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

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.

i

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.

iii

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.

iv

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.

v

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

vi

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

vii

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

viii

List of Figures

Figures

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

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

p.3

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

into their sub-basins [Goncharov, 2004; Marshall & Lang, 2013; Google

Earth, 2015].

p.3

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

of extension [after Buck et al, 1988].

p.5

1.4. Strength-depth profiles contrasting lithospheric necking and lithospheric

faulting [after Lavier & Manatschal, 2006].

p.6

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

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

p.6

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

Kusznir, 2004].

p.8

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 &

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

p.10

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

down faults [Lavier & Manatschal, 2006].

p.9

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

2013; Manatschal et al, 2015].

p.11

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

‘depth dependent stretching’ end members of crustal extension [after

Nagel & Buck, 2007; Reston & Pérez-Gussinyé, 2007; Huismans &

Beaumont, 2011; Doré & Lundin, 2015].

p.12

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

of a ‘fast’ rift event.

p.13

ix

1.13. Diagrams showing the difference between structure of an idealised

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

p.14

1.14. Evolutionary section showing the evolution of asymmetric rifted margins

[Brune et al, 2014].

p.15

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

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

p.20

1.16. Summary image showing the sequence stratigraphic classification for the

NW shelf and palaeogeographic maps of the NW shelf [compiled from

Longley et al, 2002].

p.21

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

2004].

p.17

1.18. Maps of the NW shelf showing sediment thicknesses and crustal

thicknesses [Goncharov, 2004].

p.18

1.19. Section through the Northern Carnavon Basin from gravity foreward

modelling [Belgarde et al, 2015].

p.19

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

2015].

p.19

3.1. Satellite free-air gravity map of the NW shelf [Sandwell et al, 2013]. p.26

3.2. Aeromagnetic anomaly map of the NW Shelf with basin outlines marked

[Petrel, 2013].

p.27

3.3. Location map showing the position of the Huntsman 1 well. p.28

3.4. Chronostratigraphic chart showing key horizons provided by Repsol

acting as a pseudo-well.

p.29

3.5. Image showing multiples in section 128_01. p.33

3.6. Image showing migration smiles in section 95_07. p.33

3.7. Image showing the effects of annealing on section 128_05. p.34

3.8. Seismic section 120_01 showing the degradation of seismic imaging with p.35

x

increasing depth.

3.9. Image showing the difference in seismic imaging between surveys

120_03 and db98_224.

p.35

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

gravity, magnetic anomaly and bathymetry maps.

p.38

4.2. Image of the Argo Abyssal Plain showing alignment of magnetic

anomalies in oceanic crust.

p.39

4.3. Image showing the 3D location of the Huntsman 1 well and its well tops. p.40

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

120_14, 120_01 and 128_03.

p.41

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

structure.

p.45

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

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

p.47

4.8. Velocity model for the NW shelf. p.49

4.9. Image showing the process of backstripping line 120_01 with a β factor of

1.

p.50

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

Base Tertiary - Top Syn-R2, Top Syn-R2 - Early Jurassic, Early Jurassic -

Intra-Triassic, and Intra-Triassic - Top Permian.

p.52

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

Google Maps, 2015].

p.56

5.3. Interpreted gravity map of the NW shelf. p.57

5.4. Interpreted magnetic map of the NW shelf. p.58

5.5. Images of section 120_01 showing: (A) section with Mesozoic/Cenozoic p.59

xi

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

section.

p.61

5.7. Representative partial section showing horizons between the Top

Permian and the Seabed, and their internal structure.

p.62

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




p.63





p.65





p.67

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

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

extents.

p.69

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

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

section 128_03.

p.70

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

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

p.71

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

and the package between them.

p.72

5.15. Map of the ‘Tres Hombres Dome’ shown in the Top Permian horixon p.73

xii

showing its symmetrical nature.

5.16. Images showing the location, bathymetric expression and seismic image

of the canyon surrounding the Wombat Plateau

p.74

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

nature of synformal structures.

p.75

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

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

p.76

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

evidenced by a cliff at the shelf edge.

p.77

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

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

p.78

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

line; (B) 2x vertical exaggerated section; (C) 1:1 section showing the true

geometries of the basement structures and the sediments above.

p.80

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

line; (B) 2x vertical exaggerated section; (C) 1:1 section showing the true

geometries of the basement structures and the sediments above.

p.81

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

the seismic line; (B) 2x vertical exaggerated section; (C) 1:1 section

showing the true geometries of the basement structures and the

sediments above.

p.82

5.25. Key to the depth converted units. p.79

5.26. Backstripped section showing line 120_01 restored to sea level. p.84



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

Top Permian horizon.

p.87

xiii

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

and ‘hyperextended’ zones.

p.88

6.3. Fully interpreted section of line 120_01 including basement structure. p.90


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

Northern Carnavon basins.

p.91



6.8. 3D image of the study area showing the geometry and lateral extents of

fault blocks along the basin.

p.92

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

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

p.92

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

encompass.

p.96

6.11. Schematic diagrams showing the interpretation of spreading direction

from magnetic anomalies.

p.98

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

events that caused them.

p.99

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

section containing a strain ellipse explaining the formation of ENE-WSW

striking normal faults at the margin.

p.100

6.14. Bathymetry map of the COB surrounding the Argo Abyssal Plain showing

the apparently small scale structural variability associated with the COB.

p.101

6.15. Schematic diagrams showing the development of the COB at oblique

transform margins.

p.102

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

2001].

p.103

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

Tres Hombres Dome.

p.106

7.1. Cross sections across the Northern Carnavon Basin taken from the

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

p.109

xiv

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

hyperextended, necked and stretched zones.

p.110

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

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

p.117

7.5. Schematic sections showing the idealised evolution of a symmetrical

polyphase rifted margin that has undergone a period of hyperextension, a

period of complete lithospheric re-equilibration, and another period of

stretching.

p.121

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

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.

p.42

7.1. Summary table showing the geodynamic and tectonic evolution of the

NW Australian shelf.

p.113

Graphs

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

different depth conversion scenarios.

p.48

Equations

1.1. Equation for β stretching factor. p.7

7.1. Metamorphic reaction equation showing the dehydration of lizardite to

form talc, forsterite, clinochlore and fluid.

p.119

xv

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

curve on the depth conversion of section 120_01.

p.135

Appendix 3. Sections showing the process of backstripping section 120_01 using

a variable β factor.

p.139

Appendix 4. Seed grids and surfaces generated for horizons: Top Permian,

Intra-Triassic, Early Jursassic, Top Syn-R2, Base Tertiary and Seabed. These

horizons have been used to generate thickness maps.

p.143

Appendix 5. A3 location map of the NW shelf is inserted as a loose sheet for

convenience.

p.149

1

1. Introduction

1. Introduction

2

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).

1. Introduction

3

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].

1. Introduction

4

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

1. Introduction

5

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].

1. Introduction

6

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].

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].

1. Introduction

7

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 =𝑡0𝑡1

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 crust-

penetrating 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).

Equation 1.1. stretching factor, where: t0 = original thickness, and t1 = present

day crustal thickness [after Davis & Kusznir, 2004].

1. Introduction

8

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].

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].

A

B

1. Introduction

9

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].

1. Introduction

10

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].

1. Introduction

11

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).

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. Introduction

12

Fig

ure

1.1

1. S

erie

s o

f d

iagr

ams

sho

win

g th

e tw

o e

nd

mem

ber

s o

f th

e cr

ust

al s

tret

chin

g

mo

del

: (A

) ev

olu

tio

n o

f a

pu

rely

‘hyp

erex

ten

ded

’ mar

gin

(fi

g. 1

.8);

(B

) ev

olu

tio

n o

f a

mar

gin

that

has

un

der

gon

e h

eav

ily

dep

th d

epen

den

t st

retc

hin

g in

wh

ich

th

e lo

wer

cru

st s

tays

du

ctil

e fo

r lo

nge

r b

ecau

se o

f as

then

osp

her

ic u

pw

elli

ng

[aft

er N

agel

& B

uck

, 20

07

; Res

ton

&

Pér

ez-G

uss

iny

é, 2

00

7; H

uis

man

s &

Bea

um

on

t, 2

01

1; D

oré

& L

un

din

, 20

15

]. N

ote

th

at a

ll t

he

stre

ngt

h-d

epth

pro

file

s re

late

to

th

e ce

ntr

al r

ift

axis

.

- D

epth

de

pen

den

t str

etc

hin

g e

nd

mem

ber.

-

Hyp

ere

xte

nsio

n e

nd m

em

ber.

B

1. Introduction

13

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 lithosphere-

asthenosphere 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.

1. Introduction

14

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].

1. Introduction

15

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].

1. Introduction

16

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

1. Introduction

17

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

<18km of sediment onto ∼4km thick crust (compared to an onshore continental

thickness of 35-40km [Goncharov, 2004], supporting the argument for extensive crustal

thinning. Proceedings from the recent APPEA 2015 conference have provided further

insights into the deep structure of the NW shelf. Belgarde et al [2015] have used gravity

forward modelling and new deep reflection seismic, and have proposed the area may

have experienced hyperextension during the Permian (fig. 1.19); the shelf is divided

into ‘stretched’ ‘necked’ and ‘hyperextended’ zones (fig. 1.20).

1.3.3.2. Jurassic-Cretaceous Extension (R2):

The late Jurassic rifting of the West Burma Block is well studied and understood as

ample, good quality seismic and well data are available for the shallow sediments

[Goncharov, 2004]. Karner & Driscoll [1999] found for the Northern Carnavon basin

that the extension and subsequent thermal subsidence pertaining to R2 is highly depth-

dependent. Although they invoke an unrealistic ramp-flat-ramp detachment fault, the

study provides a maximum lithospheric β factor of 2.65 and a maximum upper crustal β

factor of 1.15.

A

Figure 1.17. Map showing the rifting on the NW Shelf Margin (NWSM). MVL

stands for Mount Victoria Land is equivalent to the West Burma Block [Stagg et

al, 2004].

1. Introduction

18

Figure 1.18. Maps of the NW shelf showing: (A) sediment thickness (in km)

taken from seismic data; (B) crustal thickness (in km) taken from seismic

velocity modelling [Goncharov, 2004]

B

A

1. Introduction

19

Figure 1.19. Proposed cross section along line A-A’ across the Northern

Carnavon Basin (shown on fig. 1.20) derived from gravity forward modelling.

[Belgarde et al, 2015]

Figure 1.20. Map showing the division of the NW shelf into rift ‘zones’ [Belgarde

et al, 2015].

Introduction 1.

20

Fig

ure

1. 1

5. S

um

mar

y im

age

sho

win

g th

e te

cto

nic

his

tory

of

the

NW

sh

elf

[co

mp

iled

fro

m

Lo

ngl

ey e

t al

, 20

02

; Hei

ne

& M

ull

et, 2

00

5; M

etca

lfe,

20

13

].

Introduction 1.

21

Introduction 1.

22

Fig

ure

1.1

6. (

pag

es 2

1 a

nd

22

) Su

mm

ary

imag

e sh

ow

ing:

(le

ft)

the

seq

uen

ce s

trat

igra

ph

ic c

lass

ific

atio

n c

har

t

for

the

NW

sh

elf

incl

ud

ing

each

seq

uen

ce s

trat

igra

ph

ic p

ack

age

and

th

eir

corr

esp

on

din

g b

ou

nd

ary

ho

rizo

ns;

(rig

ht)

pal

aeo

geo

grap

hic

map

s o

f th

e N

W s

hel

f th

rou

gh v

ario

us

stag

es o

f it

s ev

olu

tio

n f

rom

th

e T

rias

sic

to t

he

Cre

tace

ou

s [c

om

pil

ed f

rom

Lo

ngl

ey e

t al

, 20

02

].

23

2. Aims & Objectives

2. Aims & Objectives

24

2.1. Aims:

To document the geodynamic and tectonic evolution of the NW Australian shelf.

To determine how many episodes of extension have been experienced in the area of

interest.

Belgarde et al [2015] propose that hyperextension has occurred during a Permian

rifting event. This study aims to test this interpretation and compare the lateral

extents of ‘hyperextended’, ‘necked’ and ‘stretched’ zones across the area of interest.

To find out whether DDS has occurred related to a second episode of rifting and to

test lithospheric β factors presented by Karner & Driscoll [1999].

If the NW shelf has experienced multiple rifting events, this study aims to investigate

the nature of the relationship between the different phases.

2.2. Objectives:

Carry out a regional 3D seismic interpretation using regional 2D seismic data,

producing surfaces and thickness maps of key correlatable horizons to show broad

structural trends.

Use free-air gravity and magnetic anomaly data to support the findings of the

seismic interpretation.

Perform a detailed 2D seismic interpretation on four key, representative seismic

lines across the area of interest.

Use backstripping software to determine a whole lithospheric β factor from any

post-rift sedimentation.

25

3. Data Quality & Availability


26

3.1. Gravity Data:

A global satellite free-air gravity grid has been used. The accuracy of the gravity

measurement is up to 1mGal with a spatial resolution of 7km (half a wavelength)

[Sandwell et al, 2013].

Fig

ure

3.1

. Sat

elli

te f

ree-

air

grav

ity

map

of

the

NW

sh

elf

wit

h b

asin

loca

tio

ns

mar

ked

. Map

mai

nly

use

d f

or

inte

rpre

tin

g b

asin

ext

ents

. Gre

en b

ox

ind

icat

es t

he

area

of

inte

rest

fo

r th

is

stu

dy.

War

m c

olo

urs

ind

icat

e gr

avit

y h

igh

s [S

and

wel

l et

al, 2

01

3].


27

3.2. Magnetic Data:

A global aeromagnetic grid has been used. The grid is available as an overlay in Petrel

2013, it has a 2 arc-minute resolution [Petrel, 2013].

Fig

ure

3.2

. Aer

om

agn

etic

an

om

aly

map

of

the

NW

Sh

elf

wit

h b

asin

ou

tlin

es m

ark

ed. G

rid

use

d p

rim

aril

y fo

r in

terp

reta

tio

n o

f th

e o

cean

-co

nti

nen

t b

ou

nd

ary.

Th

e gr

een

bo

x in

dic

ates

the

area

of

inte

rest

war

m c

olo

urs

ind

icat

e m

agn

etic

hig

hs

[Pet

rel,

20

13

].


28

3.3. Well Data:

3.3.1. Huntsman 1 Well:

Well data for across most of the NW shelf has been provided by Repsol as part of the

‘2012 Acreage Release’ data package from Geoscience Australia. Of these data, only one

in the area of interest is time-converted and contains the appropriate marker horizons:

the ‘Huntsman 1’ well (fig. 3.3). The well was drilled in 2007 by Woodside in the Beagle

Sub-Basin and penetrates to a depth of 4343.8m (4137.2ms-TWT).

Fig

ure

3.3

. Lo

cati

on

map

sh

ow

ing

the

po

siti

on

of

the

Hu

nts

man

1 w

ell.

Lin

e 1

20

_01

is t

he

clo

sest

sei

smic

lin

e to

th

e w

ell s

o w

ill b

e u

sed

to

tie

(se

e se

ctio

n 4

).


29

3.3.2. Pseudo-Well:

Five key, regionally extensive horizons have been provided by Repsol in the form of a

pseudo well in the same location as the Huntsman 1 well (fig. 3.4).

Figure 3.4. Chronostratigraphic chart showing key horizons provided by Repsol

acting as a pseudo-well.


30

3.4. Seismic Data:

3.4.1. Data Availability:

Over 4,000 individual seismic lines within 89 surveys (app. 1) have been provided by

Repsol, compiled from Geoscience Australia’s NW shelf dataset; all of these lines are in

Two Way Time (TWT). Of these surveys, 6 have been used (table 3.1). The data were

provided as a Petrel 2013 project in coordinate system: WGS1984, UTM 51°S.

Survey 110.

Survey Name: AGSO Survey 110

(SNOWS-2); Barrow/Dampier M.S.S.

Acquired By: Australian Government

Survey Organisation (AGSO).

Year of Survey: 1990.

Survey Location: Barrow and

Dampier sub-basins (release area

W07-01).

Spacing: N/A

Polarity: Positive.

Survey Depth: 16 s-TWT.

Survey 128.

Survey Name: AGSO Survey 128.

Acquired By: AGSO.


Survey Location: Northern Carnavon

Basin, Roebuck Basin, Browse Basin

(release area W11-07 and W11-08).

Spacing: N/A

Polarity: Positive.

Survey Depth: 16s-TWT.


31

Survey 095.

Survey Name: AGSO Marine Survey

95; Canning/Exmouth.

Acquired By: AGSO.




(release area W07-18).

Spacing: N/A

Polarity: Positive.

Survey Depth: 6-9s-TWT.

Survey 120.

Survey Name: AGSO Survey Marine

120 (SNOWS-3); Southern North

West Shelf.

Acquired By: AGSO.


Survey Location: Roebuck Basin

(release area W07-11).

Spacing: N/A

Polarity: Positive.


Survey 119.

Survey Name: AGSO Marine Survey

119; Browse Basin M.S.S.

Acquired By: AGSO.


Survey Location: Browse (release

area W07-09).

Spacing: N/A

Polarity: Positive.



32

Survey dc98.

Survey Name: Deep Water North

West Shelf Spec M.S.S.

Acquired By: GHD-Gardline Surveys

Party Ltd.




(release areas W11-7, W11-8 and

W11-9).

Spacing: 10km.

Polarity: Negative.


3.4.2. Processing:

Processing information is not available for the seismic data provided, however there are

indications within the seismic as to the way the data has been processed:

(1) Amplitude Gain Correction – The strength of some multiples suggests that an

amplitude gain may have been applied to AGSO surveys: 128, 95, 120 and 119

(fig. 3.5).

(2) Migration – All surveys show migration smiles in the deep section (fig. 3.6).

(3) Annealing – Has been used in order to create coherent reflectors. This can be

seen when viewing faults, where a fault may be well defined in shallow parts of

the section, in the deep section these faults may be masked due to annealing (fig.

3.7).

Table 3.1. Shows location maps for each of the surveys and basic information

about them.


33

Figure 3.5. Seismic section 128_01 showing seabed multiples in the section (5x

vertically exaggerated).

NW SE

16s

3s

SW NE

9s

3s

Figure 3.6. Seismic section 95_07 showing large and strong migration smiles in

the deep section (5x vertically exaggerated).


34

3.4.3. Data Quality:

The aforementioned processing artefacts all affect the quality of the seismic and,

consequently increase the uncertainty in interpretation. Aside from these factors, there

is also a definite degradation in data quality towards the deeper section from the

shallow section (fig. 3.8). There is also a discrepancy in data quality between different

surveys (fig. 3.9). Note, all seismic images will be shown in the ‘red white blue’ colour

scheme where blues represent peaks.

NNW SSE 2s

9s

Figure 3.7. Part of seismic section

128_05 showing the effects of

annealing. The faults in the

shallow section are well defined

and easily picked however in the

deeper section (about 4-5

seconds) there appears to be a

series of coherent reflectors. This

could also be due to a

detachment horizon however

(discussed in section 5) (5x

vertically exaggerated).


35

Figure 3.8. Seismic section 120_01 with a dashed black line showing the depth

beneath which the seismic section becomes very unclear and shows very little

structure (10x vertically exaggerated).

NW SE

0s

16s

Figure 3.9. Comparison between lines of two different surveys that run alongside

one another: (A) is a part of line 120_03, a deep regional survey. Very few faults

are visible in this section; (B) is a part of line dc98_224, a shallower regional

survey; (C) shows the faults that can be interpreted in line dc98_224 that are not

visible in line 120_03 (5x vertically exaggerated).

A B

C

1.5s

7s

36

4. Methodology

4. Methodology

37

4.1. Flow Chart:

Table 4.1. Flow chart showing the order in which the following methods were

carried out, and the feedback between the different methods.

4. Methodology

38

4.2. Gravity Interpretation Methods:

A free-air gravity grid has been input into Petrel and inverted. Free-air gravity includes

a signature from: seafloor bathymetry, short wavelength features such as basement

highs or deep basins that would be best imaged using bouguer gravity, and moho

fluctuations usually imaged using isostatic residual gravity. Thus, to interpret short

wavelength features as required by this study, the gravity must be cross referenced

with the magnetic anomaly and bathymetry for said area (fig. 4.1).

Figure 4.1. Image of a gravity high showing the cross-referencing process

between: (A) gravity; (B) magnetic anomaly; (C) Bathymetry. As there is no

bathymetric expression but there is both a gravity and magnetic high, this

feature is interpreted as a basinal feature.

250km

4. Methodology

39

4.3. Magnetic Interpretation Methods:

A magnetic anomaly grid has been loaded as an overlay in Petrel As well as being used

alongside gravity data to interpret basement highs and other sedimentary features, the

magnetic anomaly of oceanic crust can provide information on the direction of seafloor

spreading (fig. 4.2).

Figure 4.2. Image of the Argo Abyssal Plain showing the alignment of magnetic

anomalies in the oceanic crust. These lineaments can aid the interpretation of

oceanic spreading direction (see section 5).

250km

4. Methodology

40

4.4. Seismic-Well Tie:

The Huntsman 1 well has been tied to lines 120_01 and dc98_107 as these are the

closest lines to the well (fig. 4.3). This well contains the depths of marker horizons ‘Base

Tertiary’ and ‘Early-Jurassic’ (see section 4.5.) at depths of 2.803s-TWT and 4.05s-TWT

respectively (fig. 4.3).

Figure 4.3. Image showing: (A) the location of the Huntsman 1 well relative to

sections 120_01 and dc98_107; (B) the depths of the well tops on the relevant

sections.

A

B

4. Methodology

41

4.5. Seismic Interpretation Methods:

Four key regional seismic sections have been interpreted in detail in order to get a

sense of any along-strike variability in structure: 128_05, 120_01, 128_03 and 120_14

(fig. 4.4), of which, section 120_01 has been analysed in most detail to determine the

structure of the Roebuck Basin. On these sections, 10 key horizons have been

interpreted, 6 of which are traceable in 3D (table 4.2). All horizon names provided by

Repsol [2015].

4.5.1. Mesozoic/Cenozoic Interpretation:

Due to the well imaged shallow seismic, the Mesozoic and Cenozoic sediments have

been interpreted (table 4.2) by loop tying round the area and jump correlating across

faults.

Figure 4.4. Map showing the locations of the four key interpreted seismic

sections: 128_05(Northern Carnavon Basin), 120_14 (Northern Carnavon &

Roebuck Basins), 120_01 (Roebuck Basin), and 128_03 (Browse Basin).

4. Methodology

42

Horizon Seismic Characteristics Reason for Picking Uncertainty in Picking Interpreted in 3D?

Seabed

Seabed reflection depends on polarity of survey (see section 3).

N/A N/A Yes

Base Tertiary

Strong trough at the base of a large coastal prograding system.

Characterised by onlaps in the coastal parts of the section, less visible in the oceanward parts.

Regionally correlatable seismic horizon so can be easily identified on most sections. Key time marker between the Mesozoic and Cenozoic (horizon provided in Huntsman 1 well and Repsol’s pseudo-well).

Lack of onlaps towards the oceanward parts of the shelf can make the horizon difficult to identify.

Yes

Top Syn-R2

Varies between peaks and troughs.

Unconformity in places. Top package with very

variable thickness.

Key tectonic event marker [Repsol, 2015]. Also a regionally correlatable seismic horizon (horizon provided Repsol’s pseudo-well).

In places, stratal truncations are not obvious and in areas there are no thickening strata making the horizon difficult to interpret.

Yes

Base Syn-R2


Unconformity in places. Top of rather continuous

thickness unit with largely correlatable horizons.

Base of package with very variable thickness.

Key tectonic event marker [Repsol, 2015].

No erosional truncations are visible in places and in some places the horizon is not present.

No

4. Methodology

43

Early Jurassic

Strong trough. In places downlaps are

present onto the top of the horizon.

In the majority of the area it is present within a relatively continuous package.

In places the horizon is heavily faulted.

Regionally correlatable seismic horizon (horizon provided in Huntsman 1 well and Repsol’s pseudo-well).

Due to lateral facies variability over such a large area as the NW Australian shelf, it can be hard to trace the horizon across the area as the seismic character can change in places. Jump correlating across large faults can also be an issue.

Yes

Late-Triassic UC.

Major unconformity in places, primarily atop structural highs.

Identified by erosional truncations.

Usually represents a boundary between well imaged (above) and largely poorly imaged seismic (below).

Key tectonic event marker showing major erosion and marks a change in general seismic imaging (horizon provided in Repsol’s pseudo-well).

As erosion is localised around structural highs, the horizon can be very hard to pick, especially across large distances. Faults are often present within the shallower sediments however they die out before this horizon.

No

Intra-Triassic

Strong trough atop a set of ∼3-4 peaks and troughs within a largely transparent package.

Heavily faulted in places.

Regionally correlatable strong seismic horizon (horizon provided in Repsol’s pseudo-well).

The horizon is within an area of very poorly imaged stratigraphy meaning that in areas this reflector is not visible. The effects of annealing within the poorly imaged stratigraphy mean faults are very hard to pick accurately.

Yes

4. Methodology

44

Top Permian


Top of a chaotic package of high amplitude, discontinuous reflectors.

Largely laterally extensive however is absent in places.

Tectonically significant and regionally correlatable strong seismic horizon (horizon provided in Repsol’s pseudo-well).

Lack of continuity in the chaotic reflectors means that picking the top of this package very difficult.

Yes

Top Oceanic Crust

Strong trough atop a series of 2-4 high amplitude reflections.

Very poorly imaged below these reflections.

Covered by largely flat sediments deposited as a layer-cake.

Picked as a continuation of ‘Top Syn-R1’.

Key regionally correlatable tectonic marker [Repsol, 2015].

Nearing the continent-ocean-boundary, the position of this reflector becomes unclear.

No

Table 4.2. Showing the key Mesozoic/Cenozoic horizons, their seismic 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.

4. Methodology

45

4.5.2. Interpretation of Deep Structure:

Due to the poor imaging of the deep section and thus the high degree of uncertainty in

its interpretation, the position of the ‘Top Basement’ and the Moho have been

interpreted using infrequent seismic reflections and inferral from patterns recognised

within the shallow parts of the basin and from assumptions (figs. 4.5 & 4.6).

4.5.2.1. Picking Top Basement (Grey Horizon):

Within the Mesozoic/Cenozoic sediments (section 4.5.1), there are a series of

asymmetric synforms, assumed to be compactional in origin, which can be used to

identify basement highs and lows (fig. 4.5).

Compactional Synform Top Basement

Reflector?

Inferred Basement

Structure

Figure 4.5. Segment of line 120_01 showing the process of inferring the

basement structure. (A) shows the section without the interpreted Top

Basement; (B) shows the section with the inferred basement (5x vertical

exaggeration).

A

B

11s

10s

0s

0s

4. Methodology

46

4.5.2.1. Picking the Moho (Red Horizon):

Warner [1987] states that, because of effects of seismic velocities and isostasy, in an

isostatically equilibrated system the Moho should always occur from 9-12s TWT. Based

on this key assumption and some sporadic reflectors around these depths, the moho has

been inferred (fig. 4.6).

Possible Moho

Reflection?

0s

12s

6s

3s

9s

0s

12s

6s

3s

9s

Figure 4.6. Segment of line 128_03 showing the process of inferring the moho.

(A) shows the lack of Moho reflectivity in the deep section; (B) shows the

inferred position of the Moho (5x vertical exaggeration).

A

B

4. Methodology

47

4.5.3. 3D Interpretation Methods:

Six regionally correlatable horizons have been picked across the area of interest: Seabed,

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

interpretations, surfaces were then made for each of these horizons using Petrel’s

‘make/edit surface’ tool with a grid increment of 200 (x & y) and using the ‘convergent

interpolation’ algorithm (fig. 4.7). To visualise the lateral sediment thickness variability,

TWT thickness maps have been made from these surfaces. The lateral discontinuity of

some horizons leads to a discrepancy in the regional extent of each horizon

interpretation, this then leads to uncertainty with thickness mapping. As a fault model

has not been created, the generated surfaces and thickness maps will only show broad

scale thickness.

Figure 4.7. Map of the NW shelf showing the seed grid for the ‘Top Permian’

horizon (pink lines) and the boundary polygons for all surfaces and thickness

maps made (yellow outline).

4. Methodology

48

4.6. Depth Conversion Methods:

Once interpreted, seismic sections 128_05, 120_01 and 128_03 have been depth

converted in Move 2015 to unearth the true structural geometries and to use in

backstripping (section 4.6). The velocity model has been derived from three sources (fig.

4.8):

(1) Averaged interval velocities from the Huntsman 1 well.

(2) Interval average of velocity modelled sections across the NW shelf [Goncharov,

2004].

(3) Global average of oceanic crust velocities given to the oceanic crust [white et al,

1992].

A different velocity model has been used for the oceanic and continental crust in which

the ‘oceanic cover’ velocity is the average of the ‘Top Syn-R2’ to ‘Seabed’ as the ‘Base

Tertiary’ is unidentifiable.

Due to the uncertainty surrounding the velocity model, different scenarios have been

run within the range of ±10% to test the model’s sensitivity to the velocity (app. 2.1).

Scenarios have also been run to test the effect of the three compaction curves available

in Move (app. 2.2). These scenarios prove that the velocity model is insensitive to

changes ±10% and to different compaction curves (graph 4.1.).

0

2000

4000

6000

8000

10000

12000

14000

16000

18000

780078507900795080008050810081508200

Dep

th (m

s)

Distance (km)

Average

Scenario2

Scenario3

Scenario4

Scenario5

Christie/Slater

Baldwin/Butler

Dixon

Graph 4.1. Showing the effect on the ‘Top Permian’ horizon for each of the

different depth conversion scenarios mentioned above (for scenario parameters

see app. 2).

4. Methodology

49

Interval -10% Average Velocity (m/s)

+10%

Water N/A 1400 N/A Base Tertiary – Seabed

1593 1770 1947

Top Syn-R2 – Base Tertiary

2493 2770 3047

Early Jurassic – Top Syn-R2

3267 3630 3993

Intra Triassic – Early Jurassic

3726 4140 4554

Top Permian – Intra Triassic

4500 5000 5500

Top Basement – Top Permian

4896 5440 5984

Moho – Top Basement

5913 6570 7227

Oceanic Crust Cover 2043 2270 2497 Oceanic Crust 6390 7100 7810 Mantle 7200 8000 8800

Figure 4.8. Velocity model for NW shelf.

4. Methodology

50

4.7. Backstripping Methods:

In order to work out a β factor for any post-rift subsidence that has occurred, sections

128_05, 120_01 and 128_03 have been backstripped to the ‘Top Syn-R2’ horizon. After

exporting the depth converted horizons, backstripping has been carried out using Flex

Decomp. This method requires the experimentation of applying different β factors in

order to restore the ‘Top Syn-R2’ to a flat datum (fig. 4.9). If the β factor varies across

the section, as is predominantly the case for passive margins, using trial and error the

section can be restored using a variable β factor (app. 3).

Figure 4.9. Image showing the process of backstripping line 120_01 with a

constant β factor of 1 to horizon ‘Top Syn-R2’ (top light blue above). (A) Original

section with 7x vertical exaggeration; (B) Section after removal of 2 post-rift

layers, the horizon is not restored to the flat datum so a higher and variable

stretching factor is required; (C) Shows the β model used.

A

B

C

Flat Datum

51

5. Results

5. Results

52

5.1. Preliminary Observations:

5.1.1. Thickness Maps:

TWT thickness maps have been created for 5 Mesozoic-Cenozoic intervals to show

general thickness variability and broad structural trends across the area (see fig. 4.7. for

location map) (fig. 5.1). The predominant structural trend derived from these maps is

NE-SW and, within the mapped area, most intervals show the thickest sediment over

the outer shelf.

Aside from uncertainty regarding the picking of horizons, the main uncertainty with

surface and thickness map generation is the issue with ‘convergent interpolation’

algorithm. Convergent interpolation does not take into account any predominant

structural trends and also falls down when interpreting areas with limited datapoints

(fig. 5.1.e); an ‘interpretive contouring’ method would be favoured. It is also important

to note that oceanic cover sediments are not included in these maps.

Figure 5.1.a. Thickness between the Seabed and Top Permian horizons. It shows

a general thinning of the sediment towards the Argo Abyssal Plain (NW) and

towards the coast (SE). Thick sedimentary accumulations are generally aligned

in a ∼NE-SW orientation with a broadening towards the SW of the map (the

Northern Carnavon Basin).

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Figure 5.1.b. Thickness between the Seabed and Base Tertiary horizons. It shows

the thinnest sediments towards the Argo Abyssal Plain and a broadly uniform

shelf with a trend of thicker sediments running ∼NE-SW. There is an area of

thickening to the west of the section.

Figure 5.1.c. Thickness between the Base Tertiary and Top Syn-R2 horizons. It

shows the same broadly NW-SE trend as (a) and (b) however the thinnest areas

are not over the Abyssal Plain, instead the minimum sediment is over the

Northern Carnavon Basin.

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Figure 5.1.d. Thickness between the Top Syn-R2 and Early Jurassic horizons.

This interval is very variable in thickness and exhibits no definitive structural

trend however the thinnest sediments are around the Argo Abyssal Plain.

Figure 5.1.e. Thickness between the Early Jurassic and Intra-Triassic horizons.

The map shows an anomalously thick area in the NW, this is an artefact of the

‘convergent interpolation’ algorithm due to lack of data constraints. Where data

is available (along the shelf), there is a broad thickening towards the Northern

Carnavon Basin and thinning in the Roebuck Basin.

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5.1.2. Bathymetry Observations:

In order to interpret basinal features, bathymetry, free air gravity and magnetic data

have been used together. The extents of the shelf and the abyssal plains are well defined

by the bathymetry map (fig. 5.2). Between the abyssal plains (dark blue) and the shelf

(light blue) there is a broad platform with more internal structure. The Wombat Plateau

(fig. 1.1) is expressed as a topographic high near the edge of this platform.

Figure 5.1.f. Thickness between the Intra-Triassic and Top Permian horizons. It

shows minimum thicknesses around the Argo Abyssal Plain and towards coastal

regions. It also shows a broadly NE-SW trend.

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56

5.1.3. Gravity Observations:

A basin’s gravity signature often reflects its basement structure and therefore, in

conjunction with bathymetry and magnetics, it can be used to identify basins and

basement highs. Fig. 5.3. shows an interpreted map showing trends in gravity anomalies.

Along the shelf there is a ‘ridge’ of high gravity trending SW-NE, bordered by: (to the SE)

a thin gravity low also trending SW-NE, and (to the NW) a broader zone of low gravity.

The Wombat Plateau appears as a gravity high on the edge of a ‘platform’ of relatively

uniform gravity. The boundary between Browse, Roebuck and the northern part of the

Northern Carnavon Basin and the abyssal plains are marked by gravity lows however to

the west of the Northern Carnavon Basin, the boundary is not as obvious.

Shelf

Abyssal

Plain W.P.

Figure 5.2. Satellite map showing the bathymetry of the NW Australian shelf. The

grey dotted lines outline interesting features and the boundary between the

shelf, a broad slightly deeper platform and the abyssal plains. W.P. – Wombat

Plateau [after Google Maps, 2015].

5. Results

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5.1.4. Magnetic Observations:

Magnetic maps can also be indicative of basement structure, the ‘sharpness’ of magnetic

anomalies is an indicator of their depth and the anomalies themselves often correspond

to basement highs. Fig. 5.4. shows the same NE-SW trending ridge as seen in the gravity.

Bordering the ridge to the NW is a platform containing little magnetic signature which

could represent a broad, thick sedimentary basin. The Wombat Plateau is expressed as a

magnetic high, as well as a gravity high, indicating that it is in fact a basinal feature. The

western boundary between the ‘platform’ and the abyssal plains is better defined in the

magnetic map than the gravity. Within the abyssal plains, there are magnetic lineaments

trending both NE-SW and NNE-SSW.

Figure 5.3. Gravity map of the NW shelf showing any major highs or lows shown,

main high-low boundaries are shown by the white dashed line. See fig. 3.1. for

the uninterpreted map with basin names. Warm colours indicate gravity highs.

5. Results

58

Figure 5.4. Magnetic map of the NW shelf showing the main features picked out

by magnetic anomalies (green dashed lines). Brown lines indicate lineaments in

the abyssal plains. See fig. 3.2. for the uninterpreted map with basin names.

Warm colours indicate magnetic highs.

5. Results

59

5.2. Mesozoic/Cenozoic Structure:

5.2.1 Line 120_01 (Roebuck Basin):

Because of the difference in interpretation confidence between the Mesozoic/Cenozoic sediments (horizons Top Permian – Seabed) and any deeper structures, it is important to separate the observations

made regarding each of them.

Figure 5.5 (a-c). Section 120_01 showing: (A) 5x vertically exaggerated interpreted Mesozoic/Cenozoic horizons and their structure; (B) ) 5x vertically exaggerated uninterpreted section showing

the locations of figures presented later in the text; (C) interpreted section with no vertical exaggeration showing the true (time domain) expression of the section. See next page for 5.5 d & e.

A

B

C

5. Results

60

D

E

Fig

ure

5.5

(d

& e

). S

ho

win

g: (

D)

a k

ey t

o t

he

seis

mic

ho

rizo

ns

and

inte

rval

s d

isp

lay

ed

abo

ve; (

E)

a lo

cati

on

map

sh

ow

ing

seis

mic

lin

e 1

20

_01

.

5. Results

61

5.2.1.1. Synformal Structures:

The most obvious feature on a large scale are the two large asymmetric synforms (fig.

5.6) separated by two highs. Within these synforms the intervals: Top Permian – Intra-

Triassic, Intra Triassic – Late-Triassic U.C., and Late-Triassic U.C. – Early Jurassic all

seem to thicken. Between the Top Permian and the Late-Triassic U.C. there are no clear

onlaps indicating a possible compactional origin, however between the Late-Triassic U.C.

and the Early Jurassic distinct onlaps are visible (fig. 5.6.b).

5.2.1.2. Nature of the Late-Triassic U.C.:

Across much of the section, the Late-Triassic U.C. marks the boundary between the

relatively well imaged sediments above and poorly imaged seismic below. The nature of

the boundary is clearly unconformable (fig. 5.7) and is also the point at which many of

the faults seen in the Intra-Triassic horizon terminate (fig. 5.7).

5.2.1.3. Nature of Faulting:

All faults in the section show a normal sense of offset. Faulting in the section is very

minor, usually showing no discrete offset, and tends to be localised around the shelf

edge. Faults between the Late-Triassic U.C. and Top Syn-R2 terminate before or at the

Top Syn-R2 horizon and few are seen penetrating the Late-Triassic U.C (fig. 5.7).

Figure 5.6. Partial sections of line 120_01 showing two synforms (A & B) present

in the section. Onlaps are indicated by red arrows, truncated horizons are

purple.

5. Results

62

The package between the Top Permian and the Late-Triassic U.C. appears heavily

faulted in places however very few penetrate the Late-Triassic U.C. and none penetrate

the Top Permian.

5.2.1.4. Nature of Top Syn-R2:

The package between the Early Jurassic horizon and the Top Syn-R2 shows onlaps at its

base and at its top it is truncated by the Top Syn-R2 horizon; the Base Syn-R2 is absent

(5.7).

Above the Top Syn-R2, the sediments form a ‘wedge’ that contains a series of

prograding clinoforms which is internally structureless (fig. 5.5).

Figure 5.7. Representative partial section showing horizons between the Top

Permian (pink) and the Seabed, and their internal structure. Purple arrows

indicate erosional truncations.

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63

5.2.2. Lateral Variability in Structure:

Seismic sections 128_05, 120_14, and 128_03 are presented to show the lateral variability in structure.



C

A

B

Line 128_05

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Fig

ure

5.8

(d

& e

). S

ho

win

g: (

D)

a k

ey t

o t

he

seis

mic

ho

rizo

ns

and

inte

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s d

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abo

ve; (

E)

a lo

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on

map

sh

ow

ing

seis

mic

lin

e 1

28

_05

.

5. Results

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C

A

B

Line 120_14

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Fig

ure

5.9

(d

& e

). S

ho

win

g: (

D)

a k

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he

seis

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and

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E)

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seis

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20

_14

.

5. Results

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Line 128_03

5. Results

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Fig

ure

5.1

0 (

d &

e).

Sh

ow

ing:

(D

) a

key

to

th

e se

ism

ic h

ori

zon

s an

d in

terv

als

dis

pla

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abo

ve; (

E)

a lo

cati

on

map

sh

ow

ing

seis

mic

lin

e 1

28

_03

.

5. Results

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5.2.2.1. Synformal Structures:

The synformal structure in the SE of line 120_01 can be carried NE along a NE-SW

structural trend to line 128_03 (5.11) where it is expressed as a fault (fig. 5.12).

The synform around the middle of line 120_01 can roughly be traced SW along a NE-

SW structural trend to line 120_14 (figs. 5.11).

Figure 5.11. Surface map of the Top Permian horizon showing the axes of the

two synforms shown in seismic line 120_01, their NE-SW trends, and lateral

extents.

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5.2.2.2. Nature of the Late-Triassic U.C.:

The erosive nature of this horizon is most evident in line 120_01 however it is evident

in lines 120_14 and 128_03 (fig. 5.13).

5.2.2.3. Nature of Faulting:

In contrast to line 120_01, where little faulting occurs, across the rest of the shelf there

is significantly more faulting. There are two main scales of faults: large, oceanward

dipping faults that cut the whole Mesozoic/Cenozoic stratigraphy, and smaller scale

faults that have less of a preferred orientation (although predominantly dip oceanward),

constrained to mainly the sediments between Late-Triassic U.C. and Top Syn-R2. The

highest degree of faulting is seen in Northern Carnavon Basin (figs. 5.8 & 5.9) however

in seismic line 120_14 (fig. 5.9), faulting seems to be localised atop antiformal structures

(see section 5.2.2.5). When mapped, these faults trend predominantly NE-SW and are

more frequent towards the edge of the basins.

Figure 5.12. Image showing coastward (SE) dipping faults cutting the Top

Permian, Intra-Triassic, Late-Triassic U.C., and Early Jurassic horizons in the SE

of section 128_03.

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5.2.2.4. Nature of Top Syn-R2:

Like in line 120_01, the faults present in lines 128_05, 120_14 and 128_03 all terminate

against the Top Syn-R2. However, where more faulting occurs, the Base Syn-R2 is

present. The Base Syn-R2 truncates horizons below in places and is the base of a

discontinuous unit, the thickness of which is heavily controlled by faults (fig. 5.14).

5.2.2.5. Other Interesting Features:

The antiformal structure in line 120_14 which localises faulting can be traced in 3D as a

perfect dome (termed the ‘Tres Hombres’ [Repsol, 2015]) (fig. 5.15), this dome is

∼37km across and has ∼1.2s vertical elevation.

Figure 5.13. Image showing the SE of line 128_03. Highlighted is the Late-

Triassic U.C. truncating the reflectors of the sediments below, stratal

terminations are marked by purple arrows. Also shown is a fanning of dip in

strata seen below the Top Permian horizon.

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Figure 5.14. Images showing the nature of the Base Syn-R2 and Top Syn-R2

horizons and the package between them. (A) Shows a thickening package

between these horizons into a large, ocean-dipping (NW) fault. It also shows the

Top Syn-R2 horizon truncating some reflectors below it, (B) Shows the same

trend with some of the smaller observed faults. The Base Syn-R2 can also be

seen truncating some of the lower reflectors in this image.

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Line 128_05 images the Wombat Plateau (fig. 5.8). This feature is made of primarily

continental crust and is bordered to the NE by the Argo Abyssal Plain and to its right by

a canyon (fig. 5.16). Into this canyon, the beds below the Early Jurassic thin and

terminate against the plateau. These are then covered by a thick post Top Syn-R2

sequence.

Figure 5.15. Map of the ‘Tres Hombres Dome’ shown in the Top Permian horixon

showing its symmetrical nature. See fig, 5.11. for location map of the Top

Permian surface.

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Figure 5.16. Images showing the location, bathymetric expression and seismic

image of the canyon surrounding the Wombat Plateau. The Wombat plateau

stands proud and the canyon surrounding it is filled in with post-Top Syn-R2

sediments.

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5.3. Deep Structure:

5.3.1. Line 120_01 (Roebuck Basin):

Assuming the synformal structures discussed in section 5.2. are compactional in origin,

it can also be assumed that the axis of said synforms will co-incide with the point of

deepest sedimentation below. The highs separating these synforms thus correspond to

basement highs (fig. 5.17).

On the NW side of the structural high separating the two synforms in line 120_01,

there is a strong reflector dipping to the NW. Above this reflector there is a ‘wedge’ of

faint reflectors that appear to show a fanning of dip (figs. 5.13 & 18). To the SE of the

same structural high, reflectors show a similar indication of a fanning dip but to a lesser

depth.

5.3.2. Lateral Variability in Deep Structure:

In the Northern Carnavon Basin (line 128_05), seismic imaging is good down to ∼12-

13s and very little basement structure is observed, instead there is a broad, flat platform

of sediment <13s thick (fig. 5.8).

In the Browse Basin (line 128_03), deep basin imaging is significantly worse than

128_05, however a similar ‘wedge’ like feature to that seen in 120_01 is also seen in the

SE of the section.

Figure 5.17. Part of line 120_01 showing the positions of basement highs based

on the nature of synformal structures.

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Figure 5.18. Part of line 120_01 showing a sediment package exhibiting a fanning

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

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5.4. Nature of the Continent Ocean Boundary (COB):

The COB is shown by the gravity, magnetic and bathymetry maps (section 5.1), however

the seismic data show that the nature of the COB is different in different areas. On line

120_01 there is a definitive boundary between the edge of the basin and the Argo

Abyssal Plain (fig. 5.19). However, lines 128_05 and 128_03 show a much less clear-cut

(fig. 5.20). From studying the regional lines that image the COB, the boundary has been

divided into areas where the contact is sharp and where it is transitional (fig. 5.21).

Figure 5.19. Image of the COB

seen in line 120_01. The

transition is sharp and

evidenced by a cliff at the shelf

edge.

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Figure 5.20. Image showing the COB in line 128_05, NW of the Wombat Plateau.

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

Figure 5.21. Map of the COB surrounding the Argo Abyssal Plain. Seismic lines

that show a sharp COB are marked by orange lines. Seismic lines that show a

transitional boundary are marked by yellow lines, their length represents the

length of the transitional zone.

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5.5. Depth Conversion:

In order to show the true geometry and depths of basin structures, seismic lines 120_01,

128_05, and 128_03 have been depth converted (figs. 5.22, 5.23, and 5.24 respectively)

using the velocity model shown in section 4.6. It is important to note that the depth

converted sections include a simplified interpreted basement (see section 6.1 for

interpretation process); a new colour scheme is also used during depth conversion (fig.

5.25).

Depth conversion suggests that the continental crust/basin sediment thickness ranges

from ∼20km thick around the COB, to ∼35km towards the coastal parts where the

basement is thicker. The 1:1 sections reveal the true nature of basement structures that

were overshallowed in the 1:1 time sections. The oceanic crust in most places is ∼10km;

near to the COB this may be thicker as transitional crust is modelled as oceanic. The

post-Top Permian sediments appear relatively unchanged.

Figure 5.25. Key to the

depth converted units.

Colour scheme has

changed to display

units used in depth

conversion. The Late-

Triassic U.C. and Base

Syn-R2 are not shown.

5. Results

80

Line 120_01

40,000m 10,000m

40,000m 10,000m

Figure 5.22. Depth converted line 120_01 showing (A) location map of the seismic line; (B) 2x vertical exaggerated section; (C) 1:1 section showing

the true geometries of the basement structures and the sediments above.

C

B

A

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40,000m

10,000m

40,000m 10,000m

Line 128_05

C

B

A



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40,000m

10,000m

40,000m

10,000m

Line 128_03

C

B

A



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5.6. Backstripping:

Backstripping has been carried out post-interpretation of the Mesozoic/Cenozoic basin

history (see section 6.2.) and assumes a rift phase ending (with breakup) at the Top

Syn-R2 horizon. Post-rift sedimentation has been removed to give a variable β factor for

seismic lines 120_01, 128_05 and 128_03 (figs. 5.26, 5.27, and 5.28 respectively) (see

section 5.5. for location maps). Calculated maximum β factors are 1.25 for lines 120_01

and 128_05, and 1.3 for line 128_03 indicating relatively uniform post-rift subsidence

across the shelf.

Although there are numerous faults within the sections, many of them are too small to

measure offset therefore any β factor estimates derived using these faults would likely

be underestimations. A study of the closely spaced shallow seismic surveys available

(app. 1) would give a more realistic estimate. All sections are 5x vertically exaggerated.

1.11.2

1.3 1.2

1.3 1.2

Flat Datum

Figure 5.28.

Backstripped section

showing line 128_03

restored to sea level.

The variable β factor

used to backstrip is

presented below, with a

maximum β factor of

1.3 ocurring above

faults so these are

probably due to

compaction, the

average β factor is

actually 1.2.

NW SE

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Res

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fact

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of

1.0

5

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5. Results

85

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86

6. Analysis

6. Analysis

87

6.1. Interpreteation of Deep Structure & Early Basin History:

6.1.1. Line 120_01 (Roebuck Basin):

From the geometries of the synforms in the Mesozoic/Cenosoic sediments and the

inferred positions of basement highs (figs 5.6 & 5.17), it is clear that the deep sediments

in the basement are heavily structurally controlled. Due to its inability to explain

present day geometries and a lack of recognition in published literature, a

compressional structural control is ruled out. It is therefore suggested that a pre-Top

Permian extension event has caused the structures seen today. Two possible scenarios

can explain the present day geometries seen in the deep basin (fig. 6.1).

Figure 6.1. Schematic diagrams showing the evolution of seismic line 120_01 to

the Top Permian horizon showing: (A) explanation of margin evolution using a

‘hyperextended’ model; (B) explanation of margin evolution using a rifted block

architecture. A.v. and B.iii. show the two interpretations on the seismic section.

The main difference between the models is the assumed original crustal

thickness. The Permian sediments are shown in pink, the upper crust in yellow,

the lower crust in brown and the lithosphere in green; serpentinized mantle is

light green.

35km

>50km

6. Analysis

88

6.1.1.1. Scenario 1 (hyperextension):

The ‘rule of thumb’ which states that the Moho, in an isostatically equilibrated system,

should occur between 9-12s [Warner, 1987] assumes a ‘normal’ original continental

crustal thickness of 30-40km. Scenario 1 builds upon this assumption and invokes large,

rotated fault blocks (faults dipping SE) that are shown detaching onto the Moho. As

these fault blocks formed and rotated, sediments have concurrently infilled the basin

causing the dip fan seen in fig. 5.18. This model is characteristic of a ‘hyperextended

margin’ where crustal-scale faults penetrate to the Moho because of coupling between

the upper and lower crust due to thinning and cooling (fig. 1.8). The faults then allow

the hydration and serpentenisation of the uppermost mantle, reducing its frictional

strength and allowing it to act as a decollement. Towards the edge of the shelf, sediment

directly overlies possible exhumed mantle. The exhumed mantle formed due to the

continued stretching and final separation of the crustal blocks (fig. 6.1.a).

Figure 6.2. Schematic diagrams showing the pitfalls in the classification of

‘necked’ and ‘hyperextended’ zones (boundary shown by the dotted line). It

shows that with detached fault blocks (C) and attached fault blocks (B), the

boundary between the necked zone is definitive across the margin. However if a

fault block is partially detached, the boundary will be at the edge of the

continent at section A-A’ however at section B-B’, the boundary is oceanward of

the fault block. This means that classification of ‘necked’ and ‘hyperextended’

zones in 3D is tenuous. Crust is coloured yellow, serpentinized mantle is light

green and lithosphere is dark green.

6. Analysis

89

Due to poor seismic imaging at depth, there is a large amount of uncertainty

surrounding the true geometry of the deep structure. The primary uncertainty lies in

whether the two inferred fault blocks are separated as this dictates how the section’s is

divided into hyperextended ‘domains’ (fig. 1.10). It is for this reason that the distinction

between ‘hyperextended’ and ‘necked’ domains (fig. 1.10) is often not clear cut (fig. 6.2).

It is also uncertain as to what the sediments within the deep basin are overlying and the

thickness of the serpentinite body as this can have large implications when depth

converting.

6.1.1.2. Scenario 2 (rifted):

Scenario 2 requires a larger crustal thickness than that assumed by Warner [1987], ergo

the Moho need not be confined to 9-12s. The model suggests steeper faults which

detach into the lower crust (fig. 6.1.b). Although controlled by steeper faults, the

basement blocks must have been tilted significantly to concur with the geometries seen

in fig. 5.18.

6.1.1.3. Comparison Between Scenario 1 & 2:

The primary factor determining the favoured scenario is crustal thickness. Goncharov

[2004] presents a crustal thickness for the Kimberly and Pilbara Blocks of ∼35-45km

which lies within the range of normal crustal thickness required by Warner’s [1987]

Moho rule. For this reason, the Moho is expected to be found at ∼10-12s which suggests

that scenario 1 is more geologically feasible as it is unrealistic that the thickness of

rifted crust be thicker than the cratons it borders. This is backed up by the depth

conversion of the section, which proves that the crustal thickness decreases from

∼35km beneath thick basement to ∼20km nearing the COB (fig. 5.22). The deep

structure of the basin therefore resembles a hyperextended rift architecture (fig. 6.3)

suggesting an early (pre-Top Permian) extensional phase in the basin.

6. Analysis

90

B A

C

Figure 6.3. Fully interpreted section of line 120_01 including basement structure. (A) – key

to the section, (B) – Location map for the section, (C) – Interpreted section. 5x vertical

exaggeration.

6. Analysis

91

6.1.2. Lateral Variability in Deep Structure:

Interpretations of lines 128_05, 120_14 and 128_03 support the theory of an early

hyperextended rift architecture, forming the deep basin’s structure. Depth conversion of

lines 128_05 and 128_03 show crustal thicknesses in the expected range of ∼20-30km

(figs. 5.23 & 5.24). Moreover, high quality imaging on line 128_05 relatively flat-lying

sediments to depths of ∼10s interpreted to be overlying exhumed mantle (fig. 6.4).

The distribution of tilted fault blocks can be mapped using the trends of synforms seen

in the Top Permian horizon and the four key seismic lines (figs. 6.5). The boundary

between the basement fault blocks (gravity and magnetic highs) and the exhumed

mantle can also be observed in the gravity and magnetic data (figs. 5.3 & 5.4). From this

mapping and analysis of lines 120_01, 128_05, 120_14, and 128_03 (figs 6.3, 6.4, 6.6 &

6.7 respectively), it is shown that the Roebuck and Browse basins both contain two

relatively large fault blocks whereas the Northern Carnavon Basin is underlain by a

large expanse of exhumed mantle (fig. 6.8).

Figure 6.5. Interpretive structure contour map of the Roebuck and north part of

the Northern Carnavon basins. Dark colours indicate structural lows. Faults and

crust are coloured and contoured separately.

Northern Carnavon Basin

Roebuck Basin

N

6. Analysis

92

All of the deep faults are interpreted to dip south east. This, combined with the uneven

lateral continuity of fault blocks, suggests that the rift architecture was either highly

asymmetric or heavily influenced by crustal inheritance; possibly a combination of the

two (see section 1.2.5 for description of inheritance). In order to test this, a detailed

study of the original basement architecture would be necessary, however initial

observations of Australia’a basement terrains reveal no nearby structural trend

orientated NE-SW (fig. 6.9) . This suggests that structural inheritance may not be a

major control on the NW shelf rift architecture.

Figure 6.8. 3D image of the study area showing the geometry and lateral extents

of fault blocks along the basin (note the faults are dipping to the NE). The NE-SW

orientated lines along the continental boundary show the position of the four

key sections used in structural contouring 6.4.

N

Figure 6.9. Map showing

the basement terrains of

Australia, used to identify

structural trends near the

NW shelf [OZ Seebase,

2005].

6. Analysis

93

A



exaggeration.

B

C

6. Analysis

94

A



exaggeration.

B

C

6. Analysis

95

A



exaggeration.

B

C

6. Analysis

96

6.2. Srtucture of the Mesozoic & Cenozoic Basin Fill:

The Mesozoic and Cenozoic sediments across the NW shelf can be divided into three

broad megasequences split by unconformities: Top Permian – Late-Triassic U.C. (MS1),

Late-Triassic U.C. –Top Syn-R2 (MS2), and Top Syn-R2 – Seabed (MS3) (fig. 6.10).

6.2.1. Megasequence 1 (MS1):

MS1 lies directly atop the early rifted sediments (see section 6.2). The Late-Triassic U.C.

is a major unconformity around structural highs (fig. 5.6), however across much of the

basin the horizons below are not truncated. In areas not truncated by the unconformity,

the strata form compactional synclines characterised by a lack of visible onlaps. The

Late-Triassic U.C. also, in places, represents a boundary between relatively well imaged

seismic data (above) and relatively poorly imaged data (below). This is most likely

because of image degradation due to the unconformity. The fact that erosional

truncations are largely limited to the vicinity of structural highs (fig. 5.6) indicates that

some uplift and possibly inversion along deeper structures suggesting the presence of a

compressional event at the time of the unconformity.

Figure 6.10. Key to the megasequences described in this section and the horizons

they encompass.

6. Analysis

97


The majority of MS2 contains continuous reflectors and broadly maintains thickness

across most of the NW shelf. However between structural highs, affected by the Late-

Triassic U.C., the strata exhibit onlaps (fig. 5.6). These characteristics suggest that this

stage of basin fill was formed by sedimentation accommodated by passive subsidence.

The relatively continuous strata of the Late-Triassic U.C. – Base Syn-R2 are heavily

faulted in places (figs. 6.3, 6.4, 6.6 & 6.7). These faults terminate against the Top Syn-R2

and show growth strata into them (fig. 5.15). The Base Syn-R2 is a minor unconformity

in places (usually truncating the sides of horst structures) and happens to be the base of

the unit containing said growth strata. The Base Syn-R2 – Top Syn-R2 interval suggests

that two extensional phases have been experienced by the basin: R1 (Top Basement –

Top Permian) and R2 (Base Syn-R2 – Top Syn-R2). This extensional event could be

responsible for continental breakup and the formation of the Argo and Gascoyne

abyssal plains (see section 6.2.4).


MS3 marks a change to unstructured, continuous deposition across most of the shelf.

The Base Tertiary horizon lies at the base of a sequence of prograding clinoforms

interpreted as an inner shelf delta, the lateral extents of this delta can be seen in the

shelf’s bathymetric expression: its shelf-edge-break marks the boundary between the

‘shelf’ and the ‘platform’ in fig. 5.2. The nature of the MS3 deposits and their position

above MS2 are indicative of post-rift subsidence deposits.

6.2.4. Nature of R2:

The magnetic anomalies in the abyssal plains (fig. 5.4) can indicate the direction of

seafloor spreading at creation. Interpretation of these magnetic anomalies indicates a

seafloor spreading direction ∼NW-SE for the Argo Abyssal Plain and a more WNW-ESE

direction for the Gascoyne and Cuvier abyssal plains. The cross-cutting nature of the

magnetic anomalies suggests the initial spreading direction was ∼NW-SE (fig. 6.11).

The backstripping of MS3 (figs. 5.26, 5.27 & 5.28) has revealed a maximum β factor of

1.25-1.3 for sections 120_01, 128_05 and 128_03 (all show increasing β factors toward

the abyssal plain) suggesting that lithospheric stretching was relatively uniform

laterally across the study area.

6. Analysis

98

There are two scales of faults that control Syn-R2 deposition: large scale, oceanward

dipping faults and minor faults that dip both ocean and coastward (fig. 5.14). All of these

faults trend ∼NE-SW with an ∼60° swing in strike (fig. 6.12). Due to the predominant

trend of the large faults dipping oceanward (NW), it is likely that the large faults cutting

Mesozoic/Cenozoic strata in line 128_03 are reactivated basement faults. There are

three causes for faulting explaining the varying strike of the faults:

(1) – Faulting due to NW-SE and subsequently WNW-ESE rifting causing the

majority of the NE-SW and NNE-SSW trending faults (fig. 6.12).

(2) – The NE-SW spreading axis shown by the magnetic anomalies (fig. 6.11) are

oblique to the edge of the Northern Carnavon Basin. This has caused normal

faults with an ENE-WSW trend sue to shearing (6.13).

Figure 6.11. Schematic diagrams showing the interpretation of spreading

direction from magnetic anomalies. (A) – Magnetic map of the shelf, (B) – line

drawing produced of magnetic anomalies from the map, (C) – Evolution of the

spreading ridge from NW rifting to WNW-ESE rifting.

6. Analysis

99

(3) – Formation of the Wombat Plateau and Tres Hombres Dome (see section 6.4)

have also caused a high degree of faulting. As the four key sections presented in

this study have been picked to show these interesting features, lines 128_05 and

120_14 show an unrepresentatively high degree of faulting (figs. 6.4 & 6.6).

Due to the effects of shearing and other features, seismic lines 120_01 and 128_03 are

most representative of the degree of brittle crustal deformation. As mentioned in

section 5.9., due to the scale of observation it is not possible to determine a crustal β

factor as many of the faults in these sections are too small to measure offset. However,

the fact that the faulting is so minor in these sections, the β factor is expected to be

relatively insignificant when compared with the β factors determined for lithospheric

stretching (section 5.9).

R2 is therefore interpreted to be a phase of rifting that is highly depth dependent in

nature, this is also supported by the transitional crust seen at places along the COB as

this is a common feature of ‘depth dependent stretched’ margins (fig. 1.11).

Figure 6.12. Stereonet showing the trends of faulting on the NW shelf and the

likely events that caused them.

6. Analysis

100

Figure 6.13. Diagram of the COB surrounding the Argo Abyssal Plain with a

magnified section containing a strain ellipse explaining the formation of ENE-

WSW striking normal faults at the margin.

6. Analysis

101

6.3. Nature of the COB:

The COB surrounding the Argo Abyssal Plain can roughly be traced using magnetic and

gravity anomaly maps, and bathymetry maps (figs. 5.2, 5.3 & 5.4). Observations of the

bathymetry show a significant amount of small-scale variability along the COB (fig. 6.14),

thus to accurately study the nature of the COB is beyond the scope of this study.

Interpretation of spreading direction from magnetic anomalies (section 6.2.4)

suggests a spreading direction of ∼NW-SE. As the COB is not completely perpendicular

to the spreading direction, it is likely that at the time of rifting some shearing of the COB

was occurring. This shearing has manifested at the COB by forming an oblique

transform margin (fig. 6.15).

Figure 6.14. Bathymetry map of the COB surrounding the Argo Abyssal Plain

showing the apparently small scale structural variability associated with the

COB.

6. Analysis

102

Seismic lines that exhibit a sharp COB (fig. 5.21) are thought to be formed from

transform parts of the margin (fig. 6.16) and ones that show transitional crust are

formed from non-sheared parts of the margin (fig. 6.15).

Figure 6.15. Schematic diagrams showing the development of the COB at oblique

transform margins. This figure explains the existence of transitional crust at

some parts of the margin (line 128_03) and the sharp COB seen in others (line

120_01). It suggests that the COB type is a function of the angle and position that

the seismic is shot relative to the margin.

6. Analysis

103

Figure 6.16. Diagram explaining the evolution of a solely transform margin. This

is the process affecting the sheared part of the NW shelf margin [Bird, 2001].

6. Analysis

104

6.4. Other Interesting Features:

6.4.1. Formation of the Wombat Plateau:

The Wombat Plateau (fig. 5.2) is interpreted as a block of continental crust (based on

gravity, magnetic and seismic data) that is separated from the Northern Carnavon Basin

by a canyon that is filled with post-Top Syn-R2 sediments. The Wombat Plateau is

consequently interpreted as a continental raft that was initially separated from the shelf

during R1. The Northern Carnavon Basin was subsequently filled causing strata to

terminate against the side of the Wombat Plateau. R2 then re-mobilised this continental

raft causing a rollover of the sediments in the Northern Carnavon Basin into the space

created (fig. 6.17). Outer-arc extension associated with this rollover of sediment is

thought to have caused the heavy faulted parts of section 128_05 (figs. 6.4 & 6.17).

6.4.2. Formation of the Tres Hombres Dome:

Although most of the features seen in the NW shelf can be explained, the Tres Hombres

Dome (see section 5.2.2.5) is anomalous. It appears as a perfect dome (fig. 5.15) that

localises faulting in MS2 but does not affect MS3, suggesting it was possibly related to

R2. The Top Permian is deformed by the structure indicating a deep structure has

formed it, and its size (∼35km across) means it is very unlikely to be a sedimentary

diapir. Two possible hypotheses have been presented (fig. 6.18) explaining its formation:

a lithospheric thermal anomaly (fig. 6.18a), and serpentine diapirism related with

differential loading due to R2 faulting (fig. 6.18b). Its size (35km across) is small

compared to that expected from a ‘hot spot’ so the serpentine diapir model is preferred.

However, to determine the true nature of this structure would require a more detailed

study.

6. Analysis

105

Figure 6.17. Schematic sections showing the evolution of the Wombat Plateau.

Line shown is an extension of seismic line 128_05. For sediments colour scheme

see fig. 6.4.

NW SE

6. Analysis

106

Figure 6.18. Schematic sections of line 120_14 showing the possible evolutions

of the Tres Hombres Dome. (A) – the lithospheric thermal anomaly model

involves the upwelling of hot asthenosphere as R2 was initiated. Uplift due to

this upwelling could be a cause of the faulting or this could be due to the thermal

and isostatic re-equilibration of the lithosphere causing subsidence. (B) – As the

margin’s sediments are interpreted to be lying atop serpentinized mantle, this

could have become mobile during R2 with faulting in the sediments above

causing differential loading. In response to this differential loading the

serpentine could have formed a dome in a similar way to that of a salt dome. The

light green area represents serpentinized mantle, the dark green indicates

mantle; for sediments and basement colour scheme see fig. 6.6.

107

7. Discussion

7. Discussion

108

7.1. Geological Evolution of the NW Australian Shelf: a comparison with

published literature:

Evolutionary models and tectonic maps, derived from the results of this study and from

published works, are presented in table 7.1 for the Northern Carnavon, Roebuck and

Browse basins. These are compared with tectonic maps shown in previous studies

(table 7.1). The NW shelf has been interpreted as a polyphase rifted passive margin,

having undergone two tectonically independent rifting events (R1 in the Permian, and

R2 in the Jurassic – Cretaceous). These rifting events have been caused by the

separation of the Cimmerian Microcontinent, the West Burma Block, and the Indian

Plate from Gondwana (table 7.1 – see page 113).

7.1.1. Comparison of R1:

Most studies [Driscoll & Karner, 1998; Karner & Driscoll, 1999; Heine, 2002; Longley et

al, 2002; Heine & Muller, 2005; Marshall & Lang, 2013] focus on the Jurassic –

Cretaceous separation of the West Burma Block and Indian Plate from Gondwana

during R2. There is a tendency to leave the pre-Triassic strata as

‘basement/uninterpreted basin’ [Geoscience Australia, 2015a & b]. Belgarde et al [2015]

have recognised that in order to accommodate <25km of ‘uninterpreted basin’

sediments in the Northern Carnavon Basin [this study], significant crustal thinning must

have occurred. This crustal thinning is thought to be accommodated by the

hyperextension of the continental crust.

The findings of this study strongly support those of Belgarde et al [2015], suggesting a

broad zone of exhumed mantle underlying much of the Northern Carnavon Basin (figs.

7.1 & 7.2) and the Browse Basin (fig. 7.2). However within the Roebuck Basin, the

findings of this study suggest a different architecture (fig. 7.2). Figures 7.1 and 7.2

divide the NW shelf into ‘hyperextended’, ‘necked’ and ‘stretched’ zones based on Sutra

et al’s [2012] classification (fig. 1.10).

7.1.2. Comparison of R2:

Rifting and separation of the West Burma Block has lead to significant amounts of post-

rift subsidence and sedimentation, however faulting in the area remains relatively

minor [Driscoll & Karner, 1998; this study]; making R2 on the NW shelf a commonly

cited example of DDS [Davis & Kusznir, 2004; Kusznir et al, 2005]. Karner & Driscoll

7. Discussion

109

[1999] present a maximum lithospheric β factor for post-R2 subsidence of 2.65 with a

negligible amount of upper crustal, fault accommodated extension; this study presents a

maximum lithospheric β factor of 1.3 with negligible upper crustal extension. Although

the results from both prove that R2 stretching is highly depth dependent, there is still a

large discrepancy between the two studies. Reasons for this discrepancy include: a

different method of calculating β factors, the sections have been backstripped to

different horizons, or the backstripping has been carried out in different parts of the

section where a thicker post-R2 package is seen.

Figure 7.1. Cross sections across the Northern Carnavon Basin taken from the

locations shown in fig. 7.2. (A-A’ on map A; B-B’ on map B). The figure shows: (A)

a cross section created using gravity foreward modelling from Belgarde et al

[2015]; (B) a schematic cross section derived from seismic interpretation

through the same area from this study (for colour scheme see fig. 6.4). The

sections show a very similar trend and the proposed hyperextended, necked and

stretched zones are in similar positions.

7. Discussion

110

Figure 7.2. Maps of the NW shelf dividing the shelf and its constituent basins into

hyperextended, necked and stretched zones. (A) is taken from Belgarde et al

[2015]; (B) has been created from this study, the interpretation has been carried

as far as confidence allows and the same colour scheme has been used.

7. Discussion

111

7.1.3. Uncertainties in the Model:

The obvious uncertainty regarding the geometry of R1 comes from the issue of image

quality. However, the newly-proposed model showing hyperextended crust underlying

the Permian sediments also adds massive uncertainty as to the nature of continental

breakup. It was previously thought that the West Burma Block rifted off the Australian

Plate [Heine, 2002; Heine & Muller, 2005; Metcalfe, 2013] however the presence of a

broad zone of hyperextension raises the question: where within this zone did breakup

occur? Logically, breakup either occurred along the edge of a continental block or within

the zone of hyperextension (fig. 7.3). This debate is by no means limited to this margin,

it also feeds into the wider debate of how significant crustal structure is in controlling

Figure 7.3. Potential models for the onset of oceanic spreading during R2 (lateral

continuity of interpretation taken as far as confidence allows). (A) shows

breakup occurring in the centre of a broad hyperextended zone; (B) shows

breakup occurring along the boundary between a broad hyperextended zone and

the West Burma Block (shown here as attached to the Lhasa Block).

7. Discussion

112

margin breakup [Manatschal et al, 2015]. A definitive answer to this question is not

available due to the political situation in Myanmar meaning little data is available

regarding the composition of the West Burma Block [Heine, 2002]. However, due to the

sinuous nature of the COB and tectonic models in the published literature, scenario 1 is

preferred.

7. Discussion

113

3D Section Evolution Tectonic Map (this study) Explanation. Tectonic Map (Published Literature)

Through the Carboniferous, the Australian Plate was part of Gondwana [Metcalfe, 2013]. These rocks form the basement in this study.

During the Permian, rifting and separation of the Cimmerian Microcontinent from Gondwana causes a failed rift (R1) between the West Burma/Lhasa Block. This model states that hyperextension has occurred within the failed rift system. Tectonic maps in the literature fail to document this Permian rifting event, thus the West Burma / Lhasa Block are shown attached to Gondwana.

From the Top Permian to the Late Triassic the margin has undergone post R1 passive subsidence and sedimentation.

N/A

N

Metcalfe, 2013

Metcalfe, 2013

- Exhumed/Serpentinized

Mantle

- Permian Sediments

- Top Permian – Intra-Triassic - Intra-Triassic – Top Triassic U.C.

Asthenosphere

Lithospheric Mantle

L = Lhasa Block

SWB = Southwest Borneo

7. Discussion

114

A Late Triassic compressive event related to the subduction of the Meso-Tethys then caused local uplift and possible inversion primarily near structural highs. This compressive event is termed the Fitzroy Movement and is the cause of the Late Triassic U.C. [Stagg et al, 2004; Chongzhi et al, 2013; Metcalfe, 2013].

Another period of subsidence then occurred post compression leading to more passive subsidence across the area. This disagrees with Loingley et al [2002] who states that during the Early and Middle Jurassic the Lhasa Block (1 in the right hand figure) rifted off the NW shelf and caused extensional structures. As no evidence of Early-Middle Jurassic structures are observed in this study, the Lhasa Block is shown attached to the West Burma Block (2 in right hand figure).

In the Late Jurassic (Oxfordian-Tithonian) the West Burma Block rifted and separated from the NW shelf in a NW-SE spreading orientation. This created the Argo Abyssal Plain and its present day COB.

Latest Jurassic.

Metcalfe, 2013

Heine & Muller, 2005

2

1

- Top Triassic U.C. – Early

Jurassic

- Early Jurassic – Base

Syn-R2 (Late Jurassic)

- Transitional Crust

- Oceanic Crust

EJ-WS=East Java–West Sulawesi

1 = Lhasa Block 2 = West Burma Block

WB = West Burma Block

Longley et al, 2002

7. Discussion

115

In the Early Cretaceous, India began to separate from the west of Australia in a WNW-ESE spreading orientation. This caused faults trending NNE-SSW across the Northern Carnavon Basin.

From the Middle Cretaceous to the present day, passive subsidence of the margin related to relaxation of the lithosphere after R2 and breakup.

N/A

Heine & Muller, 2005

- Base Tertiary - Present - Early Cretaceous –

Base Tertiary

Table 7.1. Summary table showing the geodynamic and tectonic evolution of the NW

Australian shelf. Column one contains schematic evolutionary sections through the red lines

indicated on the figures in column 2 (the southwest section line is the front section)

showing the 3D evolution of the shelf. These sections are roughly extensions of seismic

lines: 128_05, 120_01 and 128_03. Column two contains tectonic maps created using a

combination of the published literature and the results found in this study, the striped

green area represents study area. Column three provides a brief explanation of each

evolutionary stage and a comparison between the tectonic maps presented in columns 2

and 4. Column 4 shows tectonic maps shown in the literature for each stage (if possible).

[Compiled from: Heine, 2002; Longley et al, 2002; Stagg et al, 2004; Heine & Muller, 2005;

Chongzhi et al, 2013; Metcalfe, 2013].

7. Discussion

116

7.2. Comparing the NW Shelf with Analogue Margins:

7.2.1. The Norwegian Margin:

The Norwegian Rifted Margin is a polyphase rifted margin has undergone Late Jurassic

– Early Cretaceous hyperextension forming a series of large tilted fault blocks

[Osmundsen, 2008; Peron-Pinvidic, 2013]. The basin was then filled by passive

subsidence until in the latest Cretaceous – Early Tertiary when a second phase of rifting

caused continental breakup; the second rifting event is highly depth dependent [Kusznir

et al, 2005].

7.2.2. The Namibian Margin:

The Namibian Margin is another polyphase rifted margin. The first phase of rifting

occurred in the Triassic and is classified as ‘non-volcanic’; the second phase occurred in

the Jurassic and was of ‘volcanic’ nature [Gladczenko et al, 1998].

7.2.3. Comparison:

Both of the aforementioned margins share similarities with the NW Australian shelf. All

three margins show an early extension phase, an ∼100Ma period of relative tectonic

quiescence, followed by another stretching event (fig. 7.4). Norway exhibits the same

‘hyperextension -› passive subsidence -› DDS and continental breakup’ evolution as the

NW shelf and is therefore its most analogous margin [Osmundsen, 2008; Peron-Pinvidic,

2013]. However, the tectonic events that caused the two rifting events are not thought

to be completely isolated and thus the margins are not perfect analogues [Peron-

Pinvidic, 2013]. The Namibian margin shows extensive plume-related volcanism related

to the second rift event and is therefore commonly termed a ‘volcanic margin’ (or active

margin). However, the earlier extensional event is classified ‘non-volcanic’ (or passive);

these tectonic events are thought to be independent. Thus, the Namibia and NW

Australian margins are partially analogous.

Comparisons with other hyperextended margins can also help to explain some of the

anomalous features seen on the NW shelf. A similar feature to the Wombat Plateau (see

fig. 6.17 for evolution) can be seen on the Brazilian margin: the Sao Paulo Plateau

[Scotchman et al, 2010]. Although this is partially attributed to a transform zone, it

shows similar characteristics to the Wombat Plateau and could be used to better

determine the formation of the Wombat Plateau [Scotchman et al, 2010].

7. Discussion

117

Serpentine diapirism is well documented in subduction zones [Kamimura et al, 2002]

and in the Porcupine Basin [Reston et al, 2004]. Based on these examples, it is plausible

that the Tres Hombres has formed from deep serpentine diapirism.

Figure 7.4. Three very simplified sections through: the NW Australian shelf, the

Norwegian Margin, and the Namibian Margin. The figure is used to identify

trends shown within each margin where each margin undergoes significant

‘passive’ rifting, followed by the accumulation of ∼100Ma of passive margin or

‘sag’ sedimentation, followed then by a highly depth dependent rifting event that

leads to continental breakup. Taken from: this study, Osmundsen et al [2008]

and Gladcsenko et al, [1998].

7. Discussion

118

7.3. Some Remarks Regarding the Evolution of Polyphase Rifted

Margins:

Comparisons of the Norwegian and NW Australian margin have revealed that the

margins share a remarkably similar extension history. Both margins have undergone a

period of hyperextension, a period of ∼100Ma of passive subsidence, followed by a

period of depth dependent stretching (DDS) leading to continental breakup

[Osmundsen, 2008; Peron-Pinvidic, 2013; this study]. As the terms ‘hyperextension’

and ‘DDS’ are the two end-members of a continual system (see fig. 1.11), there must be a

reason for the differences in deformation style between the two phases of extension. As

explained in section 1.2.5, higher heat flow (and therefore faster rifting) will lead to

more stretching in the lithospheric mantle than in the crust.

The second extensional phase (that leads to continental breakup and shows DDS) at

both margins is preceded by a cooler (and possibly slower) first phase of rifting that has

possibly lead to the hydration and serpentinization of the uppermost mantle. It is

thought that the two phases of rifting in the Norwegian margin are not completely

isolated [Peron-Pinvidic, 2013] and neither margin shows perfectly symmetric rifting

[Peron-Pinvidic, 2013; this study (section 6.1.2)]. However, for the purpose of simplicity

in this model the two rift phases at each margin will be treated as symmetrical and

isostatically unrelated.

A simple way a pre-existing rift architecture can influence a second rift phase is the

formation of a lithospheric heterogeneity in rift phase one which is then re-exploited

during the second phase. This would lead to rapid localisation of lithospheric extension,

leading to fast lithospheric necking and heavily depth dependent stretching (fig. 7.5 –

see page 120).

The aforementioned explanation allows for higher heat flow in the second rifting

phase. However, depth dependent stretching requires a decoupling between the upper

crust and lithospheric mantle (maintained by the upwelling of hit asthenosphere) (figs.

1.11 & 1.12); if hyperextension has occurred the lower crust will have been removed.

The second way pre-existing hyperextension can affect the nature of a second rift is

the formation of serpentinized mantle. Serpentinites found at hyperextended rifted

margins are of lizardite composition [Escartín et al, 1997 (discovered at the Iberian

7. Discussion

119

margin)], which has a coefficient of friction of 0.3-0.45 [Escartín et al, 1997] (upper

continental crust plots from 0.65-1 [Bürgmann & Dresen, 2008]. Although this is not as

weak as ductile lower continental crust [Bürgmann & Dresen, 2008], it provides a

decoupling horizon that does not depend on temperature. In contrast to lower crustal

decoupling horizons (that require high heat flow to remain viscous), lizardite is most

stable at low temperatures [Caruso & Chernosky, 1979; Pérez-Gussinyé & Reston, 2001].

If lizardite is present, this relieves the dependency on high heat flow in order to have an

efficient detachment and thus, relieves the temperature and speed dependency of DDS

(fig. 7.5).

Thick accumulations of sediment over time can pose uncertainty for this theory and

close to 30km of sediments are found in the Roebuck Basin on the NW Australian shelf

[this study]. Dehydration of lizardite occurs at ∼550-600°C and, following a typical

continental geotherm of ∼25°C/km, at the base of the sedimentary column the

temperature is expected to be around 750°C meaning the lizardite would have

undergone retrograde metamorphism back to its olivine phase (eq. 7.1) [Caruso &

Chernosky, 1979]. However following the same typical geotherm, the brittle-ductile

transition is expected to occur at ∼15-20km depth [Burov & Watts, 2006] meaning that

the base of the same presented sedimentary column would be part of the ductile lower

crust. In this situation, the free water expelled by the serpentine during retrograde

metamorphism (eq. 7.1) [Caruso & Chernosky, 1979] would hydrate the lower crust

possibly forming partial melt. This could then allow for a possible reduction in the lower

crustal friction coefficient, leading to the lower crust becoming a more efficient

detachment and having a higher solidification temperature. A higher solidification

temperature would act to further reduce the temperature dependency of the decoupling

horizon.

10Mg5•5Al1•0Si3•5O10(OH)8 (Lizardite) = 2Mg3Si4O10(OH)2 (Talc) +

12Mg2SiO4 (forsterite) + 5Mg5Al2Si3O10(OH)8 (clinochlore) + 18H2O (fluid)

Equation 7.1. Metamorphic reaction equation showing the dehydration of

lizardite to from talc, forsterite, clinochlore and fluid.

7. Discussion

120

If one were to use the combined effects of lithospheric heterogeneity and serpentine

acting as a temperature-independent slip surface as a predictive tool. In a system that

has undergone a phase of ‘hyperextension’ and mantle exhumation, any second phase

that were to occur would likely be relatively depth dependent.

The recognition of this trend has wider reaching implications for the debate

surrounding active (volcanic) and passive (non-volcanic) rifting. Active rifting is

commonly attributed to active mantle upwelling and/or plume related volcanism

[Turcotte & Emerman, 1983], however the recognition of a previous extensional rift

phase could provide a means for the generation of an ‘active’ margin without

dependence on lithospheric thermal anomalies.

7. Discussion

121

Fig

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7. Discussion

122

7.4. Suggestions for Further Work:

Belgarde et al [2015] present a gravity forward modelled a section through the

Northern Carnavon Basin. In order to determine any lateral variability in the deep

structure it is advised that some forward modelling be carried out along sections in

the Roebuck and Browse Basins.

To determine the point of oceanic spreading initiation relative to the hyperextended

zone, work needs to be done on the West Burma Block.

In order to correctly determine the depth dependency of R2 stretching, restorations

need to be done across the shelf to determine the real brittle crustal beta factor and

how it varies across the area to also see how it varies laterally.

In order to determine the true nature of the COB a detailed bathymetric or sidescan

survey should be carried out alongside a more detailed seismic mapping to

determine where shearing has occurred and where transitional crust is present.

As the Tres Hombres Dome is so poorly understood, a detailed study dedicated to

determining the origin of this structure using gravity forward modelling and seismic

interpretation is advised.

A detailed fault interpretation across the Northern Carnavon, Roebuck and Browse

basins should be carried out, using the closely spaced 2D lines (app. 1), to show the

true orientation and abundances of faulting in the area.

123

Conclusions

8. Conclusions

124

8.1. Concluding Remarks:

The NW Australian shelf is interpreted as a polyphase rifted margin, having undergone

two distinct phases of rifting separated by a period of ∼100Ma of tectonic quiescence.

Up to thirty kilometres of sediment have accumulated atop Carboniferous basement and

proposed serpentinized mantle. These sediments have been divided into broad

megasequences dependent on their causative tectonic event: syn-rift-1 (R1) sediments,

post-R1 passive subsidence deposits (MS1), Jurassic passive subsidence and syn-rift-2

(R2) sedimentation following a Late Triassic compressive event (MS2), post R2 passive

subsidence (MS3).

The first phase of extension (R1) occurred in the Permian and lead to the

hyperextension of the continental crust, the exhumation and possible serpentinization of

the lithospheric mantle, and the formation of large, tilted fault blocks. Results presented

in this study have found that R1 was likely highly asymmetric and possibly influenced by

crustal inheritance. The shelf has been divided into zones of ‘hyperextended’, ‘necked’

and ‘stretched’ domains based on the Sutra et al [2013] classification (fig. 1.10). This

division has been compared with that of Belgarde et al [2015] (fig. 7.2).

Estimates of whole lithospheric stretching from backstripping have provided a variable

β factor with a maximum value of 1.3. This is significantly lower than Karner & Driscoll

[1999], who propose a lithospheric β factor of 2.65. However, both studies document a

very minor amount of upper crustal faulting suggesting that R2 stretching was highly

depth dependant.

Comparisons with analogue margins have revealed that the NW Australian shelf shares

similarities with two polyphase, east Atlantic rifted margins: the Norwegian and

Namibian margins. The Norwegian margin is the NW shelf’s strongest analogue, sharing

a very similar history. The Namibian margin has associated plume-related volcanism

and is therefore only partially analogous.

A common trend in the Norwegian and NW Australian passive margins has been

identified. Both exhibit an early extensional phase that causes hyperextension and

possible exhumation of lithospheric mantle, followed by ∼100Ma of passive subsidence,

followed by a highly depth dependent stretching event. This trend is attributed to the

generation of lithospheric heterogeneities and a serpentinite detachment during the first

rift phase.

125

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132

Appendix 1

Maps showing all provided surveys.

1.1. All AGSO Regional Lines:

133

1.2. All ‘2D Browse’ Lines:

134

1.3. All ‘2D Carnavon’Lines:

135

Appendix 2

Results from different depth conversion scenarios.

2.1. Sensitivity to Velocity Models (sections are 2x vertically exaggerated):

2.1.1. Average Velocity Model:

2.1.2. -10% Scenario Velocity Model:

136

2.1.3. +10% Scenario Velocity Model:

2.1.4. Alternating 1 Scenario Velocity Model:

137

2.1.5. Alternating 2 Scenario Velocity Model:

2.2. Sensitivity to Compaction Curves (all compaction curve scenarios

modelled from the ‘average velocity’ scenario):

2.2.1. Christie/Slater Compaction Curve:

138

2.2.2. Baldwin/Butler Compaction Curve:

2.2.3. Dixon Compaction Curve:

139

Appendix 3

The process of backstripping: an example from line 120_01. Section will

be backstripped to Top Syn-R2.

3.1. Original Section Interpretation:

3.2. β factor of 1 does not restore any of the section to sea level:

Flat Datum

140

3.3. Restored to β=1.05:


Flat Datum

Restored to sea level using a

beta factor of 1.05

Flat Datum


beta factor of 1.05

Restored to

sea level using

a beta factor

of 1.1

141



Flat Datum


beta factor of 1.05

Restored to

sea level using

a beta factor

of 1.1

Restored to

sea level

using a beta

factor of

1.15

Flat Datum


beta factor of 1.05

Restored to

sea level

using a beta

factor of 1.1

Restored to

sea level

using a beta

factor of

1.15

Restored to

sea level

using a beta

factor of 1.2

142

3.7. Restored to β=1.25 (finished product):

Restored to sea level using

a beta factor of 1.05

Restored to

sea level

using a beta

factor of 1.1

Restored

to sea level

using a

beta factor

of 1.15

Restored to

sea level

using a beta

factor of 1.2

Restored to sea level using

a beta factor of 1.25

Flat Datum

143

Appendix 4

Seed grids (top image) and generated surfaces (bottom image) for key

horizons.

4.1. Top Permian:

144

4.2. Intra-Triassic:

145

4.3. Early Jurassic:

146

4.4. Top Syn-R2:

147

4.5. Base Tertiary:

148

4.6. Seabed:

149

Appendix 5

Supplementary location map as an insert to use whilst reading document.

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

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