Geological Society of Australia
ABSTRACTS Number 1 9
international Conference on Deformation of Crustal Rocks
Mt. Buffalo - Australia 2 - 6 February 1987
LIST OF CONTENTS
Page CONFERENCE PROGRAMME ALPHABETICAL LIST OF AUTHORS ABSTRACTS
i xiii 1
CONFERENCE PROGRAMME
MONDAY 2.2.87 SESSION #1
"THE LOCAL SCENE - STRUCTURAL STUDIES IN S.E. AUSTRALIA"
CHAIRMAN:
Stephen Cox
AM
V.J. Morand & D.R. Gray
8.45
PROVOCATEUR:
Chris Powell
TECTONIC EVOLUTION OF THE OMEO METAMORPHIC COMPLEX, NORTHEAST VICTORIA. 9.15
C.L. Fergusson EARLY PALAEOZOIC BACK-ARC DEFORMATION IN THE BENAMBRA TERRANE, EASTERN VICTORIA.
9.45
E.I. Prendergast AN EARLY PALAEOZOIC SUBDUCTION COMPLEX ON THE NEW SOUTH WALES SOUTH COAST.
10.15
C.L. Fergusson & D.R. Gray FOLD PATTERNS AND THEIR TECTONIC IMPLICATIONS, TABBERABBERA DISTRICT, EASTERN VICTORIA.
10.45
MORNING TEA
11.00
M.P. Stubley FAULT AND KINK-BAND RELATIONSHIPS.
11.30
C.McA. Powell, J.P. Cole, G.J. McDermott & E.I. Prendergast ABNORMAL VERGENCE IN REFOLDED TURBIDITES
Noon 12.00
T.J. Fowler SUPERPOSED FOLDING IN THE ROCKLEY DISTRICT, LACHLAN FOLD BELT, NSW.
11
PM
12.30
J.W. Creasey & T.J. Cudahy FRACTURE DEVELOPMENT IN THE NOWRA SANDSTONE, SOUTHERN SYDNEY BASIN, AUSTRALIA: CHRONOLOGICAL, KINEMATIC AND TECTONIC MODELS.
1.GO
LUNCH
SESSION #2
CHAIR^N: 2.00
"BRITTLE-DUCTILE DEFORMATION, FLUIDS AND FAULTS"
Tim Bell
PROVOCATEUR:
Bruce Hobbs
G.J. Fischer DILATANCY DURING BRITTLE AND DUCTILE DEFORMATION OF SOME CRUSTAL ROCKS.
2.30
N.H.S. Oliver, P.J. Pearson, V.J. Wall, S.D. Golding & R.J. Holcombe STRUCTURAL CONTROLS ON FLUID INFILTRATION AND METASOMATISM IN THE MARY KATHLEEN FOLD BELT; IMPLICATIONS FOR LARGE-SCALE FLUID MIGRATION DURING DEFORMATION.
3.00
R.K. Valenta & B.E. Hobbs BRITTLE-DUCTILE DEFORMATION IN THE HILTON MINE, NW QUEENSLAND: THE ROLE OF SHEAR ZONES IN VEINING AND LARGER SCALE DEFORMATION PATTERNS.
3.30
AFTERNOON TEA
3.45
G.J. Fischer PERMEABILITY CHANGES IN SOME CRUSTAL MATERIALS DURING DEFORMATION AT ELEVATED TEMPERATURES.
4.15
J.A. de Roo METAMORPHIC FLUID CONDUITS ALONG ZONES OF STRAIN LOCALIZATION: GENERATION OF REEF-LIKE SILICA-CARBONATE ALTERATIONS AND ASSOCIATED MINERALIZATION DURING FOLDING.
Ill
4.45
Songfa Liu A STUDY ON A BRITTLE-DUCTILE TRANSITIONAL FAULT ZONE IN DANING GRANODIORITE, GUANGXI, CHINA.
5.15
B.E. Hobbs CONSTITUTIVE RELATIONS FOR SLIDING FRICTION
6.30
DINNER
SESSION #3
CHAIRMAN: 8.00
POSTER SESSION - VERBAL PRESENTATIONS OF POSTERS
Peter Fleming
F. Van der Hor THE INTERPRETATION OF SYNKINEMATIC ANDALUSITE PORPHYROBLAST MORPHOLOGY: EXAMPLES FROM THE BALCOOMA METAVOLCANICS, NORTHEAST AUSTRALIA
8.05
M.R. Rickard STRUCTURE OF THE YARINGA METAMORPHICS - WESTERN BASEMENT TO THE MT. ISA BASIN.
8.10
E.J. Hill REFOLDED FOLDS, EASTERN MARY KATHLEEN FOLD BELT, NW QUEENSLAND.
8.15
E.J. Hill THE TOMMY CREEK AREA, NW QUEENSLAND.
8.20
G.P. Price 6e P.F. Williams CALCITE C-AXIS FABRICS AND THE PHOTOMETRIC METHOD.
8.25
M. McLennan BRITTLE STRUCTURE EVOLUTION AT THE CASSILIS GOLD MINE, SWIFTS CREEK, VICTORIA.
IV
SESSION #4
"COMPUTER PROGRAMS AND MODELLING"
CHAIRMAN: 8.30
Graham Price
PROVOCATEUR:
Gordon Lister
D.W. Durney DEVELOPMENTS IN INCREMENTAL STRAIN AND STRESS DISTRIBUTION ANALYSIS: PROGRAMS PYRITE AND HOLEXY.
9.00
R.H. Groshong, Jr. & S.I. Usdansky THRUSTRAMP, A COMPUTER PROGRAM TO MODEL RAMP ANTICLINES.
9.30
S.Raiser, B.E. Hobbs & A. Ord COMPUTER MODELLING OF MICROFABRIC DEVELOPMENT IN POLYCRYSTALLINE AGGREGATES.
10.00
"WIZZ-BANG - Open Demonstration of Computer Programs, Software Tools and Games"
TUESDAY 3.2.1987 SESSION # 5
CHAIRMAN: AM
8.45
"STRUCTURAL STUDIES IN THE MT. ISA DISTRICT"
Alison Ord
PROVOCATEUR:
Mike Etheridge
R.J. Holcombe, P.J. Pearson & N.H.S. Oliver THE MARY KATHLEEN FOLD BELT, NORTHWEST QUEENSLAND: AND TIMING OF DEFORMATION.
9.15
P.J. Pearson, R.J. Holcombe & N.H.S. Oliver THE MARY KATHLEEN FOLD BELT, NORTHWEST QUEENSLAND: PRODUCT OF CRUSTAL EXTENSION?
9.45
GEOMETRY
D1 - A
G.S. Lister, A.D. Thomas & J. Dunn THE SIGNIFICANCE OF TRANSPRESSIONAL STRIKE-SLIP FAULTING IN THE LAKE JULIUS AREA, MOUNT ISA INLIER
V
10.15
MORNING TEA
10.30
D.E. Bettess THE STRUCTURAL EVOLUTION OF THE DEIGHTON THRUST NAPPE COMPLEX, MOUNT ISA, QUEENSLAND.
11.00
R.L. Hammond, S.C. Lang, I.W. Withnall & L.P. Black TEAR FAULT CONTROLLED BASIN DEVELOPMENT ABOVE AN ACTIVELY OVER-RIDING BASEMENT COMPLEX, NORTHEASTERN AUSTRALIA.
11.30
R.J.H. Loosveld A COMPLEX FOLD NAPPE WITHIN THE SOLDIERS CAP GROUP, MOUNT ISA INLIER, QUEENSLAND, AUSTRALIA.
Noon
PM
12.00
LUNCH
1.00
FIELD TRIP (BY BUS) TO MT. HOTHAM
WEDNESDAY 4.2.1987 SESSION #6
CHAIRMAN: AM
8.30
"STRUCTURAL STUDIES IN CENTRAL AUSTRALIAN-
David Gray
PROVOCATEUR:
Mike Rickard
R.D. Shaw INTRACRATONIC BASIN FORMATION IN CENTRAL AUSTRALIA.
9.00
R.J. Korsch & J.F. Lindsay RELATIONSHIPS BETWEEN DEFORMATION AND BASIN EVOLUTION IN THE AMADEUS BASIN, CENTRAL AUSTRALIA.
9.30
A.J. Stewart & R.Q. Oaks, Jr. NAPPES IN THE NORTHEASTERN AMADEUS BASIN, NORTHERN TERRITORY.
VI
10.00
P. Ding & P.R. James THE HARTS RANGE MOBILE BELT: A SHORT-LIVED PROTEROZOIC INTRAPLATE OROGEN FROM THE EASTERN ARUNTA INLIER OF CENTRAL AUSTRALIA.
10.30
MORNING TEA
10.45
P.R. James & P. Ding AN EARLY-MIDDLE PROTEROZOIC SHORT-LIVED EXTENSION-COLLISION INTRAPLATE OROGENY: THE STRANGWAYS OROGENY OF THE EASTERN ARUNTA INLIER, CENTRAL AUSTRALIA.
11.15
C. Amri, B.E. Hobbs & S. Raiser THE IWUPATAKA COMPLEX: A KEY AREA OF THE PROTEROZOIC AND PALEOZOIC THRUST SYTEM IN THE SOUTH ARUNTA BLOCK AND ITS KINEMATIC SIGNIFICANCE.
11.45
B.D. Goscombe REGIONAL ROTATIONAL SHEAR ACCOMPANYING HIGH GRADE METAMORPHISM IN THE EAST STRANGWAYS RANGE, ARUNTA BLOCK.
Noon 12.15
P.R. James, P. MacDonald & M. Parker STRAIN AND DISPLACEMENT IN THE HARTS RANGE DETACHMENT ZONE: A STUDY OF THE BRUNA GNEISS AND ITS BOUNDING FAULTS FROM THE EASTERN MARGIN OF THE ENTIA DOME, CENTRAL AUSTRALIA.
12.45
W.J. Collins & C. Teyssier CRUSTAL-SCALE DUCTILE FAULT SYSTEMS IN THE ARUNTA INLIER, CENTRAL AUSTRALIA.
1.15
LUNCH
Vll
SESSION #7
CHAIRMAN: 2.15
"EXPERIMENTAL STUDIES"
Othmar Tobisch
PROVOCATEUR:
Mervyn Paterson
J.L. Davidson, B.E. Hobbs & A. Ord CREEP OF ZINC SULPHIDE IN A CONTROLLED THERMODYNAMIC ATMOSPHERE.
2.45
J.C. Newton-Howes, A.C. McLaren & R.J. Fleming THE EFFECT OF BUBBLE FORMATION ON THE CONDUCTIVITY OF SYNTHETIC QUARTZ.
3.15
F.C. Luan & M.S. Paterson FABRICATION AND DEFORMATION OF SYNTHETIC QUARTZ AGGREGATES
3.45
AFTERNOON TEA
4.00
J. Gerretsen THE UPTAKE AND NATURE OF WATER AT ELEVATED PRESSURE AND TEMPERATURE IN QUARTZ AND SOME IMPLICATIONS FOR THE HYDROLYTIC WEAKENING MECHANISM.
4.30
J.D. Fitz Gerald, A. Ord, J.N. Boland & A.C. McLaren THE WATER-WEAKENING EFFECT IN EXPERIMENTAL DEFORMATION OF QUARTZ.
5.00
A. Ord & B.E. Hobbs THE PLASTIC DEFORMATION OF QUARTZ.
6.30
DINNER
Vlll
SESSION #8
"SYNTHESES AND MODELS"
CHAIRMAN: 8.00
Geoff Taylor
PROVOCATEUR:
Win Means
N.B. Woodward & E. Rutherford, Jr. STRUCTURAL LITHIC UNITS IN EXTERNAL OROGENIC ZONES.
8.30
G.S. Lister MIOCENE DUCTILE STRETCHING OF AN OVERTHICKENED CONTINENTAL CRUST AT THE TERMINATION OF THE ALPINE OROGENY, AEGEAN SEA, GREECE: A NEW SAGA IN THE EVOLUTION OF CONCEPTS CONCERNING EXTENSION TECTONICS.
9.00
B.E. Hobbs & A. Ord CRUSTAL MODELLING
9.30
M.A. Etheridge THE GEOMETRY AND TECTONIC SIGNIFICANCE OF TRANSFER FAULTS IN CONTINENTAL EXTENSION TERRANES.
10.00
G.S. Lister, M.A. Etheridge & P.A. Symonds DETACHMENT MODELS FOR THE FORMATION OF PASSIVE CONTINENTAL MARGINS.
THURSDAY 5.2.1987 SESSION #9
CHAIRMAN: AM
8.45
"STRUCTURAL PATTERNS, MICROSTRUCTURES AND FABRICS"
Brian Marshall
PROVOCATEUR:
Ron Vernon
S. Raiser, B.E. Hobbs & A. Ord EXPERIMENTAL DEFORMATION OF A QUARTZ MYLONITE: OF ORIENTATION.
THE EFFECT
IX
9.15
S.F. Cox MICROSTRUCTURAL DEVELOPMENT IN SINGLE CRYSTAL GALENA (Pbs) DURING HIGH-TEMPERATURE <100> COMPRESSION CREEP.
9.45
J.H. Kruhl PLAGIOCLASE DEFORMATION:
10.15
TEXTURES AND LATTICE ROTATION.
W.D. Means TRANSMITTED LIGHT MICROSCOPY OF DEFORMING CRYSTALLINE MATERIALS: PROGRESS AND PROSPECTS.
10.45
MORNING TEA
11.00
M. Jessell A SIMULATION OF FABRIC DEVELOPMENT IN DYNAMICALLY RECRYSTALLIZING AGGREGATES.
11.30
P.G. Lennox CONJUGATE OBLIQUE PLANAR FABRICS IN ROCK ANALOGUES - BY KINKING OR NOT BY KINKING?
Noon 12.00
P.F. Williams & G.P. Price KINKS AND CRENULATION CLEAVAGE IN SIMPLE SHEAR EXPERIEMNTS,
PM
12.30
Y. Liu & P.R. James DUCTILE DEFORMATION OF AMPHIBOLE IN THE FLORENCE SHEAR ZONE, SOUTHERN HARTS RANGE, CENTRAL AUSTRALIA.
1.00
LUNCH
2.00
T.H. Bell
CHAIRMAN:
Neil Mancktelow
REACTIVATION OF EARLIER FOLIATIONS AND DECRENULATION DUE TO SHIFTING PATTERNS OF DEFORMATION PARTITIONING.
2.30
J.H. Kruhl ROTATIONAL DEFORMATION IN THE ALPINE 'ROOT ZONE' (WESTERN ALPS) AND ITS BEARING ON PLAGIOCLASE AND QUARTZ FABRICS.
3.00
P.F. Williams & J.L. Urai CURVED VEIN FIBRES - AN ALTERNATIVE EXPLANATION.
3.30
AFTERNOON TEA
3.45
T.H. Bell, A.C. Duncan & J.V. Simmons DEFORMATION PARTITIONING, SHEAR ZONE DEVELOPMENT AND THE ROLE OF UNDEFORMABLE OBJECTS.
4.15
J. Reinhardt & M.T. Rubenach GROWTH OF PORPHYROBLASTS RELATIVE TO PROGRESSIVE DEFORMATION, TEMPERATURE INCREASE AND TIME DURING PROGRADE METAMORPHISM.
4.45
C. Steinhardt LACK OF PORPHYROBLAST ROTATION IN NON-COAXIALLY DEFORMED SCHISTS FROM PETREL COVE, SOUTH AUSTRALIA, AND ITS IMPLICATIONS.
5.15
R.H. Vernon EVIDENCE OF SYNDEFORMATIONAL CONTACT METAMORPHISM FROM PORPHYROBLAST-MATRIX MICROSTRUCTURAL RELATIONSHIPS.
FRIDAY 6.2.1986 SESSION #10
CHAIRMAN: AM
8.45
"STRUCTURAL STUDIES FROM AROUND THE WORLD"
Pat James
PROVOCATEUR:
Paul Williams
R.H. Groshong, Jr. & D.J. Patterson KINEMATIC MODEL OF THE BIRMINGHAM ANTICLINORIUM.
XI
9.15
K.C. Hill THE MULLER ANTICLINE, PAPUA NEW GUINEA; BASEMENT-CORED, INVERTED EXTENSIONAL FUALT STRUCTURES WITH OPPOSITE VERGENCE.
9.45
M. Coli & L. Matteini STRUCTURAL ANALYSIS OF THE CARRARA MARBLE IN THE ORTO DI DONNA AREA (APUAN ALPS, NORTHERN APPENNINES - ITALY)
10.15
L.R. Rankin, A.R. Martin & A.J. Parker IDENTIFICATION OF A MAJOR CRUSTAL SHEAR ZONE, NORTHWEST GAWLER CRATON, SOUTH AUSTRALIA.
10.45
MORNING TEA
11.00
C.N. Winsor THE RELATIVE TIMING OF TECTONIC OVERPRINTS IN THE IRREGULLY FORMATION, BANGEMALL BASIN, WESTERN AUSTRALIA.
11.30
B. Marshall, N.E. Odling, A. Reinsbakken & F.M. Vokes PSEUDOSTRATIGRAPHY AND THRUSTING AT JOMA MINE, NORWAY.
Noon 12.00
O.T. Tobisch & S.R. Paterson MINERALOGICAL/CHEMICAL CHANGES DURING MYLONITE GENESIS IN ITYPE PLUTONS, CENTRAL SIERRA NEVADA, CALIFORNIA: A PRELIMINARY REPORT.
12.30
T.O. Wright & R.D. Dallmeyer EPIZONE - ANCHIZONE GRADE DEFORMATION, CLEAVAGE DEVELOPMENT AND ''°Ar/^®Ar RELEASE SPECTRA OF SLATES FROM NORTHERN VICTORIA LAND, ANTARCTICA.
1.GO
LUNCH CHAIRMAN:
2.00
Rod Holcombe
C.N. Winsor & A.D. Antonovsky COMPRESSIONAL STRESS RESPONSE SURFACES: INCIPIENT CLEAVAGE EXAMPLES FROM THE IRREGULLY FORMATION, BANGEMALL BASIN, WESTERN AUSTRALIA.
Xll
2.30
N. Mancktelow THE SIMPLON FAULT ZONE: MAJOR EXTENSIONAL, TRANSVERSE MOVEMENTS IN THE PENNINE ALPS OF SWITZERLAND AND NORTHERN ITALY.
3.00
S.R. Paterson & O.T. Tobisch REGIONAL STRAIN ANALYSES IN LITHOLOGICALLY COMPLEX TERRANES,CENTRAL SIERRA NEVADA, CALIFORNIA.
3.30
AFTERNOON TEA
3.45
W.D. Roots, K.M.C. Matthews, G. Whalan & C. Morley PHANTOM OROGENIES - ^UNCONFORMITIES' PRODUCED BY FOLDING OF CONFORMABLE SEDIMENTARY LAYERS.
4.15
G.L. Clarke BASEMENT CONTROL OF COVER FOLDING IN THE ADELAIDE GEOSYNCLINE, SOUTH AUSTRALIA.
5.00
CLOSING REMARKS BY THE CHAIRMAN OF SGTSG.
5.30
DEPARTURE FOR POST-CONFERENCE EXCURSION FOR THOSE PARTICIPATING.
6.30
DINNER
Xlll
ALPHABETICAL LIST OF AUTHORS
Page Amri et al.
56
Bell
94
Bell et al.
99
Bettess
41
Clarke
125
Coli & Matteini
108
Collins & Teyssier
62
Cox
84
Creasey & Cudahy
11
Davidson et al.
64
de Roo
18
Ding 6c James
53
Durney
30
Etheridge
78
Fergusson
2
Fergusson & Gray
6
Fischer
13
Fischer
17
Fitz Gerald et al.
68
Fowler
10
Gerretsen
67
Goscombe
58
Groshong & Patterson
104
Groshong & Usdansky
32
Hammond et al.
43
Hill
25
XIV
Hill
26
Hill
106
Hobbs
21
Hobbs & Ord
77
Holcombe et al.
35
James & Ding
55
James et al.
60
Jessell
88
Korsch 6c Lindsay
49
Kruhl
86
Kruhl
96
Lee
126
Lennox
89
Lister
75
Lister et al.
39
Lister et al.
80
Liu
19
Liu & James
93
Loosveld
45
Luan 6c Paterson
66
Mancktelow
120
Marshall et al.
112
McLennan
28
Means
87
Morand & Gray
1
Newton Howes et al.
65
Oliver et al.
14
Ord & Hobbs
70
XV
Paterson & Tobisch
122
Pearson et al.
37
Powell et al.
9
Prendergast
4
Price 6c Williams
27
Rankin et al.
109
Raiser et al.
33
Raiser et al.
82
Reinhardt & Rubenach
100
Rickard
23
Roots et al.
124
Shaw
47
Steinhardt
102
Stewart 6c Oaks
51
Stubley
7
Tobisch 6c Paterson
116
Valenta 6c Hobbs
16
Van der Hor Vernon
2 2
103
Williams 6c Price
91
Williams 6c Urai
98
Winsor
110
Winsor 6c Antonovsky
119
Woodward 6c Rutherford
73
Wright 6c Dallymeyer
118
TECTONIC EVOLUTION OF THE OMEO METAMORPHIC COMPLEX, NORTHEAST VICTORIA
V.J. Morand and D.R. Gray Department of Earth Sciences, Monash University, Clayton, 3168, Victoria, Australia.
The Omeo Metamorphic complex is a pre-Late Silurian (Bolger et al., 1983), low pressure (andalusite-sillimanite) schist and gneiss terrain (Vallance, 1967). The lower greenschist to upper amphibolite metamorphics are derived from Ordovician quartzose clastics and are intruded by S- and I- type granites. The complex is cut by conjugate shear zones which show complex transcurrent movements related to fault activation and reactivation during the various compressional events that have affected it. Two major tectonic events, one in the Early Silurian (Benambran orogeny) and one in the Early Devonian (Bowning Orogeny) have produced folding, metamorphism, and granite intrusion. The first tectonic event (Dl) is associated with E-W isoclinal folding, concurrent with high grade metamorphism (Ml) and S-type granite intrusion derived by partial melting of the metasedimentary sequence. The metamorphics represent a region of localised high heat flow, probably mantle-derived. Isogradal surfaces appear to form several domes (Fagan, 1979), most being now truncated by faults. There are no Early Silurian rocks preserved in the region, but by the Middle to Late Silurian much of the metamorphic complex was covered by intermediate to silicic volcanics (Mitta Mitta Volcanics and Thorkidaan Volcanics) and shallow marine sediments (Wombat Creek Group and Enano Group). Conglomerates within these units contain no metamorphic clasts (Bolger et al., 1983) indicating that the metamorphics had probably not been unroofed by the Late Silurian. The second major event (D2), of Early Devonian age, caused tight to isoclinal, generally N-W trending folds in both basement and cover rocks. This was accompanied by greenschist facies metamorphism of the cover and widespread retrogression in high grade metamorphics (M2). Early Devonian mafic dykes intruding the metamorphics show the imprint of this low grade metamorphism. Crustal heating at this stage was associated with intrusion of granite (mostly I-type) and diorite into the metamorphics and resetting of K-Ar systems in some of the older granites (Richards and Singleton, 1981). Much of the Omeo Metamorphic Complex is bounded by major fault zones, but unlike Cordilleran metamorphic core complexes, these faults (Kiewa, Kancoona and Indi Faults) are neither low angle thrusts nor low angle normal faults, but steeply dipping structures. Lineations and S-C fabric relations on the Kiewa and Kancoona Faults indicate mainly dextral strike-slip; those on the west dipping Indi Fault indicate both reverse fault motion and a component of sinistral strike-slip. The absence of structures indicating vertical movement along bounding faults on the west side of the metamorphic complex suggests there was little uplift of the high grade rocks relative to the low grade Ordovician against which they now abut. Thus during Ml the metamorphics were probably not significantly deeper than the adjacent low grade rocks, this being consistent with low pressure metamorphism
in a n area of l o c a l i s e d h i g h h e a t f l o w . The K i e w a Fault w a s n o t initiated u n t i l a f t e r M l , a n d the K a n c o o n a a n d I n d i Faults also p r o b a b l y post-date M l . M a j o r t e x t r a l m o v e m e n t on the K i e w a and K a n c o o n a Faults o c c u r r e d during D 2 , w i t h 50 km offset e s t i m a t e d for the K i e w a F a u l t . A l o n g the I n d i Fault at this t i m e , rocks of the m e t a m o r p h i c complex w e r e thrust o v e r S i l u r i a n v o l c a n i c s a n d sediments,and some s i n i s t r a l m o v e m e n t also o c c u r r e d . Thus during the Bowning Orogeny the Omeo M e t a m o r p h i c Complex was e m p l a c e d into its p r e s e n t p o s i t i o n b y d o m i n a n t l y southwards m o v e m e n t , w i t h east-directed thrusting along the eastern margin. D o w n f a u l t i n g of the W o m b a t Creek Group p o s t - d a t e d D2 a n d p o s s i b l y d e p o s i t i o n of the Dartella Volcanic Group (Early D e v o n i a n ) . Clasts of k n o t t e d phyllite in this group (Bolger et a l . , 1983) indicate that the m e t a m o r p h i c complex was b e g i n n i n g to be u n r o o f e d at this t i m e , coincident w i t h g r a b e n formation. R e a c t i v a t i o n of the K a n c o o n a Fault in the M i d - D e v o n i a n is indicated b y 7 k m of s i n i s t r a l offset of the M i d - D e v o n i a n Y a c k a n d a n d a h G r a n i t e . M o v e m e n t along the I n d i F a u l t , truncating the Early D e v o n i a n Snowy R i v e r V o l c a n i c s , p r o b a b l y also occurred at this t i m e . A N-S c r e n u l a t i o n cleavage of M i d - D e v o n i a n (Tabberabberan) age is d e v e l o p e d in low grade rocks throughout eastern V i c t o r i a a n d overprints m y l o n i t e s in the K i e w a , Kancoona a n d I n d i Fault z o n e s . Thus m o v e m e n t along these f a u l t s , a n d f i n a l emplacement of the Omeo M e t a m o r p h i c C o m p l e x , h a d c e a s e d b y the M i d - D e v o n i a n . D e x t r a l m o v e m e n t on the Beechworth a n d Tawonga Faults p r o b a b l y o c c u r r e d in the Early Carboniferous K a n i m b l a n O r o g e n y , w h i l e dip-slip m o t i o n on these a n d o t h e r faults during the Cainozoic m a r k e d the last stage of tectonism in the r e g i o n . References: Bolger, P.F., Thorne, H.R., Wood, P.D., Cook, C.E. and Rogerson, R.J. 1 9 8 3 . Palaeozoic geology of the D a r t m o u t h Dam a r e a , northeastern Victoria. Proc. R . Soc. Vict. 95, 259-271. F a g a n , R . K . 1 9 7 9 . D e f o r m a t i o n , m e t a m o r p h i s m a n d anatexis of an Early Palaeozoic flysch sequence in n o r t h e a s t e r n V i c t o r i a . P h . D . thesis (unpubl.), U n i v . New E n g l a n d , A r m i d a l e . R i c h a r d s , J . R . a n d S i n g l e t o n , O . P . 1 9 8 1 . Palaeozoic V i c t o r i a , A u s t r a l i a - Igneous r o c k s , ages a n d their i n t e r p r e t a t i o n . J . G e o l . S o c . A u s t . 2 8 , 395-421. V a l l a n c e , T . G . 1 9 6 7 . Palaeozoic low-pressure r e g i o n a l m e t a m o r p h i s m in s o u t h e a s t e r n A u s t r a l i a . M e d d . fra D a n s k G e o l . F o r e n i n g , Kobenhaun, 17, 494-503.
2a
Fig. 1. Geological map of the Omeo Metamorphic Complex. B = Beechworth Fault, K = Kancoona Fault, T = Tawonga Fault.
WEST OF KIEWA & KANCOONA FAULTS!
OMEO METAMORPHIC COMPLEX
EAST OF INDI FAULT
500 Deep marine sediments Shallow marine clastics '.'.
Limestone
® oVo Coarse tenrestial sediments O0o V V v v Silicic volcanics ^
Granite
Fig. 2. Time-space plot of Palaeozoic history of the Omeo Metamorphic Complex and adjacent areas.
EARLY PALAEOZOIC BACK-ARC DEFORMATION IN THE BENAMBRA TERRANE, EASTERN VICTORIA
Christopher L. Fergusson Department of Geology, University of Wollongong, P.O. Box 1144, Wollongong, NSW, 2500, Australia.
Much of the eastern part of the Lachlan Fold Belt in southeastern Australia consists of the Benambra terrane which in the Ordovician contained an eastward facing island arc with a back-arc basin to the west. Part of the back-arc basin is preserved in the Tabberabbera Belt of the southwestern Benambra terrane. The southern Tabberabbera Belt is dominated by east-west trending folds that formed in a Silurian back-arc deformation and is divided by a fault (the Wonnangatta Line) into two major domains. The southwestern domain is dominated by shallowly plunging steeply inclined folds with wavelengths of up to 10 km. These folds gradually tighten towards the north where an early recumbent fold is rooted in a zone of tectonic melange up to 2 km in width at the domain boundary. The horizontal enveloping surfaces and the low amplitude-to-wavelength ratios of regional folds are consistent with a thin-skinned tectonic style and fold vergence indicates tectonic transport towards the south. The northeastern domain consists of upright close to isoclinal folds with large amplitude-to-wavelength ratios reflecting thick-skinned deformation. There is no overall sense of tectonic transport and younger structures have caused significant multiple deformation.
The Silurian back-arc deformation developed due to Chilean-style oblique-slip subduetion along the convergent boundary east of the Ordovician island arc. Regional dextral shear caused the east-west folding and also formed several major northwest-trending strike-slip faults that caused southwest translation of the Benambra terrane. Structural contrasts across the Tabberabbera Belt reflect southwesterly thickening pre-Ordovician crust that probably formed as the western passive continental margin of the back-arc basin. The tectonic pattern is consistent with strike-slip translation and convergence along the Early Palaeozoic eastern Gondwanaland margin.
AN EARLY PALAEOZOIC SUBDUCTION COMPLEX ON THE NEW SOUTH WALES SOUTH COAST
Elaine I. Prendergast Australian Plate Research Group, Macquarie University, 2109, NSW.
The Palaeozoic "coastal gre3rwacke and slate" of the NSW South Coast, represents an eastward-prograding submarine fan conformably overlying the Wagonga Beds. The "coastal greywacke and slate" consists of terrigenous, quartzo-feldspathic turbidites deposited on a fan in which the main palaeocurrents flowed toward the east. At the toe of the fan the palaeocurrents flowed oblique to the main trend and towards the south-southeast (Prendergast et al., in prep.). These currents deposited an axial channel turbidite sequence that interfingered with cherts, muds and silts of the Wagonga Beds. The Wagonga Beds are abyssal plain sediments comprising cherts, siltstones, mudstones, black shale, mafic volcanics, limestones, pelites and quartzose greywackes. Newly-discovered fossils within the Wagonga Beds (Bischoff and Prendergast, in press; Prendergast et al., in prep.) provide ages for many of the rock units. Conodonts, in the chert at the boundary of the submarine fan and the abyssal plain sediments, range in age from latest Cambrian to Early Ordovician. Limestones, associated with a mafic volcanic unit in the Wagonga Beds, contain fossils of Middle to Late Cambrian age. The volcanics are basaltic breccias formed as debris flows on the flanks of an exotic seamount and contain many limestone clasts and two parautochthonous limestone beds. The faunas extracted from the limestones include conodonts, inarticulate brachiopods, molluscs, trilobites and echinoderms. Previously, the Wagonga Beds had been considered to be Eastonian to Early Bolindian in age based on the presence of graptolites in intercalated black shale units; these Late Ordovician ages are now considered to reflect continuation of the accretionary process well into the Late Ordovician. Interfingering of the submarine fan and abyssal plain sediments occurred at least as early as latest Cambrian. At that time abyssal plain sediments, rafted on a westerly moving oceanic plate, were carried into a site in which they were swamped by axial channel turbidite deposits. It has been suggested (Prendergast et al., in prep.) that this lithological association represents a trench-related coarsening-upward succession similar to that reported from modern trenches of active subduction zones. The junction of the submarine fan and the abyssal plain sediments represents the toe of the accretionary prism. Early E-W oriented folds, that occur rarely within the Wagonga Beds, have been interpreted in previous literature to have been formed during oblique subduction. Overprinting the early deformation are three post-subduction deformations. The first two resulted from E-W compression and produced N-S trending folds and their associated cleavages. The third and last deformation, a kink related event resulted in kink folds with E-W trending axial surfaces, warps and conjugate crenulation cleavages.
The present relative positions of rocks, for which ages are known, imply at least one thrust fault within the Wagonga Beds. Alternations of older and younger rock units occur in an otherwise westwardyounging rock association. The fault may have been active during the subduction episode. Fossil ages, the upward-coarsening sequence from abyssal plain sediments to terrigenous clastics and the presence of at least one thrust fault implied by the present fossil locations are evidence for a subduction complex that was active from at least latest Cambrian to latest Ordovician. References: Bischoff, G.C.O. and Prendergast E. I. (in press). Newly discovered Middle and Late Cambrian fossils in the Wagonga Beds of New South Wales, Australia. Neues Jahrbuch fur Geologie und Palaeontologie. Prendergast, E.I., Bischoff, G.C.O., Conaghan, P.J., Powell, C.McA. and Stewart, I. (in prep.). The toe of a Palaeozoic accretionary prism.
FOLD PATTERNS AND THEIR TECTONIC IMPLICATIONS, TABBERABBERA DISTRICT, EASTERN VICTORIA
Christopher L. Fergusson^ and David R . Gray Department of Earth Sciences, Monash University, Clayton, Victoria 3168, Australia. ^ Present address: Department of Geology, University of Wollongong, P.O. Box 1144, Wollongong, NSW, 2500, Australia. Regional and mesoscopic folds reflect the kinematics of deformation in orogenic belts. Variable fold relations and geometries at Tabberabbera, Eastern Victoria indicate that pre-existing basement structure, dextral shear and folding of the regional unconformity surface have a l l influenced fold patterns in this polydeformed segment of the Tabberabbera belt of the Benambra Terrane. The dominant regional structure at Tabberabbera is the upright, isoclinal Mitchell Syncline which contains Emsian Wentworth Group strata, and formed in a period of locally intense Middle Devonian deformation. The north-northeast trend of the Mitchell Syncline is anomalous compared to the other regional Middle Devonian structures which have a northwest to north-northwest orientation. This probably reflects the Mitchell Syncline mimicking the trend of a pre-existing structure (half-graben?) which controlled sedimentation of the Wentworth Group. The intensity of Middle Devonian deformation in the Tabberabbera district suggests that it was part of a mechanically weak zone which has accommodated sinistral strike-slip motion along the northern part of the Kiewa Fault. Either side of the Mitchell Syncline is the Ordovician Hotham Group which contains F^ tight folds of probable Early to Middle Silurian age. These structures are truncated by an angular unconformity surface at the base of the overlying Wentworth Group. West of the Mitchell Syncline the F^ trends are east-west and gradually swing to north-south immediately adjacent to the unconformity surface. In this area the F^ folds are anomalously moderately to steeply plunging and face consistently to the south or southwest. These relationships indicate dextral shear west of the Mitchell Syncline during the Middle Devonian deformation and accompanying rotation of F folds from shallow to steep plunges. East of the Mitchell Syncline F^ folds are upright and gradually become more inclined as the unconformity surface is approached. This relationship is also anomalous and is probably related to folding of the unconformity surface and the underlying F^ folds during the Middle Devonian deformation. This deformation also formed north-south shallowly plunging upward and downward facing open F folds in the Hotham Group.
FAULT A N D KINK-BAND RELATIONSHIPS
M . P . Stubley A u s t r a l i a n Plate R e s e a r c h G r o u p , Macquarie U n i v e r s i t y , N S W , Australia.
2109,
Spectacular conjugate k i n k bands are e x p o s e d in a s m a l l c o a s t a l strip (600 m ) of O r d o v i c i a n turbidites, south of M y s t e r y Bay o n the N S W south c o a s t . W i t h i n the k i n k z o n e , F^ a n d F^ trends exhibit a s e g m e n t e d , counterclockwise rotation of approximately 55° about a n e a r - v e r t i c a l axis from r e g i o n a l m e r i d i o n a l t r e n d s . The k i n k zone is faulted against a c h e r t , pelite a n d basic v o l c a n i c s u c c e s s i o n to the n o r t h . This n o r t h e r n b o u n d a r y fault m a y be the s e a w a r d e x t e n s i o n of the Tantawangalo F a u l t , w h i c h has b e e n shown to the south to h a v e 16 k m d e x t r a l offset p r i o r to the deposition of U p p e r D e v o n i a n s e d i m e n t s . The turbidite succession w i t h i n the k i n k zone is t r a n s e c t e d b y v e r t i c a l conjugate faults h a v i n g orientations implying ENE-WSW compression; these directions are consistent w i t h r e g i o n a l MidD e v o n i a n transcurrent f a u l t s . F^ structures w i t h i n the k i n k z o n e , b e l i e v e d to c o r r e s p o n d to Early Carboniferous d e f o r m a t i o n elsewhere in the south coast r e g i o n , are clearly offset b y the conjugate faults w h i c h implies a similar fault system active at d i f f e r e n t t i m e s , o r a l t e r n a t i v e l y , an invalid correlation of F^ structures throughout the region. K i n k - b a n d development is directly c o n t r o l l e d b y fault l o c a t i o n . S i n i s t r a l kinks are p a r a l l e l to d e x t r a l faults a n d v i c e v e r s a . In g e n e r a l , k i n k bands are n o t offset b y the conjugate fault s y s t e m , n o r are the faults k i n k e d . S t a t i s t i c a l analysis of a l p h a , the angle b e t w e e n k i n k plane a n d e x t e r n a l f o l i a t i o n , reveals d i s t i n c t l y different means for kinks in adjacent fault b l o c k s . The d i s t r i b u t i o n of alpha is approximately n o r m a l in fault b l o c k s b o u n d e d b y only s i n i s t r a l faults a n d n o n - n o r m a l in blocks adjacent to d e x t r a l f a u l t s . The n o n - n o r m a l distributions are characteristic of b i m o d a l populations. Kink-plane orientations suggest a m a x i m u m c o m p r e s s i o n d i r e c t e d h o r i z o n t a l l y along the foliation, a n d in direct o p p o s i t i o n to that responsible for fault m o v e m e n t . As fault n u c l e a t i o n clearly predates k i n k i n g , three possible deformation histories c a n b e postulated: a) E - W c o m p r e s s i o n causing fault m o v e m e n t , f o l l o w e d b y r e v e r s a l of m a x i m u m a n d m i n i m u m p r i n c i p a l stresses causing k i n k - b a n d f o r m a t i o n , b ) R e g i o n a l E - W c o m p r e s s i o n causing fault m o v e m e n t w i t h synchronous kinkb a n d formation resulting from l o c a l stresses w i t h i n m o v i n g , wedges h a p e d fault b l o c k s , a n d c) Multiple reversals of r e g i o n a l s t r e s s e s . None of the models can be conclusively e l i m i n a t e d , h o w e v e r c i r c u m s t a n t i a l evidence supports a synchronous relationship (Model b ) . If a single or multiple r e v e r s a l of r e g i o n a l stresses o c c u r r e d , r e a c t i v a t i o n of pre-existing faults o r development of n e w faults w i t h an opposite sense of m o v e m e n t c o u l d be e x p e c t e d , w i t h at least some examples suggestive of N-S compression b e i n g p r e s e r v e d . Faults m i g h t a p p e a r k i n k e d . None of these features has b e e n o b s e r v e d at M y s t e r y B a y . As intersecting conjugate faults can n e v e r operate at p r e c i s e l y the same t i m e , m o v e m e n t on each w o u l d impart differently o r i e n t e d stresses to the adjacent r o c k s . This c o u l d e x p l a i n the single
populations of kink angles in blocks bounded only by sinistral faults and bimodal populations in areas also adjacent to dextral faults, assuming a synchronous relationship between fault movement and kinking. Kinking is viewed as a fundamental process in the development of an orogenic belt. During late stages of fold-belt development, single or conjugate faults may develop in response to continued compression normal to the fold belt. Where extension parallel to the belt is locally restricted, internal distortion of the fault blocks must occur to compensate for fault movement. In anisotropic rocks, this strain could appear as kink bands. Kink bands can only be used for local strain determinations and stress inferences. Extrapolation of this information to scales incorporating other anisotropies (e.g. faults, intrusions) may be erroneous.
Fig. 1. Idealised geometry of outcrop-scale kinks and faults at Mystery Bay. Total post-F^ strain is extensional along layering.
ABNORMAL VERGENCE IN REFOLDED TURBIDITES C. M c A . Powell, J.P. Cole, G.J. McDermott and E.I. Prendergast Australian Plate Research Group, School of Earth Sciences, Macquarie University, NSW, 2109, Australia. Outcrop-scale second-generation folds in highly-foliated Ordovician turbidites in the coastal region of southeastern Australia show abnormal vergence relationship to the macroscopic folds with which they are genetically related: the sense of vergence observed in outcrop is opposed to the position of the outcrop in the regional folds. Thus, minor folds indicating antiform to the west lie in a position where there is a syncline to the west. This reversed vergence relationship is documented in four east-west structural profiles from the coastal outcrops (broadly antiformal) to the Budawang Synclinorium. The observed vergence can be explained by considering the orientation of the highly-foliated turbidites prior to the second-generation folding, which occurred in the Early Carboniferous. Angular relations across the unconformity at the base of the Upper Devonian Lambie Group show that the Ordovician turbidites were deformed into upright or steeply inclined, close to locally very tight folds with an axialsurface segregated crenulation cleavage prior to the Late Devonian. The Upper Devonian sediments comprise a group of thin to thicklybedded quartzose sandstones with intercalated siltstones and shales (slates), with an aggregate thickness greater than 3 k m . The sandstones are stacked in thick (several hundred metres) bundles and are likely to have been the dominant mechanical layers controlling fold shape during the second generation of folding. Bedding-plane slip would have been concentrated in, or at, the sandstone-shale boundaries. The bedding and first-generation cleavage in the Ordovician turbidites, being upright or inclined moderately steeply towards the west prior to folding of the Upper Devonian sediments, was constrained to deform by the developing folds in the quartzose sandstones. The constraint is such that surfaces at a high angle to the mechanicallyactive bedding surfaces in the Upper Devonian rocks are deformed into folds with vergence sense opposite to that to be expected in layers parallel to the fold-shape controlling slip surfaces.
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SUPERPOSED FOLDING IN THE ROCKLEY DISTRICT, LACHLAN FOLD BELT, NSW
T.J. Fowler University of New South Wales, Kensington, 2033
The Rockley district lies in the Hill End Synclinorium, immediately south of the Bathurst Granite, and is believed to have been continuous with the Palaeozoic Hill End Trough. The area encloses a thick (10 km) unfossiliferous sequence of multiply deformed ?Late Ordovician to ?Early Devonian labile clastic sediments and products of silicic and mafic volcanism. Deep-water conditions appear to have prevailed throughout a history of uninterrupted deposition. The exposed Palaeozoic sequence includes probable Late Ordovician feldspathic greywacke and slate conformably overlain in turn by intratrough basalts of Early to Middle Silurian age, and dacitic/rhyolitic volcaniclastics and slate of Middle Silurian to probable Early Devonian age. The entire sequence has experienced two phases of macroscopic folding, both of which must therefore postdate the Silurian. The first folding event (Dl) produced variably trending, discontinuous, upright, ?open F1 macroscopic folds lacking both axial plane cleavage and mesoscopic folds. F1 hinges were located by analysis of mapped bedding-cleavage intersection lineation trends produced during the second folding event (D2). F1 macroscopic folds with north-northeast trends predominate and the reconstructed map of F1 fold hinges suggests that they may have been formed by simultaneous shortening in two directions at right angles, with maximum shortening in the WNW-ESE direction. The second deformation D2, has generated a fanning northwest-trending slaty cleavage S2 which is axial plane to steeply plunging tight mesoscopic F2 folds. Refolding of F1 has caused F1 plunge inversion and has formed steeply plunging D2 macroscopic folds on the limbs of tightened F1 folds. The distribution of strain during D2 deformation is remarkably inhomogeneous with the greatest intensity in the north while in the south open F1 folds are obliquely overprinted by S2 cleavages without refolding on any scale. The D2 event was accompanied by greenschiust facies regional metamorphism which reached biotite zone temperatures throughout the district (Ptot=2.5 kb; T = 470 ± 35 C). The equidimensional probable S-type Davys Creek Granite in the east of the district was intruded in the final stages of D2 deformation and has developed at best a weak tectonic foliation. The granite was preceded and accompanied by a swarm of syntectonic rhyodacite dykes. The intrusion has been accommodated by flattening, rotation and torsion of the dyke-intruded schist envelope. Latest structural events in the region are the development of S3 crenulations which are not axial plane to any mesoscopic folds and are imprinted on S2 foliations departing from the regional trend. Finally S4 kink bands and chevron folds have formed in the vicinity of reverse scissor faults that predate the intrusion of the Bathurst Granite.
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FRACTURE DEVELOPMENT IN THE N0V7RA SANDSTONE, SOUTHERN SYDNEY BASIN, AUSTRALIA: CHRONOLOGICAL, KINEMATIC AND TECTONIC MODELS
J.W. Creasey^ and T.J. Cudahy^ ^ CSIRO, Division of Mineral Physics and Mineralogy, North Ryde, 2113, NSW, Australia. ^ School of Earth Sciences, Macquarie University, North Ryde, 2113, NSW, Australia.
The lower Permian Nowra Sandstone is a 120 metre thick, flat-lying unit that blankets both older Sydney Basin sediments and complexly folded basement rocks of the Lachlan Fold Belt. This sandstone unit forms extensive plateau surfaces in the hinterland region west of Ulladulla, producing excellent exposures of fractured outcrop. Fractures, with extents ranging from centimetres to tens of kilometres (fracture lineaments) are invariably the only structures present in the Nowra Sandstone. To study the spatial and geometrical pattern of this range of fracturing we interpreted various scales of black and white aerial photography and Landsat imagery, as well as measured fractures in outcrop. The study found that the fracture arrays at different scales formed a systematic pattern consisting of well definable fracture and lineament sets. A total of five lineament and five fracture sets were distinguished, three of which were common at all scales. The three common sets were NNE-SSW, ESE-WNW and NW-SE, and formed singular and zonal lineaments. A further ENE-WSW fracture and lineament set was defined only at larger scales along with a N-S fracture set that was also the most regionally developed and frequent fracture set at outcrop scale. Each set had unique characteristics (style elemnts) that differentiated the sets and provided evidence to interpret their kinematic and tectonic development. The variations in the style elements of the sets included, a) orientation both in the vertical and horizontal planes, b) distribution (pervasive, zonal, patchy, singular), c) spacing between consecutive fractures, d) vertical and/or horizontal extent, e) fracture surface texture (rough or smooth), f) planar or curved surfaces, g) spatial arrangement (singular, en echelon, stepping), h) surface markings and i) architecture (termination and/or refraction relationships with the other sets). Many of these features provided evidence for, (i) stress directions and magnitudes, (ii) type of failure (shear, extensional or hybrid) and (iii) anisotropic influences including fracture chronology. Chronological order of fracture and lineament development can be determined by fracture architecture (Hancock, 1985). Younger fractures abut older fractures or change orientation as they cross the older fractures. At all scales fractures and lineaments showed a constant ratio of 1:8 for spacings versus length. Furthermore this relationship of size and spacing was consistent with the.chronological order of fracture formation; the shortest and closest spaced set was the last to develop.
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The first formed NNE-SSW trending fractures formed very long and widely spaced lineaments that parallel and spatially correlate with prominent basement structures. The last generation of fractures were the regionally pervasive N-S fractures. There was no evidence for conjugate pairing or sheared or faulted offsets of fractures in this set. Thus it is likely that this set formed by extensional failure. Tectonically, the mid-Cretaceous rifting of the Tasman Sea was the probable event that caused this last episode of fracturing, particularly in that the increased geothermal gradients (Embelton et al., 1985) would increase pore fluid pressure and so promote tensile failure. References: Cudahy, T.J. and Creasey, J.W. 1986. The role of basement structure in controlling structural and sedimentary patterns in the Southern Sydney Basin. CSIRO Inst, of Energy and Earth Res. unpubl. rep. Embelton, B.J.J., Schmidt, P.W., Hamilton, L.H. and Riley, G.H. 1985. Dating volcanism in the Sydney Basin: evidence from K-Ar ages and palaeomagnetism. J. Geol. Soc. Aust. NSW Div. Publ. No. 1. Hancock, P.L. 1985. Brittle microtectonics: principles and practice, J. Struct. Geol. v. 7 (3/4) pp. 437-457.
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DILATANCY DURING BRITTLE AND DUCTILE DEFORMATION OF SOME CRUSTAL ROCKS
G.J. Fischer Research School of Earth Sciences, Australian National University, P.O. Box 4, Canberra, ACT, 2601.
A device has been built for monitoring the volume of pore fluid moving in or out of a specimen during deformation at high temperatures and variable pore pressures. Operated at a constant reservoir pressure (connected to the pore system) this device enables us to measure dilatancy at pressures and temperatures above the brittle-ductile transition, provided that a sufficient permeability is preserved. Investigations into the influence of pore pressure, temperature and strain rate of dilatancy have been performed on Carrara marble, Solnhofen limestone, Gosford sandstone, Anita Bay dunite and Delegate aplite at temperatures ranging from 20°C to 1200°C, pore pressures from 40 to 280 MPa and strain rates of 10" "^s"^ and 10" ^s"^. Confining pressures up to 300 MPa were applied. A general pattern of compaction at small strains followed by dilatation has been observed. The intensity of both the compaction and dilatation increases with increasing pore pressures, while increasing the temperature decreases the dilatation and increases the strain and, more noticeably, the stress at which the transition from compaction to dilatation takes place. Opening of new void spaces continues through the brittleductile transition insofar as high pore pressure is maintained and indications are that even the permeability is increased relative to the virgin rock. The law of effective confining pressure has not been found to hold at these strain rates.
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STRUCTURAL CONTROLS ON FLUID INFILTRATION AND METASOMATION IN THE MARY KATHLEEN FOLD BELT; IMPLICATIONS FOR LARGE-SCALE FLUID MIGRATION DURING DEFORMATION
N.H.S. Oliver^, P.J. Pearson^, V.J. Wall\ S.D. Golding^ and R.J. Holcombe^ ^ Monash University, Clayton, 3168, Australia. University of Queensland, St. Lucia, 4067, Australia.
The influence of deformation on the localisation of the flow of metamorphic fluids is recognised as an important factor in the chemical and structural evolution of metamorphic belts (e.g. Etheridge et al., 1983). Examples of structurally enhanced infiltration metasomatism are common in the Mary Kathleen Fold Belt (MKFB) in the Mt. Isa Inlier, northwest Queensland. The Fold Belt predominantly comprises meta-intrusives of the Wonga and Burstall Granites, mafic meta-intrusives, acid meta-volcanics of the Argylla Formation, and the calc-silicate-rich Corella Formation, a former evaporite-carbonate sequence. A zone of moderate to intense deformation, the MKFB has experienced two major thermal peaks (reaching amphibolite facies) and associated deformation during the Proterozoic (Holcombe et al., this conference). Our studies indicate that metasomatism accompanying the deformations is localized in discrete structural and lithological zones, in which relationships between structural mechanisms and fluid transport mechanisms may be inferred. Three main categories are evident - shear zones (cm to lO's of metres scales), veins and breccias (cm to lOO's of metres), and pervasively altered meta-intrusives (granitoids and dolerites up to several km ). Anastomosing D^ shear zones in Wonga Granite show textures typical of mylonites, as well as spectacular chemical changes accompanying the formation of a quartz-oligoclasesphene assemblage from the potassic granite host. The transformations involved, including the localized presence of Cl-rich scapolite, biotite and amphibole, indicate the throughput of hypersaline Ca-Narich fluids. As there is a strong correlation between the amount of shear strain and the degree of development of the metasomatic assemblage, it can be inferred that processes related to the shearing (grainsize reduction, reaction softening, microcracking) have increased permeability in the zones and allowed fluid infiltration and concomitant high fluid/rock (F/R) ratios. The fluid was initially sourced externally to this thick meta-igneous dominated part of the sequence, and may have come from the overlying Corella Formation evaporite-carbonate sequence. During the D^ deformation, large shear zones developed in calcsilicates, particularly adjacent to competent, pre-D^ meta-intrusives. The chemical and textural changes involved are similar to those described above, with near complete albitization in the zones of highest shear strain. Albitized and brecciated calc-silicates and abundant calcite veins are also present in dilatant strain-shadow zones abutting the ends of the pre-D^ intrusive bodies. Fluid inclusions, mass transfer calculations and stable isotopic data from shear zones, veins and breccias, indicate the throughput of saline fluids apparently out of equilibrium with the surrounding unaltered
15
calc-silicates, implying at least moderate distances of fluid transport. Within the adjacent meta-intrusives, the pervasive development of syn-D^ Cl-rich alteration phases (scapolite in dolerite, Cl-rich amphibole in granite) further attests to throughput of externally derived saline fluids at high F/R ratios. Simple modelling of possible stress distributions around and within the competent meta-intrusives, in conjunction with the distribution of the metasomatic zones, can place constraints on the mechanisms of fluid transfer. The areas of inferred highest F/R ratios are located in zones of shear and dilation in calc-silicates adjacent to competent meta-intrusives, whereas the meta-intrusives have been subjected to infiltration through a system of pervasive microcracks, with a contribution from reaction-enhanced permeability. It is apparent from this modelling that lithological and related structural heterogeneities in the metamorphosing pile have a strong influence on the focussing of metamorphic fluids. Furthermore, the nature and extent of metasomatism in the fold belt support the hypothesis that large fluid volumes may be transported over many kilometres through structurally controlled plumbing systems.
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BRITTLE-DUCTILE DEFORMATION IN THE HILTON MINE, NW QUEENSLAND: THE ROLE OF SHEAR ZONES IN VEINING AND LARGER SCALE DEFORMATION PATTERNS
R.K. Valenta^ and B.E. Hobbs^ ^ Department of Earth Sciences, Monash University, Clayton, Vic. 3168 CSIRO Division of Geomechanics, P.O. Box 54, Mt. Waverley, Vic. 3149
Studies of fold, shear zone, and vein geometries in the Hilton mine are directed towards understanding the relationship between structure and mineralization. In the Hilton area, a large scale brittle-ductile shear zone separates rocks of the Mount Isa Group from chloritic greenstones of the Eastern Creek Volcanics. Associated minor shear zones within the Mount Isa Group occur parallel to the main shear zone and in conjugate pairs symmetrically disposed about the main shear zone. Fold/cleavage and shear zone relationships indicate that shear zones formed early in the deformation history, and continued deforming through at least two fold phases. Fold and vein geometries in shear zones are different from those occurring in the surrounding rocks. Folds outside shear zones are generally upright, with clockwise asymmetry looking north. Folds within shear zones are commonly gentle and symmetrical, with axial surfaces perpendicular to the shear zone walls. Shear parallel veins formed periodically throughout the shearing history, as evidenced by the occurrence of early folded and boudinaged veins in shear zone centres, grading outwards into undeformed veins at shear zone margins. Shear parallel veins and fold axial surfaces oriented perpendicular to shear zone walls imply compression sub-parallel to the shear zone. This would require a significant local change in stress regime. Periodic episodes of high fluid pressure may be responsible for the formation of shear zone parallel veins, whereas local rotation of fold axial surfaces to an orientation perpendicular to the shear zone walls may be attributed to mesoscopic scale minor changes in shear zone orientation, producing shear zone-parallel extension in the case of convergence and shear zone-parallel compression in the case of divergence. Shear zones show a strong variation in deformation intensity. As deformation increases, the ratio of carbonate to carbonaceous material decreases. This suggests a large amount of syndeformational volume loss through carbonate dissolution. The study area shows a strong lateral variation in intensity and abundance of shear zones. This variation is controlled by the movement geometry of larger scale minor shear zones, which roughly divide the area into high strain and low strain zones. The distribution of these zones has had a strong influence on later circulation of hydrothermal fluids.
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PERMEABILITY CHANGES IN SOME CRUSTAL MATERIALS DURING DEFORMATION AT ELEVATED TEMPERATURES
G.J. Fischer Research School of Earth Sciences, Australian National University, P.O. Box 4, Canberra, ACT, 2601.
The results of experimental work supported by geological evidence indicate that high pore pressures and progressive deformation can result in an increase of the permeability of crustal materials even at or above the brittle-ductile transition. We have developed a new laboratory method for measuring permeability of tight rocks. In this method we use the change in the amplitude and the phase shift between pressure oscillation induced in the upstream reservoir and the oscillatory response monitored in the downstream reservoir connected to the upstream one through a specimen. We have built a pressure transducer and corresponding electronics to resolve pressure variations of 500 Pa (0.005 bars) imposed on up to 700 MPa (7 KBar) background. Thus using the analytical solution of the equation of flow for appropriate boundary conditions and using the FFT to evaluate the phase shift, we can measure permeability as low as 10"^^ m^ (lOpD). Measurements of the permeability of a suite of crustal materials are now being taken during deformation at various pore pressures and temperatures up to 1200°C.
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METAMORPHIC FLUID CONDUITS ALONG ZONES OF STRAIN LOCALIZATION: GENERATION OF REEF-LIKE SILICA-CARBONATE ALTERATIONS AND ASSOCIATED MINERALIZATION DURING FOLDING
Jacob A. de Roo Geology Department, James Cook University, 4811 Townsville, Qld., Australia.
At the Elura Ag-Pb-Zn Mine in New South Wales syntectonic funnelling of large-scale basin dewatering during metamorphism controlled the concentration of precipitation from ore-bearing hydrothermal solutions in a structural trap. The two resulting massive sulphide orebodies occupy the cores of two doubly plunging anticlines, situated in a regionally folded sequence of terrigenous clastics. A vertical cleavage lies in the axial plane to these folds and was established prior to the mineralization. Unmineralized, less intensely doubly plunging folds surround the orebody anticlines, and their geometry designates a gradient of increasing strain towards the orebodies. This intensification of the domal folding is accompanied by progressive concentration of deformation into vertically elongated, lensoidal zones marked by increasingly contorted and segmented bedding. These lensoidal zones developed across the short limbs of doubly plunging asymmetrical folds, preferentially in the western limbs of large-scale anticlines. The selective strain localization in the east vergent folds was constrained by the asymmetrical morphology of the deforming basin. At Elura syntectonic dewatering of basinal metamorphic fluids involved focussing of the transition from regional diagenesis to low-grade metamorphism towards the progressively narrowing lensoidal "funnels" of intensifying bedding disruption. Fluid discharge along the "funnels", especially into the two anticlinal culminations of the highest strain, supported build-up of a high fluid pressure, illustrated by penetrative hydraulic microcracking and associated vein structures. This preferential localization of the high-pressure hydrothermal fluids controlled development of a zone of concentrated carbonate alteration and minor silicification in both doubly plunging anticlines. Continued fold attenuation by vertical stretching involved extensive microcracking in the relatively brittle carbonate alteration "reef". The microcracking concentrated into the most altered domains, enhancing further localization of alteration and channeling of fluids. The mineralizing fluids were fed along the highly strained western limbs into the two anticlinal cores, which acted as a silica-carbonate reservoir of high permeability and transient porosity due to the transient hydraulic microcracking. Mounting strain-rate and fluid pressure were associated with the localized replacement process and resulted in increasing competency and brittle failure. The largescale tensional gaping involved is proposed to account for local formation of vein-hosted collapse breccias within the mineralized domain, which demonstrate significant volumetric expansion.
19
A STUDY ON A BRITTLE-DUCTILE TRANSITIONAL FAULT ZONE IN DANING GRANODIORITE, GUANGXI, CHINA
Songfa Liu^ Department of Geology, Guilin College of Geology, Guilin, Guangxi, China.
A study by optical and HTEM observations of fault rock microstructures and by petrofabric and chemical analyses of samples across the fault zone has been made on a brittle-ductile transitional (BDT) fault zone in Daning granodiorite in Guangxi, China. The BDT rocks along this fault zone differ from the fault rocks of the cataclasite series (products of brittle deformation) and those of the mylonite series (products of ductile deformation). Brittle and ductile deformations are associated in the BDT fault rocks, named by the author a 'cataclasite-mylonite series' which is subdivided into protocataclasite-mylonite, cataclasite-mylonite and ultracataclasitemylonite according to the development of deformation. From protocataclasite-mylonite to ultracataclasite-mylonite, deformation mechanisms changed from predominantly brittle deformation to predominantly ductile deformation. Brittle deformation evolved from ^unstable fracturing' in protocataclasite-mylonite to ^stable fracturing' in ultracataclasite-mylonite. With the development of deformation, ductile deformation-induced microstructures such as quartz subgrains, dynamically recrystallized grains, kink bands and mechanical twins of feldspar, pressure solution and alteration of feldspars became more and more important in strongly strained fault rocks, and more and more ductile matrices were produced. In this fault zone, strain softening processes such as hydrolytic softening and grainsize reduction stimulated the transition of deformation mechanisms from brittle to ductile. In fragments of quartz in protocataclasite-mylonite, crossing of dislocations is much stronger, the density of dislocations is greater and there are fewer subgrains, when compared with quartz fragments in cataclasitemylonite. Water caused stress corrosion in crack corrosion in crack tips and diffused into lattices of minerals in fault rocks (hydrolytic softening), and also resulted in pressure solution and alteration of feldspars and other minerals. Increasing ductile matrices gradually controlled the deformation mechanisms of BDT fault rocks, changing them to predominantly ductile deformation. With the development of deformation and in strongly strained fault rocks (ultracataclasite-mylonite), C-axis fabrics of quartz fragments remained isotropic, presenting a striking contrast to the fabric development of fault rocks of the mylonite series. This is due to cataclastic deformation of quartz fragments and their ^floating' in ductile matrices. The mineral assemblage of granodiorite was gradually replaced mainly by sericite and quartz in ultracataclasitemylonite. Obvious changes in composition with development of deformation from protocataclasite-mylonite to ultracataclasitemylonite are as follows: (1) water content in the fault rocks Present address: Department of Geology, LaTrobe University, Bundoora, Victoria, Australia, 3083•
20
increases (H^O is 4.52% in ultracataclasite-mylonite, but 2.97% in protocataclasite-mylonite and 2.96% in lightly strained granodiorite); (2) increase in K (K^O: 3.00-4.59%) and decrease in Na (Na^O: 3.621.17%) are associated with the formation of sericite; (3) decrease in Si (SiO^: 66.26-59.37%) and increase in Al (Al^Og: 14.23-16.79%) may be associated with pressure solution and alteration of feldspars; (4) finally, in the strongly strained BDT fault rocks (ultracataclasitemylonite), Ti and P decrease, but TFe, Mg and Mn increase. Palaeostress estimation from recrystallized grain size, subgrain size and dislocation density indicates that the differential stress was about 65-86 MPa in the Zhanggongling BDT fault zone. The strain rate of ductile deformation in this zone is estimated as 5.7x10"^^ 3.2x10"^^/sec, but the strain rate of the whole zone may be much greater because of the brittle deformation in it. Based on the establishment of BDT fault rocks, the author suggests the classification of fault rocks with primary cohesion into three series. These are: a cataclasite series with predominantly brittle deformation, a cataclasite-mylonite series with associated brittle and ductile deformation and a mylonite series with predominantly ductile deformation. The fault rocks with little strain in the cataclasitemylonite series are similar to those of the cataclasite series, but the ones with large strain may be similar to those of the mylonite series.
21
CONSTITUTIVE RELATIONS FOR SLIDING FRICTION
B.E. Hobbs CSIRO, Division of Geomechanics, P.O. Box 54, Mt. Waverley, Vic. 3149.
Classical views of friction consider the coefficient of friction to be independent of the velocity of sliding of one surface relative to another. However, studies over the past five years or so have established a strong dependence of the coefficient of friction upon the velocity of sliding. Two contrasting types of behaviour are recognized: (i) velocity weakening behaviour where the coefficient of friction decreases with increasing velocity and (ii) velocity strengthening behaviour where the coefficient of friction increases with increasing velocity. Velocity weakening behaviour leads to instability and stick-slip behaviour for a loading machine of sufficiently low stiffness. Velocity strengthening behaviour is always associated with stable sliding. The transition from velocity weakening to velocity strengthening behaviour is pressure and temperature dependent and the conditions for this transition seem to correspond approximately with those where classical Amonton friction also breaks down so that the shear stress becomes independent of the normal stress. The above statements assume the normal stress is constant during sliding. Experiments are described where the normal stress is varied during sliding. The shear stress response to changes in normal stress is time dependent with a slow evolution of the surface towards a new state always associated with a step change in normal stress. The memory effects associated with changes in normal stress are considered and compared with the memory effects recognized for step changes in velocity at constant normal stress. Computer modelling of these constitutive relations illustrates the transitional stages between stable sliding and true stick-slip behaviour with transitions into chaotic behaviour for some conditions. New experimental data are presented to explain the well known static friction/kinetic friction transition in terms of velocity dependent friction. The implications of this work for earthquake mechanisms, for defining the base of the seismogenic zone and for setting an upper limit for stress in the lithosphere are considered. Relations between the brittle-plastic transition and the transition from velocity weakening to velocity strengthening behaviour are also considered.
22
THE INTERPRETATION OF SYNKINEMATIC ANDALUSITE PORPHYROBLAST MORPHOLOGY: EXAMPLES FROM THE BALCOOMA METAVOLCANICS, NORTHEAST AUSTRALIA (Poster)
F. Van der Hor James Cook University of North Queensland, Queensland, 4811.
Apart from crystallographic factors, the morphology of metamorphic porphyroblasts is also strongly affected by external chemico-physical variables. Consequently, the shape of particular porphyroblastic mineral species can differ considerably in rocks of different composition, texture, or structural-metamorphic setting. This poster describes the morphology of synkinematic andalusite porphyroblasts in metapelites from the Balcooma Metavolcanics, Northeast Australia. Three types of crystal shapes are present 1) (hyp)idiomorphic, 2) ellipsoidal xenomorphic and, 3) irregular xenomorphic. A model for the morphological development of andalusite as a function of rock permeability, and texture and mineralogical composition of the matrix is presented. The significance of the use of porphyroblast morphologies for the interpretation of porosity and permeability during deformation and metamorphism is discussed.
23
STRUCTURE OF THE YARINGA METAMORPHICS - WESTERN BASEMENT TO THE M T . ISA BASIN (Poster) M . R . Rickard Department Geology, Australian National University, Canberra, A C T , 2600.
The Yaringa metamorphics comprise highly deformed and retrogressed metaturbidites and migmatites intruded by granites. In a cover sequence the McNamara Group oversteps the Judenan Beds to lie unconformably on the Yaringa basement thereby indicating a buried rift margin to the M t . Isa basin (Fig. 1). Early structures of the Yaringas include tight to isoclinal folds (Fl) with now vertical NE axial planes and steep plunges, schistosity SI, and more open folds (F2). Granulite (?) metamorphism and migmatization occurred at 1900 my^^. Intrusion of the Big Toby granite plutons at 1800 my created complex marginal sheeted zones. One pluton is overlain unconformably by a rhyolite porphyry dated at 1680 my*'':" that correlates well with the Carters Bore Rhyolite to the east (Fig. 1). In a second episode of folding (Fig. 2) the Yaringa metamorphics are sharply folded (F3) and crenulated, with wide-spaced (few metres) NW axial-plane zones (Figs. 1 & 2). Minor N-trending folds (F4) also occur locally (Fig. 2). This deformation caused retrogression of the schists and the granites were extensively sheared. The rhyolite was also strongly foliated by S3. The Yaringa inlier is framed by McNamara Group conglomerates, quartzites and shales, unconformably in the w e s t and faulted in the east (Fig. 1). These sediments were deformed by simple N-S folds with gentle plunges; foliation is weak even in pelite layers. Aberrant NW cross folds and a steep foliation transect the beds in the southeast (Fig. 1) giving a steep intersection lineation. The tectonic relationships and deformational history are outlined in Figure 2. The basement has probably induced stress reorientations so that conflicting interpretations of the history are possible.
Unpublished dates by courtesy of R . Page A N U / B M R .
24
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25
REFOLDED FOLDS, EASTERN MARY KATHLEEN FOLD BELT, NW QUEENSLAND (Poster)
E.J. Hill University of Queensland, Brisbane.
In the Mt. Isa Inlier, east of the Wonga Belt, the rocks (dominantly calc-silicates and marbles) have undergone multiple deformation resulting in patterns of refolded folds. A detailed study of these patterns has revealed an early deformation (Dl) overprinted by the regional north-south folding event (D2), which folds are generally non-cylindrical. Mesoscopic scale Dl fold orientations were originally extremely variable. Analysis of macroscopic scale folding patterns in an area of detailed mapping between Knobby mine and Kundi mine showed some variability in Dl fold orientations, which are interpreted to be due to effects of folding around pre- or syn-tectonic intrusions. Dl folding is strongest in a narrow, elongate, north-south zone extending at least from Kundi mine, in the south, to Copper Blonde mine in the north. This zone parallels the Wonga Belt to the west, and recent work in the area (R. Holcombe pers. comm.) suggests that Dl is synchronous with early extensional deformation in the Wonga Belt.
26
THE TOMMY CREEK AREA, NW QUEENSLAND (Poster)
E.J. Hill Bureau of Mineral Resources, Canberra.
The Tommy Creek area consists of thick, generally unlayered masses of felsic ignimbrite interbedded with calc-silicates, sandstones, marbles and carbonaceous pelitic sediments. These are overlain by a sequence of mafic and intermediate volcanics and tuffs. The sequence has been deformed by two phases of thrust faulting, and recumbent and inclined folding (D1 and Dll); accompanied by Amphibolite grade metamorphism of the Sillimanite zone and resulting in anatectic granitic veins and pegmatites. D1 is associated with the formation of a strong to mylonitic axial plane foliation, breccias and shear zones. This early phase of deformation is overprinted by the Mt. Isa regional, upright, north-south folding event (D2). The possibility of an early, pre-Dl extension event is also considered. Unexpectedly young U-Pb age dates for the felsic volcanics of around 1600 to 1620 Ma (Page 1983) are proposed as the D1 metamoprhic age. And the previously mapped "Tommy Creek Microgranite" (Derrick et al., 1971) is reinterpreted on the basis of field evidence and thin section studies to be a recrystallised porphyritic ignimbrite. References: Page, R.W. 1983. Timing of superposed volcanism in the Proterozoic Mt. Isa Inlier, Australia. Precambrian Research, 21, 223-245. Derrick, G.M., Wilson, I.H., Hill, R.M., Glikson, A.Y. and Mitchell, J.E. 1971. Geology of the Marraba 1:100 000 sheet area, northwest Queensland. BMR Aust. Geol. Geophys. Record 56.
27
CALCITE C-AXIS FABRICS AND THE PHOTOMETRIC METHOD (Poster)
G.P. Price and P.F. Williams CSIRO, Division of Geomechanics, P.O. Box 54, Mt. Waverley, Vic. 3149.
The photometric method of deriving c-axis fabric patterns has been successfully developed for the mineral quartz. The potential exists for the method to be applied to other uniaxial, colourless minerals, in particular to calcite. However the distinctly different optical properties of calcite could result in the method being inconvenient or inaccurate. The prospects for applying the photometric method to calcite are here evaluated. The mineral calcite is uniaxial negative which results in transmission profiles 90° out of phase with those for quartz (which is uniaxial positive). This presents no difficulty since the matching of experimentally observed and theoretical transmission profiles is performed by summing the squares of differences in the fourier coefficients for the two profiles and in terms of these coefficients an intensity minimum aligned with the c-axis has no disadvantage compared with an intensity maximum. Calcite has extreme birefringence, some nineteen times higher than the birefringence of quartz. This means that the optimum microscope/optical conditions for analysing quartz fabrics (wavelength of incident light 465 mm, retardation of the gypsum plate 540 mm and thin-section thickness 17.5 micrometres) are reproduced for calcite only at thin-section thickness of 0.75-1.0 micrometres. The difficulties of routinely producing thin-sections of this thickness would make the method unsuitable for general use. This problem can be overcome by using 1/4 wavelength plate (retardation 145 nm) instead of the gypsum plate and by extending the choice of incident light wavelengths. By adding two monochromatic filters; e.g. 545 nm and 645 nm; and allowing the choice of any one, or a combination of any of the three filters, acceptable optical conditions can be found for thin-section thicknesses in the range 2-7 micrometres. This is a much m.ore acceptable thickness. The only remaining problem is that the high birefringence of calcite means that the light transmission intensities vary strongly for small changes in thin-section thickness. Therefore it is important that the thickness be measured accurately (±0.1 micrometres) and that this thickness is used in calculating the data set of theoretical transmission profiles. Given the above problems and their suggested solutions what are the possibilities of applying the photometric method to the analysis of calcite c-axis fabrics and how will the resulting pole figures compare with manually measured pole figures? Progress in resolving this question will be reported.
28
BRITTLE STRUCTURE EVOLUTION AT THE CASSILIS GOLD MINE, SWIFTS CREEK, VICTORIA (Poster)
Michael McLennan Geology Department, Monash Univesity, Clayton, Victoria, 3168, Australia.
The Cassilis gold deposit in north-east Victoria comprises goldsulphide -quartz veins hosted by a sequence of pelitic schists and psammites of Ordovician age. Two fold generations have been identified within the Cassilis region. Late Ordovician-Early Silurian regional deformation (D^) folded marine turbidite successions into upright, shallowly plunging east-west trending folds (F^). Development of a strong, penetrative, axial surface schistosity (S^) is defined by an anastomosing foliation of sillimanite and K-feldspar about cordierite, biotite and andalusite. A second regional deformation (D ) of Middle Devonian age produced open folds (F ) with a near vertical axial plane, reorientating the F^ fold hinge about 180°. F^ parasitic folds were a prominent development during D^. These wrap over the F^ hinge, the plunge of which adopts the dip of the F^ limbs. Development of a crenulation cleavage (S^) is associated with D^ and retrograde muscovite growth. The Cassilis mine is located on the north dipping limb of an F^ antiform. Bedding and schistosity strike approximately 107° and mineral lineation within S^ is subhorizontal. Ten brittle structural styles have been recognised in the mine area. 1) Fractures, which show no obvious displacements of host rock comprise empty fissures forming sets of intermediate to high angles to each other; 2) infrequent hairline cracks, and 3) and 4) which have acted as fluid pathways as evidenced by development of alteration envelopes and deposition of amorphous fill respectively. Faulting is recognised by development of fault gouge characterised by a fine clay material along planar zones. Two fault types have been classified primarily on orientation and relative timing and include: 1) faults which strike subparallel to fractures; and 2) faults which have subhorizontal slickensides indicating strike slip movement, strike almost perpendicular to, and displace fractures and veins. Both fault types show displacements of bedding. Vein systems which vary between 0.1 to 1.5 cm in width can be planar with internal crustification of mineralogies or highly irregular with sporadic sulphide deposition. Relatively late, barren quartz veinlets can be observed cross-cutting the mineralised structures. A considerable proportion of the mineralisation at Cassilis is associated with relatively thick (0.3-2.5 m) quartz zones (i.e. Blacksmith and Snake reefs). Sulphides are disseminated throughout these quartz zones and often rim metasedimentary fragments within the quartz. Irregular veins and thicker quartz zones associated with Blacksmith and Snake reefs indicative of high fluid flow or dilation. Metasedimentary fragments in the larger zones were formed by hydraulic brecciation since no shear fabrics were developed. Breccia zones indicate intense cataclasis or shearing of the country rock. The bulk of the mineralisation at Cassilis is disseminated throughout some of these prominant shears (i.e. Cassilis-, Crisps and Ceresa reefs), although not all shears have evidence of fluid movement. Closer
29
examination of Crisps reef shows it to have experienced episodic cataclasis (initially controlled by high angle fracture sets). Fracture types, type 1 faulting, veins and dilational zones are predominantly sub-parallel, striking between 335° and 343°, dipping steeply west but occasionally east. Shear zones generally strike 325° and dip west at high angles although the Cassilis reef (shear) strikes 005° and dips steeply east. The F fold axis is almost perpendicular to all fracture sets, veins and dilational zones whereas breccia zones have formed at intermediate angles between the fold axis and these brittle structures. Textures and geometries shown by fracture types, veins and dilational zones indicate they developed by displacement normal to a fracture plane (i.e., extension). Extensional joints (fractures) geometrically related to folds may or may not originate during folding (Hobbs et al. 1976, p. 299). Assuming extensional and shear structures formed synchronously at Cassilis, calculations of principle stress orientations show compression required to produce these brittle structures is subhorizontal from the north-north-west and south-southeast. Development of upright folds striking approximately 075° would have required compression from a similar orientation. Consequently the compressive stresses (cr^) needed to produce both brittle structures and the F^ fold were almost co-axial. The minor principal stress components (a^ and a^) have however exchanged position in both deformational events, a^ was vertical during folding but horizontal through brittle failure. This clearly indicates fracturing was not synchronous with F^ folding as is further evidenced by high grade metamorphism during folding. Brittle structure evolution would require probable uplift and subsequent cooling following F^ folding and prograde metamorphism. Associated with uplift, erosion and removal of overburden would cause reversal of the minimum and intermediate principal stress components to give the stress configuration necessary for brittle structure development. A proportion of these structures then acted as channelways for later fluids producing vein systems, dilational zones and mineralised shear zones. Lithological control on brittle failure is evident by the general restriction of fractures to psammitic layers. Larger quartz filled structures (i.e., reef systems) are continuous through the interbedded metasediments although tend to "pinch out" when traversing pelitic layers.
30
DEVELOPMENTS IN INCREMENTAL STRAIN AND STRESS DISTRIBUTION ANALYSIS PROGRAMS PYRITE AND HOLEXY
D.W. Durney School of Earth Sciences, Macquarie University, North Ryde, NSW 2109, Australia.
Two computer-based structural analysis methods are described, one for computing incremental strain from pressure-shadows and one for thematic representation of stress distributions around circular inclusions. Examples illustrate the input requirements, outputs and uses of the two methods. The programs are written in standard Fortran IV and currently run on a VAX 11-780 computer with output to a DEC lineprinter or matrix printer. Program PYRITE Program PYRITE uses an extended version of the 'Pyrite' model of Durney & Ramsay (1983) and Gray & Durney (1979) to calculate 2D incremental and progressive strains from measurements of rigid-fibre pressure-shadow fibres around pyrite grains. The calculations assume that local displacements and host body rotations correspond to those of the surrounding matrix. Cumulative incremental strain plots show the class and sense of flow (simple shear, pure shear, etc.), while progressive strains provide a measure of total strain and strain history of the rock. Agreement with independent measurements of total strain is usually good except for complex shadows in XZ section. Originally written for simple XY plane analysis, the program has been extended to deal with increments of any specified type in XY and XZ sections. Other capabilities include the treatment of oblong hosts, reflexive fibres and rotating shadows. The mainframe program can be used in a teaching environment and a version has been written (D.R. Gray) for use on a PC computer. Program HOLEXY Exact formulae for stresses and displacements around simple shaped openings and inclusions are amongst the most fundamental applications of the theory of elasticity to problems of rock mechanics and structural geology (Jaeger & Cook, 1979). Program HOLEXY uses these formulae to solve for 2D stresses in an infinite elastic or viscous medium surrounding a circular inclusion, which may be a cavity, a fluid under pressure or solid of stiffness different from that of the medium. Stresses are determined at xy grid points and plotted as lineprinter characters to represent intensity fringes of isochromatics, isopachics, isoclinics and potential brittle failure. Trajectories and tabulated principal values may also be obtained. Brittle failure is determined from a composite Griffith-Coulomb strength criterion. The loading conditions, material properties, plot field and plot scale are infinitely variable. This stress analysis program is primarily of interest for demonstrating and analysing features of heterogeneous stress fields in an easily visualised manner; e.g. trajectories, neutral points, maxima/minima and boundary values. It also serves as a basic model
31
for stress c o n c e n t r a t i o n , fracture development and stability around h a r d o b j e c t s , fluid inclusions, volcanic centres and u n d e r g r o u n d e x c a v a t i o n s , and as a reference for checking n u m e r i c a l and physical m o d e l s . Current development is aimed at m o d e l l i n g solution-transfer diffusion effects and upgrading the graphics. References D u r n e y , D . W . and R a m s a y , J . G . , 1973. Incremental strains m e a s u r e d by syntectonic crystal growths. In K . A . deJong and R . Scholten (eds). Gravity and T e c t o n i c s , W i l e y , New Y o r k , 67-96. G r a y , D . R . and D u r n e y , D . W . , 1979. Investigations on the m e c h a n i c a l significance of crenulation cleavage. T e c t o n o p h y s , 58, 35-79. J a e g e r , J . C . and C o o k , N . G . W . , 1979. Fundamentals of R o c k M e c h a n i c s , 3rd e d n . , Chapman & H a l l , L o n d o n . W h i t e , S.H. and W i l s o n , C.J.L., 1978. Microstructure of some quartz pressure fringes. N . J b . M i n e r . A b h . , 134, 33-51.
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32
THRUSTRAMP, A COMPUTER PROGRAM TO MODEL RAMP ANTICLINES
Richard H. Groshong, Jr. and Steven I. Usdansky Dept. of Geology, The University of Alabama, Tuscaloosa, AL 35487-1945 and Dept. of Geosciences, Murray State University, Murray, KY 42071
The latest THRUSTRAMP program includes three ramp-anticline models: an asymmetric fault-bend fold after Suppe (1983), a symmetric faultbend fold after Usdansky and Groshong (1983) and a fault-propagation fold after Suppe (1985). The program is written for IBM-compatible microcomputer systems, requires a minimum of 256 K RAM and produces CGA-compatible color graphics output in 4 colors at medium resolution or 1 color at high resolution. A model is created by inputting the scale, dip of the fault ramp, elevations of the upper and lower flats, displacement on the lower flat and the elevations on the monitor. The program also finds the displacement on the upper flat and the dip spectra of the top stratigraphic unit. A single model requires less than 2 minutes for input and display. The power of the program lies in its ability to accept additional faults at any location in the cross section and to mix styles on successive faults; for example, a fault-propagation fold could be cut by a symmetric fault-bend fold. The program is ideal for investigating the geometry of duplex structures. Widely spaced ramps in which the backlimb of the leading ramp anticline deforms only part of the forelimb of the trailing ramp anticline results in a "bumpy-roofed" duplex. Closely spaced ramps in which the backlimb of the leading ramp anticline deforms four or more trailing horses causes the trailing horses to fold into the "S" shape seen in many duplexes. They resemble listric thrusts if the upper detachment is the surface of the ground or is eroded away. A quantitative comparison between the symmetric and asymmetric faultbend folds reveals that the differences between the models are quite small for ramp dips of 20° or less. Significant differences are seen at greater ramp dips. The asymmetric anticline model is applicable only up to ramp dips of 30°, whereas the symmetric anticline model provides solutions for ramps dipping up to 80 degrees. At a 60° ramp dip the symmetric anticline model predicts an upper detachment with zero displacement, analogous to a fault-propagation fold. References Suppe, J., 1983. Geometry and kinematics of fault-bend folding: Am. Jour. Sci., V. 283, p.684-721. , 1985. Principles of Structural Geology: Prentice-Hall, Inc., 537 p. Usdansky, S.I., and Groshong, R.H., Jr., 1986, Geometric and kinematic model of thrust-ramp anticline formation: ms. in review.
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COMPUTER MODELLING OF MICROFABRIC DEVELOPMENT IN POLYCRYSTALLINE AGGREGATES
Steven Raiser^, Bruce Hobbs^ and A. Ord^ ^ Dept. Earth Sciences, Monash University, Clayton, Vic. 3168. ^ CSIRO Division of Geomechanics, P.O. Box 54, Mt. Waverley, Vic. 3149.
Deformation of polycrystalline aggregates has been simulated using an explicit, time marching continuum code; FLAG (Fast Lagrangian Analysis of Continua, Cundall, 1986). This is a two dimensional model with one slip system per grain, and allows deformation within the aggregate to be heterogeneous, even though the overall deformation is homogeneous. Deformation is accommodated on a subgrain scale by slip on slip planes or by deformation of the matrix of the subgrains as a perfectly plastic material. This ensures that grains remain in contact throughout the deformation history. Specimens have been deformed which show either an initial random distribution or an initial strong preferred orientation of slip planes. In the latter, slip planes are oriented predominantly parallel (PS), 45° (45S), or normal (NS) relative to the shortening direction for axial shortening and for biaxial coaxial strain histories, or relative to the extension direction for axial extension, or the shear plane for simple shearing. The model shows the development of grains elongate normal to the shortening direction for coaxial strain histories, and elongate approximately parallel to the finite extension direction for noncoaxial strain histories. A number of specimens develop shear zones; these reflect the incorporation of a few grains of specific favourable orientations, suitably positioned relative to each other, into the model. Slip planes rotate towards the normal to the shortening direction for coaxial strain histories, or towards the shear plane for noncoaxial histories. The developed fabric reflects both the initial distribution of slip planes and the strain history. In specimens deformed with a coaxial strain history the initial orientations of the slip planes determine to what extent they move towards the extension direction; those furthest from the extension direction move furthest. Specimens deformed by simple shearing show two types of behaviour; specimens with slip planes in the quadrant containing the finite extension direction (NS, 45S-sinistral shear, PS) show smooth reorientation trajectories towards the shear plane while those specimens with slip planes in the quadrant containing the finite shortening direction (random, 45S-dextral shear) show initially more chaotic behaviour, with many sudden jumps in orientation. The specimen deformed 45S-dextral passes through a stage showing a random orientation of slip planes. Weaker specimens (PS in compression, PS and 45S-dextral in simple shearing) deform preferentially by slip, whereas stronger specimens show an increasing component of the strain being accommodated by deformation of the matrix of the grains.
34
The features observed in this modelling are all very similar to those observed in experimentally deformed specimens (Raiser et al., 1987). This indicates that FLAG is a very powerful tool to help interpret fabric development in both experimentally and naturally deformed rocks. References:
Cundall, P. 1986. FLAG (Version 1.00) Fast Lagrangian Analysis of Gontinua. Itasca Gonsulting Group Inc., Minneapolis, Minnesota. Raiser, S., Hobbs, B.E. and Ord, A. 1987. Experimental deformation of a quartz mylonite: the effect of orientation. This Volume.
35
THE MARY KATHLEEN FOLD BELT, NORTHWEST QUEENSLAND: TIMING OF DEFORMATION
GEOMETRY AND
R.J. Holcombe^, P.J. Pearson^ andN.H.S. Oliver^ ^ University of Queensland, St. Lucia, Queensland, 4067. ^ Monash University, Clayton, Victoria, 3168.
The Mary Kathleen Fold Belt is a somewhat vaguely defined belt of upright, north-south trending folds within the eastern half of the Proterozoic Mt. Isa Inlier of northwestern Queensland. In recent years various workers have postulated early thrust-dominated deformation pre-dating the regional upright folding event, D^. The timing of the proposed thrusting is generally unknown (except for some recently published data for the western part of the inlier), but it is commonly thought to be about 1610-1620 ma, being a precursor to the D^ upright folding event at about 1550 ma. The central axis of the fold belt is the Wonga Belt, a long (>100 km), narrow (5-10 km) zone dominated by intensely deformed, metasediments and metavolcanics, intrusive metadolerite, and characteristic gneissic granitoids. It is in the core of a large D^ anticlinorium which is completely defined by the map pattern of the overlying Ballara Quartzite, the basal unit of the somewhat less deformed Mary Kathleen Group. The structural fabric within the Wonga Belt is superficially very simple, but is a composite of two generations of structures, D^, and a pervasive earlier structure, D^. The fabric is dominated by a very regular, subvertical, gneissic foliation commonly containing two lineations. However this is a composite ^^ which the shape anisotropy defining the foliation is dominantly a D^ feature that has undergone rigid body rotation into the D^ orientation. D^ strain has enhanced the fabric to varying degrees but its principal contribution is a constantly oriented, subvertical extension lineation. A second lineation is invariably present and has both intersection and extension characteristics. It is a combination of three lineations with the same orientation: parallel D^ intersection and extension lineations, enhanced by the fortuitous parallelism of the D^ intersection lineation. Thus D^/D^ fold superposition invariably produces Ramsay type 3 refolded fold patterns. Prior to D^, the original S^ foliation was subhorizontal and stratigraphy parallel. The S^ foliation wraps around major D^ folds and the Wonga Belt zone itself reappears in the core of an antiform about 5 km to the east. The same structural elements occur in the Mary Kathleen Group rocks at some distance from the central Wonga Belt but D^ fabrics, when recognized, are considerably weaker and much more variable in orientation. The dominant folds are D , but local areas of superposed folds encompass virtually all of the Ramsay refolded fold patterns, reflecting the variability of D^ fold orientations. The transition to these less deformed rocks from the Wonga Belt structures, defined in terms of gneissic granitoids, fabric orientation and intensity of deformation, is relatively abrupt.
On
36
the western limb of the Wonga Belt anticlinorium the boundary lies approximately 500 m below the relatively undeformed Ballara Quartzite (i.e. within the Wonga Granite). On the eastern limb of this anticlinorium the boundary is generally within mylonitic remnants of the Ballara Quartzite, although Wonga Belt fabric orientations commonly persist for several hundred metres above this. There is considerable evidence that most of the intrusive rocks within the area, both mafic and felsic, are synkinematic with the formation of the D^ fabric. All of the currently available U-Pb (zircon) dates for the granitoids in this area are within the range 1760-1670 ma, thus constraining the timing of the D^ deformation to the same range. That is, there is considerable evidence that the earliest foliationforming deformation in the vicinity of the Wonga Belt predates any of the early thrust-related deformations suggested by previous workers. We interpret the D^ fabric in the Wonga Belt as having formed within a subhorizontal shear zone with a north-south movement direction. The shear is interpreted to be within a detachment surface that is broadly coincident with the Ballara Quartzite over most of the area, but locally is transgressive. In an accompanying paper, Pearson et al. present the evidence for shear, and the sense of shear, and explore possible models for the tectonic evolution of the D^ fabric.
37
THE MARY KATHLEEN FOLD BELT, NORTHWEST QUEENSLAND: CRUSTAL EXTENSION?
D1 - A PRODUCT OF
P.J. Pearson^, R.J. Holcombe^ andN.H.S. Oliver^ ^ University of Queensland, St. Lucia, Qld., 406; ^ Monash University, Clayton, Victoria, 3168.
The Mary Kathleen Fold Belt is an area of upright, north-south trending folds within the eastern half of the Proterozoic Mt.Isa Inlier of northwestern Queensland. The core of the fold belt is the Wonga Belt, a long (>100 km), narrow (5-10 km) zone dominated by intensely deformed, metasediment and metavolcanics, intrusive metadolerite, and characteristic gneissic granitoids. As outlined in an accompanying paper (Holcombe et al., this volume) the zone of pervasive, early D^ fabrics defining the Wonga Belt occupies a thick, broadly stratigraphy-parallel, zone that has been folded into a tight anticlinorium during D^. D^ fabric development was characterized in most lithologies by a strong, to mylonitic, subhorizontal foliation bearing a broadly north-south oriented stretching lineation. The boundary between these Wonga Belt fabrics and the overlying, less deformed rocks is relatively abrupt, broadly coinciding with the Ballara Quartzite, but locally transgressing this boundary. There is some persistence of Wonga Belt fabric orientations for several hundred metres above the strain discontinuity. The pre-D^ geometry of the fabrics, in conjunction with kinematic indicators, suggests that the zone of mylonitic rocks was developed during a bulk non-coaxial strain history. Shear sense indicators suggest an upper plate to the north sense of shear. D^ fabrics are defined by amphibolite facies mineralogy in pelite and calcsilicate rocks. The syn-D^ regional metamorphic grade is upper amphibolite facies but there is some evidence that metamorphic conditions within the Wonga Belt during D^ were also upper amphibolite facies. The observed plastic deformation of feldspar, and the development of high temperature quartz fabrics during this event supports this assertion. The development of the early fabric was accompanied by synkinematic intrusion of large volumes of I- and A-type granitoid, and tholeiitic gabbro/dolerite as sills and dykes. The U/Pb (zircon) dates in various granitoid phases cover a large span of time (1759±22ma to 1671±8ma) yet there is considerable evidence that the D, fabric was being developed throughout this period. We interpret the Wonga Belt to be a mid-crustal, shallowly dipping mylonite zone around a detachment surface, with an upper plate to the north sense of shear. Intense ductile deformation in this zone was accompanied by synkinematic intrusion of biomodal magmas into the shear zone. The two most commonly accepted environments for deformation of this style are compressional regimes (thrust and fold nappes) or low angle extensional regimes.
38
We propose that the subhorizontal shear zone geometry, the absence of older-on-younger structures, the extended period of deformation, and the intimate association of melts with this early deformation argue strongly for the Wonga Belt having an early extensional history. The model envisaged is that of Wernicke (1982), with lithospheric extension via a gently dipping shear zone, or series of shear zones. The Wonga Belt is envisaged as the mid-crustal manifestation of such a detachment zone that has been strongly folded and subsequently exhumed by later crustal shortening and uplife. The relatively undeformed granitoids intruded into the Mary Kathleen Group rocks are considered to be a result of extension and dilation in the "semi-brittle" upper plate. Another manifestation of extension in this plate is the ubiquitous set of east-west trending folded extension veins in the calcsilicate rocks of the Corella Formation. It has been previously proposed (and accepted by us) that the Middle Proterozoic history of the Mt.Isa orogen began with the opening of north-south trending rifts in the period 1880 ma to 1760 ma, following an earlier cycle of rifting and orogenesis. The north-south transport direction suggested by us for the foliation-forming extensional event between 1760 ma and 1670 ma is at 90° to both the earlier rifting direction and the later D^ crustal shortening direction. Clearly a tectonic model involving simple opening and closing of an ensialic rift is not strictly applicable to this situation, although variations on this theme may be valid.
39
THE SIGNIFICANCE OF TRANSPRESSIGNAL STRIKE-SLIP FAULTING IN THE LAKE JULIUS AREA, MOUNT ISA INLIER A contribution to the BMR Mount Isa Regional Tectonic History Report
G.S. Lister^, A.D. Thomas and J. Dunn. Bureau of Mineral Resources, Geology and Geophysics, Canberra, Australia.
Wrench faulting is an important element of active tectonic domains near the margin of modern plate boundaries, for example the Alpine Fault in New Zealand, or the San Andreas Fault in California. Field observations in these areas are limited by the average depth of erosion. Consequently we know comparatively little about the associated deep level processes. However, in the Mount Isa area is a deeply eroded terrain which allows an opportunity to examine the effect of processes that took place at 10-15 km depth during transpressional and transtensional wrench faulting. The principal lesson learned is that movements on (brittle) strikeslip faults were associated with significant ductile deformation over large areas. Transpression led to re-existing D^ folds being substantially tightened, distorted, and disrupted. One syncline, cored by Mount Isa shale, narrows from 1 km to less than 100 m, brittley truncating the adjacent anticline in the process. D^ folds with vertical extension directions were also formed. The most remarkable features of this transpressional zone are thrusts associated with "pop-out" structures. One block rotated 70-80° counter-clockwise, as it was thrust through the rocks to the west, defining a major "pop-out" bounded by thrusts, and terminated to the north by a vertical oblique-slip fault, against which rotation of the block of rock to the south took place. Because the original fault ran NS parallel to a D^ axial zone before again cutting NW across structure, the result of these movements is to produce an EW trending syncline bounded to the north by a fault. This lateral "pop-out" is characterized by strata which terminate at the northern oblique-slip fault, only to reappear with the same trend south of the thrust block. This rotational thrusting produces structures which superficially have the characteristics of D^ thrust-ramp-synclines, but are in fact strongly rotated D^ folds. Not only has it been possible to demonstrate that these D^ thrusts and faults cut older D^ thrusts, but it has been shown that these faults are blind i.e. relative displacements along the thrusts gradually decrease towards their terminations. Relative displacements of only 5-8 km have been involved. Similar thrust geometries have been recognized in other transpressional terrains. The Dg event in the ductile transpressional zones in the Mount Isa inlier involved east-west shortening and vertical extension. Shallow dipping fractures dilated, producing vertical fibre lineations. Such dilational traps may serve as focussing agents for mineralizing solutions, and it is interesting therefore to note that minor Au prospects in the Lake Julius area occur in or near D^ dilated D^ thrusts. One such prospect ocurs south of the thrusted Argylla Volcanics, close to the intersection of the D^ thrust with the
40
transpressional zone at the D^ Quilalar Fault. The other prospects were of more significance and were the site of numerous (undisturbed) old diggings. Some gold was panned from this locality. It occurs near vein fillings where Quilalar Formation has been thrust over the Mount Isa Group. We suggest that the Seven Dwarfs thrust has dilated during D^ and acted as a dilational trap focussing the flow of mineralizing solutions. In this case, some attention should be paid to other localities in the inlier where we suspect transpressional strike-slip faulting has taken place. Regionally, a common feature seems to be D^ fault trends which switch from NE or NW, to NS. These NS segments should also be transpressional. The Mount Isa Fault itself may bound one of the NS transpressional segments of the D^ strike-slip fault. Transpression may also be important east of Lake Moondarra, where a NW-trending strike-slip fault terminates, forming another "pop-out" structure. The other zone of predicted transpression is at the southern termination of the Fountain Range Fault. Some fault movements have taken place as recently as the Mesozoic, and the extensions of some of these old strike-slip faults must have affected the Palaeozoic and Mesozoic evolution of the adjacent Eromanga Basin, for example controlling Carboniferous transpressional or transtensional strike-slip faulting, as well as later reactivation of specific faults. It is important to understand the relation of these structures, since important hydrocarbon traps can be so defined.
* Now at the Department of Earth Sciences, Monash University, Clayton, Vic. 3168.
41
THE STRUCTURAL EVOLUTION OF THE DEIGHTON THRUST NAPPE COMPLEX, MOUNT ISA, QUEENSLAND.
D,E. Bettess Earth Sciences, Monash University, Clayton, Victoria, 3168.
A partial duplex, in the central Mount Isa Inlier has defined a westerly movement for D^. This is in contrast to the southerly movement of Bell's (1983) duplex in the western Mount Isa Inlier. The duplex is developed within Proterozoic upper greenschist to lower amphibolite felsic volcanics, quartzites and calcsilicates of the Mary Kathleen Group, within the Mount Isa Inlier. D^, a major deformational phase, resulted in the formation of the Deighton thrust nappe complex. This complex is a partial duplex consisting of a series of imbricate thrusts, to the west and a roof thrust to the east (Loosveld and Schreurs, 1986, also this study). D^ was a phase of east-west shortening, producing north-south trending upright folds. This deformation steepened the thrust sheets and folded the roof thrust, producing the Deighton syncline. A late deformation phase consisted of northeast-southwest and northwestsoutheast conjugate sets of strike-slip faulting. The geometry of the imbricate thrust sheets and the orientation of the stretching lineation indicate movement directions during D^. The thrust sheets consist of steeply east dipping thrust fronts, with lateral or oblique ramps which dip towards the south or southwest, respectively. Stretching lineations, defined by micaceous minerals and oblate quartz grains, are easterly plunging. The movement direction indicated by the thrust geometry and lineations is towards the west. Rotation of the stretching lineation during D^, as suggested by Bell (1986), does not appear to have affected the orientation of the lineation, as it can be traced in a consistent orientation across D^ folds. Balanced cross-sections of the imbricate thrust stack indicate that the shortening during D^ was approximately 36%. Thickening during D^ and D^ is restricted to the upper regions of the crust, probably above the decollement zone. The decollement zone is estimated to be at the base of the Argylla Formation, which is at a depth of approximately 3 km. Shortening of this zone during D^ would increase the thickness from 3 km to 4 km. This thickens the average crustal thickness of 35 km (Loosveld and Schreurs, 1986) to 39 km. Shortening during D^ is approximately 30-40%. Hence, the cumulative shortening during D^ and D is approximately 70%. Therefore the total thickness of the crust after D^ would be approximately 40 km. This is in agreement with the current crustal thickness, estimated from geophysical data, of 40-50 km (Lister et al., 1985). Displacement along the imbricate thrusts is approximately 2-2H km.
42
References Bell, T.H. 1983. Thrusting and duplex formation at Mt. Isa, Qld. Aust. Nature, v. 304; 493-497. Bell, T.H. 1986. Foliation development and refraction in metamorphic rocks: reactivation of earlier foliations and decrenulation due to shifting patterns of deformation partitioning. Submitted J. Met. Petrol. Lister, G.S., Etheridge, M.A. and Stewart, A.J. 1985. Structure and tectonics of the Precambrian Mount Isa Inlier, northwest Queensland. Abstract, 14th BMR Symposium, BMR Rec. 1985/33, Loosveld, R. and Schreurs, G. 1986. Discovery of thrust klippen. Northeast of Mary Kathleen, Mt. Isa Inlier, Australia, Submitted to Geology.
43
TEAR FAULT CONTROLLED BASIN DEVELOPMENT ABOVE AN ACTIVELY OVER-RIDING BASEMENT COMPLEX, NORTHEASTERN AUSTRALIA R.L. Hammond^, S.C. Lang^, I.W. Withnall^ and L.P. Black^ ^ James Cook University, Townsville, Queensland, 4811. ^ Geological Survey of Queensland, 61 Mary Street, Brisbane, Queensland, 4000. ^ Research School of Earth Scineces, A.N.U., P.O. Box 4, Canberra City, ACT. 2601.
The Graveyard Creek Subprovince in northeastern Queensland originated as a more abruptly tapered, tear fault bounded portion of an overthrusting basement complex with an indented leading edge (Fig. 2A). This geometry resulted in a localised depression when basement was thrust over a more planar footwall (Fig. 2B). Syn-tectonic sedimentation continued in this localised basin preserving a prethrusting marine sequence disconformably beneath its basin fill. In the Camel Creek Subprovince (CCS) immediately to the east (Fig. 1) this marine cover sequence was initially over-ridden and ultimately imbricated and tightly folded by eastward advance of the basement, thus juxtaposing weakly deformed rocks of the Graveyard Creek Subprovince (GCS) against strongly deformed rocks of the CCS. The leading edge of basement to the GCS is exposed as the Gray Creek Complex (Fig. 1) and the Ordovician rocks enclosing it, whereas only basement rock is exposed north of the Graveyard Creek Subprovince's bounding tear fault. None of the original marine cover is preserved north of this fault (the Teddy Mount Fault). The southern margin of the over-thrust belt (i.e. Camel Creek Subprovince) is a sinistral shear zone which passes west into a more abrupt fault on the southern margin of the GCS. This fault, the Clarke River Fault Zone, is a major tear fault along which the overthrust belt may be displaced to the east (offshore). Penetrative deformation in basement rocks north of the GCS has been dated by Rb-Sr whole rock methods at 408 ± 6 Ma (asterix on Fig. 1 marks site) which is broadly consistent with the widespread resetting of basement rocks to Rb-Sr and K-Ar ages of approximately 400 Ma. This age represents the initiation of crustal shortening and is coeval with the influx of compositionally immature sediments into the GCS in the early Devonian. Bounding tear faults on the northern and southern margins of the GCS isolated it from penetrative deformation during the thrusting event. Sedimentology of the GCS also indicates uplift along its northern margin throughout much of the depositional history and much more subdued topography otherwise surrounding the GCS. The culmination of tectonism is indicated by an influx of coarse detritus into the GCS reflecting rapid uplift along its eastern margin. This uplift was a result of the imbrication of the marine succession in the Camel Creek Subprovince.
44
•r
SD?
/ jy
/
"w
/
/ /,
A .V .1/
Figure 1
Figure 2
45
A COMPLEX FOLD NAPPE WITHIN THE SOLDIERS CAP GROUP, MOUNT ISA INLIER, QUEENSLAND, AUSTRALIA
Ramon J.H. Loosveld Geology Department, Australian National University, Canberra, ACT, 2601.
The structure of the Proterozoic Mount Isa Inlier is mainly characterized by upright, N-S trending folds which is widely attributed to the second regional deformation phase. Agreement on the pre-D2 history, however, is still far-off; both isolated thin-skinned thrust complexes and isolated extensional detachment-like geometries have been described. Detailed structural mapping in the Soldiers Cap Group (S.C.G.) belt has led to the recognition of new D1 structures. The S.C.G. belt, the easternmost part of the Mount Isa Inlier, is characterized by tight to isoclinal N-S trending folds (e.g. the Snake Creek Anticline; the Weatherly Creek Syncline and the Middle Creek Anticline; Fig. 1), except in the central part, in the CLONCURRY sheet area, where tight to isoclinal E-W trending folds (the Toole Creek Syncline) are developed in the highest member of the S.C.G., the Toole Creek Volcanics. Glikson & Derrice [1] and Wilson et al. [2] regarded the Toole Creek Syncline as a limb of a large NE trending fold, with the Weatherly Creek Syncline being the complementary limb (Fig. 1), but do not agree on the deformation sequence. Much of the confusion springs from the emphasis given to orientation in correlating structures. New data from rather straightforward structural work argue against the contemporaneity of the Weatherly Creek and Toole Creek Synclines. Firstly, since isograds are roughly bedding-parallel and peakmetamorphic assemblages in the higher-grade rocks (sill.±Kfsp; andal., staur.,garn.) define the oldest recognizable tectonic fabric elements, SI and the extension lineation LI, this metamorphic event is of Dlage. As the Sl/Ll fabric is axial plane to the Snake Creek Anticline, and is folded around the Weatherly Creek Syncline, the former is a D1 fold while the latter is younger. Secondly, the "high-grade" Sl/Ll fabric, which is axial plane to the NNW trending Snake Creek Anticline, can be traced upwards through the stratigraphy into a strong slaty cleavage, axial planar to the E-W trending Toole Creek Syncline. This makes the Toole Creek Syncline a D1 fold too. D1 then is characterized by completely different structures. The Snake Creek Anticline, originally a NNW closing recumbent fold overriding a decollement, and rotated by D2, is asymmetrical, reclined and slightly overturned. This simple-shear dominated domain passes into the tight to isoclinal upright folds of the Toole Creek Syncline system, which is barely affected by D2 and the picture of a fold nappe with a coaxially shortened front emerges (Fig.2). The movement direction of the nappe was probably towards the NNW.
46
References [1]
[2]
G l i k s o n , A . Y . and D e r r i c k , G . M . 1970. The Proterozoic metamorphic rocks of the Cloncurry 1:100 000 Sheet area (Soldiers Cap B e l t ) , northwestern Q l d . BMR R e c . 1970/24. W i l s o n , I.H., G r i m e s , K . and R y b u r n , R . (in p r e p ) . 1:100 000 geological map commentary, Cloncurry, Q l d . B M R .
Fig. 1 1A0°30'
1A1°00'
Fig. 2
47
INTRACRATONIC BASIN FORMATION IN CENTRAL AUSTRALIA
R.D. Shaw Research School of Earth Sciences, Australian National University, Bureau of Mineral Resources, Canberra, ACT, 2600
An examination of the Late Proterozoic to mid-Palaeozoic central Australian basins casts doubt on the view that intracratonic basins are formed by a single mechanism. A stratigraphic and subsidence history established in the northern Amadeus Basin is used to assess the development of this and the neighbouring basins in central Australia. The sequence is divisible into lithologically distinct units separated by unconformities, each with a characteristic subsidence history. The sub-basin shape also differs for each depositional interval. This periodic deposition suggests that the Amadeus Basin is not a simple basin formed by a single mechanism, but is a complex basin made up of a succession of sub-basins formed by discrete tectonic driving forces. Late-stage thrusting has changed the final basin shape and made sub-basin reconstruction difficult. The subsidence mechanism that initiated the Amadeus Basin is unclear. Mafic dyke swarms preceding subsidence imply limited extension, but the lack of rapid facies and thickness variations suggest minimal upper crustal extension. The smooth regional non-conformity at the base of the sequence is consistent with a simple thermal model of the type applied to the Michigan Basin. Subsequent sub-basin shapes in the Amadeus Basin differ from those in the neighbouring Ngalia and Georgina Basins, even though the basins show parallel stratigraphic development. The Amadeus and Ngalia Basins form broad, asymmetrical east-west basins with lateral continuity of facies suggestive of compressive tilted-fault block basins (e.g. Wind River Basin, USA). The sub-basins of the Georgina Basin are narrow and trend northwest consistent with stike-slip faulting within the same northerly stress field that formed the other basins. Large-scale extension at a continental margin to the east in the late Early to early Middle Cambrian led to subsidence well inboard of the margin and extrusion of plateau basalts at the periphery of the region near the craton margin. A widespread transgression over much of central Australia corresponds to a sharp increase in the subsidence rate which then progressively decreases. Such a subsidence history is suggestive of a thermally contracting lithosphere, but the region lacks evidence of upper crustal extension such as normal faulting or rapid lateral changes in facies. This enigma is possibly explained if the basins formed in the plate overlying an extensional detachment zone initiating at the continental margin. Such a detachment zone would need to be at a sufficiently shallow depth to cause crustal thinning giving rise to slight and extensive uplift followed by thermally induced subsidence. Subsidence driven by a cooling and contracting lithosphere possibly continued from late Cambrian to mid-Ordovician, but a short-lived hiatus in the late Cambrian suggests compression corresponding to folding at the continental margin. Oblique-slip faulting may have
48
accompanied this compressive phase in the western Georgina Basin. An increase insubsidence in the mid-Ordovician suggests renewed crustal thinning. In the late Ordovician and continuing into the Silurian, a period of erosion was followed by slow sedimentation as the lithosphere continued to cool and thicken. Regional tilting and possible faulting may have partly controlled sedimentation in this period. Major reactivation of faults culminating at about 350 Ma north of the Amadeus Basin, resulted in a marginal fault-block basin filled with siltstone, sandstone and finally a thick polymictic molasse-like conglomerate. An oblique compressional fault basin formed in the western Georgina Basin. While the exact mechanism controlling each sub-basins development is not always clear it is apparent that changes in mechanism have occurred. Because stresses propagate over large distances, the central Australian intracratonic basins will have been affected by changing stresses and displacements at the Australian plate margin due to repeated plate rearrangement over a 600 Ma period. Such changes may be dramatic if shallow extensional detachment zones form well inboard of the plate margin. It is likely that the central Australian basins are typical of many intracratonic basins and that a closer examination of them will reveal a succession of sub-basins formed by more than one mechanism.
49
RELATIONSHIPS BETWEEN DEFORMATION AND BASIN EVOLUTION IN THE AMADEUS BASIN, CENTRAL AUSTRALIA
R.J. Korsch and J.F. Lindsay Division of Continental Geology, Bureau of Mineral Resources, GPO Box 378, Canberra, ACT, 2601, Australia.
Sedimentary basins contain a record of deformation in the upper part of the crust which has traditionally been interpreted through the recognition of unconformities. However the sedimentary succession itself contains a very sensitive record of deformational events and more information can be obtained than has been recognised previously. Raw subsidence curves for total sediment thicknesses, and backstripped curves which remove the effects of eustatic sea level, palaeobathymetry and compaction to determine the amount of tectonic subsidence, are used here to indicate periods in the history of the Amadeus Basin when the basin has been subjected to extensional or compressional events. The basin developed during an initial period of extension in the Late Proterozoic (Stage 1), was subjected to another period of extension at about Proterozoic-Cambrian boundary time (Stage 2), and was shortened during the Late Devonian-Early Carboniferous (Stage 3) (Lindsay and Korsch, in prep.). Stage 1 extension was associated with rift basin sediments and volcanics that previously were regarded as basement to the Amadeus succession. These rocks crop out in the NW and SW corners of the basin. In the metamorphic Arunta Complex, on the northern margin of the basin, dolerite dykes with a predominantly northerly trend, form the Stuart Dyke Swarm and are considered to be emplaced during this event. Extension in a N-S direction associated with Stage 2 occurred only in the northern part of the basin; at the same time, a major compressional event, the Petermann Ranges Orogeny, was occurring along the southern margin. The sediment fill at this time thickens rapidly towards the northern margin, implying a half-graben structure and the presence of a major bounding fault. Extensional veins in Stage 1 rocks may have developed at this stage. In the SW, the Petermann Ranges Orogeny caused detachment of the sediments above the salt horizon in the Bitter Springs Formation, northward transportation, and tight folding of the sediment pile. The basin succession below the decollement plus the basement were involved in major basement-cored nappes with a northward transportation direction (Forman, 1966). During Stage 3, a foreland basin associated with southward directed overthrust sheets formed in the northern part of the basin. Horizontal shortening was in the order of 50-100 km (Teyssier, 1985) and because of this deformation the present basin margins are structural rather than depositional. The structures that developed during this stage dominate the structural pattern of the basin, and can be divided into several structural styles including 1. thinskinned nappes and thrust plates in the NE, 2. thrust-related ramp anticlines above a single decollement in the central part, and 3. a complex fold pattern reminiscent of interference patterns in the west.
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Thus, throughout the history of the Amadeus Basin, sedimentation and basin evolution have been intimately related with deformational events. References: Forman, D.J. 1966. The geology of the south-western margin of the Amadeus Basin, Central Australia. Bureau of Mineral Resources Report 87, 54p. Lindsay, J.F. and Korsch, R.J. in prep. Evolution of the Amadeus Basin, central Australia. Teyssier, C. 1985. A crustal thrust system in an intracratonic tectonic environment. Journal of Structural Geology, v. 7, p. 689-700.
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NAPPES IN THE NORTHEASTERN AMADEUS BASIN, NORTHERN TERRITORY
A.J. Stewart^ and R.Q. Oaks, Jr.^ ^ Bureau of Mineral Resources, G.P.O. Box 378, Canberra, ACT., 2601 ^ Department of Geology, Utah State University, Logan, Utah, 84322, USA.
Allochthonous sheets or nappes with southward translations of tens of kilometres in the northeastern Amadeus Basin were recognized by BMR geologists in 1964, during 1:250 000 regional mapping. Early interpretations of the lateral extent of the sheets appeared in BMR Bulletin 100 (1970) and on the 1:5 000 000 Tectonic Map of Australia and New Guinea published by the Geological Society of Australia in 1971. Detailed mapping and sedimentological studies of the area by Oaks and graduate students at Utah State University has led to a new interpretation of the extent and number of sheets, and of their structural history. The northeastern Amadeus Basin contains Late Proterozoic to Ordovician shallow-marine strata, mainly sandstone, shale, and carbonate, and Ordovician to Devonian aeolian to fluvial sandstone and conglomerate, totalling about 7000 m thick. Evaporites present at two levels, one low in the Proterozoic, the other low in the Cambrian, allowed detachment and southward sliding of the allochthonous sheets. The detachment surface began in the lower evaporite horizon, cut up section, and continued along the upper evaporite horizon. Figure 1 shows the various sheets currently identified, and Figure 2 depicts four cross - sections, at natural scale. Translation ranges from 30 km in the west to 60 km in the east. The present interpretation regards all the sheets as parts of a single large parent sheet, separated into klippen by later folding and erosion. The klippen are bounded by north-dipping thrust faults at the front, by south-dipping low-angle normal faults (lags) at the back, and by lateral ramps or strike-slip faults at the sides. Because of lack of exposure, the southward extent of the parent sheet is unknown; it is shown as continuing south beyond the area of Figure 1. The two klippen forming the N'Dhala Sheet are separated by an antiformal stack of imbricate thrust slices (a duplex) which overlies a concealed autochthonous antiform. South-dipping thrust faults at the northern margin of both the Camel Flat Sheet and the autochthon south of the N'Dhala Sheet are interpreted as later back-thrusts that cut the parent sheet after its arrival from the north. Emplacement is thought to have been caused not by thrusting, but by large-scale gravity sliding off the Arunta Inlier in late Devonian-Carboniferous time.
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53
THE HARTS RANGE MOBILE BELT: A SHORT-LIVED PROTEROZOIC INTRAPLATE OROGEN FROM THE EASTERN ARUNTA INLIER OF CENTRAL AUSTRALIA.
P. Ding and P.R. James Department of Geology and Geophysics, University of Adelaide, S.A., 5000
We propose that in the Harts Range area of the Arunta Inlier of central Australia, a basement and cover relationship previously described by us (Ding and James, 1985), represents a thick complex detachment zone between the Early-Middle Proterozoic Strangways Orogenic Belt (SOB) and the younger Harts Range Orogenic Belt (HROB). Rocks of the SOB were deposited and metamorphosed to a maximum grade of granulite facies at about 1800 Ma during an early phase of crustal extension. They were subsequently intruded by significant acidic magmas at 1767 Ma and deformed under a compressional regime to form a distinctively NE-SW trending orogenic belt. We have further recognised and now document the evidence for an ancient E-W trending shallow north dipping detachment zone of intense and repeated deformation; the Harts Range Detachment Zone (HRDZ), which formed initially during a significant period of N-S crustal extension as a major low angle normal shear/detachment zone within, and crossing at a high angle to, the basement rocks of the SOB. Extensional movement on this zone initiated a new orogenic cycle. A supracrustal cover sequence, the Harts Range Cover (HRC), formed in the major basin which developed on the subsiding upper plate of the HRDZ. Continued crustal extension juxtuposed the cover sequence, of amphibolite facies and granulite facies basement of the HRDZ before a fundamental change in movement sense led to crustal contraction and a collision related, thrusting phase. The HRC and HRDZ were then internally deformed over a relatively very short time span (15-20 Ma) during the Harts Range Orogeny (HRO). The HRO shows a peculiar orogenic style of crustal contraction which was largely taken up by reverse sense discrete thrusting and shearing associated with the HRDZ. Displacement on the HRDZ was a stick-slip process with repeated punctuation of thrust-slip movement by pinning at points located progressively to the south. Each period of cessation of thrust-slip movement was followed by folding of discrete thrusts, of captured basement lobes within the thick HRDZ and of the overlying supracrustal sequence. The HRC and HRDZ define a significant intraplate mobile belt, the Harts Range Mobile Belt. Reference: Ding, P. and James, P.R. 1985. Structural evolution of the Harts Range area and its implication for the development of the Arunta Block, central Australia. Precambrian Res. 27, 251-276.
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FIG. 1
GEOLOGY OF THE HARTS RANGE AREA, N.T.
I Recent alluvium Irindina supracrustal assemblage I Reworked basement I (OonagalabI Tongue) Unreworked basement I Florence/Leaky Shear Zone Bruna gneiss
Central Australian Craton Harts Range Mobile Belt Early Middle Proterozoic S t r a n g w a y s Orogenic Belt (Harts Range Basement) unworked Entia Dome and undifferentiated basement
reworked )onagalabi ongue
Middle Proterozoic Harts Range Orogenic Belt Harts Range Detachment Shear Zone
Harts Range Supracrustal cover
Muller movement ^Bruna 'movement
Maud 'movement
Oonagalab movement
Florence movement
55
AN EARLY-MIDDLE PROTEROZOIC SHORT-LIVED EXTENSION-COLLISSION INTRAPLATE OROGENY: THE STRANGWAYS OROGENY OF THE EASTERN ARUNTA INLIER, CENTRAL AUSTRALIA.
P.R. James and P. Ding Department of Geology and Geophysics, University of Adelaide, S.A., 5000
In the Harts Range area of the eastern Arunta Inlier of central Australia, an infrastructural basement and a supracrustal cover were previously described by us (Ding and James, 1985) and recently proposed to represent two related orogenic belts, the Strangways Orogenic Belt (SOB), and the Harts Range Orogenic Belt (HROB) (James and Ding, in prep.). In this paper, details of the SOB are presented and a model is developed to demonstrate the evolution of a short-lived intraplate extension-collision orogeny and its consequent high grade metamorphic complex. The SOB is characterized by predominent orthogneiss intercalated with subordinate metamorphic supracrustals which show peak metamorphism, earlier than the emplacement of the orthogneisses and at higher than or equivalent metamorphic grades to them. The peak metamorphism of the SOB decreases from granulite facies in the west to amphibolite facies in the east. Retrograde metamorphism from granulite facies to amphibolite facies is found in certain areas where the mafic granulite is intercalated with granitic gneiss of amphibolite facies. We consider that the early evolution of the SOB represents its formation in an extensional tectonic environment associated with a major lithospheric detachment, and that the metamorphic gradation and early intense LS fabric development is associated with the slow continuous subsidence of the supracrustal sequence on the upper plate of the detachment and its subsequent metamorphism. The massive orthogneiss injection appears to have occurred near to peak metamorphic conditions and also close to the change from an extensional to a contractional movement on the detachment. The changing stress fields may have aided the mechanism of plutonic injection. The metamorphosed supracrustals and the acidic orthogneiss were subsequently involved into all the major fold events which include four fold generations in the western part of SOB and three fold generations in the east. These recumbent and mainly isoclinal events are related to the continuous contractions of the belt. The lineations and fold axes trend predominantly in a NNE-SSW direction, thus indicating a NNE-SSW trending orogenic belt. Asymmetric profiles of the major folds imply that the ESE dipping low angle detachment may be exposed further to the west and that the SOB was transported from WSE to WNW during the collisional deformation. References: Ding, P. and James, P.R. 1985. Structural evolution of the Harts Range area and its implication for the development of the Arunta Block, central Australia. Precambrian Res. 27, 251-276. James, P.R. and Ding, P. (in prep.). "Caterpillar Tectonics" in the Harts Range area: a kinship between two sequential Proterozoic orogenic belts within the eastern Arunta Inlier of central Australia.
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THE IWUPATAKA COMPLEX: A KEY AREA OF THE PROTEROZOIC AND PALEOZOIC THRUST SYSTEM IN THE SOUTH ARUNTA BLOCK AND ITS KINEMATIC SIGNIFICANCE
C. Amri^, B.E. Hobbs^ and S. Raiser^ ^ Department of Earth Sciences, Monash University, Clayton, Vic. 3168 ^ CSIRO Division of Geomechanics, Kinnoull Grove, Syndal, Vic. 3150
The Iwupataka Complex of the South Arunta Block consists of a 3 km thick sheet of flat lying Proterozoic gneisses, metasediments and meta volcanics. Horizontal thrusting during Proterozoic deformation produced a strong piano-linear fabric. Detailed field observations and the map pattern define an imbricate thrust structure. Most of the area comprises amphibolite grade rocks separated by retrograde shear zones. The Rungutjirba Gneiss Complex is separated from the Chewings Range and Simpsons Gap metasediments by the Jay Creek deformed zone. This zone of intense deformation is characterized by a strong mylonitic foliation and an elongation mineral lineation showing retrograde assemblages. Repetition of the stratigraphic sequences occurred during D thrusting and is followed by the strongly penetrative D^ deformation. D^ is responsible for most of the bulk strain observed in the rocks. was progressive and highly heterogeneous and is associated with thrusting. The finite strain averages 100-150% stretch in plane strain conditions. Asymmetric feldspar porphyroclasts and S/C shear plane relations define a NE sense of shear during Microstructural observations and quartz c-axis fabric asymmetry support this interpretation. Subsequently, the complex underwent an E-W striking period of upright F^ folding, resulting in elongate basin like klippes, subparallel to the elongation lineation. N-S trending dolerite dykes cut across the previous structure and marked the end of igneous activity in the inlier. They are extensively exposed as linear outcrops in the South Arunta Block and are one of the more useful tools for differentiating between the Proterozoic and Paleozoic orogenies. The Devonian-Carboniferous Alice Springs Orogeny (ASO) produced the last penetrative deformation in the Block. In Iwupataka Complex this orogeny is expressed as narrow steeply dipping brittle zones. Mylonites related to the ASO show more plastic deformation north from the Charles River Fault. This northward increase in plastic deformation is related to the southward sense of movement which brought the rocks from depth towards the surface. It is responsible for the tilting and overturning of the Proterozoic structures. The problems of tectonic evolution in the inlier are clarified by realizing that there is an opposed sense of movement during the Proterozoic and Devonian-Carboniferous deformations, and that both of these orogenies are expressed as crustal scale thrust systems. An important key to understanding the structure is the recognition of specific structures in the Iwupataka Complex than can be correlated across the Block. Our hypothesis of early northward (Proterozoic), and later southward (Paleozoic) overthrusting, implies shortening of
57
the crust during the Proterozoic and Paleozoic orogenies, and is consistent with the idea that the Arunta Inlier is the result of an intracratonic crustal dislocation process.
58
REGIONAL ROTATIONAL SHEAR ACCOMPANYING HIGH GRADE METAMORPHISM IN THE EAST STRANGWAYS RANGE, ARUNTA BLOCK
Ben D. Goscombe Department of Geology, University of Melbourne, Parkville, Vic. 3052.
Structural and metamorphic studies in the East Strangways Range lead to a reinterpretation of the tectonic history of the granulite gneisses in the central tectonic zone of the Arunta Block. This terrain of supracrustal and igneous gneisses is estimated to be 20002100 Ma old [1]; it was metamorphosed in the granulite facies and subsequently cooled, probably isobarically. Peak conditions are recorded at 1800 Ma [2], in the Strangways Event. No early or contemporaneous deformation related to this metamorphic event is observed. The region was subsequently affected by a series of closely related tectonic events, D1-D3, contemporaneous with a second episode of high grade metamorphism. Accompanying Dl, partial but pervasive regional hydration occurred, with the SI mineral assemblage recording a higher P and lower T than the Strangways Event. Dl deformation is regional; it produced very tight to isoclinal folds, and folds of strongly elongate flattened sheath geometry on both outcrop and kilometre scale (Fl). The folds are steeply reclined to the south-east and plunge steeply eastward. The regional tectonic fabric (SI) fabrics are commonly mylonitic with large degrees of grain refinement. A regional mineral and elongation lineation (LI) is contained within SI and plunges steeply to the east. The long axes and marginal hinge axes of sheath structures, and the fold axes of isoclinal folds are all parallel to LI. Fold - fabric relationships and microstructural features suggest this deformation is the result of rotational shear of high values of shear s train with transport from east over west. Localized D2 deformation followed the regional Dl event, and is characterized by east plunging, tight to isoclinal folds with southeast dipping axial planes. No linear or planar mineral fabrics were developed during D2. Macroscopic upright north-west trending, southeast plunging open folds (D3) are observed. D4 deformation involved movement along steeply east plunging mineral lineations in steep east/west trending shear zones also involving granulite facies metamorphic assemblages. These zones possibly accommodated relative vertical movements during isostatic uplift that resulted from thickening of the crust during Dl, D2 and D3. D1-D4 events are interpreted to be the deep crustal expression of the one collisional orogeny involving crustal thickening. The rocks presently exposed at the earth surface remained at depth within the crust following this orogeny. Further uplift of the terrain occurred during the Yamba Event at 500-345 Ma [2], and emplacement of the terrain in the uppermost crust occurred during the Alice Springs Orogeny at 310-330 Ma [2].
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References [1] [2]
Windrim, D.P. and McCulloch, M.T. 1983. Geol. Soc. of Aust., Abstracts Series, 9, 192-193. Black, L.P. et al., 1983. BMR J. of Aust. Geology and Geophysics, 8, 129-137.
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STRAIN AND DISPLACEMENT IN THE HARTS RANGE DETACHMENT ZONE: A STUDY OF THE BRUNA GNEISS AND ITS BOUNDING FAULTS FROM THE EASTERN MARGIN OF THE ENTIA DOME, CENTRAL AUSTRALIA
P.R. James, P. MacDonald and M. Parker Department of Geology and Geophysics, University of Adelaide, S.A.,
5000 Detailed geological mapping has been carried out on the Bruna granitic gneiss and its contacts with the underlying Entia gneiss complex and overlying Irindina supracrustal assemblage from the eastern margin of the Entia Dome in the eastern Harts Ranges. This zone has been described recently as comprising the Harts Range Detachment Zone, a major low-angle lithospheric detachment which during an early extension phase created the major basin into which the Irindina supracrustals were deposited and emplaced to form the Harts Range Cover sequence of the Harts Range Mobile Belt, and during a later contractional phase, this supracrustal sequence was juxtaposed by overthrusting with the underlying older Strangways Orogenic Belt basement. The basement Entia gneiss complex in the area consists of discontinuous lenses and layers of supracrustal schists, gneisses and amphibolites which suffered an early high-grade metamorphism and layer-parallel fabric development before being intruded by voluminous acid-intermediate magmas. Three repeated isoclinal and largely recumbent ductile fold events folded this sequence into a gneissic pile before the development of the HRDZ and the intrusion and structural emplacement of the Bruna gneiss protolith. The Entia gneisses are clearly truncated on a regional scale by the lower boundary of the Bruna gneiss. On a small scale the contacts are generally conformable and the Bruna gneiss is structurally interleaved with the Entia gneiss as well as showing intrusive relationships. The Bruna gneiss has a consistent moderately to intensly developed ductile fabric which varies from an augen streaky foliation through a ribbon gneiss to protomylonitic and mylonitic, in what is predominantly a granitic gneiss. At the top of the Bruna gneiss is a conformable distinctive dark-coloured mega-crystic gneiss layer which appears to have been intruded independantly from the bulk of the Bruna gneiss and has also suffered a different subsequent strain history. Principal stretch lineations are consistently north plunging or horizontal N-S except where folded. The Bruna gneiss fabrics are complexly folded about axes in an asymmetric manner which suggests interruption of ductile slip and crumpling of the fabrics followed by continuing slip on other discrete surfaces and shear zones. The fabric in the Bruna gneiss displays asymmetric augen, pressure shadows and C-planes which reveal a complex history of displacement sense variation. Most of the sense indicates displacement of the overlying Harts Range Cover in a southerly direction over the Entia gneiss basement. Reverse sense movements occur in some zones and especially in the higher megacrystic genies layer. Strain measurements of elongated feldspars, feldspar aggregates and quartz leaves using direct measurement and Fry plots, indicate moderate to high strains with predominantly oblate flattening to plane strain symmetries. In
61
the granitic gneisses pure oblate strains increase in intensity towards both the upper and lower margins, while higher strains with Flinn K values of .4 - .5 dominate the megacrystic gneiss. Strain integration over the 300-500 m detachment zone produces total displacements of only a few kilometres, suggesting that much of the overall indicated displacement is partitioned into the discrete boundary thrusts.
62
CRUSTAL-SCALE DUCTILE FAULT SYSTEMS IN THE ARUNTA INLIER, CENTRAL AUSTRALIA.
W.J. Collins^ and C. Teyssier^ ^ School of Earth Sciences, Macquarie University, NSW. 2109 ^ Dept. of Geology and Geophysics, University of Minnesota, 31 Pillsbury Drive S.E., Minneapolis, Minnesota, 55455, USA.
Fault systems in sedimentary basins are now well described and understood but are still poorly documented in underlying metamorphic basements, even though the latter are commonly dissected by complex networks of anastomosing shear zones. The Arunta Inlier in central Australia is a typical Proterozoic terrane of granulite and amphibolite facies metamorphic rocks that form basement to an upper Proterozoic-Palaeozoic cover sequence. In Late Devonian-Early Carboniferous times, the Alice Springs Orogeny produced south-verging folds, thrusts and nappes above a decollement in the cover rocks at the southern margin of the Arunta Inlier. These thin-skinned structures extend into the basement to the north (towards the hinterland) as anastomosing shear zones that reflect extensive ductile faulting of the crust. Detailed observations along two major traverses (each > 100 km long) in the east and west of the Arunta Inlier, show that ductile faults in the basement constitute crustal-scale thrust systems involving consistent geometrical and kinematic frameworks. Accordingly, two structural regions have been distinguished; a northern and southern province. The southern Arunta is characterised by a series of basement duplexes best exposed in the eastern part of the belt (Arltunga Nappe Complex) where the relationship between basin-thrust tectonics and subhorizontal mylonite zones of the basement is particularly clear. Although the lower crust is involved in the overthrust, most of the 100 km shortening is accomodated in the upper crust, above a major, complex mid-crustal detachment that separates underlying granulites from an amphibolite-facies terrane. Mid-Palaeozoic deformation in the northern Arunta is fundamentally different from that in the south. Ductile reverse faults are generally narrower (10-100 m), steeper, and show both north - and south - vergence, apparently resulting from a more coaxial deformation history. In the Reynolds-Anmatjira range region, the reverse faults fan from north-to south-dipping, producing a symmetrical, crustalscale "pop-up" structure with granulite facies rocks defining its main axis. Kilometre-wide, strike-slip fault zones separate the north and south Arunta but the extent and amount of displacement along these zones is now known. They are offset by major NW- trending faults which extend through the south Arunta, as confirmed by geophysical evidence. These faults are associated with deformation in the north Arunta and indicate that shear zone development in this area postdated that in the south. An important result of this work is that the distribution of granulite facies rocks throughout the Arunta Inlier can be explained largely by faulting associated with the mid-Palaeozoic orogeny. This is consistent with deposition of the extensive late Proterozoic and Early Palaeozoic cover sequence on granites and amphibolite facies rocks.
63
rather than granulites. Also, published K-Ar, "^^Ar-^^Ar and Rb-Sr mineral age determinations indicate Palaeozoic deformation for all shear zones, even those containing kyanite ± staurolite assemblages. There is no need to invoke extensive reactivation of older (Proterozoic) shear zones, although older zones of weakness would have existed.
64
CREEP OF ZINC SULPHIDE IN A CONTROLLED THERMODYNAMIC ATMOSPHERE
J.L. Davidson^, B.E. Hobbs^ and A. Ord^ ^ Department of Earth Sciences, Monash University, Clayton, Victoria, 3168. ^ CSIRO, Division of Geomechanics, P.O. Box 54, Mt. Waverley, Victoria, 3149.
Experiments conducted to investigate the effects of chemical environment on the creep behaviour of polycrystalline zinc sulphide (ZnS) demonstrate that its strength is dependent on the sulphur fugacity of its environment during deformation. Sixty fine grained, 5-10/zm diameter, pure synthetic ZnS specimens have been deformed at atmospheric pressure and temperatures of 940 to 1050K by uniaxial compressive stresses of 25 to 100 MPa. Specimens were deformed to approximately 10% strain at instantaneous strain rates of 1 0 " t o 1 0 " T h e sulphur fugacity (fS^) in the specimen chamber is buffered at 10'^^ Pa by liquid zinc and at 10'^ Pa by zinc oxide in an atmosphere of flowing high purity nitrogen. A low impurity level of 0^ in the specimen chamber is established by passing high purity nitrogen through a molecular sieve to remove the H^O and then over aluminium held at 775 K before its introduction to the specimen chamber. The concentrations of native crystal defects are fixed by the temperature and sulphur fugacity within the specimen chamber. Defect isotherm diagrams have been constructed, indicating the experimental range of sulphur fugacity to lie within the [Vzn"] = [Vs* '] neutrality range. The deformation is described by a power law equation in which both the stress exponent (n) and the activation energy (Q) are sulphur fugacity dependent. At fS^ = 10"^ Pa the stress exponent increases from 2.6±0.1 at 941 K to 3.3±0.1 at 1049 K while at fS^ = 10"^^ Pa the stress component remains constant at 2.7±0.1 over the same temperature range. The activation energy at fS = 10"^ Pa increases from 179±3 kJ/mole at a stress of 27 MPa to 22l±14 kJ/mole at a stress of 58 MPa while at fS^ = 10"^^ Pa the activation energy decreases from 321±1 kJ/mole at a stress of 35 MPa to 211±12 kJ/mole at a stress of 58 MPa. The range of published activation energies for the self diffusion of Zn in ZnS at fS^ = 10"^^ Pa is from 260 kJ/mole (Bansagi et al., 1968) to 405 kJ/mole (Secco, 1958) or 29 to 174 kJ/mole higher than the activation energy for creep reported here. This is inconsistent with the possibility of Zn related defects being rate controlling in the creep of ZnS at this fS^. No data exist for the activation energy for self diffusion of S at this fS_ nor do any exist for Zn or S at fS^ = 10-2 p^ 2 2 References: Bansagi, T., Secco, E.A., Strivastava, O.K. and Martin, R.R. 1968. Kinetics of hexagonal-cubic phase transformation of zinc sulphide in vacuo, in zinc vapor and in sulfur vapor. Can. J. Chem., 46, 2881. Secco, E.A. 1964. Gas solid exchange reactions; zine vapor and polycrystalline zine sulphide. Can. J. Chem., 42, 1396.
65
THE EFFECT OF BUBBLE FORMATION ON THE CONDUCTIVITY OF SYNTHETIC QUARTZ
J.C. Newton-Howes^, A.C. McLaren^ and R.J. Fleming^ ^ Department of Physics, Monash University, Clayton, Victoria, 3168. ^ Research School of Earth Sciences, Australian National University, Canberra, 2601.
The electrical conductivities of samples of synthetic quartz have been measured over a range of frequencies and complex impedance anlaysis has been used to determine the bulk conductivities. Measurements were made over a range of temperatures at room pressure. The conductivity is shown to be anisotropic and to vary between different growth regions of the crystal. Changes in conductivity occur when the samples are heated above 550°C. These changes appear to be related to the formation of bubbles and the associated increase in dislocation density which occurs inwet synthetic quartz at these temperatures [1]. Anomalous behaviour has been observed in samples from the -X growth region which has a very high (OH) content. The microstructures of the annealed samples were examined by transmission electron microscopy and these are discussed in relation to the conductivity data. Reference A.C. McLaren, R.F. Cook, S.T. Hyde and R.F. Tobin, 1983. Minerals, 9, 79-94.
Phys. Chem.
66
FABRICATION AND DEFORMATION OF SYNTHETIC QUARTZ AGGREGATES
E.G. Luan and M.S. Paterson Research School of Earth Sciences, The Australian National University, P.O. Box 4, Canberra, A.C.T. 2601
Pure quartz aggregates with different water content and controlled grain size have been fabricated from natural quartz powder and from silica gel by isostatic hot-pressing at 300 MPa. The porosity of both natural quartz-origin and gel-origin samples was minimized by selecting appropriate hot-pressing conditions. The starting material for hot-pressing of quartz powder was pure beach sand. A finely ground powder of different particle size was hot pressed both with and without added water at 1173-1473K for varying periods. The silica gel initially contained about 13wt% water and its particle size was less than one micron. After hot-pressing, about 0.1wt% water was retained in the newly formed quartz aggregates whose grain size ranged from 5 to 70/im, depending on the hot-pressing conditions. Preliminary experiments have shown that the natural quartz-origin samples are stronger by about an order of magnitude than the gelorigin samples when the grain size in both types of samples is similar. The gel-origin samples are comparable in strength to synthetic quartz single crystals. The pronounced difference in strength is accompanied by a difference in water content and distribution in the two types of synthetic polycrystals. It is suggested that it is not only the content, but perhaps rather the distribution of water in quartz which is an important factor in the water-weakening of quartz, influencing the weakening processes by controlling the availability of water to the intracrystalline defects such as dislocations in the quartz crystals. The activity of these intracrystalline defects is assumed to be responsible for the rheological properties of the material.
67
THE UPTAKE AND NATURE OF WATER AT ELEVATED PRESSURE AND TEMPERATURE IN QUARTZ AND SOME IMPLICATIONS FOR THE HYDROLYTIC WEAKENING MECHANISM
J. Gerretsen Research School of Earth Sciences, Australian National University, P.O. Box 4, Canberra, A.C.T. 2501.
The uptake of water at 1.5 GPa confining pressure, 1173 K and high water fugacity, over times up to 24 hours, has been investigated using a newly developed assembly to prevent microcracking from occurring. It was found that the uptake is small and beyond the detectability of the presently used technique of infrared spectroscopy and serial sectioning. This observation reflects either a low value for the diffusivity of solubility or a combination of both and is in agreement with Kronenburg et al. (1986) and Rovetta et al. (1986) and brings into question the results of the early experiments on hydrolytic weakening by Griggs and Blacic (1964) and recent estimates of the solubility and diffusivity by Mackwell and Paterson (1985). Preliminary results of a combined T.E.M., light scattering and infrared spectroscopy investigation of as-grown wet synthetic quartz and heat-treated wet synthetic quartz at 0.1, 300 and 1500 MPa confining pressure and 1173 K, strongly suggests that water in quartz is essentially in the form of H^O in inclusions pointing to a low value for the solubility. Based on these observations the role of water in the weakening of quartz is reviewed and alternative models for hydrolytic weakening are suggested. References: Griggs, D.T. and Blacic, J.D., 1964. The strength of quartz in the ductile region. Trans. Am. Geophys. Union. 48, 102. Kronenburg, A.K., Kirby, S.H., Aines, R.D. and Rossman, G.R., 1986. Solubility and diffusional uptake of hydrogen in quartz at high water pressures: implications for hydrolytic weakening. J. Geophys. Res., in press. Mackwell, S.J. and Paterson, M.S., 1985. Water related diffusion and deformation effects in quartz at pressures of 1500 and 300 MPa. In Schock, R.N. (ed.). Point Defects in minerals. Am. Geophys. Union, Geophys. Monogr., 31, Min. Phys., 1. Rovetta, M.R., Blacic, J.D. and Rolloway, J.R., 1986. Solubility of hydroxyl in natural quartz annealed in water at 900°C and 1.5 GPa. Geophys. Res. Lett., 13, 145.
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THE WATER-WEAKENING EFFECT IN EXPERIMENTAL DEFORMATION OF QUARTZ
J . D . Fitz G e r a l d ^ , A . O r d ^ , J . N . Boland^ a n d A . C . McLaren^ ^ Research School Earth Sciences, A u s t r a l i a n N a t i o n a l U n i v e r s i t y , Canberra. C S I R O , Division of G e o m e c h a n i c s , P.O. Box 54, M t . W a v e r l e y , 3149, Victoria.
Ord and Hobbs (1986) reported a series of deformation experiments using n a t u r a l quartz single crystals deformed in the presence of H^O vapour in a solid-medium apparatus operated at 1.64 GPa confining p r e s s u r e . In those experiments, mechanical strength and deformed microstructure were shown to be dependent u p o n the type of solid oxygen buffer incorporated in the specimen a s s e m b l i e s . U s i n g samples from these experiments, a detailed m i c r o s t r u c t u r a l study is now b e i n g undertaken in an attempt to understand which (if any) of the existing models of hydrolytic weakening (e.g. dopant c o n c e n t r a t i o n , diffusivity, m i c r o f r a c t u r e , etc.) b e s t explain the observed v a r i a t i o n in m e c h a n i c a l strength and style of deformation. While heterogeneous deformation may be a problem in the specimens, it does afford the opportunity to sample materials deformed to quite different strains. Important changes in dislocation and recrystallized grain structures appear related to the b u f f e r e d e n v i r o n m e n t , although local chemical anslyses at the 0 . l-/im-scale p o i n t to some complications. Further details of samples deformed under the infuence of Ta (low fO^ and f H ^ O , h i g h f H ^ ) and M n (high fO^ and f H ^ O , low f H ^ ) buffers are p r e s e n t e d . Ta: Extensive Dauphine twin planes (lOO^um apart) define bands which deformed b y w a y of fine (5/im) diagonal lamellar structures. The corresponding dislocation structure consists of dense tangles w i t h < a > Burger's v e c t o r s , separated b y regions of v e r y low dislocation densities (Fig. 1) inside the coarse twin structure. Mn: Recrystallization is extensive at one or b o t h ends of all specimens. The less-deformed interiors feature h e a l e d axial fractures m a r k e d b y arrays of unloading microfractures (Fig. 2). Zones of dislocation activity surround the h e a l e d fractures. Far from the h e a l e d 'plane' (~ 50/im) , < a > dislocations form familiar tangled structures (Fig. 3). Approaching the h e a l e d fracture, dislocation structures (<a> with < c > ) change progressively towards a wellrecovered state (Fig. 4 ) . The presence of fluid-filled inclusions and buffer particles on the h e a l e d fractures, along w i t h the narrow accompanying zones of p l a s t i c i t y , highlight a p o t e n t i a l role of microfracture in initiating pervasive deformation. Referenc e: O r d , A . and H o b b s , B . E . 1986. p p . 51-72 in H o b b s , B . E . and H e a r d , H . C . (eds). M i n e r a l and Rock Deformtion: Laboratory Studies, Geophysical Monograph 36, A G U .
69
Fig. 1." Darkfield TEM image, Ta-buffered specimen. High and low dislocation densities (scale bar of 1 micron).
Fig. Polarized light micrograph, Mn-buffered specimen. Healed fractures run E-W and unloading (?) micro-fractures N-S (scale bar of 50 micron).
Fig. 3" Brightfield TEM image, Mn-buffered specimen. Region 'far from healed fracture plane (scale bar of 1 micron).
Fig. 4 Darkfield TEM image Mn-buffered specimen. Healed fracture region (scale bar of 1 micron).
For each micrograph, observation direction was an a-axis, c-axis is oriented N-S so that the direction of applied stress is aligned E-W in the plane of the figure.
70
THE PLASTIC DEFORMATION OF QUARTZ
A. Ord & B.E. Hobbs CSIRO, Division of Geomechanics, P.O. Box 54, Mt. Waverley, Vic. 3149.
It may seem trivial to geologists to be reminded that natural quartz may deform plastically, by dislocation processes of slip and climb. However, it is an important point which we wish to emphasize, as it presently appears to be in question. In particular, the suggestion (Kirby and Kronenberg, 1984) that the deformation of initially dry, natural quartz, in a hydrous environment, is a brittle process and is controlled by microcracking appears to be foremost in the thoughts of some students of quartz. This recent emphasis should not be allowed to obscure the point that such hydrothermally annealed quartz does deform plastically, and that microcracking, if it is important, simply opens up the crystal so that shorter distances must be travelled by solid-state diffusion of one or more water-related defects before the quartz is in such a state that it will deform plastically. The present contentious discussions about the role of microcracking in the deformation of quartz are therefore something of a red herring. Our major aim is to explain the mechanism(s) by which the presence of water affects the dislocation processes of slip and climb within the quartz. Throughout it is assumed that water is present in excess in the environment of deformation. It is also becoming increasingly clear that more attention than is warranted is being paid to the role of high concentrations of molecular water in quartz. It appears that the presence of only a small (of the order of 30 to 60 H/10®Si) quantity of some waterrelated defect facilitates plastic deformation under conditions of high oxygen (fO^) fugacity (therefore high water (fH 0) fugacity and low hydrogen (-fH ) fugacity). Molecular water itself appears to take no part in the plastic deformation process although molecular water may be the main mode of incorporation of the impurity in quartz. Single crystals of dry natural quartz annealed and deformed normal to (1010) in talc assemblies within the a-quartz field at a confining pressure of 1.65 GPa and a strain-rate of 10~®s~^ display contrasting strengths and microstructures at 800°C and at 950°C. Specimens deformed at 800°C have high strengths ((o^-a^) about 2 GPa), low (150 H/lO^Si) (OH) concentrations, of sharp-peak type only, and very low dislocation densities (less than 10^ cm"^). They do not exhibit optically visible plastic deformation features, microcracks (fresh or healed, other than obvious extension cracks) or inclusions. In contrast to these strong and essentially elastic, low temperature specimens, specimens deformed at 950°C have low strengths less than 0.5 GPa), comparatively high (560 H/10®Si) (OH) concentrations, of broad-band as well as sharp-peak type, and moderate dislocation densities (1 x 10®cm"^ to 5 x 10®cm"^). They display undulose extinction, extension cracks, and inclusions. Deformation lamellae are present in areas free of microcracks. They are also present in areas which contain microcracks (fresh and healed). An increase in the background absorption measured using infra-red spectroscopy in these specimens is associated with the presence of inclusions. The quartz has plastically deformed.
71
The specimens described by Ord and Hobbs (1986) and Fitz Gerald et al. (1987) were deformed under a controlled thermodynamic environment at 1.65 GPa and 800°C like the strong specimens described earlier, but at a strain-rate of an order of magnitude faster. The simplest prediction is therefore that these specimens would not deform. However, and surprisingly so in the light of the earlier experience with talc, they do deform plastically, with a range in differential strengths from 200 MPa to 1,5 GPa. (OH) concentrations vary correspondingly from about 250 H/10®Si to 60 H/10®Si as fO decreases, and broad band absorption and apparently the number of inclusions decreases. Similar deformation experiments but on a quartz mylonite by Raiser et al. (1985) consistently result in specimens of lower strengths, more recrystallized and equant grains and subgrains, and a larger recrystallized grain size with high fO^, and specimens of higher strengths, few recrystallized grains, a smaller recrystalized grain size, flattened grains, and deformation lamellae with low f . The preferred crystallographic orientation of c-axes for these specimens also changes with environment, with a maximum parallel to A^ preferred at low fO^ and partial small circles preferred at high fO^, consistent with the changes observed by Tullis et al. (1973) with increasing temperature. The experiments described by Ord and Hobbs (1986) were exploratory, but the consistency in their results with those of Raiser et al. (1985) and subsequent, mostly unpublished, results of Ord (but see Ord and Hobbs, 1985a) still requires consideration of the control of the plastic deformation of quartz by its thermodynamic environment. The results of the earlier talc experiments may then be explained by the relatively low fO^ and high fH^ of that system (Ord and Hobbs, 1985b). We believe that we now have enough experimental experience to indicate that fO^ exerts a crucial control over the strength of quartz in water present systems; commonly fO^ is not known by the experimentalist and can be far from what he/she expects it to be. It is proposed that all those scientists involved in deformation experiments on quartz participate in a program designed to measure the gas fugacities of their specimen chamber before and during deformation, so that deformation results may be more suitably compared between the different laboratories. There is no excuse for not doing this as it is relatively easy to do using fugacity sensors based on solid oxygen buffers. References: Fitz Gerald, J.D., Ord, A., Boland, J.N. and McLaren, A.C. 1987. The water-weakening effect in experimental deformation of quartz. This volume. Kirby, S.H. and Kronenberg A.K. 1984. Hydrolytic weakening of quartz:uptake of molecular water and the role of microfracturing. EOS, AGU Trans., 65, 277. Ord, A. and Hobbs, B.E. 1985a. Solubility of (OH) in quartz. EOS, AGU Trans., 66, 373. Ord, A. and Hobbs, B.E. 1985b. Experimental control of hydrolytic weakening. EOS, AGU Trans., 66, 1139-1140. Ord, A. and Hobbs, B.E. 1986. Experimental control of the waterweakening effect in quartz. In Mineral and Rock Deformation: Laboratory Studies, edited by B.E. Hobbs and H.C. Heard. Geophysical Monograph 36, pp. 51-72, AGU, Washington.
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Raiser, S., Hobbs, B.E. and Ord, A. 1985. Effects of chemical environment on the experimental deformation of a quartz mylonite. EOS, AGU Trans., 66, 1084. Tullis, J., Christie, J.M. and Griggs, D.T. 1973. Microstructures and preferred orientations of experimentally deformed quartzites. Geol. Soc. Am. Bull., 84, 297-314.
73
STRUCTURAL LITHIC UNITS IN EXTERNAL OROGENIC ZONES
Nicholas B. Woodward and Everett Rutherford Jnr. Department of Geological Sciences, University of Tennessee, Knoxville, TN 37996.
Structural style changes are typical in external orogenic zones, and are inferred here to be related to changes in stratigraphic sequences. Obvious structural style changes occur from external to intermediate thrust sheets. Similar changes in style are also seen and most easily interpreted along-strike within individual regional thrust sheets. This work is based on studies in the Idaho-Wyoming-Utah (Central US Cordillera) and the Tennessee (Southern US Appalachian) fold and thrust belts. The way to evaluate the material properties of a highly anisotropic stratigraphic sequence is a complicated question, even in simply deformed areas. The method used here is to subdivide the stratigraphic sections into structural lithic units. The subdivisions are based on the dominant structural styles vertically in individual cross-sections. Currie, Patnode, and Trump (1962) defined structural lithic units originally as lithologic intervals which acted as structurally coherent packages during folding. Interlayer viscosity contrasts, interlayer relative thicknesses, interlayer spacing and interlayer cohesion are also important in defining structural intervals. Maj or flats represent boundary zones between structural lithic units in stepped fault paths. A major goal is to understand the along-strike variations between thrusts which propagated as stepped fractures and those which may have propagated as folds. Four regionally significant structural lithic units are identified in Idaho-Wyoming-Utah and five are identified in Tennessee. The characteristics of the structural lithic unit packaging in both belts are similar. The internal composition of structural lithic units (composition includes lithology, bedding characteristics and thicknesses) is a major variable in their deformation style. Individual structural lithic units are of three types: 1) a uniform interval; 2) a three-part interval with a strong center and variable upper and lower conforming members; and 3) a three-part interval with strong upper and lower parts and a variable conforming middle member. The stacking sequence of lithologic types and the resulting structural intervals is another major variable in determining the deformation style. Areal changes in either internal composition or in stacking sequence will cause lateral changes in deformation styles. In east Tennessee, lateral facies changes from carbonate-dominated to shale-dominated intervals occur in both the Middle Cambrian and in the Middle Ordovician. The stratigraphic changes are coincident with changes in thrust ramp positions (connected by major lateral ramps), and with thrust zone geometries (imbricate thrusts in one area versus a duplex zone along strike). They are also associated with changes in thrust shapes, from listric shapes to stepped shapes. The critical taper of major dominant members regionally controls where structural style changes will occur. Weak dominant members are major evaporite horizons, or thick shales and their pinch-outs are related
74
to structural boundaries. Strong dominant members have a major influence on structural style when they change regionally also. The regional change in thrust geometries (spacing, folding styles, etc.) from north to south in the Idaho-Wyoming-Utah thrust belt is the result of different structural lithic units dominating the thrust sheets in the two areas. The Upper Paleozoic structural unit (a type 3 unit) dominates the northern thrust belt, but thins southward. The Lower Mesozoic structural unit (a type 1 unit) thickens from north to south and dominates the southern part of that thrust belt. Thus the three-dimensional structural lithic unit architecture determines the dominant deformation styles of external orogenic zones. A thorough understanding of the stratigraphic facies realms may enable us to predict resultant structural styles.
75
MIOCENE DUCTILE STRETCHING OF AN OVERTHICKENED CONTINENTAL CRUST AT THE TERMINATION OF THE ALPINE OROGENY, AEGEAN SEA, GREECE: A NEW SAGA IN THE EVOLUTION OF CONCEPTS CONCERNING EXTENSION TECTONICS
G.S. Lister^ALamont-Doherty Geological Observatory, Columbia University, Palisades, N.Y., 10964, USA.
The southern Aegean Sea is dotted by the islands of the Cyclades, in which deep-seated crystalline rocks formed during Alpine high pressure "blueschist" metamorphism are exposed. These formerly deeply buried metamorphic rocks underwent a period of very rapid uplift of at least 7-12 km during the Miocene, between 12-8 Ma, until they reached the surface. Yet, during this period, there was no vast shedding of erosional debris, even though at least 10^-10® km^ of overlying rock had to have been removed. The rate of sedimentation is not unusually high during this period. Hence this period of rapid uplift was not simply the result of erosion, and we are forced to consider alternative explanations for the rapid uplift and exposure of the formerly deep-seated metamorphic rocks now exposed. An important clue as to the nature of tectonic processes at that time is given by the evidence that these rocks were "overthrust" during this uplift phase by essentially unmetamorphosed rocks. These low-angle faults may be detachment faults, as recognized in the Basin and Range Province in western North America. Tectonic denudation as the result of movement on these faults, caused by continental extension, can explain such rapid uplift of metamorphic and igneous rocks, while subsidence or a minimal rate of erosion takes place at the surface. New concepts pertaining to extensional tectonics have radically changed our thinking about many aspects of the orogenic process, for example oscillations between compressional and extensional orogeny may be of general importance in the formation and subsequent exposure of "blueschist" terrains such as exist in the Cyclades. As the result of discussions held during the course of an introductory structure course at Columbia University, it was decided that we would travel to the Aegean to evaluate this hypothesis. Specifically we would visit the island of Naxos and determine whether or not we can describe it as a metamorphic core complex similar to those observed in the Basin and Range of the western United States. The evidence is indeed that the domal exposures of deep-seated metamorphic rocks throughout the southern Aegean, and in the eastern Aegean, are in fact metamorphic core complexes, but these core complexes have a peculiar Aegean flavour. We could find no compelling evidence that would force us to kinematically relate the brittle and ductile parts of the geological history for example, except for the short time-frame under which these events took place. The core of the "thermal" dome recognized on Naxos by previous workers is in large part structurally controlled by a kilometer thick carapace of mylonites formed subsequent to the peak of the M^ metamorphism. More importantly, the pattern of stretching lineations in the Cycladic blueschist belt has been formerly attributed to ductile deformation of the crust associated with Eocene subduction. However, our reconnaissance study of lineation patterns in different island groups shows the same pattern of stretching lineations in young (10-12 Ma) Itype granitoids. This, and other structural relations suggest that
76
the lineations can be attributed to a major ductile event which started in Oligo-Miocene times during a regional Barrovian-type metamorphism (M^) and continued until ca 10 Ma, with very large strains accumulating during the last million years. During this period, the ubiquitous high pressure metamorphic rocks of the Cyclades were rapidly unroofed. Hence, the lineation pattern, long thought to have been the result of collision during the early stages of Alpine collisional events, is in fact a relatively young phenomenon produced as the result of ductile stretching of the crust during Miocene continental extension, ending at around 10 Ma. Oscillation between compressional and extensional events in this part of the Alpine orogeny provides a ready explanation for the otherwise enigmatic rapid uplift (2-5 km/M.Yr) of deep-seated metamorphic rocks which has been observed. The crust was probably 60-70 km thick at the termination of the compressive phase of the Alpine orogeny. A major continental extension event subsequently took place, as a result of retreat of the flexure in the subducting oceanic lithosphere of the advancing African plate, with obduction of the continental crust accompanying such "roll-back", giving rise to the Mediterranean Ridge. As a result of continental extension deep-seated metamorphic rocks were dragged to the surface along crustal shear zones zones (in the "lower" plate), while enigmatically, sedimentation continued on the "upper" plate, leading to the formation of a paired metamorphic belt separated by an active sedimentary basin.
^ Now at the Department of Earth Sciences, Monash University, Clayton, Vic. 3168.
77
CRUSTAL MODELLING
B.E. Hobbs and A. Ord CSIRO, Division of Geomechanics, P.O. Box 54, Mt. Waverley, Vic. 3149.
Using available experimental data for the mechanical behaviour of various rock types, rheological models for oceanic and continental lithosphere are presented for the range of geothermal gradients from 5C° km"^ corresponding to cool, stable lithosphere to 500° km"^ corresponding to hot, deforming lithosphere. The upper limit for stress in the lithosphere is assumed to be defined by the transition from velocity weakening to velocity strengthening constitutive behaviour. Also presented are rheological models for subduction zones. In continental lithosphere with a hot spot in the lithosphere, a "Christmas tree" shaped distribution of strength is developed with strong rheological contrasts developed across lithological boundaries and also across boundaries within homogeneous lithologies. Both of these kinds of boundaries are considered to act as detachment zones during both extension and shortening of the lithosphere. The transition from one kind of boundary to another produces ramps in the detachment zones leading to contrasting zones of dilation during extension and shortening tectonics. These contrasting zones of dilation explain the development of bimodal volcanism in rift zones and the development of large flat lying syntectonic intrusions in regions affected by thrust tectonics. Variations in thermal structure along the strike of a tectonic zone lead to relative displacement of the basic "christmas tree" rheological structure along the zone. This relative displacement explains the development of transfer faults, of upper and lower plate margins in rift terrains and of lateral ramps in thrust terrains. In oceanic lithosphere with a hot spot in the lithosphere a "keel" shaped distribution of strength is developed with a horizontal region of strong rheological contrast which could act as a detachment zone. Addition of new material at a ridge crest is also associated with addition of new material at the keel so that although the keel is fixed in position by the regional thermal structure, material particles are constantly moving relative to the keel. This provides a mechanism for plate tectonics. The structure of the oceanic lithosphere may be regarded as two horizontally extensive zones, an upper one which has always behaved in a brittle manner and a lower one which, depending on two alternative views of regarding flow in the mantle, may consist of horizontally foliated mylonites or a system of syntectonic intrusions. Variations in thermal structure along the strike of a ridge lead to relative changes in the thickness of the upper zone and in the position of the keel relative to the ridge axis. This leads to the development of transform faults. Thus, a consideration of the rheological properties of various rock types, coupled with variations in thermal structure throughout the lithosphere leads to an integrated view of global tectonics and, in particular, a rational mechanism for plate tectonics that does not involve "ridge pushing" or "trench pulling" mechanisms.
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THE GEOMETRY AND TECTONIC SIGNIFICANCE OF TRANSFER FAULTS IN CONTINENTAL EXTENSION TERRANES M.A. Etheridge Bureau of Mineral Resources, GPO Box 378, Canberra, ACT, 2501, Australia. Extension of the upper part of the continental lithosphere takes place largely on two sets of normal faults; large area, irrotational faults with low initial dip (detachment faults), and smaller rotational normal faults that commonly sole onto the detachment (listric or domino faults). Variations in the along-strike geometry of both fault sets result in a third set of accommodation structures known as transfer faults. Transfer faults are kinematically analogous to oceanic transform faults. Most of the published accounts of continental extension concentrate on the normal and detechment fault geometries, and the purpose of this paper is to highlight the importance of transfer faults to the extension process, to extensional basin formation and to the subsequent tectonic history of extensional terranes. Transfer faults can accommodate along-strike variations in the position, spacing and dip of normal and detachment faults. Since they are essentially accommodation structures, their geometry is largely determined by the geometry and displacements of the normal faults that terminate against them. They do not extend horizontally or vertically beyond the outer terminating normal or detachment faults, but transfer fault planes may have unusual and irregular shapes, especially where the dip direction of the normal faults switches across them. Strain compatibility requires that the displacement vectors of intersecting normal and transfer faults be parallel. Where the normal faults are close to pure dip slip, the transfer faults will therefore be steeply dipping and perpendicular to them (Bass Strait Basins, Etheridge et al., 1985, in press). However, where there is significant oblique slip on the normal faults, the transfer faults will also be oblique slip and the fault traces will not be orthogonal. The latter geometry is typical of regions of oblique extension or transtension (Viking and Moray Graben, North Sea, Gibbs, 1984, in press; East African Rift, Bosworth, 1985). We distinguish between 1st order and 2nd order transfer faults, mainly on the basis of tectonic significance. 1st order transfer faults are those which accommodate reversal in dip of the master detachment fault (e.g. Gippsland Basin, US Atlantic margin), and/or substantial offsets of the detachment. Whole rift basins may terminate against 1st order transfer faults, with their extension transferred to a separate basin (e.g. Bass Basin). By far the majority of transfer faults result in only small offsets of the detachment, but accommodate small to moderate along-strike variations in the geometry of the listric or domino faults. These 2nd order transfer faults, however, exert a significant influence on the intrabasin structure (e.g. Bass Basin; Lewis Trough, Northwest Shelf). The Bass, Gippsland and Otway Basins show the influence of 1st and 2nd order transfer faults on the geometry of intraplate rift basins at all scales. The three basins form a branched or linked rift system, in which the master detachment faults are linked by a group of 1st order
79 transfer faults. These major transfer faults accommodate switches in normal fault dip (and structural asymmetry) both within and between basins, and transfer the upper crustal extension from one basin to the other. This model requires detachment and transfer faults to pass across interbasin highs, and evidence for this is presented. Within each basin, the 2nd order transfer faults control the geoemtry of the individual normal faults and their associated tilt-blocks, and also significantly influence post-rift sedimentation and structuring. They play an important role in the development of hydrocarbon plays, both as a result of reactivation, and because they provide a means of along-strike closure. Transfer faults are an important feature of passive continental margins. 1st order transfer faults control the major along-strike variations in margin architecture. They accommodate reversals in detachment dip direction and therefore in gross structural asymmetry, juxtaposing upper plate margin segments against lower plate segments (Lister et al., 1986). They also accommodate major changes in detachment position and shape within a single segment. Examples of these variations in architecture will be described from the Australian and Atlantic passive margins. Some transfer faults can be matched on conjugate margins, and they provide probably the most precise control on pre-drift continental interior. A number of topographic lineaments visible on Australian digital terrain models are aligned with 1st order transfer faults on the margin, and are considered to have resulted from small-displacement propagation of the transfer faults in the compressive stress field that characterises the Cainozoic Australian plate. Reactivation of marginal transfer faults can also explain some enigmatic intraplate seismicity (e.g. Charleston, South Carolina). References Bosworth, W. 1985. Geometry of propagating continental rifts. Nature, v. 316, p. 625-627. Etheridge, M.A., Branson, J.C. and Stuart-Smith, P.G. 1985. Extensional basin-forming structures in Bass Strait and their importance for hydrocarbon exploration. APEA Journal, v. 25, p. 344-361. Etheridge, M.A., Branson, J.C. and Stuart-Smith, P.G. 1987. The Bass, Gippsland and Otway Basins, southeast Australia: A branched rift system formed by continental extension. Mem. Canad. Soc. Petroleum Geol., in press. Gibbs, A.D. 1984. Structural evolution of extensional basin margins. J. Geol. Soc. London, v. 141, p. 609-620. Gibbs, A.D. 1987. Linked tectonics of the northern North Sea basins. Mem. Canad. Soc. Petroleum Geol., in press. Lister, G.S., Etheridge, M.A. and Symonds, P.A. 1986. Detachment faulting and the evolution of passive continental margins. Geology, v. 14, p.246-250.
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DETACHMENT MODELS FOR THE FORMATION OF PASSIVE CONTINENTAL MARGINS
G.S. Lister-^, M.A. Etheridge and P.A. Symonds Bureau of Mineral Resources, Geology and Geophysics, Canberra, ACT, 2600, Australia.
New models for continental extension based on the operation of detachment and/or shallow-dipping crustal shear zones, predict structural asymmetry on all scales in an extended terrain. Application of these models to the formation of passive continental margins leads to the principle that opposing passive margins should exhibit complementary asymmetry, and in the simplest case, it should be possible to recognize upper-plate and lower-plate passive margins. These will have contrasting structure, as well as different uplift/subsidence histories. Upper-plate margins should be relatively devoid of structure and be uplifted if simple subcrustal lithospheric stretching models are combined with the effect of igneous underplating. Lower-plate margins, on the other hand, should be highly structured and will generally subside during the rift phase of margin development. The basement underlying the post-extension sag basin on a lower-plate margin will be characterized by tilted fault blocks adjacent to half-graben filled with syn-rift sediments. These rocks will be overlain by the gently dipping strata deposited during the post-extension, subsidence or sag phase of margin development. Marginal plateaux or rifted upper plate margins exhibit little or no upper crustal structure while extension of the crust and lithosphere below a mid-crustal detachment can explain uplift during the rift phase, and rapid subsidence thereafter. Opposing passive margins will exhibit complementary elements of these architectural styles. A number of examples illustrate complementary asymmetry of structure and uplift-subsidence histories of opposing passive margins. The best example may be the contrasting uplift/subsidence histories of the margins of the Tasman Sea, comparing Lord Howe Rise and the south-east Australian margin. The rift basins of the western Lord Howe Rise define a lower-plate margin while the uplifted margin of southeastern Australia is the corresponding upper-plate margin. Another possible example exists across the margins of the South Atlantic Ocean, comparing and contrasting the Eastern Argentinian margin with the Southwest African margin. The African margin is uplifted whereas the Argentinian margin has subsided. Other examples may be found by juxtaposing margins of the central North Atlantic Ocean. Uplift of the continental hinterland adjacent to an upper-plate margin leads to the formation of passive margin mountains, defining some of the greatest landforms on earth. Extensive peneplains existed prior to break-up of the supercontinents, so great escarpments, related to erosional retreat of major normal fault scarps, are a common feature of such passive margin mountains. This uplift may be explained as the result of substantial igneous underplating of the upper plate of the detachment system during breakup, or as the result of subcrustal extension of the lithosphere. The Palaeozoic supercontinents probably had deep mantle roots. These must have been torn or stretched during break-up, with the result that substantial rise of the asthenosphere took place. We numerically model these effects, as well as the effect of igneous underplating.
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It is difficult to explain peneplanation of tilt blocks in rift basins on passive margins, or the widespread occurrence of "break-up" unconformities, except by calling on uplift related to igneous underplating. An underplated lower-plate passive margin will remain at or above sea level during the phase of margin development, but during the subsequent sag phase, it will be quickly inundated. If the lower-plate margin is underplated by 5-15 km of mafic granulites with density 3.0 x 10'^ kg.m"^ the observed uplift-subsidence characteristics can be explained. The lower-plate margin will emerge above sea-level during the extension phase, but will rapidly subside below sea-level during the subsequent sag phase as the thermal anomalies associated with break-up decay. The upper plate margin will be subjected to an uplift of 0.9-1.4 km as the result of underplating of 10-15 km mafic granulites, additional to uplift caused by thermal buoyancy induced by rise of the mantle geotherm. Thermal buoyancy may induce uplift as much as 3.1 km at the cessation of the rift phase but thermal anomalis will subsequently decay, so maximum uplift of passive margin mountains may now be expected to lie in the range 1.4-2.9 km, as observed.
^ Now at the Department of Earth Sciences, Monash University, Clayton, Vic. 3168.
82
EXPERIMENTAL DEFORMATION OF A QUARTZ MYLONITE: ORIENTATION
THE EFFECT OF
S. Raiser^, B.E. Hobbs^ and A. Ord^ ^ Earth Sciences, Monash University, Clayton, Victoria, 3168. ^ Division of Geomechanics, CSIRO, P.O. Box 54, Mt. Waverley, Victoria, 3149.
Samples of a pure quartz mylonite have been axially shortened 60% in four orientations relative to the initial foliation and lineation, and hence relative to the initial strong crystallographic preferred orientation (Fig. 1): PSL-parallel to the foliation and lineation; 45SL-45° to the foliation and lineation; NSL-normal to the foliation and lineat ion; PSNL-parallel to the foliation, normal to the lineation. Specimens were deformed in nickel jackets with 50/^1 water added, at temperatures of 600-900''C, strain rates of 10'^-10"^ s" ^ , and a confining pressure of 1.64 GPa in a Tullis-modified Griggs solid medium deformation apparatus.
45SL Contours
1
1.64GPa PSNL
Fig. 2
60 ^ S t r a i n
The strength of the mylonite depends on specimen orientation, as well as on temperature and strain rate. At 700 and 800°C, specimens deformed PSL are weakest, those deformed 45SL are slightly stronger, and specimens deformed NSL and PSNL are 200-400 MPa stronger, at 15% strain (Fig. 2). At 900°C and lO'^s"^ all specimens show similar strengths. Many specimens show strain softening at higher strains. The microstructures developed in all specimens are similar to those observed in previous experimental studies, with the development of strongly flattened grains, and the degree of recrystallization increasing as temperature is increased and/or strain rate is decreased. The initial c-axis fabric (Fig. 1), typical of many mylonites, is an asymmetric girdle normal to the mylonitic foliation and lineation. c-axes in specimens deformed NSL (Fig. 3) and PSNL (Fig. 4) show little change in orientation with deformation; strengthening of the maximum parallel to the shortening direction is the major change. The c-axes in these specimens are inferred to be in stable orientations with respect to the shortening direction. Specimens deformed in these two orientations are the stronger of the four orientations studied.
83
PSL
'TFig. 4
tFig. 5
•^g. 6
In specimens deformed PSL (Fig. 5) a new girdle develops parallel to the initial foliation, with subsidiary girdles running from the shortening direction to normal to the initial foliation and lineation. The more complex fabrics observed in specimens deformed 45SL (Fig. 6) are associated with the triclinic symmetry of the overall deformation. No consistent changes occur, although the major fabric changes are: 1) the maxima on the perimeter rotates towards the shortening direction; 2) c-axes in the centre migrate towards the shortening direction; 3) a maximum develops at a high angle to the shortening direction. The fabrics are inferred to develop by initial kinking of grains, at a high angle to the shortening direction, into orientations more suitable for dislocation slip. Slip occurs on both basal and prism planes, with prism slip becoming more important at higher temperatures and lower strain rates. Rhomb slip may have a minor influence. In all cases slip parallel to <a> is consistent with the observed fabrics. The complexity of the fabrics developed at these high strains (60%) indicates that quartz can still show a memory for fabrics developed in previous deformations.
84
MICROSTRUCTURAL DEVELOPMENT IN SINGLE CRYSTAL GALENA (PbS) DURING HIGH-TEMPERATURE <100> COMPRESSION CREEP
S.F. Cox Research School of Earth Sciences, The Australian National University, P.O. Box 4, Canberra, ACT, 2601.
The development of deformation microstructures during high-temperature creep has been investigated in several types of natural and synthetic galena single crystals which have been shortened parallel to <100> at temperatures between 325°C (0.43T^) and 750°C (0.73T^), and at stress differences ranging from 100 MPa to 3 MPa. Strains up to 70% shortening have been investigated at creep rates ranging from 10'^ sec"^ to 10~® sec"^. The microstructural analysis has employed dislocation etch-pitting techniques in conjunction with optical microscopy and SEM. Two dislocation creep regimes have been distinguished: (1) a high stress dislocation creep regime in which subgrain structures do not develop, and (2) a low stress dislocation creep regime characterized by subgrain formation. Over the range of conditions investigated, the constitutive relations between strain-rate and stress difference may be described by power laws in which the stress exponent (n) ranges from about nine to six. In the high stress dislocation creep regime n has high values, but falls to around six at the transition to the low stress dislocation creep regime. In several types of natural galena, n is maintained close to this value at stress differences as low as 3 MPa and at temperatures as high as 0.73T . In contrast, the synthetic galenas exhibit an increase in n with decreasing stress and increasing temperature in the low stress dislocation creep regime. In both the low stress and high stress deformation regimes transient creep is characterized by a heterogeneous dislocation substructure and rapid dislocation multiplication during {110)<li0> glide. Essentially steady creep is attained at low strains in both flow regimes and corresponds to the development of a steady state dislocation density in a large proportion of the sample volume. The steady state unbound dislocation density is proportional to the square of the stress difference, and is substantially independent of temperature, dopant compositional variations, and initial dislocation density. In the low stress creep regime subgrain boundaries begin to form at low strains, however steady state subgrain structures are formed only after an extended interval of steady state creep. The steady state subgrain diameter is inversely proportional to the stress difference, and is largely independent of temperature, impurity composition and variations in the power-law stress exponent. Subgrain boundary migration at high strains maintains an essentially equant subgrain structure. At stresses around 50 MPa the strain necessary to develop subgrain structures increases markedly, and there is a transition to the high stress dislocation creep regime. At high strains in the low stress deformation regime, progressive subgrain rotation leads to the development of dynamically recrystallized microstructures. Slow boundary migration maintains a dominantly equant recrystallized grain structure. At high
85
temperatures rapid boundary migration produces dynamically recrystallized grains which initially have very low dislocation densities and are up to several times larger than rotation recrystallized grains. The marked stress dependence of steady state dislocation density, subgrain diameter, and recrystallized grainsize, together with the relative insensitivity of these microstructural parameters to temperature and impurity content, indicate that deformation microstructures in naturally deformed galena can provide sensitive indicators of stress histories in low temperature upper crustal deformation regimes.
86
PLAGIOCLASE DEFORMATION:
TEXTURES AND LATTICE ROTATION
J o r n H . Kruhl Institut fur Geowissenschaften der U n i v e r s i t a t Salzburg, Hellbrunner Strafe 34, A-5020 Salzburg, A u s t r i a .
Plagioclase (andesine/labradorite composition) from meta-gabbros of the Ivrea Zone (Southern A l p s ) has b e e n strongly deformed during prealpine medium greenschist to low amphibolite facies m e t a m o r p h i s m . K i n k b a n d s , deformation lamellae, mechanical twins, subgrains and recrystallized grains are developed. During kinkin g mostly (010) is the glide p l a n e , rarely (001). H o w e v e r , the orientation of the kink axes and the slip directions are influenced by the local shear and/or flattening direction. Deformation lamellae are to some extent related to m e c h a n i c a l albite and pericline twinning. In general, they are oriented in the zone (iOO) with maxima near (001) and (021). In addition to the albite and pericline law mechanical twins possibly occur as albite-carlsbad twins. The subgrains - and the recrystallized grain s, too - are developed b y glide processes rotating small domains of the *host' grains around distinct axes. In m o s t cases w i t h i n a single domain one glide system is dominant, mainly (010)/a/, (001)/a/, (010)/N^/ as w e l l as b - g l i d e on the rhombic section. Plagioclase deformed at rigid garnet or clinopyroxene clasts shows a local lattice rotation in relation to the compression d i r e c t i o n . A g a i n , it can be inferred that mostly only one glide system has b e e n active. The grain domains are continuously rotated around one distinct axis even if the relative position b e t w e e n the lattice and the compression direction becomes unfavourable for the active glide system. The angular velocit y determined on the basis of local strain and the rotational angle of the lattice is about 0.5° per 1% flattening. This v e l o c i t y seems to be independent of the relative p o s i t i o n b e t w e e n the compression direction and the active glide p l a n e . A similar value has b e e n recently obtained for naturally deformed quartz. In general, the type of rotation of plagioclase crystal domains during natural deformation leads to the conclusion that (i) a great v a r i e t y of glide systems may be active b u t only one is dominant in a distinct grain domain and (ii) strain variations on a micro-scale m i g h t play an important part for the development of preferred crystallographic orientations. Some of the recently published p r e f e r r e d plagioclase orientations may be explained in this w a y .
87
TRANSMITTED LIGHT MICROSCOPY OF DEFORMING CRYSTALLINE MATERIALS: PROGRESS AND PROSPECTS
W.D. Means State University of New York at Albany, 1400 Washington Avenue, Albany, N.Y. 12222, USA.
The past decade has seen a doubling every three years of papers on transmitted light microscopy of deforming crystalline materials. This new approach to microstructural problems offers the advantage that the microstructural history and the history of the displacement field become parts of the data, instead of parts of the interpretation as in more conventional experiments. Eight types of "see-through" apparatus have been described. They accommodate samples from 800 to 5 microns thick and provide plane deformations with strains of the order of 50% routinely. A stagemounted apparatus capable of pure or simple shearing at temperatures to 300°C is available commercially for a little over $1000 (US). Organic and inorganic starting materials have been employed, including camphor, napthalene, paradichlorobenzene, octachloropropane, norbornene, biphenyl, sodium nitrate, and ice. All of these develop microstructures similar (at least on the optical scale) to the familiar microstructures of silicate and carbonate rocks. A start has been made at measuring strain fields in recrystallizing polycrystals and in following lattice reorientation trajectories of grains. The role of recrystallization in development of grain shape foliation has received some study. Models for the direction of grain boundary migration have been generated and partly tested. Various phenomena have been observed closely for the first time, such as recrystallization without grain-size reduction, "grain migration", reduction of grain size by "dissection", reversal of the direction of grain boundary migration, static migration of subgrain boundaries, and "edge-wise propagation" of subgrain boundaries. There is also the appreciation gained by anyone who sees some of these experiments, of the transient character of much high-temperature microstructure and of the rapid recovery effects that can occur if stress is relaxed but temperature remains high. Teaching applications of the technique are obvious and have begun to be exploited. Opportunities for future work are numerous. There is a need for experiments on schist-like and granite-like two-phase materials, in their ductile, brittle, and intermediate regimes. Materials that undergo phase changes during deformation are clearly worth study, as are materials in which two phases react with each other in the solid state to produce porphyroblasts of a third phase. Experiments with a fluid or melt phase present are also attractive, as are experiments of any of the above types comparing the results of coaxial and noncoaxial strain accumulation. The first step in most of these new lines of research is discovery of suitable starting materials and means of fashioning them into thin samples.
88
A SIMULATION OF FABRIC DEVELOPMENT IN DYNAMICALLY RECRYSTALLIZING AGGREGATES
Mark Jessell Department of Geological Sciences, State University of New York at Albany, Albany, New York, 12222, USA.
Existing models of fabric development assume that a single process can account for the variety of measured rock fabrics. The most widely applicable models are based on the lattice rotations associated with crystal glide, and ignore the possible importance of dynamic recrystallization. There is increasing evidence that grain boundary migration can be crystallogrpahically controlled in deforming polycrystals, which would provide a competing fabric modifying process. The nature of this competition has been unclear. A new computer simulation of the development of grain shape and crystallographic preferred orientations is presented. The model combines homogeneous strains, simplified versions of the lattice rotations predicted by the Taylor-Bishop-Hill model, and recrystallization processes; and allows the progressive development of the fabric to be followed. Comparisons will be drawn between model fabrics and those measured in nature and experiments. By combining lattice rotations and dynamic recrystallization, this model is able to account for several common fabrics, and can also provide an explanation for fabric transitions associated with progressive deformation.
C>
mm]'
.00 7 INC
NO.O
2.85 INC
NO . 15
89 CONJUGATE OBLIQUE PLANAR FABRICS IN ROCK ANALOGUES - BY KINKING OR NOT BY KINKING?
P.O. Lennox Department of Applied Geology, School of Mines, U.N.S.W. P.O. Box 1, Kensington, Sydney, 2033.
Synthetic phlogopite-quartz mixtures have been deformed in a gas medium deformtion apparatus at high pore fluid pressures at temperatures of 550°C, confining pressures of 300 MPa and strain rates between 10"^ and lO'^sec"^. In all runs a microfabric consisting of the platy mineral component normal to the shortening direction develops. T.E.M. studies showed this microfabric consisted of grossly aligned submicron to micron-sized mica plates whose degree of alignment increased at higher degrees of shortening. This dominant micro-fabric is often cross-cut by a conjugate oblique planar fabric (C.O.P.F.). At low magnication using an optical microscope this fabric appears to be a pair of planar fabrics oriented approximately 45° either side of the dominant planar fabric. One oblique fabric is invariably better developed than the other. Near the resolving limits of an optical microscope this C.O.P.F. is seen to consist of submicron kink band boundaries surrounding elongated micron-sized blebs of kinked mica. These elongated blebs are oriented at approximately 45° to the dominant microfabric. In spite of the sand weight percentage varying between 5 and 23 in representative runs shortened up to 56% the C.O.P.F. is regularly developed. Thin section examination of these runs shows that the quartz grains are illustrated in different parts of the specimen. Even so across the whole thin section from quartz grain-rich to quartz grain-poor sections there is no discernible variation in the angular relationship of the C.O.P.F. to the shortening direction. This may indicate that fabric development reflects the finite strain experienced by the specimen rather than the localised heterogeneous strain field near each quartz grain. C.O.P.F. in the form of conjugate crenulation cleavages have been observed in rocks (Tobisch & Fiske, 1976, Tewksbury, 1986) and in the form of planar fabrics in soils (Brewer, 1964). In some rocks the m.ica within the cleavage is oriented oblique to the conjugate cleavage zones (Powell, 1982), whilst in other cases the micas are parallel to the conjugate cleavage zone (Cox, 1979). In both these cases the conjugate cleavage has formed so that the shortening direction bisects the obtuse bisector unlike these rock analogue experiments. The Cobbold et al. (1971) model for the development of conjugate normal kinks by compression normal to layers with a high degree of anisotropy does not yield angles between the kinked micas and the shortening direction consistent with those observed from these experiments. However, these angles may not be significant since studies in metals and ceramics indicate that conjugate shear zones develop at a variety of angles to the shortening direction (Backofen, 1972, Vardoulakis, 1984).
90
Paterson and Weiss's (1966) model involving shortening parallel to the phyllite foliation (cf. current experiments where shortening is normal to synthetic phlogopite fabric) results in mica basal planes oblique and kinks normal to the shortening direction after 50% shortening can be used to explain the observed microstructures. At moderate strains the C.O.P.F. is formed whilst at higher strains (>55%) in some specimens kink band boundaries and basal planes of synthetic phlogopite are observed oriented normal to the shortening direction. The following model of kinking could be envisaged to occur during the deformation of the synthetic phlogopite oxide-quartz mixtures. Soon after micron-sized plates of synthetic phlogopite are crystallized, submicron-sized conjugate kinks develop (especially in areas adjacent to quartz grains) with kink band boundaries migrating during further deformation resulting in collision and isolation of micron-sized blebs in conjugate sets containing kinked mica. The generation of conjugate micron-sized kinks, their boundary migration and subsequent collision, cannibalisation or growth at the expense of surrounding mica are continuing processes during the deformation of the specimens.
91
KINKS AND CRENULATION CLEAVAGE IN SIMPLE SHEAR EXPERIMENTS
P.F. Williams and G.P. Price CSIRO Division of Geomechanics, PO Box 54, Mt. Waverley, 3149, Australia.
Artificial schists composed of 70% KCl or NaCl and 30% mica have been deformed in simple shear to angular shear strains of 30°. Specimens are initially 50mm x 50mm x 60mm rectangular blocks and they have a rectilinear grid on 2mm centres stamped on the two sides perpendicular to the shear plane and containing the movement vector. This grid facilitates determination of local strain. All specimens were deformed at room temperature and the main series of experiments described in this paper was conducted on dry KCl "schist" at an angular shear strain rate of ca 2°/hour (average shortening rate 4.8 X 10"^ sec"^) and normal load of 20 MPa. The starting material for this series was strongly foliated and the initial orientation (9) of the foliation (SI) with respect to the shear plane was varied in 10° increments from G = 0° - 170° such that the line of intersection of the foliation and the shear plane was perpendicular to the movement vector. With respect to the instantaneous strain ellipse, angles e = 0° - 90° lie in the shortening field and angles 9 = 90° - 180° lie in the extensional field. Specimens with 9 values of 10° - 90° developed kinkbands during deformation. Between 9 = 20° and 9 = 70° the kinkbands form a conjugate pair with the most even development of the two orientations between 9 = 40° and 9 = 60°. For 9 = 10°, 80° and 90° there is only one orientation of kinkband and for a dextral bulk shear the kinks are sinistral for both the low and the high 9 values. Similarly, for 9 values where both orientations of kinkband are present, if one orientation is better developed than the other it is consistently the sinistral one. Specimens with 9 values between 110° and 170° developed crenulation cleavage during deformation. Considering dextral bulk shear the crenulation is steep, relative to the shear plane, and sinistral for low values of 9 and gently dipping and dextral for high values of 9. Both orientations and senses of shear are consistent with the crenulation cleavages being Reidel shears. Which of the two shear planes develops is dependant on the orientation of the original foliation (SI) but the orientations of the shear planes are otherwise insensitive to the orientation of SI. Specimens with 9 = 0 ° and 100° deformed without the development of any obvious structures except for a weak folding in the former. This reflects a 3% shortening parallel to the shear plane and a departure therefore from the intended experimental conditions. Analysis of this material indicates four deformation mechanisms: (1) slip on SI (2) bulk flattening orthogonal with Si (3) kinking (4) ductile faulting, i.e. development of crenulation cleavage. These are not totally independent mechanisms; kinking for example involves slip on SI; but they are mechanisms that differ sufficiently to be distinguished by the resultant structures and strain. Which mechanism
92
or mechanisms operate or dominate appears to be a complex function of their relative strengths, orientation of stress axes and constraints. For example, we can recognise two factors that influence the development of kinks. First, the conjugate kink orientations must bracket the finite extension direction and the kinkbands must have appropriate senses of shear. This limits their possible orientations. Second the orientation of SI must be suitable for kinkband development. A kink must be symmetrical or there are volume problems in the kinkband. Thus if a possible kink orientation is at 45° to SI a kinkband can form that will result in a large displacement while remaining symmetrical. If the potential kink plane is normal to SI any kinkband of that orientation will immediately have volume or strain compatibility problems and is therefore inhibited.
93
DUCTILE DEFORMATION OF AMPHIBOLE IN THE FLORENCE SHEAR ZONE, SOUTHERN HARTS RANGE, CENTRAL AUSTRALIA
Y. Liu and P.R. James Department of Geology and Geophysics, University of Adelaide, S.A. 5000.
Amphibole minerals usually exhibit brittle deformation behaviour in low temperature mylonites. Observation of microstructural features on some mafic mylonites collected in the Florence Shear Zone of central Australia, has revealed that the amphibole grains within the mylonites display extensive ductile behaviour which is characterised by intensely-strained, fracture free amphiboles with pronounced intracrystalline strain features and a strong crystallographic preferred orientation. The preferred orientation figures presented, along with the observation and measurement of deformation bands and mechanical twins which are parallel or subparallel to the (100) [001] planes of amphibole crystals, suggest that these grains have rotated into parallelism with the shear plane for easy slip during mylonitization, resulting in geometrical softening. The factors responsible for the ductile behaviour of amphibole have been explored and discussed. High temperatures during the mylonitization played an important role in enhancing the ductility of amphiboles as well as high confining pressure and a minor contribution of geometrical softening. Chemical softening may also have an effect on the ductile behaviour of amphiboles, as a result of the formation of new minerals along the grain margins and intracrystalline deformation boundaries.
94
REACTIVATION OF EARLIER FOLIATIONS AND DECRENULATION DUE TO SHIFTING PATTERNS OF DEFORMATION PARTITIONING
T.H. Bell Department of Geology, James Cook University, Townsville, Queensland, 4811, Australia.
Reactivation of early foliations accounts for much of the progressive strain at more advanced stages of deformation. Its role has generally gone unrecognized because evidence is best preserved only where porphyroblasts containing inclusion trails are present. Reactivation occurs when progressive shearing, operating in a synthetic anastomosing fashion parallel to the axial planes of folds, changes to a combination of coarse and fine scale zones of progressive shearing, some of which operate antithetically relative to the bulk shear on a fold limb. Reactivation of earlier foliations occurs in these latter zones. Reactivation decrenulates pre-existing or just-formed crenulations generating shearing along the decrenulated or rotated pre-existing foliation planes. Partitioning of deformation within these foliation planes, such that phyllosilicates and/or graphite take up progressive shearing strain and other minerals accommodate progressive shortening strain, causes dissolution of the latter minerals. This results in concentration of the phyllosilicates in a similar, but more penetrative manner, to the formation of a differentiated crenulation cleavage, except that the foliation can form or intensify on a fold limb at a considerable angle to the axial plane of synchronous macroscopic folds. Reactivation can generate bedding-parallel schistosity in multideformed and metamorphosed terrains without associated folds. Heterogeneous reactivation of bedding generates rootless intrafolial folds with sigmoidal axial planes from formerly through-going structures. Reactivation causes refraction of axial-plane foliations in those beds or zones in which an earlier foliation has been reactivated and results in destruction of the originally axial-plane foliation at high strains. Reactivation also provides a simple explanation for the apparently "wrong sense", but normally observed "rotation" of garnet porphyroblasts, whereby the external foliation has undergone rotation due to antithetic shear on the reactivated foliation. Alternatively, the rotation of the external foliation can be due to its reactivation in a subsequent deformation event. Porphyroblasts with inclusion trails commonly preserve evidence of reactivation of earlier foliations and can therefore be used to identify the presence of a deformation that has not been recognized by normal geometric methods, because of penetrative reactivation. Reactivation commonly reverses the asymmetry between pre-existing foliations and bedding on one limb of a later fold leading to problems in the geometric analysis of an area when the location of early fold hinges is essential. The stretching lineation in a reactivated folition can be radically reoriented, potentially causing major errors in determining movement directions in mylonitic schistosities on folded thrusts.
95
Geometric relationships resulting from reactivation of foliations around porphyroblasts can be used to aid timing of the porphyroblasts relative to deformation events. However, other aspects of reactivation can lead to complications in timing of porphyroblasts if the presence of this phenomenon is not recognized; for example, D2grown porphyroblasts may be dissolved against reactivated SI and hence appear to have grown syn Dl.
96
ROTATIONAL DEFORMATION IN THE ALPINE 'ROOT ZONE' (WESTERN ALPS) AND ITS BEARING ON PLAGIOCLASE AND QUARTZ FABRICS
Jorn H. Kruhl Institut fur Geowissenschaften der Universitat Salzburg, Hellbrunner Strafe 34, A-5020 Salzburg, Austria. Previous studies in the *Root Zone' of the Western Alps (west of Lago Maggiore, N. Italy) have shown that a mainly rotational deformation occurred under alpine greenschist and amphibolite facies conditions, first, during increasing and later during decreasing temperatures, causing steeply NW dipping cleavage planes and lineations of variable orientation. Since the deformation is inhomogeneous textures from different metamorphic stages are preserved. Especially plagioclase and quartz textures and preferred orientations reflect the details of the deformation history. During an early low temperature stage kinking of quartz produced caxis double maxima patterns. Large magmatic plagioclase grains are rotated with their flat (010)-sides parallel to the plane of flattening. At increased temperatures the a directions of recrystallized plagioclase grains show a steep plunging on the first schistosity parallel to a rarely developed preferred dimensional orientation of micas. (010) poles form a broad incomplete girdle around a and (001) poles a maximum vertical to the plane of flattening (Fig. 1). These patterns are interpreted as the result of plane strain conditions during a first alpine deformation event. During subsequent stages of deformation under medium amphibolite to upper greenschist facies conditions these plagioclase patterns are rotated around the approximately horizontal intersection line of early and later schistosity planes. The orientation of the quartz fabrics differs significantly from that of the plagioclase fabrics. They are not related to the mica stretching but to the intersection line of the schistosites: Quartz c-axes show crossed-girdles approximately perpendicular to it indicating the quartz stretching being parallel to the intersection line. Obliquities of maxima against the pole of the schistosity may be related to the sense of rotation around the intersection line inferred from macro- and micro-textures. Specimens with a moderately plunging stretching additionally show c-axis crossed-girdle patterns with one strongly developed branch indicating a shear parallel to the stretching. These two asymmetries may be interpreted as resulting either from two subsequent shear events or from two shear components of the same deformation event. During the last stage of deformation during decreasing greenschist facies temperatures rarely a subvertical stretching (micas) is developed. Asymmetric c-axis patterns point to mainly dextral shear parallel to the stretching (viewed to the NE). Although horizontal or moderately plunging lineations dominate the picture of the Alpine *Root Zone' west of Lago Maggiore most of the shearing in this zone is interpreted to be directed about vertically, in particular during the last stage of deformation and metamorphism. Probably these movements are related to the uplift of the Central Alpine block in relation to the Southern Alps.
97
Moreover, the study of plagioclase fabrics seems to be a powerful method to get information especially on the early (and hightemperature) stages of deformation history.
n=231
quartz- c
Fig. 1. Plagioclase and quartz fabrics from sample 2818. The plane of projection is the plane of flattening. Plagioclase measurements are from the three sections parallel and vertical to the sl/s2intersection line and parallel to the plane of flattening, quartz measurements are from the sections 1 and 2. Equal area projection.
98
CURVED VEIN FIBRES - AN ALTERNATIVE EXPLANATION
P.F. Williams^ and J.L. Urai^ ^ Dept. Geology, U.N.B., Fredericton, N.B., Canada E3B 5A3 ^ I.V.A. Rijksuniversiteit, Utrecht, The Netherlands. Veins occurring at the edge of the dextral Indian Islands Fault (Newfoundland) are folded in response to a fault-parallel, ductile shear. The veins, which locally constitute a penetrative fabric element at outcrop scale, are composed mainly of calcite and have a narrow rim of quartz and chlorite. Both the calcite and quartz are generally fibrous and lattice distortion in the calcite is not commensurate with the observed degree of fibre curvature. This observation supports the popular view that curved fibres grow curved as they track the vein opening vector. It is demonstrated however, that the curvature, in the material described here, is due to deformation and that the lack of lattice distortion is due to recrystallisation. A l l observations are consistent with both the calcite and the quartz fibres having grown perpendicular to the vein walls. The chlorite grains in the vein have grown with their long dimension (the trace of (001)) inclined to the length of the quartz fibres and their orientation is controlled by the orientation of micas that define a pre-vein cleavage (SI) in the country rock. In some veins markers make it possible to define the net opening direction and, in a l l examples, it is demonstrated that the direction has not been tracked by the fibres. Since this conclusion may be more generally applicable, caution should be exercised in interpreting kinematics on the basis of fibre geometry. Where vein density is high, the veins and country rock screens separating them from a multilayer sequence that is folded into fairly harmonic folds. The pre-vein cleavage (SI) has the appearance of a convergently fanning axial plane foliation, when the fold profile is viewed, but it is inclined to the fold axis. The folds therefore have the morphology of cleavage transected folds b u t , despite the fact that the cleavage satisfies a l l the geometric criteria for being contemporary with folding, it is demonstrated that the cleavage is older than both the veins and the folds.
99
DEFORMATION PARTITIONING, SHEAR ZONE DEVELOPMENT AND THE ROLE OF UNDEFORMABLE OBJECTS
T.H. Bell^, A.C. Duncan^ andJ.V. Simmons^ ^ Department of Geology, James Cook University, Townsville, Queensland, 4811, Australia. ^ Department of Civil and Systems Engineering, James Cook University, Queensland, 4811, Australia.
The formation of a mylonite zone by strictly progressive simple shear alone is generally impossible because the deformation will always partition. However, in a zone of rock undergoing bulk simple shear the presence of rigid bodies spreads and homogenizes the stress field such that the instantaneous strain field is essentially homogeneous about the rigid body at a scale approximately four times the diameter of the rigid object. Therefore, that portion of the rock deforms by strictly progressive simple shear and the rigid object is forced to rotate. This has considerable implications for rotation of unstrained porphyroblasts and porphyroclasts and what they tell us about the sense of shear as well as the deformation history that the rocks have undergone. Superficially similar matrix porphyroclast geometric relationships can indicate exactly the opposite sense of shear leading to considerable problems. It also has implications for the origin of blueschist minerals preserved in garnet porphyroblasts, the respective roles of strain softening and hardening during mylonitization due to the destruction of feldspar porphyroclasts and the growth of garnet porphyroblasts respectively.
100
GROWTH OF PORPHYROBLASTS RELATIVE TO PROGRESSIVE DEFORMATION, TEMPERATURE INCREASE AND TIME DURING PROGRADE METAMORPHISM
J. Reinhardt and M.J. Rubenach Department of Geology, James Cook University of North Queensland, Townsville, Queensland, 4811.
Sequential growth of different porphyroblasts in any particular zone in relation to progressive foliation development can be demonstrated by comparison of their inclusion trails (S.) to the external foliation (S ) (Bell and Rubenach, 1983). The interrelationships of progressive deformation, temperature, time and porphyroblast growth in an orogenic zone can be further examined by comparing S. and S^ for a particular mineral from its isograd through higher grade zones. Complicating factors include the heterogeneity of strain in space and time, and reactivation of existing foliations. However the exercise may be statistically meaningful if a large number of samples are examined, particularly those where S^ has reached an advanced stage of crenulation cleavage development. In two recently studied Proterozoic terrains with different metamorphic characteristics and different tectonic histories, a microstruetural feature has been observed which cannot be simply explained by heterogeneous strain. In the schists of the Corella Formation (Rosebud Syncline, northwest Queensland) and in the Robertson River Formation (north-central Queensland), the foliations preserved as inclusion trails in porphyroblasts are on average in a more * advanced' deformation stage in lower-grade zones than they are in the same porphyroblast species in higher-grade zones. In the Corella schists, the S^ inclusion trails within the syn-D^ andulusites are either straight or show incipient crenulation in the higher-grade zones, whereas sharp bends of S^ in the outer porphyroblast rims, millipede structures, etc. are typically encountered in lower-grade zones. Thus, the D^ imprint preserved in the andalusites is strongest in the lowest-grade zones. In the Robertson River Formation, porphyroblasts overprint the latest stage of foliation development at their respective isograds, but commencing immediately upgrade of their isograds they overprint quite early stages. (For example, mineral X, Fig. 1). Such microstructures may result from a low rate of temperature increase relative to foliation development. However, reaction and growth rates must be relatively rapid, as in general a particular porphyroblast mineral preserves one rather than several stages of foliation development in the one specimen. Reference: Bell, T.H. and Rubenach, M.J. 1983.
Tectonophysics 92: 171-194.
101
Temp. (T)
^
j THERMAL RE-EQUILIBRATION 1
DEFORMATION
& UPLIFT (EARLY)...(LATE) (POST)
15V
— ^ ^ ^ 11 1iI
i
—
1
Time (t)
Fig. 1. Model of temperature-time relations during homogeneous crustal thickening. Schematic T-t curves for localities A, B, C in zones of different metamorphic grade. Mineral X has grown syndeformational at A (during t^) and B (during t^), and postdeformational at C.
102
LACK OF PORPHYROBLAST ROTATION IN NON-COAXIALLY DEFORMED SCHISTS FROM PETREL COVE, SOUTH AUSTRALIA, AND ITS IMPLICATIONS
C. Steinhardt Department of Geology, James Cook University of North Queensland, Townsville, Queensland, 4811, Australia.
Inclusion trails representing an S^ cleavage demonstrate the lack of porphyroblast rotation during subsequent highly non-coaxial deformations. The pelitic schists of the Kanmantoo Group at Petrel Cove, South Australia contain two generations of porphyroblasts. The first one consists of cordierite porphyroblasts that formed early in D and contain straight to slightly sigmoidal inclusion trails of S^. The second generation consists of andalusite porphyroblasts that overgrew crenulated S^ late during D^. Several hundred inclusion trail traces from cordierites measured from oriented specimens taken throughout a strongly folded area show a horizontal great circle distribution when plotted and contoured on a stereographic projection. Hence, S^ was planar and horizontal prior to D^. S^ measurements on limbs and hinges of a mesoscale D^ foldpair show that folding had no effect on porphyroblast orientation as the S^ orientation remained constant and subhorizontal around the fold. Hence, porphyroblasts have not rotated during any of the non-coaxial deformations accompanying and following their growth. This is interpreted as a result of the partitioning of the deformation around them. S in the matrix has been totally destroyed by the formation of S^ as a fully differentiated crenulation cleavage. What has previously been regarded as inconsistent senses of shear recorded by porphyroblasts around folds is resolved by the fact that the matrix foliation rotated rather than the porphyroblasts due to the effects of deformation partitioning. The presence of a subhorizontal S^ foliation suggests horizontal movements (e.g. thrusting or detachment faulting) during the earliest phase of the Adelaidian orogeny.
103
EVIDENCE OF SYNDEFORMATIONAL CONTACT METAMORPHISM FROM PORPHYROBLASTMATRIX MICROSTRUCTURAL RELATIONSHIPS
R.H. Vernon School of Earth Sciences, Macquarie University, Sydney, NSW, 2109, Australia.
Cordierite porphyroblasts in three contact metamorphic aureoles in SE Australia show microstructural evidence of having grown during the development of a crenulation foliation. The evidence includes curvature of straight inclusion trails into crenulated matrix folia, and deflection of crenulation folia (axial surfaces) around porphyroblasts. In some rocks, coarsening has obliterated evidence of crenulations in the matrix, owing to heating having outlasted deformation. The crenulations are confined to the contact aureoles of granitoid plutons that have not undergone penetrative tectonic deformation, and even plutons in areas that have undergone no postintrusion deformation. Therefore, the crenulations appear to have resulted from local deformation caused by the intrusion.
104
KINEMATIC MODEL OF THE BIRMINGHAM ANTICLINORIUM
Richard H. Groshong Jr. and Daniel J. Patterson Dept. of Geology, The University of Alabama, Tuscaloosa, AL 35487-1945
The Birmingham anticlinorium is a first-order fold system in the southern Appalachian Valley and Ridge fold and thrust province. It is approximately 20 km wide, extends about 160 km along strike, and has a maximum structural relief of 2.6 km. The anticlinorium consists, from foreland to hinterland, of a newly recognized gently dipping forelimb; a narrow, overturned sequence associated with a major thrust fault, the Opossum Valley fault; a flat crest; another significant thrust fault, the Jones Valley fault; and a long, gently dipping backlimb called the Cahaba synclinorium (see Figure). Internal strain within beds is negligible except within the overturned sequence.
A balanced and restorable cross section for a transect through the vicinity of Birmingham, Alabama has been developed using an analytical fault-bend fold model after Suppe (1983). Fault-bend folding is suggested by the dip-domain geometry of the anticlinorium, the clearly thin-skinned, flat-bottomed syncline in the adjacent foreland, and by the regional prevalence of wide, gently dipping backlimbs interpreted to overlie major low-angle ramps. A dip isopleth map, contouring areas of equal dip, indicates that individual dip domains occur over distances along strike of at least 50 to 60 km, though they are interrupted locally by smaller-scale structures. The analytical fault- bend fold model is quite rigorous; so far only one solution has been obtained that closely satisfies the surface geometry. Dipspectral analysis indicates that the fundamental cut-off angle is 8°; the presence of third-order dips indicates a minimum of three superimposed imbrications. The exposed thrusts are interpreted to root in the backlimb, explaining the second- and third-order dips found there. The major structural relief and the gently dipping forelimb are attributed to two blind thrusts that root in an older basement-involved graben beneath the Cahaba synclinorium. A higher cut-off angle, 16°, for the ramp of one of these thrusts helps explain high-order dips in that area. The kinematic solution implies that the Opossum Valley fault is later than the subjacent blind thrust, so that the fault sequence is at least in part a break-back sequence. This interpretation rests upon the assumption that hinges related to deepseated fault bends must penetrate the entire overlying sequence. If
105
the mechanical properties of the sequence prevent this, the Opossum Valley fault could be a break-forward thrust. The total transport on the four thrusts is a minimum of 40 km, giving a shortening of 72% across the anticlinorium. Reference: Suppe, J. 1983. Geometry and kinematics of fault-bend folding: Jour. Sci., V. 283, p. 684-721.
Am.
106
THE MULLER ANTICLINE, PAPUA NEW GUINEA; BASEMENT-CORED, INVERTED EXTENSIONAL FAULT STRUCTURES WITH OPPOSITE VERGENCE
Kevin C. Hill Department of Geology, University of Melbourne, Parkville, Victoria, 3052, Australia.
Throughout the NW-SE trending Papuan Fold-Belt, two dominant structural styles are present. In the NE the multiple imbricate 1 kmthick-slices of Miocene limestone attest to the classical thin-skinned nature of the deformation. In the SW adjacent to the undeformed foreland, structures are 1 or 2 orders of magnitude greater in size, are strongly asymmetrical and commonly have relatively thicker stratigraphic sections on the crest. Such structures are interpreted as Mesozoic to Palaeogene extensional faults, along the original NE rift margin of the Australian continent, that were inverted during the Mio-Pliocene compression. Regional traverses from the undeformed foreland heading NE across the 150 km by 50 km Muller Range in western PNG provided detailed surface structural data. When combined with analysis of existing geological maps, exploration wells, regional gravity surveys and extensive seismic in the foreland, moderately constrained balanced and restorable cross-sections through the Muller Anticline could be constructed. Gravity data show the Muller Anticline to be cored by basement and seismic data extrapolated beneath the mountains show the outcrop of basement in the centre of the Muller Range to be elevated 8 kms above regional. The Eastern Muller Anticline has a shallow NE limb and steeper SW limb indicating an original NE-dipping extensional fault (a) such that basement was subsequently thrust to the SW (b) The Cecilia, Wai Asi, Juha and Lavani anticlines to the SW of the Muller anticline are interpreted to be due to shortening of the whole sedimentary section to balance the shortening in basement. There are no such structures to the SW of the Western Muller Anticline where the SW limb is gentle dipping and the NE limb is steep. Hence, the Western Muller Anticline is interpreted as having originally been a SW dipping extensional fault (a) along which basement was subsequently thrust to the NE (b). The lateral ramp separating Eastern and Western Muller Anticline coincides with the NW end of the Cecilia and Lavani anticlines and was originally a transfer zone between the 2 extensional faults. At the SE end of the Muller Range there is a major lateral ramp in basement causing a drop in elevation of all the structures into the Tari Basin. Associated with this lateral ramp are further structures detached at top basement involving the entire sedimentary section. These, too, may have originally been roll-over anticlines above extensional faults. As behind (northeast of) the Muller Anticline, the Darai duplex is exposed northeast of these Tari Basin structures.
107
TRANSFER ZONE
a) EXTENSION
TEAR FAULT
s y y
b) INVERSION
MULLER RANGE BASEMENT DEFORMATION SKETCHES TO SHOW EXTENSION & INVERSION
108
STRUCTURAL ANALYSIS OF THE CARRARA MARBLE IN THE ORTO DI DONNA AREA (APUAN ALPS, NORTHERN APPENINES - ITALY). M. Coli and L. Matteini Dipartimento di Scienze della Terra, Universita degli Studi di Firenze, Via G. La Pira 4 - 50121 Firenze, Italy.
A structural map at 1:2000 scale has been carried out in the Carrara Marble of the Orto di Donna area. The mesostructural analysis was followed by micro-structural observations (TOM) on oriented samples. At Orto di Donna different lithotypes are recognizable in the Carrara Marble: marble with schists, with abundant phyllosilicates in the main schistosity; marble with dolomite, with levels of dolomite in the schistosity; veined marble, which can be further subdivided into strong, normal and weak veined marble, with dolomite and second-phase particles in the schistosity (within the veined marble there are levels of deformed - flattening - marble breccias and of white marble); "Bardiglio", homogeneously grey marble with a higher % of quartz, oxides and iron sulphides. Each lithotype shows a different average grain size (<0.03 to 0.4 mm) and a different texture. Regions with larger crystals of calcite (0.5 to 1 mm) lay in the main schistosity and are partially elongated parallel to X; here grain boundaries are strongly irregular and sutured, and twinning and gliding surfaces are also present. Grain boundaries are straight in the marble with schists (with mica control on size dimension - average 0.05 mm - and orientation of the calcite) and become gently curved being concave outwards in the veined marble, while they become polygonal in the Bardiglio. When the average grain size is larger (0.2 to 0.4 mm) smaller grains (<0.1 mm) have polygonal boundaries; relict of twinning and gliding surfaces can be observed in grains with size ranging from 0.1 to 0.4 mm. The main structural surfaces recognizable in the marble appear to have a strict relation with the principal planes of finite strain obtained from previous studies carried out in the whole Northern Apuan Alps. Those studies revealed the presence of three syn-metamorphic and schistogenic fold generations interfering at every scale. Marble lithotypes depict an isoclinal antiform which fits well with the structural framework of the whole Northern Apuan Alps. The microstructural observations allowed to recognize different stages of crystalline deformation. A hypothesis of deformation history, during which processes such as recovery, primary and secondary recrystallization, new grain growth and inclusion inhibition where acting, has been put forward. Finally, an attempt has been made to correlate lithotypes, deformation history, structural pattern and sedimentary heredity.
109
IDENTIFICATION OF A MAJOR CRUSTAL SHEAR ZONE, NORTHWEST GAWLER CRATON, SOUTH AUSTRALIA
L.R. Rankin, A.R. Martin and A.J. Parker S.A. Dept. Mines and Energy, P.O. Box 151, Eastwood, Sth. Australia, 5063.
The Karari Fault Zone is a major linear tectonic feature defining the northwestern margin of the Gawler Craton. The fault zone is defined by a pronounced linear aeromagnetic anomaly, the magnitude and form of which in the Tallaringa-Ooldea region suggest that it is reflecting a deep, steeply-dipping, dyke-like body; either a major basic/ultrabasic intrusion or a magnetic shear zone up to 7 km wide and 300 km long. Field mapping and drilling indicate that the fault zone separates amphibolite-granulite facies gneisses of the Mulgathing Complex in the southeast from relatively unknown Precambrian basement covered by up to 1 000 m of Palaeozoic sediments of the Officer Basin to the northwest. The S.A. Geological Survey drilled two stratigraphic holes in 1985 to examine the fault zone and identify the source of the aeromagnetic anomaly. Ooldea 2 was sited to intersect a known occurrence of quartz + sapphirine-bearing gneisses northwest of the fault zone, and intersected a sequence of quartz + feldspar + garnet gneisses with minor quartz + feldspar + hypersthene granulite and 2pyroxene granulites. Ooldea 3 was sited to intersect the major aeromagnetic anomaly associated with the fault zone. This hole intersected a major mylonite zone, with recovery of almost 300 m of metasedimentary gneisses, iron formation and pegmatites with heterogeneous development of a mylonitic fabric. The dominant lithologies intersected by Ooldea 3 were quartz + feldspar + biotite + sillimanite + garnet gneiss and well-layered quartz + magnetite gneiss with variable feldspar content, representing a sequence of upper amphibolite facies semi-pelitic and banded iron formation bearing metasediments. Interlayered with the metasediments are minor bands of intrusive pegmatite and amphibolite. The amphibolites exhibit a weak mylonitic fabric, while the pegmatites typically exhibit an intense, anastomosing mylonitic foliation (Sm) intersected by common microshear planes oblique to Sm. Type II S-C mylonite fabrics are common within quartz + feldspar gneisses which have undergone intense sericitisation during mylonitisation. All kinematic indicators, including asymmetric megacrysts, elongation lineations, S-C planes and microfabric development indicate subvertical movement of the fault zone during mylonite development, with downthrow of the northwestern block. The timing of this episode of ductile/brittle shearing is interpreted as being equivalent to the D^ deformation of the Early Proterozoic Kimban Orogeny; the deformation responsible for development of the Kalinjala Mylonite Zone on the coast of Eyre Peninsula. Later brittle faulting occurred along the Karari Fault Zone during the Palaeozoic and Tertiary at shallow crustal levels. During these later movements, the northwestern block was downthrown, with deposition of a thick sequence of clastic sediments in the Officer Basin.
110
THE RELATIVE TIMING OF TECTONIC OVERPRINTS IN THE IRREGULLY FORMATION, BANGEMALL BASIN, WESTERN AUSTRALIA
C.N. Winsor Geology Department, The University of Western Australia, Nedlands, Western Australia, 60089.
Carbonates are very responsive to stress under low temperature/strain conditions and thus are readily affected by vertical and horizontal compressional, or extensional forces. Dolostones and orthoquartzites in the Proterozoic Irregully Formation, Bangemall Basin, Western Australia, have undergone a complex deformation history involving recurrent pressure solution, buckling, dilation, shear and cataclasis. The order of tectonic events and their macroscopic relation has been reconstructed through petrographic examination of pressure responses, comprising overprints in the tensional (TSR) and compressional (CSR) stress regimes (Logan, 1984). The Irregully Formation is well exposed in Irregully Gorge, where there is macroscopic evidence of slightly asymmetrical, localized folding (F^) about a gently, northwest plunging axis. Two other folding episodes have resulted in localized mesoscopic buckles. The interpreted history of events affecting the Formation after sedimentation comprise eight overprints, within a paragenetic sequence related spatially and in time to macro- and mesofolding. These events are: (1)
TSR overprint, development of fine dolomite veins parallel and normal to bedding. Upward and lateral movement of soluble material.
(2)
CSR overprint (vertical stylolite peaks), dissolution via overburden pressure possibly accompanying dolomitization. Further upward and lateral movement of soluble material.
(3)
Weakly developed buckling, cataclasis, CSR development (horizontal peaks) and related extensional vein (TSR) formation. Upward and lateral movement of soluble material. Localized silicification.
(4)
Second buckling event, macro- and mesobuckling, cataclasis, CSR development (horizontal peaks) and related development of extensional veins (TSR).
(5)
Third localized buckling event and associated shear, cataclasis, minor CSR and extension veins (TSR) formation.
(6)
CSR development (vertical peaks) dissolution via overburden pressure.
(7)
TSR event, dilation in particular horizons, producing secondary porosity.
(8)
Uplift resulting in the development of joints following inherent weaknesses.
Ill
The method used to establish the deformation history should provide a valuable procedure for mineral and hydrocarbon exploration in weakly deformed carbonate terrains, providing information relevant to the timing and location of these deposits. Reference: Logan, B.W. 1984. Pressure responses (deformation) in carbonate sediments and rocks Analysis and application. Canning Basin. In: The Canning Basin W.A. (P.G. Purcell, editor) Geol. Soc. Aust./Pet. Expl. Soc. Aust. Symposium, Perth: 235-251.
112
PSEUDOSTRATIGRAPHY AND THRUSTING AT JOMA MINE, NORWAY
Brian Marshall^, N.E. Odling^, A. Reinsbakken^ and F.M. Vokes^ ^ Department of Applied Geology, New South Wales Institute of Technology, P.O. Box 123, Broadway, 2007, Australia. ^ Geologisk Institutt, 7034 Trondheim - NTH, Norway.
Joma Mine is situated some 250 km NE of Trondheim in the Grong district of Norway, close to the Swedish border. It is one of a group of mines which exploits bodies of stratiform Cu/Zn mineralization within Lower Palaeozoic nappes of the Norwegian Caledonides. The Joma orebody is hosted by the central member of three metabasites intercalated with phyllite, chloritic schist and albite laminite, within the Silurian Leipikvatnet nappe. The host rocks have undergone two main periods of deformation: the principal event (D^) produced isoclinal folds with shallow SW-dipping hinge surfaces and NW-trending hingelines, a penetrative cleavage/schistosity, and a mineral lineation; the next event (D^) generated open to tight folds with steep NW-dipping hinge surfaces and NE-trending hingelines, and a variable penetrative crenulation fabric. A regional D^ fold extending NE into Sweden is termed the Joma Synform. Late-stage kinking/crenulation (D^) exists, and there is limited evidence for a pre-cleavage/schistosity event (D^). Regional stratigraphic inversion (based on deformed pillows - Kollung, 1979) and minescale inversion (based on "sulphide stratigraphy" Olsen, 1980), both ascribed to D, are not easily reconciled with regional and minescale vergence observations (Odling, unpublished data). This disparity is ascribed to the wrongful application of "sulphide stratigraphy" to an orebody now comprising a succession of thin (- 1 m) thrust slices. An event-sequence, essentially consistent with that to the E in Sweden, is in Table 1. Evidence for pre- or early syn-D^ thrusting is provided by: ore pseudostratigraphy varying rapidly between, and sometimes within, levels; knife-edge ore/silicate contacts truncating bedding and banding in silicate and sulphide rocks respectively; lenticularity of layering in intercalations of sulphides and silicates (duplex effects); retrogression of metabasites adjacent to pyritic massive sulphide layers; ductile segregation of pyrrhotite and development of durchbewegt ore along some sulphide/silicate interfaces; and many of these relationships (including pseudostratigraphy) being form surface to D^ folds and overprinted by D^ cleavage/schistosity. In essence, most sulphide/silicate and sulphide/sulphide contacts are tectonic. For much of their exposed length, the thrusts follow bedding-controlled flats before climbing across sequence on^shallow ramps. Such braided flat and ramp combinations lead to multiple duplex structures and result in the ore layer comprising a complex stack of thrust slices. Evidence for small scale D^ thrusting is provided by tracking D^ folds from metabasite into the ore layer. Folding is accommodated in highly ductile marginal sulphide layers and does not transmit further into the ore across sulphide/sulphide layer interfaces; hinge surfaces flatten and hingelines rotate towards the transport direction; fabrics akin to those in S-C mylonites (type 2) are developed in sulphides with thin silicate interlaminations; and ductile segregation of
113
pyrrhotite and developments of durchbewegt ore again characterize sulphide/silicate interfaces. Transport distances are on the order of a few metres and have not significantly contributed to the development of pseudostratigraphy, although some reactivation of pre-existing thrusts probably occurred. References: Kollung, S. 1979. Stratigraphy and major structures of the Grong District, Nord-Troendelag. Norges geologiske undersokelse, 354, 1-51. Olsen, J. 1980. Genesis of the Joma stratiform sulfide deposit, central Norwegian Caledonides. Proc. Fifth Quadrennial lAGOD Symposium E. Schweizerbart'sche Verlagsbuchhandlung, Stuttgart, pp. 745-757.
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DEFORMATION
TABLE
EVENT
D2
SEQUENCES
COMPONENT
-
MESOSCALE
EVIDENCE
SUPPORT
EVENT
(a)
Isoclinal, large amplitude folding.
None at mesoscale
(b)
Thrusting along and shallowly oblique to S^ forming imbricate stacks.
Strong but D^ age not proved.
(a)
As in (b) above alternatives.
(b)
Tight to isoclinal folding with short limbs exceeding 30 m. in level scale folds.
Very strong
(c)
Thrusting along old and newly formed surfaces.
Strong
(a)
Folding with close to isoclinal styles and moderate to shallow NW dipping hinge surfaces.
Very strong
(b)
Thrusting along old (?) and newly formed surfaces.
Strong
(c)
Folding with open to close styles and moderate to steep SE dipping hinge surfaces.
Limited
(d)
Relaxation faulting and development of dilation veins - NW dips for both structures.
Weak
(a)
As above in (d) alternatives.
Weak
(b)
Folding with open style and shallow SE dipping hinge surfaces.
-
(b) and (a) are
Strong for pre-S2
D3
—
(d) and (a) are
Very weak
115
TABLE
1
DEFORMATION
COMPONENT
EVENT
D2
SEQUENCES
-
MESOSCALE
EVIDENCE
SUPPORT
EVENT
(a)
Isoclinal, large amplitude folding.
None at mesoscale
(b)
Thrusting along and shallowly oblique to S^ forming imbricate stacks.
Strong but D^ age not proved.
(a)
As in (b) above alternatives.
(b)
Tight to isoclinal folding with short limbs exceeding 30 m. in level scale folds.
Very strong
Thrusting along old and newly formed surfaces.
Strong
(a)
Folding with close to isoclinal styles and moderate to shallow NW dipping hinge surfaces.
Very strong
(b)
Thrusting along old surfaces.
Strong
(c)
-
(b) and (a) are
(?) and newly formed
Strong for pre-S2
D3 (c)
Folding with open to close styles and moderate to steep SE dipping hinge surfaces.
Limited
(d)
Relaxation faulting and development of dilation veins - NW dips for both structures.
Weak
(a)
As above in (d) alternatives.
Weak
(b)
Folding with open style and shallow SE dipping hinge surfaces.
—
(d) and
(a) are
Very weak
116
MINERALOGICAL/CHEMICAL CHANGES DURING MYLONITE GENESIS IN I-TYPE PLUTONS, CENTRAL SIERRA NEVADA, CALIFORNIA: A PRELIMINARY REPORT
Othmar T. Tobisch and Scott R. Paterson Earth Sciences Board, University of California Santa Cruz, California, 95064, USA.
A suite of quartz diorites of presumed I-type intrude low grade metamorphic rocks in the western metamorphic belt of the central Sierra Nevada. Timing of emplacement varies from probable pretectonic through syn- to post-tectonic. One of the larger of these bodies, a probable pretectonic composite pluton varying in composition from hornblende quartz diorite to biotite quartz diorite with local appearance of granodiorite, shows a tectonic foliation (Dl) which varies from very weak to moderately strong. Subsequent shear zones are present, and locally develop into intense mylonite fabrics which are oriented essentially parallel to the tectonic foliation. Development of the main (Dl) foliation usually is a function of the deformation of quartz, which manifests itself as plastic deformation (initially as undulose extinction), and evolves into ribbon structures with widespread polygonization forming in the more intensely deformed specimens. Feldspars behave largely as rigid bodies, exhibiting sericite or epidote alteration, undulatory extinction, locally bent or microfaulted twins, and break up at higher strains. Hornblende and biotite often polygonize or may alter to actinolite or secondary biotite/chlorite as the foliation intensifies. These changes can be observed in all stages of development. S-C mylonite fabrics, common in many deformed plutons in other terranes, are poorly developed to absent in these rocks. Instead, shear zones develop in the foliated plutons along which substantial chemical/mineralogical changes have taken place. These structures first appear as minute zones (<1 mm wide) separating lozenges of rock showing only (Dl) foliation. As the strain in the zones intensifies, they widen and locally encompass the whole rock over areas as wide as 2-3 meters or more. Chemical/mineralogical changes that accompany this later deformation are often extreme, strongly suggesting the deformation was accompanied by high fluid activity. Changes observed indicate partial to nearly complete removal of alkalis, magnesium, and ferric iron, with substantial increase in calcium relative to the parent rock. Aluminium and silica appear to remain more or less constant. These geochemical changes, when observed in the case of the most fully developed mylonite, result in a rock composed essentially of epidote group minerals and quartz, with or without minor actinolitic amphibole. The chemical changes observed in the shear zones within the pluton can be explained by a model which invokes volume loss (alkalis, etc.) occurring during deformation, thereby enriching the mylonite zones in calcium originally contained in the parent rock. Preliminary calculations suggest that significant volume loss may have taken place in the most highly deformed zones. The main (Dl) foliation in the pluton is coeval with slaty cleavage in the surrounding wall rock. Subsequent deformation (D2) in the wall rock is expressed as domainal crenulations and folds. In a large
117
shear zone (1-2 km wide) bordering the east side of the pluton, D2 structures are the dominant fabric. Chiastolite in this shear zone shows an involved structural and metamorphic history, indicating renewed movement along the slaty cleavage and intense development of D2. These observations suggest that the mylonites in the plutons may be, at least in part, coeval with wider (D2) deformation found in the wall rock.
118
EPIZONE - ANCHIZONE GRADE DEFORMATION, CLEAVAGE DEVELOPMENT AND RELEASE SPECTRA OF SLATES FROM NORTHERN VICTORIA LAND, ANTARCTICA Thomas 0. Wright^ and R. David Dallmeyer^ ^ Division of Earth Sciences, National Science Foundation, Washington D.C. 20550. Department of Geology, University of Georgia, Athens, Georgia.
O
The Robertson Bay terrane is the easternmost allochthonous terrane identified in northern Victoria Land, and consists of distal turbidites and hemipelagic slates. The rocks are folded about northwest axes and possess a single, well-developed, upright, nearaxial plane cleavage over most of the 30,000 sq km area of exposure. Illite crystallinity, conodont alteration index and petrologic data establish that the metamorphic grade reached was approximately at the Epizone - Anchizone transition and was remarkably constant over the area. During the development of cleavage, pressure solution was accompanied by recrystallization of fine-grained detrital phyllosilicates into strongly oriented very fine-grained white micas and chlorite. Under these conditions (approximately 300°C) all inherited argon is expected to be lost from detrital fine-grained phyllosilicates, and the recrystallized new micas formed below temperatures where argon diffusion is thought to become effective. Thus release spectra are interpreted to date the cleavageforming event. These ages indicate that the cleavage formed 500 Ma in the western part of the terrane and 460 Ma in the east, suggesting diachronous deformation. If so, the deformation swept eastwards at about 1 cm/yr. The western margin of the Robertson Bay Terrane is marked by a several kilometer-wide band of well-developed polydeformed rock that is spatially associated with a major thrust that placed the Bowers terrane eastwards over the Robertson Bay terrane. This thrust faulting was younger than the previously discussed folding and upright cleavage because it overprints these features, and includes the formation of a strong crenulation cleavage near the fault. The metamorphic grade during thrusting was no higher than during the formation of the first cleavage. This second cleavage involves major pressure solution removal of material and passive rotation of first cleavage micas into second cleavage orientations, however there is no evidence of formation of new phyllosilicates. Ar release spectra from these fault rocks are indistinguishable from the single deformed samples away from the fault. These results are interpreted to mean that the method is incapable of dating this particular deformation, despite its apparent intensity, because no new micas were forming. This study illustrates the need to know the operative mechanism and the conditions during deformation in order to adequately interpret age spectra. It also provides an example of extensive pressure solution dominated deformation imposed on already well cleaved and folded metasedimentary rock.
119
COMPRESSIONAL STRESS RESPONSE SURFACES: INCIPIENT CLEAVAGE EXAMPLES FROM THE IRREGULLY FORMATION, BANGEMALL BASIN, WESTERN AUSTRALIA.
C.N. Winsor^ and A.D. Antonovsky^* ^ Geology Department, The University of Western Australia, Nedlands, Western Australia, 6009. Electron microscopy centre, The University of Western Australia, Nedlands, Western Australia, 6009.
Compressional stress response (CSR) surfaces are a product of pressure solution, shear or some combination of these (Logan, 1984). In the Irregully Formation, Bangemall Basin there are three main orientations of CSR-surfaces: bedding-parallel, subhorizontal and subvertical. The surfaces, with normal to subnormal stylolite peaks, can be geometrically related to the deformation history. CSR-surfaces are evident at all scales in the Irregully Formation and have formed during tectonic and overburden pressure events. Carbonates in the formation commonly have a micron-scale grain size, therefore observing CSR surfaces and evidence of pressure solution is assisted by the aid of a scanning electron microscope. Carbonate is preferentially dissolved along CSR-surfaces leaving mainly quartz as an insoluble residual material (i.e. stylocumulate, Logan, 1984). This may react with metallic elements not readily dissolvable and recrystallize to form phyllosilicates as reactate (Logan, 1984). The development of phyllosilicates along CSR-surfaces at various times in the structural evolution is interpreted as the first event in the production of a spaced solution cleavage (e.g. Alvarez et al., 1978). This incipient cleavage may be crenulated by later compressional events, resulting from either a horizontally directed shortening or the effect of overburden pressure. References Alvarez, W., Engelder, T. and Geiser, P.A., 1978. Classification of solution cleavage in pelagic limestones. Geology, 6: 263-266. Logan, B.W., 1984. Pressure response (deformation) in carbonate sediments and rocks Analysis and Application, Canning Basin, In: The Canning Basin W.A. (P.G. Purcell, editor) Geol. Soc. Aust./Pet. Expl. Soc. Aust. Symposium, Perth: 235-251.
* present address - Alcoa Australia Aluitiinium R e f i n e r y , K w i n a n a , W.A.
Ltd.
Kwinana
120
THE SIMPLON FAULT ZONE: MAJOR EXTENSIONAL, TRANSVERSE MOVEMENTS IN THE PENNINE ALPS OF SWITZERLAND AND NORTHERN ITALY
Neil Mancktelow Geologisches Institut, ETH-Zentrum, CH-8092 Zurich, Switzerland.
The Simplon Fault Zone (SFZ) is a major, low-angle extensional fault separating basement nappes of the Upper and Lower Pennine Zones. Its existence was first recognized due to a clear and sudden jump in the metamorphic grade and isotopic mineral ages across a so-called "Simplon Line". In the most complete profile through the SFZ, along the Zwischbergen Valley, the following divisions can be recognized: 1) a hangingwall block to the SW (the Upper Pennine Zone) which preserves older Alpine structures and mineral ages. Quartz textures are clearly unrelated to the Simplon movements and the ductile imprint of the Simplon event is absent. The structures in this block are clearly truncated by 2) a structural discontinuity dipping ca. 30° towards 240°. Where well exposed, this discontinuity ("the Simplon Line") is seen to consist of a narrow zone of cataclastics, with brecciated ductile mylonite blocks and rare lenses of brecciated (Triassic?) dolomitic marble. 3) below this discontinuity is a fairly narrow zone (<50m) of strongly foliated but poorly-lineated, green-grey, retrograde mylonites, with occasional narrow transecting zones of cataclastics. 4) this grades into a broad zond (1-2 km) of planar foliated and variably lineated mylonites and mylonitic gneisses. Quartz veins have clearly asymmetric single girdle c-axis textures indicating a normal fault sense of movement. This is consistent with microstructural shear criteria (shear bands, mica "fish", oblique shape fabric to the newly recrystallized quartz grains etc.), the younger isotopic mineral ages and the higher metamorphic grade of this footwall block. 5) this zone grades imperceptibly over several kilometres into variably overprinted gneisses of the Lower Pennine Zone, which preserve earlier Alpine and pre-Alpine structures but in which the quartz veins often still show asymmetric textures consistent with SFZ movement. By tracing these features to the NW and SE and utilizing the 2000 m topographic relief, it can be established that in the Simplon region the SFZ forms a half-dome plunging SW. The continuation of this dome to the NE is complicated by increasing metamorphic grade, resulting in almost complete annealing of the all important quartz veins. However, a strongly foliated zone continuous with the SFZ mylonites can be mapped which completes this structure as an elongate, 55 km long NE-SW dome. The asymmetry of the quartz textures from the sugary, coarsergrained quartz veins is not always clear but, in general, they appear to give similar results to the Simplon region: i.e. the upper block (the Upper Pennine Zone) has moved SW relative to the underlying units.
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The jump in isotopic mineral ages and in the metamorphic grade across the SFZ suggests a vertical component of ca. 7 km. The true offset parallel to the zone itself is clearly dependent on the original average dip angle. From the regional geometry, this could not have been more than 15°, giving a minimum displacement of ca. 30 km. Such movement along a low-angle normal fault must reflect a major extension of the Alpine chain parallel to its length and therefore effectively perpendicular to the original nappe emplacement direction. This extension occurred late in the Alpine deformation history (ca. 19 Ma?), well after the main peak of metamorphism (Lepontine event, ca. 35 Ma), and is synchronous with, and possibly related to, the opening of the western Mediterranean.
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REGIONAL STRAIN ANALYSES IN LITHOLOGICALLY COMPLEX TERRANES, CENTRAL SIERRA NEVADA, CALIFORNIA
Scott R. Paterson and Othmar T. Tobisch Earth Science Board, University of California, Santa Cruz, California, 95064, USA.
Because orogenic belts may consist of numerous terranes, it is important to understand how terranes interact during amalgamation and subsequent deformation. To this end over 200 finite and some incremental strains have been measured in sedimentary and volcanic rocks from three adjacent terranes (Foothills, Sullivan Creek, Merced River) in the western metamorphic belt. Sierra Nevada, California. These data have been used to examine the structural characteristics of each terrane and the contacts between the terranes. Our approach of analyzing strains includes applying corrections for the presence of primary fabrics and matrix-clast viscosity contrasts, grouping strains by locality, lithology, and structural setting, and averaging each group using the techniques outlined by Oertel -(1981). Results show that strain intensities and symmetries in individual samples and averaged data are strongly controlled by lithology. In any one domain the lowest intensities are recorded in sandstones or clast-rich conglomerates, and increasing intensities recorded in lapilli tuffs, pebbly mudstones, and slates. Flattening symmetries are common in slates and sandstones suggesting volume losses of 45% to 65% and 0% to 26%, respectively. Symmetries in lapilli tuffs are indicative of plane strain and less than 10% volume loss. These differences in volume losses and tectonic strains indicate that most if not all lithologicl contacts in this region represent some form of strain discontinuity. Regional patterns of strain are distinct despite the lithologic control. In western portions of the Foothills terrane, strain intensities are high with values of shortening around 50%, and directions of greatest extension plunging steeply. Strain intensities drop to low values in eastern portions of this terrane (0% to 30% shortening) and directions of greatest extension commonly plunge moderately. The presence of a large ductile fault, different strain characteristics, and different rock ages between the eastern and western domains raises the possibility that this terrane consists of two smaller fragments amalgamated during regional deformation. In both the Sullivan Creek and Merced River terranes, strain increases to values as high or higher than those in the western Foothills belt with shortening ranging between 50% to 60%. Lithologic correlations, along with the similarity in the styles and ages of strain and structures in these two terranes indicate that they are in fact a single terrane. Large ductile-brittle, steep-dipping faults with heterogeneous but commonly intense strains border these terranes. Initial studies suggest that the faults record east-over-west, reverse motion with only a small strike-slip component. Thus, each terrane displays a unique regional strain/structural pattern and is bordered by large ductile faults. The regional strain patterns in the Foothills and Sullivan Creek-Merced River terranes show differences in strain intensities and orientations, but bulk strain symmetries probably approach plane strain in both terranes. A
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comparison of finite and incremental strains in the terranes and bordering faults suggests that the strains formed largely due to pure shear but with increasing components of simple shear in the fault zones.
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PHANTOM OROGENIES - ^UNCONFORMITIES' PRODUCED BY FOLDING OF CONFORMABLE SEDIMENTARY LAYERS
W.D. Roots, K.M.C. Matthews, G. Whalan and C. Morley School of Earth Sciences, Macquarie University, North Ryde, 2109, Australia.
We have subdivided the belt of m-Ordovician to u-Silurian rocks between Goulburn and Mt. Shivering into six Formations as follows Silurian
:
Ordovician :
Whipbird Creek Fm Argyle Fm Cobra Fm
-
mudrock qtz. arenite massive silty-mudrock, u-Sil. fossils at base qtz. arenite (top at least is probably Silurian) mudrock, u-Ord. graptolites qtz. arenite, m-Ord. conodonts
Burra Burra Ck. Fm
-
Bubalahla Fm
-
Jocks Ck. Fm
-
All formations are of marginal sea turbidite facies. At many locations, perfect exposures show that each boundary is conformable and gradational. In particular, the Burra Burra Ck. Fm./Cobra Fm. boundary is gradational in both lithology and fossil content. However, angular discordance also occurs at many locations between these same units, always with boundary-parallel faulting. We find that the clearest gradational boundaries occur around major fold noses, while faulted and discordant boundaries occur typically on the limbs of folds. We interpret these patterns as the result of the post-depositional folding of a set of totally concordant units, accompanied by boundary faulting caused by fold style contrasts between units of differing ductility. Close spaced parasitic folds may help to key adjacent units together at major fold noses, although there is little need for boundary slip to occur at fold noses. On the limbs of major folds, where parasitic folds are less frequent, planar boundary faults will pass obliquely through isolated parasitic folds. Fault movement will then juxtapose a faulted parasitic half-fold against planar beds, creating the appearance of an unconformity if the half-fold lies in the older unit. We have located such half-folds above and below boundary faults. This interpretation suggests that no orogeny affected these rocks before the latest Silurian. Pre-end-of-Silurian folding may appear indicated by the apparently more complex nature of Ordovician folding. However, we note that where the Bubalahla Fm. occurs in the core of either a major anticline or syncline, it forms steeply plunging parasitic folds that contrast with the gentle plunges of both the younger Burra Burra Ck. Fm. or the older Jocks Ck. Fm. Also, parasitic folds in the Bubalahla Fm. plunge but gently on the flanks of major folds, indicating that apparent fold complexity of a ductile unit may be caused by proximity to a major fold hinge rather than a more complex fold history (than the enclosing units).
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BASEMENT CONTROL OF COVER FOLDING IN THE ADELAIDE GEOSYNCLINE, SOUTH AUSTRALIA
Geoffrey L. Clarke Department of Geology, University of Melbourne, Parkville, Victoria, 3052, Australia.
The Adelaide Geosyncline comprises upper Proterozoic to Cambrian sediments deposited on lower to middle Proterozoic basement. The rocks of the Geosyncline were deformed and, in some areas, metamorphosed during the Cambro-Ordovician "Delamerian" Orogeny. The Geosyncline now has a sigmoidal shape, with an additional northward spur into the northern Flinders Ranges. The purpose here is to explain this sigmoidal shape. The N-trending structures within the Mt. Lofty Ranges, which constitute the central portion of the geosyncline, were generated during westward thrusting of the rocks of the Geosyncline towards and onto the mid-Proterozoic Gawler Craton. The eastern margin of the Gawler Craton, the N-trending Torrens Hinge Zone, represents'the tectonic boundary of the "Delamerian" Orogeny. To the NE, the NEtrending cover folding in the Mt. Lofty-Olary arc is interpreted to be due to E-striking dextral wrench faults in the basement. Evidence for these wrench faults is observed in the basement inliers comprising the Willyama Complex. The exposure of the basement here is controlled by the western boundaries of these inliers, which are E-dipping thrust faults displaying westward movement, consistent with that in the central portion of the geosyncline. The E- to NE-trending folding in the southern portion of the geosyncline is interpreted to be similarly controlled by another basement wrench fault zone. Folding in the northern Flinders Ranges exhibits the interference of N trending folds, related to cover shortening by westward directed folding and thrusting, with NE-trending folds generated by basement wrench faulting. The possibility of wrench faulting also accommodating basement shortening and initiating a cover-basement decollement, provides a tectonic framework in which the piercement structures observed in this region may form at the same time as fold "flower" structures. The observation of areas within the Adelaide Geosyncline where structure is controlled by wrench faulting, suggests that transcurrent faults could possibly be a means of displacing this tectonic boundary of the "Delamerian" Orogeny, to continue in other parts of Gondwanaland, for example, rocks comprising the Ross Orogen in Antarctica.
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KINEMATICS OF DUCTILE EXTENSION IN THE METAMORPHOSED LOWER PLATE OF THE NORTHERN SNAKE RANGE METAMORPHIC CORE COMPLEX, NEVADA, USA.
Jeffrey Lee Department of Geology, Stanford University, Stanford, CA, 94305, USA.
Metamorphic core complexes of the western US Cordillera are characterized by an upper plate of unmetamorphosed rocks cut by imbricate normal faults and a lower plate of ductilely thinned and stretched metasedimentary and metaigneous rocks. Separating these two plates is a low angle detachment fault. Much debate has focused on the amount of displacement along these low angle detachments and the kinematics of the ductilely deformed rocks below. A detailed study of finite strain, mesoscopic structures, microstructures, quartz c-axis fabrics and geochronology from the lower plate rocks of the northern Snake Range metamorphic core complex place constraints on the nature, geometry, kinematic history and timing of ductile extensional deformation. These data provide new insights into the processes of deep-seated ductile extension beneath supracrustal normal fault mosaics in highly extended regions. Beneath imbricate normal faults of Tertiary age, ductile extension has greatly thinned and stretched quartzites and schists in the lower plate of the northern Snake Range. The lower plate rocks are well developed L-S tectonites with a N60W elongation lineation defined by elongate quartz grains, stretched pebbles, pressure shadows and boudinaged staurolite and biotite porphyroblasts. There is a single mesoscopic bedding-parallel foliation that is gently arched across the range. A transect of finite strain measurements, taken parallel to the stretching lineation, on strained detrital grains and pebbles indicates that the lower plate underwent plane strain with a subvertical Z-axis and a WNW-ESE X-axis. There is a dramatic west-toeast increase in strain from a low on the west flank of the range of 5.8:1 (X:Z) to a high on the east flank of the range of 31:1 (X:Z), but apparently there is no vertical strain gradient. Bedding and foliation are everywhere parallel, and the stratigraphic sequence is thinned 30-90%. These data alone are compatible with either a pure shear necking model and/or a moderately southeast dipping shear zone. The nature and geometry of microstructures and quartz c-axis fabrics show a progressive change from west-to-east. In the low strain rocks on the west flank of the range microstructures are characterized by a single bedding parallel foliation (S^) defined by flattened detrital grains and recrystallized quartz grains, and micas. C-axis fabrics from these rocks show symmetric, both in skeletal outline and density distribution, cross girdle patterns. Globular grains are rare and have their c-axes oriented subparallel to the Z-axis of the finite strain ellipsoid. East of these samples in slightly higher strain rocks the c-axis fabrics are symmetric cross girdles with an asymmetric density distribution. Microstructures are the same as to the west and globular grains are still present with their c-axes parallel to Z. On the east side of the range in the highly strained rocks four foliations (not all present in the same thin section) are observed. At high structural levels in the lower plate there is a bedding parallel foliation (S^) that is defined by ribbon-like grains. A second foliation, C-planes, dips more steeply to the southeast. At deeper structural levels ribbon-like grains have been completely
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recrystallized into an oblique, quartz grain shape foliation that defines the third foliation (S^). The S^ foliation dips more steeply to the northwest than bedding, and where observed the C-planes parallel bedding. The fourth foliation (shear bands) is rare, cross cuts both the C and S foliations and dips moderately to the southeast. The asymmetry of the S-C orientation consistently indicates top-tothe-east shear. This is supported by the orientation of asymmetric mica "fish", stair-step mica "fish" tails and drag of S and C foliations adjacent to shear bands. C-axis fabrics from these high strain rocks show asymmetric dog-leg single girdle patterns which also indicate top-to-the-east shear. The kinematic information provided by the microstruetures and quartz c-axis fabrics suggests that the progressive change in the nature and geometry of microstructures and quartz c-axis fabrics from low to high strain indicates a strain path with an early history of coaxial deformation that is followed by noncoaxial deformation. ^^Ar/^^Ar geochronology on lower plate hornblendes and micas suggest that temperatures of deformation were <530°C, but >280°C and increased with depth. The age data indicates that ductile deformation is post latest Cretaceous and was probably ongoing 22-26 Ma. Any model that attempts to chart the evolution and deformational history of the lower plate of the northern Snake Range must take into account the following relationships in addition to the data above. (1) Upper plate normal faults soled into a subhorizontal detachment, the northern Snake Range decollement (NSRD). Flat lying metasediments beneath and parallel to the NSRD and the lack of stratigraphic omission across the NSRD rules out large translations on a surface that originally cut down section to the east, but does not preclude bedding parallel movement (Miller et al., 1983). (2) Discontinuous, subhorizontal reflections in the middle and lower crust imaged on seismic data from the northern Snake Range have been interpreted as indicating that ductile extension is distributed throughout the entire crustal column (McCarthy, 1986) which supports the interpretation that strain may not die out with depth. All available data have been used to propose a highly simplified model for the evolution of the lower plate rocks of the northern Snake Range (see Fig.). In the early Oligocene the first generation of upper plate normal faults root into a subhorizontal zone of decoupling, the NSRD. Ductile deformation below this zone occurred because of locally elevated temperatures. The thermally weakened lower plate rocks deformed primarily by pure shear necking with subhorizontal extension and subvertical shortening perpendicular to bedding. The second generation of normal faults were active during the late Oligocene to early Miocene. In the lower plate the locus of thermal weakening appears to have shifted eastward resulting in the "freezing in" of pure shear fabrics on the west flank. Second generation normal faults root into a diffuse zone of down-to-the-southeast ductile shear on the east flank. As these regionally thinned and stretched lower plate rocks are uplifted non-coaxial deformation overprints the earlier formed coaxial fabrics. This data along with seismic reflection data suggest that ductile extensional deformation occurred because of heat input into shallow levels of the crust, and that these rocks probably represent the top of a regional metamorphic terrane of Tertiary age.
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References: Miller, E.L., Cans, P.B. and Garing, J.D. 1983. The Snake Range decollement: An exhumed mid-Tertiary ductile-brittle transition: Tectonics, v. 2, p. 239-263. McCarthy, J. 1986. Reflection profiles from the Snake Range metamorphic core complex: A window into the mid-crust: Reflection Seismology: The Continental Crust, Geodynamics Series, v. 145, p. 281-292.
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