The Tasman Fold Belt System in Victoria

Tectonophysics, 48 (197 8) 267-297 @ Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netheriands

THE TASMAN

267

FOLD BELT SYSTEM IN VICTORIA

A.H.M. VANDENBERG Geological Survey of Victoria, Melbourne, (Received for publication

Victoria (Australia)

January 5, 1978)

ABSTRACT VandenBerg, A.H.M., 1978. The Tasman Fold Belt System in Victoria. In: E. Scheibner (Editor), The Phanerozoic Structure of Australia and Variations in Tectonic Style. Tectonophysics, 48: 267-297.

To provide a background to a model of the evolution of the Tasman Fold Belt System in Victoria, brief descriptions are given of the various tectonic elements, arranged in eight structural zones, and of the structural deformation affecting them. The evolution extended over four main depositional episodes, punctuated by erogenic activity. (1) The Cambrian to Late Ordovician depositional episode involved the diachronous formation of four large troughs (Stawell, Ballarat, Wagga, Melbourne Troughs). Early Cambrian mafic and ultramafic igneous activity was followed by thick diachronous flysch * sedimentation. The Cambro-Ordovician Delamerian Orogeny stabilized part of the Stawell Trough and caused eastward shift in sedimentation into the newly formed Ballarat Trough. Late Middle Ordovician diastrophism led to cessation of sedimentation in the Ballarat Trough, and to the formation of the Wagga and Melbourne Troughs. The episode was terminated by the Benambran Orogeny, which folded and stabilized the Ballarat and Wagga Troughs, and caused onset of rapid subsidence of the Melbourne and newly formed Buchan and Grampians Troughs. (2) Silurian deposition was confined to the Grampians, Melbourne and Buchan Troughs. The Melbourne Trough received a thick, relatively deep marine sequence, while in the other two troughs, acid volcanicity was followed by non-marine and shallow-marine sedimentation in the Grampians, and flysch and shallow-marine sedimentation in the Buehan Trough. Widespread diastrophism and granite intrusion marks the Bowning Orogeny, which stabilized the Grampians Trough and changed the Buchan Trough from erogenic to transitional character. The Melbourne Trough remained relatively unaffected. (3) Deposition during the Early Devonian episode was mainly confined to the Melbourne and Buehan Troughs. Eastward shift of the Melbourne Trough axis was folfowed by a gradual change from flysch to non-marine sedimentation. In eastern Victoria, formation of a large volcanic arch, superimposed on the Buchan Trough, was followed by widespread shallow-marine sedimentation. The Middle Devonian Tabberabberan Orogeny stabilized the Melbourne and Buchan Troughs, and caused major translocation along the Indi-Long Plain Fault System in eastern Victoria. (4) The Late Devonian to Early Carboniferous episode includes intrusion of granite in * The term “flysch” is used in Eastern Australia for thick sequences of alternating arenite, aleurolite and pellite, with less common psefite, in which the arenite is often turbidite. (Editor.)

268 the Ballarat and Melbourne Troughs; extrusion of volcanics in large cauldron subsidences in the area of the former Melbourne Trough; volcanicity and “redbed” sedimentation in the newly formed Howitt Trough; and some “redbed” sedimentation in eastern Victoria. Mild deformation of these rocks in the Kanimblan Orogeny marked the close of the evolution of the Tasman Fold Belt System in Victoria. INTRODUCTION

The Tasman Fold Belt System underlies the whole of Victoria, but exposure is limited to an east-west belt of hills and highlands. In western and west-central Victoria it is partly obscured by Cainozoic basalt. It is concealed to the northwest by flat-lying Cainozoic sediments of the Murray Basin, and to the south by thick Cretaceous and Tertiary sediments of the Otway and Gippsland Basins, which are traversed by east-west block faults. Permian glacigene sediments are confined to smafl grabens in the highland belt and Murray Basin. To provide a background to the tectonic development of the fold belt described in the last chapter, brief descriptions of the various tectonic elements are given. Elements are lettered according to chronology and rock type, and organized into eight structural zones (Table I; Fig. 1). Most zones have stratotectonic significance, but some have boundaries arbitrarily chosen for convenience of description.

I

I

KANMANTOO

FOLD

G Gienelg

BELT

Zone

0 L

/

/ 80

KlLOUElRLS 100

Fig. 1. Distribution of structural zones in Victoria,

LACHLAN 1 2 3 4 5

FOLD

BELT

Gromptans Zone Ararar - Btndtgo Zone Melbourne Zen+ H owqua Zone vi oggo Zone a Tobbrrobbera SubzonE b Benombro Subzone 6 Buchon Zone 1 Molong Zone 36”

I

sequences:

X

x X

Lower Palaeozoic flysch and ophiolite-flysch (t) Middle to Upper Ordovician (11) Cambrian to Middle Ordovician (u) Cambrian

in the various zones

x

of structura1 elements

Tertiary-Quaternary sediments and basalts Otway and Strzelecki Gps (L. Cret.) Coleraine Trachyte (Juras.) Mt. Leinster Igneous Complex (Trias.) Permian glacials and periglacials U. Devonian-Carboniferous redbeds and volcanics Tabberabberan intrusives (U. Dev.) Central Victorian cauldron complexes (U. Dev.) Middle Devonian dykes and granites Wentworth and Buchan Groups (Lr. Dev.) Snowy River Volcanics (Lr. Dev.) Bowningan granites (Lr. Dev.) “Reedy River Volcanics”, Jemba Rhyolite (Lr. Dev.) (m) Wombat Creek and Cowombat Beds (U. Sil.) Mitts Mitta Volcanics (Lr. Sil.?) ’ Melbourne Trough sediments (M. Ord.-M. Dev.) Kerrie Conglomerate (Sil. or Lr. Dev.?) Grampians Group (al.?) Rocklands and Wickliffe Rhyolites (Sil.?) Benambran granites (Lr. Sil.) Delamerian granites (Lr. Ord.) Cambrian? mafic/ultramafic intrusives Glenelg River Metamorphic Complex (Cambro-Ordovician)

Elements

Distribution

TABLE

x

x

x x

x

x

X

x

x

x

270

DATA ON INDIVIDUAL

STRUCTURAL

ZONES

Kanman too Fold Belt Glenelg Zone (Fig. 2) Zone margins: Western margin hidden

under Cainozoic cover; eastern margin placed at western boundary of Rocklands Rhyolite, partly obscured by Cainozoic cover. Elements: Cambrian? strata (element u) are partly altered to schist and gneiss (element w) and are intruded by Ordovician granite (element x). Stratigraphy : Element u is an unknown thickness of terrigenous sediments, partly flysch and partly shallow marine, with some carbonate and volcaniclastics (Wells, 1956). They may be as young as Early Ordovician, but are more probably Cambrian or perhaps even Precambrian. Igneous activity: Episode 1: Element y consists of mafic and ultramafic dykes intruding elements u and w (Wells, 1956). Episode 2: Element x includes granodiorite, partly gneissic, intruding element w, and leucocratic granite intruding element u. The latter has given a K/Ar age of 475 f 9 my. (Bowen, 1975), establishing a minimum Early Ordovician age for the deformation of element u. Metamorphism : Phase 1: Greenschist facies metamorphism has produced albite, tremolite, diopside, chlorite, talc and magnesite in element y. A pod of serpentinite rimmed by talc schist may be altered peridotite or pyroxenite (Wells, 1956). Phase 2: The Glenelg River Metamorphic Complex (element w) consists of schist and gneiss reaching low almandine amphibolite facies. It is presumably derived from element u (Wells, 1956). Deformation : The deformation and regional metamorphism that produced element w is assumed to have resulted from the Cambro-Ordovician Delamerian Orogeny. Foliation is the most obvious structure; bedding is not always determinable. Both have a northwesterly regional strike, with dips usually exceeding 50” (Wells, 1956). The terminal Delamerian Orogeny was the basis for inclusion of this structural zone into the Kanmantoo Fold Belt. Lachlan

Fold Belt

1. Grampians Zone (Fig. 2) Zone margins: Sharply defined,

to the west by the margin of the Rockto the east by the Woorndoo Fault. Elements: Post-erogenic Silurian? strata (element q) are folded and blockfaulted and have been shortened by <5%. They are intruded by granite (element k) and lie with disconformity on element r and angular unconformity on element u. Silurian(?) rhyolite (element r) with unknown structure and thickness lies on elements w and u with unconformity. Cambrian(?) sedilands Rhyolite,

ZONE

GLENELG

GRAMPIANS Granite (k)

EARLYDEVONIAN

Grampims

SILURIAN

ZONE

Gmup

(q)

Rocklands and Wick/if&

f

Granite(x) EARLYORDOVICIAN

Granite

G/em/g River

Complex (w)

Sandstone. sittstone. minor shale. limestone

CAMBRIAN

Ultramefic

415?9

K/Ar

be/t

Schist

slate. R

(v)

(y)

m

age determination,

Rhydite

(r)

(x?) (w?)

sandstone,

sinsta7e

Ultramafic

and/or

(V) mati

belt3 (y)

m.y.

n HOPSHAM

GRAMPIANS ZONE

\

CASTERTON

0 I

10 20 30 40 50 KILOMETRES ’ ’ ’ ’ I

r’

Normal

Fig. 2. Simplified map of the Glenelg and Grampians Zones, with Permian and younger elements deleted.

212

ments and andesitic volcanics (element u) of unknown thickness and structure form the basement in the eastern part of the zone. To the west, the basement is schist (element w). The postulated shortening in both is >50%. Stratigraphy: The Grampians Group (element 4) consists of 6000 m of non-marine and shallow-marine quartz-rich elastics, partly of “redbed” type. Poorly preserved fossils suggest a Devono-Carboniferous age (Talent and Spencer-Jones, 1963; Spencer-Jones, 1965), but recent datings of intruding granite (Fig. 4) suggest a Silurian or perhaps earliest Devonian age. Element u is unfossiliferous terrigenous flysch with some basic volcanics and chert, of possible Cambrian age (Singleton, 1965). Igneous activity : Episode 1: Poorly documented Cambrian(?) andesite occurs in narrow fault belts (Spencer-Jones, 1965). It is assumed to form the base of element u. Episode 2: The Rocklands and Wickliffe Rhyolites (element r) consist of massive to flow-banded porphyritic rhyolite. The Wickliffe Rhyolite is 60 m thick, the other much thicker (Spencer-Jones, 1965). They are post-Early Ordovician and pre-Early Devonian, probably Silurian. Episode 3: Element k consists of granodiorite, granite and dyke rocks of Early Devonian age, intruding the Grampians Group (Fig. 4) (Spencer-Jones, 1965). Metamorphism: Fhase 1: The regional metamorphics of the Glenelg River Complex, which outcrop at the western zone margin, are described on p. 270. Phase 2: Intrusion of element Fzgranites produced narrow contact aureoles of quartzite and quartzdiopside hornfels (Spencer-Jones, 1965). Deformation: Phase 1: Deformation of element u produced steep dips with regional north--south strike. Phase 2: The Bowning Orogeny produced broad, open north-trending folds in the main outcrop belt of element 4, and closer folds and steeper dips in outliers. Fold wavelength varies from 30 to 3 km. Block faults form the lateral boundaries of element 4, the largest being the Woomdoo Fault with a vertical offset of at least 2500 m near Halls Gap. 2. Ararat-Bendigo Zone (Fig. 3) Zone margins: Western margin sharply defined by Woomdoo Fault; eastern margin defined by the Mt. William Fault (= Mt. Ida, McIvor Fault) in the north, and gradational across Upper Ordovician rocks in the south. Elements: Post-erogenic Upper Devonian(?) acid volcanics (element g) occupy a small cauldron subsidence at the eastern margin. Silurian or Devonian strata (element p) with gentle folding outcrop nearby, and both overlie elements u and o with angular unconformity and are intruded by element f. Granites intruding element u in the eastern half of the zone belong to element f, while those of the western half, intruding element u, belong to element k. Cambrian and Ordovician strata (elements u and u) are about 4500 m thick, and have been shortened by about 60% to 70% by deformation.

I

0

10

20

30

40

50 KILOMETRES

LATE DEVONIAN

Gran&e (k)

EAflLY D~NIAN

Schist A@ uncartaln

Granite

SILURIAN of DEVONIAN MIDDLE bD LATE ORDOVICIAN

uerrfeCongIonlsnrrn (p) Sandstone, Wfsttme, shah,slate.somecxmg&nemlu CO)

EARLY to MIDDLE ORDOVICIAN CAMBRIAN f

Fig. 3. Simplified map of the Ararat-Bendigo deleted.

Zone,

with Permian and younger elements

274

Strutigruphy: Element p is the Kerrie Conglomerate, an unfossiliferous 150-300 m thick non-marine conglomerate and sandstone of probable Silurian or Early Devonian age (Thomas, 1932; VandenBerg, 1976). Element u forms the basement in the eastern half of the zone. Cambrian volcanics at the base are overlain by 730 m of Middle to Late Cambrian volcaniclastics, shale and chert, and about 1800 m of Lower to Middle Ordovician graptolitic terrigenous flysch and shale (Hills and Thomas, 1954; Wall and Ceplecha, 1976; V.J. Wall, written communication, 1977; VandenBerg and Wilkinson, in prep.). Element v, which forms the basement of the western half, is broadly similar but is probably Cambrian (Singleton, 1965). Igneous activity: Episode 1: The Lower Cambrian(?) greenstone suite of the Heathcote Axis consists of 2000+ m of spilite, albite dolerite, andesite, pyroclastics, acid dykes and talc schist which Thomas and Singleton (1956) considered to be altered ultramafics. Banded dolerite or gabbro occurs near Geelong (Coulson, 1930). Little-known “greenstone”, apparently including andesitic and basaltic types, occurs in the Mount Stavely Axis near Stawell (Spencer-Jones, 1965). Episode 2: The dated granites of the western half of the zone are all Bowningan (Early Devonian; Fig. 4). Episode 3: The Macedon Rhyodacite (element g) consists of at least 1000 m of homogeneous hypersthene rhyodacite (see Singleton, 1967). Close similarity with cauldron volcanics of the Melbourne Zone suggests a Late Devonian age. Episode 4: The discordant granites of the eastern half of the zone are apparently all post-Tabberabberan (Late Devonian, Fig. 4). Metamorphism Phase 1: Greenschist facies metamorphism has affected much of the greenstone suite of the Heathcote Axis. Albite, actinolite and chlorite are the main silicates, with small occurrences of talc schist (Nicholls, in Thomas et al., 1976). Comparison with the Mount Wellington Axis suggests a Cambrian age for the metamorphism. Phase 2: Schist, associated with Early Devonian granites, forms a northtrending belt of up to 25 km wide near Wedderburn, and occurs as narrower aureoles around granites in the Moliagul-Kooyoora area and at Mount Ararat. Phase 3: Intrusion of the post-Tabberabberan granites produced contact aureoles up to 2 km wide (Beavis, 1962a; Beavis and McAndrew, 1967). Deformation : Phase 1: The main erogenic phase may have been Benambran or Bowningan. In the eastern half of the zone, it produced tight close V-shaped folds with apical angles of about 40” and homoclinal limbs. Fold axes are vertical or near-vertical, persist at depth, and have average wavelength of 400 m and amplitude of about 600 m. Penetrative slaty cleavage extends across much of the zone (Talent and Thomas, 1967; Beavis, 1967; Beavis and Beavis, 1968; Wilkinson et al., 1974). Phase 2: The Tabberabberan Orogeny has produced a simple asymmetric U-shaped syncline in element p, with dips of up to 30” in the eastern limb,

-

-

-

z

”

:

$j

2 ,” >>>

-L7 c.7 <.?e”

t

2

Q 2 >>>

s

- ” ”

.

C TURAL

-YT 2 1,

:; 2 -‘ >,

”

t

. .

.

2

“o

.

“7 ” 5 ;

.

.

-’

i

_ ”

/ +

2 J

.

+

_ ~_ g = ” ~ 18

+

ZONE5

t2 >9

” cc

Egg “,‘“o

.

.

I

:-’ 2: JZ$ZZ

l

;;

ez ”

z

-I

I

Benanibran

Bownlng

Tobberobberon

Fig.

4. Radiometric datings of crystalline rocks arranged in structural zones. Datings are by K/Ar method (dots) and Rb/Sr method (crosses): vert,ical lines represent error hars. Names of plutons are those most commonly used in the literature. Sources: (1) McDougall, in Marsden, 1967; (2) Stewart, 1971; (3) Brooks and Leggo, 1972; GA numbers from Evernden and &chards, 1~62; VAlJ numbers from Bowen, 1975.

SAMPLE

460

440

420

400 -

380 -

360

340 -

NUMBERS

EARLY

-7-

MUDDLE

3RDOVICIAN

z

z n 2

z 0 > w 0

z 4

_7-

LATE

STRII

2

276

and a near-ho~zontal western limb (Thomas, 1932). The orogeny probably also produced the complex pattern of brachy-anticlino~a and brachysynclinoria with wavelengths of 3-6 km and amplitudes of 1500-2500 m, superimposed on the earlier tight folds of element u. A set of meridional steep west-dipping reverse faults, traversing the eastern half of the zone, may also be Tabberabberan. The Whitelaw Fault is one of the largest, with a stratigraphic displacement of up to 3500 m. 3. Melbourne Zone {Fig. 5) Zone margins: Western margin: see p. 272. Eastern margin partly defined by faults, partly gradational. ~Ze~e~~s: Post-erogenic Upper Devonian acid volcanics (element g), situated in cauldron subsidences, are moderately to steeply tilted (up to 70”) and block faulted, resulting in shortening of
LATE DEVONIAN

Marine sandstone, siltstone, shale. rare

(-B

SILURIAN to MIDDLE DEVONIAN

/ I

MIDDLE to LATE

i

a

congkW?Wate; non-m8rine sandstone dt.srone at top (0) &vie Conghmerate (1 Sandstone. s&tone, some congkhner8te (‘proximal turbidite facies7 (0)

364t6 K/Ar age detemtination. my.

iTRES

Fig. 5. Simplified

map of the Melbourne

Zone,

with post-Palaeozoic

elements

deleted.

Subgroup) (Birch et al., 1970). The cauidron is partly surrounded by a ring dyke of granite which closely matches the collapse-phase volcanics in composition. In the adjoining Acheron Cauldron, the Cerberean Subgroup forms part of the pre-collapse and initial collapse phases, and is followed by about 1000 m of hypersthene rhyodacite (Dudley, 1971). The only fossils of note in element g are Late Devonian fishremains (Hills, 1929). Episode 2b: The numerous post-Tabberabberan granite plutons are typically discordant, with the two largest complex batholiths (Strathbogie and Gembrook) showing pronounced east-west elongation. Five plutons intrude cauldron volcanics of proven or assumed Late Devonian age, and there is no reason to doubt that others are similar, despite the Middle Devonian aspect of K/Ar determinations (Fig. 4). Metamorphism : The post-Tabberabberan granites have narrow hornfels contact aureoles where they intrude elements u and o. Some more extensive hornfels areas are interpreted as roofed granites. Where they intrude cauldron volcanics, these have been propylitized, and in some cases have been converted to schist (Edwards, 1956). Deformation: Phase 1: The Tabberabberan Orogeny produced V and U-shaped folds in elements u and o, with wavelengths between 2 and 6 km, and amplitudes from 1.5 to 5 km. In the eastern part of the zone, major folds are ill-defined, usually consisting of clusters of close “parasitic”’ folds with wavelengths of 0.1-l km. Slaty cleavage is rare, mainly confirmed to the eastern part of the zone. The zone is traversed by several large reverse faults which experienced displacement during both the Bowning and Tabberabberan Orogenies. The largest are the Mt. William Fault along the westem margin, and the Enochs Point Thrust of the Mt. Easton Axis, with maximum stratigraphic offsets of 13 km and 2.5 km, respectively. Phase 2: Deformation during extrusion of the Upper Devonian cauldron volcanics is expressed mainly as block faulting and tilting. Some faults are rejuvenated phase 1 faults, while others are associated with ring dyke intrusions. The cauldrons are envisaged as broad saucer-like structures (Cerberean and Acheron) or as deep synclines (Mount D~denong). Radial block faults were operative during extrusion of the Cerberean Cauldron sequence (Birch et al., 1970). 4. Howqua Zone (Fig. 6) Zone margins: Western margin partly defined by faults; eastern margin arbitrarily placed near eastern boundary of the Upper Devonian of the Howitt Trough. Elements: Post-erogenic Upper Devonian to Lower Carboniferous sediments and volcanics (elements e and g) are mildly to moderately folded, have several unconformities, and overlie elements h, o, t and u with angular unconformity. Progressive deformation has resulted in shortening of up to 25% in the older, <5% in the youngest rocks. Granite of element h intrudes elements o, t and u. Element o is Middle to Upper Ordovician black shale,

EARLY DEVONIAN Age uncertain SILURIAN or EARLY DEVONIAN IR~VICIAN

or ~LURIAN m

MIDDLE to LATE ORDOVICIAN EARLY OROOVICIAN

Sam

tifkfcme -

sfae bsft

(U)

stack &ate. rue Sam d&e faciesl (01

!zz5z@l fWack is3

Samistone, Mstone, minor bksckshale f%wbfdileMj

m

SandS~.

ftl

i

sihtone. slate (U)

399’11

*‘p*

KILOMETRES

Fig. 6. Simp~i~ed map of the Howqua Zone, with post-Palaeozoic

efemenis deleted.

280

and includes a poorly known Silurian sandstone. Element t is Middle to Upper Ordovician tightly deformed flysch. Element u includes a dismembered Cambrian ophiolite and thick Lower Ordovician strata. Relationships between elements o, t and u are obscured by faulting. Deformation has resulted in shortening in them of 50-80X. Stratigraphy : Element e consists of Upper Devonian to Lower Carboniferous non-marine “redbed” sediments and acid volcanics. A moderate to strong unconformity separates Upper Devonian from Lower Carboniferous rocks in the north, but it is absent from the southern part of the belt. In the northern segment, the Upper Devonian rocks consist of sediments and acid volcanics, including a cauldron subsidence sequence, whose variable thickness and distribution reflect contemporaneous diastrophism. The overlying Mansfield Group consists of 1800-2400 m of sediments with Early Carboniferous fish remains (Marsden, 1976). In the southern segment, the Avon River Group includes the whole of element e. It consists of 20-240 m of sediments (Moroka Glen Formation), 300-900 m of acid volcanics (Wellington Rhyolite), and up to 3000 m of monotonous “redbed” sediments (Mt. Kent Conglomerate, Snowy Plains Formation) with Late Devonian fish remains in the lower three formations (Neilson, in Marsden, 1976). Element o consists of Middle to Upper Ordovician black shale and some chert (Mt. Easton Shale), restricted to fault slices in the western half of the zone, and a littleknown patch of Lower Silurian sandstone (Harris and Thomas, 1954). Element t is a thick Middle to Upper Ordovician flysch sequence, extending east from the Mount Wellington Axis. Element u consists of Cambrian? ophiolite and ultramafic intrusives, overlain by Middle Cambrian volcaniclastics with interbedded limestone (Garvey Gully Tuff, Dolodrook Limestone) and Lower Ordovician black shale and chert (Howqua Chert) (Teale, 1919,192O; Harris and Thomas, 1938, 1940, 1954; Thomas and Singleton, 1956). The rocks are so disrupted that no thickness estimate is possible. The “Mount Useful Beds” consist of a thick Early Ordovician flysch (Serpentine Creek Sandstone) and unfossiliferous siltstone which may be Silurian or Ordovician (Donnellys Creek Beds). Igneous activity: Episode 1: The oldest rocks of the zone are Cambrian(?) spilite, albite dolerite, basic volcaniclastics and shale (Jamieson Greenstone), and serpentinized peridotite, dunite and pyroxenite (“Wellington Serpentine”) (Teale, 1919, 1920; Boxer, 1973; Vinycomb, 1973). Episode 2: The rare granite plutons of the zone are similar to those of the Melbourne Zone in having strongly discordant boundaries and narrow hornfels aureoles. The Mt. Buller Granodiorite is largely hornblende granodiorite with subordinate diorite and gabbro (Marsden, 1967). The two dated granites are somewhat older than Bowningan granites elsewhere (Fig. 4). Episode 3: Upper Devonian acid volcanics extend over the entire zone. The Wellington Rhyolite consists of 300-900 m of rhyolite, agglomerate, tuff, fragmental ignimbrite, and sediments with Late Devonian fish (Neilson, in Marsden, 1976). In the complex northern segment, the oldest volcanics

281

(Hollands Creek Rhyodacite) consist of 1200 m of interbedded ignimbritic rhyodacite and “redbed” sediments, lying unconformably on elements u and t. The unconformably overlying Tolmie Igneous Complex compares with cauldron sequences of the Melbourne Zone. It includes O-90 m of basal conglomerate, 600 m of rhyolite and rhyodacite (Ryans Creek Rhyolite), and 1800 m of collapse-phase biotite rhyodacite (Toombullup Rhyodacite), intruded by the Barjarg Granite. Elsewhere in the northern segment, the volcanics consist of much thinner lenticular rhyolite and rhyodacite sequences with interbedded sediments (Marsden, 1976). Episode 4: The post-Tabberabber~ Barjang Granite intrudes the Toombullup Rhyodacite and older rocks, and underlies Mansfield Group rocks. Metamorphism: Greenschist facies metamorphism has produced chlorite, actinolite, albite, zoisite and rare epidote in the Cambrian volcanics. The intrusives are largely converted to serpentinite. At Wellington River, the metamorphism predates the Middle Cambrian Garvey Gully Tuff which contains serpentinite clasts (Teale, 1919,192O; Boxer, 1973; Vinycomb, 1973). Phase 2: Contact aureoles of granite plutons are similar to those of the Melbourne Zone. Deformation: Phase 1: Diapiric intrusion of serpentinite, associated with metamorphic phase 1, took place at the initial shearing and faulting along the complex Mount Wellin~on Axis. Phase 2: Intense deformation during the Benambran or Bowning Orogeny produced asymmetric to recumbent folds with west-dipping axes and penetrative slaty cleavage in element u. Major faulting along the several Cambrian axes probably happened during this phase. Phase 3: Folding in earliest Late Devonian time produced dips of up to 45” in the older part of element e (Hollands Creek Rhyodacite, Kevington Creek Beds). Uplift of the horst-like Howqua-Rose High had marked influence on sedimentation (Marsden, 1967, 1976). Phase 4: Deformation during the Kanimblan Orogeny produced widely spaced north to no~hwest-trending folds with ill-defined U-shaped hinges in element e. Folds have wavelengths of 5-10 km; dips are mostly
Wentworth Group (i)

EARLY DEVONIAN

LATE SILURIAN or EARLY DEVONIAN Age uncettaln

[q

Granite Omeo Metamorphic Complexand schist of Tallangana district

EARLY SILURIAN

(1)

1=

I_ a MIDDLE to LATEOR~VI~IAN

Gmnite (s)

ia 38~

ssndsofk?. si~rom?,sf?&, slate (t)

K/A? and Rb/ Sr aga datannin&tfon.m.y.

/

Normal

‘1

Fauit.5

BENAMBRA

TABEERABBERA

0

10 20 30 40

SUBZONE

S

50 KlLOMETftES

Fig. 7. Simplified map of the Wagga Zone, with Permian and younger elements deieted.

283

ment t with angular unconformity. Deformation has resulted in shortening in them of up to 80%. Lower Devonian volcanics (element I) rest with unconformity on element s and appear to be little deformed. Most of the numerous granites intruding element t are discordant and belong to elements s or h. Others are concordant with the regional strike of foliation and belong to element s. Element t consists of Middle to Upper Ordovician tightly folded strata, and regional metamorphics derived from them. Stratigruphy: The Avon River Group (element e) continues into the Wagga Zone from the adjacent Howqua Zone with similar strati~aphy, with the notable difference that the Wellington Rhyolite thins rapidly eastwards and is absent from the easternmost outcrops. A small outcrop of Upper Devonian? conglomerate occurs near Tallangatta. The Wentworth Group (element i) consists of more than 1000 m of shallow marine elastics and rare limestone with a rich Early Devonian fauna (Talent, 1963). Element t consists of poorly documented sandstone-rich flysch with a probable thickness of several thousand metres. Rare black shale and chert contain latest Middle to Late Ordovician graptolites. Igneous activity: Episode 1: Benambran granites are of two types. The first type is concordant gneissic granite which, with some exceptions, is not distinguished from the Omeo Metamorphic Complex. Plutons of the second type are discordant and generally unaffected by metamorphism, even though many have wide schist aureoIes. The largest batholiths, near the New South Wales border, consist of two-mica granite and less abundant granodiorite (Brooks and Leggo, 1972). Farther south, plutons are much smaller and consist predominantly of granodiorite. Episode 2: Granites of the Bowning Orogeny are fewer and are all discordant. They include leucocratic and normal granodiorite and perhaps some diorite. Available datings suggest a considerable age spread (Fig. 4). Similar dates obtained from high-grade metamorphics of probable Benambran age may reflect Ar loss due to secondary heating. Extrusion of the Jemba Rhyolite (element 1) was closely associated with intrusion of the Bowningan Pine Mo~tain Granite and with a large dyke swarm intruding the Corryong Batholith. It consists of some 600 m of rhyolite, apparently contained in a cauldron subsidence. The dyke swarm includes a modified dolerite-granophyre suite and an acid suite (Brooks and Leggo, 1972). Episode 3: The Middle Devonian Tabberabbera Dyke Swarm intrudes element i and underlies element e. It includes dykes of quartz diorite, hornblende porphyrite and quartz porphyrite, striking parallel to element i (Talent, 1963). Middle Devonian KjAr dates derived from two granites in the zone are considered unreliable by Evernden and Richards (1962). Episode 4: Rare small dykes of lamprophyre and quartz porphyry intruding element e must be of post-Late Devonian age (Talent, 1963). ~etu~~r~~is~: Phase 1: The Omeo Metamorphic Complex, which characterizes the Benambra Subzone of the Wagga Zone, consists of regional metamorphics of hornblende “hornfels” and amphibolite facies. The subzone

284

has a largely unmetamorpho~d central area, surrounded to the north and east by plutons with wide schist aureoles, and to the west and southwest by a 25-45 km wide belt of high-grade metamorphics. In this belt, migmatitic (“permeation”) gneiss outcrops in three broad domes, connected and surrounded by schist which shows increasing grade towards the gneiss contacts (Tattam, 1929; Crohn, 1950; Beavis, 1962b; Leggo and Beavis, 1967). Beavis and Beavis (1976) recognised three metamorphic stages: MI, development of schistosity and layering; MZ, emphasis of layering and growth of cordierite, garnet and sillimanite; and MS, retrograde metamorphism with widespread pinitization of cordierite. Granite plutons outside the Benambra Subzone have hornfels aureoles, with the exception of the Mt. Baldhead aureole, part of which consists of ho~fels-like knotted schist. Phase 2: Schist aureoles are developed around the Early Devonian Mt. Nunniong pluton, and around the East Kiewa and Niggerheads plutons, which Beavis and Beavis (1976) regard as Late Devonian. Other post-Benambran plutons have normal hornfels aureoles. Deformation: The best documented structures producing during the Benambran Orogeny are from within the metamorphic complex. The belt of high-grade rocks forms a broad anticlinorium, with gneiss outcropping in the cores of broad domes. Beavis and Beavis (1976) have postulated three phases of folding in the Kiewa region: P, produced tight, frequently isoelinal folds with gently plunging nosh-trending axes, and penet~tive schistosity parallel to bedding, in the lowest grade rocks. Pz produced crenulation cleavage by folding the PI schistosity about steeply to gently plunging west to northwest trending axes, and produced PI style folds in high-grade rocks. P3 produced Pz style folds in the high-grade rocks but did not generate foliation. The lowgrade metamorphics east of Omeo are tightly folded about axes with wavelength of about 200-400 m (Crohn, 1950). Very little is known about the structure outside the metamorphic complex. Phase 2: The Benambra Subzone is traversed by numerous large faults which strike northeast and northwest, forming a subrectangular lozenge pattern. Most have had strike and dip displacement at different times, and some have been active in Cainozoic time. The faults frequently form boundaries between different elements, so that their chronology is still poorly known. Some may be late Benambran, but the main faulting appears to have taken place during the Bowning and Tabberabberan Orogenies. The Kiewa Fault is the longest, traceable for 185 km, and forms the western boundary to the metamorphic complex. Near Mt. Feather-top it is a steep east-dipping reverse fault with a 1.5 km wide mylonite zone, intruded by the Niggerheads Granite (Beavis, 196213). Farther north it is a sinistral wrench which has displaced the Early Devonian Yackandandah Basin Granite by 8 km. The northeast-striking Baranduda Fault, which crosses it, may have had considerable movement before it displaced the post-Early Devonian strike slip of the Kiewa Fault by 3 km. (R.L. King, personal communication,

285

1976). The Mt. Hopeless Fault near the eastern margin of the zone dips steeply in part, but at its southern extremity near Bindi it is a subhorizontal thrust with a total southward displacement of the upper plate of perhaps 28 km. It is part of a northwest-striking fault system which marks the southern termination of the Indi Fault System. Phase 3: The Tabberabberan Orogeny was undoubtedly responsible for some of the faulting described above. It also produced a Noah-trending V-shaped syncline, with dips of 60-80”, in element i. South of Tabberabbera, this syncline changes into a south-plunging synclinorium with amplitude of over 1000 m (Talent, 1963). Phase 4: Mild Kanimblan deformation produced low amplitude southplunging folds and small block faults in element e. The two main folds in the zone have a half wavelen~h of 23 km. Dips generally do not exceed 10”. 6. Buchan Zone (Fig. 8) Zone margins: Sharply defined

by the boundaries

of elements

n, m, 1 and

j. Elements: Lower Devonian strata (element i) occupy a synclinorium and small scattered grabens and half-grabens, and overlie element j with apparent conformity. Folding along north-trending axes has resulted in shortening of about 25%. Lower Devonian acid volcanics and sediments (element j) lie with unconformity on elements h, 1, m and t. Broad folding and block faulting has resulted in shortening of perhaps 20%. Siluro-Devonian volcanics (element I) have unconformable and faulted contacts with elements j and m. Thick Upper Silurian strata (element m) are tightly folded along north and east trending axes. Contacts with element t are faulted. Early Devonian granite (element Iz) intruding element t has a hornfels and schist aureole. Upper Ordovician strata are similar to those of the Wagga and Molong Zones. Stratigraphy: The Buchan Group (element i) consists of 200-370 m of limestone with thin basal volc~iclastics (Buchan Caves Limestone}, and 550-750 m of muddy limestone (Taravale Formation) with a limestone lens (Murrindal Limestone) (Teichert, in Teichert and Talent, 1958). The rich Early Devonian fauna is summarized in VandenBerg et al. (1976). Element j is mainly volcanic but includes three important sedimentary phases, two at the base, and one at the top. The Seldom Seen Beds consist of terrigenous conglomerate and sandstone with a rhyolite flow (dyke?), lying with slight unconformity beneath the Snowy River Volcanies. The Timbarra Formation consists of 2000 m of terrigenous and volcanigenic sediments with thin rhyodacite flows, faulted against the Snowy River Volcanics (Fletcher, 1963). An upward fining sequence of volcaniclastics (Boundary Creek Conglomerate, Wulg~mer~g Tuff) forms the top of the Snowy River Volcanics in the northern part of the volcanic belt (Ringwood, 1955). Element m consists of marine sediments with Late Silurian fossils. The Wombat Creek Beds, confined to a graben near Benambra, consist of 1000 m of terrigenous sediments and lenticular limestone, with rare clasts of rhyolite,

N B. Contacts

0

BUCHAN

LATE DEVONlAN

10 20 30 40

50

between eh?ments t.

KJLOMETRES

ZONE

-I-

EARLY DEVONIAN I

a SILURIAN -i_

pgJ

Age mmain MIDDLE to LATE WlOV1CIAN

Fig. 8. Simplified deleted.

Wombat Creek

and

Coowombat beds

(rn)B

Wombat Creek MS equivalent (m)

A-+-+-

Grmire

iszl

Sandstone,sittstone sJate

map of the Buchan and Molong Zones, with post-Palaeozoic

(t)

elements

granite and slate (Whitelaw, 1954; Talent, 1959; Singleton, 1965; P. &tiger, personal communication, 1976). The less well-known Cowombat Beds include a thick flysch sequence with conglomerate and limestone lenses, and 1000 m of partly terrigenous, partly volcanigenic elastics with some limestone (Whitelaw, 1954; Talent, 1959). The Mount Tambo Beds comprise a thick conglomerate, sandstone and mudstone sequence. Although usually regarded as Late Devonian (Crohn, 1950), their structural and lithological similarity with the Cowombat Beds nearby suggests a Silurian age. Igneous activity: Episode 1: The Banimboola Diorite at the northern extremity of the zone is intruded by the Mitta Mitta Volcanics and is clearly Benambran (Talent, 1965). Episode 2: Extrusion of the Mitta Mitta Volcanics (element n) marks the earliest post-Benambran event. They form a thick sequence of rhyodacite with some volcaniclastics with shale and diorite clasts. Field evidence suggests that the volcanics post-date elements t and s, and predate element m (Singleton, 1965; P. Bolger, personal communication, 1976). Episode 3: The “Reedy River Volcanics” (element 1) were previously included in the Snowy River Volcanics, but recent mapping suggests that they underlie the latter with angular unconformity. They are best known from a northeast trending fault block between Bindi and Cowombat Plain, and consist of acid volcanics with some andesite. Their.age is probably earliest Devonian. A single K/Ar date derived from the Mt. Nunniong Pluton suggests a similar age (Fig. 4). Episode 4: Intrusion of granite plutons of the Kosciusko Batholith at the eastern margin of the zone occurred during the Early Devonian (Fig. 4), and may belong to episode 3. Episode 5: The Snowy River Volcanics (element j) consist of 3000 m of predominantly rhyodacite with some rhyolite, andesite, rare quartz latite, and thin tuff and conglomerate (Cochrane and Samson, 1950; Ringwood, 1955; Fletcher, 1963; Bradley, 1969). The group is mostly non-marine but contains several marine intercalations near the top. Extrusion may have taken place along faults within the volcanics (Ringwood, 1955), and through a large unmapped dyke swarm outside the zone. Metamorphism: Phase 1: Regional metamorphics of the Omeo Complex occur at the northweste~ margin of the zone. Phase 2: The Suggan Buggan Schist consists of schist and gneiss which Talent et al. (1964) considered to be derived from element m. The Early Devonian Mt. Nunniong pluton has a narrow aureole of low grade schist to the north, and a wide belt of retrograde schist to the west. Deformation: Phase 1 and 2: Marked contrast between structures produced by the Benambran and Bowning Orogenies exist only along the Indi Fault Zone, which separates metamorphics from unmetamorphosed sediments of element 172,and at Wombat Creek, where penetrative slaty cleavage occurs in element t but not in element m (P. Bolger, personal communication, 1976). Elsewhere in the zone, both elements have comparable struc-

ture, suggesting that the Benambran Orogeny had less effect than the Bowning (VandenBerg, 1976). The Bowning Orogeny includes two phases. In the first, tight folding of element m produced fracture cleavage in some areas. In the second phase, block faulting and erosion produced an angular unconformity between elements 1 and j. Phase 3: The effects of the Tabberabberan Orogeny are most pronounced along the Indi Fault Zone, where large-scale translocation (see p. 293) produced intense shearing and cleavage. Elsewhere in the zone, different styles of deformation between. elements j and i reflect differences in competence. Element j occupies a large north-trending syncline which runs along the centre of the volcanic belt. Dips decrease from about 50” near the margins to 30” and less near the synclinal axis. Block faults appear to be common (Fletcher, 1963), but are difficult to detect outside the limestone areas. The largest preserved belt of element i lies along the synclinal hinge between Buchan and Murrindal, where the sediments are tightly folded along north-trending axes with U-shaped hinges and wavelengths between 300 and 600 m. Dips vary from 10” to 50” (Teichert, in Teichert and Talent, 1958). Outside the synclinal axis, the Buchan Group is preserved in half-grabens with homoclinal dips. 7. Molong Zone (Fig. 8). Zone margins. Western margin defined by eastern boundary of elements m and j; eastern margin lies beneath the Tasman Sea. Elements. Post-erogenic Upper Devonian strata (element e) are moderately deformed by folding and block faulting which has resulted in shortening of <15%. They lie with unconformity on elements t, k and f. The Gabo Island Granite (element f) intrudes element t. Lower Devonian strata (elements j and i) form a narrow, steeply dipping belt along the Combienbar Fault. Granite plutons of element k include almost all intrusions into element t. Element m is represented by a poorly known conglomerate outcrop. Element t is thick Upper Ordovician flysch, locally metamorphosed to schist and gneiss, and folded about northeast-trending axes. Stratigraphy. Element e includes the Combienbar and Genoa River Beds which consist of non-marine “redbed” type sediments with Late Devonian(?) plant remains and have a thickness of 900 m and 1800 m, respectively (Spencer-Jones, 1967; Douglas, 1974). Elements i and j are represented by 500-800 m of volcaniclastics and marine sediments with Early Devonian fossils (Thomas, 1949; Talent, 1969). Element m is documented from asmall area near the western margin of the zone, and consists of shallow-marine sediments correlated with the Cowombat Beds (Douglas, 1974). Element t is a thick flysch sequence with some shale and chert with Late Ordovician graptolites (Talent, 1969). Igneous activity. Episode 1: Early Devonian granites (Fig. 4) occur as elongate northeast-trending plutons with hornfels aureoles, and with a belt of regional metamorphics.

Episode 2: The Gabo Island Granite is the only known post-Tabberabberan pluton in the zone. It intrudes Middle Devonian Eden Rhyolite in New South Wales. Metamorphism. The “Kuark Metamorphics” (Douglas, 1974) form a northeast-trending belt 40 km long and 5-8 km wide, associated with the Early Devonian Ellery Granite. Other plutons in the zone have hornfels aureoles. Deformation. Virtually no structural information is available for rocks of elements t and m. Elements i and j dip to the west at 70-80”, reflecting their proximity to the Combienbar Fault. As in other zones, block faulting has taken place at various times. The Combienbar Beds are preserved in three en echelon north-trending asymmetric synclines, truncated by north and northeast-trending faults. Dips range from 5” to 20” but are much steeper along the boundary faults (Spencer-Jones, 1967). The Genoa River Beds occupy a half-graben and dip to the west at 5-20” (Douglas, 1974). TECTONICS

The development of the Tasman Fold Belt System in Victoria extended over four main depositions episodes, punctuated by . erogenic activity (Fig. 9). la. Cambrian to Late Ordovician

depositional

episode

This episode is characterized by deposition of thick diachronous flysch over the entire fold belt. Three large depositional troughs (Stawell, Ballarat, Wagga Troughs; Fig. 10) became established in succession, and a fourth (Melbourne Trough) began to form towards the close of the episode. The flysch rests on greenstone suites composed of mafic to andesitic volcanics with some ultramafic and serpentinite intrusions, generally regarded as Early Cambrian. The greenstone is exposed only in several long, narrow fault zones, locally known as “axes”, and may either represent fault-emplaced oceanic crust, or may be volcanic arc-like structures (Harrington et al., 1974; Crook and Felton, 1975; Crook and Powell, 1976). Three of the belts (Heathcote, Waratah Bay and Mount Wellington Axes) were tectonically active during the succeeding sedimentation. In the Stawell Trough, the volcanics are overlain by thick terrigenous flysch, which probably extends eastwards under the Ordovician of the Ballarat Trough. Farther east, on the Heathcote, Waratah Bay and Mount Wellington Axes, the volcanics are succeeded by relatively condensed Middle Cambrian to earliest Ordovician volcaniclastics, shale, chert and some limestone. There is evidence that parts of these axes were exposed during this period. The Cambro-Ordovi~i~ Delamerian Orogeny stabilized the western half of the Stawell Trough, and may have affected the eastern half. It caused east-

8

$2

Fig. 9. Lithofaeies correlation chart across the various structural zones.

DEVORIAH

BEN~BRAN ,

DELAMERIAN

_

I

I

_

-

-TABBERABEERAt

I

OROGEWIES

291 1

1

I

'. 150

148'

14b”

1AA' B

troughs of omgenic

a as.

Troughs

roughs of omgenrc

domain plus tronsitionoldomoin

of transitvmal

domain

osrnr with Mcsozoic.C~inozo~c

S

StowelI

G

Grompians

8A

Bollorat

M

hbibourne

H

Hewitt

W

Wogg. Trough

BU

Buchan Trough

hatform

cover

340

Trough Trough Trough Trough

Trough

38” 38”

144"

Fig. 10. Distribution

of the main sedimentary

146’

basins of the Tasman

Fold

Belt in Victoria.

ward shift of sedimentation into the newly formed Ballarat Trough, in which 2000 m or more of flysch was deposited during the Early and Middle Ordovician. The eastern limit of the flysch is not known, but possible outliers outcrop within the Melbourne Trough on the ~ornington Peninsula, and along the Mount Wellin~on Axis. It seems possible that the flysch underlies a substantial part of the Melbourne Trough, which, prior to the Late Ordovician, was not a separate entity but formed the eastward continuation of the Ballarat Trough. Widespread diastrophism towards the close of the Middle Ordovician caused major changes in the pattern of sedimentation and was responsible for the formation of the Mebourne and Wagga Troughs. Flysch sedimentation appears to have ceased at this time in the Ballarat Trough, while the Wagga Trough contains no sediments of proven pre-latest Middle Ordovician age. Late Ordovician sediments are of three distinct facies: (1) a proximal flysch facies (Riddell Grits) with coarse sandstone and conglomerate with channel structures, confined to the southwes~rn margin of the Melbourne Trough; (2) a pelagic black shale facies (Mt. Easton Shale), widely distributed over

292

the eastern part of the Melbourne Trough, extending as far west as the Mornington Peninsula; and (3) thick terrigenous flysch, extending over the entire Wagga Trough. The boundary between (2) and (3) lies close to the Mt. Wellington Axis, which formed a barrier between the sediment-starved Melbourne Trough and the rapidly filling Wagga Trough. 1 b. Benambran

Orogeny

(Early Silurian)

The first main depositional episode was terminated by the Eenambran Orogeny, during which the Wagga and Ballarat Troughs were stabilized by tight folding, accompanied in the Wagga Trough by regional metamorphism. Evidence for strong uplift or perhaps deformation of the Ballarat Trough is the sudden influx, in the Early Silurian, of proximal flysch with marine slump and channel conglomerate along the western margin of the Melbourne Trough. These sediments, derived from the west, contain well-rounded clasts of quartz, quartzite, sandstone, chert and some acid igneous rocks. More significantly, they contain clasts of limestone which point to the former existence of shelf limestone near the trough margin. The beginning of the Silurian marked the onset of rapid subsidence and sedimentation, suggesting that the Melbourne Trough is an extensional basin produced by crustal stretching during the Benambran Orogeny. The scanty information available for the Buchan Trough points to a similar origin. In contrast, the Grampians Trough is a basin of transitional domain, lying on previously stabilized basement. 2a. Silurian depositional

episode.

Silurian deposition was confined to the Grampians, Melbourne and Buchan Troughs (Fig. 10); the areas in between may have been source regions. In the Grampians and Buchan Troughs, the cycle began with extrusion of acid volcanics, followed by deposition of thick elastics, non-marine and shallow-marine in the Grampians, and shallow marine and flysch in the Buchan Trough. The elastics were derived from a sedimentary/igneous terrain, with some contribution of acid volcaniclastics and limestone in the Buchan Trough. In the Melbourne Trough, sedimentation appears to have continued without interruption from the Late Ordovician into the Silurian, although the effects of the Benambran Orogeny are reflected in the sediments (see above). The Silurian sequence consists of relatively deep marine mudstone with two prominent flysch pulses, and is much thicker in the western part of the trough than in the eastern. 2b. Bowning

Orogeny

(Early Devonian)

The most notable Bowning events were widespread block faulting and intrusion of granite (Fig. 4); the only areas without Early Devonian granite

293

are the Ballarat Trough and the Melbourne Structural Zone. In the Stawell and Wagga Troughs, intrusion produced belts of regional metamorphics. In the Grampians Trough, intrusion followed moderate folding and block faulting. In the Melbourne Trough, where sedimentation was not interrupted, the orogeny is reflected by upfaulting of the Waratah Bay Axis which received limestone and shed material to the north; by influx of coarse shallow-marine elastics at Heathcote; and by faulting along the Moormbool and Sunday Creek Faults which separate contrasting Early Devonian facies (VandenBerg et al., 1976). The Bowning Orogeny changed the character of the Buchan Trough from an erogenic to a transitional tectonic domain. Tight folding of the Silurian sediments was followed by extrusion of the “Reedy River Volcanics”. 3a. Early to Middle Devonian

depositional

episode

Deposition during this episode was mostly confined to the Melbourne and Buchan Troughs. In the Melbourne Trough, sedimentation became increasingly confined to the east, following an eastward shift of the trough axis in the Pragian. The shift coincided with influx of feldspathic elastics and, in the northeast, slump and channel conglomerate with some allochthonous limestone (Walhalla Group), probably derived from the northeast or east. The episode ended with non-marine sedimentation (Cathedral Beds). In eastern Victoria, extrusion of the thick talc-alkaline Snowy River Volcanics formed a volcanic arch superimposed on the Buchan Trough. It was followed by widespread shallow-marine sedimentation (Buchan Group). At the same time, a small but deep “trough”, opening to the south, formed in the Wagga Structural Zone and received thick shallow-marine elastics (Wentworth Group). The character of this “trough” suggests it is an extensional basin. 36. Tabberabberan

Orogeny

(Middle Devonian)

The folding and block faulting of the Melbourne Trough sequence and of the Lower Devonian rocks of eastern Victoria marks the last major erogenic event; younger rocks all formed in a transitional tectonic domain. In eastern Victoria, the main effects were translocation and associated deformation along the Indi-Long Plain Fault System. If the Buchan Trough is considered as a southern extension of the Cowra Trough (Packham, 1969), its present position is best explained by dextral translocation of about 150-180 km along the Indi-Long Plain system, so that the Cowombat Beds were originally roughly on strike with Tumut. A little-known serpentinite outcrop near Cowombat could then be part of the Coolac-Kiandra ultramafic belt of New South Wales. The translocation appears to have been accommodated by major thrusting along the Wombat-Indi and Mt. Hopeless Faults. This model still fails to explain the present position of the Wombat Creek Beds.

294

Talent et al. (19’75) proposed a different model, in which major dextral translocation along the Mount ~ellin~on Axis caused separation of the originally contiguous Melbourne and Cowra-Buchan Troughs. The complex pattern of anticlinoria and synclinoria in the Ararat-Bendigo Zone may be due to broad Tabberabberan folding superimposed on the tight, closely spaced Benambran folds. 4a. Late Devonian to Early Carboniferous episode Rocks of this episode formed in a transitional tectonic domain. The episode is marked by igneous intrusion and extrusion, and non-marine sedimentation, with strong regional differences. It includes: (1) intrusion of high-level granites in the Ballarat Trough; (2) granite intrusion in the Melbourne Trough, culminating in many cases in extrusion of thick volcanic piles in cauldron subsidences; (3) formation of the Hewitt Trough (Fig. lo), which received thick non-marine sediments with major intercalations of talc-alkaline volcanics; and (4) scattered non-marine sedimentation in the Molong Zone. 4b. Kanimblan Orogeny (Early Curboniferous) Mild to moderate block faulting and folding of the Upper Devonian and Lower Carboniferous rocks marked the close of development of the Tasman Fold Belt in Victoria. ACKNOWLEDGEMENTS

I wish to thank members of the I.G.C.P. Southwest Pacific Basement Correlation working group, who provided observations and interpretations of the western and central Victorian sequences; and my colleagues, who discussed and criticized various aspects of the manuscript. Dr. D. Spencer-Jones kindly placed various facilities of the Victorian Mines Department at my disposal, and gave permission for publication. REFERENCES Beavis, F.C., 1969. The Tawonga Fault, North-east Victoria. Proc. R. SOC. Victoria, 72: 95-100. Beavis, F.C., 1962a. Contact metamorphism at Big Hill, Bendigo, Victoria. ProC. R. SW. Victoria, 75: 89-100. Beavis, F.C., 196213. The geology of the Kiewa area. Proc. R. Sot. Victoria, 75: 349-410. Beavis, F.C., 1967. Structures in the Ordovician rocks of Victoria, Proc. R. Sot. Victoria, 80: 147-182. Beavis, F.C. and Beavis, J.H., 1968. Structural geology and graptolites of the Ordovician rocks at Steiglitz, Victoria, Australia. Proc. R. Sot. Victoria, 81: 97-118. Beavis, F.C. and Beavis, J.C.H., 1976. Structural geology in the Kiewa region of the Metamorphic Complex, northeast Victoria. Proe. R. Sot. Victoria, 88: 61-76.

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