Elsevier Encyclopedia of Geology

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Figure 3 Geological map of the Tasman Orogenic Belt, showing the dominant lithologies, granite intrusives, major fault traces, and

trend lines from regional aeromagnetic datasets.

AUSTRALIA/Tasman Orogenic Belt 241

inclined and polydeformed as they approach major

faults. Metamorphism is greenschist facies or lower

across the Lachlan Orogen, except in the fault-bounded

Wagga-Omeo and several smaller (Cooma, Camba-

long, Jerangle, and Kuark) metamorphic complexes,

where high-temperature–low-pressure metamorphism

is characterized by andalusite–sillimanite assemblages.

Thomson Orogen

The Thomson Orogen is meant to be coeval with, as

well as the northern continuation of, the Lachlan

Orogen (Figure 2). The orogen is hidden under a

cover of younger sedimentary basins that make up

the Great Artesian Basin, but geophysical aeromag-

netic imagery shows that it has eastwards-trending

structures that truncate the trends of the Lachlan

Orogen. Drill samples and limited surface exposures

have shown that it consists of low-metamorphic-

grade turbiditic sandstone and mudstone lithologies

similar to those of the Lachlan Orogen, and like the

Lachlan Orogen it is intruded primarily by Silurian

and Devonian granites.

New England Orogen

The New England Orogen is the youngest and

most easterly part of the Tasman Orogen (Figure 2)

and incorporates arc, fore-arc, and accretionary

complexes. The New England Fold Belt was tec-

tonically active from the Early Carboniferous to the

mid-Triassic (a period of ca. 130 Ma). Westwards-

directed Permian–Triassic thrusting caused interleaving

and imbrication of the arc magmatic belt (Connors–

Auburn Belt) and fore-arc (Yarrol–Tamworth Belt) and

oceanic assemblages, including subduction complexes

(Wandilla–Gwydir Belt) and ophiolite (Gympie Belt).

Subduction-complex assemblages show a strong

thrust-related fabric, polyphase deformation, and

greenschist to amphibolite facies metamorphism. The

Hunter–BowenOrogeny(ca.265–230 Ma) consolidated

the terranes with Australia and resulted in the de-

velopment of a Permo-Triassic foreland basin (the

Sydney–Bowen Basin) (Figure 3).

Lithofacies

The Tasman Orogen is dominated by turbidites that

were once part of large submarine fans overlying

oceanic crust (Figure 3). Dismembered ophiolitic

rocks of Neoproterozoic to Cambrian age and mafic

to ultramafic affinities are preserved as slivers along

major faults (Figure 4). The mafic rocks dominantly

include tholeiitic pillow basalts and dolerites with

some andesitic and boninitic volcanics, and are gen-

erally associated with hemipelagic black shales and

cherts. These are interpreted as remnants of the

oceanic crust that regionally underlies the Cambro-

Ordovician turbidite fans. The ophiolitic slivers were

incorporated into the turbidites as offscraped slices,

as imbricated fault–duplex slices, and as blocks in

lange. In the eastern Lachlan Orogen Ordovician

shoshonitic andesites are structurally interleaved with

the Ordovician turbidites (Figure 3).

Inboard of the turbidites are Upper Proterozoic

(Adelaidean) intracratonic rift sequences of marine

to deltaic sandstones and shales, lagoonal evaporites,

dolomites, and limestone, overridden by the Delamer-

ian Orogen (Figure 3). These sediments were trans-

gressed by Lower Cambrian shelf sediments that are

transitional into deep-water sandstones and mud-

stones of the Kanmantoo Group. Outboard of the

turbidites the New England Orogen consists

of a collage of deformed and imbricated terranes of

largely Middle to Upper Palaeozoic and Lower Meso-

zoic marine to terrestrial sedimentary and volcanic

rocks, as well as strongly deformed flysch, argillite,

chert, pillow basalts, ultramafics, and serpentinites.

Deformation

Based on unconformities, sedimentary facies changes,

and, most recently, geochronology (Figure 5), deform-

ation across the Tasman Orogenic Belt occurred in the

Late Cambrian (Delamerian Orogen), Late Ordovi-

cian–Silurian and Early Devonian (Lachlan Orogen),

and Late Permian–Middle Triassic (New England

Orogen). The deformation was partitioned into re-

gional-scale migrating thrust systems. Tectonic ver-

gence, recorded largely by the dips of the major

faults, is craton-directed for the Delamerian and

New England Orogens, whereas the Lachlan Orogen

shows mixed vergence, but with thrusting largely

directed away from the craton (Figure 6).

The deformation style is chevron folding cut by high-

angle reverse faults. Fault zones are characterized by

higher than average strain and intense mica fabrics,

transposition foliation, isoclinal folds, and polydefor-

mation with overprinting crenulation cleavages.

Metamorphism

Metamorphic grade is generally low across the Tasman

Orogen (Figure 7), with greenschist facies (epizonal)

and subgreenschist facies (anchizonal) metamorphism

of the turbidite sequences. Most of the turbidites are

within the chlorite zone, with localized development

of biotite in contact aureoles around granites. High-

temperature–low-pressure metamorphism is indicated

by localized migmatites and alkali feldspar–cordierite–

andalusite–sillimanite gneisses in the Mount Lofty

242 AUSTRALIA/Tasman Orogenic Belt

Ranges, Glenelg Zone, and Moornambool Complex

(Delamerian Orogen), Wagga-Omeo Complex and

Cooma, Cambalong, and Kuark belts (Lachlan Oro-

gen), and Wongawibinda Complex (New England

Orogen) (Figure 7). Delamerian metamorphism is

characterized by peak conditions of 650–700



Cand

4–5 kbar in the Glenelg Zone, and 540–590



C and ap-

proximately 7–8 kbar in the Moornambool Complex.

Peak metamorphic conditions in the Lachlan Orogen

are 700



Cand3–4 kbar in the Wagga-Omeo Meta-

morphic Belt and Eastern Metamorphic complexes

(e.g. Cooma Complex), but these belts are intimately

associated with S-type granitic bodies.

Turbidites from the low-grade parts of the Lachlan

Orogen show intermediate pressure series meta-

morphism, based on b

or x-ray diffraction lattice

spacing measurements of phengitic micas, and are in-

consistent with previous interpretations of widespread

low-pressure ‘regional-aureole’ metamorphism. Mod-

erately high-pressure metamorphism has been inferred

from co-existing chlorite and actinolite in metabasites

from one of the major fault zones. Furthermore,

Figure 4 Geological map of the Tasman Orogenic Belt, showing the distribution of rocks with mafic and ultramafic affinities

(dismembered ophiolites), major fault traces, and trend lines from regional aeromagnetic datasets.

AUSTRALIA/Tasman Orogenic Belt 243

Figure 5 Geological map of the Tasman Orogenic Belt showing

geochronological data.

low-temperature–intermediate-pressure blueschist

facies metamorphism is recorded by blueschist blocks

(knockers) in serpentinite–talc-matrix me

langes

within other major fault zones of the Lachlan Oro-

gen. Winchite (blue sodium–calcium amphibole) and

glaucophane (blue sodium amphibole)–actinolite

assemblages give estimated pressures of 6–7 kbar

and 5–6.5 kbar, respectively, with temperatures of

less than 450



C. These conditions match those in-

ferred from metabasite blueschists from the Francis-

can Complex, California and the Sanbagawa Belt of

Japan. This low-temperature–intermediate-pressure

metamorphism is coincident with the regional de-

formation at around 450–445 Ma (Lachlan Orogen).

Magmatism

Distinct pulses of magmatism are recorded by the

granitic rocks that occupy up to 35% of the exposed

area of the Tasman Orogenic Belt (Figure 8). Granitic

bodies tend to be large (up to 10 000 km

) and com-

monly elongated subparallel to the structural grain.

Regional aureole, contact aureole, and subvolcanic

field associations, as well as S and I types based on

geochemistry and mineralogy, have been recognized.

They are largely post-tectonic with undeformed and

narrow (1–2 km wide) contact aureoles, although

some (e.g. the Wagga-Omeo Metamorphic Com-

plex and the Eastern metamorphic complexes of the

Lachlan Orogen; Figure 7) are coeval with regional

deformation. In the eastern Lachlan Orogen some

granites coincident with the Eastern Metamorphic

Belt were emplaced along major westwards-dipping

shear zones. Some (e.g. the Central Victorian Mag-

matic Province; Figure 8) are subvolcanic granites

associated with rhyolites and ash flows of similar

composition, and reflect intrusion to shallow crustal

levels (ca.1–4 km). The regional-aureole types are

less common and are associated with the high-tem-

perature–low-pressure metamorphism, migmatites,

and alkali feldspar–cordierite–andalusite–sillimanite

gneisses (e.g. Mount Lofty Ranges, Glenelg Zone,

Moornambool Complex, Cooma, Cambalong, and

Kuark belts, Wongawibinda Complex; Figure 7).

Granite belts with distinctive shapes (Figure 8)

reflect distinct tectonic regimes. The north-north-west-

wards-trending granites of the Central Lachlan Orogen

(1 in Figure 8), the more northwards-trending granites

of the eastern Lachlan Orogen (2 in Figure 8), and the

northwards- to north-north-westwards-trending mid-

Permian to Triassic granites of the New England

Orogen (3 in Figure 8) are representative of arc and

continental-margin–arc associations comparable to the

Cordilleran batholiths of the western USA. Mid- to

lower-crustal melting is due to crustal thickening and/

or subduction, with some granitic activity – in particu-

lar the Silurian–Early Devonian granites of the eastern

Lachlan Orogen – being linked to subduction rollback.

Major magmatic activity occurred in the Cambrian–

Ordovician (Delamerian part), Silurian–Early Devon-

ian, Late Devonian, and Carboniferous (Lachlan part),

and Carboniferous and mid-Permian–Triassic (New

England part) (Figure 8).

Eastern Australian Plate Tectonic

Evolution in the Gondwanan Context

During the Palaeozoic the Tasman Orogen was

part of a greater Gondwanan oceanic accretionary

system (Figures 9 and 10). The cycle of extension,

sedimentation, and orogeny that formed eastern

244 AUSTRALIA/Tasman Orogenic Belt

Figure 6 Geological map of the Tasman Orogenic Belt, showing tectonic vergence based on major fault dip. The major fault traces

and trend lines from regional aeromagnetic datasets are also shown.

Australia was preceded during the Late Neopro-

terozoic by initial rifting between cratonic Australia

and another large continental plate, probably Laur-

entia (North America) within the supercontinent of

Rodinia (Figure 9A).

p9000 Tasman Orogen tectonic evolution has been

broken down into a number of significant phases

described below and is also presented as a series of

Gondwanan-based maps (see Figures 9 and 10).

Rodinia Breakup (750–650 Ma): Rifting of Eastern

Australia in the Late Neoproterozoic

Rifting began at about 800 Ma but did not lead

to separation of the continents until about 700 Ma

(Figure 9A). The early phases of rifting were epicontin-

ental and are expressed as rift sequences in the Adelaide

Rift and other graben structures. After separation,

this sequence takes on a character that is more like

that of a passive margin. In the Early Cambrian

back-arc spreading in the south-east formed the

deep-water Kanmantoo Basin, which rapidly sub-

sided and was filled with clastic sediments. Very shortly

after deposition of the Kanmantoo sediments the

basin was inverted in the first stage of the Delamerian

Orogeny.

Basin Inversion along the Gondwanan Margin

(520–500 Ma): Ross–Delamerian Orogeny

The Delamerian orogenic event involved closure of the

Kanmantoo Basin by folding and thrusting, meta-

morphism, partial melting, and intrusion of granitic

AUSTRALIA/Tasman Orogenic Belt 245

plutons (Figure 9B). The driving force for this orogenic

event was presumably related to subduction that

resulted in basin inversion. Subduction on the Pacific

margin of the orogen is suggested by the presence of arc

volcanics and plutons. The ultimate closure of the

Kanmantoo Basin may have resulted from the collision

of the margin with formerly rifted continental frag-

ments that form the basement of the Glenelg Terrane

or Tasmania or from ophiolite obduction. The Dela-

merian event began at about 520 Ma, continued until

about 490 Ma, and involved westwards-directed

thrusting. It is uncertain whether any oceanic crust

was consumed between cratonic Australia and rifted

continental ribbons in western Victoria, or whether the

Kanmantoo Basin was developed only on thinned con-

tinental crust. The early phases of deformation were

dominated by folding, metamorphism, and thrusting,

followed shortly by syntectonic plutonism. The ‘or-

ogeny’ culminated in local high-temperature–low-

pressure metamorphism involving partial melting in

Figure 7 Simplified metamorphic map of the Tasman Orogenic Belt, showing the distribution of the main regional metamorphic

facies and the locations of the high-grade metamorphic complexes.

246 AUSTRALIA/Tasman Orogenic Belt

the highest-grade areas (at about 506 Ma) and was

followed by post-tectonic plutonism (ca.490–480 Ma).

Back-arc Basin Formation (520–500 Ma): Evolution

of the Lachlan Orogen

A magmatic arc grew on the eastern side of the Dela-

merian Orogen, starting at about 510–500 Ma, and

included plutons and volcanic rocks of the Stavely

Belt in Victoria (Figure 4). Shortly after this major

crustal thickening the Delamerian belt began to ex-

tend rapidly. Extension is suggested by, first, the dom-

inantly alkaline and extension-related nature of the

post-tectonic plutons (intrusion ages younger than

about 490 Ma), second, the inferred rift setting of

the Mount Stavely volcanic complex, and, third, ther-

mochronological data indicating relatively rapid cool-

ing of the higher-grade areas. Post-orogenic extension

may have been caused by subduction roll-back after

the Delamerian event, and this probably formed most

of the oceanic (back-arc) basement for the Lachlan

turbidites that were deposited during the Late Cam-

brian and Ordovician. Most of the mafic metavol-

canic rocks and plutons of the central Victorian

basement give ages of about 500 Ma. Extension in-

duced by slab roll-back may also have rifted small

continental fragments away from the cratonized

Delamerian Orogen and distributed them within the

developing back-arc basin.

Figure 8 Ages and distribution of granite intrusives associated with the Tasman Orogen.

AUSTRALIA/Tasman Orogenic Belt 247

Figure 9 Plate-tectonic reconstructions at (A) 750–650 Ma, (B) 520–460 Ma, and (C) 450–420 Ma. Adapted and reprinted, with

permission,fromtheAnnualReviewofEarthandPlanetarySciences,Volume28,ß2000byAnnualReviews,www.annualreviews.org.

The plate positions and continental outlines were calculated and drawn using data in Paleogeographic Information System/MacTM

verison June 7, 1997 by MI Ross & CR Scotese (PALEOMAP Project, Arlington Texas).

248 AUSTRALIA/Tasman Orogenic Belt

During this time large turbidite fans grew off the

shore of the Ross–Delamerian mountain chain and

spread onto the mafic crust of the developing mar-

ginal ocean (Figure 9B). Some thousands of kilo-

metres off shore subduction had initiated within the

oceanic plate, leading to the development of a vol-

canic arc complex at about 485 Ma, now represented

by the Ordovican shoshonites in the eastern Lachlan

Orogen, and an associated accretionary complex

at about 445 Ma (Narooma Accretionary Complex),

exposed in south-eastern New South Wales.

Back-arc Basin Closure (450–420 Ma): Evolution

of the Lachlan Orogen

The Lachlan Orogen formed by amalgamation of a

series of thick turbidite-dominated thrust wedges and

volcanic-arc terranes in a tectonic setting similar to

that of the Philippine, Mulucca, and New Guinea

sectors of the western Pacific today (Figure 9C). The

Lachlan Orogen resulted from the closure of a small

ocean-basin–arc system situated along the Pacific

margin of Gondwana, inboard of a larger major

long-lived Palaeozoic subduction system that is now

exposed in the New England Orogen (Figure 10A).

The total amount of subducted oceanic lithosphere

was relatively small (less than 1000 km).

Basin closure involved Woodlark Basin-style

double divergent subduction (Figure 9C), inferred

from the multiple subduction-related thrust systems

(Figure 6) and the presence of the blueschist blocks in

the serpentinite-matrix me

lange along major faults

(Figure 7). The thrust systems developed by duplexing

Figure 10 Plate-tectonic reconstructions at (A) 400–380 Ma and (B) 365–340 Ma. Adapted and reprinted, with permission, from the

AnnualReviewofEarthandPlanetarySciences,Volume28,ß2000byAnnualReviews,www.annualreviews.org.Theplatepositions

and continental outlines were calculated and drawn using data in Paleogeographic Information System/MacTM verison June 7, 1997

by MI Ross & CR Scotese (PALEOMAP Project, Arlington Texas).

AUSTRALIA/Tasman Orogenic Belt 249

in the oceanic crust and imbrication in the overlying

turbidite wedge. Shallow-angle subduction had prob-

ably initiated in the west, under the backstop of the

Delamerian Orogen, by about 460 Ma, when major

faults began forming in the western Lachlan Orogen.

Sedimentation was continuing outboard in the basin

as the western Lachlan sediment wedge thickened

and the basal de

collement propagated eastwards.

A magmatic arc associated with this westwards-dip-

ping subduction zone did not develop until Late Si-

lurian times, probably because the subduction was of

very shallow dip and the ocean basin that closed was

relatively small, so the amount of subduction was

limited. Shallow-dipping subduction is consistent

with the moderately high metamorphic pressure–

temperature ratios. The eastwards-dipping subduc-

tion system of the central Lachlan Orogen resulted

in the high-temperature–low-pressure metamorphism

and elongated north-westwards-trending Late Silur-

ian to Early Devonian granitoids of the Wagga-Omeo

Metamorphic Belt (Figures 7 and 8) as well as the

emplacement of ophiolite slices (e.g. Howqua,

Dookie, and Tatong greenstones; Figure 4) and the

Howqua blueschists (Figure 7).

Andean-type Margin (400–380 Ma): Evolution of

the Lachlan Orogen

During the Early Devonian, at around 400 Ma, the

last undeformed part of the marginal basin in the west

collided with the arc–metamorphic complexes of the

central Lachlan Orogen (Figure 10A). This caused

some crustal thickening and deformation of the

younger sediments (former Melbourne ‘trough’).

Combined structural thickening and the removal of

the oceanic lithosphere of the marginal basin formed

crust of continental thickness and character, and led

to a switch to continental sedimentation and the de-

velopment of a major regional fold-belt-wide angular

unconformity. These effects are related to the Tabber-

abberan orogenic event.

Collision followed by attainment of freeboard over

most of the Lachlan Orogen led to the development

of an Andean-type margin (Figure 10B). Continued

westwards-dipping or craton-directed subduction of

proto-Pacific lithosphere beneath this part of the

Gondwanan margin along the major outboard long-

lived subduction zone caused the magmatism that was

responsible for the extensive northwards-trending

elongated granitoids in the eastern Lachlan Orogen

(Figure 8). An easterly younging in these granitoids

suggests associated slab rollback during the mid-Late

Silurian to Early Devonian, with rollback rates of the

order of 6 mm year

1

. Thrusting was also taking place

in the eastern Lachlan Orogen at this time.

Roll-back and Gondwanan Margin Post-Orogenic

Extension (365–340 Ma)

Marked crustal-scale extension at around 380 Ma

caused rifts, basin-and-range faulting, voluminous ex-

plosive volcanism and caldera development, and high-

level plutonism in the western and central Lachlan

Orogen (e.g. the Central Victorian magmatic pro-

vince; Figure 8). By 340 Ma subduction in the eastern

Lachlan Orogen had probably stopped, and reefs were

growing around the volcanic edifices. The major sub-

duction zone at this time probably stepped east more

than 1000 km to form the New England Orogen

(Figure 10B).

Andean Margin (350–280 Ma) and Arc–Continent

Collision (260–230 Ma): Evolution of the New

England Orogen

During the Carboniferous, westwards-dipping sub-

duction beneath the Gondwanan margin led to silicic

and intermediate volcanism from 350 Ma to 310 Ma,

coincident with an inboard belt of Late Carboniferous

rocks (330–325 Ma). At around 300 Ma magmatic

activity migrated outboard into the developing associ-

ated Devonian–Carboniferous accretionary prism.

Migration of the volcanic arc was associated with

widespread extension in the Late Carboniferous to

Early Permian due to slab retreat along the long-lived

subduction zone, with the development of an outboard

intraoceanic arc (Gympie Belt). Collision of this arc

with the Gondwanan margin in the Late Permian led

to the Hunter–Bowen Orogeny (260–230 Ma), which

juxtaposed and accreted the Carboniferous flank–fore-

arc sequence (Yarrol–Tamworth Belt), the Devonian

arc complex, and the Devonian–Carboniferous sub-

duction complex (Wandilla–Gwydir Belt) by craton-

directed overthrusting. Margin overthrusting led to

molasse sedimentation in a developing foreland

basin (Sydney–Bowen Basin). Cordilleran-style gran-

ites (e.g. New England batholith) reflect renewed

subduction magmatism at this time, with the devel-

opment of a Late Permian–Early Triassic magmatic

arc. This was followed by Late Triassic exten-

sion, which is recorded by silicic caldera-related

volcanism and granitic plutonism as well as the

development of small extensional basins with bimodal

volcanism.

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