Excursion Leaders:
Prof. Neil S. Mancktelow Prof. Christoph A. Heinrich Editor:
Michael Schirra
Department of Earth Sciences (D-ERDW) Geological Institute & Institute of
Geochemisty and Petrology ETH Zürich
Ch-8092 Zürich, Switzerland
Available at www.research-collection.ethz.ch – search Schirra, Mancktelow & Heinrich (2018) Open Access
Acknowledgements
Students and organizers would like to thank the following geologists, mining managers and companies for their help in planning this field trip, for granting access to active mine sites, and providing accomodation and great company. Without their help, this field trip would not be possible.
Dr. Alfredo Camacho for guidance around Alice Springs; Mark McGeough and Theresa Ruatoka of Chinova Resources; Brad Miller, Trevor Shaw and Daniel Taylor from Glencore for initiating visits led by Vanessa Sexton, Jessica Cooper and their colleagues at the Ernest Henry and Mt. Isa mines; Neal Valk and colleagues at MMG’s Dugald River mine;
Tom Evans, Richard Hatcher and David A'Izzedin of Capricorn showing us the Mount Gordon
mines. Dr. Nick Olivier, former professor at James Cook University and now consultant with
HCOV Global led part of the excursion and helped with advice and contacts, along with
Mark Hinman and Rick Valenta from the University of Queensland.
Contents
A. List of Participants and Program 1
B. The Petermann Orogeny and its Foreland Basin 3
1. Tectonic Overview 3
2. The intraplate Orogeny Enigma 3
3. Foreland Basins of the Petermann Orogen 7
C. Woodroffe Thrust and Sheath Folds 9
1. Introduction
2. Geology 10
3. Field Observations 10
4. Discussion 12
D. Pseudotachylytes in the Musgrove Ranges, Central Australia 15 1. Introduction: Why is it important to know about Pseudotachylites? 15
2. Pseudotachylites in Australia 17
E. The Central Australian Superbasin 21
1. Introduction 21
2. Tectonic Evolution of Proterozoic Australia 21
3. The Formation and break-up of Supercontinents 22
5. Snowball Earth Theory 26
F. Intracontinental Orogeny: Comparing the Palaeozoic Alice Springs Orogeny and the
Mesoproterozoic Mount Isa Belt 30
1. Tectonic History 30
2. Mesoproterozoic Mt. Isa (Isan) Orogeny 30
3. Palaeozoic Alice Springs Orogeny 32
G. Overview of the Geologic History of the Mount Isa Inlier 37
1. Location 37
2. Palaeoproterozoic Basement 37
3. Superbasins 37
4. Isan Orogeny and Proterozoic Evolution of the Inlier 39
H. Paleoproterozoic Basement to Mesoproterozoic Sedimentation in the McArthur
River – Mount Isa Pb-Zn±Ag Province 44
1. Introduction 44
2. Tectonic History 45
3. Conclusions 50
2. Metamorphic Evolution 52
3. Magmatism 52
4. Post-Isan Orogeny Proterozoic Evolution of the Inlier 53
K. Regional Geology of Lawn Hill Area including the Lawn Hill Circular Structure 55
1. Regional Geology 55
2. The Lawn Hill Impact Structure 57
L. The Century Zinc Deposit 61
1. Introduction 61
2. Regional Geology of Century Deposit 62
3. Century Deposit Geology and Stratigraphy 63
4. Orebody Stratigraphy and Sedimentology 63
5. Ore Forming Processes 66
M. Dugald River Zn-Pb Deposit 67
1. Introduction 67
2. Geological setting 67
3. Ore Formation 69
N. Genesis of the Mount Isa Pb-Zn-Ag Deposits 71
1. Introduction 71
2. Geological setting 73
3. Lead-zinc Mineralization 78
O. Mount Isa Copper Deposits, Syn- to Postmetamorphic Fluid Flow and Hydrothermal
Alteration 89
1. Location and geological Context 89
2. Economics, Discovery and Processing 90
3. Local Mine Geology and regional Alteration Assemblage 91
4. Mt. Isa Cu Ore Deposit Formation and related Processes 95
5. Comparison between Mt. Isa, Hilton, and Mammoth Cu Ore 98
P. The Mammoth and Esperanza Cu Deposits 101
1. Introduction 101
2. Geological Setting 101
3. Economics 107
4. Deposit Formation 107
Q. Structure, Regional Metamorphism and Granite Plutonism in the Eastern Succession of the Mount Isa Inlier, including the Mary Kathleen U-REE Deposit and the Corella Meta-
Evaporites 110
1. Introduction 110
2. Chronology of Deformation, Metamorphism and Granite Plutonism in Eastern Mt. Isa 110
3. Mary Kathleen U-REE Deposit 113
R. Iron Oxide Copper Gold Deposits in the Eastern Mt. Isa Inlier 118
1. Iron Oxide Copper Gold Deposits in Australia 118
2. Recap on IOCG’s 120
3. IOCG Genesis: Granite Plutonism and regional Metamorphism 120
4. Hydrothermal System 120
5. Ore Mineralogy and Petrology 122
6. Geology of the Eastern Mount Isa Inlier 123
7. Broken Hill versus Mt Isa 123
S. The Osborne Deposit 129 1. Location, Mining History and Regional Geology of the Osborne Deposit 129
2. Deposit Formation and Mine Geology 132
3. Economics and Processing 133
T. The Ernest-Henry Copper-Gold Deposit 138
1. Location and geological Context 139
2. Economics, Discovery and Processing 139
3. Mine-scale Geology 141
4. Deposit Formation Processes 143
U. Post-orogenic (<1500 Ma) Erosion of the Mt. Isa Province and the Formation of
Cambrian Phosphorites 145
1. Introduction 145
2. Post-orogenic Evolution of the Mt. Isa Province 145
3. Global Events of Phosphogenesis and the Cambrian Transgression 146
4. Mt. Isa Phosphorites 146
Chapter A
1 A. List of participants and program
Surname First Name Position Phone Number
1 Heinrich Christoph
Andreas
Professor +41 79 337 24 26
heinrich@erdw.ethz.ch 2 Mancktelow Neil
Sydney
Professor +41 79 730 68 69
neil.mancktelow@erdw.ethz.ch 3 Bongiovanni Mauro Msc
Student
+39 34 095 49798
maurobo@student.ethz.ch
4 Bruni Elena Msc
Student
+41 79 886 71 38
brunie@student.ethz.ch 5 De Selva-
Dewint
Thomas Theodore
Msc Student
+32 49 639 77 17
desethom@student.ethz.ch 6 Dieterle Michael
Andreas
Msc Student
+41 76 435 72 38
michael.dieterle@erdw.ethz.ch
7 Farsky David Msc
Student
+41 78 607 35 92
farskyd@student.ethz.ch 8 Fleischmann Sarah Msc
Student
+41 79 615 53 18
flsarah@student.ethz.ch
9 Frei Dominik
Roger
Msc Student
+41 79 851 42 13
freido@student.ethz.ch
10 Fux Michael Msc
Student
+41 79 683 12 32
mfux@student.ethz.ch
11 Gay Nicolas
Guillaume
Msc Student
+41 79 508 02 65
gayni@student.ethz.ch 12 Gülcher* Johanna
Pia
Msc Student
+41 78 685 61 23
guanna@student.ethz.ch
13 Jones Nina
Sarah
Msc Student
+41 76 432 45 26
jonesn@student.ethz.ch 14 Kuntze Calvin
Cox
Msc Student
+41 76 801 05 07
ckuntze@student.ethz.ch 15 Melendez
Castellanos
Maria Paula
Msc Student
+41 77 929 90 26
casmaria@student.ethz.ch 16 Morgenthaler Joel Lukas Msc
Student
+41 79 517 34 24
mjoel@student.ethz.ch 17 Rebecchi Marco
Maria
Msc Student
+41 79 480 49 78
marco.rebecchi@gmx.de 18 Reyes
Alvarez
Julian Mauricio
Msc Student
+41 77 949 67 29
rejulian@student.ethz.ch 19 Sartori Gino
Simone
Msc Student
+41 79 853 64 67
sartorig@student.ethz.ch 20 Schirra Michael PhD
Student
+41 76 408 66 17
michael.schirra@erdw.ethz.ch 21 Schmid
*Timothy
Chris
Msc Student
+41 79 257 82 93
schmidti@student.ethz.ch 22 Schulthess Claudio Msc
Student
+41 77 461 84 87
claudio.schulthess@gmx.ch
23 Sitabi Amar
Brandon
Msc Student
+41 77 973 38 63
sitabia@student.ethz.ch
24 Tanner Thomas
Martin
Msc Student
+41 79 661 25 88
tannerth@student.ethz.ch
*only during first part (structural geology) of excursion: 19.08 – 26.08.2018
Chapter A
2
Date Program Accomodation
19.08.2018 Sun Flight Zürich - Dubai Air
20.08.2018 Mon Flight Dubai - Melbourne Air
21.08.2018 Tue Flight Melbourne - Alice Springs; Uluru foreland sediments & Petermann Orogeny
Yulara Campground 22.08.2018 Wed Kata Tjuta formations, travel to Mulga Park Camping in creek 23.08.2018 Thu Woodroffe Thrust, Mt. Fraser mylonites &
pseudotachylytes
Camping in creek 24.08.2018 Fri Travel to Alice Springs, Red Bank deformed
zone on the way
Wintersun Campground 25.08.2018 Sat Ormiston Gorge & Ellery Creek (section
through Neoproterozoic Amadeus Basin)
Wintersun Campground
26.08.2018 Sun Flight Alice Springs – Brisbane Bunk Brisbane Hotel 27.08.2018 Mon Flight Brisbane – Mount Isa, Logistics, drive to
Osborne mine
Osborne mine site 28.08.2018 Tue Osborne iron oxide Cu-Au deposit: drill core
and surface geology
Osborne mine site 29.08.2018 Wed Merlin core & waste dump, underground visit in
subgroups, drive to Cloncurry
Cloncurry Wagon Wheel
30.08.2018 Thu Ernest Henry deposit: drill core, waste dump &
surface geology
Cloncurry Wagon Wheel
31.08.2018 Fri Dugald River Pb-Zn deposit: visit mine &
flotation plant, drive to Mary Kathleen
Dugald River mine site
01.09.2018 Sat Mary Kathleen U-REE: metasomatism in Rosebud Creek & relation to IOCG
Camping in creek 02.09.2018 Sun Drive to Mt. Isa, red rock alteration of ECV &
late-metamorphic fluids related to Cu
Mt. Isa campground 03.09.2018 Mon Mount Isa Cu / Hilton Pb-Zn mine visits in
subgroups of 6 people
Mt. Isa campground 04.09.2018 Tue Mount Isa Cu / Hilton Pb-Zn mine visits in
subgroups of 6 people, drive to Mt. Gordon
Mt. Gordon mine site 05.09.2018 Wed Gunpowder / Mt. Gordon Cu mine Mt. Gordon mine site 06.09.2018 Thu Drive to Lawn Hill (approx. 10h), free time Adels Grove camp 07.09.2018 Fri Century Zn mine: surface geology, alleged
impact structure
Adels Grove camp
08.09.2018 Sat Drive back to Mt. Isa Mt. Isa campground
09.09.2018 Sun Flight from Mt. Isa over Brisbane & Dubai to Zürich
Air
Photos c/o Jessica Cooper, Glencore - Mount Isa
Chapter B Gülcher, Johanna
3
B. The Petermann Orogeny and its Foreland Basin
Weakening, failure and deformation in continental interiors, ultimately leading to orogeny, are conspicuous as they are the most obvious exceptions to the main principle of simple plate tectonics, i.e. that plates are rigid and deform only at their margins. The Ediacaran–Cambrian Petermann Orogeny recorded in Central Australia is an exceptional example of intraplate orogenesis. The Petermann orogenesis (~560 - 520 Ma) was responsible for the exhumation of the crystalline basement of the Musgrave Province from beneath the formerly contiguous Centralian Superbasin. Until this day, the evolution of the Petermann Orogeny remains enigmatic and still puzzles many geologists, mainly due to a lack of empirical data, particularly with regard to the duration, rate, and physical and thermal conditions of this event.
1. Tectonic Overview
The central Australian geology can be summarized as large inliers of Paleoproterozoic (2.5 - 1.6 Ga) to Mesoproterozoic (1.6 - 1.0 Ga) crystalline basement, e.g. the Arunta and Musgrave regions, separated and surrounded by Neoproterozoic (1000 - 541 Ma) to Paleozoic (541 - 252 Ma) sedimentary basins (fig. B-1). The stratigraphic continuity between the sedimentary Amadeus Basin and Officer Basin suggests that they comprised part of an extensive intracratonic sedimentary basin initiated during the assembly of Rodinia, named the Centralian Superbasin (e.g. Wade et al., 2008; chapter E). The Superbasin remained intact until fragmentation occurred during late Neoproterozoic, i.e. the Petermann Orogeny, exhuming the Musgrave Block. The Petermann Orogeny is the first of two major intraplate orogenies that have affected the central Australian region. The second major episode, in the Devonian and Carboniferous (430 - 400 Ma), is known as the Alice Springs orogeny (Chapter F).
The Petermann Orogeny involved high temperature and pressure (granulite and sub- eclogite facies) metamorphism (e.g. Raimondo et al., 2010) and N-S shortening exceeding 100 km, accommodated by substantial crustal thickening (Flottmann et al., 2004). North-vergent shortening was mainly concentrated along the south-dipping Woodroffe Thrust (Chapter C), while south directed overthrusting was accommodated along the southern margin of the Musgrave Block (fig. B-2 and B-3). The series of E-W trending fault structures, showing a dextral transpressive shear system, dissect the deep crust on the northern margin of the Musgrave block and thereby creates a crustal-scale ‘flower structure’ that exposes the deepest crustal rock in its core (fig. B-2 and B-3).
2. The intraplate Orogeny Enigma
In the absence of local plate-margin interactions, the tectonic evolution of continental interiors must be controlled by either the transmission of horizontal plate-boundary stresses across large distances through the lithosphere, or the development of independent intraplate stresses such as vertical forces in the mantle (Raimondo et al., 2014).
For in-plane stresses generated at plate boundaries to be transmitted to continental interiors, the lithosphere acts as an effective stress guide. This implies a strong lithospheric mantle rheology, in order to account for far-field stress propagation through the discontinuous upper crust and to enable the support of thick uplifted crustal wedges.
The alternative hypothesis suggests that intraplate deformation is driven by vertical forces
related to mantle subduction or gravitational (Rayleigh-Taylor) instabilities caused by
convective flow in the mantle lithosphere. In this scenario, the mantle from one of two colliding
continental fragments becomes detached from the overlying lighter crust and is forced
downwards. This results in surface uplift and thickening due to a reduction in lithospheric
loading and accompanying heating caused by asthenosphere upwelling following mantle
delamination. Yet, predictions of temporal and spatial scales for orogenesis of the latter model
of intraplate stress generation, primarily involving mantle downwelling, are inconsistent with
the observed records of deformation of the Peterman orogeny (Raimondo et al., 2014 and
references therein), and the transmission of horizontal plate-boundary stresses is often thought
a more likely scenario for central Australian deformation.
Chapter B Gülcher, Johanna
4 The evolution of the Petermann Orogeny is often tied to the progress of Gondwana assembly - one of Earth’s most dramatic periods of tectonism. Yet, uncertainties regarding the exact assembly of Gondwana make defining a specific tectonic driver not straightforward.
oblique motion of India relative to its western margin as part of the Neoproterozoic-Cambrian Kuunga Orogeny (Meert, 2003; Aitken et al., 2009). Another proposition for Gondwana assembly recognizes the dominance of transpressional orogenic belts and proposes that oblique subduction along the Pacific margin of Gondwana from ca. 560 Ma onwards led to counter-rotation of continental blocks in Gondwana (Veevers, 2003).
Fig. B-1: Tectonic map of Australia showing the basins and basement provinces and the area of interest discussed in this chapter. The Officer, Amadeus, Ngalia, southern Georgina and Murraba Basins (and smaller unmarked outlies in the east Kimberly region) were likely linked as the intracratonic Centralian Superbasin during the Neoproterozoic, but the depositional boundary of this entity is difficult to delineate. Modified from Haines et al. (2016).
Chapter B Gülcher, Johanna
5
Fig. B-2: Regional map and cross-section (X–Y) of Central Australia, showing key structural relationships of the Petermann and Alice Springs Orogens and the distribution of basement and cover regions. Also shown are the locations of axial orogenic zones containing mid- to lower-crustal rocks. Abbreviations: AB, Amadeus Basin; GT, Gardiner Thrust; HF, Hinckley Fault; MF, Mann Fault; MT, Munyarai Thrust; Musgrave P., Musgrave Province; NB, Ngalia Basin; NT, Napperby Thrust; OB, Officer Basin;
RBSZ, Redbank Shear Zone; UT, Uluru Thrust; WT,Woodroffe Thrust. Modified from Raimondo et al. (2014).
Chapter B Gülcher, Johanna
6
Fig. B-3: (a) Regional geologic map of the Musgrave Block, showing its position relative to the intracratonic Amadeus and Officer basins. Also shown are the locations of key E‐W trending fault structures of the Petermann Orogen and the boundaries of the field areas discussed in this study. (b) Schematic cross section (Y‐Y′) across the central Musgrave Block, showing the structural arrangement of major faults and lithologies. Note the overall crustal‐scale dextral transpressive shear system, involving significant Moho displacement and deep exhumation along the Woodroffe Thrust/Mann Fault.
Abbreviations: AB, Amadeus Basin; BBZ, Bloods Back Thrust Zone; CL, Caroline Lineament; HF, Hinckley
Fault; LL, Lindsay Lineament; MAF, Mount Aloysius Fault; MF, Mann Fault; OB, Officer Basin; PDZ, Piltardi Detachment Zone; WDZ, Wankari Detachment Zone; WHL, Wintiginna‐Hinckley Lineament; WL, Wintiginna Lineament; WT, Woodroffe Thrust.From Raimondo et al. (2010)
Chapter B Gülcher, Johanna
7 3. Foreland Basins of the Petermann Orogen
Foreland basins are sensitive indicators of the isostatic and geodynamic processes of orogenic belts and therefore provide an important record of orogenic evolution (Aitken et al., 2009 and references therein). The Petermann orogenesis was accompanied by deposition of syn-tectonic siliciclastic sedimentary package in adjacent depocenters such as the Officer and Amadeus basins. The basins that flank the Musgrave Province therefore should record the evolution of the Petermann orogeny. Two spectacular and culturally significant Australian geographic landmarks are traditionally described as these fluvial deposits: Uluru (Ayers Rock) and Kata Tjuta (the Olgas).
The Mutitjulu Arkose (at Ulu ṟ u) and inferred correlative Mount Currie Conglomerate (at Kata Tju ṯ a), shown in Fig. B.4, have a maximum depositional age of about 1 Ga based on published detrital zircon data (Haines et al., 2016). Deposition during the 560 - 520 Ma Petermann
Orogeny is commonly assumed based on their syntectonic character and location near the boundary between
Mesoproterozoic basement of the Musgrave region, which was uplifted during the Petermann Orogeny, and the Amadeus Basin.
Furthermore, the Mount Curie conglomerate appears to uncomformably overly the Winall beds of the
Amadeus Basin, although this contact is
not well exposed. Such
an interpretation requires that the conglomerate was deposited late in the Petermann Orogeny — in the early Cambrian, or even later. Yet the top of the preserved Mount Currie Conglomerate (estimated to be 6 km thick) and the Mutitjulu Arkose (> 2.5 km thick) are metamorphosed to greenschist facies (abundant authigenic epidote in the matrix) indicating significant burial (Forman, 1966;
Edgoose et al., 2004;
Sweet et al., 2012).
Interestingly, there are no reports of meta- morphism in the supposedly underlying
Fig. B-4: Top: Image of the Australian landmarks Kata Tjuta (The Olgas) and Uluru (Ayers Rock). Bottom: map of Uluru national park and inferred correlation between the Mount Currie conglomerate (at Kata Tjuta) and the Mutitjulu Arkose (at Uluru).
Chapter B Gülcher, Johanna
8 Winnall beds. The apparent position of the low metamorphic grade rocks above non- metamorphosed rocks is at odds.
The Mount Currie conglomerate and Mutitjulu Arkose contain cobble and pebble clasts of fresh igneous and metamorphic rocks including granite, basalt, and other lithologies. The arenaceous components contain abundant labile minerals such as plagioclase and biotite.
Fresh rock, as seen in caves, has a grey colouration indicative of reducing conditions. In contrast, better studied syn-Peterman deposits (Supersequence 4) were confirmed to be deposited under conditions facilitating the breakdown of most labile minerals leaving relatively quartz-rich sediments, coloured red by iron oxides. The relative mineralogical maturity of these syn-Petermann deposits suggests warm humid conditions and deep weathering under oxidising conditions. This is consistent with palaeomagnetic data, which indicate that central Australia was equatorial during deposition of the Arumbera Sandstone (Mitchell et al., 2010).
The survival of more labile minerals and less oxidising conditions in the Mount Currie Conglomerate and Mutitjulu Arkose would be consistent with deposition under significantly colder and thus potentially higher latitude conditions.
These sedimentological differences, the odd placing above the unmetamorphised Winnal beds, as well as recent zircon studies by Haines et al. (2012; 2016) led to the suggestion that the Mount Currie Conglomerate and Mutitjulu Arkose do not date from the Petermann Orogeny at all, but rather have been deposited during the latest Mesoproterozoic within what has been termed the Ngaanyatjarra Rift (c. 1. Ga), further west (Haines et al., 2016). They were once buried by thick Amadeus Basin Neoproterozoic sediments, and fault emplaced into their current structural position during the Petermann Orogeny. This would evidence for deep burial and subsequent exhumation. Syn-rift sedimentary and low-grade metasedimentary rocks from the top of the Tjauwata Group contain similar mineralogically immature lithologies as the Mount Currie conglomerate and the Uluru Arkose. This interpretation would require that the previously inferred relationship between the Mount Currie Conglomerate and Winnall beds is incorrect, i.e. a tectonic contact rather than an unconformity. A final resolution, however, requires more data on field relationships (Haines et al., 2016).
References
Aitken A.R.A., Betts P.G., Ailleres L. (2009): The architecture, kinematics and lithospheric processes of a compressional intraplate orogen occurring under Gondwana assembly: The Petermann orogeny, central Australia. Lithosphere 1 (6), 343–357.
Flöttmann T., Hand M., Close D., Edgoose C., Scrimgeour I.R. (2004): Thrust tectonic styles of the intracratonic Alice Springs and Petermann orogenies, Central Australia. In Thrust Tectonics and Hydrocarbon Systems, edited by K. McClay, AAPG Mem., v. 82, p. 538-557.
Haines, P.W., Allen, H-J., Grey, K. and Edgoose, C. (2012): The western Amadeus Basin: revised stratigraphy and correlations. In Central Australian Basins Symposion III.
Haines, P.W., Kirkland, C.L., Wingate, M.T.D., Allen, H., Belousova E.A. and Gréau, Y. (106): Tracking sediment dispersal during orogenesis: A zircon age and Hf isotope study from the western Amadeus Basin, Australia. Gondwana Research, v. 37, p. 324-347.
Mitchell, R.N., Evand, D.A.D. and Kilian, T.M. (2010): Rapid Early Cambrian rotation of Gondwana.
Geology, v. 38, p. 755-758.
Raimondo, T., Collins, A.S., Hand, M., Walker-Hallam, A., Smithies, R.H., Evins, P.M. and Howard, H.M.
(2010): The anatomy of a deep intracontinental orogen. Tectonics, v. 29, TC4024
Raimondo, T., Hand, M. and Vollins, W.J. (2014): Compressional intracontinental orogens: Ancient and modern perspectives. Earth-Science Reviews, v. 130, p. 128-153
Veevers, J.J. (2004): Gondwanaland from 650-500 Ma assembly through 320 Ma merger in Pangeo to 185-100 Ma breakup: supercontinental tectonics via stratigraphy and radiometric dating. Earth. Sci.
Rev. v. 68 (1-2), p.1-132.
Wade, B.P., Kelsey, D.E., Hand, M. and Barovich, K.M. (2008): The Musgrave Province : Stitching north,
west and south Australia. Prec. Res., v. 166, p. 370-386.
Chapter C Schmid, Timothy
9
C. Woodroffe Thrust and Sheath Folds 1. Introduction
In compressional regimes, large-scale deformation is associated with either thick-skinned tectonics, which is characterized by basement-cored nappes or thin-skinned tectonics, which is characterized by an alternating flat-ramp thrust geometry, respectively. For both models, thrust planes are predicted to initiate with an angle of about 30° to the axis of maximum shortening. The Musgrave Block in central Australia experienced a major intracontinental compression event during the Peterman Orogeny (chapter B) at around 560 - 520 Ma with a first early phase of thin-skinned deformation and subsequent thick-skinned tectonics (Wex et al., 2017 and references therein). The Woodroffe Thrust is considered to be one of the largest thick-skinned shear zones at crustal-scale, well exposed in the central Musgrave Block, Australia.
The Woodroffe thrust is assumed to approximately extend a length over 600 km and so far, only 150 km have been established along strike and about 60km in thrusting direction, respectively. Dip measurements show that the thrust plane only dips shallowly to the south with a dip angle of ~6° (Wex et al., 2017) along several tens of kilometres. Such low-angle thrusts require the rocks along the thrust to be weak. However, field observations in this area (Bell & Etheridge, 1976; Bell, 1978; Bell & Johnson, 1989; Wex et al., 2017) argue for a strong behaviour, which contradicts the general assumption.
Fig. C-1: Geological map around the Woodroffe Thrust: Rocks from the footwall are of amphibolite facies gneisses whereas the hanging wall consists of granulite facies gneisses (Wex et al., 2017).
Chapter C Schmid, Timothy
10 2. Geology
Bound to the north by the Neoproterozoic to Palaeozoic Amadeus Basin and to the south and west by the Neoproterozoic to Palaeozoic Officer Basin, the Musgrave Block consists mainly of quartzo-feldspathic gneisses and granitoids with subordinate meta-dolerites, mafic gneisses and meta-pelites. Protholites of the gneisses were dominantly felsic volcanics, sediments and intrusives with depositional emplacement ages of roughly 1550 Ma (see also chapter B and E). Further, felsic magmatism is syn-tectonic to post-tectonic to the Musgravian Orogeny (~1200 Ma) subsequent episodes of bimodal magmatism and mafic magmatism (1080 to 1050 Ma and ~800 Ma) are widespread. During the Musgravian Orogeny, rocks were regionally metamorphosed into upper amphibolite facies in the footwall (Mulga Park Subdomain) to granulite facies in the hanging wall (Fregon Subdomain) gneisses representing different crustal levels of the same terrane, rather than being distinctly different blocks (Camacho & Fanning, 1995; Scrimgeour et al., 1999).
A second major deformation phase which affected the Musgrave Block was the Petermann Orogeny around 560 - 520 Ma (chapter B), during which the Woodroffe thrust developed together with other ductile shear zones such as the Mann and Hinckle Faults, the Davenport and Ferdinand shear zones, and the Wankari and Piltardi detachment zones (fig. C-1). The Woodroffe thrust separates the Mulga Park Subdomain (north of Woodroffe thrust) in the footwall from the Fregon Subdomain in the hanging wall. Both, footwall and hanging wall predominantly consists of granitoids (mostly in the footwall) and quartzo-feldspathic gneisses (mostly in the hanging wall) of different metamorphic grade. Metamorphic conditions attributed to the Petermann Orogeny are of high-pressure amphibolite to subeclogite facies.
Metamorphic conditions of mylonitization are estimated to be midcrustal (520 – 620 °C and 0.8 - 1.1 GPa) to lower crustal (650°C and 1.0 – 1.3 GPa; Wex et al., 2017).
Evidence for an influence of the later mid-Palaeozoic Alice Springs Orogeny in the Musgrave Block is sparse (Edgoose et al., 2004).
3. Field Observations
The mylonitic zone of the Woodroffe Thrust is defined by an association of protomylonites, mylonites, ultramylonites, and blastomylonites with degree of mylonitization decreasing into the footwall (Wex et al., 2017). The thickness of this zone is variable from several tens to hundred meters in the north to several hundreds of meters in the south. Pseudotachylytes and dolerite dykes are generally overprinted by the mylonitic foliation with pristine pseudotachylytes that crosscut and brecciate the mylonitic fabric (Wex et al., 2017). Measurements of foliation and stretching indicate N-S shortening, locally varying between NNE-SSW and NNW-SSE (fig.
C-2). General sense of movement is top-to-north in the main mylonitic zone. However, occasionally, stretching lineations from isolated shear zones show a top-to-south shear sense.
Mylonitic foliation is shallowly dipping and can be interpreted on a broad scale as a fold
interference, which produces a dome-and-basin like pattern. The thrust plane is generally flat
but undulate and thus, characterized by two local depressions creating the Kelly Hills and
Mount Fraser klippen, whereas the thrust plane steepens toward an average dip of ~30° in the
south (fig. C-2). Folds with a steep fold axial plane suggest a component of WNW-ESE
shortening postdating the development of the Woodroffe Thrust mylonites.
Chapter C Schmid, Timothy
11
Fig. C-2: Satellite imagery of the central Musgrave Block. Orientation data of the mylonitic foliation and stretching lineation are projected in lower hemisphere Schmidt nets. Measurements indicate N-S shortening, locally varying between NNE-SSW and NNW-SSE (Wex et al., 2017).
Further, a distinctly different mylonitic zone has been observed consisting of a coarser grain size with a roughly ESE dipping foliation, ENE to ESE plunging stretching lineation and a top- to-west shear sense (fig. C-3 left). These top-to-west mylonites overprint the dolerite dykes, which, in the footwall, are folded with roughly NE-SW trending fold axial planes. Later, top-to- west mylonites and the folded dolerite dykes are themselves overprinted, and orientated into parallelism with the Woodroffe Thrust. Orientation of fold axial planes into parallelism with the Woodroffe Thrust is described by Bell (1978) and references therein: Fold axes originally form at a high angle to the mineral-elongation direction (parallel to shear sense) and are passively rotated with increase in strain into parallelism with mineral-elongation direction. Changes in orientation of the fold axis greater than 100° within the fold axial plane were reported by Bell (1978), forming sheath folds, highly non-cylindrical structures associated with shear zones.
Sheath folds generally form in simple shear regimes and are indicative of high finite strain. An
essential detail to form sheath folds is the initial stage where pre-existing fold hinges are slightly
curved causing an instability or local anisotropy, which acts as a nucleus in which the sheath
fold will develop (Wex et al., 2014). If pre-existing fold hinges lie at a high angle to the flow
extensional eigenvector (Passchier, 1997), they tend to rotate towards it, forming sheath folds
(fig. C-3 right). A change in kinematics is also seen in strike-slip shear zones in the Musgrave
Block, indicating two different shortening directions (NNE-SSW and WNW-ESE).
Chapter C Schmid, Timothy
12
Fig. C-3: Left. Comparison between the top-to-west mylonites showing folded dolerite dykes and the top-to-north Petermann mylonites (Wex et al., 2017). Right. Sketch of sheath fold growth. Pre-existing fold axes are bent into parallelism with the lineation (Bell and Hammond, 1984).
4. Discussion
Present metamorphic facies and estimated temperature and pressure according to mineral assemblages and microstructures in of the Woodroffe Thrust are typical for a deep midcrustal shear zone. Mylonites and shear zones with Petermann kinematics show two distinctly different microstructures. Microstructures that still preserve pristine, ribbon-like aggregates of quartz and feldspar are abundantly observed along the N-S footwall (type-B micro-structure).
Microstructures where the mylonitic structure is annealed are only present in the southern footwall (type-A micro-structure). Textural and metamorphic differences along N-S support the idea that the two fabrics represent different times slices during shearing at lower crustal (annealed mylonite structure) and midcrustal conditions, representing progressively deeper crustal levels to the south. It is proposed that high-strain microstructures at the periphery of the Woodroffe Thrust formed during early lower crustal shearing and annealed as deformation progressively localized into the interior of the mylonitic zone at midcrustal levels (Wex et al., 2017). Geothermal gradients show no thermal perturbation and seems to be low but reasonable for old stable continental crust (~17°C/km).
Small temperature differences between two types of microstructures along an N-S profile (fig. C-4) indicate either a flat ramp or a low-angle ramp geometry that steepens to the south.
The Woodroffe Thrust was after the displacement of ~60 km at least partially thermally re-
equilibrated, which is rather unusual for a hanging wall, which overthrusts a flat-ramp
geometry.
Chapter C Schmid, Timothy
13
Fig. C-4: Geographical distribution of mylonitic type-A and type-B microstructures and temperatures from thermometry (Wex et al., 2017 and references therein).
Wex et al. (2017) propose a model where the footwall is underthrusted below the hanging wall as the Woodroffe thrust represents part of an in-sequence thrust system (fig. C-5). Thus, further shortening and crustal thickening is accommodated along progressively younger underlying thrust planes. Larger scale undulations of the Woodroffe Thrust leading to the Kelly Hills and Mount Fraser Klippen are likely due to a folding, which postdates mylonitization by a late-stage WNW-ESE directed shortening. The Woodroffe Thrust shows no evidence for retro- grade metamorphism as expected when a midcrustal package is exhumed. Partial exhumation of the entire region may have occurred by syntectonic erosion during the Petermann Orogeny and could explain the lack of any retrogressional metamorphism.
The low-angle geometry observed in the rather thick-skinned Woodroffe Thrust requires a weak precursor structure along which the thrust plane nucleates. The extent of the Woodroffe Thrust suggests such a structure to be a single horizon that was over 600 km long and
Fig. C-5: N-S cross section of the Woodroffe Thrust after the interpretation of Wex et al. (2017). Shortening and crustal thickening is accommodated along progressively younger underlying thrust planes.
Chapter C Schmid, Timothy
14 characterized by a high abundance of pseudotachylytes. If this horizon represents a single seismic rupture event (of a magnitude of 8-9), it remains unclear how such an event affects an intracontinental area such as the Musgrave Block.
References
Bell T.H. & Etheridge M.A. (1976): The Deformation and Recrystallization of Quartz in a Mylonite Zone, Central Australia. Tectonophysics 32, 235-267.
Bell T.H. (1978): Progressive Deformation and Reorientation of Fold Axes in a Ductile Mylonite Zone:
The Woodroffe Thrust. Tectonophysics 44, 285-320.
Bell T.H. & Johnson S.E. (1989): The role of deformation partitioning in the deformation and recrystallization of plagioclase and K-feldspar in the Woodroffe Thrust mylonite zone, central Australia. Journal of Metamorphic Geology 7, 151-168.
Camacho A. & Fanning, C.M. (1995): Some isotopic constraints on the evolution of the granulite and upper amphibolite facies terranes in the eastern Musgrave Block, central Australia. Precambrian Red 71, 155-181.
Edgoose C.J., Scrimgeour I.R., Close D.F. (2004): Report 15: Geology of the Musgrave Block, Northern Territory, North. Territ. Geol. Surv., Darwin.
Passchier C.W. (1997): The fabric attractor. Journal of Structural Geology 19, 113-127.
Scrimegour I.R., Close D.F., Edgoose C.J. (1999): 1:250,000 Geological Map Series and Explanatory Notes. Petermann Ranges SG52-7, 2
nded., North. Territ. Geolo. Surv., Darwin.
Wex S., Passchier C.W., De Kemp E.A., Ilhan S. (2014): 3D visualization of sheath folds in Ancient Roman marble wall coverings from Ephesos, Turkey. Journal of Structural Geology 67, 129-139.
Wex, S., Mancktelow N.S., Hawemann F., Camacho A., Pennacchioni G. (2017): Geometry of a large-
scale, low-angle, midcrustal thrust (Woodroffe Thrust, central Australia). Tectonics 36 AGU
Publications.
Chapter D Bruni, Elena
15
D. Pseudotachylytes in the Musgrove Ranges, Central Australia 1. Introduction: Why is it important to know about Pseudotachylites?
Pseudotachylyte (PST) is a rock that appears similar to a basaltic glass (“tachylyte”) and is usually dark and aphanitic. Although the origin of PST is still controversial, it is generally considered to be the product of solidification from a friction-induced melt produced at high strain rates as can occur during seismic faulting (“co-seismic slip”). Other geological processes are also reported, such as melting due to the shock waves emitted by hyper-velocity impacts of meteorites, but this is a less common scenario than earthquakes. Most PST are found spatially related to exhumed faults. Such PST is referred to as “tectonic pseudotachylyte” and is therefore considered as a type of fault rock. However, a clear separation from ultracataclasite or ultramylonite is only possible by means of microscopy (Sibson, 1975; Spray, 1992).
In the field, PST is found in the form of veins, with a distinction made between fault veins that lie on the generation surface of the melt and injection veins that branch off the fault veins into the surrounding host rock and form by melt that is extruded into the host rock along preexisting or newly formed (tensional) cracks.
As tectonic PST are considered to be a marker for fossil earthquakes, geologists started to study PST in order to better understand earthquake rupture dynamics by “direct” observation of fault rocks in addition to “indirect” geophysical monitoring of seismicity and numerical modeling.
Fig. D-1: Field examples of pseudotachylytes: (a) Pseudotachylyte breccia disrupting mylonitic foliation (26.3877 S, 131.7091E). (b) Late stage pseudotachylyte localizing at the boundary of a sheared dolerite dyke, creating a duplex- like structure with all planes of movement decorated by pseudotachylytes (N is up, 26.3408 S, 131.5255 E). (c) Polished slab with caramel coloured pseudotachylyte including fragments of quartzofeldspatic gneiss and of a mafic granulite. Note the internal foliation and elongation of clasts (26.3853 S, 131.7105 E). (d) Sheared pseudotachylyte in an otherwise undeformed gabbro (N is up, 26.3528 S, 131.8419 E) (Hawemann et al., 2017).
Chapter D Bruni, Elena
16 What limits the usefulness of PST in inferring earthquake source parameters is that (i) cataclastic deformation, which in many studies is found as precursor to PST formation, can occur both during aseismic creep or as the first increment of seismic deformation and it is essential to be able to distinguish between the two and (ii) that PST may be formed heterogeneously along the fault zone and it is difficult to assess a characteristic PST vein thickness (Di Toro et al., 2005). Additionally, subsequent deformation overprinting pristine PST and the lack of offset markers make it even more difficult to calculate earthquake parameters.
Since PST have been reported in different geotectonic environments and ambient conditions of formation, different mechanisms have been proposed to explain the production of PST and the associated earthquake. Whereas many PST occurrences are linked to brittle faults in the upper crust, others are found in spatial association with ductile shear zones and mylonites in mid-crustal depths (Di Toro et al., 2009). As PST is also found in highly metamorphosed terranes, it has been proposed that also intermediate and deep earthquakes can produce PST (Austrheim & Boundy, 1994; Lund & Austrheim, 2003). The proposed mechanisms for PST formation are:
(i) Frictional heating on a sliding surface at high strain rates (e.g. seismic faulting), occurring over such a short time that the process is adiabatic, leading to the production of frictional melts.
(ii) Mechanical energy that is dissipated as heat during ductile deformation in a shear zone, which is referred to as strain heating (Brun & Cobbold, 1980) or shear heating (Thielmann et al., 2015), which leads to a weakening of the deformed rock. This phenomena is called thermal softening and can result in an increased rate of deformation, which in turn leads to enhanced thermal softening. This is what is understood as thermal feedback. If temperature rises locally very quickly because the heat produced exceeds the heat that can be transported away by conduction, then highly localized catastrophic failure occurs, which is called thermal runaway (Braeck & Podladchikov, 2007).
(iii) Ultracataclasis that produces fine grained material that is no longer crystalline but amorphous. This process is called amorphization and is considered an alternative mechanism to form PST below the melting temperature.
Fig. D-2: Overview of the Prototerozoic cratons in Australia. The Musgrave block is located between two Archean cratons on the Proterozoic orogen (Hawemann et al., 2017).
Chapter D Bruni, Elena
17 Metamorphic dehydration reactions liberating a fluid from a previously hydrated subducting slab would lead to an increase of the pore fluid pressure, which acts against the lithostatic pressure and thereby lowers the effective stress, promoting brittle failure. This mechanism is referred to as dehydration embrittlement and has been used to explain the origin of double seismic zones (Hacker et al., 2003).
2. Pseudotachylites in Australia
The Woodroffe thrust is an E-W trending structure located within Meso-Neoproterozoic metamorphic rocks of the northern part of the Musgrave Block, central Australia (fig. D-1). The Musgrave Block is dominated by the geologically distinct Fregon and Mulga Park subdomains, which are separated by the Woodroffe thrust (Edgoose et al., 1993; Camacho et al., 1995).
The Fregon subdomain is composed of granulite facies gneiss derived from interlayered felsic volcanic rocks, while the Mulga Park subdomain consists of amphibolite facies gneiss and porphyritic granites (fig. D-2; Gray, 1978; Edgoose et al., 1993). The granulites are variably overprinted by the subsequent ecologite-facies deformation (T = 650°C and P = 12 kbar) related with the Woodroffe thrust (Ellis & Maboko, 1992), with high-strain bands, truncating and deflecting the granulite foliation. Sm-Nd mineral isochrons and
40Ar-
39Ar data show that the ecologite facies high strain overprint occurred ~550 Ma (Camacho et al., 1997).
The Woodroffe thrust zone consists of numerous mylonitic shear zones that anastomose around less shear-deformed or undeformed bodies of granulitic gneiss and granite (Fig. D-2;
Ellis & Maboko, 1992). The main (widest) mylonitic shear zone occurs at the bottom of a sequence of highly deformed rocks and contains a NE-SW to E-W striking mylonitic foliation and a prominent mineral lineation plunging 20–30° to the south, which is defined by the preferred alignment of clasts (fig. D-2c). The mylonite zones are generated interlayered with undeformed granulite facies rocks (fig. D-2c). The thickness of an individual mylonite zone varies from 10 cm to 10 m and the total thickness of the mylonite-ultramylonite zone is >300 m in the study region.
Large volumes of pseudotachylyte are developed in the Musgrave Range, which are bounded with the granulite facies rocks on the hanging wall and amphibolite facies rocks on the footwall within the Woodroffe thrust zone in the study region (fig. D-2a). The pseudotachylyte-containing block can also be recognized in a Landsat image, in which pseudotachylyte-bearing areas (blue colors) are bounded by granulite facies gneiss (dark- gray) (fig. D-2b). The block width is up to 3.5 km and the shear zone including M-Pt, Um-Pt and C-Pt veins are developed in a wide zone of >1.5 km within the Woodroffe thrust zone (fig.
D-2a). Based on structural and textural features, vein morphology, and host-rock lithology, four
types of pseudotachylyte are recognized, that are cataclasite-related (C-Pt), mylonite-related
(M-Pt), ultramylonite-related (Um-Pt) pseudotachylytes (Lin et al., 2005) and granulite-related
pseudotachylyte (G-Pt; fig. D-4).
Chapter D Bruni, Elena
18
Fig. D-3: (a) Geologic map of the central Musgrave Block (modified from Camacho et al., 1995; Shimamoto and Arai, 1997). (b) and (c) show enlargements of selected sections through the Woodroffe thrust (Lin et al., 2005).
Chapter D Bruni, Elena
19
Fig. D-4: Photographs showing the occurrences of C-Pt, M-Pt, G-Pt veins, mylonite. (a) C-Pt generation zone bounded by two main fault planes. C-Pt veins are injected and stopped in the generation zone. (b) Multiple generations of the C-Pt veins cut the M-Pt and G-Pt veins. The order of C-Pt-3, C-Pt-2, C-Pt-1, M-Pt, and G-Pt veins indicates the formation stages from younger to older in timing. The youngest Pt-3 veins cut all other stage pseudotachylyte veins (Lin, 2008).
References
Austrheim H. & Boundy T.M. (1994): Pseudotachylytes generated during seismic faulting and eclogitization of the deep crust. Science 265(5168), 82-83.
Braeck S. & Podladchikov Y.Y. (2007): Spontaneous thermal runaway as an ultimate failure mechanism of materials. Physical Review Letters, 98(9).
Brun J.P. & Cobbold P.R. (1980): Strain heating and thermal softening in continental shear zones: a review. Journal of Structural Geology, 2(1-2), 149-158.
Camacho A., Vernon R.H., Fitz Gerald J.D. (1995): Large volumes of anhydrous pseudotachylyte in the Woodroffe Thrust, eastern Musgrave Ranges, Australia. Journal of Structural Geology 17(3), 371–
383.
Di Toro G., Pennacchioni G., Teza G. (2005): Can pseudotachylytes be used to infer earthquake source parameters? An example of limitations in the study of exhumed faults. Tectonophysics, 402(1-4 SPEC. ISS), 3-20.
Di Toro G., Pennacchioni G., Nielsen S. (2009): Pseudotachylytes and earthquake source mechanics.
International Geophysics 94, 87-133.
Edgoose C.J., Camacho A., Wakelin-King G.A., Simons B.A. (1993): Kulgera, Northern Territory:
250,000 Geological Series, Northern Territory Geological Survey Explanatory Notes, SG 53–6, Darwin.
Ellis L., Maboko M.A.E. (1992): Precambrian tectonics and physico-chemical evolution of the continental crust, I. The gabbro-ecologite transition. Precambrian Research 55, 491–50.
Gray C.M. (1978): Geochronology of granulite facies gneisses in the western Musgrave Block, Central Australia. J. Geologic. Soc. Australia 25, 403–414.
Hacker B.R., Peacock S.M., Abers G.A., Holloway S. D. (2003): Subduction factory 2. Are intermediate- depth earthquakes in subducting slabs linked to metamorphic dehydration reactions? Journal of Geophysical Research: Solid Earth, 108(B1).
Hawemann F., Mancktelow N.S., Wex S., Camacho A., Pennacchioni G. (2017): Pseudotachylyte as field evidence for lower crustal earthquakes during the intracontinental Petermann Orogeny (Musgrave Block, Central Australia). Solid Earth Discussions 1–25.
Hottarek M. (2016): P-T-Conditions of pseudotachylyte formation in mafic and ultramafic host rocks of Alpine Corsica. ETH Zürich.
Lin A. et al. (2005): Propagation of seismic slip from brittle to ductile crust: Evidence from pseudotachylyte of the Woodroffe thrust, central Australia. Tectonophysics 402(1–4 SPEC. ISS), 21–
35.
Lin A. (2008): Seismic slip in the lower crust Inferred from granulite-related pseudotachylyte in the woodroffe thrust, Central Australia. Pure and Applied Geophysics 165(2), 215–233.
Lund M.G. & Austrheim H. (2003): High-pressure metamorphism and deep-crustal seismicity: Evidence from contemporaneous formation of pseudotachylytes and eclogite facies coronas. Tectonophysics 372(1-2), 59-83.
Menegon L. et al. (2017): Earthquakes as Precursors of Ductile Shear Zones in the Dry and Strong
Lower Crust. Geochemistry, Geophysics, Geosystems 18(12), 4356–4374.
Chapter D Bruni, Elena
20 Sibson R. H. (1975): Generation of Pseudotachylyte by Ancient Seismic Faulting. Geophysical Journal
of the Royal Astronomical Society 43(3), 775-794.
Spray J.G. (1992): A physical basis for the frictional melting of some rock-forming minerals.
Tectonophysics 204(3), 205-221.
Thielmann M., Rozel A., Kaus B.J.P., Ricard, Y. (2015). Intermediate-depth earthquake generation and shear zone formation caused by grain size reduction and shear heating. Geology 43(9), 791-794.
Wenk H.-R. (1978): Are pseudotachylites the product of fracture or fusion? Geology 6, 507–511.
Wenk H.R. & Weiss L.E. (1982): Al-rich calcic pyroxene in pseudotachylite: An indicator of high pressure and high temperature? Tectonophysics 84(2–4), 329–341.
* not published
Chapter E Tanner, Thomas
21
E. The Central Australian Superbasin 1. Introduction
Neoproterozoic sedimentary basins cover a large area of central Australia with the largest of them being the Amadeus, Georgina, Ngalia, Officier and Savory Basins (see fig. B-1). Acritarch biostratigraphy, isotope chemo- stratigraphy and conventional litho- stratigraphy of these basins allow a synthesis of the evolution of each basin, leading to the conclusion that they were once one single, Central Australian Superbasin (CAS; Walter et al., 1995).
Figure E-1 shows the inferred extent of the Neoproterozoic to Palaeozoic CAS and the currently preserved constituent basins.
To understand and interpret the sedimentary record in the Amadeus Basin, it is necessary to have basic knowledge of the tectonic evolution of Australia, the sedimentary history preserved in the CAS and the ideas behind the “Snowball Earth”
Hypothesis.
2. Tectonic Evolution of Proterozoic Australia
Proterozoic Australia has long been interpreted as a single intact continent with tectonic and magmatic activities being of intracratonic origin. Extensive research in various fields (e.g.
Proterozoic tectonics, supercontinent assembly and break-up, magnetostratigraphy etc.) over the last 30 years proposes a completely different story.
Starting in the Paleoproterozoic around 2500 Ma, according to Myers et al. (1996) numerous fragments of Archaean crust such as the Yilgarn, Pilbara or Gawler craton (fig. B- 1b) were already established by rifting and fragmentation. Over the next 700 Ma, these fragments were subsequently assembled into three cratons. Australia then consisted basically of a Western Australian Craton (formed by the Capricorn Orogeny starting at 1840 Ma), a South Australian Craton (assembled with the Kimban Orogeny at 1845 Ma) and a North Australian Craton (formed during the Hooper and Halls Creek Orogeny at 1865 and 1830 Ma), all of them surrounded by ocean (fig. E-3a).
Between 1700 and 1300 Ma, the southern margin of the North Australian Craton experienced repeated terrane accretion and orogenic activity. Additionally, intracratonic rifting created both the McArthur Basin and the Mount Isa rift and later deformation lead to the Isan Orogeny (see chapter F & G).
Tectonic activity between 1300 and 1100 Ma combined first the North Australian Craton with the West Australian Craton, incorporating the Rudall Complex. Later, the combined West and North cratons were joined to the South Australian Craton along the Albany-Fraser Orogen (fig. E-3b). This amalgamation of the three cratons led to the final assembly of Proterozoic Australia at approximately 1100 Ma as an early component of the supercontinent Rodinia.
3. The Formation and break-up of Supercontinents
Rodinia and Pangaea are the two supercontinents that we know to have included almost all the continents on Earth. There are some remarkable similarities between them: (1) Both had a lifespan of about 150 M.y. with Rodinia from ca. 900 Ma to 750 Ma and Pangaea from ca.
320 Ma to 180-160 Ma, (2) The break-up of both supercontinents started with broad mantle
Fig. E-1: Extent of the Neoproterozoic Central Australian Superbasin (Camacho et al., 2015).
Chapter E Tanner, Thomas
22 upwellings (or superplumes) beneath them, resulting in widespread bimodal magmatism and continental rifting.
These superplumes are generated by mantle avalanches, caused by the sinking of stagnated subduction slabs, which accumulated at the mantle transition zone surrounding the supercontinent, plus thermal insulation by the supercontinent. Such a superplume was formed about 40-60 M.y. after the completion of Rodinias assembly. As a result, widespread continental rifting occurred with episodic plume events (e.g. the Gairdner dykes at 825 Ma in figure E-3c).
This observation concludes that superplumes or clusters of plumes lead to the break-up of supercontinents and that therefore the lifespan of such a superplume is likely linked to the timespan of the related supercontinent cycle. The Pangean superplume (beneath Africa) started between 250 and 200 Ma and lasts to sometime into the future and the Rodinian superplume (beneath then South China and Australia) started between 860 and 820 Ma and lasted to at least around 600 Ma.
Fig. E-2: The supercontinent Rodinia at its peak assemblage and the theory of break-up caused by a superplume (Li et al., 2008).
Chapter E Tanner, Thomas
23 After a period of tectonic stability and a depositional hiatus of about 200 M.y., a rising superplume (related to the formation of Rodinia) uplifted much of central Australia around 900 Ma, leading to peneplanation of the uplifted region and the generation of large volumes of sand-sized clastic materials (Lindsay, 2002). This sand, known as the Heavitree Quartzite, forms the lowest stratigraphic unit of the CAS, supports the hypothesis of a rising superplume (Lindsay, 1999) and will be discussed later in this chapter. The decline of the superplume some 100 M.y. later led to thermal recovery and the development of a sag basin beginning around 800 Ma (fig. E-3c).
Following a relatively short sag phase of around 20 M.y. (and a possible hiatus of as much as 50 – 100 M.y.), compressional tectonics reactivated earlier thrust faults (such as the Woodroffe Thrust, see chapter C) which disrupted the CAS, causing uplift of basement blocks and breaking it into the four smaller fault-bounded basins that are still visible today. This compressional tectonic is most likely related to the early stages of break-up of Rodinia around 750 Ma and shaped the appearance of the Australian Continent significantly (fig. E-3d).
Time wise, around 750 Ma is also the onset of identified Neoproterozoic glaciations (“Snowball Earth”, will be discussed later) and it is likely that uplifted interbasinal basement blocks acted as nucleation sites for ice sheets (Lindsay, 2002).
Finally, the Petermann Orogeny (chapter B) at the Proterozoic-Phanerozoic boundary and the later Alice Springs Orogeny (chapter F) during the Devonian to Carboniferous time period, are two major intraplate orogenies that have affected the central Australian region as part of the final tectonic chapter of Australia, with the Alice Springs Orogeny closing the various basins at around 290 Ma (Lindsay, 2002).
Fig. E-3: Four major events in the tectonic history of Australia (Myers et al., 1996).
Chapter E Tanner, Thomas
24 4. Sedimentary History of the Central Australian Superbasin
The Neoproterozoic to Palaeozoic basins of central Australia (first the CAS and later its sub- basins) developed as a direct result of the assembly and dispersal of the supercontinent Rodinia (Lindsay, 2002). The basin(s) persisted for almost half a billion years leaving behind a sedimentary record of regional climatic and tectonic events. This sedimentary record is sometimes discontinuous within the same basin or it is hard to correlate similar sedimentary units of two or more different basins. Nevertheless, Walter et al. (1995) reconstructed a Neoproterozoic stratigraphy of the CAS by combining different sedimentary formations within the same basin into a “supersequence” and then correlating these supersequences across the various basins (fig. E-4). Discussing each formation within the various basins would go beyond the scope of this field guide, therefore this chapter focuses solely on the sediments within the Amadeus Basin.
Fig. E-4: The correlation of different units across the various subbasins in the CAS and sedimentary hiatus (Walter et al., 1995).
The Amadeus Basin is about 800 km long in an east-west direction, covers an area of 170’000 km
2and contains a maximum preserved sediment thickness of about 14 km. This sediment is of predominantly Neoproterozoic to Devonian-Carboniferous age and consists of almost entirely shallow marine or terrestrial deposits (Laurie et al., 1991). The stratigraphy described is also visible in figure E-5.
The oldest unit and also the base of supersequence 1, which is the only sequence consistent across the whole CAS (Lindsay 2002), is the Heavitree Quartzite or its lateral equivalent, the Dean Quartzite (these units are generally not metamorphosed, the term
“quartzite” is used in Australia for clan quartz arenites that are well cemented by silica, Walter
et al., 1995). The Heavitree Quartzite and its correlative formations record deposition in a sag
basin (the CAS) that subsided synchronously across much of the Australian Craton over a time
interval of about 40 M.y. in response to a decaying mantle plume prior to the break-up of
Rodinia. The large supply of quartz sand resulted from peneplanation associated with the
mentioned plume and the lack of soil-stabilising plants (Lindsay, 1999). The transition to the
top formation of supersequence 1, the carbonate-dominated Bitter Springs Formation, was
Chapter E Tanner, Thomas
25 gradual and conformable, indicating that subsidence gradually outstripped sediment supply and the water depth increased. Once the sand was redistributed by fluvial and tidal processes, the sag basin was starved of clastic sediment and sedimentation switched to evaporates and carbonate deposition.
There appears to be a pronounced hiatus in the sedimentary record after the first supersequence with a time gap as large as 100 M.y. in some basins (Lindsay, 2002). The Sturtian glaciation and the subsequent widespread flooding of the Australian plate left a distinctive sedimentary record. This is used to define supersequence 2, with glacial deposits marking the base unit (Walter et al., 1995). In the northern Amadeus Basin, the tillitic Areyounga Formation occurs disconformably above the Bitter Springs Formation and consists of up to 250 m of diamictite and conglomerate of glacial origin. The uppermost unit is a dolostone, which was referred to as the lower marker “cap dolomite” (Laurie et al., 1991).
Overlying and forming the top of supersequence 2 is the Aralka Formation consisting predominantly of shale and siltstone, indicating the aforementioned flooding.
Glacial deposits mark again the base of the next supersequence 3. These diamictites within the Olympic Formation (and its lateral equivalent the Pioneer Sandstone) have been correlated continent-wide and it seems that the Marinoan glaciation has been the most severe that the Earth has ever experienced (Walter et al., 1995). The distinctive “upper marker cap dolomite”
overlies the glacial deposits in the Amadeus Basin. As with supersequence 2, the glacial deposits are overlain by sedimentary products from a eustatic rise in sea level following deglaciation, visible in the Pertatataka Formation (consisting mostly of red and green shales and siltstones). The Pertatataka Formation shallows up into the Julie Formation, a widespread, relatively thin sequence of limestone.
Supersequence 4 is dominated by sandstone (Arumbera Sandstone), records the “Ediacara fauna” in its lower parts and has an Early Cambrian age in its top units (Walter et al., 1995).
The Arumbera Sandstone forms the base of the Pertaoorrta Group (Haines et al., 2012). These sand-rich sediments were transported into the Amadeus Basin from the uplifted Petermann Ranges and Musgrave Block during the Petermann Orogeny around 580 – 545 Ma (Lindsay, 2002). Supersequence 4 terminates the story of the Neoproterozoic but sedimentation continued for more than 200 M.y.
Because Walter et al. (1995) did not continue with the sedimentary history of the CAS, Lindsay (2002) continued with an overview of the Palaeozoic history. He used a slightly different categorization (e.g. combined supersequence 2 and 3 into one megasequence) and attributed the sequences to orogenic events. Walters supersequence 4 is equal to Lindsays megasequence 3 and the transition to megasequence 4 is not really visible in the Amadeus Basin (somewhere in the Arumbera Sandstone). Megasequence 4, the remainder of the Pertaoorrta Group spans most of the Cambrian and is comprised of a mixed carbonate- siliciclastic fossiliferous marine succession (Haines et al., 2012).
Megasequence 5 consists of sediments associated with the Late Cambrian Delamerian Orogeny. These sandstones of the upper Goyder Formation and the following five units of the Larapinta Group are composed of predominantly siliclastic sediments deposited in a shallow intracratonic sea (Laurie et al., 1991). This Group is economically important, especially the Pacoota Sandstone is the main reservoir for hydrocarbons.
Finally, the Palaeozoic Megasequence 6 is associated with the Alice Spring Orogeny. First, the Mereenie Sandstone represents marginal marine to terrestrial facies (Haines et al., 2012) or a possible arid desert across most of the basin (Laurie et al., 1991). Coarsening upward siltstones, sandstones and conglomerates of the Pertnjara Group represents molasses sediments accumulated in a fordeep at the flank of the uplift during the Orogeny (Haines et al., 2012).
Most of the deformation of the Amadeus Basin in the form of thrust faulting, folding and
diapirism occurred during the Alice Spring Orogeny and it has remained relatively stable since
this time. In the end, extensive Mesozoic and Tertiary erosion generated the present landscape
(Laurie et al., 1991).
Chapter E Tanner, Thomas
26
Fig. E-5: Stratigraphy of the North-Eastern Amadeus Basin (modified from Laurie et al., 1991).
5. Snowball Earth Theory
The appeal of the snowball earth hypothesis, first introduced by Joseph Kirschvink in 1992, is that it provides credible explanations for many previously enigmatic features of Neoproterozoic Earth history. It explains: (1) the widespread distribution of LNGD (Late Neoproterozoic glacial deposits) on virtually every continent, (2) the palaeomagnetic evidence that glacial ice lines reached sea level close to the equator for long periods, (3) the stratigraphic evidence that glacial events began and ended abruptly, (4) the reappearance of iron formations, exclusively within glacial units, after an absence of 1.2 billion years, (5) the world- wide occurrence of cap carbonates with unusual features, resting sharply above successive LNGD and (7) the existence of very large positive and negative delta
13C anomalies before and after each glacial event, respectively (Hoffman & Schrag, 2002).
Figure E-7 shows the widespread distribution of LNGD for the three major glacial
successions, a paleogeographical reconstruction for the presumed age period in which these
three glaciations occurred, and different schematic stratigraphic profiles taken from all over the
world.
Chapter E Tanner, Thomas
27 The following image E-6 shows a typical abrupt conformable contact between a glacial stratigraphic unit (here with dropstones within the Ghaub diamictite unit in north-west Namibia) and its cap dolostone (or cap carbonate). Most LNGD are capped by these continuous layers of pure dolostone, meters to tens of meters thick.
The idea behind these cap carbonates (and its delta
13C anomaly) is roughly the following:
A physically stratified ocean (for a long period of time during glaciation) switched to a turnover mode during and particularly after glacial events. In stratified mode, the biological pump (descent of organic particles from the surface) drove the delta
13C of dissolved inorganic carbon (DIC) in the surface waters to higher values and partial remineralization of the organic “rain” in the water column produced deep water with
13C depleted DIC. This increased the contrast between surface and deep-water DIC reservoirs, explaining the observed (positive and negative) excursions in delta
13C before and after a glaciation event. When turnover began (hence when glaciation retreated), anoxic, deep water upwelled to the surface, releasing CO
2and precipitating cap carbonates with low δ
13C values. The release of large amounts of greenhouse gases in the aftermath of glaciation enhanced silicate and carbonate rock erosion and large amounts of calcium were transported into the oceans (Hoffman & Schrag, 2002).
Fig. E-6: Abrupt contact between a glacial stratigraphic unit and its cap dolostone (Hoffman &
Schrag, 2002).
Chapter E Tanner, Thomas
28
Fig. E-7: Upper: Paleogeographical reconstructions showing the position of Australia for 750 and 580 Ma (Etienne et al., 2007). Lower Left: Modern distribution of Surtian, Marinoan, and Gaskiers glacial successions and association of Band Iron Formations (BIFs; Etienne et al., 2007). Lower Right: Locations (A) of schematic profiles (B) showing stratigraphic range, simplified lithologies, distribution of glacial deposits and occurrence of Ediacaran fossils (Halverson et al., 2005).
