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Structure of the Blue Lake Fault Zone, Otago Schist,New ZealandA Henne a , D Craw a & D MacKenzie aa Geology Department , University of Otago , Dunedin, New ZealandPublished online: 31 Aug 2011.

To cite this article: A Henne , D Craw & D MacKenzie (2011) Structure of the Blue Lake Fault Zone, Otago Schist, NewZealand, New Zealand Journal of Geology and Geophysics, 54:3, 311-328, DOI: 10.1080/00288306.2011.577080

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Structure of the Blue Lake Fault Zone, Otago Schist, New Zealand

A Henne, D Craw* and D MacKenzie

Geology Department, University of Otago, Dunedin, New Zealand

(Received 3 February 2011; final version received 24 March 2011)

The Blue Lake Fault Zone occurs at a major crustal boundary between Otago Schist and Torlesse greywacke. This zone has beenactive since the middle Cretaceous (c. 112 Ma), when extensional exhumation of the Otago Schist belt was initiated. The BlueLake Fault Zone is dominated by a set of normal faults that have caused juxtaposition of rocks of different metamorphic grade,from prehnite-pumpellyite facies turbidites to pervasively recrystallised greenschist facies schists. This metamorphic transitionhas been thinned from c. 15 km to c. 2 km by normal fault motion on the scale of kilometres. This condensed section is thenarrowest such section on the Otago Schist margin. The faults in this condensed section are defined by gouge zones that arehundreds of metres wide and constitute about 30% of the section. Gouge zones were locally reactivated as thin Late Cenozoicreverse faults on the centimetre to metre scale.

Keywords: broken formation; gouge; Manuherikia; normal fault; Otago Schist; stromatolite; structure; Torlesse

Introduction

The boundary between the Mesozoic Otago Schist and

Torlesse greywacke terrane to the northeast is a major

structural feature in South Island geology, and reflects a

discontinuity in crustal structure and thickness (Fig. 1A;

Upton et al. 2009). The boundary is a complex structural

zone that has been periodically tectonically active for more

than 100 million years (Gray & Foster 2004; Mitchell et al.

2009; Upton et al. 2009). On a regional scale, the boundary

between greywacke and schist along the north-eastern

margin of the schist belt appears to be gradational, with

progressive increase in metamorphic grade towards the

southwest (Bishop 1972; Forsyth 2001). The present width

of the surface expression of this apparent transition varies

widely along the schist margin, but is typically of the order

10 km (Bishop 1972; Forsyth 2001). However, this transition

zone becomes as narrow as 2 km wide in the Manuherikia

valley of Central Otago (Fig. 1B). This anomalously narrow

greywacke-schist transition is the focus of this paper. The

surface expression of the major crustal structural boundary

between schist and greywacke (Fig. 1A) is defined by two

principal structures at the surface: the Hawkdun Fault Zone

and the Blue Lake Fault Zone (Figs. 1B, 2). The Hawkdun

Fault Zone has the most prominent present topographic

expression, with a distinctive fault scarp (�1 km high) on

the southwest side of the Hawkdun Range (Fig. 1B).

However, the Hawkdun Fault Zone has little lithological

change across it, and generally juxtaposes greywacke against

greywacke (Figs. 1B, 2). In contrast, the Blue Lake Fault

Zone defines at the surface the lithological boundary

between the Torlesse greywacke terrane and the Otago

Schist (Fig. 1B; MacKenzie & Craw 2005).The boundary between greywacke and schist also coin-

cides with a major change in Late Cenozoic and Recent

tectonic deformation as a result of the underlying crustal

differences (Upton et al. 2009). The Otago Schist is currently

deforming into a set of northeast-trending antiformal ridges

separated by synformal basins (Fig. 1B; Jackson et al. 1996;

Craw et al. 2007). These folds terminate at their north-

eastern ends against the northwest-striking Blue Lake and

Hawkdun Fault Zones (Fig. 1B). Strands of the Blue Lake

Fault have Late Cenozoic and Recent (active) offset (Figs.

1C, 1D; Lindqvist 1994; Forsyth 2001). The structural

change from folded schist to faulted greywacke is linked to

the lithological variations at the small to medium scale and

to the major crustal discontinuity at the larger scale (Upton

et al. 2009).Despite the regional significance of this major structural

zone in Central Otago, there is little detailed information on

the structure and lithological variations within the zone.

This paper reports on observations from a detailed structur-

al section through the Blue Lake Fault Zone, based on a

previously undescribed extremely well-exposed river section

that cuts across the fault zone from Torlesse greywacke to

lower greenschist facies Otago Schist. Parts of the section

are overlain by Miocene sediments, and these sediments

were deposited during active deformation along the Blue

Lake Fault Zone. Some sedimentary facies are directly

linked to fault activity (Lindqvist 1994; Youngson et al.

1998), and some of these sediments contain important

vertebrate fossils (Worthy et al. 2006). This paper provides

*Corresponding author. Email: dave.craw@stonebow.otago.ac.nz

New Zealand Journal of Geology and Geophysics

Vol. 54, No. 3, September 2011, 311�328

ISSN 0028-8306 print/ISSN 1175-8791 online

# 2011 The Royal Society of New Zealand

DOI: 10.1080/00288306.2011.577080

http://www.tandfonline.com

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some additional structural context for the sediments which

contain these significant fossils.

Geological setting

The Mesozoic Otago Schist belt formed during Jurassic

collision and metamorphism of Torlesse and Caples Terrane

turbiditic complexes that are dominated by greywacke

(Mackinnon 1983; Mortimer 1993; Gray & Foster 2004;

Little et al. 2009). Metamorphic grade increases from

prehnite-pumpellyite facies greywackes to the southwest

(Caples Terrane) and northeast (Torlesse Terrane) of the

belt to upper greenschist facies, with biotite and garnet, in

the core of the belt (Craw 1998; Mortimer 2000). The

metamorphic gradient on the margins of the schist belts

includes pumpellyite-actinolite facies semischists and lower

greenschist facies (chlorite zone) schists (Fig. 2; Bishop

1972).The increase in metamorphic grade on the north-

eastern margin of the belt is accompanied by progressive

metamorphic recrystallisation and foliation development,

and this gradient has been subdivided for regional mapping

purposes into textural zones (Bishop 1972; Turnbull et al.

2001). Prehnite-pumpellyite facies greywackes and asso-

ciated argillites (Textural Zone 1, TZ 1) are unfoliated and

sedimentary textures associated with turbiditic origins are

preserved with little recrystallisation other than in the

sedimentary matrix (Mackinnon 1983). Lowest grade schists

(TZ 2A) of pumpellyite-actinolite facies have a weak

foliation in greywackes, outcrops are commonly dominated

by bedding and metamorphic recrystallisation is minor.

More foliated low-grade schists, where foliation dominates

rock fabric (TZ 2B), have more extensive metamorphic

recrystallisation and sedimentary features are largely ob-

scured. Lower greenschist facies schists (TZ 3) are almost

pervasively recrystallised to metamorphic quartz, albite,

muscovite, chlorite, epidote, titanite and calcite. TZ 3 schists

are pervasively foliated, and locally tightly folded with

incipient development of a second foliation parallel to fold

axial surfaces (Turnbull et al. 2001; MacKenzie & Craw

Figure 1 Locality diagrams for the Blue Lake Fault Zone. A, Location of the area of the Blue Lake Fault Zone of this study, in relation to

the crustal discontinuity between Torlesse greywacke and Otago Schist (Upton et al. 2009) in the South Island of New Zealand. B, Generalgeology (partly after Forsyth 2000) of the area surrounding the Manuherikia river gorge, which extends from Falls Dam to the Manuherikiaflats near Loop Road. C, D, Sketch cross-sections through active strands of the Blue Lake Fault Zone, at localities marked in B.

312 A Henne et al.

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2005). TZ 4 schists, not discussed in this paper, are more

recrystallised and coarser grained upper greenschist facies

metamorphic rocks (Turnbull et al. 2001).The Otago Schist was exhumed in the early to middle

Cretaceous as the latter stages of metamorphism gave way

to extensional tectonics (Gray & Foster 2004; Little et al.

1992). Relative exhumation of rocks of different meta-

morphic grades and textural zones occurred along normal

faults on the margins of the schist belt (Deckert et al. 2002;

Gray & Foster 2004; Mitchell et al. 2009). The high-grade

core of the schist belt was exhumed relative to the adjacent

lower greenschist facies (TZ 3) and lower grade rocks by ca

112 Ma (Gray & Foster 2004; Mitchell et al. 2009). This was

the time of initiation of the northwest-striking fault zones

along the north-eastern margin of the schist belt, including

the Hawkdun and Blue Lake Fault Zones (Deckert et al.

2002; Gray & Foster 2004; Mitchell et al. 2009).Minor regional extension continued from late Cretaceous

to middle Cenozoic, resulting in basement subsidence and

marine planation (Landis et al. 2008). Tectonic deformation

began again in Central Otago in the Late Cenozoic, related to

development of the Alpine Fault (Fig. 1A; Cooper et al. 1987;

Craw 1995). Fluvial sediments related to associated uplift

were deposited on the low-relief surface and in channels cut

into that surface, as the early to middle Miocene Dunstan

Formation (Douglas 1986). The Dunstan Formation is

dominated by quartz gravels, commonly with placer gold,

and fluvial sands, silts, muds and lignite (Douglas 1986;

Youngson et al. 1998). This formation is overlain by, and

locally interfingered with, lacustrine sediments of the middle

to late Miocene Bannockburn Formation, which

were deposited in the regionally extensive lake complex of

LakeManuherikia (Douglas 1986). LateMiocene to Pliocene

uplift of the Torlesse greywacke mountains along the north-

west-striking faults on the northern margin of the schist belt

yielded large volumes of greywacke detritus that at least

partly filled Lake Manuherikia and formed extensive fan

deposits: the Maniototo Conglomerate (Fig. 1B) in the

northern part of Central Otago (Douglas 1986; Youngson

et al. 1998).The Maniototo Conglomerate, Dunstan and

Bannockburn Formations, the low-relief unconformity and

underlying schist basement are now being folded into broad

antiforms and synforms (Figs. 1B, 2) that have been active

since the Quaternary (Jackson et al. 1996; Craw et al. 2007).

The basement ranges, uplifted sediments and intervening

unconformity are currently rising on folds and faults and

shedding debris to form a thin veneer of fan deposits in the

intervening basins (Fig. 1B; Youngson et al. 1998; Forsyth

2001).

Figure 2 Cross-sections through the northeast margin of the Otago Schist belt. A, Regional cross-section along a line shown in Fig. 1B,

across the Hawkdun and Blue Lake Fault Zones. Different textural zones and metamorphic grades in the basement and the Mioceneunconformity (dotted) with preserved remnants of overlying Miocene sediments (thick black lines) are depicted. B, Cross-section through theBlue Lake Fault Zone (as indicated in A), compiled from the Manuherikia gorge river section. Approximate locations of diagrams in thispaper are indicated.

Structure of the Blue Lake Fault Zone 313

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Methods

This paper is based primarily on field observations in a rock-walled gorge (20�30 m deep) of the Manuherikia Riverdownstream of Falls Dam (Fig. 1B). The gorge persiststhrough the strath terraces of Fiddlers Flat (Fig. 1B) andends on the downthrown side of the Blue Lake Fault Zonewhere broad low-relief alluvial terraces predominate(Manuherikia flats near Loop Road; Fig. 1B). The near-continuous, largely unweathered outcrop along thisManuherikia river section provides an excellent basis forstructural observations of four internally coherent structuralblocks of basement rocks (Table 1; Fig. 2B) and the locallyoverlying Tertiary sediments. Fault zones, including softgouge zones, are exposed in many places along this section(Fig. 2B). The continuity of outcrop is interrupted spor-adically by minor landsliding of fault rocks and fissileschists, typically on a scale of 10 m laterally.Lithological and structural observations were made

through the section, with locations and distances determinedwith tape measure where practical and by pacing inintervening sections. Observations were necessarily madealong the winding river course, and these were thencompiled into a summary section drawn perpendicular tothe general northwest strike (Fig. 2B). Minerals and textureswere examined in standard polished thin sections, with fine-grained mineral confirmation by semiquantitative electronmicroprobe scanning.Fault gouges are highly comminuted and fine grained, so

determination of gouge protoliths was done geochemicallyin comparison to unfaulted rocks within the gorge. Freshrocks (c. 200 g) were sampled for this geochemical analysisfrom clean exposures. Major elements were analysed by X-ray fluorescence on fused glass disks which were manufac-tured by melting of rock powder and lithium metaborateflux in platinum crucibles. Analyses were carried out using aPhillips PW2400 spectrometer at the Geology Department,University of Otago. Volatile components were determinedby loss on ignition after heating at 1100 8C for 1 hour.

Carbonaceous (graphitic) material, which is locally abun-dant, may not have been completely volatilised by thismethod. High clay mineral contents of gouges may not havefully dehydrated, resulting in slightly low totals for someanalyses. Representative analyses are presented in Table 2.Supplementary data on the clay fraction in fault rocks

were derived from X-ray diffraction analyses, carried out onthe PANalytical X’Pert PRO MPD Diffractometer at theDepartment of Geology, University of Otago. The clayfraction was extracted from fault rock samples to derive aset of four samples each. One sample of each set was heatedto 150 8C, one was heated to 550 8C and one sample wasglycolated for 24 hours for confirmation of clay mineralidentifications.Non-carbonate carbon (or total organic carbon, TOC)

and sulphur contents were determined on 3mg powderedsamples. Powders were pre-treated with hot (80 8C) HCl(10%) overnight to dissolve carbonates, then washed anddried. TOC and S data were obtained by oxidation andvolatilisation by flash combustion with a Carlo Erber CHNSElemental Analyser Detection limit is 0.1 wt% for TOC and0.3 wt% for S. Representative results are presented in Table3, with the associated carbonate and opaque mineralsspecified.

Basement rock types and structures

Deformed turbidites

The turbidites that dominate the north-eastern part of thesection (Table 1; Figs. 2A, 2B) have preserved clastictextures and sedimentary structures such as graded bedding,fine laminations, cross-bedding and rare flame structuresand load casts. Most of the section has massive (c. 20 m)greywacke beds with only minor, thin-bedded (usually B 50cm) argillite. The greywacke/argillite proportion increasesdownstream (southwest) and outcrops become increasinglydominated by thin-bedded (10�40 cm) alternating argilliteand greywacke beds interbedded with minor, equally

Table 1 Summary of principal lithological and structural features of basement rocks in the Manuherikia River section (FAS: fold axialsurface).

Structural block Metamorphic facies Textural zone Rock types Principal fabric Subordinate fabric

Turbidite Prehnite-pumpellyite TZ 1 Greywacke, siltstone,argillite

Bedding Spaced shears inargillites

Broken formation Pumpellyite-actinolite? TZ 1 to 2A Sheared argillite;

greywacke, conglomerateboudins

Sheared bedding Micaceous shear

foliation in argillaceousshear zone

Low-grade schist Pumpellyite-actinolite TZ 2B Psammitic, pelitic schists Foliation (S1) parallel to

bedding; transposition

Spaced cleavage (S2)

FAS to open folds of S1in pelitic schist

Medium-grade

schist

Greenschist TZ 3 Quartzofeldspathic,

micaceous schists

Pervasive foliation, S1 Micaceous cleavage (S2)

FAS to tight recumbentfolds of S1

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thin-bedded siltstone. The strike of bedding varies consider-

ably from north to west, with moderate to steep dips

(Fig. 3A). Sedimentary structures suggest that the sequence

youngs towards the southwest, and some packets are over-

turned. A prominent zone of 10 m scale folds can be traced

over approximately 300 m along the Manuherikia River

(Fig. 3A, 3B) where the bedding is folded about a northwest-

trending fold axis into two close chevron folds with straight

limbs and narrow hinge zones. Faults with 1�20 cmbrecciated zones mark the contact between 10 m scale

greywacke slabs and the bedded turbidite sequence, and

similar faults run subparallel to bedding in argillites in tight

fold hinge zones (Fig. 3A, 3C).Outcrops of the adjacent structural block (Fig. 2B; Table

1) consist of turbidites with block-in-matrix fabric (Fig. 4A).

Some argillites have a weakly developed cleavage, associated

with folds of greywacke in dismembered beds (Fig. 4B).

These structures are typically called ‘broken formation’

(Nelson 1982; Watson & Grey 2001; Vannucchi & Bettelli

2002), and have been described in Torlesse Terrane turbi-

dites elsewhere (Mackinnon & Howell 1984; Silberling et al.

1988). The only recognisable metamorphic fabric in most

Table 2 Representative whole rock X-ray fluorescence analyses (wt%) of rocks from the Manuherikia gorge section (Turbidite block is TZ1; BF: broken formation; LOI: loss on ignition). Sample numbers refer to samples curated by University of Otago (OU).

Block Turbidite Turbidite Turbidite Turbidite Turbidite Turbidite TZ 2B TZ 2B TZ 2B TZ 1 BF TZ 2B

Rock Greywacke Greywacke Siltstone Siltstone Argillite Argillite Schist Schist Schist Gouge Gouge Gouge

Sample 81154 81190 81191 81192 81138 81139 81121 81125 81131 81197 81198 81199SiO2 75.76 68.43 65.12 63.30 59.28 58.71 56.18 57.95 76.58 59.81 57.50 58.78TiO2 0.37 0.46 0.76 0.84 0.91 0.88 0.81 0.81 0.38 0.83 0.97 0.80

Al2O3 11.76 10.76 16.81 18.01 19.75 19.71 18.55 17.81 7.20 17.78 21.10 17.58Fe2O3* 3.42 2.42 6.01 5.85 6.89 7.07 9.38 8.93 2.70 7.15 4.15 6.38MnO 0.05 0.12 0.09 0.07 0.09 0.11 0.05 0.07 0.09 0.08 0.04 0.08

MgO 0.96 0.76 1.93 2.10 2.54 2.63 1.28 1.52 0.74 2.59 1.36 2.55CaO 1.77 7.42 1.74 1.23 1.76 2.29 0.85 0.95 5.41 1.58 0.21 1.95Na2O 3.49 3.52 2.95 2.81 2.45 2.37 0.53 0.71 1.31 1.86 0.94 0.12

K2O 1.26 1.15 2.96 3.50 3.98 4.01 4.05 3.18 1.01 3.37 4.25 3.38P2O5 0.02 0.10 0.23 0.19 0.09 0.14 0.10 0.06 0.09 0.17 0.06 0.18LOI 1.57 4.01 2.39 2.19 2.74 2.55 8.40 8.12 5.10 4.56 7.31 5.74Total 100.43 99.15 100.99 100.09 100.48 100.47 100.18 100.11 100.61 99.78 97.89 97.54

1All iron is expressed as ferric

Table 3 Principal carbonate and opaque minerals in selected samples from the Manuherikia gorge section, with non-carbonate carbon(TOC) and sulphur (S) analyses (both in wt%). (BF: broken formation; DPy: diagenetic pyrite; Dcarb: detrital carbonaceous material;MGraph: metamorphic graphite; MPy: metamorphic pyrite; MPy porph: porphyroblastic metamorphic pyrite.)

Block Rock type Carbonate Opaque minerals TOC S

Turbidite Argillite Calcite DPy, Dcarb B0.1 B0.3Turbidite Argillite Calcite DPy, Dcarb B0.1 B0.3

Turbidite Argillite Calcite DPy, Dcarb B0.1 B0.3Turbidite Argillite Calcite DPy, Dcarb B0.1 B0.3Turbidite Siltstone Calcite DPy, Dcarb 2.1 B0.3BF Black shear Calcite, ankerite MGraph 5.18 B0.3

BF Black shear Calcite, ankerite MGraph 5.78 B0.3BF Black shear Calcite, ankerite MGraph 3.44 B0.3BF Black shear Calcite, ankerite MGraph 1.55 B0.3

TZ 2B Pelitic schist Calcite MGraph, MPy 5.45 B0.3TZ 2B Pelitic schist Calcite MGraph, MPy 1.1 B0.3TZ 2B Pyritic pelitic schist Calcite MGraph, MPy porph 1.66 4.55

TZ 2B Pyritic pelitic schist Calcite MGraph, MPy porph 1.13 1.48TZ 2B Pyritic pelitic schist Calcite MGraph, MPy porph 0.72 0.3TZ 2B Pyritic pelitic schist Calcite MGraph, MPy porph 2.77 3.12

TZ 2B Pyritic pelitic schist Calcite MGraph, MPy porph 2.05 3.43TZ 3 Micaceous schist Ankerite MGraph 0.48 B0.3TZ 3 Micaceous schist Ankerite MGraph 0.64 B0.3TZ 3 Micaceous schist Ankerite MGraph 0.36 B0.3

Structure of the Blue Lake Fault Zone 315

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outcrops is a weak pervasive cleavage in some argillaceousrocks. Limited data suggest that bedding strikes are highlyvariable with moderate to steep dips (Fig. 4A).The south-western part of the broken formation block

contains abundant sheared argillite with additional meta-morphic recrystallisation and shearing, yielding a distinctiveblack shear zone (Fig. 2B). Two subparallel and intersectingshear sets strike westwards across argillaceous brokenformation (Fig. 4B), disrupting the block-in-matrix fabric.The shear fractures are in-filled with black semi-cohesivefoliated gouge which also envelopes phacoidal clasts ofgreywacke and conglomerate with a thin, often sheared andslickensided, veneer. The veneer consists of graphite, mus-covite, chlorite, rutile and quartz (optical, electron microp-robe and X-ray diffraction confirmation). Thesemetamorphic minerals have been formed and/or recrystal-lised during deformation to give a complex fabric on boudinmargins (Fig. 5). The gouge displays a clear foliation definedby muscovite, chlorite, pervasive foliation-parallel quartzveins and occasional foliation-parallel graphite smears (Fig.5). Crosscutting quartz veins are also common within thisfabric. Total organic carbon (TOC) analyses of this boudinveneer material range from 1.3 to 5.8 wt% (Table 3).

Schist

The low-grade schist structural block (Fig. 2B; Table 1)consists of psammitic and pelitic schists distinguished

primarily by the differing proportions of micaceous(muscovite, chlorite) and quartzofeldspathic components(Figs. 6A, 6B). Some pelitic schists are micaceous but alsohave abundant quartzofeldspathic laminae (Fig. 6A, 6D).The metamorphic fabric has been locally enhanced anddisrupted by the development of foliation-parallel meta-morphic quartz veins (Fig. 6B). The combination ofstructural transposition and quartz vein emplacement hasresulted in prominent colour-banding in many outcrops,resembling metamorphic segregation (Figs. 6A, 6B, 6D). Inaddition, numerous post-metamorphic quartz veins cut thefoliation, especially in psammitic layers (Fig. 6B). The S1foliation generally dips gently-to-moderately northwards(Fig. 6C).A set of low-grade schist samples (TZ 2B) was analysed

for geochemical comparison to turbidites further upstream(Table 2). Both sets of analyses show very similar geochem-ical trends, reflecting the same general lithological variationsbetween greywacke and argillite (Figs. 7A�7D). Rocks withgreywacke protoliths have high silica contents (70�80 wt%),and more argillaceous protoliths have progressively lowersilica and higher MgO (increasing chlorite) and higher K2O(increasing muscovite) (Figs. 7A, 7B). The most micaceousTZ 2B pelitic schists have higher K2O and lower silica thantypical argillites (Fig. 7B). Many of these pelitic schists areprominently enriched in graphite and pyrite and TOCcontents (Table 3) range up to 5.5 wt% and sulphur contentsrange up to 11.5 wt% for these micaceous schists. This is

Figure 3 Section through the folded sequence of TZ 1 turbidites near Fiddlers Flat. A, Cross-sectional view of the wall of the Manuherikiariver gorge. B, Lower hemisphere, equal area stereonet of poles to bedding in the folded area in A (arrowed). Open circle is the axis of thisfold, with associated great circle. C, Annotated photograph of the dashed box in A, showing complexly folded and faulted bedded turbidites

adjacent to two thick greywacke slabs.

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distinctly elevated relative to background values which were

below the detection limit (B 0.3wt%). Graphite occurs

within the metamorphic foliation (S1), and rarely in quartz

veins. Pyrite occurs disseminated within the matrix and in

syn- to post-metamorphic quartz veins. The elevated pyrite

contents of these micaceous schists are also reflected in the

high iron contents of the rocks compared to the typical

greywacke-argillite range (Fig. 7C). The relatively high

amounts of pyrite and graphite in the TZ 2B micaceous

schists also results in elevated loss on ignition (Fig. 7C).

Medium grade schist (TZ 3; Table 1) is exposed at the

southwest of the Manuherikia gorge section (Fig. 2B), where

the gorge ends at the Manuherikia flats (Fig. 1B). Conse-

quently, outcrop is relatively poor compared to the up-

stream section. The pervasive foliation (S1) has been

extensively deformed on the decimetre to metre scale by

recumbent tight to isoclinal folds with shallow-plunging

westward trends (Fig. 8A). An S2 fabric is particularly

strongly developed on attenuated fold limbs, where it locally

dominates the fabric at outcrop (Fig. 8A). Both S1 and S2

Figure 4 Broken formation in the Manuherikia gorge section. A, Sketch made from an oblique photograph, showing the characteristic block-

in-matrix fabric that dominates this structural block. Kaolinitised basement rock underlies the Miocene Dunstan Formation at the top of theoutcrop. Stereonet inset shows poles to bedding. B, Conglomerate boudin (pale, top) with underlying matrix of sheared argillite (Sh. arg).Matrix contains small boudins of less-sheared argillite (Arg. boudin; margins shown with dashed white lines). Inset stereonet shows poles to

argillite shears in the black shear zone of the broken formation.

Structure of the Blue Lake Fault Zone 317

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have metamorphic quartz veins parallel to them (Fig. 8A),and the folded S1 quartz veins are strongly lineated wherethey have been folded. The S2 fabric is generally flat-lying ordips gently (B208) to the north or south (Fig. 8A).

Gouge zones

Fiddlers Flat gouge zone

This is the most prominent and well-exposed gouge zone inthe Manuherikia gorge section, and extends over c. 200 m atthe boundary between TZ 1 turbidites and the brokenformation block (Fig. 2B). The gouges are best developed inthe TZ 1 rocks, and exposures of gouges on the brokenformation side of the boundary are poor. The gouge zonebecomes progressively developed downstream (southwest) inTZ 1 rocks. Isolated bedding-parallel shears and small gougezones (B 5 m wide) are focused in argillite beds that arelocally tectonically thickened (up to 2 m). Apparentlyrandomly oriented small-scale (1�3 m) faults crosscut thebedding with small offsets (B 2 cm) with a normal sense ofoffset. Fault rocks include fine-grained gouges, coarsergrained cataclasites and breccias with clasts up to 5 cm indiameter, and are commonly cemented with calcite and/orankerite.In the main gouge zone (Fig. 9A), the fault rocks consist

of variably brecciated greywacke and sheared argilliteembedded in grey and black incohesive, fine-grained, clay-rich gouge. Gouge bodies up to 2 m across are partially

cemented with calcite. The primary mineralogy of blackgouge is quartz, muscovite, chlorite, smectite, kaolinite andminor albite (X-ray diffraction identification). Major ele-ment data (Table 2) suggest that the gouge is derived fromargillites (Figs. 7A�7D).The northeast margin of the gouge zone dips moder-

ately northeast, and bedding within the turbidites has beenrotated into approximate parallelism with this margin(Fig. 9A). Sheared rocks, particularly black argillaceousmaterial, define a gouge fabric within the gouge zone, andthis fabric has a wide range of orientations (Figs. 9A�9C).Cemented and uncemented gouge is locally deformed intocomplex folds defined by crushed rock of different shadesof black and dark grey (Fig. 9C). The gouge fabric isgenerally parallel or subparallel to adjacent fault strandsthat cut through the gouge zone (Fig. 9A). Bending ofrelict bedding and gouge fabric immediate adjacent tothese subsidiary faults is consistent with a normal sense ofmotion within the gouge zone (Fig. 9A, 9B).The folded gouge is crosscut and offset by a second

generation of narrow southwest-dipping shears (Fig. 9C).The shears consist of thin (B 5 cm) seams of black, fine-grained, incohesive gouge. Displacement along the shears istypically less than a metre, with a reverse sense (Fig. 9C).

Other gouge zones

The boundaries between broken formation and TZ 2B rocksand between TZ 2B and TZ 3 rocks are both defined by

Figure 5 Photomicrograph (transmitted light) of the sheared margin of a greywacke boudin in the black shear zone within the broken

formation block. Greywacke is at the base, and material added and recrystallised during shearing is at the top. Black material is variablysheared rock with metamorphic graphite, rutile, muscovite and chlorite. White material is quartz (Q) and/or coarse-grained metamorphicmuscovite (M). The metamorphic muscovite, chlorite and graphite impart a shear foliation (horizontal) to the rock. This is cut by late-stage

quartz veins.

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A

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Figure 6 Low-grade schist (TZ 2B) outcrops in the Manuherikia gorge section. A, Laminated TZ 2B schist with alternatingquartzofeldspathic and micaceous horizons, showing fine-scale transposition of bedding into parallelism with foliation (S1). B, Massivepsammitic schist with metamorphic and post-metamorphic quartz veins. C, Stereonet with poles to TZ 2B foliation. D, Post-metamorphic

shear zone in a micaceous pelitic schist layer sandwiched between less-deformed psammitic schist (top) and pelitic schist with abundantquartzofeldspathic laminae (bottom).

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gouge zones, but the exposure of these gouge zones is poorerthan that at Fiddlers Flat. Deformation associated withthese gouge zones appears up to 200 m from the main zones,and is typically focused in weaker rocks in narrow (B 1 m)zones. These zones generally become progressively morecommon towards the main gouge zones.Broken formation rocks are juxtaposed against textural

zone 2B lithologies, with an abrupt change from brokenformation to undisturbed textural zone 2B semischist within50 m along the Fiddlers Flat section. A distinct fault tracewas not observed, but the boundary coincides with thesynmetamorphic black shear zone in the broken formationblock. Parts of the black shear zone have been redeformedto form steeply dipping zones (metre scale) of grey,incoherent cataclastic rock which is made up of clasts ofthe foliated gouge. The gouge and clasts are commonlycemented with ankerite, and some cemented zones have beenredeformed to yield ankeritic fragments (1�10 cm scale)

within the gouge zones. No gouge involving TZ 2B rockswas observed, although micaceous schist layers are locallysheared and disrupted.The boundary between TZ 2B and TZ 3 schists occurs at

the end of the Manuherikia gorge, where outcrop isrelatively poor. This boundary is better exposed at Penny-weight Hill, 4 km along strike to the southeast (Fig. 1B),where the bounding gouge zone dips steeply northeast (Fig.1C). Gouge zones occur over 300�400 m near to thisboundary in the Manuherikia gorge (Fig. 2B). Narrowdeformation zones in TZ 2B first appear at least 300 mupstream of the boundary between the textural zones, andthese are typically confined to micaceous rocks (Fig. 6D).These narrow zones are spaced at 2�10 m intervals and havea wide range of orientations, partly controlled by thefoliation orientation which is generally shallow-dipping(c. 308). However, some of these narrow gouge zones dipsteeply northeast with a west to north-westerly strike. The

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Figure 7 Geochemical plots showing the ranges of compositions defined by the primary sedimentary variation between greywacke andargillite in TZ 1 turbidites (black squares). Low-grade schists (TZ 2B; small grey circles) show a similar range, although more extreme peliticcompositions occur. Gouge samples (large open circles) are all derived from argillite. A, Silica�MgO variations. B, Silica�K2O variations.C, Silica�total iron variations. D, Alumina�loss on ignition variations.

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river at the downstream end of the gorge follows one ofthe prominent west to northwest striking gouge zones for�50 m within TZ 2B rocks, with a 5�20 cm thick shearplane which juxtaposes semi-pelitic against psammitic schist.Bending of foliation and gouge fabric at minor faults withinthese gouge zones suggest a normal sense of displacement.However, rare thin (centimetre scale) shears cutting throughthe gouge zones appear to have a reverse sense of motion,based on bending of gouge fabric.

Most outcrops of TZ 3 schists on the Manuherikia flatsdownstream of the gorge have some degree of brecciationand gouge formation cutting across the flat-lying S2foliation (Fig. 8B). The gouge zones have a wide range oforientations, generally with a westerly strike and dipsranging from steep to gentle (Fig. 8B). These gouge zonesbreak the outcrops up into relatively intact blocks of schist(0.5�10 m scale), separated by breccia and gouge zones thatare also typically 0.5�10 m across (Fig. 8B). Additional

A

B

Figure 8 Structure of greenschist facies schist (TZ 3) outcrop at Manuherikia flats. A, Foliation S1 is folded, and an incipient fold axialsurface foliation (S2) dominates the fabric. Both foliations are accentuated by millimetre-scale quartz veins. Inset stereonet showsorientations of fold axes (squares) and poles to S2 foliation. B, Strongly brecciated and gouge-filled zones cut the outcrop between more

intact schist blocks. Location of intact schist in A is indicated with a white box. Inset stereonet shows poles to gouge zones in the boundarybetween TZ 3 and TZ 2B schists.

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narrow gouge zones (centimetre scale) cut across these largergouge zones (Fig. 8B).Gouges in TZ 2B and 3 rocks consist of quartz,

muscovite, chlorite, smectite, kaolinite and little or noalbite. Many gouges show a very weak incipient shear fabricparallel to the strike and dip of the individual gouge zones.The fabric of the matrix anastomoses around clasts rangingin size from 20 mm to 4 cm with a weak preferredorientation. Clasts comprise single crystals to remnantrock fragments. Clasts in TZ 3 gouge are generally morerounded to phacoidal shaped than in textural zone 2B, atleast partly because of the greater abundance of meta-morphic quartz veins in TZ 3 schists (Fig. 8A). Composi-tions of gouges in TZ 2B and TZ 3 are similar to the FiddlersFlat gouge (Table 2), and are apparently derived fromargillaceous rocks (Fig. 7A�7D). Most TZ 3 gouge ispervasively cemented with ankerite, and ankerite veinsextend into nearby fractured host rocks.

Tertiary sediments

Miocene sediments of the Manuherikia Group (Douglas1986; Youngson et al. 1998) rest unconformably on some ofthe rocks of the Blue Lake Fault Zone. Rocks beneath the

unconformity are variably altered to clays for up to 20 mbelow the sediments. The altered rocks and their veneer ofsediments, where preserved, provide a useful marker forpost-Miocene deformation of the area (Fig. 2A). Miocenesediments and the underlying unconformity are preserved intwo places in the Manuherikia gorge: at Fiddlers Flat andon the Manuherikia flats downstream of the Manuherikiagorge (Fig. 1B). Exposures of similar geological features tothe latter locality are visible 4 km along-strike at Penny-weight Hill (Fig. 1C).At Fiddlers Flat, quartz pebble conglomerate, sand and

silt of the basal Dunstan Formation overlie the clay-alteredunconformity on the broken formation (Fig. 2A, 4A). Thesediments are preserved as a shallow syncline whose south-western limb dips c. 258 northeast (Fig. 4A) towards theFiddlers Flat gouge zone (Fig. 2B). Sediments and clay-altered unconformity are preserved on deformed brokenformation gouge (Fig. 9A), but have been largely erodedand/or offset above the TZ 1 part of the gouge zone.At the Manuherikia flats, the Dunstan Formation was

juxtaposed against TZ 3 schist by a strand of the Blue LakeFault that was active during Miocene sedimentation (Fig.1D; Lindqvist 1994). The plane of this fault strand is notwell exposed, but variably sheared and tilted Dunstan

Figure 9 Structures within the Fiddlers Flat gouge zone between TZ 1 turbidites and the broken formation block. A, Sketch section throughthe gouge zone, showing orientations of gouge fabric (dashed lines) and faults (heavy black lines, with inferred sense of motion). B, Shearedargillite (dark grey) and less-deformed greywacke (pale) in a normal fault zone near the south-western edge of the gouge zone (located in A).

Both argillite and greywacke are locally coated by calcite (white). C, Zone of complexly folded black and grey gouge (derived from argillite)with abundant secondary calcite impregnation, near centre of gouge zone (located in A). White dashed lines accentuate gouge fabric that hasbeen folded. Southwest-dipping later shears (full white lines) displace sheared argillaceous gouge. One of these later shears has ademonstrable reverse sense of motion (heavy dashed arrow).

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Figure 10 Miocene unconformity on TZ 3 schist at Manuherikia flats. A, Outcrop photo showing the Bannockburn Formation restingunconformably on kaolinitised schist. Mudstone (M) fills interstices between stromatolites (S). B, Close-up view of stromatolites and

mudstone in black box in A. Arrows point in the direction of stromatolite encrustation on schist cobbles. C, Cartoons (not strictly to scale) ofa northeast to southwest section across the Blue Lake Fault at the outcrop in A show possible Miocene evolution of the site. In the first panel,stromatolites are in life position on a fault scarp with encrustations growing upwards. The second panel shows dead stromatolites redeposited

after continued fault movement.

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Formation sandstones and quartz pebble conglomerates

define the downthrown side. On the upthrown side of this

fault, the Miocene unconformity dips southwest at 10�208and is overlain by boulders of TZ 3 schist and also silcrete

eroded from the pre-existing Dunstan Formation (Lindqvist

1994). Bannockburn Formation lacustrine mudstone fills

interstices between boulders, in a layer up to 50 cm thick

(Figs. 1D, 10A). Miocene stromatolites have nucleated on

many of the cobbles (5�20 cm) of TZ 3 schist and silcrete.Some stromatolites fully coat their core cobble but most

have only encrusted three sides, leaving an uncoated

portion. The uncoated portions of the stromatolites face in

a wide variety of directions, including directly upwards (Fig.

10A, 10B). TZ 3 schist cobbles within stromatolites are

variably kaolinitised, similar to the underlying basement.

Discussion

Polyphase deformation in the Blue Lake Fault Zone

Rocks in the Blue Lake Fault Zone show evidence for

numerous generations of deformation, each with character-

istic features. These characteristic features allow for sub-division of the deformation into three main stages in thestructural evolution of the area: metamorphic deformation,wide gouge zone deformation and deformation affecting the

Miocene unconformity. Not all deformation features withinthe fault zone can be assigned easily to one of these stages,but the overall development of the fault zone becomestractable with this simplification. These three stages aresummarised in Figs. 11A�11C, with chronological order ofdeformation stages depicted from bottom to top of thediagram.Deformation that accompanied or immediately post-

dated metamorphism is different for each of the differentstructural blocks defined in the section (Table 1; Fig. 2B).This deformation is of a regional nature and thereforeunrelated to the development of the Blue Lake Fault Zone,

but is discussed here in order to distinguish it fromstructures related to the Blue Lake Fault Zone. In TZ 1,synmetamorphic deformation involved folding of bedding,with localised development of brittle faults and sheared

zones (Figs. 3A�3C). Fault zones are typically dominated bybrittle lithic breccias, rather than soft gouges. The main

Figure 11 Summary of the structural evolution of the Otago Schist margin at the Manuherikia gorge section. A, Present active structures.B, Cretaceous structures. C, Original metamorphic transition in C. The thick metamorphic section C became extremely condensed by normal

faults to the present narrow section in B, which was reactivated as in A.

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observable large-scale fold in the TZ 1 turbidites (Figs. 3A,3B) is steeply north-plunging, and is geometrically andrheologically unrelated to the large northwest-striking gougezones further to the southwest in the section (Fig. 2B).In TZ 2B and TZ 3, development of foliation(s) was a

regional scale effect of metamorphic deformation underductile deformation conditions. This ductile deformationoccurred at considerable depth, ranging up to greenschistfacies conditions of c. 3508C and�15 km depth (Fig. 11C;Mortimer 2000). The broken formation formed undermetamorphic conditions intermediately between TZ 1and TZ 2B & 3, and involved minor metamorphiccleavage development and abundant shearing and breccia-tion. The black shear within the broken formation blockshows the extreme extent of the broken formation style ofdeformation, with formation of foliated gouge and loca-lised metamorphic foliation on boudin margins (Fig. 5).This metamorphic foliation involved crystallisation andrecrystallisation of muscovite, chlorite and graphite (Fig.5) to a much greater extent than elsewhere in the brokenformation zone.The above-described metamorphism and metamorphic

deformation prepared the host rocks for development ofpost-metamorphic structures that make up the Blue LakeFault Zone. The Late Cenozoic component of this deforma-tion was small, as the Miocene unconformity is largelypreserved over the general area (Fig. 2A). The Blue LakeFault Zone is dominated by gouge zones that were formedbefore the Miocene unconformity developed, as outlined inthe next section.

Cretaceous extension and the condensed metamorphic section

The gouge zones that make up about 30% of the 2 kmsection from greywacke to schist (Fig. 2B) represent majorfault movement that resulted in juxtaposition of the differentstructural blocks. In contrast, each structural block isinternally broadly homogeneous. The structural and meta-morphic transition from TZ 1 to TZ 3 that presumablyexisted in some form (e.g. Fig. 11C) has been extensivelydisrupted and tectonically thinned to c. 15% of its originalwidth (Fig. 11B). Textural zone 2A rocks, abundant else-where on the northeast schist margin (Bishop 1972; Forsyth2001), have apparently been completely excised from theManuherikia gorge section. This requires kilometre-scalerelative movement on the faults represented by the gougezones. Similar fault disruptions of metamorphic gradationsare common throughout the Otago Schist (Craw 1998;Mortimer 2000), but the Blue Lake Fault Zone is the mostthinned and disrupted such section in Otago.The gouge zones have a broad northwest strike and

moderate to steep northeast-wards dip (Figs. 2A, 2B, 9A).The northeast side (footwall) of each of the zones isrelatively lower grade and therefore has been relativelydownthrown. Hence, these gouge zones primarily represent

normal faults. Evidence for normal sense of motion is

common in the gouge zones (e.g. Figs. 9A, 9B), althoughthe gouge zones are so internally complex and chaotic (e.g.

Fig. 8B, 9C) that obtaining consistent motion senses from

outcrops is difficult. The gouges are soft and incohesive andcontain clay minerals (kaolinite and smectite) that have

altered from metamorphic minerals: mainly muscovite,

chlorite and albite. Hence, the gouges are clearly post-metamorphic in origin and have formed at shallower crustal

levels than any of the host metamorphic rocks.On a regional scale, the most prominent tectonic event in

the Otago Schist that involved post-metamorphic normal

faults with kilometre-scale offset occurred in the middle

Cretaceous (Deckert et al. 2002; Gray & Foster 2004;Mitchell et al. 2009). This event resulted in exhumation of

the core of the schist belt via normal faults on the margin of

the belt, and was active on the northeast side of the belt c.112 Ma (Gray & Foster 2004; Mitchell et al. 2009; Tulloch

et al. 2010). The major deformation in the Blue Lake Fault

Zone, which formed the gouge zones, was probably part ofthat regional event. Middle Cretaceous sediments associated

with these normal faults are locally preserved on the schist

margin, including a thick pile (Kyeburn Formation) c. 20km east of the Manuherikia section (Bishop & Laird 1976;

Mitchell et al. 2009). Normal fault movement persisted in

Central Otago to at least 96 Ma (Barker et al. 2010).

Late Cenozoic fault reactivation

Deformation of the Miocene unconformity has been minorand highly localised. Offset on faults has been on the 1�100m scale, and the results of this deformation are largely

visible as the present relief of the area surrounding theManuherikia gorge (Figs. 2A, 11A). The gorge itself has

been cut because of the ongoing uplift on the south-

easternmost strands of the Blue Lake Fault (Fig. 1C, 1D,11A) that define the boundary between the schist gorge and

the sediment-coated Manuherikia flats immediately down-

stream (Figs. 1B, 2B).The faults responsible for the offset of the Miocene

unconformity are narrow (centimetre to metre scale) and are

hosted in the wide gouge zones of the pre-existing normal

faults. Hence, the Miocene�Recent component of movementon the Blue Lake Fault Zone is localised reactivation of pre-

existing gouge zones. The contrast between these two

different types of fault zones can be seen in Fig. 9C, wherecentimetre-scale shears with reverse motion (Miocene�Recent) cut across the wide, multiply-deformed Cretaceousgouge zone. Cenozoic reactivation has occurred in this

Fiddlers Flat gouge zone to the extent that the syncline in

Miocene sediments has been truncated on its north-easternlimb and a greywacke hill (�50 m high) has been uplifted

on that side (Fig. 11A). Reactivation in the gouge zone

between TZ 2B and TZ 3 at the edge of the Manuherikia

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flats has resulted in only minor offset (B10 m) of theMiocene unconformity (Figs. 1D, 11A).There is no preserved evidence for post-Miocene reacti-

vation in other gouge zones in the Manuherikia gorge, butlate-stage centimetre-thick shears are common in thesegouges and some appear to have reverse sense of motion.However, Mackie et al. (2009) report Cretaceous compres-sional structures in the schist of North Rough Ridge (Fig.1B), so it is not possible to link reverse-fault reactivationfeatures to Miocene�Recent deformation without observingan offset of Miocene features. The Miocene unconformityappears to have been warped over the Manuherikia gorgebetween the two known reactivated fault strands (Fig. 11A).It is possible that post-Miocene folding of the schist base-ment and overlying unconformity has occurred betweenthese fault strands, similar to that which is occurring in theantiformal schist ranges such as Blackstone Hill and NorthRough Ridge to the south (Fig. 1B; Jackson et al. 1996).Small-scale reactivation steps in gouge zones throughout thegorge may have facilitated this broad folding. However,warping of the Miocene unconformity near the Blue LakeFault Zone has a northwest trend, rather than the northeasttrend of antiformal ranges to the south (Fig. 1B).The Miocene reactivation of normal faults was signifi-

cant for the stromatolites located at the contact of TZ 3schist and Miocene sediments (Fig. 1D, 10A, 10B). Thestromatolites owe their existence to the formation of afavourable habitat on the Miocene lake margin by the initialdevelopment of a small (metre-scale) bedrock scarp by faultreactivation in TZ 3 schist (Lindqvist 1994). Their photo-synthetic growth at that time presumably resulted inencrustations on the tops and sides of cobbles (Fig. 10C,panel 1). Their current positions are therefore not lifepositions (Figs. 10A, 10B), and some form of disruptionhas occurred at this site. The stromatolites have interstitialmudstone of the Bannockburn Formation, so the disruptionmust have occurred during Miocene deposition of thatsediment. A possible disruption event, related to ongoingfault reactivation, is depicted in Fig. 10C (panel 2). Withincreasing tectonic relief on the lake margin, the stromato-lites were uplifted relative to the water level of the lake,which would have led to the demise of the colony. The deadstromatolites were probably redeposited by mass flow orsurficial creep on the steepening slope.

Lithological controls on deformation

Throughout the structural evolution of the Manuherikiagorge section, there has been strong lithological control ondeformation style. In particular, argillites and pelitic schistsderived from argillites have been important for localisingfaults, starting under metamorphic conditions. Greywackeand psammitic schists have formed more resistant blocks ofrock at a range of scales, around which the argillites havedeformed and thickened (Fig. 3C). This effect is most

pronounced in the broken formation block, where strainduring metamorphism was so strongly focused into argillites(Fig. 4B) that greywacke and conglomerate boudins did notdevelop any cleavage. Focusing of synmetamorphic defor-mation into argillite reached an extreme in the brokenformation with development of the black shear zones (Fig.2B, 5), and this may have been facilitated and accentuatedby simultaneous formation and recrystallisation of micac-eous minerals and graphite.Argillites have also controlled the location and develop-

ment of the Cretaceous gouge zones, the most importantfaults in this section. Geochemical analyses show that gougeis derived from argillaceous material (Figs. 7A�7D). TheFiddlers Flat gouge zone occurs mainly in TZ 1 turbiditeswhere argillite is relatively common, rather than in agreywacke-dominated part of the section (Figs. 3A, 11B).Likewise, the gouge zone between TZ 2B and TZ 3 ispreferentially concentrated in pelitic schists on the TZ 2Bside of the fault (Fig. 6C). The TZ 3 schists are so fissile thatthis distinction becomes less significant in schists of thatmetamorphic grade (Fig. 8B).Once deformation zones developed in argillites and

pelitic schists, later reactivation was controlled by thesezones of weakened rocks. The synmetamorphic black shearin the broken formation helped to localise the post-metamorphic fault which juxtaposes the broken formationagainst TZ 2B rocks (Figs. 11B, 11C). The effects of thisreactivation are observable over at least 100 m in theargillaceous black shear in the broken formation, but thereis little evidence for deformation along this fault boundaryin the psammitic schists of TZ 2B in the hanging wall.Likewise, Miocene�Recent reverse faulting has been focusedinto argillaceous gouge zones of the Cretaceous normalfaults (Figs. 11A, 11B). While the regional-scale control onthe location of the Blue Lake Fault Zone was the majorcrustal boundary between Torlesse greywacke and OtagoSchist (Fig. 1A; Upton et al. 2009), on a more local scale itwas the argillites and pelitic schists in the rock sequence thatcontrolled the location (and subsequent reactivation) ofspecific fault strands.

Conclusions

Three major northwest-striking fault strands of the BlueLake Fault Zone are well exposed in a gorge of theManuherikia River in Central Otago. These faults juxtaposeslices of basement rocks of increasing metamorphic grade,from largely unmetamorphosed Torlesse turbidites togreenschist facies schist. The turbidite slice (prehnite-pum-pellyite facies; TZ 1) consists mainly of well-bedded grey-wacke with minor argillite. The proportion of argilliteincreases up-section, and these argillaceous rocks havecontrolled formation of a fault with a prominent gougezone (200 m wide) at Fiddlers Flat. This fault juxtaposes the

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turbidite sequence against broken formation (probablepumpellyite-actinolite facies).The broken formation consists of uncleaved greywacke

and conglomerate boudins (1�20 m scale) in a matrixdominated by sheared and weakly cleaved argillite. Moreextreme shearing in this broken formation has yielded ablack argillaceous shear zone with a fabric containingrecrystallised muscovite, chlorite and graphite. This blackshear zone has, in turn, localised a later set of clay-bearinggouges constituting a fault zone that juxtaposed the brokenformation against low-grade schists (pumpellyite-actinolitefacies, TZ 2B). These schists are dominated by a pervasivefoliation that is subparallel to transposed bedding. Morepelitic schists within this slice are locally folded withdevelopment of a spaced axial surface cleavage. Some ofthese pelitic schists contain more abundant metamorphicpyrite and graphite than argillites at lower grade. However,the TZ 2B schists are generally geochemically similar to thelow-grade turbidites. TZ 2B psammitic schists show feweffects of the fault juxtaposition to the broken formationblock. However, pelitic schists at the other side of the TZ 2Bstructural block have controlled development of a wide (300m) gouge zone where these schists were juxtaposed againstmicaceous greenschist facies schists (TZ 3). The TZ 3 schistsare also extensively deformed, with abundant gouge zonessubparallel to and crosscutting the pervasive foliation.The three main gouge zones make up about 30% of the

narrow (2 km wide) structural transition from low- to high-grade rocks. This structural section has been stronglythinned to c. 15% of its original metamorphic width bynormal faulting, and is now the narrowest such section onthe Otago Schist margin. These normal faults were initiatedduring middle Cretaceous tectonic unroofing of the OtagoSchist. The resultant Blue Lake Fault Zone is part of thesurface manifestation of a major crustal boundary betweenOtago Schist and Torlesse greywacke that formed in theCretaceous.The Blue Lake Fault Zone was reactivated in the

Miocene, in narrow (centimetre- to metre-scale) faults withmetre-scale reverse offsets. These offsets locally affectedthe deposition of fluvial and lacustrine sediments of theManuherikia Group, and facilitated growth of stromato-lites on a palaeo-lake shore at a fault scarp in basementschists. Ongoing deformation caused stromatolite deathand redeposition in this setting. Miocene�Recent deforma-tion has caused, and continues to cause, minor foldingand faulting (1�100 m scale) of the basement topographyand overlying Miocene sediments.

Acknowledgements

Funding for this research was provided by the Foundation for

Research, Science and Technology with additional support from

the University of Otago. This paper has benefited from discussions

with Daphne Lee and Alan Cooper. Thanks are due to Damian

Walls and Brent Pooley for laboratory assistance and to Tim

Blackler, James McCarthy and Zoe Reid Lindroos for field

assistance. Two journal referees provided constructive reviews

that substantially improved the presentation of the paper.

References

Barker SLL, Sibson RH, Palin JM, FitzGerald JD, Reddy S, WarrLN, van der Pluijm BA 2010. Cretaceous age, composition,and microstructure of pseudotachylyte in the Otago Schist,New Zealand. New Zealand Journal of Geology and Geo-physics 53: 15�29.

Bishop DG 1972. Progressive metamorphism from prehnite�pumpellyite to greenschist facies in the Dansey Pass area,Otago, New Zealand. Geological Society of America Bulletin83: 3177�3198.

Bishop DG, Laird MG 1976. Stratigraphy and environment ofdeposition of the Kyeburn Formation (Cretaceous), a wedgeof coarse terrestrial sediments in Central Otago. Journal of theRoyal Society of New Zealand 6: 55�71.

Cooper AF, Barreiro BA, Kimbrough DL, Mattinson JM 1987.Lamprophyre dike intrusion and the age of the Alpine fault,New Zealand. Geology 15: 941�944.

Craw D 1995. Reinterpretation of the erosion profile across thesouthern portion of the Southern Alps, Mt Aspiring area,Otago, New Zealand. New Zealand Journal of Geology andGeophysics 38: 501�507.

Craw D 1998. Structural boundaries and biotite and garnet‘isograds’ in the Otago and Alpine Schists, New Zealand.Journal of Metamorphic Geology 16: 395�402.

Craw D, Burridge C, Waters J 2007. Geological and biologicalevidence for drainage reorientation during uplift of alluvialbasins, central Otago, New Zealand. New Zealand Journal ofGeology and Geophysics 50: 367�376.

Deckert H, Ring U, Mortimer N 2002. Tectonic significance ofCretaceous bivergent extensional shear zones in the Torlesseaccretionary wedge, central Otago Schist, New Zealand. NewZealand Journal of Geology and Geophysics 45: 537�547.

Douglas BJ 1986. Lignite resources of Central Otago. N.Z. EnergyResearch and Development Committee, University of Auck-land, Auckland, New Zealand, Publication P104: 368 pp.

Forsyth PJ 2001. Geology of the Waitaki area. Institute ofGeological and Nuclear Sciences 1:250 000 Geological Map19. 1 sheet � 64 p.

Gray DR, Foster DA 2004. 40Ar/39Ar thermochronologic con-straints on deformation, metamorphism and cooling/exhuma-tion of a Mesozoic accretionary wedge, Otago Schist, NewZealand. Tectonophysics 385: 181�210.

Jackson J, Norris RJ, Youngson JH 1996. The structural evolutionof active fault and fold systems in Central Otago, NewZealand: evidence revealed by drainage patterns. Journal ofStructural Geology 18: 217�234.

Landis CA, Campbell HJ, Begg JG, Mildenhall DC, Paterson AM,Trewick SA 2008. The Waipounamu Erosion Surface: ques-tioning the antiquity of New Zealand land surface andterrestrial fauna and flora. Geological Magazine 145: 173�197.

Lindqvist JK 1994. Lacustrine stromatolites and oncoids: Manu-herikia Group (Miocene), New Zealand. In: Bertrand�SafariJ, Monty C eds. Phanerozoic Stromatolites 11. KluwerAcademic Publishers. Pp. 227�254.

Little T, Mortimer N, McWilliams M 1999. An episodic Cretac-eous cooling model for the Otago�Marlborough Schist, NewZealand, based on 40Ar/39Ar white mica ages. New ZealandJournal of Geology and Geophysics 42: 305�325.

Structure of the Blue Lake Fault Zone 327

Dow

nloa

ded

by [

Ana

dolu

Uni

vers

ity]

at 1

6:57

22

Dec

embe

r 20

14

MacKenzie DJ, Craw D 2005. Structural and lithological con-tinuity and discontinuity in the Otago Schist, Central Otago,New Zealand. New Zealand Journal of Geology and Geo-physics 48: 279�293.

Mackie C, MacKenzie DJ, Craw D 2009. Structural and litholo-gical controls on gold mineralization at Oturehua on thenortheastern margin of the Otago Schist, New Zealand. NewZealand Journal of Geology and Geophysics 52: 43�47.

Mackinnon TC 1983. Origin of Torlesse and related rocks, SouthIsland, New Zealand. Geological Society of America Bulletin94: 967�985.

Mackinnon TC, Howell DG. 1984. Turbidite facies in an ancientsubduction complex: Torlesse Terrane, New Zealand. Geo-Marine Letters 3: 211�216.

Mitchell M, Craw D, Landis CA, Frew R 2009. Stratigraphy,provenance, and diagenesis of the Cretaceous Horse RangeFormation, east Otago, New Zealand. New Zealand Journalof Geology and Geophysics 52: 171�183.

Mortimer N 1993. Jurassic tectonic history of the Otago Schist,New Zealand. Tectonics 12: 237�244.

Mortimer N 2000. Metamorphic discontinuities in orogenic belts:example of the garnet�biotite�albite zone in the Otago Schist,New Zealand. International Journal of Earth Sciences 89:295�306.

Nelson D 1982. A suggestion for the origin of mesoscopic fabric inaccretionary melange, based on features observed in theCrystalls Beach Complex, South Island, New Zealand. Geo-logical Society of America Bulletin 93: 625�634.

Silberling NJ, Nichols KM, Bradshaw JD, Blome CD 1988.Limestone and chert in tectonic blocks from the Esk Head

subterrane, South Island, New Zealand. Geological Society ofAmerica Bulletin 100: 1213�1223.

Tulloch AJ, Ramezani J, Mortimer N, Mortensen J, van denBogaard P, Maas R 2010. Cretaceous felsic volcanism in NewZealand and Lord Howe Rise (Zealandia) as a precursor tofinal Gondwana breakup. Geological Society of LondonSpecial Paper 321: 89�118.

Turnbull IM, Mortimer N, Craw D 2001. Textural zones in theHaast Schist � a reappraisal. New Zealand Journal of Geologyand Geophysics 44: 171�183.

Upton P, Koons PO, Craw D, Henderson CM, Enlow R 2009.Along-strike differences in the Southern Alps of New Zealand:Consequences of inherited variation in rheology. Tectonics 28:TC2007. DOI:10.1029/2008TC002353

Vannucchi P, Bettelli G 2002. Mechanisms of subduction accretionas implied from the broken formations in the Apennines, Italy.Geology 30: 835�838.

Watson JM, Grey DR 2001. Character, extent and significance ofbroken formation for the Tabberabbera Zone, central LachlanOrogen. Australian Journal of Earth Sciences 48: 943�954.

Worthy TH, Tennyson AJD, Archer M, Musser AM, Hand SJ,Jones C, Douglas BJ, McNamara JA, Beck RMD 2006.Miocene mammal reveals a Mesozoic ghost lineage on insularNew Zealand, southwest Pacific. Proceedings of the NationalAcademy of Sciences 103: 19419�19423.

Youngson JH, Craw D, Landis CA, Schmitt KR 1998. Redefini-tion and interpretation of late Miocene�Pleistocene terrestrialstratigraphy, Central Otago, New Zealand. New ZealandJournal of Geology and Geophysics 41: 51�68.

328 A Henne et al.

Dow

nloa

ded

by [

Ana

dolu

Uni

vers

ity]

at 1

6:57

22

Dec

embe

r 20

14