The initiation and evolution of the transpressional ...

Journal of Structural Geology 30 (2008) 410e430www.elsevier.com/locate/jsg

The initiation and evolution of the transpressional Straight Rivershear zone, central Fiordland, New Zealand

Daniel S. King a,*, Keith A. Klepeis a, Arthur G. Goldstein b,George E. Gehrels c, Geoffrey L. Clarke d

a Department of Geology, University of Vermont, Burlington, VT 05405-0122, USAb Department of Geology, Colgate University, Hamilton, NY 13346, USA

c Department of Geosciences, University of Arizona, 1040 E 4th Street, Tucson, AZ 85721, USAd School of Geosciences, F09, University of Sydney, NSW 2006, Australia

Received 12 July 2007; received in revised form 5 December 2007; accepted 5 December 2007

Available online 23 December 2007

Abstract

Structural data and U/Pb geochronology on zircon from central Fiordland, New Zealand show the role of pre-existing structural heteroge-neities in the kinematic evolution of a newly discovered zone of transpression. The Straight River shear zone consists of steep zones of highstrain that are superimposed onto older fabrics across a 10� 80 km region. The older foliation formed during two periods of tectonism: con-traction and magmatism of mostly Carboniferous (w312e306 Ma) age and Early Cretaceous batholith emplacement ending by 113.4� 1.7 Mafollowed by extension that ceased by 88.4� 1.2 Ma. The primary mechanism for the formation of steep shear zone foliations was the folding ofthese older fabrics. Conjugate crenulation cleavages associated with the folding record shortening at high angles to the shear zone boundaries.Fold axial surfaces and axial planar cleavages strike parallel to the shear zone with increasing strain as they progressively steepened to subvert-ical. In most areas, shear sense flips from oblique-sinistral (east-side-down component) to oblique-dextral (west-side-down) across zones ofintermediate and high strain. High strain zones display subvertical mineral lineations, steep strike-slip faults and shear sense indicators thatrecord strike-slip motion across the steep lineations. These patterns reflect triclinic transpression characterized by narrow zones of mostlystrike-slip deformation and wide zones of mostly contraction. Zones of high strain align with offshore traces of late Tertiary strike-slip faults,suggesting that a previously undocumented component of late Tertiary shortening and strike-slip motion is accommodated within Fiordland.� 2007 Elsevier Ltd. All rights reserved.

Keywords: Transpression; Shear zone; Strain partitioning; U/Pb geochronology

1. Introduction

Transpression refers to deformation that accommodatessimultaneous flattening and shearing and is commonly ob-served at oblique plate margins (Harland, 1971). One of thefirst three-dimensional kinematic models for transpressionwas introduced by Sanderson and Marchini (1984), andmany subsequent studies have modified this model and its

* Corresponding author. Present address: University of Minnesota, Depart-

ment of Geology and Geophysics, 310 Pillsbury Drive SE, Minneapolis,

MN 55455, USA. Tel.: þ1 612 626 0572; fax: þ1 612 625 3819.

E-mail address: [email protected] (D.S. King).

0191-8141/$ - see front matter � 2007 Elsevier Ltd. All rights reserved.

doi:10.1016/j.jsg.2007.12.004

boundary conditions to explore the complex three-dimensionalstrain patterns that are possible with transpression (e.g. Fossenand Tikoff, 1993; Jones et al., 1997, 2004; Robin and Cruden,1994; Jiang et al., 2001). These and most other mathematicalmodels generally assume homogeneous deformation withinthe block being deformed. However, zones of continentaldeformation typically are characterized by heterogeneousstrain patterns, including displacements that are distributednon-uniformly across large areas. Kinematic partitioning,where components of strike-slip and dip-slip motion occurin different places and on separate structures, is especiallycommon in zones of oblique convergence and has beendocumented in many orogens and continental transforms

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411D.S. King et al. / Journal of Structural Geology 30 (2008) 410e430

(e.g. Mount and Suppe, 1987; McCaffrey, 1992; Goodwin andWilliams, 1996; Butler et al., 1998; Norris and Cooper, 2001;Bhattacharyya and Hudleston, 2001; Claypool et al., 2002;Fuis et al., 2003; Holdsworth et al., 2002; Czeck and Hudleston,2003; Barnes et al., 2005; Sutherland et al., 2006). Previouswork has shown that the controls on kinematic partitioning,and the evolution of obliquely convergent zones in general,can include far-field plate boundary conditions (e.g. Teyssieret al., 1995; Jiang et al., 2001; Tikoff et al., 2002), rheologicalcontrasts in the crust (e.g. Coke et al., 2003; Marcotte et al.,2005), and the presence of mechanical heterogeneities suchas old faults and shear zones (e.g. Mount and Suppe, 1987;Vauchez et al., 1998; Tavarnelli et al., 2004). Determiningwhich of these factors exerts the dominant control on zonesof oblique convergence, and at which scale, is importantfor understanding how orogenic belts develop in differentsettings.

Variations in the style or degree of strike-slip partitioningcommonly occur along the strike of major fault zones suchas the Alpine fault, which has accommodated at least460 km of dextral movement since 20e25 Ma (Wellman,1953; Sutherland, 1999). Oblique-slip on the central sectionof the Alpine fault (Fig. 1a) provides an example of a systemcharacterized by a low degree of strike-slip partitioning(Norris et al., 1990; Berryman et al., 1992; Teyssier et al.,

Westland/Nelson

FiordlandOtago

AlpineFault

George Sound

0 20

N

a b

167°00'E44°30'S Milford Sound

Doubtful Sound

Caswell Sound

1

Study Areaskm

Breaksea Entrance

Tasman Sea

SI

RI

MI

Dusky Fault

Fig. 1. (a) Sketch map showing location of Fiordland and WestlandeNelson regio

Island; RI e Resolution Island; MI e Mt. Irene.

1995; Little, 1996; Norris and Cooper, 1995, 2001). Alongthis segment, the Alpine fault strikes to the northeast (55�),dips moderately to the southeast and displays a slip directionthat plunges w22�. The remaining motion is distributed onthrust and oblique-slip faults in a >100 km-wide zone locatedmostly east of the Alpine fault. In contrast, the Fiordland(southernmost) segment of the Alpine fault is nearly verticaland accommodates almost pure strike-slip motion (Barneset al., 2001, 2005). Folds and reverse faults west and east ofthe Alpine fault accommodate contraction, indicating thatthis part of the Alpine fault system is strike-slip partitioned(Norris et al., 1990; Markley and Norris, 1999; Claypoolet al., 2002; Barnes et al., 2001, 2005). However, the amountof shortening accommodated by these structures has beendifficult to quantify (Norris and Cooper, 2001).

In this paper, we examine how lithologic heterogeneity andpre-existing mechanical anisotropies controlled the initiationand evolution of a large transpressional shear zone of apparentlate Tertiary age in Fiordland, New Zealand. The StraightRiver shear zone (Fig. 1), which is located southeast of thesouthernmost segment of the Alpine fault, was first identifiedby Oliver and Coggon (1979) as the Straight River [thrust]fault and is interpreted here as a transpressional structure onthe basis of new data. This reinterpretation shows the existenceof a previously undocumented component of contraction

e

Lake Manapouri

Lake TeAnau

Early Mesozoic Arc Rocks

Paleozoic Basement

Structures

Western Fiordland Orthogneiss (WFO) & Breaksea Gneiss (Early Cretaceous)

Undifferentiated metasediment andgneiss (Cambrian-Permian)

Mafic-intermediate orthogneiss(Triassic-Early Cretaceous intrusiveswith some Paleozoic host rock)

Plutonic and volcanosedimentaryrock of the MedianTectonic Zone(Triassic-Early Cretaceous)

Anita Shear Zone

68°00'E46°00'S

A piF u

ln

alt

Thrust fault

Extensional Shear Zone,includes Doubtful Sound andResolution Island shear zones

Straight River Shear Zone

ns. (b) Geological map of Fiordland (after Bradshaw, 1990). SI e Secretary

412 D.S. King et al. / Journal of Structural Geology 30 (2008) 410e430

accommodated in Fiordland in the early stages of the forma-tion of the Alpine fault. An especially interesting aspect ofour study is the analysis of outcrops that record the progressivedevelopment of a heterogeneous array of structures that recorddifferent combinations of contraction and strike-slip deforma-tion. In particular, a progressive tightening of asymmetricfolds and the rotation of fold axial planes during shorteningat high angles to the shear zone boundaries controlled thedevelopment of steep foliation planes, down-dip and obliquemineral lineations, and other structures commonly viewed asdiagnostic of transpression. This field-based model illustratesa mechanism for the formation of vertical structures in rockswith a pre-existing foliation. Our results provide informationon the types and scales of anisotropies that influence theformation and evolution of ductile structures in a newly dis-covered zone of transpression that affects at least 800 km2 ofcrust. This style of transpressional deformation is character-ized by smaller-scale partitioning of deformation than thedocumented partitioning between the Alpine fault systemand offshore fold-thrust belts.

2. Regional tectonic history of Fiordland

Central Fiordland (Fig. 1) records a history of orogenesisthat occurred between w481 and w334 Ma (Oliver, 1980;Gibson et al., 1988; Ireland and Gibson, 1998). Crustal thick-ening is indicated by kyanite-bearing assemblages (recordingpressures of 7e9 kb) that overprint higher-temperature andlower pressure (P¼ 3e5 kb) sillimanite-bearing assemblagesin Paleozoic gneiss (Gibson, 1990). The relatively high-T,low-P event occurred at w360 Ma (Ireland and Gibson,1998; Gibson and Ireland, 1999), the high-P phase of metamor-phism (7e9 kb) occurred at w330 Ma (Ireland and Gibson,1998).

Superimposed on these Paleozoic metamorphic rocks isa record of Early Cretaceous arc magmatism, upper amphibo-lite to granulite facies metamorphism, and contraction (Oliver,1977; Bradshaw, 1989; Gibson and Ireland, 1995; Mortimeret al., 1999; Daczko et al., 2001; Klepeis et al., 2004). A majorbatholith was emplaced into the middle and the lower crustfrom w126 to w113 Ma and subsequently deformed toform the Western Fiordland Orthogneiss (Mattinson et al.,1986; McCulloch et al., 1987; Gibson et al., 1988; Gibsonand Ireland, 1995; Tulloch and Kimbrough, 2003; Holliset al., 2004). Similar intrusions comprise the protolith of theBreaksea Gneiss (Fig. 1). Mineral assemblages indicate thatmetamorphism at pressures of at least 12e14 kb occurred inboth batholiths (Oliver, 1977; Bradshaw, 1989; Clarke et al.,2000; Daczko et al., 2001).

By w114 Ma western New Zealand was dominated byextension and crustal thinning (Tulloch and Kimbrough,1989; Spell et al., 2000). The extensional Doubtful Soundand Resolution Island shear zones formed after w111 Ma (Ol-iver and Coggon, 1979; Oliver, 1980; Gibson and Ireland,1995; Ireland and Gibson, 1998; Klepeis et al., 2007).

Exhumation of rocks representative of Early Cretaceousmiddle and lower crust occurred mostly during the Mid-Late

Cretaceous as a result of extension (Flowers et al., 2005;Klepeis et al., 2007). Metamorphic core complexes formedduring this period (Tulloch and Kimbrough, 1989; Alliboneand Tulloch, 1997; Spell et al., 2000; Turnbull and Allibone,2003; Kula et al., 2005; Klepeis et al., 2007). The extensionculminated in the rifting of New Zealand from Australia andAntarctica and the initiation of seafloor spreading in the Tas-man Sea by w84 Ma (Gaina et al., 1998; Kula et al., 2005).Exhumation following Late Cretaceous extension was slowuntil the onset of late Cenozoic transpression (House et al.,2002). In northern Fiordland, this exhumation history is re-corded by superposed structures in the Anita shear zone(Fig. 1; Hill, 1995; Klepeis et al., 1999).

By w52 Ma, sea floor spreading in the Tasman Sea hadended (Gaina et al., 1998). The PacificeAustralia plate bound-ary developed as a spreading center south of New Zealand by47e45 Ma (Sutherland et al., 2000). From 30 to 11 Ma,spreading became progressively more oblique and the bound-ary evolved into a transform (Lamarche et al., 1997). Subduc-tion beneath Fiordland and formation of the Alpine fault beganas early as 25e20 Ma (Lebrun et al., 2003; Lamarche andLebrun, 2000).

3. Structural and kinematic analysis

In this section we describe structures from the DoubtfulSound region (Fig. 1) where we studied the SRSZ and olderstructures in detail. We compare these elements to those ex-posed at Breaksea Entrance to evaluate their along-strikevariability. The analysis of Paleozoic and Mesozoic structuresallows us to determine how older structures influenced theevolution of subsequent phases of deformation. The DoubtfulSound region is characterized by three structural domains(Fig. 2). Domain I preserves structures associated with Paleo-zoic deformation, granitic magmatism, and metamorphism.Domain II preserves structures and metamorphic mineralassemblages associated with Early Cretaceous magmatismand mid-Cretaceous extension. Domain III preserves struc-tures associated with the formation of the younger SRSZ.

3.1. Paleozoic rock units and structures (domain I)

Domain I (Fig. 2a) contains schist, paragneiss, and orthog-neiss of the Deep Cove Gneiss. At the west end of DoubtfulSound, and on Secretary Island (Fig. 3), the thickest unit isa micaceous meta-arenite. Garnet amphibolite, calc-silicate,and marble layers are also present. In most areas of domainI, bedding (S0) is overprinted by deformation and upper am-phibolite facies metamorphism. However, near the southeastheadland of Secretary Island we observed relict cross-bedding(Fig. 4a) and graded beds in tightly folded (F1) meta-arenitelayers (Fig. 4b). The folds are overturned and recumbent,and display an axial planar foliation (S1) defined by biotitein schist and hornblendeþ plagioclase aggregates in garnetamphibolite units. Aligned biotite and hornblende define min-eral elongation lineations (L1) in these units, respectively(Fig. 4b, inset).

Febrero Pt.Secretary Is.

Cascada BayWestern Bauza Is.

ThompsonSoundFirst Arm

BradshawSound

CrookedArm

Description/Assemblage

S0

S1,L1

Granitoid/AmphiboliteSheets

F2 RecumbentFolds

Intr

usiv

es a

nd

Ext

ensi

on(M

esoz

oic)

SRSZ

Str

uctu

res

(Mes

ozoi

c/C

enoz

oic)

Bri

ttle

Faul

ting

(Ter

tiary

)

SWFO

GRZ veins

SGRZ

SDS, LDS, FDS

Secretery Is.Syncline

FSR

Scr1,Scr2

SSR,LSR

Faults

Fractures

NW SE

Domain I Domain II Domain III

biotite

graded bedding

biot, musc,calcite,amph

biotite

amph +/- cpx +/- biot

garnet, cpx

biot, amph, qtz

epidote faultsurfaces

0 1 km

Thompson Sound

Bradshaw Sound

First A

rm

Febrero Point

Casca

da B

ay

Secretary Is. (SE)

Dom

ain

III

Dom

ain

I

Domain I

Domain II

Domain II

Domain II

Crooked Arm

Secr

etar

y Is

.(NW

)

?

Bauza Is.

a

b

167°

00’E

166°

50’E

45°20’ S

45°15’ S

45°20’ S

45°15’ S

167°

00’E

166°

50’E

N

unmapped

Stru

ctur

es W

ithin

M

etas

edim

ents

(P

aleo

zoic

)

Fig. 2. (a) Map showing structural domains surrounding Secretary Island. (b) Fabric elements associated with each domain. See text for definitions.


At least two generations of granitic dikes intrude the DeepCove gneiss. The first cuts the L1eS1 fabric and is folded intotight inclined-recumbent folds (F2) (Fig. 4c). The folds displayan axial planar foliation (S2). The second generation of dikes

cuts the first and forms thick (several meters), weakly de-formed tabular bodies (Fig. 4d). One of the largest of these lat-ter bodies is the Deas Cove Granite (Fig. 3), which yielded anRb/Sr whole-rock age of 372� 12 Ma (Oliver, 1980). These

36

40 32

42

33

15

29

27 54

60

242370

2187

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25

87

32

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79

16

7240

8673

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6856

5048

50

26

59 12

63

47

76

78

53

20

65

4226

20

86

60

31

33

53

57

580866

1822

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3846

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36

68

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5515

60

85 90

78

155725

86

76

2668

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60

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4469 90

8050

35

47

47

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60

86

39

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6467

68

54

61

5540

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22

2410

13

32

1012

05

25

13

05

30

24

45

26 82

35

50

20

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65

43

36

1025

04

25

14

24

40

40

A

A‘

B

B‘

04-53

04-60

Doubtful Sound

Thom

pson Sound

Bradshaw Sound

First A

rm

North

Crooked Arm

Bauza Island

Cascada B ay

Febrero Point

05-135

Geologic Symbols

LWFO (mineral)

LSR (mineral)

L1 (mineral)

LDS (mineral)

Lcr (intersection)

45

45

45

Foliations

Lineations

Contacts

Fold A xes

Dashed where inferred

SWFO

S1

SDS

SSR

45

Other data sourcesOliver, 1980Claypool et al., 2002Wood, 1960

Plunging antiform Plunging synform

Inferred synformunknown plunge

Faults and Shear Zones

Straight Rivershear zone

(SRSZ)

Doubtful Soundshear zone

(DSSZ)

dashed where inferredbars on downthrown side and sense

of offset are shown where known

ductile shear zones

0 1 2 3Kilometers

DeepCoveGneiss:

Western Fiordland Orthogneiss166º

50’

E

55’

E

167º

00’

E

Deas Cove granite/ undifferentiated granitoid

Garnet amphibolite

Hornblende-epidote gneiss

Quartz-biotite schist and gneiss (meta-arenite) intruded by granitic dikes

Garnet, mica sillimanite schist

Garnet-mica schist

Quartz-plagioclase gneiss,calc-gneiss and schist

Marble

Lithologic Units

Location of profilesin Fig. 7

45º 20’ S

45º 15 ’ S

45º 20 ’ S

05’

E

167º

00’

E

166º

50’

E

45º15’ S

DSSZ

DSS

Z

DSS

Z

DSSZDSS

Z

SRSZ

SRSZ

SRSZ

brittle faultticks on downthrown side

40

30

SecretaryIsland

32

2819

60

69

85

18

20

38

10

14 62

Fig. 3. Geological map showing major structures and lithologic units in Doubtful, Thompson, and Bradshaw Sounds. Maps of the interior of Secretary Island were

completed by I. Turnbull and others at the Institute for Geological and Nuclear Sciences in Dunedin. Cross-sections AeA0 and BeB0 are shown in Fig. 5. Samples

locations are indicated (Wood, 1960).


observations show that the regional fabric in domain I formedas mid-Paleozoic granitoids intruded a thick sequence of vol-canic and sedimentary rocks.

3.2. Mesozoic intrusive rock and extensional structures(domain II)

The Western Fiordland Orthogneiss contains several distinc-tive mineral assemblages, including garnet- and clinopyroxene-

bearing veins that record garnet granulite facies metamorphismat depths >45 km (Oliver, 1977, 1980; Gibson and Ireland,1995; Hollis et al., 2004). These garnet granulite assemblagesare recrystallized within the retrogressive, upper amphibolitefacies Doubtful Sound shear zone (Figs. 3 and 5e, f). Klepeiset al. (2007) showed that these superposed assemblages recorda progressive change in temperature, pressure, and fluid condi-tions in the lower crust and create a regional gneissic layeringthat characterizes domain II (Figs. 2 and 5feh).

Fig. 4. Structures exposed along Thompson Sound that predate the SRSZ. (a) Overturned cross-bedding (S0) in meta-arenite. (b) Folded (F1) meta-arenite layer

in garnet (gt)þ clinozoisite (cz) schist with an axial planar cleavage (S1). Inset shows L1 mineral lineations and the average S1 cleavage orientation plotted on

an equal area lower hemisphere projection. (c) First generation of granitic dikes are folded into tight F2 folds. (d) Second generation of dikes are weakly deformed

and of mostly Carboniferous age (Fig. 11g).


The Doubtful Sound shear zone is composed of folded, at-tenuated and locally mylonitic layers of marble, calc-silicategneiss and orthogneiss. The orientation of high strain zonesis variable. Along Doubtful Sound they dip 20e30� to theeastenortheast (Fig. 5b, f, h). Top-down-to-the-northeast and-southwest sense-of-shear indicators are consistent with inter-pretations of the shear zone as a ductile normal fault (Gibsonet al., 1988; Oliver, 1990; Klepeis et al., 2007).

On Resolution Island, (within the southernmost study site,Fig. 6) the Cretaceous Breaksea Gneiss is composed primarilyof granulite and, locally, eclogite facies metagabbro and meta-diorite that is similar to, but more mafic than, the metagabbroof the Western Fiordland Orthogneiss. Unretrogressed parts ofthis unit have yielded peak metamorphic pressures rangingfrom 11 kbar to at least 15e16 kbar (Allibone et al., 2005;Milan et al., 2005; M. De Paoli, personal communication,2007), the highest pressures recorded in Fiordland. Like theDoubtful Sound shear zone, the Resolution Island shear zone(Fig. 6) is defined by heterogeneous upper amphibolite facieshigh strain zones that form a regional flat to moderately dip-ping foliation (Klepeis et al., 2007).

3.3. The Straight River shear zone (domain III)

The Straight River shear zone (SRSZ) is a new name givento an array of steep high strain zones between ThompsonSound and Resolution Island (Figs. 3 and 7a). Oliver (1980)suggested that the part of this structure south of DoubtfulSound (the Straight River fault) was a major thrust fault thatparallels the Doubtful Sound shear zone. We present newdata that show the regional extent of the shear zone, its kine-matic evolution, and its crosscutting relationship with theDoubtful Sound shear zone (Fig. 5). We completed fourdetailed transects across the shear zone and developed criteriafor defining low, medium and high strain zones that qualita-tively define the gradational boundaries of the SRSZ (Figs. 7and 9).

3.3.1. Thompson Sound transectAlong Thompson Sound, the SRSZ lies entirely within Pa-

leozoic meta-arenite that is intruded by minor granitoids(Fig. 7b). This transect is important because the relative uni-formity of the meta-arenite lithology (Fig. 7b) allowed us to

B‘

?

0

400800

-400-800

2 4 6 8 10 12 14 16

0

400800

1200

-400-800

A A'

??

2 4 6 8 10 12 14 16

B

Cascada Bay

Bauza Island Thompson Soundn=7

n=19 n=29

Joseph Point Secretary Island Synform Bradshaw Sound

Poles to SDS (n=13):(with best fit great circle)

Mineral LDS (n=10):

Poles to SDS (n=10):(with best fit great circle)

Mineral LDS (n=10):

Pole to S1 (n=43):

Fold axis:

N

N N

N N N

SRSZDSSZ

(a)

(b)

(c) (d)

(e)

(f) (g) (h)

SSR

SSR

SSR

Fig. 5. (a) Cross-section AeA0 (location in Fig. 3) showing the SSR cutting the Doubtful Sound shear zone. (b), (c), (d) Poles to SSR foliation (circles) in the SRSZ

with best fit great circle at the locations of three detailed sketches shown in Fig. 7. Plots are lower hemisphere, equal area projections. White squares are quartzebiotite mineral lineations (LSR). (e) Section BeB0 showing antithetic/synthetic pair of extensional shear zones at Doubtful and Bradshaw sounds. (f) Poles to SDS

foliation (circles) with best fit great circle and LDS (squares). (g) p-diagram of poles to S1 defining a F2 fold axis. (h) Poles to SDS foliation (circles) with best fit

great circle and LDS (squares) at Bradshaw Sound.


rule out the effects of different lithologies as a cause of struc-tural variability. At northern and southern boundaries of theshear zone (Fig. 3), pre-existing S1 foliation is cut by a steepshear zone foliation that dips steeply away from the center ofthe shear zone (SSR). The outer w1 km of domain III atThompson Sound (Fig. 2a) is characterized by zones up to sev-eral meters wide composed of steep shear zone foliationplanes. Pre-existing S1 foliation planes are deflected, foldedand transposed into the SSR shear zone foliation in these zones.Between these discrete zones the fabric is dominated by moregently dipping pre-existing S1 foliation planes. This pattern ofdeformation allowed us to determine the orientation of theboundaries of high strain zones.

In the center of domain III, all Paleozoic structures areoverprinted by SRSZ structures. Fig. 9a shows a monocline

exposed in a low strain zone northwest of the center high strainzone along Thompson Sound. Compositional layering isfolded into monoclines and small-scale asymmetric foldsthat display northwest dipping axial planes. The oppositeasymmetry (i.e. southeast-dipping axial planes of monoclines)occurs on the southeast side of the SRSZ on Bauza Island(Fig. 7c). The orientations of folded pre-existing S1 foliationplanes (Fig. 9b) from within a w50 m section of folds displaya southwest-plunging calculated fold axis that coincides withmeasured fold hinges. The axial planes dip to the northwest.We interpret these monoclines to characterize a low strainzone that records an early stage in the evolution of shearzone folds.

Crenulation cleavage occurs throughout the transect (Figs.8c and 9c). The cleavage is best developed in the micaceous

661E’63

45 32’ N

N

1 k m

66

52

40

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71

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28 85

70

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70

4546 73

28

05

2575

6558

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8250

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67

ResolutionIsland

Breaksea Entrance

Breaksea Sound

location of profile in Fig. 7e

Stra

ight

Riv

ersh

ear

zone

Undifferentiated Deep Cove Gneiss with marble layers (Paleozoic)

Undifferentiated Breaksea and Resolution gneiss (Early Cretaceous)

Western Fiordland Orthogneiss (Early Cretaceous)

Lithologic Units

45

45

45

SWFO

S1

SBRG

SSR

45

LWFO (miner al)

LSR (miner al)

Resol. I

s.sh

ear zone

Fig. 6. Simplified geological map of Breaksea Entrance showing the southern strand of the SRSZ. Geologic symbols are the same as in Fig. 3 except foliation with

white boxes, which represents part of the extensional Resolution Island shear zone. Map of Resolution Island combines data collected along the coastlines by the

authors and data from the interior of the island from Turnbull et al. (2005), Allibone et al. (2005) and Milan et al. (2005).


meta-arenite layers where dominant crenulation cleavageplanes (Scr1) strike w110� and dip w65� to the southwest.The subordinate crenulation cleavage planes (Scr2) strikew48� and dip w80� to the northwest (Fig. 8e). The twosets always occur together and form angles of 112� and 68�

with one another. These zones, with two well-developed setsof crenulation cleavage and relatively tight, inclined folds, de-fine our intermediate strain zones (Fig. 9c, f).

Within high strain zones at the center of the zone of defor-mation, Scr2 is more pronounced than it is elsewhere and Scr1 isless defined if present at all. The center of the SRSZ is definedby zones where pre-existing foliation is completely transposedinto a steep shear zone foliation that is parallel to the Scr2

plane. Shear zone fold hinges are nearly vertical and foldsare significantly tighter than the intermediate strain zones oneither side of the high strain zone. Between these high strainzones of steeply dipping shear zone foliation and isoclinalfolds are intermediate strain zones where both crenulationcleavage planes are visible. In the transition zones near shearzone boundaries, both crenulation cleavage orientations coex-ist and pre-existing S1 is folded into recumbent and monocli-nal shear zone folds with axial planes that dip away from thecenter of the zone of deformation.

3.3.2. Cascada Bay transectLocated 2 km inland from the Tasman Sea (Fig. 3), Cascada

Bay (Fig. 3) displays evidence for the localization of strainwithin a marble-rich sequence and a strain gradient definedby variations in fold tightness and a decrease in the degreeof transposition toward the center of the shear zone. East of

the bay, the Doubtful Sound shear zone foliation (SDS) withinthe Western Fiordland Orthogneiss is folded into broad, openSRSZ folds (FSR) (Fig. 7d). Close to the contact between theorthogneiss and the marble-rich sequence (Fig. 7d), folds aretight and the steep shear zones are common, defining a qualita-tive increase in strain to the west toward Cascada Bay at highangles to the SRSZ boundaries. Structures exposed at CascadaBay indicate that strain is focused into a w250 m-wide zonewithin the marble-rich sequence. In low and intermediatestrain zones, SRSZ fold axial planes dip to the SW and NEin recumbent and inclined monoclinal folds (Fig. 9d). In zonesthat record higher strain closer to the center of the shear zone,SRSZ fold axial planes are nearly vertical and are subparallelto the SRSZ foliation (Fig. 7d). Fold hinges plunge w80� tothe northenorthwest. The contact between the marble-rich se-quence and the calc-silicate gneiss to the west is sheared anddisplays the subvertical SRSZ foliation. Farther west, thepre-existing S1 foliation is folded into SRSZ folds and locallytransposed into the shear zone foliation. These folds graduallyopen up to the west defining a gradational boundary betweendomains III and I (Fig. 2a).

3.3.3. Straight River shear zone structures in the WesternFiordland Orthogneiss

Structures associated with the SRSZ in the Western Fiord-land Orthogneiss occur up to w6 km east of the high strainzone at Cascada Bay (Fig. 10). The Doubtful Sound shearzone foliation (SDS) is folded and transposed into the steepSRSZ foliation (SSR) in 10 m-wide high strain zones (e.g.Fig. 9e) that are spaced a few hundred meters to several

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Fig. 7. (a) Simplified map showing the known extent of the SRSZ and the locations of four profiles at (b) Thompson Sound (c) Bauza Island (d) Cascada Bay, and (e) Resolution Island. Map patterns are Paleozoic

gneiss (shaded grey) and Early Cretaceous Western Fiordland Orthogneiss and Breaksea Gneiss (crosses). Profiles show orientations of fabric elements (S1, Scr1, Scr2, FSR, and SSR) defined in text. Shaded areas in

(b) are granitic dikes. Other than the dikes, this transect is composed of a relatively uniform meta-arenite lithology. The other three transects contain multiple lithologies.

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Fig. 8. Evolution of SRSZ structures in thin section. (a) Meta-arenite outside the shear zone at Thompson Sound. (b) C0eS fabric within high strain zone showing

an oblique-dextral component of motion. Thin section surface parallels lineation and is perpendicular to foliation. (c) Conjugate crenulation cleavages (Scr1, Scr2) in

a low-intermediate strain zone, indicating a component of shortening across the shear zone. This section was cut perpendicular to the Scr2 foliation marked in the

figure (orientation: 065 80 SE). (d) Conjugate semi-brittle faults indicating a component of shortening across the shear zone at Resolution Island. This section was

cut perpendicular to the shear zone foliation marked in the figure (orientation: 047 85 NW). Lower hemisphere, equal area projection showing the two orientations

of conjugate crenulation cleavages (e), conjugate faults (f) and mineral lineations, including quartz slickenfibers (f). (g) Block diagram summarizing the relation-

ship among faults.


Fig. 9. Structures observed along two NWeSE transects of the SRSZ at Thompson and Doubtful sounds. (a) Photograph of an asymmetric monocline (FSR) in zone

of low strain at edge of the shear zone at Thompson Sound. (b) Lower hemisphere, equal area projection showing measurements of FSR axial planes and S1, and L1

structures associated with the monocline in (a). (c) Photograph of asymmetric, inclined FSR fold and crenulation cleavage in zone of low-intermediate strain. (d)

Photograph of inclined asymmetric folds (FSR) in marble sequence in zone if intermediate strain at Cascada Bay (location in Fig. 3). Inset sketch shows SE-dipping

axial plane. (e) Sketch showing vertical profile of oblique-dextral, east-side-down transpressional shear zone. Shear zone deforms older foliation of the Doubtful

Sound shear zone (SDS). (f) Summary sketches taken from the profiles shown in Fig. 7 illustrating the criteria that define zones of relative low, intermediate, and

high strain that define the SRSZ. (g) Plot showing changes in the strike (black diamonds) and dip (squares) of shear zone cleavage with increasing strain. Note 20e35� change in cleavage dip between low and high strain domains. In contrast the average strike of cleavage in these same areas remains approximately parallel to

the shear zone boundaries, with a few exceptions. (h) Plot showing changes in the pitch of mineral lineations (diamonds) and fold axes (squares) with increasing

strain. Data are from NE-striking shear zones exposed along Thompson and Doubtful sounds. See text for discussion.

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Fig. 10. (a) Perspective diagram showing the steepening of fold axial surfaces and the increase in the pitch of fold axes with increasing strain. Black plane rep-

resents a strike-slip fault. Note that intermediate strain zones record evidence of both contractional and strike-slip components of motion. See text for discussion.

(b) Map showing the location and kinematics of steep zones of the SRSZ. The hanging wall is shaded grey in each equal area, lower hemisphere projection. The

arrow shows the direction of motion of the upper-plate (shaded) relative to the lower-plate as indicated by measured mineral lineations. Great circles are measured

orientations of shear zone boundaries. Note that the group to the east of First Arm has an east-side-down component of motion with sinistral slip, and the group to

the west has west-side-down component of motion with dextral slip.


kilometers apart. Between these shear zones, the dominantfabric elements are a gently northeast-dipping Doubtful Soundshear zone foliation and a northeast-plunging Doubtful Soundshear zone hornblende lineation. This crosscutting relationshipis evidence that the SRSZ is younger and unrelated to the de-formation that produced the Doubtful Sound shear zone. Thepresence of these structures also shows that, whereas we ob-serve SRSZ structures across a wide zone, the deformationis expressed differently in the various lithologies. The morecompetent dioritic orthogneiss accommodates strain in rela-tively narrow, widely separated shear zones across several ki-lometers (Fig. 10), whereas strain is highly localized intoa w250 m-wide zone within the less competent marble se-quence at Cascada Bay.

3.3.4. Bauza Island transectBauza Island, located w2 km northeast of Cascada Bay

(Fig. 3) displays features that are similar to those at CascadaBay. Deformation is localized within a w500 m-thick layerof highly strained and brecciated marble that lies betweenthe Western Fiordland Orthogneiss with Doubtful Sound shear

zone foliation and Paleozoic gneiss with pre-existing S1 folia-tion (Fig. 7c). The Doubtful Sound shear zone foliation withinthe orthogneiss varies from dipping shallowly to the northeastto shallowly to the northwest defining broad, open SRSZ folds.We interpret this as a low strain zone showing evidence for theearly stages of the SRSZ overprinting the Doubtful Soundshear zone.

As at Cascada Bay, structures related to the SRSZ (FSR,SSR, LSR) are best developed within the marble layer. Thepre-existing S1 foliation in the marble-rich sequence is foldedinto SRSZ folds. To the east of the area shown in the profileare a series of SRSZ monoclines and recumbent folds witheastesoutheast-dipping axial planes (e.g. Fig. 7c). High strainzones of SRSZ shear zone foliation overprint the folds. Thecontact between the marble sequence and the calc-silicategneiss to the west is highly sheared. Angular fragments ofthe competent gneiss are suspended within a highly shearedmarble matrix.

Farther west, the SRSZ folds in the calc-silicate gneissgradually becomes open and axial planes dip moderately tothe northwest. Measurements of the S1 foliation around a large


recumbent synform w500 m west of the marble sequence de-fine an axis that plunges shallowly to the northeast with an in-terlimb angle of w38�. The transition from upright, isoclinalfolding near the contact (Fig. 7c) to more open folds with axialplanes that dip to the NW qualitatively suggest a gradual de-crease in strain away from the marble sequence where mostof the strain is localized.

3.3.5. Resolution Island transectOn Resolution Island and the north shore of Breaksea En-

trance (Fig. 6), the high strain parts of the SRSZ are definedby steep semi-brittle fault zones and a steeply dipping, locallymylonitic greenschist facies foliation that has overprinted allolder foliations, including the low-angle extensional Resolu-tion Island shear zone. Quartz forms dynamically recrystal-lized ribbons and plagioclase displays brittle microfaults andasymmetric pressure shadows of recrystallized quartz and bi-otite (Fig. 8d). Whereas the dominant foliation is consistentlysteep within the high strain zones of the SRSZ, biotite andquartz stretching lineations are variably oriented, in part dueto the complexity of fault orientations. Nevertheless, two dis-tinctive groups of orientations are recognizable. Quartzebiotite mineral stretching lineations plunge variably to thesouthwest, west and the northeast. Within mylonite zones ofthe SRSZ on Resolution Island, hornblende and plagioclasemineral stretching lineations plunge steeply to the northeastand southwest suggesting that, locally, the SRSZ may recordamphibolite facies conditions. These two groups of lineationsthat record different metamorphic grades are indicative of sig-nificant exhumation during deformation.

Like the other transects, the geometry of the greenschist fa-cies SRSZ foliation is variable across the shear zone (Fig. 7e).On Resolution Island (Fig. 6), the SRSZ separates the Paleo-zoic host rock to the west from the Western Fiordland Orthog-neiss to the east. Along w1 km of shoreline the orthogneiss isinterlayered with the metasedimentary host rock of probablePaleozoic age. High strain zones are localized within marbleand biotite-rich metasedimentary layers. Within these zones,the foliation dips more steeply to the west than outside thesezones. To the west of the center of the shear zone the SRSZfoliation dips gently to the west. To the east, a gneissic folia-tion within the orthogneiss dips variably to the east, northeastand northwest (Fig. 6) and is folded and transposed into theshear zone foliation close to the center of the SRSZ.

Crosscutting the mylonitic zone are networks of brittle andsemi-brittle faults that form several distinctive sets. Two ofthe most prominent sets form near vertical conjugate pairsof minor sinistral and dextral strike-slip faults that strike athigh angles to the dominant mylonitic fabric (R1, R2, respec-tively; Fig. 8d, f, g). Another set of minor thrust faults dipsgently to the east and west. Both the conjugate faults andthe gently dipping thrusts record contraction at high anglesto the shear zone boundaries (Fig. 8f, g). Within and nearsteep high strain zones, two sets of crenulation cleavage andsemi-brittle shear bands overprint the foliation. Here, greens-chist facies fault zones deform mylonitic foliation planes intoasymmetric lozenges. A well-developed steeply dipping to

subvertical set of crenulation cleavage and semi-brittle faultzones also overprints older foliation planes. Adjacent to thefault zones, hornblende-bearing fabrics are retrogressed tochlorite-, epidote- and biotite-bearing assemblages. In thesefault zones, biotite and quartz mineral lineations plungegently to the northeast on shear band foliation planes. Boudin-aged amphibolite layers and pegmatites indicate that thesedirections are true stretching directions.

3.3.6. KinematicsChanges in the geometry of structures across strain do-

mains (defined in Section 3.3 and Figs. 7 and 9) allowed usto interpret the kinematics of deformation within the SRSZ.Low strain domains generally lack shear zone mineral linea-tions (LSR) and display asymmetric monoclines. In areas ofmica schist, they also are characterized by two crenulationcleavages (Scr1, Scr2). The geometry of these cleavages indi-cates a component of shortening at high angles to the shearzone boundaries (Fig. 8e). Experimental studies of crenulationcleavage show that conjugate sets form with the axis of max-imum principle compression (s1) bisecting a w110� angle be-tween the two cleavage planes (Zheng et al., 2004 andreferences therein). From this relationship, we infer that s1

was oriented northwestesoutheast at a high angle to the shearzone boundaries at Thompson Sound (Fig. 8e). In support ofthis interpretation, the orientation of the conjugate dextraland sinistral faults, as well as thrust faults on Resolution Is-land, also indicate compression at high angles to the shearzone boundaries (Fig. 8eeg).

In general, sense-of-shear indicators are poorly preserved inlow and intermediate strain domains. Where present, they sug-gest complex three-dimensional displacements. In low strainareas, asymmetric crenulation cleavages and biotite fishshow maximum asymmetry on surfaces viewed perpendicularto foliation and perpendicular to fold axes and crenulation lin-eations. The asymmetries indicate that opposite senses ofshear occur across fold axes: a top-down-to-the-southeastsense-of-shear occurs on the east side of antiforms anda top-down-to-the-northwest sense-of-shear on the west sideof antiforms. This pattern, and evidence of thickening in thehinges of folds, suggests that flexural flow during NWeSEshortening resulted in folds that evolved into steep high strainzones.

Variations in cleavage orientation and fold geometries pro-vide additional information on the importance of shortening.As finite strain increases across domains, fold axial surfaces(FSR) and the dominant crenulation cleavage (Scr2) rotatefrom moderately dipping (�60�) to subvertical (80e90�)(Fig. 9g). In contrast, the strikes of these structures maintainan approximately constant orientation that parallels the steepboundaries of the shear zone. These patterns suggest thatshortening was the dominant control on the evolution ofshear zone folds and foliations in most places. Nevertheless,in a few intermediate strain zones, fold axes and cleavagestrikes are oriented 20e30� from the strike of the shearzone (Fig. 9g), suggesting that the contribution of strike-slip motion also is discernable.


In intermediate strain zones where quartzebiotite minerallineations are present, shear bands (C0) record the greatestasymmetry on surfaces that parallel the lineation and are per-pendicular to foliation. The gentle-moderate plunge (20e30�)of these mineral lineations (LSR, Fig. 9h) and the kinematic in-dicators show that both reverse and strike-slip components oc-curred in these domains. For example Fig. 8d shows a C0eSfabric in a meta-arenite from Thompson Sound that recordsa top-down-to-the-northwest, oblique-dextral sense-of-shearparallel to the mineral lineation. Similar displacements occurat Cascada Bay (Fig. 3). West of this bay, shear senses flipback and forth along the south shore of Doubtful Sound ina manner that is similar to that which occurs on the oppositesides of fold axes in low strain zones at Thompson Sound(Fig. 10b). West of First Arm, shear sense indicators show ob-lique-dextral shear with a component of west-side-down nor-mal motion (Fig. 10b). East of First Arm, they showoblique-sinistral shear with a component of east-side-downnormal motion. These alternating oblique shear senses areconsistent with a style of non-coaxial contraction involvingNWeSE shortening and a subordinate component of strike-slip deformation. Similar patterns of oblique-slip associatedwith flexural slip folds have been described in other orogenswhere they have been interpreted to be related to triclinictranspression (Holdsworth et al., 2002).

Shear sense indicators and changes in the pitch of minerallineations and fold axes with increasing strain also show thatcombinations of contraction and strike-slip deformation occurin high strain domains (Fig. 9h). Fold axes display gentle andmoderate pitches on axial surfaces in areas of low strain. In in-termediate strain domains, quartzebiotite mineral lineationsdisplay moderate pitches on cleavage (SSR) planes(Fig. 10a). The pitches of fold axes in these latter domainsare variable, reflecting the two sets of crenulation cleavage.One set parallels the mineral lineations, the other is obliqueto this lineation. In high strain domains, mineral lineations dis-play moderate (50e60�) to steep (80�) pitches, and fold axespitch steeply (70e85�). Kinematic indicators in these highstrain domains include C0eS fabric, biotite fish, asymmetricdeflections of foliation planes, and asymmetric tails on por-phyroblasts. In many of these zones the greatest asymmetryoccurs on surfaces viewed parallel to the mineral lineationand perpendicular to foliation. However, in some areas it oc-curs on surfaces viewed oblique to the lineation, indicatingthat some strike-slip motion occurs across steeply plunginglineations (Fig. 10a). This latter pattern, and the increase inlineation pitch with increasing strain, are expected in zonesof triclinic transpression (Robin and Cruden, 1994; Linet al., 1998) where rapid switches in the direction of finite ex-tension are common (e.g. Tikoff and Greene, 1997; Holds-worth et al., 2002; Marcotte et al., 2005).

On the basis of these observations, we conclude that theSRSZ is a heterogeneous zone of dextral transpression that re-cords contraction at high angles to the shear zone boundaries.Strain is partitioned into a series of spaced, relatively narrowhigh strain zones that accommodate mostly dextral strike-slip motion with components of flattening and vertical

extension. Elsewhere, contraction dominates (non-coaxialcontraction, involving mostly shortening with a subordinatecomponent of strike-slip motion) and is accommodated overa much broader region.

4. U/Pb geochronology

To constrain the numerical age of rock fabrics and struc-tures we analyzed zircon with laser-ablation ICPMS. The col-lector configuration allows simultaneous measurement of204Pb in a secondary electron multiplier while 206Pb, 207Pb,208Pb, 232Th, and 238U are measured with Faraday detectors.The samples are from two pegmatite dikes that are deformedby the SRSZ along the southern shore of Doubtful Soundand one felsic dike within the host rock from the western shoreof Thompson Sound (sample locations in Fig. 3). All age un-certainties are reported at the 2s level (analytical methods arein the Supplementary material).

Sample 04-60 is from a pegmatite dike in a narrow zonedominated by SSR. This site lies east of the main zone of con-tinuous deformation associated with the SRSZ in domain III(Fig. 3). The dike cuts the Doubtful Sound shear zone foliationin the Western Fiordland Orthogneiss and displays a steeplydipping SRSZ foliation. These crosscutting relationships indi-cate that the dike was emplaced after the formation of theDoubtful Sound shear zone foliation and prior to or duringthe formation of the SRSZ foliation. Thirteen analyses wereobtained from zircon grains in sample 04-60. The grainsshow no sign of inheritance and yielded an age of88.4� 1.2 Ma (Fig. 11a, b). High U/Th ratios (Fig. 11c),with most between 30 and 120, suggest metamorphic zircongrowth, although these ratios do not rule out igneous crystal-lization at the same time (Rubatto et al., 2001; Williams,2001).

Sample 04-53 is from a pegmatite dike west of the center ofthe SRSZ along the boundary between domains I and III(Fig. 3). This dike cuts the pre-existing S1 and S2 foliationsin locally migmatitic Paleozoic metasediment and is faulted.Thirty-eight analyses were conducted on cores and tips of zir-con grains. Tips and some young cores define a well-con-strained igneous age of 113.4� 1.7 Ma (Fig. 11d, e). Mostcores show older inherited ages. Robust peaks defined by mul-tiple analyses are shown on the age probability plot (Fig. 11g)at 335 Ma, 555 Ma, and 767 Ma. Additional age peaks arepresent, but defined by only one analysis. Most U/Th valuesare typical of igneous zircon grown (U/Th< 10), but sometips have extreme U/Th values (Fig. 11f), suggesting thatsome metamorphic zircon growth accompanied or followedthe igneous crystallization of most of the zircon. We interpretthe igneous age to reflect zircon crystallization during partialmelting in the migmatite and/or contact metamorphism duringthe intrusion of Early Cretaceous plutons associated with theWestern Fiordland Orthogneiss.

Sample 05-135 is from a felsic (quartzþ plagioclase) dikewithin the metasedimentary host rock. The sample location iswithin domain I near the boundary with domain II from thewestern shore of Thompson Sound (Fig. 3). The dike cuts

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05-135 05-135

Concordia Age = 88.83 ± 0.86 Ma (2σ)88.8 ± 1.2 Ma (with systematic errors)

MSWD (of concordance) = 0.92

Age = 88.4 ± 1.2 MaMean = 88.4 ± 0.9 Ma

MSWD = 0.64 (2σ)

Age = 113.4 ± 1.7 MaMean = 113.4 ± 1.2 Ma

MSWD = 0.39 (2σ)

Age = 312.45 + 17.95 - 8.33 Ma(97.9% conf. from coherent group of 10)

box heights are 1σ

Fig. 11. U/Pb isotopic data collected from zircon using laser-ablation ICPMS. Plots (a), (d) and (h) are concordia plots that show all analyses from samples 04e60, 04e53, and 05e135, respectively. See Fig. 3 for sample

locations. Error ellipses on these plots are presented at 68.3% confidence. Plots (b) and (e) show the distribution of 206Pb/238U ages measured for samples 04e60 and 04e53. Inset in plot (h) shows an age distribution plot for

a coherent group of ten anlayses from sample 04e53. The ages are shown with two uncertainties. Those labeled ‘‘age’’ include all errors, while the ‘‘mean’’ includes only the random (measurement) errors. The age with a

larger uncertainty is reported in the text. Note that the uncertainties are reported at the 2s level while the error bars for each analysis are shown at the 1s level. Plots (c) and (f) show U/Th versus 206Pb/238U ages. See text for

a description of how these plots help us discriminate between igneous and metamorphic zircon. Plot (g) is an age probability plot of core analyses from sample 04e53. Plot (i) is an age distribution plot of all analyses.

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the dominant flat foliation (S1 and S2) but is also tightly foldedwithin the F2 folds that deform S1 and S2. From this fieldrelationship, we interpret the dike to have formed after theS1 foliation and before F2 folds. Thirty-two analyses were con-ducted on zircon grains, 27 from cores of grains and five fromtips or rims. The age distribution plot (Fig. 11i) shows thatthere is significant spread in the ages. Cores of zircon grainsrecord age peaks of Proterozoic and Paleozoic age with thestrongest peak at w306 Ma. A group of 10 coherent analysesdefine an approximate age of 312.45þ 17.95, �8.33 Ma(Fig. 11h).

5. Discussion

5.1. Interpretation of U/Pb geochronology

The analyses of zircon from sample 05-135 (Fig. 11h, i)show that the S1 and S2 foliations of domain I records severaltectono-thermal events. The zircon is complex due to the ef-fects of both Pb loss and inheritance, and record a range ofProterozoic and Paleozoic ages. The youngest recorded eventis Carboniferous with an average of 312.45þ 17.95/�8.33 Maand a peak at 306 Ma according to the zircon density distribu-tion (Fig. 11i). These ages are slightly younger than those ob-tained by Ireland and Gibson (1998) and Gibson and Ireland(1999) who showed that high-P metamorphism occurred atw330 Ma following a period (340e370 Ma) of arc magma-tism. The lack of zircon overgrowth or the crystallization ofMesozoic and younger zircon shows that this section of thecrust remained relatively cool and undisturbed by fluids sincew306 Ma. The results also show that the dominant foliation indomain I records only minor effects of Cretaceous extensionand magmatism (cf. Klepeis et al., 2007).

The results of zircon analyses from sample 04-60(Fig. 11aec) allow us to constrain the duration of extensionin Fiordland. The ages and crosscutting relationships of thedike that yielded this sample indicate that extension ceasedby 88.4� 1.2 Ma. This age is younger than other publishedzircon 206Pb/238U ages from the Western Fiordland Orthog-neiss and represents the first determination of when extensionin Fiordland ended using U/Pb techniques. Ion probe analysesof zircon from the Western Fiordland Orthogneiss by Gibsonet al. (1988) yielded 206Pb/238U ages of 119� 5 Ma (1s) inan amphibolite facies mylonite, and 126� 3 Ma (1s) in a gran-ulite facies orthogneiss. SHRIMP analysis of zircon by Holliset al. (2004) yielded 206Pb/238U ages of 115.6� 2.4 Ma (2s) ina metadioritic sample of the orthogneiss, and 114� 2.2 Ma(2s) in a sample of sheared garnet granulite in the orthogneiss.U/Pb analyses of zircon from a syntectonic dike obtained byKlepeis et al. (2007) indicated that deformation in the DoubtfulSound shear zone began before and outlasted 102.1� 1.8 Ma.Our result that extension ended by w88 Ma is compatiblewith these published ages. It also indicates that the SRSZformed after 88.4� 1.2 Ma.

The results of zircon analyses from sample 04-53 allowedus to evaluate the relative proximity of rocks located west ofthe SRSZ to Early Cretaceous plutons. The 113.4� 1.7 Ma

igneous age from this sample (Fig. 11g) represents one ofthe youngest ages associated with Early Cretaceous arc mag-matism and contact metamorphism in Fiordland. Similar crys-tallization ages from the Western Fiordland Orthogneiss inthe range 116e113 Ma have been obtained from elsewherethe granulite belt (Gibson et al., 1988; Tulloch andKimbrough, 2003; Hollis et al., 2004). Hollis et al. (2004) ob-tained SHRIMP analyses of zircon grains from calc-silicateparagneiss above the orthogneiss at Crooked Arm, whichyielded a strong peak in 206Pb/238U ages at 117.7� 2.8 Ma(2s) and lesser peaks at c. 600e500 Ma and 1100e900 Ma.Zircon from Crooked Arm yielded a pronounced peak at116� 2 Ma (2s) with Paleozoic inheritance, most notablya peak at w480 Ma (Ireland and Gibson, 1998). These age re-lationships and the occurrence of migmatite suggest that thisrock experienced igneous crystallization, contact metamor-phism, and dike emplacement. These observations place it inclose proximity to either the Western Fiordland Orthogneissor the Breaksea Gneiss during the final stages of the emplace-ment of their protoliths.

The results from samples 04-53 and 05-135 (Fig. 11bei) alsoallow us to evaluate the possible terrane affiliations of rocks oneither side of the SRSZ. The broad similarity of the protolith li-thology and the pattern of inherited zircon ages from these sam-ples (especially the young Carboniferous ages) with otherpublished ages from metasedimentary rock at Doubtful Sound(Ireland and Gibson, 1998; Gibson and Ireland, 1999) supportsthe interpretation that the Paleozoic metasediments locatedwest and east of the SRSZ are part of the same, or similar,Paleozoic terrane. Confirmation of this interpretation awaitsadditional analyses on both sides of the shear zone.

The interpretations that sample 04-53 was in close prox-imity to Early Cretaceous plutons and that Paleozoic hostrock on both sides of the SRSZ are similar implies that de-formation in the SRSZ involved strike-slip displacements ofonly a few kilometers. One hypothesis is that the migmatiteat site 04-53 originated from an area where the westernmoststrand of the SRSZ has cut across and displaced the contactaureole of the Breaksea Gneiss a few kilometers south ofDagg Sound (Fig. 7a). Alternatively, oblique top-down-to-the-northwest displacement on the SRSZ at the CascadaBay could have dropped migmatitic host rock down fromabove the Western Fiordland Orthogneiss along DoubtfulSound. Both possibilities are consistent with our findingthat deformation in the SRSZ was dominated by contractionat high angles to its boundaries and involved less than 20 kmof strike-slip displacement and only a few kilometers of ver-tical motion. Nevertheless, the exact magnitude of displace-ment along the SRSZ is unknown and awaits detailedmapping and the dating of units between Breaksea andDagg sounds.

5.2. Interpretation of transpressionalshear zone evolution

Any kinematic model of the SRSZ must explain the follow-ing observations: (1) contraction at high angles to the shear


zone boundaries; (2) fold axial surfaces and cleavage planesthat strike parallel to the shear zone at all stages of develop-ment, but progressively steepen to subvertical with increasingstrain (Fig. 9g, h); (3) The flipping of shear sense from obli-que-sinistral (east-side-down component) to oblique-dextral(west-side-down component) across zones of intermediateand high strain (e.g. Fig. 10a, b); and (4) The occurrence ofhigh strain zones where mineral lineations and fold axes pitch

60° (SSR)

60° (FSR)

60° (FSR)

late f

Scr2

Scr1

Scr2

D

DDD

UU

UUU

SSRlow strain

low strain

low strain

intermediate strain

intermstr

high

NW

NWc

b

a

Scr2

Scr2

Scr2

Scr1

Scr1

Scr1

S1S1

Fig. 12. Block diagrams showing the progressive evolution of structures in the SR

crenulation cleavages record shortening at high angles to shear zone boundaries. Sh

Continued deformation results in the progressive tightening and steepening of fold

zones of high strain. Shear sense varies on either side of the fold hinges as a result of

that accommodates oblique-slip. (c) As strain accumulates, cleavage rotates to subv

shearing and shortening occur in these zones of high strain. At a regional scale, sho

over a large region. In contrast, zones of mostly strike-slip deformation are localized

and minor thrusts in high strain zones also reflect this localization of strain.

steeply and kinematic indicators record strike-slip motionin narrow zones. Below we present a kinematic model(Fig. 12) of triclinic transpression where zones of mostlystrike-slip deformation are narrow and contraction is accom-modated heterogeneously over a relatively large region. Themodel also illustrates how the folding of pre-existing foliationsand the steepening of axial surfaces during shortening resultedin steep, spaced zones of high strain during transpression.

60°-90° (LSR)

10°-30° (LSR)

80°-90° (SSR)

70° (SSR)

Thrust Fault

ault

D

D

DownDU

U

ediateain

strain

NWSE

SE

SE

shear zone

Scr2DD

UU

SZ. (a) Layer parallel shortening forms monoclines and box folds. Conjugate

ear sense varies on either side of the fold hinges as a result of flexural slip. (b)

s. Cleavage intensifies in the steepened limbs of tight-isoclinal folds, forming

flexural flow. Combined shearing and shortening creates a shear zone cleavage

ertical and mineral lineations and fold axes plunge steeply. Combined dextral

rtening with a subordinate component of strike-slip deformation is distributed

into relatively narrow zones. The formation of late conjugate strike-slip faults


Deformation is inferred to have begun with the formationof asymmetric monoclines and open folds of pre-existing re-gional foliations (S1/S2, SDS) (Fig. 12a). The axial planes ofthe monoclines and crenulation cleavages (Scr1, Scr2) dip mod-erately (60�) to the northwest and southeast (e.g. Figs. 7c and9a). With continued shortening, the monoclines tightened intoasymmetric folds (FSR) and their axial planes rotated to sub-vertical (Fig. 12b). Conjugate crenulation cleavages and minorthrusts developed that record shortening at high angles to theshear zone boundaries. Slip or ductile shear between rocklayers during folding resulted in shear senses that changefrom oblique-west-side-down to oblique-east-side-downacross the hinges of the folds. This type of kinematic patterncommonly results where folds form by flexural slip or flexuralflow between layers in a rock, and is well-documented inzones where a pre-existing mechanical anisotropy controlsfolding (Holdsworth et al., 2002; Coke et al., 2003).

Continued shortening resulted in the rotation of FSR foldaxial planes to near vertical as strain increased (Fig. 12c). Apenetrative axial planar foliation (SSR) developed from theflattening and the intensification of the steeper of the twocrenulation cleavages. These zones of SSR foliations andtight-isoclinal folds define steep zones of high strain. Pre-ex-isting fabrics are transposed into the SRSZ foliation. Highstrain zones accommodated both shortening and componentsof strike-slip motion and near vertical extrusion. Conjugatebrittle faults (R1, R2) and minor thrusts formed and recordshortening and oblique-slip in narrow, branching fault zones.

This style of strain partitioning invites a comparison toother zones of partitioned transpression. At Marble Cove inNewfoundland, Goodwin and Williams (1996) reporteda broad domain in a transpressive shear zone where stretch-ing lineations plunge steeply and accommodate mostly re-verse motion with a component of dextral shear. Narrowzones of high strain display shallowly plunging lineationsand accommodated mostly dextral shear with a componentof reverse motion. This style, where shortening is accommo-dated over a broader region than the strike-slip component, issimilar to that which we observed in the SRSZ. However, thegeometry of lineations and distribution of sense-of-shear in-dicators we observed are different than those in the MarbleCove example. We observed the steepest lineations in narrowzones of high strain. More shallowly plunging lineations oc-cur heterogeneously in zones of intermediate strain. This pat-tern may partly reflect the poor preservation of minerallineations outside of high strain domains. However, it alsoappears to reflect a situation where strike-slip deformationis partitioned into narrow zones in both intermediate andhigh strain domains. A similar pattern, where areas domi-nated by contraction develop punctuated strain heterogene-ities that reflect some strike-slip motion, has beendescribed by van Noorden et al. (2007). These authors attri-bute the pattern to the incipient stages of strain partitioningwhere organized domains of contraction and strike-slip mo-tion have not yet formed. Our observations, where this pat-tern occurs in some intermediate strain areas, are consistentwith this interpretation.

In our model, vertical lineations reflect triclinic bulk strainwhere components of shortening and vertical extension controlthe orientation of structures at the margins of high strain do-mains (e.g. Tikoff and Greene, 1997; Holdsworth et al.,2002; Marcotte et al., 2005). In the centers of these domainssteep faults and narrow zones of ductile strain accommodatethe strike-slip component. Some strike-slip motion is evidenceby shear indicators that occur across the steep lineation awayfrom the faults. Similar high strain zones where triclinic trans-pression is partitioned into zones of non-coaxial contraction(contraction at high angles to the shear zone boundaries withcomponents of dip-slip simple shear and strike-slip simpleshear) have been described by Holdsworth et al. (2002). Ourobservations are compatible with this study and suggest a het-erogeneous partitioning of transpressional deformation intobroad zones of non-coaxial shortening (shortening plus subor-dinate components of vertical extension and strike-slip mo-tion) and narrow zones of strike-slip-dominated deformation.

The Fiordland example also illustrates how lithologic vari-ability and competency contrasts influenced the evolution ofa zone of transpression. Fig. 7a shows that the SRSZ closelyfollows the contacts between the Western Fiordland Orthog-neiss/Breaksea Gneiss and Paleozoic host rock, where rheo-logical contrasts are most pronounced. Within Paleozoic hostrock, where compositional layers of different competencythat define the inherited Paleozoic foliation are thin (<1 m),the shear zone folds are short-wavelength and tight. In con-trast, where metadioritic rocks of the Western Fiordland Or-thogneiss and Doubtful Sound shear zone consist of thicklayers of gneiss and mylonite (>10 m), long-wavelength,open folds accommodate layer-perpendicular shortening.

5.3. Regional significance of transpression

The small amount of isotopic data that constrain the abso-lute age of transpression in Fiordland allow for only tentativecorrelation with other structures. Our U/Pb zircon analyses in-dicate that the SRSZ is younger than 88.4� 1.2 Ma. U/Pbanalyses of rutile from Crooked Arm (Fig. 3) have yieldedages from 73.0� 0.5 Ma to 65.8� 0.5 Ma, suggesting thattemperatures in this section of the crust cooled to beloww400e450 �C by these times (Flowers et al., 2005). Sincemineral assemblages within the SRSZ record metamorphismmostly at the greenschist facies and lower, these rutile agescould represent a lower limit for the age of the SRSZ. How-ever, transpression is inconsistent with the documented re-gional tectonic setting at this time, which was dominated byextension and the opening of the Tasman Sea (Gaina et al.,1998; Kula et al., 2005).

The style of deformation in the SRSZ invites comparisonwith both the Anita shear zone (Fig. 1) and to offshore strandsof the Alpine fault. In the latter, Klepeis et al. (1999) identifieda phase of contraction-dominated dextral transpression (theirD3 event) that is similar to what we have documented insome parts of the SRSZ. Both shear zones show domainsthat are dominated by folding, subhorizontal shortening, anda subvertical foliation. However, although it postdates Late


Cretaceous extension, the numerical age of the transpressionalfabric in the Anita shear zone also is unknown.

The SRSZ occurs inboard of and parallels active traces of theAustralianePacific plate boundary located offshore (Frohlichet al., 1997; Barnes et al., 2001, 2005; Cande and Stock,2004). In central Fiordland, the Alpine fault is w10e30 kmfrom the shoreline and is composed of many branching faultsegments (Barnes et al., 2001, 2005). Several major strike-slipfaults of the Resolution Ridge segment occur along the strikeof the SRSZ to the south (Sutherland, 1999; Barnes et al.,2001; Lebrun et al., 2003; Barnes and Nicol, 2004). This align-ment and similarities between the SRSZ and transpressionalstrands of the Alpine fault suggests that the former has playeda role in accommodating some of the obliquely convergentmotion between the Australian and Pacific Plates. This alsoseems plausible given that the SRSZ displays faults that are ki-nematically similar to late Tertiary faults exposed near MilfordSound (Sutherland and Norris, 1995; Norris and Cooper, 2001;Claypool et al., 2002). We therefore suggest that at least themost recent phase of deformation in the SRSZ is related tothe late Tertiary dextral motion along the Alpine fault. If cor-rect, our results suggest that a significant, albeit unquantified,component of margin-perpendicular shortening is accommo-dated inside Fiordland by networks of steep transpressionalzones. Regardless of whether this is correct, the SRSZ is thelargest transpressional shear zone yet discovered insideFiordland.

6. Conclusions

A 10 km-wide zone of transpression, the SRSZ, affects atleast 800 km2 of continental crust inboard of the southernmostsegment of the Alpine fault in Fiordland, New Zealand.Regionally, the SRSZ traces the boundary between Early Cre-taceous granulite facies orthogneiss and Paleozoic metasedi-mentary rock. The SRSZ is composed of steep, narrowzones of relatively high strain. The geometry of folds, conju-gate faults, secondary thrusts, and conjugate crenulation cleav-ages in the shear zone record contraction at high angles to itsboundaries. The dominant process by which the steep zones ofhigh strain formed involved the development of moderatelyinclined, asymmetric folds that were progressively tightenedand rotated to vertical as strains accumulated. The folding ofpre-existing layering resulted in a kinematic pattern character-ized by shear senses that alternate from oblique-sinistral (east-side-down component) to oblique-dextral (west-side-downcomponent) across high strain zones. These steep zones ac-commodated both dip-slip and strike-slip components ofmotion at scales of 10e100 m. The shear zone records a styleof partitioned triclinic transpression where strike-slip motionlocalized into relatively narrow zones while non-coaxial con-traction (flattening plus a subordinate component of strike-slip motion) was accommodated over a large region. It alsoillustrates how pre-existing structural and lithologic heteroge-neities are important factors in controlling the evolution ofsubvertical structures commonly viewed as diagnostic oftranspression.

At its northern end, the SRSZ formed in Paleozoic rockwhere a regional pre-existing fabric formed during a periodof contraction and magmatism of mostly Carboniferous(w312e306 Ma) age. Along its central and southern seg-ments, the shear zone cuts across Early Cretaceous plutonsand gently dipping extensional shear zones. New U/Pb zirconanalyses indicate that Cretaceous magmatism had ended by113.4� 1.7 Ma and extension had ceased by 88.4� 1.2 Ma.Zircon analyses show similarities in protolith age and igne-ous and metamorphic histories across the shear zone, indicat-ing that it is not a major terrane boundary associated with thelarge-scale (>20 km) dispersal of rock units along themargin.

The age and style of transpression in the shear zone invitecomparisons with offshore segments of the Alpine fault sys-tem. Within the limits of available ages, the similarities amongthese structures suggest that transpression could have occurredduring the late Tertiary, indicating that a significant amount ofshortening at high angles to the Alpine fault may be accommo-dated within Fiordland.

Acknowledgements

This work resulted in a MSc thesis by DSK. Funding wasprovided by the National Science Foundation (EAR-0087323, EAR-0337111 to KAK and EAR-0443387 to GG);Geological Society of America funding to DSK; and anAAPG Grant-in-Aid to DSK. We thank Mo Turnbull and R.Jongens for providing detailed maps and data from the interiorof Secretary Island (Fig. 3). A. Tulloch and N. Mortimer pro-vided helpful assistance. A. Schoonmaker provided commentson the manuscript. We are grateful to Laurel Goodwin andMike Williams for thoughtful reviews. We thank the Depart-ment of Conservation in Te Anau for permission to visit andsample localities in Fiordland. M. De Paoli, G. ‘Rusty’Rigg, and D. Karn helped with field work.

Appendix A. Supplementary data

Supplementary data associated with this article can befound, in the online version, at doi: 10.1016/j.jsg.2007.12.004.

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