40ar/39ar thermochronologic constraints on deformation, metamorphism and cooling/exhumation of a...

www.elsevier.com/locate/tecto

Tectonophysics 385 (2004) 181–210

40Ar/39Ar thermochronologic constraints on deformation,

metamorphism and cooling/exhumation of a Mesozoic accretionary

wedge, Otago Schist, New Zealand$

D.R. Graya,*, D.A. Fosterb

aVIEPS School of Earth Sciences, University of Melbourne, Melbourne, Vic. 3010, AustraliabDepartment of Geological Sciences, University of Florida, Gainesville, FL 32611-2120, USA

Received 27 October 2003; accepted 6 May 2004

Available online 2 July 2004

Abstract

Structural thickening of the Torlesse accretionary wedge via juxtaposition of arc-derived greywackes (Caples Terrane)

and quartzo-feldspathic greywackes (Torlesse Terrane) at f120 Ma formed a belt of schist (Otago Schist) with distinct

mica fabrics defining (i) schistosity, (ii) transposition layering and (iii) crenulation cleavage. Thirty-five 40Ar/39Ar step-

heating experiments on these micas and whole rock micaceous fabrics from the Otago Schist have shown that the main

metamorphism and deformation occurred between f160 and 140 Ma (recorded in the low grade flanks) through 120 Ma

(shear zone deformation). This was followed either by very gradual cooling or no cooling until about 110 Ma, with some

form of extensional (tectonic) exhumation and cooling of the high-grade metamorphic core between 109 and 100 Ma.

Major shear zones separating the low-grade and high-grade parts of the schist define regions of separate and distinct

apparent age groupings that underwent different thermo-tectonic histories. Apparent ages on the low-grade north flank

(hanging wall to the Hyde-Macraes and Rise and Shine Shear Zones) range from 145 to 159 Ma (n=8), whereas on the

low-grade south flank (hanging wall to the Remarkables Shear Zone or Caples Terrane) range from 144 to 156 Ma (n=5).

Most of these samples show complex age spectra caused by mixing between radiogenic argon released from neocrystalline

metamorphic mica and lesser detrital mica. Several of the hanging wall samples with ages of 144–147 Ma show no

evidence for detrital contamination in thin section or in the form of the age spectra. Apparent ages from the high-grade

metamorphic core (garnet–biotite–albite zone) range from 131 to 106 Ma (n=13) with a strong grouping 113–109 Ma

(n=7) in the immediate footwall to the major Remarkables Shear Zone. Most of the age spectra from within the core of

the schist belt yield complex age spectra that we interpret to be the result of prolonged residence within the argon partial

retention interval for white mica (f430–330 jC). Samples with apparent ages of about 110–109 Ma tend to give

concordant plateaux suggesting more rapid cooling. The youngest and most disturbed age spectra come from within the

‘Alpine chlorite overprint’ zone where samples with strong development of crenulation cleavage gave ages 85–107 and

f101 Ma, due to partial resetting during retrogression. The bounding Remarkables Shear zone shows resetting effects due

to dynamic recrystallization with apparent ages of 127–122 Ma, whereas overprinting shear zones within the core of the

schist show apparent ages of 112–109 and f106 Ma. These data when linked with extensional exhumation of high-grade

0040-1951/$ - see front matter D 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.tecto.2004.05.001

$ Supplementary data associated with this article can be found, in the online version, at doi: 10.1016/j.tecto.2004.05.001.

* Corresponding author. Tel.: +61-3-8344-6931; fax: +61-3-8344-7761.

E-mail addresses: [email protected] (D.R. Gray), [email protected] (D.A. Foster).

D.R. Gray, D.A. Foster / Tectonophysics 385 (2004) 181–210182

rocks in other parts of New Zealand indicate that the East Gondwana margin underwent significant extension in the

f110–90 Ma period.

D 2004 Elsevier B.V. All rights reserved.

Keywords: Convergent margin tectonism; Accretionary wedge; Cretaceous tectonics; 40Ar/39Ar geochronology; Tectonic exhumation; Cooling

history; Otago Schist; New Zealand

1. Introduction ranes; Bradshaw, 1989) in the hanging wall to a long-

Within the deeper level of accretionary sediment

wedges, the processes of structural evolution and

exhumation of metamorphosed and transposed inter-

bedded sandstone and mudstone sequences are only

partly understood. Structures deep within these wedges

are characterized by intense schistosity development,

large-scale thrust-nappes and crosscutting shear zones

(e.g. Hara et al., 1990; Kusky et al., 1997; Aoya and

Wallis, 2003). Establishing which structures and fab-

rics relate to thickening of the schist wedge, that is, by

underplating in a simple wedge model, versus those

structures that form by vertical thinning and/or related

extensional collapse with exhumation is problematic.

Various hypotheses for the structural metamorphic

evolution of wedges have testable predictions. For

example, models of underplating combined with ver-

tical shortening suggest fabrics dominated by coaxial

strain and symmetrical flattening, uniform gradual

cooling and exhumation via erosion (e.g. Feehan and

Brandon, 1999; Ring and Brandon, 1999); whereas

models of shortening via nappe development at depth

followed by extensional exhumation and erosion sug-

gest non-coaxial deformation overprinted by exten-

sional shear zones and nonuniform cooling histories. In

this paper, we address some of these issues for the

Otago Schist belt of New Zealand (Fig. 1) using

mesoscopic structural overprinting relationships,40Ar/39Ar step heating experiments on micas and

whole rocks from selected fabric elements, and then

linking the overall timing of fabric development with

tectonic parameters external to the schist belt.

The Otago Schist belt provides a well-exposed

example of the exhumed deeper levels of a Jurassic–

Cretaceous accretionary wedge that developed along

the Mesozoic margin of East Gondwana (Coombs et

al., 1976; Bradshaw, 1989; Mortimer, 1993a,b). It

represents a period of continuous sedimentation and

accretionary prism accretion (Rakaia and Pahau Ter-

lived subduction system that bordered the then Gond-

wana continent. Consisting largely of monotonous

quartzo-feldspathic schist with minor intercalated mi-

caceous schist, greenschist and metachert, the Otago

Schist includes variously metamorphosed rocks of

Torlesse (quartzo-feldspathic greywackes), Caples

(volcaniclastic greywackes) and Aspiring (chert-mafic

volcanics) affinities. These have undergone a mutual

syn- to post-amalgamation regional metamorphism to

become part of the Otago Schist (Mortimer, 1993a,b,

2000; Graham and Mortimer, 1992).

Exposed as a region of domed flat-lying foliation

(Fig. 1b) within intensely deformed quartzo-feldspath-

ic greywacke the Otago Schist crops out as an f150

km wide, elongate, NW-trending, metamorphic belt

(Fig. 2) cored by garnet–biotite–albite greenschist

facies schist coincident with a medial antiform (Mor-

timer, 1993a,b, 2000). Mineral parageneses indicate

P–T conditions of 450 jC and 8–10 kbar (Mortimer,

2000) suggesting burial to depths of f20–30 km.

Previous K–Ar and Ar–Ar geochronology applied

to whole rock and partial mineral separates from the

Otago Schist argue for a Jurassic age for schist

metamorphism and a Cretaceous age for uplift and

final closure of the isotopic systems (see Harper and

Landis, 1967; Adams et al., 1985; Adams and Gabites,

1985). Stratigraphic and provenance data indicate that

exhumation of the Otago Schist was complete by

the late Early Cretaceous (f105 Ma, Adams et al.,

1985). Recently, recognition of extensional shear

zones (Deckert et al., 2002; Forster and Lister, 2003)

has led to models of core complex-related exhuma-

tion (Forster and Lister, 2003), although Mortimer

(1993a,b, p. 242) had previously argued for extension-

al exhumation.

Controversy still exists over how the Otago Schist

belt formed, the relative components of crustal thick-

ening and thinning, the rate and nature of cooling and

the process of exhumation. New thermochronologic

Fig. 1. (a) Geologic map of New Zealand showing the key tectonic elements, the main terranes and provinces, the distribution of plutonic rocks

and Torlesse terrane components. The Otago Schist deformation/metamorphism overprints the Caples terrane and part of the Rakaia terrane

defining a NW-trending schist belt. (Map modified from tectonostratigraphic terrane map of IGNS Qmap Series map). (b) Geological profile

showing the crustal architecture of the southern part of the South Island (based on seismic reflection profile from Mortimer et al. 2003, Fig.8).

D.R. Gray, D.A. Foster / Tectonophysics 385 (2004) 181–210 183

and geochronologic data presented in this paper further

constrain the timing of regional deformation and

metamorphism, as well as the cooling history and

exhumation of the schist belt. These data show that

much of the deformation is significantly younger than

previously considered and that apparent cooling ages

from the high-grade schist core coincide with the

timing of overall extension in other parts of the New

Zealand landmass.

2. Geological background

2.1. Geological makeup

The islands of New Zealand (Fig. 1a) consist of

distinct terranes (Coombs et al., 1976; Bishop et al.,

1985; Frost and Coombs, 1989) that represent dif-

ferent parts of the arc-trench-subduction system that

was active along the Gondwana margin from Triassic

Nevis Bluff

Fig. 2. Map of the Otago Schist showing the Caples–Torlesse boundary, the general schist structure, including axial surface traces of both

Cretaceous fold-nappes and Cenozoic folds, the areal distribution of the north and south flanks to the schist core, and the Ar–Ar sample

locations (based on maps in Mortimer, 1993a,b and Fig. 24 of Turnbull, 2000). RSSZ: Rise and Shine shear zone; HMSZ: Hyde-Macraes shear

zone; RemN: Remarkables fold-nappe; BenN: Bendigo fold-nappe; ManN: Manorburn fold-nappe; BrN: Brighton fold-nappe.


to Late Cretaceous times (e.g. Bradshaw, 1989;

Mortimer, 2004). These terranes include:

Median Batholith (former Median Tectonic Zone):

‘Cordilleran-style’ composite batholith consisting

of Triassic–Early Cretaceous subduction-related I-

type plutonic, volcanic and sedimentary rocks that

intrude and separate Permian of the Eastern

Province Brook Street Terrane from the lower to

mid-Palaeozoic Gondwana margin assemblages of

the Western Province (Kimbrough et al., 1994;

Muir et al., 1988; Mortimer et al., 1999).

Brook Street terrane: Permian arc sequence of

layered ultramafic-gabbro sequences, diorites and

volcaniclastic sediments (roots of an intra-oceanic

arc).

Murihiku terrane: Triassic to Jurassic volcanogenic

sandstone, siltstone and tuff (probable forearc basin

sequence).

Dun Mountain-Matai terrane: Permian ophiolite

melange, and volcanogenic sediments.

Caples terrane: Permian to Triassic/Jurassic?

volcaniclastic marine flysch (trench slope or trench

floor deposit).

Rakaia/Older Torlesse terrane: Permian to Middle

and Late Triassic turbiditic quartzo-feldspathic

sandstone and argillites (submarine fan sequence).

2.2. Crustal structure

The fault-bounded ophiolite and melange of the

Dun Mountain-Matai terrane defines a steeply N-


dipping interface between the gently folded, inter-

leaved arc– forearc sequences (Median Batholith,

Brook Street and Murihiku terranes) to the south

and the deformed Torlesse composite-terrane subma-

rine fan sediments to the north (Fig. 1b). The Otago

Schist occupies a domal culmination in the immediate

hanging wall to the Livingstone Fault and has lower

metamorphic grade units (Caples—south flank and

Rakaia—north flank) structurally overlying the high-

grade schist core. Deep crustal seismic profiling (see

Fig. 8 of Mortimer et al., 2003) indicates the schist

core is centered on the medial domal antiform defined

by a broad warp in flat-lying schistosity, with a

maximum subsurface width of f220 and f20 km

of structural relief (Fig. 1b). Northwards beyond the

Waihemo Fault, there is a transition into the tectoni-

cally imbricated and weakly metamorphosed Perm-

ian–Triassic greywacke sequence of the Rakaia

(Older Torlesse) terrane.

South of the Livingstone fault the crustal section

is composed of a f10–15-km-thick succession of

Murihiku forearc sediments overlying a f10-km-

thick arc sequence of Brook Street volcanics,

intruded to the south by the Median Batholith that

constitutes the largely Jurassic–Cretaceous arc. To

the north of the Dun Mountain ophiolite slice, the

crustal section is composed of structurally thickened

(f20 km thickness), Permian to Triassic/Jurassic

sediments of the Rakaia terrane with an overlying

f10-km-thick wedge of trench sediments (Caples

terrane) immediately adjacent to the Livingstone

fault.

2.3. Otago Schist structure

Structure of the Otago Schist is dominated by

schistosity and transposed layering at the mesoscale

(Fig. 3), whereas shear zones and apparent recum-

bent isoclinal hinges generally in a transposition

layering occur at the macro- or regional scale

(Means, 1963, 1966; Wood, 1963). Flat-lying schis-

tosity of the Otago Schist core consists of L-S

tectonite with a dominant foliation and marked

stretching and/or rodding lineation (Mortimer,

1993a,b). The schist displays variable morphology

due to variations in strain, transposition, metamor-

phic differentiation and metamorphic grade (Bishop,

1972; Mortimer, 1993a,b, 2003; Norris and Bishop,

1990). Important boundaries between structural

domains within the Otago Schist are defined by

shear zones (Norris and Craw, 1987; Craw, 1998;

Mortimer, 2000; Deckert et al., 2002). Major bound-

aries include (1) the Caples–Torlesse boundary (Fig.

2), which is defined by a high strain zone typified by

the Remarkables Shear Zone (Cox, 1991); (2) the

northern boundary with the high grade core is

marked by the Hyde-Macraes Shear Zone in eastern

Otago and the Rise and Shine Shear Zone in central

Otago (Deckert et al., 2002) (see HMSZ and RSSZ,

Figs. 2 and 3) further northwest, the boundary is

marked by a shear zone separating the biotite–

garnet–albite assemblages in multiply transposed

psammitic schist of the Torlesse terrane from pelitic

schist intercalated with metavolcanics containing rare

biotite of the Aspiring association (see Fig. 4a of

Craw, 1998).

Other shear zones, characterized by markedly

higher strain than the surrounding schist, appear as

narrow, crosscutting, phyllonite zones (cf. Forster

and Lister, 2003, Fig. 7d). These truncate zones of

steep enveloping surface (e.g. Niger Nappe, Fig. 4a)

that have been interpreted as macro-folds (Means,

1963, 1966; Turnbull, 1981) in the core of the schist

(see Cretaceous fold axial surface traces, Fig. 2).

These folds, or half-folds (after Mortimer, 1993a,b,

p. 242) show a visible strain gradient increase from

upper limb to hinge to lower limb (e.g. Remarkables

Nappe, Fig. 5; Brighton nappe, Fig. 6). The Man-

orburn structure of Means (1963, 1966) (see ManN,

Fig. 2) shows a change from transposition layering

to ‘‘mylonitic’’ fabrics towards the lower limb (Fig.

7). More highly strained zones develop ‘‘platy’’

schist (Fig. 3b) where quartz–albite veins are bou-

dinaged and strung out subparallel to, and help

define the dominant schistosity, giving the schist

the appearance of a pseudo-metamorphic layering.

Transitional to these more highly strained zones,

folds in the quartz–albite veins develop sheath-like

form with curvilinear hinges (Fig. 7e) that give

irregular rodding intersection lineations on the schis-

tosity surface (‘‘herringbone’’ rodding pattern; after

Norris, personal communication, 1992). Shear bands

and other shear sense indicators are rare in the

Otago Schist (Mortimer, 1993a,b), but shear bands

tend to be localized in zones of overprinting, higher

strain with phyllonitic or mylonitic schistosity (see

Fig. 3. Schistosity of the Otago Schist. (a) Typical tor outcrops of flat-lying Otago Schist, Gorge Creek, near Roxburgh (see Fig. 2 for location).

(b) Schistosity defined by quartz–albite layering, formerly veins, near Roxburgh, central Otago. Lens cap for scale has 6.5 cm diameter.


Deckert et al., 2002; and Figs. 7b and 8a of Forster

and Lister, 2003).

The Caples terrane (south flank of the schist

domal culmination, Fig. 2) consists of a large

overfold into a basal shear zone (Remarkables

fold-nappe: RemN, Fig. 2) associated with NE-

directed thrusting (Norris and Craw, 1987; Cox,

1991; Mortimer, 1993a,b; Little and Mortimer,

2001). Relative to other parts of the Otago Schist

it shows lower strain with recognizable bedding and

a slaty-type cleavage (Fig. 5a). To the north, the

Rakaia terrane (north flank) also shows lower strain,

recognizable bedding with a spaced cleavage tran-

sitional into a transposition layering (Fig. 4b,c) with

overprinting crenulation cleavages (Bishop, 1972;

Norris and Bishop, 1990).

2.4. Schist structural chronology

Various attempts have been made to establish a

structural chronology across the Otago Schist

(Means, 1963; Wood, 1963; Norris, 1977; Craw,

1985; Mortimer, 2003, Fig. 2), and there are clear

relationships of folding of bedding, early cleavage

and schistosity (see Fig. 2 of Means, 1966) as well

as early-formed high strain zones (e.g. greenschist

Fig. 4. (a) Oblique view of Niger Nappe cut by shear zone (DDZ: ductile deformation zone), Matukituki Valley. This is typical of the fold-

nappes or half-folds of Mortimer (1993a,b). (b) and (c) are photographs of structures from the low-grade northern flank, Otago Schist. (b)

Differentiated layering axial surface to isoclines in bedding, shoreline on Lake Hawea. Lens cap for scale has 6.5 cm diameter. (c) Transposed

layering, Danseys Pass. Coin for scale has f3 cm diameter.


fold core at Ophir tip). Most fold-nappes are second

generation structures (e.g. ‘Manorburn’ generation of

Norris, 1977).

Norris established generations of structures

based on overprinting relationships, but these were

difficult to apply across the whole schist belt due

to poly-deformation related to ‘‘transposition cy-

cling’’ (after Tobisch and Paterson, 1988; see Mor-

timer, 1993a,b) and overprinting shear zones (e.g.

Forster and Lister, 2003). As a consequence, Craw

(1985) has argued that fabrics cannot be reliably

correlated from one point in the schist to another

and adopted a dominant fabric (Sm) approach.

Most recently, Forster and Lister (2003) based on

observations in the Dunstan Range of central Otago

argued for episodic, multistage deformation with

Fig. 5. Photographs of the Remarkables thrust-nappe. (a) View looking east at the Remarkables Range showing the south-dipping Remarkables

Shear Zone or ductile deformation zone (DDZ) and the positions of photographs 1, 2 and 3. Note the ski field road (zigzags) for scale. ALA:

Aspiring Lithologic Association or Aspiring terrane. (b) Weakly deformed schist with quartz–albite veins, Lake Alta. (c) Asymmetrically folded

layering in 1 with new axial surface-parallel veins, Ski Road at position 2. (d) Intensely deformed layering adjacent to the shear zone underlying

the Remarkables thrust-nappe at location 3 in (a). Lens cap for scale has 6.5 cm diameter.


three generations of recumbent folds and four gen-

erations of shear zones.

2.5. Schist metamorphism

The Otago Schist has a well-documented and

well-defined metamorphic zonation (e.g. Hutton

and Turner, 1934; Brown, 1967, 1968; Bishop,

1972, 1974; Yardley, 1982; Mortimer, 2000). Meta-

morphic grade increases towards the center of the

Otago Schist from prehnite–pumpellyite facies on

the northern and southern flanks (Torlesse and

Caples Terranes, respectively) to biotite grade

greenschist facies, through epidote–amphibolite fa-

cies in the broad central portion (Mortimer, 2000).

Former estimates of maximum pressure and temper-

ature, based on mineral assemblages, are 4.5 kbar

and 400 jC, respectively for the central part of the

belt (Yardley, 1982). Geobarometry on garnet–bio-

tite–albite zone assemblages that are now more

widely recognized in the schist give possible pres-

sures as high as 8–10 kbar (Mortimer, 2000). These

estimates place the Otago Schist in a moderate–high

P/T metamorphic series (Barrovian-type).

Relict blueschist assemblages occur in mafic

greenschist/chert sequences close to the Caples–Tor-

lesse boundary (Yardley, 1982). Riebeckite and cross-

ite relicts occur mantled by actinolite indicating an

earlier high-pressure intermediate type metamorphism

overprinted by the Barrovian moderate–high P/T

metamorphism.

3. Previous geochronology/thermochronology

K–Ar dates of whole rock samples and mineral

concentrates from the Otago Schist/Torlesse terrane,

New Zealand were previously used to argue for a

Jurassic age for schist metamorphism and a Cretaceous

age for uplift and final closure of the isotopic systems

(see Harper and Landis, 1967; Sheppard et al., 1975;

Adams et al., 1985; Adams and Gabites, 1985; Adams

Fig. 6. Schematic profile and field photographs of different elements of the Brighton fold-nappe, coastal Otago. (a) Structural profile with40Ar/39Ar ages (see Table 1 for errors on ages) and sketch enlargements of structural elements at three different structural positions

corresponding to the photographs. S0: bedding trace, Sm: dominant foliation or schistosity trace, Scc: crenulation cleavage trace, (b), (c), and (d),

respectively (designated by the letter inside the circle). (b) Asymmetric vergence folds on the upper limb with axial surface crenulation cleavage.

Hammer for scale has 33 cm length. (c) Intense rodding fabric within the hinge zone of the Brighton fold-nappe, foreshore at Brighton. Pen for

scale has 14 cm length. (d) Upper limb to hinge transition showing cuspate folds in psammite bedding, foreshore south end of Brighton.

Notebook for scale has 19 cm length.


and Robinson, 1993; Adams and Graham, 1996,

1997). Key interpretations based on the previous K–

Ar data are:

(1) initial regional metamorphism of the Otago Schist

is at least 170 Ma old and may be as old as 200–

225 Ma (Early–Middle Jurassic) with subsequent

cooling over f45 Ma, and a possible hydrother-

mal event at f120 Ma (e.g. Adams and Graham,

1997);

(2) K–Ar ages show an inverse correlation with

metamorphic grade where the ages date either the

time of peak metamorphism at a particular

structural level or a stage of the post-metamor-

phic cooling (e.g. Adams et al., 1985; Adams and

Robinson, 1993);

(3) deformation in the North Island is diachronous

from west to east, with deformation occurring

over 20 Ma, and overprinted by shear zones in the

period 100–135 Ma (see Fig. 4 of Adams and

Graham, 1996).

K–Ar dates from the north flank of the Otago

schist range from f200 to 150 Ma (Adams et al.,

Fig. 7. Manorburn fold-nappe after Means (1966). (a) Schematic profile showing the gross geometry, location of samples and apparent ages

of mica separates from the samples illustrated (see Table 1 for errors on ages). (b) Strong rodding fabric in L-S tectonite from shear zone

truncating the lower limb, Ophir. Lens cap for scale has 6.5 cm diameter. (c) Steep enveloping surface in folded layering of the hinge

zone, upper Manorburn Reservoir. Base of photo is f3 m. (d) Crenulated layering and vergence folds, Alexandra. Base of photo is f40

cm. (e) Curved hinge line of sheath-fold in quartzite layer, hillside south of Ophir Tip. Lens cap for scale has 6.5 cm diameter. L:

lineation trace.


1985), with rocks near Lake Wanaka and Lake Hawea

(Fig. 2) interpreted to have undergone peak metamor-

phism at 173 Ma, followed by gradual cooling

through 120 Ma (Adams and Gabites, 1985). Rb–Sr

isochron ages from Danseys Pass (Fig. 2) are 145F3

and 152F9 Ma (see Fig. 6 of Graham and Mortimer,

1992), whereas K–Ar ages there are 154.8F1.8 and

159.3F2.5 Ma (see Table 2 of Graham and Mortimer,

1992).

Whole rock K–Ar and white mica 40Ar/39Ar

apparent ages across the Caples–Torlesse/Aspiring

boundary shear zone near Glenorchy (Fig. 2) are

between 135 and 145 Ma (Graham and Mortimer,

1992; Adams and Robinson, 1993; Little et al., 1999).

At Nevis Bluff (Fig. 2) immediately below the

Remarkables shear zone, schist samples gave K–Ar

ages of f117 and f114 Ma and an Rb–Sr age of

111F5 Ma (Graham and Mortimer, 1992); whereas40Ar/39Ar analyses of white mica have yielded an age

of 133F5 Ma (see Fig. 6 of Little et al., 1999). Near

Queenstown (Fig. 2) schist has yielded an Rb–Sr age

of 129F9 Ma and a K–Ar age of 141F2 Ma (Graham

and Mortimer, 1992). Such discordance in apparent

ages for the K–Ar and previous 40Ar/39Ar data were

attributed to hydrothermal fluids, excess argon as well

as effects of poly-deformation (Adams and Robinson,

1993, Little et al., 1999). Some of the discordance

may have been due to relatively large sample sizes

and associated heterogeneity.

Based on K/Ar data for the Otago Schist, Adams

and Graham (1997, p. 281) argued that the schist

either (1) underwent progressive burial to higher

metamorphic grade during a single event, followed

by long and continued uplift, or (2) early burial


metamorphism at f200 Ma was followed by later

deformation and metamorphism between 196 and 115

Ma, followed by rapid uplift. Little et al. (1999), based

on 40Ar/39Ar data from Marlborough (Fig. 1), Gle-

norchy and Queenstown (Fig. 2), as well as thermal

modeling, interpreted polyphase metamorphism of the

Otago Schist to involve regional metamorphism and

mica growth at 200–170 Ma, uplift and rapid cooling

at 135 Ma, followed by reheating at 110–70 Ma

(Mortimer, 2000). They argued against a slow and

continuous cooling history. 40Ar/39Ar data in the

schist core gave ages of 130.1F2.3 Ma (Taieri River,

Hindon, Fig. 2) and 129F1.3 Ma (north of Queens-

town) (Little et al., 1999).

Most recently 40Ar/39Ar geochronology from the

Dunstan Mountain (Fig. 2) part of the Otago Schist

(Forster and Lister, 2003) has been used to argue for

episodic recumbent folding and shear zone develop-

ment at different times. Mica ages imply recumbent

folding at f120 Ma (Bendigo fold-nappe hinge) and

f115 Ma (hinge near Clyde, Fig. 2) followed by the

development of younger shear zones that have appar-

ent ages of f110 and f97 Ma (see Northburn Shear

Zone 40Ar/39Ar spectra in Fig. 5 of Forster and Lister,

2003). Age spectra from most of these samples are

highly discordant (see Figs. 9 and 10 of Forster and

Lister, 2003) presumably a consequence of heteroge-

neous mica breakdown during dynamic recrystalliza-

tion and retrogression associated with the overprinting

shear zones.

4. 40Ar/39Ar step heating experiments

In this study, 40Ar/39Ar geochronology was used to

date micas selectively from different foliations and

structural levels in the schist pile, and from over-

printing high strain zones. Cleavage zones were also

dated using carefully selected small whole rock chips

in the low-grade outer flanks because of the very fine

grain size of the white micas (cf. Foster et al., 1999).

4.1. Analytical technique

40Ar/39Ar analyses of the samples were undertaken

at La Trobe University, Australia (Foster et al., 1999)

and the University of Nevada Las Vegas, following

standard methods (e.g. McDougall and Harrison,

1999). Samples analyzed at La Trobe University were

irradiated in a core position of the IRR-1 reactor,

Soreq Nuclear Research Center, Israel along with the

flux monitor GA1550 biotite. Samples were heated in

a computer-controlled double vacuum resistance fur-

nace with a tantalum crucible, or with a 7 W argon-ion

laser. Laser-step heating was done with a defocused

beam and by changing the power output of the laser.

Gas was expanded into a stainless steel clean-up line

and purified with two 10 l/s SAES getters. Argon

isotopes were measured using a VG3600 mass spec-

trometer with a Daly photomultiplier, operating at a

sensitivity of 7�10�4 A/Torr. Data were corrected for

machine background determined by measuring system

blanks, and mass discrimination determined by ana-

lyzing atmospheric argon. Extraction line blanks were

typically <2�10�16 mole 40Ar. Correction factors for

interfering isotopes were determined by analyzing

K2SO4 and CaF2 salts irradiated with the samples.

Samples analyzed at the University of Nevada Las

Vegas were wrapped in Sn foil and stacked in fused

silica tubes with the neutron fluence monitor FC-2

(Fish Canyon Tuff sanidine). Samples were irradiated

at the Ford reactor, University of Michigan for 6 h in

the L67 position. Correction factors for interfering

neutron reactions on K and Ca were determined by

repeated analysis of K-glass and CaF2 fragments

included in the irradiation. Measured (40Ar/39Ar)Kvalues were 1.56 (F38.21)�10�2. Ca correction fac-

tors were (36Ar/37Ar)Ca=2.79 (F6.09)�10�4 and

(39Ar/37Ar)Ca=6.61 (F0.21)�10�4. Samples were

heated using a double vacuum resistance furnace.

Reactive gases were removed by two GP-50 SAES

getters prior to expansion into a MAP 215-50 mass

spectrometer. Peak intensities were measured using a

Balzers electron multiplier. Mass spectrometer dis-

crimination and sensitivity was monitored by repeated

analysis of atmospheric argon aliquots from an online

pipette system. The sensitivity of the mass spectrom-

eter was 6�10�17 mol/mV. Line blanks averaged

17.33 mV for mass 40 and 0.06 mV for mass 36.

4.2. Results

A summary of the samples analyzed, calculated

ages and comments on the interpretation of the data

are given in Table 1. The raw 40Ar/39Ar data are

included in Appendix 1 (see online version). Age

Table 1

Summary of mica and whole rock Ar–Ar data

Samplea Location Grid ref Calculated age

(Ma)bCommentsc

North flank

DG91-93 (wr) Dansey’s Pass 141/961767 156.8F0.8 TFA

DG99-300 (wr) Tiroiti 142/952471 158.7F0.9 partial plateau of saddle shaped spectrum

DG91-137A Lake Hawea S107/005426 152F1 average age of flat portion of age spectrum

(f80% of gas), high-T ages suggest a minor

detrital mica component

DG91-117 Lake Hawea S107/030416 146.8F0.8 plateau and isochron age

DG91-111 (wr) Lake Hawea S107/038409 151.3F0.4 partial plateau segment

DG99-298 Oturehua Stone Quarry H41/658811 145.1F0.8 TFA: possible recoil effects

DG89-341E Eweburn Dam H41/791741 146.4F1.0 TFA: age gradient from f150–140 Ma

DG90-13D Lindis Valley G40/323065 144.6F0.2 partial plateau of saddle shaped spectrum

DG91-123 Lindis Valley G40/322066 143.6F0.7 TFA of slightly saddle-shaped spectrum

DG99-264 Lindis Valley G40/339948 144.5F0.7 plateau age

South flank

DG91-162 (wr) Taeri River mouth 145/924579 158.8F0.8 TFA: discordant, hump shaped age spectrum

due to recoil?

DG90-82F (wr) Kuri Bush 145/956611 156.1 F0.3 plateau segment with mica K/Ca ratios,

some recoil problems and detrital mica

component with ages f175 Ma for high-T steps

DG94-93 (wr) Kuri Bush 145/956611 151.8F0.8 TFA: hump-shaped spectrum with minor recoil?

DG94-95 (wr) Brighton 145/034703 143.1F0.7 TFA: saddle shaped spectrum

DG92-90 Lake Onslow spillway G43/437118 139.5F0.6 TFA: discordant age spectrum

DG90-77B Gorge Creek G43/189302 134.3F0.7 isochron age

DG99-258 Old Man Range G42/157340 134.4F0.1 plateau-like spectrum

DG90-69A Lake Alta F41/805633 125.0F0.6 TFA: discordant, multiple generations of white mica

DG92-21 Alex Bridge G42/277447 122.7F0.6 plateau age

DG90-150B Remarkables Ski road F41/792674 121.8F0.3 partial plateau age, with a lower T gradient

to f107 Ma (two generations of white mica)

Schist core

DG90-75A Bendigo G41/172789 122.2F0.9 high-T plateau segment, with low-T age gradient

DG92-45A Upper Manorburn Dam G42/445330 121.6F1.1 plateau age

DG91-155 Thompsons Pass G42/365765 111.1F0.8 TFA: average of a cooling gradient from about

113 to 107 Ma

DG90-73 Gentle Annie Creek F41/980676 113.8F0.3 partial plateau

DG92-89B Cromwell Gorge G42/206573 112.6F1.4 flat segment of spectrum comprising about 80%

of the gas, with cooling gradient to f100 Ma

DG94-115 Gold Monument,

Cromwell Gorge

G41/121648 111.5F0.6 plateau age

DG99-294 Cromwell Gorge G42/206574 110.8F0.6 TFA: discordant spectrum, most steps give ages

between 113 and 105 Ma

DG97-215 Kawarau Gorge F41/992690 111.8F0.2 plateau age

DG89-353C Nevis Bluff F41/948669 109.6F1.1 TFA: average of a cooling gradient from

about 110 to 107 Ma

DG89-356 Cromwell Gorge G42/206560 108.9F1.1 TFA: >90% of gas about 110 Ma

DG89-326 Treble Cone S114/745187 108.2F0.2 high-T plateau segment, with low-T cooling

gradient to f85 Ma

DG92-73 Ophir Tip G41/418603 107.4F0.9 TFA: minor saddle shape to age spectrum

DG91-95B Ophir Tip G41/418606 113.8F0.6 TFA: plate age spectrum with most steps f113 Ma

DG99-267 Lake Wanaka S115/799171 102.1F0.2 plateau age

DG99-268 Matukituki Valley S114/730308 100.9F0.6 plateau age



spectra diagrams for individual samples are given in

Appendix 2 (see online version), and typical age

spectra are grouped with respect to structural/geo-

graphic position (Fig. 8). 40Ar/39Ar data from the

Otago Schist show a range of apparent ages from

160 to f100 Ma.

The oldest apparent ages are associated with the

lowest grade rocks that still show bedding and a

primary cleavage. These ages occur along the north-

ern and southern flanks of the Otago Schist core

(Table 1 and Fig. 9). In the southern flank, or the

Caples terrane, the oldest apparent ages (f156–151

Ma) come from the Brighton coastline south of

Dunedin (n=4) (Figs. 2 and 9); however, the spectra

from these samples are partly discordant due to 39Ar

recoil effects, so placing precise error bounds on

these ages is not possible. Along the coast, there

appears to be a northwards change from older ages

of about 156–151 Ma (samples DG91-162, DG90-

82F, DG94-93) to f144 Ma (sample DG94-95)

(Figs. 2 and 9) in the rodded-schist that is part of

the hinge of an apparent fold nappe at Brighton (Fig.

6c); i.e. towards the major Caples–Torlesse terrane-

bounding shear zone (dashed line in Fig. 9). Further

to the west, the same northwards decrease in appar-

ent age occurs towards the Remarkables shear zone

segment (RMS in Fig. 9), with a change from ages

of about 134 Ma (samples DG90-77B, DG92-90,

DG99-258) in central Otago to f122 Ma (sample

90-150B) within the shear zone. In the hanging wall

to the Remarkables shear zone, the Remarkables

fold/thrust-nappe of Cox (1991) has bedding and a

slaty-type cleavage (Fig. 10a) preserved in the upper

limb. This schist yielded a mildly discordant spectra

with an average age of 125F0.6 Ma (sample DG90-

69A, Fig. 10e), whereas adjacent to the bounding

shear zone micas associated with the intensely trans-

posed ‘‘mylonitic-like’’ layering (Fig. 10d) give a

well-defined plateau age of 122F1 Ma (sample

DG90-150B, Fig. 10f). Detrital mica is considered

rare in Caples greywackes even at very low grade

(see Little et al., 1999, p. 316), and the age spectra

Notes to Table 1:

Grid ref: map #/grid reference (based on 1:50,000 map series or 1:63,000aPicked white mica or white mica concentrate, except where noted asbErrors are one sigma.cTFA=total fusion age.

from these samples appear to reflect degassing of

argon from metamorphic mica only. The effects of a

small fraction of partly degassed detrital mica can be

seen however, in some spectra with steps as old as

f175 Ma (e.g. sample DG90-82F, Fig. 11).

Apparent ages on the low-grade north flank

(hanging wall to the Hyde-Macraes- and Rise and

Shine Shear Zones) range from about 145 to 159

Ma (n=8) (Table 1, Figs. 8 and 9). There is some

discordance in the age spectra due to a variable

fraction of partly degassed detrital mica along with

the neocrystalline metamorphic mica in the samples.

Sample DG99-300 (Fig. 12a,b) yields a saddle-

shaped spectrum where the pseudo-plateau age

gives a maximum age for the age of metamorphic

mica, and the older steps that are as old as f170

Ma due to mixing with partially reset detrital mica

that remains in the sample. Samples with ages of

about 145–150 Ma, also in the hanging wall, could

be a better record of the metamorphic mica age

(Fig. 12b). The f145 Ma ages (samples DG99-

264, DG90-13D) are also typical of the hanging

wall of the Hyde-Macraes Shear Zone. Some age

spectra with significant proportions of the gas

released giving about 145 Ma are relatively concor-

dant like these samples, while others are discordant

(sample DG91-123, Table 1 and Appendix 2 (see

online version)). The concentration of apparent ages

around 144–147 Ma for the north flank samples

that lack detrital mica contributions suggests that the

metamorphic micas either formed in these samples

at that time, or that they cooled through mica

closure at that time. The metamorphic temperature

conditions of the low-grade north flank overlap the

closure temperatures for argon in white mica, which

supports an interpretation that the micas grew at this

time. Samples with overprinting discrete crenulation

cleavages (Scc in samples DG99-300 and DG89-

341E, Fig. 12) give the age of primary slaty

cleavage development, as these younger fabrics are

due solely to pressure solution removal of quartz

and do not involve new mica growth.

series, designated with an S).

whole rock (wr).

Fig. 8. Grouped 40Ar/39Ar spectra from different structural positions in the Otago Schist, (a) Schist core, (b) northern and southern flanks. Note

the groupings of data at f120 and f115–105 Ma in (a) and f145–155 and 130–120 Ma in (b).


Micas from the fabrics in the high-grade core of the

Otago Schist give consistent apparent ages of f122–

123 andf112–110Ma (Table 1 and Fig. 9). Fabrics in

the hinges of the Manorburn generation ‘‘folds’’ (e.g.

Manorburn fold of Means, 1966 and Bendigo fold-

nappe of Paterson, 1971) give apparent ages of 122F1

Ma (samples DG90-45A and DG90-75A, Table 1; Fig.

13). Overprinting shear zones or regions with appre-

ciably higher strain (Fig. 14) and therefore younger

fabric development give apparent ages of f112 Ma

(samples DG92-89B, DG94-115, DG99-294: Crom-

well Gorge Shear Zone or Northburn Shear Zone;

Fig. 9. 40Ar–39Ar age distribution map showing sample locations and calculated ages (for errors see Table 1) with respect to the schist core and

major bounding shear zones. The location of the shear zone boundary (dashed line) is the Caples-Torlesse boundary after Mortimer and Roser

(1992).


Fig. 10. Photomicrographs showing strain dependent microfabrics in the Remarkables fold/thrust-nappe, from samples from Lake Alta to the

shear zone (DDZ: ductile deformation zone) from the Remarkables Ski field road, near Queenstown. (a) to (d) shows increasing strain with

accompanying 40Ar/39Ar age spectra (e and f ). Base of photomicrographs is f7 cm in each case.


Fig. 14a,b), and f114 Ma (sample DG91-95B) and

f107 Ma (sample DG92-73, Fig. 14c, d) from local-

ized shear zones in the Ophir region of the Manorburn

fold-nappe. In the footwall to the Remarkables Shear

Zone (e.g. Nevis Bluff; see Fig. 2 for location) isocli-

nally folded greyschist (Fig. 15a,b,c) structurally inter-

leaved with greenschist (Fig. 15a) gives an age spectra

of 109F1 Ma (sample DG89-353C; Fig. 15d). These

may define further periods of ductile shearing, where

the micas are recrystallized enough that the older gas

was completely lost, in the schist core. This younger

group of samples tends to give relatively flat age

spectra with plateau segments, or age gradients that

differ by only a few million years suggesting relatively

rapid cooling after shearing and recrystallization. We

therefore interpret the concentration of ages between

aboutf113 andf107 Ma to indicate that there was a

period of rapid cooling and exhumation of the schist

core that started at this time.

4.3. Interpretation of step heating experiments

40Ar/39Ar data for individual samples from the

Otago Schist potentially reflect the timing of mineral

Fig. 11. Typical fabric of low-grade Otago Schist from a coastal exposure in the Caples Terrane, Kuri Bush south of Brighton. (a) Photograph of

thin section showing thin siltstone laminations in bedding (S0), a primary slaty cleavage (S1) and a weakly developed crenulation cleavage (Scc).

Base of thin section is 3.5 cm. (b) Outcrop photograph showing curvilinear bedding intersection traces (heavy black lines) on the flat-lying

cleavage planes indicating high strain. Pen for scale has 14 cm length. (c) 40Ar/39Ar age spectrum from sample shown in (a) (sample DG90-

82F).


growth, the timing of cooling and exhumation, or the

timing of mica ‘‘recrystallization’’. These events may

have been part of a single progressive deformation or

of a more complex, episodic history of poly-defor-

mation. Potential complications with interpretation of

some of the age spectra may be related to (1) the

detrital mica component, particularly in the outer low

grade flanks where metamorphic temperatures were

<280 jC (Fig. 9, Kuri Bush), (2) partial argon loss

during gradual/slow cooling through the Ar partial re-

tention zone between temperatures of f400–320 jC

(schist core), and (3) the effects of partial dy-

namic recrystallization in the overprinting shear

zones.

There are two important possible effects on

schist microstructure to be kept in mind when

interpreting the 40Ar/39Ar data. Firstly, dynamic

recrystallization of schist microstructure in shear

zones, and secondly thermal annealing of micro-

structure due to metamorphic heat input outlasting

the deformation. Both effects may be important

because the schist microstructure is typically either

Fig. 12. Typical fabrics and 40Ar/39Ar age spectra of low-grade Otago Schist from northern flank. (a) Slate (sample DG99-300) with transposed

bedding, isoclinal folds in thin psammitic layers, an obliquely intersecting discrete crenulation cleavage (Scc) as well as boudinaged quartz veins

now subparallel to S0 (bedding) and Sm (dominant foliation), Kokonga-Middleborough road, northeast Otago. Base of photomicrograph is 7.5

cm. (b) Whole rock 40Ar/39Ar spectrum for mica-rich domain in sample DG99-300. (c) Psammitic slate (sample DG89-341E) with transposed

bedding, an isoclinal fold-hinge and an obliquely crosscutting discrete crenulation cleavage. Base of photomicrograph is 7.5 cm. (d) Whole rock40Ar/39Ar spectrum for mica-rich domain in sample DG89-341E. Discordance in the age spectrum of DG99-300 is due either to some recoil

distribution of 39Ar because of the fine grain size, or a fraction of partly reset detrital mica; whereas, the less severe discordance between steps in

DG98-341E is most likely due to minor 39Ar recoil redistribution.


‘‘clean’’ looking and annealed or lacking undulose

extinction (see photomicrographs of Norris and

Bishop, 1990; Mortimer, 2003). There is also

growth of new mica in phyllonitic zones and

crenulation hinges (see Fig. 8 of Forster and Lister,

2003), as well as ‘‘compacted’’, deformed older

metamorphic mica in phyllonitic zones (see Fig. 7b

of Forster and Lister, 2003). The presence of de-

formed old micas, as well as new mica growth

during deformation, potentially complicates interpre-

tations of the schist K–Ar and 40Ar/39Ar age data,

such that interpretations of slow cooling related to

position in the structural pile or wedge (e.g. Adams

and Graham, 1997) are problematic (cf. Little et al.,

1999).

4.3.1. Apparent age distribution

The oldest 40Ar/39Ar ages come from the low-

grade flanks of the Otago Schist (Fig. 9). Micas in

the schist flanks grew below the blocking temper-

Fig. 13. (a) Outcrop photograph of hinge zone with steep enveloping surface, Manorburn fold-nappe, Upper Manorburn Reservoir. Base of

photo is 3 m. (b) Thin section photograph of layering folded in (a). Base of photomicrograph is 5 cm. (c) 40Ar/39Ar age spectrum is from the

mica fraction of the sample in (b) (sample DG92-45A).


ature for argon in white mica (f350 jC), presum-

ably without subsequent argon loss, and are there-

fore interpreted to be mica crystallization ages;

except those with detrital mica contamination (cf.

Little et al., 1999). Samples from the high-grade

core, however, yield cooling ages affected locally

by high strain zones and metamorphic overprinting

during transposition cycling. Most of the latter show

complex age spectra because of partial retention

during a protracted cooling path through the mica

partial annealing zone (see Fig. 8 of Little et al.,

1999). In some areas of the schist, dynamic recrys-

tallization below the closure temperature interval

(i.e. at lower temperatures) caused heterogeneous

or wholesale argon loss, resulting in completely

disturbed spectra such as those shown in Figs. 9

and 10 of Forster and Lister (2003).

Sample DG90-45A (Fig. 13) is typical of the

samples with f120 Ma ages. The f120 Ma Ar–

Ar ages could be due to a thermal or deformation

event in the Otago Schist, or a partial retention

age between the Late Jurassic metamorphic age

and exhumation. The samples with f110 Ma

ages, which tend to give concordant age spectra

with broad plateaux (e.g. DG89-353C of Fig. 15),

are generally located in the footwall adjacent to

the Remarkables shear zone or shear zone defin-

ing the Caples–Torlesse boundary. These samples

Fig. 14. Typical 40Ar/39Ar spectra from more strongly deformed schist. (a) Crenulated schist (sample DG92-89B) developing into rodding fabric

with disruption of hinges (view normal to Sm and Lm, where Sm is the dominant foliation and Lm is the stretching lineation). Cromwell Gorge,

near Clyde, central Otago. Base of photo is 6.5 cm. (b) 40Ar/39Ar age spectrum for sample DG92-89B. (c) High strain phyllonitic greyschist

(sample DG92-73) with transposed veins and layering now all subparallel and defining the foliation. Hillside above Ophir tip, central Otago.

Base of photo is 7cm. (d) 40Ar/39Ar spectrum on mica separate from DG92-73.


probably record rapid cooling associated with

exhumation driven by extension. 40Ar/39Ar data

from Forster and Lister (2003) also showed ages

grouped at f120 and f110 Ma. Samples with

f102 Ma ages (DG99-268, Fig. 16) are inter-

preted to reflect a distinctly younger (Late Creta-

ceous) medium-pressure overprint affecting the

Alpine schist and western part of the Otago schist

(Fig. 17a).

4.3.2. Effects of metamorphism

Mortimer (2000) argues that the Haast Schist

has experienced two overprinting periods of meta-

morphism, a younger Alpine event (f110 Ma)

superimposed on an older Otago (f180 Ma) event

which is now exposed as the largely unretrog-

raded, high-grade schist core [peak conditions of

f8–10 kbar/400 jC]. This interpretation is sup-

ported by the youngest 40Ar/39Ar ages in this

Fig. 15. Nevis Bluff locality with outcrop photographs showing (a) intercalated greenschist and greyschist bands, representing transposed and

isoclinally folded layering at the outcrop scale, grs: greenschist band; gry: greyschist band. (b) Isoclinally folded greyschist. Lens cap for scale

has 6.5 cm length. (c) Photomicrograph of the greyschist. Base of photomicrograph is 7.5 cm. (d) 40Ar/39Ar age spectrum from mica separates

from the greyschist shown in (c) (sample DG89-353C).


study (f101–102 Ma in Figs. 16 and 17) occur-

ring associated with crenulated schist (Scc fabrics)

within the chlorite zone overprint of Mortimer

(2000), where biotite is replaced by chlorite. These

data suggest that the K–Ar systems were rejuve-

nated during the retrogression event, either by

thermal degassing and/or reaction-enhanced volume

diffusion.

4.3.3. Effects of dynamic recrystallization during

subsequent shear zone development

When rocks are deforming at temperatures within

and below the partial retention zone for argon in

white mica (e.g. 430–330 jC), microscale deforma-

tion mechanisms cause argon loss in micas. Over-

printing shear zones in the Otago Schist are marked

by strongly deformed L-tectonites with intense rod-

ding fabrics and recrystallized mica fabrics (cf.

Forster and Lister, 2003). 40Ar/39Ar age spectra on

mica from these fabrics show discordance giving a

range of apparent ages. For example, the Cromwell

Gorge or Northburn Shear Zone, an inferred high

strain zone (see Fig. 2 of Forster and Lister, 2003)

along the lower limb of the Northburn fold-nappe

(see Fig. 6 of Turnbull, 1981) shows disturbed age

spectra yielding apparent ages of f112 Ma (sam-

ples DG92-89B, DG99-294, DG94-11, respectively;

compare with Figs. 9 and 10 of Forster and Lister,

2003). Likewise, shear zones overprinting the Man-

orburn fold-nappe of Means (1966) give ages of

113.8F0.6 and 107.4F0.9 Ma (DG91-95B and

DG92-73; Table 1).

Fig. 16. Typical 40Ar/39Ar age spectra from crenulated schist, central Otago Schist. (a) Outcrop photograph and 40Ar/39Ar age spectrum from

white mica (sample DG99-267), west side of Lake Wanaka. (b) Outcrop photograph and 40Ar/39Ar age spectrum (DG91-155), Thompsons

Pass road, Dunstan Range. Sm: dominant schistosity; Scc: crenulation cleavage. Lens cap for scale in both photographs has 6.5 cm length.


4.3.4. Depth in the pile and cooling

Inverse relationships between metamorphic grade

and isotopic age have been used to argue for gradual

cooling following the metamorphic climax (e.g. Harp-

er and Landis, 1967). Metamorphic grade should

equate approximately with structural depth in the

schist pile with the highest metamorphic grade, and

therefore youngest 40Ar/39Ar ages, coinciding with

the deepest levels (Fig. 17b). Plots of age versus

structural depth (Fig. 18) have been used to determine

the bulk ‘‘cooling’’ history of the schist (e.g. Fig.7 of

Little et al., 1999 and Fig. 8 of Mortimer, 2003). As

the schist is clearly heterogeneously deformed (cf.

Fig. 2 of Mortimer, 2003; Forster and Lister, 2003),

such plots are difficult to interpret and the arguments

above preclude a simple cooling history for the entire

pile. The regional schistosity is clearly cut by younger

shear zones (e.g. marked by DDZ in Fig. 4a, Matu-

kituki Valley; Fig. 5a, the Remarkables; and Fig. 7a,

Ophir) involving ductile deformation where the schist

has undergone further dynamic recrystallization, and

by overprinting crenulation cleavages (e.g. Treble

Cone, Wanaka, and Thompsons Pass; Fig. 16) involv-

ing new mica growth and/or neocrystallization. These

deformations result in complex 40Ar/39Ar spectra

because of mixtures of argon released from mica

grains that may retain the regional cooling age, but

also subjected to later partial argon loss with mica

neocrystallization (see also Little et al., 1999; Forster

and Lister, 2003). These younger heterogeneous

Fig. 17. Plots of the 40Ar/39Ar data from this paper on (a) a metamorphic map (after Fig. 8 of Mortimer, 2000), and (b) a schist ‘‘isopach’’ map

(after Fig. 7 of Mortimer, 2003). ‘‘Isopach’’ thickness is a measure of structural relief relative to the Remarkables shear zone interface between

the Caples and Torlesse terranes. +ve ‘‘isopach’’ values indicate elevation above this shear zone (i.e. correspond to positions in the north and

south flanks of the schist belt), whereas �ve values indicate depth below this shear zone (i.e. correspond to positions in the schist core).


Fig. 18. Depth versus apparent age plot of the 40Ar/39Ar data from this paper. Compare with Fig. 7 of Little et al. (1999) and Fig. 8 of Mortimer

(2003). Approximate depth values were determined by inspection (see Fig. 17b) from the Schist ‘‘isopach’’ map of Mortimer (2003).


‘‘events’’ contribute to the overall scatter in the data

plotted in Fig. 18; scatter may also relate to the errors

in schist ‘‘isopach’’ determinations (see Mortimer,

2003).

5. A revised structural evolution of the Otago

Schist

Interpreting the 40Ar/39Ar data using both outcrop

relationships and microstructural considerations, we

suggest the following sequence of deformational

events for the Otago Schist:

(1) cleavage and folding in the low grade flanks:

f150–130 Ma,

(2) juxtaposition of Caples and Torlesse: f130–120

Ma,

(3) fold-nappes and shear zones in schist core:

f140–120 Ma,

(4) overprinting shear zones in schist core with

extensional reactivation of shear zones and

development of crenulation cleavages (Scc):

f114 and f106 Ma,

(5) overprinting crenulation cleavages in Alpine

chlorite zone overprint: f102–101 Ma.

Overall cooling of the schist core is considered to

occur through f80 Ma.

These new timing relationships are depicted sche-

matically in a structural cartoon (Fig. 19). The struc-

tural configuration is based on Craw (1985), Norris

and Craw (1987) and Roser and Cooper (1990). These

new 40Ar/39Ar data suggest that peak metamorphism

in the Otago Schist occurred at 150–140 Ma (Late

Jurassic), and not as previously argued in the Early

Jurassic (e.g. Adams et al., 1985) or the Middle

Jurassic (Little et al., 1999) (Fig. 20).

6. Fabric development during wedge-thickening

versus exhumation-related extension

Within the monotonous Otago Schist, one of the

problems is establishing which structures and fab-

rics relate to thickening of the schist wedge (i.e.

underplating in a simple wedge model) versus

vertical thinning and/or extension-related structures

formed during exhumation. Petrofabric investiga-

tions (Cooper, 1995) provide insight into this

problem. High strain fabrics related to the Caple–

Torlesse shear zone boundary all show non-coaxial

top-to-the-ENE or -NE transport (Cooper, 1995;

Fig. 19. Cartoon illustrating the geometry and timing of different elements that make up the Otago Schist. Timing is based on data presented in

this paper. ALA: Aspiring Lithologic Association or Aspiring terrane after Norris and Craw (1987).


Little and Mortimer, 2001). These shear sense

determinations support (1) the Caples fold-nappe

overthrusting mechanism argued by Cox (1991)

based on increasing strain and top-to-the-NE shear

sense inferred from rotation of quartz veins in the

Caples terrane towards the Remarkables shear zone,

as well as (2) the high strain overthrusting model

proposed by Mortimer (1993a,b) where the Caples

terrane is thrust northwards over the Torlesse

terrane.

Exhumation has been linked to mid Cretaceous

extensional faulting (Mortimer, 1993a,b, p. 244) and

most recently to a metamorphic core complex scena-

rio with implied tectonic switching from shortening

and crustal thickening to one or more periods of

crustal extension (Forster and Lister, 2003). This

model implies that the schist core has been pulled

out from beneath the Caples terrane with reactivation

of shear zones, in particular the Remarkables shear

zone interface between the Caples and Torlesse/As-

piring terranes.

Recognition of shear zone reactivation is problem-

atic in general, and particularly in the monotonous

Otago Schist. Localized development of shear bands

with shear sense reversal has provided the evidence

(Deckert et al., 2002), but these reactivated fabrics

have not been specifically dated. Above Cromwell

Gorge, shear zone reactivation is shown by rare shear

bands as well as brittle normal faults (see Fig. 5a,e,

and f of Deckert et al., 2002), and along the Rise and

Shine Shear Zone near Bendigo by marked develop-

ment of shear bands (see Fig. 5b,c, d of Deckert et al.,

2002). Further evidence of shear strain reversal was

found in the footwall to the contact or transition zone

between the Caples and Aspiring terranes in the

Richardson Mountains, where more highly deformed

Aspiring terrane rocks showed top-to-the-W/SW shear

sense (Cooper, 1995, p. 211). This may reflect exten-

sional reactivation (i.e. shear sense reversal relative to

the top-to-NE overthrusting mechanism) along this

zone.

7. Tectonic implications

A revised time–space diagram for the Haast/Otago

Schist (Fig. 20) integrates the Ar–Ar data presented in

this paper with timing and/or age-constraint data from

other tectonic elements that make up the Western and

Eastern Provinces of New Zealand (Fig. 1). In this

framework, Caples metamorphism and fabric devel-

opment is considered to occur between 150 and 140

Ma with Torlesse/Aspiring terrane metamorphism and

deformation between 150 and 145 Ma (Older Tor-

Fig. 20. Time–space plot of 40Ar/39Ar data from this paper, plotted with events that have affected other tectonic elements that make up the

South Island of New Zealand today. Data sources are listed in the figure.



lesse). This timing of metamorphism is in general

agreement with a maximum age of Haast Schist peak

metamorphism at 152F5 Ma based on unpublished

whole rock-mineral Rb–Sr isochron data from gar-

net–oligioclase zone metabasites (Grapes and Wata-

nabe, 1992, p. 177). We infer that peak metamorphism

in the Otago Schist high-grade core occurred at this

time, with shear zone development and mica recrys-

tallization from 115 to 106 Ma. Extensional exhuma-

tion was possibly initiated at about 110 Ma.

At 140 Ma East Gondwana was a subduction-

accretion margin (Bradshaw, 1989) involving struc-

tural thickening of forearc wedge/accretionary prism

and continued sedimentation outboard (Pahau Ter-

rane), with final Caples–Torlesse terrane amalgam-

ation at f120 Ma. Juxtaposition of the Caples and

Torlesse terranes is probably part of an inferred

collision at 125–118 Ma originally considered be-

tween the Median Tectonic Zone (Median Batholith)

and the Western Province (see Waight et al., 1998a,b).

The generally accepted tectonic evolutionary model

for New Zealand (e.g. Coombs et al., 1976; Bradshaw,

1989; Roser and Cooper, 1990) requires convergence

of the Permian–Jurassic Torlesse fan with arc–fore-

arc-trench slope sequences (Brook Street, Murihiku

and Caples terranes) due to subduction of Permian

oceanic crust beneath the Gondwana margin. This

subduction caused structural thickening of the Tor-

lesse turbidite fan in the Late Jurassic (f150–140

Ma), with Early Cretaceous (130–120 Ma) structural

interleaving of Caples and deformed Torlesse sedi-

ment resulting in final accretion of the turbidite fan to

the margin. Erosional exhumation and some exten-

sional ‘‘collapse’’ occurred in the late Early Creta-

ceous (110–90 Ma).

Pulses of magmatism within the Median Batholith

occurred with subduction-related, Late Jurassic–Early

Cretaceous intrusion of gabbro-noritic to granitic plu-

tonic rocks (f147–137 Ma) and by late Early Creta-

ceous adakitic magmatism (125–105 Ma) possibly

associated with crustal thickening of the margin (Kim-

brough et al., 1994; Waight et al., 1998a,b; Mortimer et

al., 1999). The late Early Cretaceous magmatism is

contemporaneous with granulite facies metamorphism

in the Western Fiordland Orthogneiss (Gibson et al.,

1988; Bradshaw, 1990; Daczko et al., 2002; Hollis et

al., 2003; Klepeis et al., 2003) attributed to crustal

thickening within the arc by granulite facies thrusting

and emplacement of sheeted plutons (Daczko et al.,

2002; Klepeis et al., 2003). Cessation of magmatism

within the Median Batholith occurred at f105 Ma

such that it does not contain 85–100 Ma Gondwana

breakup igneous suites (Mortimer et al., 1999).

Cooling ages and/or recrystallization ages during

shear zone development in the high-grade core of the

schist coincide with core complex and basin develop-

ment in other parts of the margin (e.g. New Caledonia

Basin; Taranaki Basin; Great South Basin: Laird,

1993). In the Paparoa Core Complex (Fig. 1), exten-

sional shear zones deformed pre-existing 114F18 Ma

granitoids, but had ceased by f108 Ma (Tulloch and

Kimbrough, 1989). Cooling of lower plate rocks took

place between 110 and 90 Ma (Spell et al., 2000),

superimposed half-graben development at 102–101

Ma (date on basal tuff) and intrusion by A-type

granitoids by 82 Ma (Waight et al., 1998a). In the

Western Province, the Fiordland granulites underwent

structural thickening between 126 and 116 Ma and

were unroofed by 90 Ma (Claypool et al., 2002; Hollis

et al., 2003). Mid-Cretaceous (114–109 Ma) pluton-

ism of the Hohonu Batholith suite (Fig. 1) has been

attributed to a change at f110 Ma from lithospheric

thickening to lithospheric extension leading to conti-

nental breakup (Waight et al., 1998b). At 110 Ma, we

suggest that the East Gondwana margin was strongly

affected by slab rollback with New Zealand and the

Lord Howe Rise becoming a lower plate margin (see

Lister and Etheridge, 1989; Forster and Lister, 2003).

Thermal decay from Tasman Sea spreading com-

menced at f84 Ma and was well underway at 70

Ma (see Figs. 19 and 20 of Johnson and Veevers,

1984).

8. Conclusions

40Ar/39Ar step heating experiments on micas and

whole rocks have provided new thermochronologic

constraints on the deformation, metamorphism and

cooling/exhumation of a Mesozoic accretionary

wedge preserved as the Otago Schist in the South

Island of New Zealand. By themselves, 40Ar/39Ar age

spectra cannot distinguish between contraction-ero-

sion models of sediment wedge development and

those involving extensional exhumation. In this paper,

we have attempted to integrate 40Ar/39Ar crystalliza-


tion age data from the low-grade schist flanks and

cooling age data from the schist core with timing and

age-constraint data from other tectonic elements that

make up present-day New Zealand.

Late Jurassic–Early Cretaceous convergence is

inferred along the Gondwana margin from f160 to

120 Ma. This led to thickening of the Rakaia sediment

wedge accompanied at the deeper levels by intense

schistosity development, major thrust-nappes and

crosscutting shear zones. The age of this structural

deformation is reflected by metamorphic mica growth

in the low-grade parts of the wedge (f160–145 Ma).

As part of this event, terrane accretion resulted from

northwards thrusting of the Caples volcaniclastic

forearc sequence over the Aspiring and Torlesse

terrane associations from f140 Ma to 120 Ma. The

timing of thrusting is constrained by metamorphic

mica growth in the Remarkables Shear Zone. Reacti-

vation of major and minor shear zones in the schist

pile took place during late Early Cretaceous extension

along the Gondwana margin from 110 to 100 Ma.

This is recorded by metamorphic mica growth in

younger crosscutting shear zones, rodded zones and

crenulation cleavage (Scc) zones.

Acknowledgements

DRG acknowledges Australian Research Council

(ARC) funding from ARC Grants A39927139 and

A39030706 and a Monash University Small Grant

during fieldwork and analytical work, and an

Australian Professorial Fellowship (ARC Discovery

Grant) during write-up. The geological framework for

this paper was established over a period of 15 years in

collaboration with Bob Gregory, Simon Cox, and

Richard Norris. This framework has been influenced

by the early work on the schist by Win Means and

Bryce Wood in particular, and more recently by Nick

Mortimer, Moses Turnbull, Graham Bishop, Dave

Craw and Simon Cox. We thank Simon Cox, Richard

Norris, Dave Craw, Nick Mortimer, Moses Turnbull

and Alan Cooper for discussions on schist structure

and evolution, on sampling locations for Ar–Ar work,

and in particular Nick Mortimer for being helpful and

also keeping us abreast with developments in Otago

Schist research. Reviews by Tim Little and an

anonymous reviewer helped improve the paper.

References

Adams, C.J., Gabites, J.E., 1985. Age of metamorphism and uplift

in the Haast Schist Group at Haast Pass, Lake Wanaka and Lake

Hawea, South Island, New Zealand. N.Z. J. Geol. Geophys. 28,

85–96.

Adams, C.J., Graham, I.J., 1996. Metamorphic and tectonic geo-

chronology of the Torlesse Terrane, Wellington, New Zealand.

N.Z. J. Geol. Geophys. 39, 157–180.

Adams, C.J., Graham, I.J., 1997. Age of metamorphism of Otago

Schist in eastern Otago and determination of protoliths from

initial strontium isotope characteristics. N.Z. J. Geol. Geophys.

40, 275–286.

Adams, C.J., Robinson, P., 1993. Potassium–argon age studies of

metamorphism/uplift/cooling in Haast Schist coastal sections

south of Dunedin, Otago, New Zealand. N.Z. J. Geol. Geophys.

36, 317–325.

Adams, C.J., Bishop, D.G., Gabites, J.E., 1985. Potassium–argon

age studies of a low-grade, progressively metamorphosed grey-

wacke sequence, Dansey Pass, South Island, New Zealand. N.Z.

J. Geol. Geophys. 36, 317–325.

Aoya, M., Wallis, S.R., 2003. Role of nappe boundaries in subduc-

tion-related regional deformation. spatial variation of meso- and

micro-structures in the Seba eclogite unit, the Sanbagawa belt,

SW Japan. J. Struct. Geol. 25, 1097–1106.

Bishop, D.G., 1972. Progressive metamorphism from prehnite –

pumpellyite to greenschist facies in the Dansey Pass area,

Otago, New Zealand. Bull. Geol. Soc. Am. 83, 3177–3198.

Bishop, D.G., 1974. Stratigraphic, structural and metamorphic rela-

tions in the Dansey Pass area, Otago. N.Z. J. Geol Geophys. 17,

301–355.

Bishop, D.G., Bradshaw, J.D., Landis, C.A., 1985. Provisional ter-

rane map of South Island, New Zealand. In: Howell, D.G. (Ed.),

Tectonostratigraphic Terranes. American Association of Petro-

leum Geologists, Circum-Pacific Council for Energy and Min-

eral Resources, Earth Science Series 1, 515–521.

Bradshaw, J.D., 1989. Cretaceous geotectonic patterns in the New

Zealand region. Tectonics 8, 803–820.

Bradshaw, J.Y., 1990. Geology of crystalline rocks of northern

Fiordland: details of the granulite facies Western Fiordland

Orthogneiss and associated rock units. N.Z. J. Geol. Geophys.

33, 465–484.

Brown, E.H., 1967. The greenschist facies in part of eastern Otago,

New Zealand. Contrib. Mineral. Petrol. 14, 259–292.

Brown, E.H., 1968. Metamorphic structures in part of the eastern

Otago schists. N.Z. J. Geol. Geophys. 11, 41–65.

Claypool, A.L., Klepeis, K.A., Clarke, G.L., Daczko, N., Dockrill,

B., 2002. The evolution and exhumation of Early Cretaceous

lower crustal granulites during changes in plate boundary

dynamics, Fiordland, New Zealand. Tectonophysics 359,

329–358.

Coombs, D.S, Landis, C.A., Norris, R.J., Sinton, J.M., Borns,

D.J., Craw, D., 1976. The Dun Mountain Ophiolite belt,

New Zealand, its tectonic setting, constitution, and origin,

with special reference to the southern portion. Am. J. Sci.

276, 561–603.

Cooper, E.K., 1995. Petrofabric studies across the Caples/Torlesse


Terrane Boundary, Otago Schist. Unpub. Msc. Thesis, The Uni-

versity of Canterbury, Christchurch, New Zealand, 273 pp.

Cox, S.C., 1991. The Caples/Aspiring terrane boundary—the tran-

sition of an early nappe structure in the Otago Schist. N.Z. J.

Geol. Geophys. 34, 73–82.

Craw, D., 1985. Structure of schist in the Mt Aspiring region,

northwest Otago, New Zealand. N.Z. J. Geol. Geophys. 28,

55–75.

Craw, D., 1998. Structural boundaries and biotite and garnet ‘iso-

grads’ in the Otago and Alpine Schists, New Zealand. J. Meta-

morph. Geol. 16, 395–402.

Daczko, N.R., Klepeis, K.A., Clarke, G.L., 2002. Thermomechan-

ical evolution of the crust during convergence and deep crustal

pluton emplacement in the Western Province of Fiordland, New

Zealand. Tectonics 21, 1–18.

Deckert, H., Ring, U., Mortimer, N., 2002. Tectonic significance of

Cretaceous bivergent extensional shear zones in the Torlesse

accretionary wedge, central Oatgo Schist, New Zealand. N.Z.

J. Geol. Geophys. 45, 537–547.

Feehan, J.G., Brandon, M.T., 1999. Contribution of ductile flow to

exhumation of low-temperature high-pressure metamorphic

rocks: San Juan-Cascade nappes, NWWashington State. J. Geo-

phys. Res. 104, 10883–10902.

Forster, M.A., Lister, G.S., 2003. Cretaceous metamorphic core

complexes in the Otago Schist, New Zealand. Aust. J. Earth

Sci. 50, 181–198.

Foster, D.A., Gray, D.R., Bucher, M., 1999. Chronology of defor-

mation within the turbidite-dominated Lachlan Orogen: impli-

cations for the tectonic evolution of eastern Australia and

Gondwanaland. Tectonics 18, 452–485.

Frost, C.D., Coombs, D.S., 1989. Ns isotope character of New

Zealand sediments: implications for terrane concepts and crustal

evolution. Am. J. Sci. 289, 744–770.

Gibson, G.M., McDougall, I., Ireland, I., 1988. Age constraints on

metamorphism and the development of a metamorphic core

complex in Fiordland, southern New Zealand. Geology 16,

405–408.

Graham, I.J., Mortimer, N., 1992. Terrane characterisation and tim-

ing of metamorphism in the Otago Schist, New Zealand, using

Rb–Sr and K–Ar geochronology. N.Z. J. Geol. Geophys. 35,

391–401.

Grapes, R., Watanabe, T., 1992. Metamorphism and uplift of the

Alpine schist in the Franz Josef-Fox Glacier area of the Southern

Alps, New Zealand. J. Metamorph. Geol. 10, 171–180.

Hara, I., Shiota, T., Hide, K., Okamoto, K., Takeda, K., Hayasaka,

Y., Sakurai, Y., 1990. Nappe structure of the Sambagawa belt. J.

Metamorph. Geol. 8, 441–456.

Harper, C.T., Landis, C.A., 1967. K–Ar ages from regionally meta-

morphosed rocks, South Island, New Zealand, and some tecton-

ic implications. Earth Planet. Sci. Lett. 2, 419–429.

Hollis, J.A., Clarke, G.L., Klepeis, K.A., Daczko, N.R., Ireland,

T.R., 2003. Geochronology and geochemistry of high-pressure

granulites of the Arthur River Complex, Fiordland, New Zea-

land: Cretaceous magmatism and metamorphism of the palaeo-

Pacific Margin. J. Metamorph. Geol. 21, 213–299.

Hutton, C.O., Turner, F.J., 1934. Metamorphic zones in north-west

Otago. Trans. Roy. Soc. N. Z. 64, 161–174.

Johnson, B.D., Veevers, J.J., 1984. Oceanic palaeomagnetism. In:

Veevers, J.J. (Ed.), Phanerozoic Earth History of Australia. Ox-

ford Monographs on Geology and Geophysics, vol. 2. Oxford

Science Publications, Clarendon Press, Oxford, pp. 17–38.

Kimbrough, D.L., Tulloch, A.J., Coombs, D.S., Landis, C.A., John-

ston, M.R., Mattison, J.M., 1994. Uranium–lead ages from the

Median Tectonic Zone, South Island New Zealand. N.Z. J. Geol.

Geophys. 37, 393–419.

Klepeis, K.A., Clarke, G.L., Rushmer, T., 2003. Magma transport

and coupling between deformation and magmatism in the con-

tinental lithosphere. GSA Today 13, 4–11.

Kusky, T.M., Bradley, D.C., Haeussler, P.J., Karl, S., 1997. Con-

trols on accretion of flysch and melange belts at convergent

margins: evidence from the Chugach Bay thrust and Iceworm

melange, Chugach accretionary wedge, Alaska. Tectonics 16,

855–878.

Laird, M.G., 1993. Cretaceous continental rifts: New Zealand re-

gion. In: Balance, P.F. (Ed.), South Pacific Sedimentary Basins.

Sedimentary Basins of the World, vol. 2. Elsevier, Amsterdam,

pp. 37–49.

Lister, G.S., Etheridge, M.A., 1989. Towards a general model; de-

tachment models for uplift and volcanism in the eastern high-

lands, and their application to the origin of passive margin

mountains. In: Johnson, R.W., Knutson, J., Taylor, S.R.

(Eds.), Intraplate volcanism in eastern Australia and New Zea-

land, pp. 297–313 Cambridge Univ. Press, Cambridge, UK.

Little, T.A., Mortimer, N., 2001. Rotation of ductile fabrics across

the Alpine Fault and Cenozoic bending of the New Zealand

orocline. J. Geol. Soc. (Lond.) 158, 745–756.

Little, T.A., Mortimer, N., McWilliams, M., 1999. An episodic

Cretaceous cooling model for the Otago-Marlborough Schist,

New Zealand, based on 40Ar/39Ar white mica ages. N.Z. J. Geol.

Geophys. 42, 305–325.

McDougall, I., Harrison, T.M., 1999. Geochronology and Thermo-

chronology by the 40Ar/39Ar Method, 2nd edition Oxford Univ.

Press, New York 269 pp..

Means, W.D., 1963. Mesoscopic structures and multiple deforma-

tion in the Otago Schist. N.Z. J. Geol. Geophys. 6, 801–816.

Means, W.D., 1966. A macroscopic recumbent fold in schist near

Alexandra, Central Otago. N.Z. J. Geol. Geophys. 9, 173–194.

Mortimer, N., 1993a. Jurassic tectonic history of the Otago Schist,

New Zealand. Tectonics 12, 237–244.

Mortimer, N., 1993b. Geology of the Otago Schist and adjacent

rocks. Scale 1:500 000. Inst. Geol. Nucl. Sci. Geol. Map 7.

Mortimer, N., 2000. Metamorphic discontinuities in orogenic belts:

example of garnet–biotite–albite zone in the Otago Schist, New

Zealand. Int. J. Earth Sci. 89, 295–306.

Mortimer, N., 2003. A provisional structural thickness map of the

Otago Schist, New Zealand. Am. J. Sci. 303, 603–621.

Mortimer, N., 2004. New Zealand’s geological foundations. Gond-

wana Res. 7, 261–272.

Mortimer, N., Roser, B.P., 1992. Geochemical evidence for the

position of the Caples–Torlesse boundary in the Otago Schist.

J. Geol. Soc. (Lond.) 149, 967–977.

Mortimer, N., Tulloch, A.J., Spark, R.N., Walker, N.W., Ladley, E.,

Allibone, A., Kimbrough, D.L., 1999. Overview of the Median

Batholith, New Zealand: a new interpretation of the geology of


the Median Tectonic Zone and adjacent rocks. J. Afr. Earth Sci.

29, 259–270.

Mortimer, N., Davey, F.J., Melhush, A., Yu, J., Godfre, N.J., 2003.

Geological interpretation of a deep crustal seismic reflection

profile across the eastern Province and Median Batholith, New

Zealand: crustal architecture of an extended Phanerozoic con-

vergent orogen. N.Z. J. Geol. Geophys. 45, 349–363.

Muir, R.J., Ireland, T.R., Weaver, S.D., Bradshaw, J.D., Evans, J.A.,

Eby, G.N., Shelley, D., 1988. Geochronology and geochemistry

of a Mesozoic magmatic arc system, Fiordland, New Zealand. J.

Geol. Soc. (Lond.) 155, 1037–1053.

Norris, R.J., 1977. Structural generations in the Otago Schist—their

definition and correlation. Geol. Soc. New Zealand Conference

Abstracts, Queenstown.

Norris, R.J., Bishop, D.G., 1990. Deformed conglomerates and

textural zones in the Otago Schist, New Zealand. Tectonophy-

sics 174, 331–349.

Norris, R.J., Craw, D., 1987. Aspiring terrane: an oceanic assem-

blage from New Zealand and its implications for Mesozoic

terrane accretion in the southwest Pacific. In: Leitch, E.C.,

Scheibner, E. (Eds.), Terrane Accretion and Orogenic Belts.

Geodyn. Ser., vol. 19, pp. 169–177.

Paterson, C.J., 1971. Geology at Bendigo, Central Otago. Unpubl.

Bsc. Hons. Thesis, The University of Otago, Dunedin, New

Zealand.

Ring, U., Brandon, M.T., 1999. Ductile deformation and mass loss

in the Franciscan Subduction Complex: implications for exhu-

mation processes in accretionary wedges. In: Ring, U., Brandon,

M.T., Lister, G.S., Willet, S.D. (Eds.), Exhumation Processes:

Normal Faulting, Ductile Flow and Erosion. Spec. Publ. - Geol.

Soc. Lond., vol. 154, pp. 55–86.

Roser, B.P., Cooper, A.F., 1990. Geochemistry and terrane affilia-

tion of Haast Schist from the western Southern Alps, New Zea-

land. N.Z. J. Geol. Geophys. 94, 635–650.

Sheppard, D.S., Adams, C.J., Bird, G.W., 1975. Age of metamor-

phism and uplift in the Alpine Schist belt, New Zealand. Bull.

Geol. Soc. Am. 86, 1147–1153.

Spell, T.L., McDougall, I., Tulloch, A.J., 2000. Thermochronologic

constraints on the breakup of the Pacific Gondwana margin: the

Paparoa metamorphic core complex, South Island, New Zea-

land. Tectonics 19, 433–451.

Tobisch, O.T., Paterson, S.R., 1988. Analysis and interpretation of

composite foliations in areas of progressive deformation. J.

Struct. Geol. 10, 745–754.

Tulloch, A.J., Kimbrough, D.L., 1989. The Paparoa Metamorphic

Core Complex, New Zealand: Cretaceous extension associated

with fragmentation of the Pacific margin of Gondwana. Tecton-

ics 8, 1217–1234.

Turnbull, I.M., 1981. Contortions in the schists of the Cromwell

district, central Otago, New Zealand. N.Z. J. Geol. Geophys. 24,

65–86.

Turnbull, I.M., 2000. Geology of the Wakitipu area, 1:250,000

geological map 18. Institute of Geological and Nuclear Scien-

ces, Lower Hutt, New Zealand, 1 sheet+72 pp.

Waight, T.E., Weaver, S.D., Maas, R., Eby, G.N., 1998. French

Creek Granite and Hohonu dike swarm, South Island, New

Zealand: Late Cretaceous alkaline magmatism and the opening

of the Tasman Sea. Aust. J. Earth Sci. 45, 823–835.

Waight, T.E., Weaver, S.D., Muir, R.J., 1998. Mid-Cretaceous gra-

nitic magmatism during the transition from subduction to exten-

sion in southern New Zealand: a chemical and tectonic

synthesis. Lithos 45, 469–482.

Wood, B.L., 1963. Structure of the Otago schists. N.Z. J. Geol.

Geophys. 6, 641–680.

Yardley, B.W.D., 1982. The early metamorphic history of the Haast

Schists and related rocks of New Zealand. Contrib. Mineral.

Petrol. 81, 317–327.

40ar/39ar thermochronologic constraints on deformation, metamorphism and cooling/exhumation of a...

Documents