Has Hyperextension Occurred on the Northwest Australian Shelf? The effects of pre-existing rift architectures on polyphase rifted margins.

Has Hyperextension Occurred on the Northwest Australian Shelf? The effects of pre-existing rift architectures on polyphase rifted margins.

Daniel Tek School of Earth and Environment, University of Leeds. Summer 2015 200632501 9959 Words Submitted in partial fulfilment of requirements for the degree of Master of Science, Structural geology with Geophysics.

Declaration of Academic Integrity UNIVERSITY OF LEEDS SCHOOL OF EARTH SCIENCES

To be attached to any essay, Dissertation, or project work submitted for assessment as part of a University examination.

I have read the University regulations on Cheating and Plagiarism, and I state that this piece of work is my own, and it does not contain any unacknowledged work from any other sources.

Name:

[printed]

Daniel Tek

signed

Date

Programme of Study:

MSc Structural Geology with Geophysics.

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Acknowledgements I would firstly like to thank Repsol and their Australia team for facilitating this project and providing the data used. I would specifically like to thank my company supervisor Dr. Oscar Frenandez for providing the opportunity to undertake this project and for providing expert advice and support throughout. Within the department, I thank my two internal supervisors: Simon Oldfield for his insight and guidance, helping me to overcome many hurdles, and Dr. Douglas Paton for his intellectually challenging suggestions helping to shape the project. Special thanks are extended to Ben Craven for all of his support, technical or otherwise, during often challenging times when using certain softwares. To all of my peers, especially my project peer William Eaton, who have helped make the project and the year an enjoyable experience I would like to express my gratitude. Finally, my appreciation goes out to my parents and my girlfriend who have helped me through this challenging yet rewarding year.

Software Used: Petrel 2013 (Schlumberger) – Seismic interpretation. Move 2015 (Midland Valley – Depth conversion. FlexDecomp (Badley Geoscience) – Backstripping. Microsoft Excel 2010 – Backstripping. CorelDraw – Cconstruction of images. ArcMap – Georeferencing images. Microsoft Word 2010 – Writing the thesis.

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Abstract The Northern Carnavon, Roebuck and Browse basins cover the majority of the northwest Australian passive margin. The margin has experienced a complex, polyphase extensional history leading to the accumulation of over 20km thick sediments in places. Although the presence of a Permian rift phase has long been documented, it remains poorly understood and thus, the pre-Triassic basin fill is often ignored. This study has attempted to determine the nature of this early, uncomprehended rift event using a suite of geophysical data and a number of geological interpretation techniques and investigate its effect on any subsequent extension. The findings of this report have revealed that, during a Permian extension event, hyperextension has occurred in the Northern Carnavon, Roebuck and Browse basins which has led to the exhumation and possible partial serpentinization of the uppermost lithospheric mantle. It is proposed that the presence of this pre-existing hyperextended rift architecture has heavily influenced the second (Late Jurassic – Early Cretaceous) rifting event that eventually led to the onset of oceanic spreading. The creation of lithospheric heterogeneities and the presence of a serpentinite slip surface from Permian hyperextension are thought to control the nature of Jurassic – Cretaceous stretching, making it highly depth dependent. Models for the evolution of uncomprehended features seen on the NW shelf, such as the Tres Hombres Dome and the Wombat Plateau have been presented. These could prove of interest for further study.

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List of Contents Preamble Declaration of Academic Integrity________________________________________________________i Acknowledgements________________________________________________________________________iii Abstract_____________________________________________________________________________________iv List of Contents_____________________________________________________________________________v List of Figures_____________________________________________________________________________viii

1. Introduction__________________________________________________________________1 1.1. Theoretical Background____________________________________________________2 1.2. Regional Setting___________________________________________________________14 1.3. Geological Background____________________________________________________16

2. Aims & Objectives______________________________________________________________23 2.1. Aims___________________________________________________________________________24 2.2. Objectives__________________________________________________________________24

3. Data Quality & Availability________________________________________________25 3.1. Gravity Data___________________________________________________________________26 3.2. Magnetic Data_________________________________________________________________27 3.3. Well Data__________________________________________________________________28 3.4. Seismic Data___________________________________________________________________30

v

4. Methodology_________________________________________________________________36 4.1. Flow Chart__________________________________________________________________37 4.2. Gravity Interpretation Methods____________________________________________38 4.3. Magnetic Interpretation Methods___________________________________________39 4.4. Seismic-Well Tie_____________________________________________________________40 4.5. Seismic Interpretation Methods_____________________________________________41 4.6. Depth Conversion Methods__________________________________________________48 4.7. Backstripping Methods______________________________________________________50

5. Results________________________________________________________________________51 5.1. Preliminary Observations_________________________________________________52 5.2. Mesozoic/Cenozoic Structure______________________________________________59 5.3. Deep Structure____________________________________________________________75 5.4. Nature of the COB___________________________________________________________________77 5.5. Depth Conversion___________________________________________________________________79 5.6. Backstripping_______________________________________________________________________83

6. Analysis_______________________________________________________________________86 6.1. Interpretation of Deep Structure and Early Basin History_________________87 6.2. Structure of the Mesozoic and Cenozoic Basin Fill__________________________96 6.3. Deep Structure___________________________________________________________101 6.4. Other Interesting Features_______________________________________________________104

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7. Discussion___________________________________________________________________107 7.1. Geological Evolution of the NW Australian Shelf: a comparison with published literature____________________________________________________________________108 7.2. Comparing the NW Shelf with Analogue Margins__________________________116 7.3. Some Remarks Regarding the Evolution of Polyphase Rifted Margins____118 7.4. Suggestions for Further Work____________________________________________________122

8. Conclusions__________________________________________________________________123 8.1. Concluding Remarks____________________________________________________________124

References______________________________________________________________________125

Appendices_____________________________________________________________________132 Appendix 1___________________________________________________________________________132 Appendix 2____________________________________________________________________________135 Appendix 3_______________________________________________________________________________139 Appendix 4_______________________________________________________________________________143 Appendix 5_______________________________________________________________________________149

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List of Figures Figures 1.1.

Map of the NW Australian shelf showing the four main basins and the

p.3

main hydrocarbon producing fields [Marshall & Lang, 2013]. 1.2.

Map of the NW Australian shelf showing the division of the main basins

p.3

into their sub-basins [Goncharov, 2004; Marshall & Lang, 2013; Google Earth, 2015]. 1.3.

Schematic sections contrasting the ‘pure shear’ and ‘simple shear’ models

p.5

of extension [after Buck et al, 1988]. 1.4.

Strength-depth profiles contrasting lithospheric necking and lithospheric

p.6

faulting [after Lavier & Manatschal, 2006]. 1.5.

Strength-depth profiles showing three variants of the ‘jelly sandwich’

p.6

model [after Burov & Watts, 2006; Lavier & Manatschal, 2006]. 1.6.

Models showing the process of depth dependent stretching [after Davis &

p.8

Kusznir, 2004]. 1.7.

Numerical model of lithospheric extension [after Kusznir et al, 2005].

p.8

1.8.

Model of the evolution of a hyperextended rifted margin [after Nagel &

p.10

Buck, 2007; Reston & Pérez-Gussinyé, 2007; Doré & Lundin, 2015]. 1.9.

Schematic section of a hyperextended rift invoking a series of convex-

p.9

down faults [Lavier & Manatschal, 2006]. 1.10.

Diagram defining domains of a hyperextended margin [after Sutra et al,

p.11

2013; Manatschal et al, 2015]. 1.11.

Series of diagrams showing the evolution of the ‘hyperextension’ and

p.12

‘depth dependent stretching’ end members of crustal extension [after Nagel & Buck, 2007; Reston & Pérez-Gussinyé, 2007; Huismans & Beaumont, 2011; Doré & Lundin, 2015]. 1.12.

Strength-depth profiles contrasting extension from a ‘slow’ rift with that

p.13

of a ‘fast’ rift event.

viii

1.13.

Diagrams showing the difference between structure of an idealised

p.14

lithosphere and a real lithosphere [Manatschal et al, 2015]. 1.14.

Evolutionary section showing the evolution of asymmetric rifted margins

p.15

[Brune et al, 2014]. 1.15.

Summary image showing the tectonic history of the NW shelf [compiled

p.20

from Longley et al, 2002; Heine & Mullet, 2005; Metcalfe, 2013]. 1.16.

Summary image showing the sequence stratigraphic classification for the

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NW shelf and palaeogeographic maps of the NW shelf [compiled from Longley et al, 2002]. 1.17.

Palaeogeographic map showing a Permian rifting event [Stagg et al,

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2004]. 1.18.

Maps of the NW shelf showing sediment thicknesses and crustal

p.18

thicknesses [Goncharov, 2004]. 1.19.

Section through the Northern Carnavon Basin from gravity foreward

p.19

modelling [Belgarde et al, 2015]. 1.20.

Map showing the division of the NW shelf into rift ‘zones’ [Belgarde et al,

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2015].

3.1.

Satellite free-air gravity map of the NW shelf [Sandwell et al, 2013].

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3.2.

Aeromagnetic anomaly map of the NW Shelf with basin outlines marked

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[Petrel, 2013]. 3.3.

Location map showing the position of the Huntsman 1 well.

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3.4.

Chronostratigraphic chart showing key horizons provided by Repsol

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acting as a pseudo-well. 3.5.

Image showing multiples in section 128_01.

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3.6.

Image showing migration smiles in section 95_07.

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3.7.

Image showing the effects of annealing on section 128_05.

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3.8.

Seismic section 120_01 showing the degradation of seismic imaging with

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ix

increasing depth. 3.9.

Image showing the difference in seismic imaging between surveys

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120_03 and db98_224.

4.1.

Image of a gravity high showing the cross-referencing process between

p.38

gravity, magnetic anomaly and bathymetry maps. 4.2.

Image of the Argo Abyssal Plain showing alignment of magnetic

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anomalies in oceanic crust. 4.3.

Image showing the 3D location of the Huntsman 1 well and its well tops.

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4.4.

Location map showing the four key interpreted seismic sections: 128_05,

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120_14, 120_01 and 128_03. 4.5.

Segment of line 120_01 showing the process of inferring the basement

p.45

structure. 4.6.

Segment of line 128_03 showing the process of inferring the moho.

p.46

4.7.

Map of the NW shelf showing the seed grid for the ‘Top Permian’ horizon

p.47

and the boundary polygons for all surfaces and thickness maps made. 4.8.

Velocity model for the NW shelf.

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4.9.

Image showing the process of backstripping line 120_01 with a β factor of p.50 1.

5.1.

Thickness maps between: Seabed - Top Permian, Seabed - Base Tertiary,

p.52

Base Tertiary - Top Syn-R2, Top Syn-R2 - Early Jurassic, Early Jurassic Intra-Triassic, and Intra-Triassic - Top Permian. 5.2.

Satellite image showing the bathymetry of the NW Australian shelf [after

p.56

Google Maps, 2015]. 5.3.

Interpreted gravity map of the NW shelf.

p.57

5.4.

Interpreted magnetic map of the NW shelf.

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5.5.

Images of section 120_01 showing: (A) section with Mesozoic/Cenozoic

p.59

x

horizons interpreted; (B) the uninterpreted section; (C) the section displayed with no vertical exaggeration; (D) a key to the interpreted seismic horizons; (E) location map of the line. 5.6.

Partial sections of line 120_01 showing two synforms present in the

p.61

section. 5.7.

Representative partial section showing horizons between the Top

p.62

Permian and the Seabed, and their internal structure. 5.8.

Images of section 128_05 showing: (A) section with Mesozoic/Cenozoic

p.63

horizons interpreted; (B) the uninterpreted section; (C) the section displayed with no vertical exaggeration; (D) a key to the interpreted seismic horizons; (E) location map of the line. 5.9.

Images of section 120_14 showing: (A) section with Mesozoic/Cenozoic

p.65

horizons interpreted; (B) the uninterpreted section; (C) the section displayed with no vertical exaggeration; (D) a key to the interpreted seismic horizons; (E) location map of the line. 5.10.

Images of section 128_03 showing: (A) section with Mesozoic/Cenozoic

p.67

horizons interpreted; (B) the uninterpreted section; (C) the section displayed with no vertical exaggeration; (D) a key to the interpreted seismic horizons; (E) location map of the line. 5.11.

Surface map of the Top Permian horizon showing the axes of the two

p.69

synforms shown in seismic line 120_01, their NE-SW trends, and lateral extents. 5.12.

Image showing coastward (SE) dipping faults cutting the Top Permian,

p.70

Intra-Triassic, Late-Triassic U.C., and Early Jurassic horizons in the SE of section 128_03. 5.13.

Image showing the SE of line 128_03 showing the Late_Triassic U.C.

p.71

truncating strata and a fanning of dip below the Top Permian. 5.14.

Images showing the nature of the Base Syn-R2 and Top Syn-R2 horizons

p.72

and the package between them. 5.15.

Map of the ‘Tres Hombres Dome’ shown in the Top Permian horixon

p.73

xi

showing its symmetrical nature. 5.16.

Images showing the location, bathymetric expression and seismic image

p.74

of the canyon surrounding the Wombat Plateau 5.17.

Part of line 120_01 showing the positions of basement highs based on the

p.75

nature of synformal structures. 5.18.

Part of line 120_01 showing a sediment package exhibiting a fanning of

p.76

dip to the NW of the large basement high in the section. 5.19.

Image of the COB seen in line 120_01. The transition is sharp and

p.77

evidenced by a cliff at the shelf edge. 5.20.

Image showing the COB in line 128_05, NW of the Wombat Plateau. The

p.78

COB is much less obvious here, it is gradational over ∼90km. 5.21.

Map of the COB surrounding the Argo Abyssal Plain.

p.78

5.22.

Depth converted line 120_01 showing (A) location map of the seismic

p.80

line; (B) 2x vertical exaggerated section; (C) 1:1 section showing the true geometries of the basement structures and the sediments above. 5.23.

Depth converted line 128_05 showing (A) location map of the seismic

p.81

line; (B) 2x vertical exaggerated section; (C) 1:1 section showing the true geometries of the basement structures and the sediments above. 5.24.

Figure 5.24. Depth converted line 128_03 showing (A) location map of

p.82

the seismic line; (B) 2x vertical exaggerated section; (C) 1:1 section showing the true geometries of the basement structures and the sediments above. 5.25.

Key to the depth converted units.

p.79

5.26.

Backstripped section showing line 120_01 restored to sea level.

p.84

5.27.

Backstripped section showing line 128_05 restored to sea level.

p.85

5.28.

Backstripped section showing line 128_03 restored to sea level.

p.83

6.1.

Schematic diagrams showing the evolution of seismic line 120_01 to the

p.87

Top Permian horizon.

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6.2.

Schematic diagrams showing the pitfalls in the classification of ‘necked’

p.88

and ‘hyperextended’ zones. 6.3.

Fully interpreted section of line 120_01 including basement structure.

p.90

6.4.

Fully interpreted section of line 128_05 including basement structure.

p.93

6.5.

Interpretive structure contour map of the Roebuck and north part of the

p.91

Northern Carnavon basins. 6.6.

Fully interpreted section of line 120_14 including basement structure.

p.94

6.7.

Fully interpreted section of line 128_03 including basement structure.

p.95

6.8.

3D image of the study area showing the geometry and lateral extents of

p.92

fault blocks along the basin. 6.9.

Figure 6.9. Map showing the basement terrains of Australia, used to

p.92

identify structural trends near the NW shelf [OZ Seebase, 2005]. 6.10.

Key to the megasequences described in this section and the horizons they

p.96

encompass. 6.11.

Schematic diagrams showing the interpretation of spreading direction

p.98

from magnetic anomalies. 6.12.

Stereonet showing the trends of faulting on the NW shelf and the likely

p.99

events that caused them. 6.13.

Diagram of the COB surrounding the Argo Abyssal Plain with a magnified

p.100

section containing a strain ellipse explaining the formation of ENE-WSW striking normal faults at the margin. 6.14.

Bathymetry map of the COB surrounding the Argo Abyssal Plain showing

p.101

the apparently small scale structural variability associated with the COB. 6.15.

Schematic diagrams showing the development of the COB at oblique

p.102

transform margins. 6.16.

Diagram explaining the evolution of a solely transform margin [Bird,

p.103

2001]. 6.17.

Evolutionary sections showing the evolution of the Wombat Plateau.

p.105

6.18.

Schematic sections of line 120_14 showing the possible evolutions of the

p.106

Tres Hombres Dome. 7.1.

Cross sections across the Northern Carnavon Basin taken from the

p.109

locations shown in fig. 7.2. (A-A’ on map A; B-B’ on map B). xiii

7.2.

Maps of the NW shelf dividing the shelf and its constituent basins into

p.110

hyperextended, necked and stretched zones. 7.3.

Potential models for the onset of oceanic spreading during R2.

p.111

7.4.

Three very simplified sections through: the NW Australian shelf, the

p.117

Norwegian Margin, and the Namibian Margin for use in comparison. 7.5.

Schematic sections showing the idealised evolution of a symmetrical

p.121

polyphase rifted margin that has undergone a period of hyperextension, a period of complete lithospheric re-equilibration, and another period of stretching.

Tables 3.1.

Location maps and basic information about each seismic survey used.

p.30

4.1.

Flow chart showing the order in which the methods were carried out.

p.37

4.2.

Table showing the key Mesozoic/Cenozoic horizons, their seismic

p.42

characteristics, the reason for picking these horizons, any uncertainty faced when picking, and whether the interpretation has been expanded across the shelf in a 3D interpretation. 7.1.

Summary table showing the geodynamic and tectonic evolution of the

p.113

NW Australian shelf.

Graphs 4.1.

Graph showing the effect on the ‘Top Permian’ horizon for each of the

p.48

different depth conversion scenarios.

Equations 1.1.

Equation for β stretching factor.

p.7

7.1.

Metamorphic reaction equation showing the dehydration of lizardite to

p.119

form talc, forsterite, clinochlore and fluid.

xiv

Appendices Appendix 1. Location maps for all the seismic surveys provided by Repsol.

p.132

Appendix 2. Sections testing the effect of the velocity model and compaction

p.135

curve on the depth conversion of section 120_01. Appendix 3. Sections showing the process of backstripping section 120_01 using p.139 a variable β factor. Appendix 4. Seed grids and surfaces generated for horizons: Top Permian,

p.143

Intra-Triassic, Early Jursassic, Top Syn-R2, Base Tertiary and Seabed. These horizons have been used to generate thickness maps. Appendix 5. A3 location map of the NW shelf is inserted as a loose sheet for

p.149

convenience.

xv

1. Introduction

1

1. Introduction

1.1. Regional Setting: The northwest Australian shelf comprises four separate basins: the Northern Carnavon, Roebuck (or Offshore Canning [Longley et al, 2002]), Browse, and Bonaparte basins (fig. 1.1). Although many of the near-coastal sub-basins hold significant hydrocarbon resources, the deeper water outer shelf remains relatively underexplored (fig. 1.1). The Northern Carnavon Basin can be divided into two zones: a series of en-echelon rift-related sub-basins bound to the SE by the Pilbara Block and to the NW by the Rankin and Exmouth platforms; the Exmouth Plateau, which lies NW of the Rankin Platform, is a broad sedimentary platform which contains little internal structure. The basin is bordered by the Argo, Gascoyne and Cuvier abyssal planes to the north, west and south respectively (fig. 1.2). The Roebuck and Browse basins are also sub-divided based on structural divisions. To its NW the Browse basin also extends into a broad platform, the Scott Plateau, which is also less explored than the rest of the shelf (fig. 1.2). Numerous studies have attempted to determine the tectonic and palaeogeographic evolution of the shelf [Longley et al, 2002; Heine & Muller, 2005; Chongzhi et al, 2013; Marshall & Lang, 2013; Metcalfe, 2013; Geoscience Australia, 2015a] however few have tried to constrain the geodynamic evolution of the area [Driscoll & Karner, 1998; Karner & Driscoll, 1999; Goncharov, 2004; Belgarde et al, 2015]. This study will focus on determining the deep structure and thus the geodynamic evolution of the lesser studied Roebuck Basin and adjacent parts of the Northern Carnavon and Browse basins (fig. 1.2).

2

1. Introduction

Figure 1.1. Map of the NW Australian basins showing the locations of major oil and gas fields in the area with field names in the legend. The largest hydrocarbon accumulations are in the coastal parts of the Northern Carnavon Basin and in the middle Browse Basin [Marshall & Lang, 2013].

Figure 1.2. Map of the NW shelf showing the locations of the four major basins, their constituent sub-basins and the surrounding abyssal plains. The pink box indicates the area of interest for this study [Goncharov, 2004; Marshall & Lang, 2013; Google Earth, 2015]. 3

1. Introduction

1.2. Teoretical Background: 1.2.1. Theoretical Development: The subsidence history of sedimentary basins has been a contentious issue since the development of the McKenzie [1978] uniform stretching model (fig. 1.3a). Commonly termed the ‘pure shear’ model [Buck et al, 1988], this theory posits that the whole lithosphere is stretched as one with deformation being accommodated by faulting in the upper crust and necking in the lithospheric mantle (fig. 1.3a). Post-rift subsidence deposits are formed from the thermal relaxation and contraction of the lithospheric mantle [McKenzie, 1978; Jarvis & McKenzie, 1980; Le Pichon & Sibuet, 1981; Houseman & England, 1986]. Although the McKenzie model holds true for several intra-continental, failed rift systems, it fails to explain thick post-rift deposits seen along many of the world’s passive margins requiring larger lithospheric stretching than that shown by the crust [Davis & Kusznir, 2004]. The Wernicke [1981] simple shear model [Buck et al, 1988] (fig. 1.3b) accounts for the discrepancy between fault-dominated crustal extension and lithospheric thermal subsidence by invoking a large, convex-down detachment that cuts to the asthenosphere [Wernicke, 1981; Wernicke & Burchfiel, 1982]. This model also provides an explanation for the highly asymmetric nature of some conjugate margins, and for vast terrains of exhumed mantle observed offshore Iberia [Brun & Beslier, 1996]. Despite allowing for thick passive margin deposits, the model is invalidated by the ‘upper plate paradox’ [Driscoll & Karner, 1998] which states that all passive margins (including conjugate pairs) correspond to the ‘upper plate’ (hanging wall). The main distinction between the two aforementioned models is the process by which the lithospheric mantle is thinned and eventually broken. The pure shear model accommodates this thinning by lithospheric necking [Zuber & Parmentier, 1986] (fig. 1.4.a) and the simple shear model by lithospheric faulting (fig. 1.4.b).

1.2.2. Rheological Structure of the Lithosphere: The ‘jelly sandwich’ model (fig. 1.5) is the generally accepted model explaining the strength of the lithosphere [Burov & Watts, 2006]. There are three variations of the model commonly used in numerical modelling (fig 1.4). Although most studies use the model shown in fig.1.5.a, the lithosphere’s inherent rheological structure can have a 4

1. Introduction large effect on rift architecture [Burov & Diament, 1995; Burov & Poliakov, 2001; Reston & Pérez-Gussinyé, 2007; Manatschal et al, 2015] so it is important to check which model is used.

Figure 1.3. Schematic models showing: (A) the McKenzie [1978] pure shear model with distributed thinning throughout the crust and lithospheric mantle; (B) the Wernicke [1981] simple shear model with a lithosphere-cutting low angle detachment providing increased subsidence in the hanging wall [after Buck et al, 1988].

5

1. Introduction

Figure 1.4. Diagrams showing the two models of deformation in the lithospheric mantle: (A) lithospheric necking, where stretching of the lithospheric mantle allows for upwelling of hot asthenosphere which raises the frictional-viscous transition thus allowing for viscous deformation of the lower lithosphere, the process is then self-perpetuating [after Zuber & Parmentier, 1986]; (B) lithospheric faulting, where the competent upper mantle acts in a brittle manner [after Lavier & Manatschal, 2006].

Figure 1.5. Diagrams showing the three commonly used variants of the ‘jelly sandwich’ model of crustal rheology: (A) brittle upper crust, weak lower crust, competent lithospheric mantle; (B) crust split into felsic and mafic each consisting of a competent ‘upper’ and weak ‘lower’, all above a competent mantle; (C) whole mafic crust is competent [after Burov & Watts, 2006; Lavier & Manatschal, 2006].

6

1. Introduction

1.2.3. Depth Dependent Stretching: Depth dependent stretching (DDS), a variant of the pure shear model, recognises a common discrepancy between whole lithospheric β stretching factor (eq. 1.1) and crustal β factor.

𝛽=

𝑡0 𝑡1

Equation 1.1.  stretching factor, where: t0 = original thickness, and t1 = present day crustal thickness [after Davis & Kusznir, 2004].

DDS requires a decoupling of deformation between the crust and the lithospheric mantle by the lower crust; the upper crust detaches onto a lower crustal shear zone (fig. 1.6). The basic principle behind DDS is that the lithospheric mantle is thinned more than the crust and the decoupling provided by the lower crust means that there is often a spatial discrepancy between the axis of crustal thinning and the axis of lithospheric necking (fig. 1.6.b). When the lithosphere thermally re-equilibrates, subsidence indicated by post rift sedimentation exceeds that indicated by upper crustal faulting [Davis & Kusznir, 2004; Kusznir et al, 2005]. Although DDS has traditionally been applied along commonly termed ‘volcanic’ margins such as NW Australia [Driscoll & Karner, 1998] and the Norwegian margin [Kusznir et al, 2005], numerical models have tried to accommodate for the exhumed mantle seen in some ‘non-volcanic’ margins (fig. 1.7). The main downfalls of DDS are: (1) although mantle exhumation can be accounted for in numerical models, it doesn’t account for the commonly observed lower crustpenetrating faults such as those offshore Norway and Angola [Osmundsen et al, 2002; Unternehr et al, 2010]; (2) DDS fails to account for the cooling and solidification of the lower crust with increasing stretching (fig. 1.8).

7

1. Introduction

Figure 1.6. Diagrams showing the process of DDS: (A) shows the lithospheric mantle being thinned massively while the crust is thinned only slightly following the same axis; (B) shows a scenario where the lithospheric thinning axis is offset from the crustal thinning axis [after Davis & Kusznir, 2004].

A

B

Figure 1.7. Numerical models of lithospheric extension in: (A) non-volcanic margin explaining the presence of exhumed mantle at the continent-ocean boundary; (B) volcanic margin where the zone of exhumation is much narrower [after Kusznir et al, 2005].

8

1. Introduction

1.2.4. Hyperextension: In contrast to DDS, hyperextension is “defined as stretching of the crust such that the lower and upper crust become coupled and embrittled, allowing major faults to penetrate to the mantle, leading to partial hydration (serpentinization) of the uppermost mantle” [Doré & Lundin, 2015, pp95]. If extension then continues, lithospheric mantle can be exhumed and this process allows for a consistent β for the entire lithosphere. Early models built on the simple shear model, invoking a series of convex-down faults (fig. 1.9) [Lavier & Manatschal, 2006] in order to solve the upper plate paradox. More recent studies [Nagel & Buck, 2007; Reston & Pérez-Gussinyé, 2007; Karner et al, 2007] have adopted lithospheric necking as the key process thinning the lithospheric mantle (fig. 1.8). The definition of the hyperextension process is defined above, but the classification of sub-terranes at hyperextended margins still lacks consensus. Sutra et al [2013] and Belgarde et al [2015] have defined zones of ‘stretching’, ‘necking’, and ‘hyperextension’ (fig. 1.10), these zones will be used in this study. Because hyperextension has been developed for commonly termed ‘non-volcanic’ margins such as offshore Iberia [Lavier & Manatschal, 2006; Sutra & Manatschal, 2012], it fails to explain the depth dependency of stretching seen at many margins [Kusznir et al, 2005].

Figure 1.9. Diagram of a rift system in which lithospheric extension is being accommodated by a series of convex-down detachment faults. Note the lithospheric structure follows that of fig. 1.5.c [Lavier & Manatschal, 2006].

9

1. Introduction

Figure 1.8. Series of schematic diagrams showing the evolution of a hyperextended rifted margin: (A) thermally equilibrated continental crust with a rheological structure equivalent to fig. 1.5.a; (B) distributed stretching accommodated by faulting in the upper crust and lithospheric necking in the lithospheric mantle, decoupled by the ductile lower crust; (C) Strain localises necking in the lithospheric mantle, the lower crust has been cooled by thinning and is now solid allowing faults to penetrate, hydrate, and detach onto the lithospheric mantle; (D) the upper crust breaks apart and exposes the exhumed mantle; (E) the remaining lithospheric mantle breaks apart and sea-floor spreading is initiated [after Nagel & Buck, 2007; Reston & Pérez-Gussinyé, 2007; Doré & Lundin, 2015]. 10

1. Introduction

Figure 1.10. Diagram defining the different domains of a hyperextended continental margin: innate crust shows no major upper crustal faulting and possibly a small amount of lower crustal thinning; stretched crust contains upper crustal faults which detach onto the lower crust; necked crust is where the lower crust has become brittle and deep crustal faults decolle onto serpentinized mantle; hyperextended terrains are those with exhumed mantle exposed under the post-rift sediments [after Sutra et al, 2013; Belgarde et al, 2015].

1.2.5. Current Theoretical Understanding: With increased understanding of volcanics at passive margins, the traditional labels of ‘volcanic’ and ‘non-volcanic’ classification is becoming redundant; it is now understood that most margins go through periods of volcanism [Davis & Kusznir, 2004; Huismans & Beaumont, 2011]. Because of this, more recent models of passive margin formation have acknowledged that DDS and hyperextension are by no means mutually exclusive but instead part of a two end-member system (fig. 1.11) [Kusznir & Karner, 2007; Huismans & Beaumont, 2011; Brune et al, 2014; Belgarde et al, 2015; Manatschal et al, 2015]. On one end of this idealised system sits pure ‘DDS’, where the lithospheric mantle reaches breakup leaving the crust fairly undeformed. At the other end lies ‘hyperextension’ where the crust and the mantle are stretched equally (fig. 1.11).

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B - Hyperextension end member.

Figure 1.11. Series of diagrams showing the two end members of the crustal stretching model: (A) evolution of a purely ‘hyperextended’ margin (fig. 1.8); (B) evolution of a margin that has undergone heavily depth dependent stretching in which the lower crust stays ductile for longer because of asthenospheric upwelling [after Nagel & Buck, 2007; Reston & Pérez-Gussinyé, 2007; Huismans & Beaumont, 2011; Doré & Lundin, 2015]. Note that all the strength-depth profiles relate to the central rift axis.

- Depth dependent stretching end member.

1. Introduction

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1. Introduction In understanding lithospheric deformation, it is important to thoroughly comprehend its structure. In simplified continental crust (fig. 1.5.a), the lithosphere-asthenosphere boundary and upper crust-lower crust boundary are both largely temperature dependent, whereas the moho corresponds to a compositional change. When the lithosphere is stretched, upwelling of asthenosphere will heat the lithosphere above and, when the crust is stretched and thinned, the surface temperature will cool the lower crust. Logically, rift timing will affect the nature of rifting [Pérez-Gussinyé & Reston, 2001] (fig. 1.12): a slow rift will allow thermal re-equilibration of the lithosphereasthenosphere boundary and therefore cooling of the lower crust is more likely leading to hyperextension; a fast rift will lead to rapid upwelling of the asthenosphere and therefore a longer decoupling between the upper crust and the lithospheric mantle. Not only does this make rifting time dependent, but also heavily temperature dependent [Manatschal et al, 2015].

Figure 1.12. Strength-depth profiles showing the effect that rift speed has on the structure of lithosphere that has been stretched by the same amount: (A) original crustal state following the jelly sandwich model (fig. 1.5.a); (B) lithospheric structure after slow rifting, where the lithospheric mantle has time to partially thermally re-equilibrate during rifting; (C) lithospheric structure after fast rifting, where hot asthenosphere heats the lithosphere.

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1. Introduction Besides rift dynamics (timing and temperature), rift-independent factors also influence the evolution of a rifted margin, broadly classified as ‘inheritance’. Manatschal et al [2015] define three types of lithospheric inheritance (fig. 1.13): (1) Thermal inheritance – the ambient crustal geotherm at the onset of rifting. This broadly corresponds to the age and thickness of the lithosphere. (2) Compositional inheritance – usually refers to the inherent strength-depth profile followed by the lithosphere. However, compositional variations are also present within the lithospheric mantle and the crust. (3) Structural inheritance – refers not only to crustal and lithospheric internal structures (faults etc.), but also the overall rheological layers of the lithosphere.

Figure 1.13. Diagrams showing the difference between (A) the idealised lithospheric structure used to model continental deformation; (B) a ‘real’ lithosphere with inheritance taken into account. This example is from the reconstructed Variscan belt along which the Iberian margin is thought to have broken [Manatschal et al, 2015].

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1. Introduction Recent theoretical developments have advanced our understanding of idealised, symmetric rifting, however complications that are still poorly understood include: the formation asymmetric conjugate margins (fig. 1.14) (Brune et al [2014] explain these using lower channel flow and rift migration); and polyphase rifting events.

Figure 1.14. Model showing how ‘rift migration’ can create asymmetry in conjugate continental margins [Brune et al, 2014].

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1. Introduction

1.3. Geological Background: 1.3.1. Tectonic History: The tectonic history is related to the breakup of Gondwana [Chongzhi et al, 2013]. Since the early Permian, the NW Australian shelf has undergone a complex subsidence history involving a Permian phase of extension (R1), a Triassic compressional event, and a second, Jurassic-Cretaceous epispode of extension (R2) (fig. 1.15: see page 20) [Heine & Muller, 2005; Metcalfe, 2013]. In places, over 20km thick sedimentary deposits have accumulated upon pre-Permian basement [Goncharov, 2004; Metcalfe, 2013].

1.3.2. Sedimentary History: Due to the scale of the NW Australian shelf, there are significant lateral geological changes, therefore making a lithostratigrpahic approach to basin-correlation unrealistic [Marshall & Lang, 2013]. In response to this, a regional sequence stratigraphic classification scheme is most commonly used (fig. 1.16: see pages 21 & 22) (first devised by Longley et al [2002]). Due to the abundant seismic and well constraints on the Mesozoic and Cenozoic stratigraphy, the sequence stratigraphic approach has become commonplace for most Mesozoic-Cenozoic sediments, and has allowed detailed palaeogeographic interpretations of the late Triassic-early Cretaceous (fig. 1.16). However, the deeper and older stratigraphy are scarcely studied and are only drilled on structural highs [Belgarde et al, 2015b]. Atop the ‘Bedout High’ within the Roebuck Basin, there is interpreted to be predominantly limestone and sandstone unit topped by the ‘Bedout Volcanics’, no thicknesses are given [Longley et al, 2002; Marshall & Lang 2013; Geoscience Australia, 2015b]

1.3.3. Geodynamic Work on NW Australian Shelf: 1.3.3.1. Permian Extension (R1): Due to good data availability and a good understanding of post-Permian Gondwana breakup, the majority of early geodynamic studies on the NW shelf focus on R2 (fig. 1.15) and the pre-Triassic is classified as ‘pre-rift’ [Longley et al, 2002, Marshall & Lang, 2013]. The generally accepted model of Permian extension is that of a large sag basin related to separation of the Cimmerian Continent [Karner & Driscoll, 1999] (fig. 1.15), however, to accommodate such thick sedimentary deposits, significant lithospheric thinning must have occurred. Stagg et al [2004] first interpreted a Permian failed rift stage between 16

1. Introduction the West Burma Block and the Australian Continent (fig. 1.17). Seismic velocity modelling (fig. 1.18) [Goncharov, 2004], and isostatic residual gravity interpretation across the shelf [Lockwood, 2004] have both shown that the NW shelf is covered by