2213

X

55 60 65SiO2wt%

70 75 80

CM

5

3

1 -

-1 -

-3

-5

-7

BTZ IV schists n = 22

• TZ III schists n = 13

-5 -3 -1

F1

1.5

1.0

O

0.5

0.0

TZ IV schists n = 22• TZ III schists n = 13

Torlesse

0.00 0.5 1.0 1.5

La/Y2.0 2.5

80

60

40

20

D

•

• /

Caples Z ^ H n * X x

/ X ^


X X

Torlesse

2 3La/Sc

Fig. 10 Trace element analyses of schist samples from the Lake Onslow region plotted on diagrams useful for discriminating Caples andTorlesse Terrane rocks (after Mortimer & Roser 1992). A, SiO2 versus K2O/Na2O. B, Discriminant function diagram of Roser & Korsch(1988). The two functions, F1 and F2, are linear combinations of major element concentrations that best separate rocks with Torlesseaffinity from those with Caples affinity. C, La/Y versus Ce/V Caples and Torlesse fields after Mortimer & Roser (1992). D, La/Sc versusTi/Zr diagram of Roser & Cooper (1990). ACM, active continental margin provenance; ARC, oceanic island arc provenance.

2.0

i

1.5

LO-

CI 5-

TZ• TZ

A

? §

IV schists n =III schists n =

•

a

22

13

X

X

X

X

XX

XX

x x

X

4.5

4.0

3.05B<J>3.0-

_l 2.552.0

1.5

1.0

0.5

5TZ• TZ

B

•qb

0 •

•

IV schistsIIV schists

n=22n=13

•

B

XX

X

XX

X

X

X

x xX

X

v X

x xX

XX

X

5 10 15 20 25 30 35 40

Distance in km NE from Clutha River5 10 15 20 25 30 35

Distance in km NE from Clutha River40

Fig. 11 2.0Schist analyses from the Lake Onslow region plottedagainst distance from the Clutha River: TZTZIIIIVschistsschistsnn==1322A, La/Y; B , La/Sc;C, Th/Sc. The Caples/Torlesse boundary, as mapped in the field, islocated c. 12 km northeast from the Clutha River.

2.0

1.5

uCO

1.0

0.5


XXXX

0.0 I0 5 10 15 20 25 30 35 40

Distance in km NE from Clutha River

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£ Foliation strike and dipr with quartz rodding lineation

Lower hemisphereequal area stereonetsI = no. of measured

lineationsf = no. of measured

fold axes

FaultDomain boundaryMineralised veins

1 Cenozoic cover

1 Greyschist

•71 Greenschist

Foliation


Poolburn Generationfold axis - M-foldsfold axis w/ vergence

Fig. 12 Block diagram of the Dunstan Mountains region. Inset: Location of blocks in relation to the Otago Schist.

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and mapped as TZ III, plot within the Caples Terrane field.To further illustrate the geochemical variation relative to thegeographic location of the mapped boundary, the sampleswere plotted according to their distance from the Clutha River(Fig. 9, an arbitrary point) in Fig. 11. In these plots there issome overlap, but the TZ IV (Torlesse Terrane) schists plot asa distinct group higher on the ordinate than the TZ III (CaplesTerrane) schists.

BLOCK DIAGRAMS OF REGIONAL STRUCTURE

The structures described in this study have a wide range oforientations, and it is not possible to include all of these in asingle cross-section. Hence, we have drawn block diagramsof key areas, to show an inferred 3D view of the featureswe describe in the following sections. The blocks, outlinedin Fig. 1, form a 3D section through the central part of theOtago Schist: Figure 12 depicts the structure of the DunstanRange, at the western edge of the region described in thispaper; the block diagrams in Fig. 6 form a north-south sectionextending from non-schistose rocks north of Blackstone Hillto higher grade schist along the Raggedy Range and NorthRough Ridge; and in Fig. 9, the section continues southwardsthrough the high-grade central axis of the schist belt, andacross the Torlesse/Caples boundary to lower grade schistsouth of Roxburgh.

Faults and geological domains in the Dunstan Range

The Dunstan Range is cut by several post-metamorphic faults(Fig. 12), which have juxtaposed schist domains with differentstructural and lithological characteristics. The principalfeatures, which highlight differences in schist across thesefaults, are the orientations of Lq (Fig. 8,12) and the sequencesof rock types found in each domain. The Thomsons GorgeFault, a shallow to moderately dipping normal fault zone(Fig. 12), is the most prominent post-metamorphic structurein the Dunstan Range. This fault separates the northernDunstan Range (TZ III) from geologically distinct TZ IVschists to the southwest. The Rise and Shine Shear Zone(Fig. 12) is traceable for >7 km in the immediate footwallof the Thomsons Gorge Fault, and is defined by a zone ofcrushed schist, up to 50 m wide, which has been variablysilicified and mineralised with Au and pyrite (Park 1908;Paterson 1971).

The north part of the Dunstan Range is composed ofquartzofeldspathic pelitic and psammitic schists with only rareoccurrences of greenschist horizons. Conglomeratic layersup to 10 m thick are recognisable in several places withinthis domain (Turnbull 2000). The schist of this domain lieswithin the lower greenschist facies, with no biotite or garnet(Mortimer 1993b). Spen is relatively flat lying and Lq plungesgenerally northeast and southwest. Mesoscopic folds arelocally present throughout the area and their fold axes trendnortheast, subparallel to Lq. (Fig. 12).

Rocks southwest of the Thomsons Gorge Fault arepredominantly massive psammitic schist interlayered on thedecimetre to metre scale with coarsely laminated pelitic schisthorizons. The rocks have been subjected to upper greenschistfacies metamorphism with biotite and garnet (Paterson1971; Mortimer 1993b; Craw 1998). Spen is pervasive andLq is well developed and trends generally east-west tonorthwest-southeast (Fig. 12). Spen is locally strongly folded

by Manorburn Generation mesoscopic folds with fold axesplunging to the east or southeast, subparallel to Lq.

The southern and eastern portions of the Dunstan Rangeconsist principally of massive TZ IV quartzofeldspathicschist with interlayered fissile micaceous schist. There arealso numerous (up to 5 vol.%) greenschist horizons andassociated metachert layers. Biotite and garnet are presentin some quartzofeldspathic schists (Mortimer 2000). Lq andsegregation intersection lineations plunge to the NNE andSSW except where locally rotated to southeast in zones ofManorburn Generation mesoscopic folds with axes subparallelto Lq. However, some overprinting Poolburn Generationmesoscopic folds have east- or southeast-trending axes,at a high angle to the earlier fold axes (Fig. 12). Similaroverprinting of post-metamorphic fold generations has beendescribed in the Cromwell Gorge, immediately southwest ofthe Dunstan Range (Fig. 12), by Winsor (1991).

The southern portion of the Dunstan Range is separatedfrom the central portion by the well-defined Green Valley Faultthat strikes southeast (Fig. 12) (Paterson 1971). This faultintersects a northeast-striking fault zone high on the easternslopes of the range, in the headwaters of Thomsons Creek(Fig. 12). This latter fault in turn intersects the southeasternextension of the Thomsons Gorge Fault (Fig. 12).

Blackstone Hill

The section (Fig. 6) starts north of Blackstone Hill in TZ ITorlesse Terrane metagreywacke and cuts through successivefault-bounded slices of TZ IIA, IIB, and III (Forsyth 2002).The faults between these textural zones are steeply dippingwhere seen in outcrops. The juxtaposition of textural zonespredates the middle Tertiary regional unconformity, butthe faults have been reactivated by late Cenozoic tectonics(Madin 1988).

At Blackstone Hill, the ridge is characterised by TZ IIIschist with a northeast-trending stretching lineation welldefined by quartz rodding and quartz segregation lineations(Grady 1968). In the northern part of Blackstone Hill,mesoscopic fold axes have easterly vergence and trendnortheast subparallel to Lq. Midway along the ridge, thesestructures are overprinted by a northwest-trending zone ofintense Poolburn Generation mesoscopic folding. Fold axesin this zone trend northwest with mainly southwest vergence,and symmetrical vergence in the most intensely foldedparts of the zone. The highly folded zone is bounded tothe south by a fault which separates TZ III schist withwestward-dipping foliation in the north from TZ IV schistwith eastward-dipping foliation in the south. South of thefold zone, Manorburn Generation mesoscopic fold axes trendSSW with a westerly vergence.

North Rough Ridge

North Rough Ridge is structurally similar to BlackstoneHill. The section (Fig. 6) traverses fault slices of TZ IIA andIIB schist and then cuts through TZ III schist in the ridge'snorthern end. The schist here dips to the west and containssome mesoscopic folds with axes subparallel to Lq (Fig. 6).Midway along the ridge, a zone of highly folded (PoolburnGeneration) schist, similar to the one at Blackstone Hill(above), overprints these structures. Here, fold axes in the foldzone trend NNW with a southwest to symmetrical vergence. Inareas where these folds are abundant, a crenulation cleavage is

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locally developed parallel to fold axial surfaces. The fold zoneis bounded on the west by a fault that separates TZ III schistwith westward-dipping foliation in the northeast from TZ IVschist with eastward-dipping foliation in the southwest.

To the south of this fold zone, the schist is relativelyplanar, with rare Manorburn Generation mesoscopic foldsthat have northeast-trending fold axes subparallel to Lq.Locally, however, the schist is highly folded mesoscopicallyin several northwest-trending Poolburn Generation fold zonesincluding the one at Dovedale Creek (Fig. 6, 7). These foldzones overprint, at a high angle, the Lq-parallel mesoscopicManorburn Generation structures. South of Dovedale Creek(Fig. 7) the schist continues to dip southward and the foliationis locally folded about northeast-trending mesoscopicManorburn Generation folds subparallel to Lq. At PoolburnReservoir (Fig. 9), another northwest-trending zone ofintense Poolburn Generation folding affects the schist andoverprints the northeast-trending Lq. Locally, the outcropsare highly deformed with a well-developed axial planarcleavage that overprints the predominant foliation. A similarzone of deformation occurs farther south near the UpperManorburn Dam in a zone that extends towards Alexandra(Fig. 9).

East of the Long Valley Fault

To the east of the Long Valley Fault and in the Lake Onslowregion (Fig. 9), the regional metamorphic fabric is notablydifferent from that described above. Lq trends consistentlynorthwest-southeast, at a high angle to the northeast stretchingdirection mapped in the Rough Ridge to Raggedy Range area(Fig. 6). This orientation is typical of schist in east Otago(Mortimer 1993a).

Mesoscopic Manorburn Generation fold axes on the eastside of the Long Valley Fault trend northwest with northeastvergence in the north and southwest vergence in the south(Fig. 9). A zone of highly folded schist trends northwesttowards the Upper Manorburn dam (Fig. 9). The asymmetricfolds immediately on the east side of the fault trendsubparallel to the prominent stretching direction. This closesubparallelism between Lq and mesoscopic fold axes persiststhroughout much of east Otago, and is distinctly different fromthe Poolburn Generation fold zones immediately northwestacross the Long Valley Fault (Fig. 9).

Old Man Range

South of Alexandra in the Old Man Range (Fig. 9) is a segmentof TZ IV schist bounded to the east by the Old Man Fault.This area is a mixture of psammitic and pelitic schist with asignificant greenschist component (5%) and is mapped as asouthern continuation of the Wanaka lithologic association(Craw 1984; Turnbull 2000). The schist is characterisedby a predominant NNE-SSW quartz rodding lineation andabundant Manorburn Generation mesoscopic folds with axestrending northeast. Along the eastern edge of the range, theOld Man Fault separates the highly folded Wanaka lithologicassociation from generally flat lying and planar, TZ III CaplesTerrane schist to the east.

To the south and west of the Old Man Range, the CaplesTerrane is a thin, gently southward dipping veneer, whichlies on top of the Wanaka lithologic association and iscompletely eroded from the top of the ridge (Turnbull 2000).Like the Caples rocks to the east of the Old Man Fault, the

Caples schist on the western side is TZ III and has relativelyplanar schistosity.

DISCUSSION

Regional extent of Poolburn Generation fold zones

Poolburn Generation folds are commonly, but not exclusively,found in discrete fold zones (Fig. 6, 9). A zone of intensefolding on Blackstone Hill (Fig. 6) appears to be part ofthe Poolburn Generation (above), overprinting ManorburnGeneration folds, which occur to the north and south ofthe fold zone. The relationships between these differentgenerations of structures are essentially identical to thosedescribed for the Dovedale Creek area (Fig. 6,7), although theabsolute orientations of the structural elements are differenton Blackstone Hill from those at Dovedale Creek (Fig. 6,7).Nevertheless, there is an apparent continuity of folded rocksbetween Blackstone Hill and North Rough Ridge, and bothzones coincide with the boundary between TZ III and IV(Fig. 6). Hence, these fold zones may represent a continuouspost-metamorphic structural feature.

The schist east of Alexandra is characterised by abundantmesoscopic folds with subtly different styles and differentrelative ages. These folds were first mapped by Means (1966)to define a zone of vergence change that he interpreted as theaxial trace of a large regional-scale recumbent fold. The axialtrace was interpreted by Means (1966) to run northwest fromthe Upper Manorburn Dam towards Alexandra, where it thencurved northwards to Ophir (Fig. 4). We suggest instead thatthis apparently curved fold zone represents the intersection ofregional-scale fold zones of the two different generations oflate metamorphic structures, with overprinting as describedabove and by Winsor (1991). Manorburn Generation foldsoccur in a northeast-trending vergence boundary running fromOphir to south of Alexandra (Fig. 9), with mesoscopic foldaxes trending subparallel to Lq (Fig. 9). East of Alexandra,the zone is overprinted by a later northwest-trending foldedzone of Poolburn Generation structures (Fig. 9). A northwest-trending Poolburn Generation zone runs from the UpperManorburn Dam through to Alexandra, where it overprintsearlier lineations and Manorburn Generation structures(Fig. 9). This Poolburn folded zone defines a vergenceboundary separating northeast-vergent folds in the southfrom southwest-vergent folds in the north. This highly foldedzone trends subparallel to several northwest-trending foldedzones near Ophir (Fig. 6). The northwest-trending zonesnear Ophir are aligned with similar folded zones of PoolburnGeneration in the Dunstan Range (Fig. 12). Hence, continuityof Poolburn structures across the intervening Manuherikiavalley is possible, although speculative.

Regional mapping of schist domains

Structural descriptions above, combined with lithologicalobservations, show that Central Otago is made up of areas ordomains of schist that are relatively uniform internally, butare distinctly different from neighbouring domains. Differentdomains are separated by the structural discontinuities thathave been described in previous sections. We have identifiedpost-metamorphic fold zones within some of these domains,but these fold zones appear to have little or no relativedisplacement across them. We suggest that these schistdomains constitute useful regional mapping units, as they

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show up and contrast areas of structural continuity from zonesof structural discontinuity. The principal domains outlinedin this study are shown in Fig. 8, in relation to domainsthat have been described by other workers elsewhere in theOtago Schist.

On the regional scale, we have identified three differentdomains on the Dunstan Range (Fig. 8,12). The southernmostof these contains abundant greenschist horizons interlayeredwith massive quartzofeldspathic schist. This domain isstructurally and lithologically continuous with the Wanakalithologic association (Fig. 1) mapped farther west by Craw(1984) and Turnbull (2000). The Wanaka lithologic associationcontinues farther southeast on to the Old Man Range, whereit is overlain by TZ III Caples Terrane rocks (Fig. 9). Fartherwest, the Aspiring lithologic association (Fig. 1)(Craw 1984;Turnbull 2000) separates Wanaka lithologic association fromCaples Terrane rocks. Caples Terrane TZ III schist has beenthrust on to biotite zone Aspiring rocks in the Lake Wakatipuarea (Fig. 1) (Cox 1991; Craw 1998).

A large domain of TZ III and IV Torlesse Terrane schistoccurs in the area of Blackstone Hill and Rough Ridge(Fig. 6). This domain (Fig. 8) is bounded to the north byTZ IIB Torlesse schist, and to the south by Caples Terraneschist. A structurally abrupt but poorly defined boundary inthe Manuherikia River valley separates this domain fromthe Wanaka lithologic association to the west. The easternboundary, at the Long Valley Fault, is described above. Alarge TZ IV Torlesse schist domain in east Otago (Fig. 8),immediately east of the Long Valley Fault, is bounded to thenorth by the Hyde-Macraes Fault Zone which is in TZ IIITorlesse Terrane schist. To the south of the east Otago domain,an enigmatic domain, the Chrystalls Beach Complex (Fig. 8),lies within the Caples Terrane but has some geochemicalcharacteristics of Torlesse Terrane rocks (Mortimer & Roser1992; Coombs et al. 2000), and unknown boundaries withthe higher grade east Otago schist.

Tectonic and structural evolution

The map pattern of internally homogeneous domains and thestructural discontinuities that have juxtaposed them in theOtago Schist Belt (Fig. 8) is the result of late metamorphicand post-metamorphic deformation during uplift of the belt.Compressional deformation persisted in the schist belt in theinitial stages of rock uplift in the late Jurassic, and possiblycontinued into the early Cretaceous (Gray & Foster 2004).Pervasive syn-metamorphic folding and recrystallisationgave way to more localised deformation and shear zonedevelopment during this uplift (Gray & Foster 2004). TheManorburn and Poolburn Generations of folds developed atthis time, as strain became more localised into specific foldzones, leading to mappable fold zones (Fig. 6, 7, 9). Latemetamorphic shear zones such as Hyde-Macraes, and Riseand Shine (Fig. 1,12), may be shallower level manifestationsof this stage of deformation as the schist passed through thebrittle/ductile transition (Craw et al. 1999).

Most of the structural discontinuities described aboveare related to Early Cretaceous faults and related shearzones (Deckert et al. 2002; Forster & Lister 2003; Gray &Foster 2004). In particular, extensional structures such as theThomsons Gorge Fault and the Footwall Fault of the Hyde-Macraes Shear Zone (Fig. 1, 8) have juxtaposed schists ofcontrasting textural zones after relative movement on thekilometre scale (Deckert et al. 2002). Faults at the north end

of Blackstone Hill and North Rough Ridge (Fig. 6) probablyformed at the same time with similar geometry. The regionalextension may have initiated the Green Valley Fault, CromwellGorge Fault, and Old Man Fault (Fig. 9,12). Offset on theselatter faults is relatively small, and they mainly juxtaposedifferent structural levels in a vertically inhomogeneousschist pile. Continued extension in the Cretaceous resultedin disruption of the schist belt on a rectangular northeast andnorthwest fault pattern (Fig. 1) (Mortimer 1993b; Turnbullet al. 1993). Some of these faults have juxtaposed differenttextural zones on the margin of the schist belt (Turnbull etal. 1993; Turnbull 2000; Forsyth 2002), but little disruptionoccurred in the core of the belt. The Long Valley Fault (Fig. 9)is the most significant of these later faults with respect tojuxtaposition of differing schist domains.

The Caples/Torlesse Terrane boundary (Fig. 1,8) is one ofthe most fundamental structural discontinuities in the OtagoSchist (Mortimer 1993a,b), but the nature of this boundaryis poorly known. The boundary is a ductile fault zone nearLake Wakatipu (Fig. 1) (Cox 1991; Craw 1998). This ductileboundary has been disrupted by younger brittle faults farthereast towards Alexandra (Fig. 1)(Craw 1998; Turnbull 2000).Likewise, the boundary appears to be a late stage fault thatjuxtaposes schists of differing textural zones in east Otago,and that has been further disrupted by the Long Valley Fault(Fig. 9). The nature and location of the boundary to thesoutheast of Alexandra is obscured by the strongly foldednature of those rocks at and near Manorburn and Poolburnfold zone intersections. However, it is clear that the maptrace of the Caples/Torlesse Terrane boundary (Fig. 1, 9) iscomposite, and is made up of segments defined by structuresof widely differing ages and tectonic origins.

CONCLUSIONS

The Otago Schist of Central Otago can be subdivided intodomains in which lithological and post-metamorphic structuralcharacteristics display continuity for tens of kilometres.These domains are separated by post-metamorphic faults.Syn-metamorphic quartz rodding lineations have broadlyconsistent orientations within domains, but change orientationsharply across faults between domains. Post-metamorphicfolding affected the foliation within domains, and kilometre-scale folded zones are mappable. These folded zones haveno detectable offset across them, although there has beenlocalised attenuation of marker layers. Juxtaposition of schistwith different degrees of textural reconstitution has occurredon faults associated with some folded zones. A set of thesefolds has axes subparallel to the quartz rodding lineation, andthis has been described as Manorburn Generation by earlierworkers. In addition, a later set of folds locally overprintsManorburn Generation structures, and these are describedherein as Poolburn Generation. Poolburn Generation foldaxes are typically at a high angle to Manorburn structures.The two generations of folds may be part of a continuum oflate metamorphic deformation.

The combination of structural/lithological domains andassociated discontinuities developed during late metamorphicand post-metamorphic deformation and uplift of the schistbelt in the Late Jurassic and Early Cretaceous. Pervasivecompressive ductile deformation evolved to more localisedfold zones, and possibly shear zones, in the early stages of thisuplift. Subsequent extensional tectonics caused regional-scale

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juxtaposition of domains with different textural zones, andmore localised extensional faulting juxtaposed schists fromslightly different structural levels with different structural andlithological features. The Caples/Torlesse Terrane boundary iscomposite, with different segments forming at different stagesthroughout the uplift history of the schist belt.

ACKNOWLEDGMENTS

This research was financially supported by the New Zealand PublicGood Science Fund Contract UOO813, and the Institute of Geologicaland Nuclear Sciences Ltd, generously facilitated by I. M. TurnbullandN. Mortimer. Discussions with R. J. Norris and S. C. Cox helpedus formulate some of the ideas in this paper. Geochemical data weregenerated with the able assistance of Damian Walls.

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structural and lithological continuity and discontinuity in the otago schist, central otago, new...

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