The Tectonic Evolution of Pegasus Basin and the Hikurangi Trench, Offshore New Zealand

THE TECTONIC EVOLUTION OF PEGASUS BASIN AND THE HIKURANGI TRENCH, OFFSHORE NEW ZEALAND

by

Sarah E. King A thesis submitted to the Faculty and the Board of Trustees of the Colorado School of

Mines in partial fulillment of the requirements for the degree of Master of Science (Geology).

Golden, Colorado

Date:______

Signed:______Sarah E. King

Signed:______Dr. Bruce Trudgill Thesis Advisor

Golden, Colorado

Date ______

Signed:______Dr. M. Stephen Enders Professor and Interim Department Head Department of Geology and Geological Engineering

ii ABSTRACT

The Pegasus Basin overlies part of the tectonic transition between oblique southwest subduction of the Paciic Plate below the East Coast of the North Island of New Zealand, and the strike-slip faulting that dominates the majority of the South Island of New Zealand. The

transition from this strike-slip zone into the actively subducting Hikurangi Trench requires a signiicant translation of plate motion from margin parallel to margin normal within the Pegasus Basin. The purpose of this research was to understand the distribution of strain along this complex transition, and to identify how shortening is manifested on structures through time.

The diferent stress regimes along the coast may correspond to diferent shortening amounts absorbed on a variety of structures that translate strain accommodation through the major tectonic transition from compressional subduction to strike-slip displacement. Interpretations of 2D seismic proiles guided by models of margins with comparable tectonic settings ensure geologically restorable interpretations and reasonable shortening values within this transition zone. Restorations of depth converted seismic cross sections constrain ages of the faults and establish controls on their timing and activation. Complexities such as a rotating forearc, a southern migrating subduction zone, and a strike-slip zone further complicate restorations past the late Pliocene-Quaternary aged sediments. Shortening values acquired from restorations align with shortening values from other studies in the area, and also align with estimates based of of the current plate motion vector movement. Economic interest in the Pegasus Basin is primarily petroleum based. Though the basin has not been tested, active seeps, pockmarks, gas chimneys, and surface slicks are numerous within the Pegasus Basin. Structural interpretation, and modelling of potential hydrocarbon accumulations and the implied luid pathways impact the feasibility of exploration and development of hydrocarbons in the Pegasus Basin.

iii TABLE OF CONTENTS ABSTRACT ...... iii

LIST OF FIGURES ...... vii

ACKNOWLEDGMENTS ...... xiii

CHAPTER 1 - INTRODUCTION ...... 1

CHAPTER 2 - GEOLOGIC SETTING ...... 3

2.1 Stratigraphy ...... 7

2.2 Structural Evolution ...... 9

2.3 Petroleum System ...... 12

2.3.1 Reservoirs and Seals ...... 12

2.3.2 Gas Hydrates ...... 13

2.3.3 Seeps and Slicks ...... 14

CHAPTER 3 - DATA RESOURCES ...... 16

3.1 Seismic Data ...... 16

3.2 Well Data...... 16

3.3 Earthquake Data ...... 19

3.4 Alternate Data ...... 21

CHAPTER 4 - SEISMIC INTERPRETATION ...... 23

4.1 Horizon Interpretation ...... 23

4.1.1 R8 (Basement) ...... 24

4.1.2 R7 ...... 24

4.1.3 R6 ...... 25

iv 4.1.4 R5C ...... 25

4.1.5 R5B ...... 26

4.1.6 R5 ...... 26

4.1.7 R3 and R4 ...... 26

4.1.8 R0 ...... 26

4.1.9 Torlesse Supergroup ...... 27

4.2 Time Depth Conversion ...... 27

4.3 Fault Interpretation ...... 28

4.4 Model Based Interpretation ...... 29

4.5 Alternative Interpretations ...... 35

CHAPTER 5 - RESTORATIONS ...... 37

5.1 Worklow ...... 37

5.2 Kinematics in Move ...... 39

5.2.1 Trishear ...... 40

5.2.2 Fault Parallel Flow ...... 41

5.2.3 Flexural Slip Unfolding ...... 41

5.3 Results ...... 45

CHAPTER 6 - DISCUSSION ...... 47

6.1 Comparison of Shortening Values...... 47

6.1.1 Comparison at 2Ma ...... 47

6.1.2 Comparison at 5Ma ...... 48

6.1.3 Plate Motion Vector Comparisons ...... 52

v 6.1.4 Active Fault Motion Comparison ...... 53

6.2 Rotation Accommodation ...... 54

6.3 Petroleum and Modelling Implications ...... 56

6.3.1 Gas Hydrate Modelling ...... 57

6.3.2 Pore Pressure Modelling ...... 58

6.4 Error ...... 58

6.5 Megathrust Earthquakes ...... 60

6.6 Accretionary Wedges Worldwide ...... 62

CHAPTER 7 - CONCLUSIONS ...... 64

7.1 Future Work ...... 65

REFERENCES ...... 67

APPENDIX A ...... 73

APPENDIX B ...... 78

vi LIST OF FIGURES Figure 2.1 An overview of the geologic setting of the islands of New Zealand. Bathymetry, provided by NIWA (National Institute of Water and Atomic Research), shows that the mini continent called “Zealandia” extends as shallow submarine continental crust, much further than the islands of New Zealand, which are outlined in blue. Plate motion vectors demonstrate the direction and magnitude of movement of the Paciic Plate relative to the Australian plate per year. The plate boundary (black lines) moves from subduction in the north, to the strike slip zone along the South Island, and back to subduction south of the islands of New Zealand (Modiied from Uruski & Bland 2011)...... 3 Figure 2.2 Faults within the study area are colored to demonstrate zones of extension and compression in various parts of the both the North and South Islands of New Zealand. Pegasus Basin is shown in purple, and covers the area that transitions from the subduction thrust into the Marlborough Fault Array along the northern South Island...... 5

Figure2.3 Crosssection A-A’, location shown in (Figure 2.2)...... 6 Figure 2.4 General chronostratigraphy of New Zealand with transects from the Marlborough region to the Wairarapa region. Compiled from Uruski & Bland (2011), Wood et al. (1989), Field & Uruski et al., (1997), Lee & Begg (2002), Rattenbury et al. (2006), and references therein...... 8 Figure 2.5 Possible reconstructions of the New Zealand plate boundaries, and the location of Pegasus Basin through time, compiled from Nicol et al. (2007) and Uruski & Bland (2011) and references therein. A. Gondwana subduction zone with location of landmasses and sedimentary basins. B. The New Zealand Gondwana sector pulls apart from Australia and Antarctica. C. A period of tectonic quiescence for Pegasus Basin. D. Paleogene and Neogene tectonic development E. Modern plate boundaries form around New Zealand (40Ma). The Emerald Basin started to open, rifting is widespread in New Zealand, and there was compression in the northwest, in the Reinga and outer Taranaki basins. F. Development of the Alpine Fault is formed, and the present subduction zone begins, and rotation of the plates begins. G. Subduction zone has rotated from NW-SE to NE-SE and motion vectors indicate signiicant rotation around poles. H. Present day tectonic setting...... 10

Figure 2.6 A. Schematic of luid pathways within a thrust and anticline in the Pegasus Basin. Taken from Barnes et al., (2010). B. Section of seismic time proile PEG019 demonstrating the strong BSR in the area, and the underlying thrust responsible for the anticlinal trap, and the luid migration pathways...... 14 Figure 2.7 (Bland et al., 2014).A map of seeps and luid indicators across Pegasus Basin. ...15 Figure 3.1 Location of seismic data within Pegasus Basin. The PEG09 survey traces are in red. The closest wells to Pegasus Basin, Titihaoa-1 and Tawatawa-1 are in the northeast corner of the map. Bottom samples and ages are shown by the black dots. Other igures are shown in yellow...... 17 Figure3.2 Welltieto PEG09 survey for Titihaoa-1 from Uruski & Bland 2011. Uninterpretedseismic proile (bottom) is shown in time. Interpretation is shown on the top, also displayed in time. Note the well location on the shelf,

vii and the various piggyback basins separated by thrust fault related ridges. Location of seismic proiles shown in Figure 3.1...... 18 Figure 3.3 Earthquake data from the past ten years with a magnitude of 3+.Additional data is available with ilters for date of occurrence, depth, and magnitude on the EQC & GNS website...... 19 Figure 3.4 Taken from Eberhart-Phillips et al., (2010). Section location shown in Figure 2.2. Seismic events from 2001-2009 shown in depth. Seismicity is observed to follow the plate boundary interface of the Paciic Plate down into the Moho. Color scheme is the result of the Eberhart-Phillips 3D seismic velocity model for the New Zealand region. The white line represents a resolution SF=3.5 (spread function)...... 20 Figure 3.5 Taken from Lamb et al., (2011). Displacement of the Hikurangi margin over the past 4Ma to present day setting. Constrained by data from inite plate motion, major fault displacement measurements, and assuming rigid body rotation of blocks. A. Displacements relative to Australian Plate. B. Displacements relative to the Paciic Plate. Both demonstrate signiicant changes in motion vectors at a hinge near the northern South Island, at the apex of the Pegasus Basin, indicating the change in plate motion vectors, and in strain accommodation...... 21

Figure 3.6 Taken from Mortimer (2014). Reconstructions demonstrating the evolution of the oroclinal bend in the South Island and the basement markers that allow measurement of ofset at their current setting. a) 105Ma with assumed colinear basement markers. b) 85Ma after rifting and initial bending and stretching of basement markers. c) 45Ma shows three options for the restoration of the Fiordland part of the Median Batholith. d) Present day alignment of basement markers...... 22

Figure4.1 Horizonsthat were used to interpret seismic time proiles are listed with their inferredage, and velocity used for depth conversion (from Barnes et al., 2010 andPlaza-Faverola et al., 2012). Tectonostratigraphic packages in this section wereused for both restoration and depth conversion. This seismic proile islocated at the deformation front on seismic proile PEG023, the second northernmostline in the PEG09 dataset (Figure 3.1)...... 23 Figure4.2 CookStrait faults (black) shown with annual movement in millimeters per year in red for currently active faults. Bathymetry is shown with 500m contours. The large red arrow is the Paciic-Australia relative plate motion vector. PEG09 survey lines PEG009 and PEG017 are shown for reference to interpretations. Taken from Wallace et al (2012)...... 28 Figure4.3 Examplesof obliquity inluence on the Coulomb wedge, taken from Graveleau et al(2012). Models that illustrate the main components and structural features ofoblique convergent margins. A.(Biagi, 1988) Map view of fault systems displaying two separate characteristics, one of frontal thrusts, and internal strike-slip faults. B. (Biagi, 1988) 3D view of internal fault structures and their crescent shaped contacts bounded by strike slip faults and a major thrust. C. (Calassou et al., 1993) Cross section view of internal fault geometry when comparing direct convergent wedges to obliquely convergent wedges (angle of obliquity 45-55°). There are fewer thrusts with steeper angles in the latter. D. (Malavielle et al., 1992) High obliquity faults display two geometries,

viii major sub-vertical faults along the backstop, and minor faults between major thrusts. E. (Martinez et al 2002) backstop height and its efect on wrench zone location, thickness, and geometry. Compare to Figure 4.2 for similarities in map view fault geometries...... 30

Figure4.4 Interpretationof seismic cross section PEG023. The top igure shows the depth converted,uninterpreted seismic line. The lower igure shows the inal interpretation of the cross section. Primary controls of this interpretation come from the clear relectors in the undeformed section of the wedge in the eastern part of the wedge. The dramatic changes in bathymetry control placement of major thrust faults, and their associated anticlines in the hanging wall. Interpretation of strike-slip faults versus oblique thrusts are identiied from the previously published fault maps. Geometries of the thrusts at depth are guided by sandbox models and previous interpretations of the seismic data. Location of seismic line can be found in Figure 3.1...... 32

Figure4.5 Interpretationof seismic cross section PEG017. The top igure shows the uninterpreted, depth converted seismic line. The lower section shows the inal interpretation of the cross section. Interpretation of the faults comes primarily from discontinuity in the seismic relectors, and the deformation of the sealoor. Previously published fault maps show thrusts that dip towards the strike slip zone on either side of a major strike-slip wrench zone. Sandbox models show similar geometries at depth of this wrench zone. Earthquake hypocenters are projected from 10km out onto the depth proile, and indicate the décollement surface at the plate interface. The location of seismic line can be found in Figure 3.1...... 33

Figure 4.6 Interpretation of seismic cross section PEG009.The top igure shows the uninterpreted depth converted section of the seismic cross section. The lower igure shows the inal interpretation of the cross section. Previous fault maps and research in the area of this line indicate steeply dipping thrust faults that connect onto thrusts at depth, that then detach onto the plate interface. Seismic relectors, tectonostratigraphic packages and restorations of observed deformation controlled horizon placement. This line is in the southwestern Pegasus Basin where seismicity is far more frequent than in other cross section. Earthquake hypocenters are projected from 10km out onto the depth proile. High magnitude earthquake hypercenters align with placement of faults in the westernmost part of the cross section. Location of seismic line can be found in Figure 3.1...... 34 Figure 4.7 Alternate interpretations of seismic line PEG017 from previous work...... 36 Figure 5.1 Restoration of seismic cross section PEG023. Internal deformation of the wedge occurs until the critical taper is reached, and faults then step foward into the wedge...... 42 Figure 5.2 Restoration of seismic cross section PEG017. Faults deform internally until critical taper is obtained, then thrusts step forward into the wedge. There is a signiicant amount of displacement on the strike-slip faults that cannot be accounted for in 2 dimensional restorations...... 43 Figure 5.3 Restoration of seismic cross section PEG009. Faults tend to step forward into the wedge through time, though out of sequence faults are required to maintain critical taper. Faults initiate as thrusts, but accommodate more strike-slip as

ix the wedge grows...... 44

Figure5.4 Diferentmethods to calculate strain based on placement of L o and Lf. For comparison to diferent ages, method A is used to calculate strain rates. For calculating total strain for each step of the reconstruction, method B is used. When determining the amount of strain between individual time steps, method C is used. The method used to determine strain rates depends on what time period the strain rates are being compared to...... 46 Figure 6.1 Shortening values obtained from this study in comparison to the plate motion vectors, and the Ghisetti et al (2016) study since 2Ma...... 48 Figure 6.2 Shortening values obtained from this study in comparison to the plate motion vectors, and rates from Nicol et al., (2007). Shortening values across the North Island are summed across proiles A-E. Final shortening estimates are shown on the right of the transects. Estimates from the 3.5+ Ma cross section in this study were used to compare with the Nicol et al., (2007) study...... 49 Figure 6.3 Shortening values parallel and perpendicular to the plate motion vector at 5Ma using angles of the plate motion vector within Pegasus Basin...... 50

Figure 6.4 Schematic cross section of transect E (Nicol et al., 2007), and the location of the PEG023 seismic cross section, with their associated percentages of the total shortening from the plate motion vector...... 51

Figure 6.5 Block model fromWallace et al., (2012) combined with rotation rates since 4Ma from Lamb et al., (2011) through paleomagnetic studies. Blocks in the south remain dominantly strike-slip in translation of strain, whereas to the north there is signiicant rotation, subduction, and transpression accommodating and dispersing strain within the Australian Plate...... 54 Figure 6.6 Faults and seismic line locations shown with the yellow dashed line, representing the separation of wedge sediments underlain by Cretaceous and Paleogene basement (north of the yellow line) and the late Cenozoic frontal accretionary wedge, from Barnes et al., (2010). The orange dashed line is the suggested change in this boundary from this study. This change is based on seismic signatures observed in southern seismic time proiles PEG009, PEG007 and PEG005...... 57

Figure 6.7 Taken fromWallace et al. (2010).The Hikurangi margin and its transition from aseismic creep in the North Island, to interseismic locking zones in the South Island. Diferent aspects of the wedge dynamics are compared as the margin moves from subduction into the strike-slip Marlborough Fault Array on the South Island. Red and blue shading indicate coupling coeicients. Red indicates areas that are currently locked and are likely to rupture in future subduction thrust earthquakes. Blue indicates smooth aseismic creep. Green contours show areas of slip in slow slip events since 2002 from Wallace and Beavan (2010). Convergence rates are shown in red (in mm/yr). The accompanying table shows along-strike variations in various subduction margin properties for the accretionary wedge. The inset shows the large scale tectonic setting. HT is Hikurangi Trench, KT is Kermadec Trench, TT is Tonga Trench, NI is North Island, and SI is South Island...... 61

Figure6.8 Takenfrom Davis et al. (1983). Theoretical linear relationships that compare the

x décollement dip (β), and the topographic slope (α), to predict the range of possible luid pressure ratios. Boxes indicate observed geometries of active wedges, used to infer luid pressure ratios within them. Heavy boxes indicate wedges with available pressure data. The theoretical linear relationship of the décollement to the topographic slope, α + Rβ = F, is controlled by the ratio of luid pressure ratios, λ to λb assuming that basal friction (μb) = 0.85 and internal friction (μ) = 1.03 (consistent with Byerlee's empirical law of sliding friction). Hikurangi prism shown in red...... 63 Figure A.1 Map showing the extent of each horizon within the undeformed region of the wedge Younger units depositional areas increase in area to the south and east. 73 Figure A.2 Map showing the extent of horizon R3 throughout the Pegasus Basin. Horizon is not continued west through the deformation front...... 73 Figure A.3 Map showing the extent of horizon R4 throughout the Pegasus Basin. Horizon is not continued west through the deformation front...... 74 Figure A.4 Map showing the extent of horizon R5 throughout the Pegasus Basin. Horizon is not continued west through the deformation front...... 74 Figure A.5 Map showing the extent of horizon R5B throughout the Pegasus Basin. Horizon is not continued west through the deformation front...... 75 Figure A.6 Map showing the extent of horizon R5C throughout the Pegasus Basin. Horizon is not continued west through the deformation front...... 75 Figure A.7 Map showing the extent of horizon R7 throughout the Pegasus Basin. The horizon is not continued past deformation, but must exist there, as it is the décollement surface for the subduction thrusts...... 76 Figure A.8 Map showing the extent of horizon R8 throughout the Pegasus Basin. The horizon dips down underneath the North Island, and represents the top of the Hikurangi Plateau...... 76 Figure A.9 Map showing the extent of the Torlesse supergroup in the Chatham Islands. The Torlesse extends much further south and east of this study area...... 77

Figure A.10 Map showing the extent of horizon L. This is the surface observed in the southern lines that represents the Oligocene sediments that the Neogene sediments onlap onto on the Chatham Rise ...... 77

xi LIST OF TABLES

Table B.1 Table showing the strain rates (in percentages) of the various L o and Lf combinations for the restoration of seismic proile PEG023...... 77

TableB.2 Tableshowing the strain rates (in percentages) of the various L o and Lf combinations for the restoration of seismic proile PEG017...... 77

TableB.3 Tableshowing the strain rates (in percentages) of the various L o and Lf combinations for the restoration of seismic proile PEG009...... 77

TableB.4 Tableshowing the changes in line length (km) of the various L o and Lf combinations for the restoration of seismic proile PEG023...... 78

TableB.5 Tableshowing the changes in line length (km) of the various L o and Lf combinations for the restoration of seismic proile PEG017...... 78

TableB.6 Tableshowing the changes in line length (km) of the various L o and Lf combinations for the restoration of seismic proile PEG009...... 78

xii ACKNOWLEDGMENTS This project would not have been possible without the guidance and support of my advisor Bruce Trudgill. Additionally, my committee members, Yvette Kuiper, and Lesli Wood were very insightful and willfully gave their input, criticism, and suggestions. I am extremely grateful for other students in the tectonostratigraphic research group for their support, and for allowing me to constantly ask their opinion, or ridiculous questions over and over again. I'd like to also thank Wesley Bucker, for passing on Move knowledge, New Zealand knowledge, and any other type of advice he could think of.

I'd like to acknowledge GNS Science and New Zealand Petroleum and Minerals, for their willingness to provide data within the study area. Earthquake Commission and NIWA allowed integration of a variety of datasets, which I could not be more grateful for. I'd like to acknowledge the New Zealand GeoNet project and its sponsors EQC (Earthquake Commission), GNS Science, and LINZ (Land Information New Zealand) for providing data and images used in this study.

A huge thank you to all my Colorado School of Mines colleagues. I have had a ton of fun, and have learned more from you all than I ever would have expected. Thank you for the support, and the time you put in, both to our educations and friendships.

I'd like to thank the Colorado School of Mines Geology Department, for the opportunity to be a Teacher's Assistant, and to participate and contribute to the wealth of petroleum geology therein. Scholarships and fellowships from Apache and Devon Energy Corp. were absolutely essential in helping fund this research.

Finally, I'd like to thank my family for listening to my rants and allowing me to talk about rocks for hours, even if they didn't know what I was talking about.

xiii CHAPTER 1 INTRODUCTION Plate boundaries host the majority of the deformation caused by large scale plate tectonic movements. Restoration of deformation allows insight into the controls and mechanisms driving the tectonic evolution of the plate boundary interactions (Nicol et al., 2007). The boundary's history can be quantiied using a variety of datasets that span several diferent time boundaries. GPS data provide data on a scale of tens of years, while fault movement analysis and active fault data give a scale of tens of thousands of years. Paleomagnetic data and large scale structures provides analysis up to millions of years. Analysis of a combination of these data allow kinematic restoration of plate boundaries (Lamb et al., 2011; Wallace et al., 2012).

The plate boundary between the Paciic Plate and the Australian Plate undergoes severe deformation as the margin transforms from a subduction zone just east of the North Island, to a strike-slip zone in the northern South Island. Seismic data recently acquired from eastern ofshore New Zealand are ideal for identifying the intense deformation along this complex margin. The survey through the associated accretionary wedge provides clear images of thrust faulting near the deformation front that become washed out as seismic resolution is inluenced by structural complexities in the western part of the basin. For the shallow, clear strata that are imaged by the seismic data, kinematically accurate restoration of the more recent deformation is possible, to better understand the present geometries associated with tectonostratigraphic growth.

Where seismic data become unreliable, model based interpretations, and existing fault maps were utilized to help guide interpretations and restorations.

Previous studies indicate the complexities that come with the restoration of the Hikurangi margin, including; as a rotating forearc, a subduction zone migrating south, buoyant continental crust that will not subduct, a deformable backstop that is diicult to identify in seismic data, and a range of ductile to brittle deformation mechanisms. All of these factors complicate restorations past the late Pliocene-Quaternary sediments (Burgreen-Chan et al., 2015, Ghisetti et al., 2016). Large-scale restorations of the entire margin require extrapolation of geologic interpretations far

1 below seismic resolution and commonly misrepresents the true shortening values of a margin

(Ghisetti et al., 2016).

Shortening values acquired from restoration of seismic cross sections within this study are well constrained through the Pliocene-Quaternary sediments. These shortening values are calculated based on correlation of horizons and restorations of cross sections. The values are compared to alternate shortening values in the same area over the same time periods. The comparisons show similar amounts of shortening when they are compared to the other regional studies. Shortening values from these restored cross sections are also compared with shortening estimates expected based of of the current plate motion vector. Shortening values that extend past 5Ma, however, are estimated, and are subject to variables that are indistinguishable within the 2D seismic lines, and therefore are only minimal estimates for shortening past 5Ma are calculated. Structural interpretations and their associated restorations are heavily inluenced by the change in structural regime observed within the basin. The interpretations and restorations provide a structurally restorable model of the margin's tectonic evolution, while providing estimates for shortening values and fault accommodation of shortening over the past 5Ma.

2 CHAPTER 2 GEOLOGIC SETTING

The present-day plate boundary between the Australian and Paciic Plates runs through the New Zealand mini-continent in a south-southwest direction. It extends from the Kermadec Trench in the north, along the Hikurangi Trench east of the North Island, through the westernmost part of Pegasus Basin, across the Cook Strait, and into the Marlborough region at the northern end of the South Island (Figure 2.1). The subduction zone north of New Zealand moves from the very deep (>9000m) Kermadec Trench, to the relatively shallow (~3000m) Hikurangi Trench that bounds the east coast of the North Island (Figure 2.1). The structures within the Pegasus Basin demonstrate the changing stress regime as the increase in right lateral

Figure 2.1 An overview of the geologic setting of the islands of New Zealand. Bathymetry, pro- vided by NIWA (National Institute of Water and Atomic Research), shows that the mini continent called “Zealandia” extends as shallow submarine continental crust, much further than the islands of New Zealand, which are outlined in blue. Plate motion vectors demonstrate the direction and magnitude of movement of the Pacific Plate relative to the Australian plate per year. The plate boundary (black lines) moves from subduction in the north, to the strike slip zone along the South Island, and back to subduction south of the islands of New Zealand (Modified from Uruski & Bland 2011).

3 strike-slip movement becomes more obvious in both outcrop and subsurface structures. The Hikurangi Trench shallows into the accretionary wedge where the strike-slip zone becomes dominant over convergent deformation in the southern Pegasus Basin. The major strike-slip zone is the Alpine Fault (Figure 2.1), which dominates the majority of the length of the South Island. It splays to the north into the Marlborough Fault Array, which is younger to the southeast (Van Dissen and Yeats, 1991; Holt and Haines, 1995; Langridge et al., 2010; Wallace et al., 2012). The Marlborough Fault Array crops out along the northeastern part of the South Island and extends northeast into the Cook Strait. Southwest of the New Zealand mini-continent, the subduction zone shifts polarity, and the Australian Plate subducts under the Paciic Plate in the Puysegur Trench (Furlong & Kamp 2009) (Figure 2.1).

Subduction and deformation styles along this margin vary signiicantly from north to south as the stress regime changes laterally, as observed in the changing fault orientations and ofset directions in Figure 2.2. The subducting crust of the Paciic Plate in the north (Hawke’s Bay area) hosts several pockmarks and seamounts and 1-1.5km of trench sediments, while to the south (Wairarapa, Pegasus Basin, and Marlborough), the trench sediments increase to 4-6km in thickness. The trench sediment and associated accretionary wedge narrow to the south as the

Chatham Rise, (a submerged extent of the Zealandia continental crust that stretches from the South Island approximately 1000km to the Chatham Islands) (Lewis et al.,1998; Plaza-Faverola et al., 2012; Ghisetti et al.,2016), creates the easternmost depositional boundary for the wedge and terminates at the South Island (Figure 2.1). The Pegasus Basin is frontal accretion dominant, and is known for its deep, slow slip events (SSEs) and strong interseismic coupling, while further north towards Hawke’s Bay the convergence rate increases and the SSEs are shallow with weak interseismic coupling (Wallace et al., 2010). Interseismic coupling is the relationship between the slipping velocity on the plate interface during seismic events, and the total long term slip rate. Strong interseismic coupling means that movement along the interface is infrequent, but is activated as high intensity seismic events, where weak interseismic coupling indicates a relatively consistent velocity of subduction and low intensity slip events (Wallace et al., 2010).

4 The vector of movement of the Paciic Plate relative to the Australian Plate is roughly 47mm/yr in the north decreasing to 42mm/yr in the south (Figure 2.1) at an oblique angle of roughly 50° to the trend of the margin (DeMets et al., 1994; Walcott, 1998; Beavan et al., 2002; Ghisetti et al., 2016). The change from plate normal motion is attributed to the clockwise rotation of the forearc region and an increase in plate convergence obliquity to the south (Wallace et al., 2004; Ghisetti et al., 2016). This rotation separates the North Island into two zones, one of convergence and compression to the south, and another of extension, and associated volcanism in

Figure 2.2 Faults within the study area are colored to demonstrate zones of extension and com- pression in various parts of the both the North and South Islands of New Zealand. Pegasus Basin is shown in purple, and covers the area that transitions from the subduction thrust into the Marl- borough Fault Array along the northern South Island.

5 the Taupo Volcanic Zone to the north (Figure 2.2) (Nicol et al., 2007).

Development of this subduction margin began 24-30Ma (Ballance, 1976; Cole and Lewis, 1981; Lewis and Pettinga, 1993; Lamb, 2011; Ghisetti et al., 2016). Most of the present day shortening is thought to occur along the décollement surface at the subduction interface

(Nicol and Beavan, 2003; Burgreen-Chan et al., 2016), though the upper plate is deformed through strain dispersed within reverse, normal and strike-slip faulting, as well as block rotation

(Nicol et al., 2007 and references therein; Burgreen-Chan et al,. 2016). The central margin of the Wairarapa coast (Figure 2.2) is a classical imbricated thrust wedge dominated by accretion (Davey et al., 1986b; Lewis and Pettinga, 1993; Collot et al., 1996; Barnes and Mercier de Lépinay, 1997; Lewis et al., 1999; Barnes et al., 2010) that narrows to the south (Figure 2.2), where the present-day transition from oblique subduction to strike-slip deformation occurs (Holt and Haines 1995; Barnes et al., 1998a; Barnes and Audru, 1999; Barnes et al., 2010).

The Pegasus Basin is comprised of the sediments within the accretionary wedge, located on the subducting Paciic Plate, along the eastern margin of the North Island of New Zealand (Figure 2.2 and Figure 2.3). The northern boundary is the southern margin of the East Coast Basin that continues southwest along the continental shelf into the Cook Strait. The southeastern boundary of the basin is deined by the onlap of the wedge sediments onto the Chatham Rise (Figure 2.1). The southwestern part of the Pegasus Basin extends into the Marlborough Fault

Figure 2.3 Cross section A-A’, location shown in (Figure 2.2).

6 Array. This right-lateral strike-slip fault array splays from the major right-lateral strike-slip fault known as the Alpine Fault (Figure 2.1) that traverses much of the length of the South Island. The splays in the Marlborough Fault Array are younger to the southeast (Wallace et al., 2012). The plate boundary in the Wairarapa area is inferred to be interseismically locked (Wallace et al., 2012) along the subduction interface down to around 40km depth (Figure 2.3). This segment is thought to have the ability to produce large megathrust earthquakes when the locked zone slips, resulting in large failures (Wallace et al., 2009).

2.1 Stratigraphy Stratigraphy of the Pegasus Basin is closely linked with tectonic events that control accommodation, provenance, and shortening related geometries. Well control is not available for stratigraphic correlation, because the closest wells to Pegasus Basin (50-100km away, chapter 3.2) do not penetrate deeper than middle Miocene sediments. Outcrops in the Wairarapa and Marlborough regions (Figure 2.2) are the most useful for understanding the stratigraphy of the Pegasus Basin, though they are fundamentally diferent in their structural history. Just ofshore of these outcrops, the shelf and upper slope of the Pegasus Basin is underlain Cretaceous and Paleogene rocks. These rocks act as a backstop for the deformation within the wedge. The backstop is slightly more rigid than the overlying Neogene sediments. The backstop moves on the basal detachment to cause contraction of the upper basin inilling sequences, and has a higher ability to support larger stresses than the inilling sediments, but still undergoes deformation in the Hikurangi Trough (Gomes et al., 2010). Neogene sediments have been deposited on top of this backstop simultaneous to deformation at the start of subduction 24Ma and are the deformed inilling sequences (Lewis 1973; Lewis and Pettinga, 1993; Uruski 1994; Barnes et al., 1997).

The basement of both the Pegasus and the East Coast Basin is a metasedimentary rock called the Torlesse Supergroup (Figure 2.4), a series of mass-low successions (Uruski & Bland 2011). Its interpreted environment of deposition was along the ancient subduction margin of Gondwana (Mortimer, 2004; Burgreen-Chan et al., 2016), dated based on the relationship with

7

Figure 2.4 General chronostratigraphy of New Zealand with transects from the Marlborough region to the Wairarapa region. Com- piled from Uruski & Bland (2011), Wood et al. (1989), Field & Uruski et al., (1997), Lee & Begg (2002), Rattenbury et al. (2006), and references therein.

8 the Mesozoic basement rocks of the Chatham Rise. The Torlesse Supergroup crops out in the Marlborough region and in the Wairarapa region, though the Marlborough region’s outcrops are slightly older in age (Uruski & Bland 2011). The Torlesse Supergroup is expected to act as a deformable backstop that is the foundation for the deposition of Neogene sediments.

From the end of the ancient phase of Gondwana subduction at about 105Ma, to 24Ma the Pegasus Basin accumulated more than 2000 meters of passive margin Late Cretaceous and Paleogene sediments along the Chatham Rise. As the source area subsided around 24Ma, the sedimentary succession became iner grained and transitions into Oligocene carbonate deposition. Neogene sediments are dominated by channel and turbidite systems that reach more than 6000 meters thick within the accretionary wedge adjacent to the East Coast Basin, in the area called the Pegasus Basin (Uruski & Bland, 2011).

The Pegasus Basin itself is comprised of the sediments within the accretionary wedge located on the subducting Paciic Plate along the eastern margin of the North Island of New Zealand. The wedge is primarily comprised of Neogene mass transport deposits, turbidites, pelagic muds, and deepwater channel systems that transport material from the continental margin of both the North and South Islands, as well as from the Cook Strait, Pegasus Canyon, and the

Chatham Rise. There are limestones associated with the structural paleohighs (Bailleul et al., 2013; Burgreen-Chan et al., 2016). The stratigraphy and associated geological events that are used for restoration of the margin are described in detail in chapter 4.1.

2.2 Structural Evolution

Prior to 30Ma, the Zealandia landmass (termed by Luyendyk, 1995; Landis et al., 2008; Furlong & Kamp 2009) was all on a single plate, although there were still various degrees and types of internal deformation across the upper plate (Figure 2.5) (Furlong & Kamp 2009). The Pegasus Basin, East Coast Basin, and Raukumara Basins were originally all connected along the ancient Gondwana margin from Permian to mid Cretaceous times (Figure 2.5A) (Bland et al., 2015). Following the Gondwana breakup, rift related thermal subsidence lead to 80 million years

9 Figure 2.5 Possible reconstructions of the New Zealand plate boundaries, and the location of Pegasus Basin through time, compiled from Nicol et al. (2007) and Uruski & Bland (2011) and references therein. A. Gondwana subduction zone with location of landmasses and sedimentary basins. B. The New Zealand Gondwana sector pulls apart from Australia and Antarctica. C. A pe- riod of tectonic quiescence for Pegasus Basin.D. Paleogene and Neogene tectonic development E. Modern plate boundaries form around New Zealand (40Ma). The Emerald Basin started to open, rifting is widespread in New Zealand, and there was compression in the northwest, in the Reinga and outer Taranaki basins. F. Development of the Alpine Fault is formed, and the present subduction zone begins, and rotation of the plates begins. G. Subduction zone has rotated from NW-SE to NE-SE and motion vectors indicate significant rotation around poles. H. Present day tectonic setting.

10 of passive margin sedimentation within the Pegasus Basin (Figure 2.5B-C) (Bland et al., 2015). Subduction along the Hikurangi margin is inferred to have commenced around 30–24 Ma (Figure 2.5E) (Nicol et al., 2007). The subduction zone has progressively migrated south (Figure 2.5F-H) and currently transitions into a strike-slip zone within the Pegasus Basin (Figure 2.5).

The Paciic/Australian plate boundary currently accommodates a relative plate-motion vector of 42–47 mm/year trending at ca. 50° to the trend of the margin (Figure 2.1 and Figure 2.5H). This orientation is a result of the forearc (North Island and the island chain along the Kermadec Trench) rotating clockwise from a WNW orientation in the early Miocene (Figure 2.5F) to its present inclination that strikes approximately NNE (Barnes et al., 1998.)

Greater than 80% of margin-normal shortening is accommodated on the subduction thrust (Nicol et al., 2007). The remainder of the margin-normal motion (10-20%) and most of the margin-parallel motion (>50%) are accommodated in the upper plate through a combination of reverse faulting, strike-slip faulting, and vertical-axis clockwise rotations (Nicol et al., 2007). The regional surface slope of the wedge is <1°, although the series of ridges caused by strike- slip and thrust faults along the coast have bathymetric relief of up to 1km (Barnes et al., 1998). The décollement surface that separates the Australian Plate from the Paciic Plate dips below the coast at approximately 3°, while the wedge taper angle is around 4°-6°. These angles are relatively low compared to global databases (Barnes et al., 1998), and indicate a very low basal friction and high pore pressure ratios (near lithostatic pressures) (Barnes et al., 1998).

In order for such high overpressuring to occur, luids from subducting sediments would need to have a conduit to migrate into the upper crust. The Hikurangi Plateau sediments are relatively cold, and luids would only be released at the Moho or deeper (Peacock, 2009; Wada and Wang, 2009; Fagereng 2011). It is therefore predicted that the luids released along the subduction front can migrate up the subduction interface. This interface doubles as the décollement for the detaching subduction thrusts, so luids continue their migration up fault planes into the Neogene wedge sediments (Fagereng 2011).

11 2.3 Petroleum System Petroleum exploration is not new in New Zealand. There have been oil and gas seeps that have been actively sought for over 100 years. New technologies such as 3D seismic combined with wildcat wells north of Pegasus Basin indicate that subsurface structures potentially hold large reserves of both oil and gas. Seismic surveys have been acquired in the East Coast Basin, along with three wells that have signiicant gas shows and a sub-commercial discovery (Griin et al., 2015). The Pegasus Basin shows similar promise, though no wells have currently been drilled. The PEG09 survey conirmed several direct petroleum indicators for petroleum systems in the Pegasus Basin. There are several lat spots, amplitude washouts, bright spots, and velocity push-down efects as well as a striking bottom simulating relector (BSR) (Figure 2.6), indicating the widespread existence of gas hydrates (Uruski & Bland, 2011). Seeps and pockmarks further conirm the release of hydrocarbons within the basin (Figure 2.7). Several owners and operators have applied and received ofshore permits for the Pegasus Basin, though more research needs to be done before exploration drilling is considered.

The Ministry of Economic Development of New Zealand released a petroleum report from the PEG09 survey that identiied six diferent play types within the Pegasus Basin. They include anticline folds along the East Coast deformation front, anticline folds along the buried

Gondwana margin, anticline folds created by blind thrusts, BSR (bottom simulating relector) traps and compound structural/BSR traps, stratigraphic pinchouts against the Chatham Slope, and stratigraphic traps along the upper Chatham Rise.

2.3.1 Reservoirs and Seals There are limited seismic data within the Pegasus Basin, and no wells have currently been drilled, so little is known about potential reservoirs. However, in the northern part of the East Coast Basin in the Hawke’s Bay area there are a few promising reservoirs with positive shows from wells drilled ofshore. The main potential reservoirs within the Pegasus Basin include Cretaceous shelfal and deepwater sandstones, Neogene shelfal and deepwater sandstones, and Neogene fractured limestones.

12 The most promising formation is the Whakataki Formation (Figure 2.4) which is Early Miocene in age, and has gas shows in the Titihaoa-1 well (section 3.2). The logs indicate thin, heterogeneous beds, and a log-derived net to gross of 8-13%. The sandstone beds encountered within this well have porosities around ~15% and permeabilities of 20+ md, which match those of samples collected from outcrops (Grifen et al., 2015). Other formations that have been tested in various wells up and down the coast include the Tuanui Formation (Middle Miocene), the Kaohauroa Limestone (Early-Middle-Late Miocene), and the Whakataki Formation (Early Miocene) (Figure 2.4) (Grifen et al., 2015).

Work compiled by Bland et al., (2014) suggests that primary reservoirs are in deep- water slope fan and basin loor fan turbidite deposits within the Neogene wedge. Sediments derived from the quartz-rich terranes of the southern South Island are expected to have desirable reservoir properties. Sediments sourced from the low-grade metasedimentary and volcaniclastic sources of the Chatham Rise, lower North Island, and upper South Island are expected to have degraded reservoir properties due to burial and diagenetic efects (Bland et al., 2014).

Seals for these reservoirs are estimated to primarily be interbedded shales, mudstones and marls. The accretionary wedge is dominated by turbidite deposits, so intraformational mudstones are estimated to have high seal potential, though the lack of 3D data and no well control limits these estimates. The base of the gas hydrate stability zone is also considered to be a seal for free gas accumulations within certain structures (Uruski & Bland 2011).

2.3.2 Gas Hydrates

There is a prominent bottom simulating relector (BSR) (Figure 2.6) that represents the base of the gas hydrate stability zone in both the Pegasus Basin, and the East Coast Basin.

Thrusts faults and their associated anticlines are the suspected luid pathways and trap respectively (Figure 2.6A and B). The BSR represents the phase change from frozen gas to free gas at the base of the stability zone, creating the large acoustic impedance that is captured in seismic data that typically mimics the sealoor geometry, though has the opposite polarity. The

13 BSR does not honor lithologic boundaries, which means that it often cross cuts the seismic

relectors from sedimentary strata (Bland et al., 2014). It is estimated that there is a large reserve of gas hydrates based on the widespread presence of the BSR, though well testing will conirm the extent of their presence. For now, the major interest in the BSR is the indication of hydrocarbon accumulations below the gas hydrates and the seal potential of the BSR on various

structures (Uruski & Bland 2011).

Figure 2.6 A. Schematic of fluid pathways within a thrust and anticline in the Pegasus Basin. Taken from Barnes et al., (2010). B. Section of seismic time profile PEG019 demonstrating the strong BSR in the area, and the underlying thrust responsible for the anticlinal trap, and the fluid migration pathways.

2.3.3 Seeps and Slicks

Both oil and gas seeps are found just onshore of the East Coast Basin (Uruski & Bland 2011) and have recently been identiied ofshore within the Pegasus Basin. The active luid seepage sites (Figure 2.7) have been associated with ridges occurring at the boundary between the accretionary wedge and the Cretaceous and Paleogene deforming backstop (Barnes et al., 2010; Bland et al., 2014). Cold luid seeps are associated with carbonate mounds (hard grounds),

14 which are primarily found on top of ridges propagating from thrust faults (Figure 2.7) (Bland et al., 2014). Pockmarks indicating the evacuation of gas and liquids on the ocean loor have also been identiied in large numbers along the Marlborough and Wairarapa margins of the Pegasus Basin (Bland et al., 2014). They occur in large numbers where the Neogene sediments onlap onto the Chatham Rise along the southern margin of the Pegasus Basin (Figure 2.7). A small number of slicks were also recognized during a recent satellite SAR study (Fugro NPA, 2009; Uruski & Bland 2011). Slicks are only found at the basin margins where deformation creates fault related migration pathways and dipping carrier beds with potential for migration (Uruski & Bland 2011).

..

Figure 2.7 (Bland et al., 2014). A map of seeps and fluid indicators across Pegasus Basin.

15 CHAPTER 3 DATA RESOURCES Data utilized in this project are primarily 2D seismic data. There is limited well control in the area, and sparse age dated bottom samples give only a limited amount of age control.

Earthquake hypocenters, GPS data, and paleomagnetic data are also considered in this study.

3.1 Seismic Data

The location of the seismic dataset is of the east coast of New Zealand. The PEG09 survey (Figure 3.1) was acquired by the New Zealand government to stimulate exploration interest in the area. The survey includes 3200km of high quality 2D seismic data that were acquired between November 2009 and March 2010. It contains 17 2D seismic lines, 5 that are parallel to the coastline, and 12 that are approximately perpendicular. The sample interval was 2ms and data were recorded for 12 seconds. The PEG09 survey was processed to pre-stack time migration (Uruski & Bland, 2011). Three of these time proiles, (PEG023, PEG017, and PEG009 (Figure 3.1)) have been depth converted and then restored to estimate shortening values in this study. When referring to a seismic line displayed in time the term proile is used. The depth converted equivalent of those seismic time proiles are termed seismic cross-sections.

3.2 Well Data

The closest wells, Titihaoa-1 and Tawatawa-1 (Figure 3.1), were drilled 50 to 100km to the north of the study area. Neither well provides a tie to pre-middle Miocene sequences, and are only tied loosely with the PEG09 survey through a variety of lines published by Uruski

& Bland 2011 (Figure 3.2). The wells were drilled on anticlines in the shallower continental shelf, and intersected relatively good reservoir quality in turbidite sandstones. Both wells had good gas shows within the reservoir type intervals. Three ODP wells have been drilled on the Hikurangi Plateau and adjacent sediment aprons north of the Chatham Islands, though there is no seismic tie from Pegasus Basin to those wells. The ODP wells provide ages for stratigraphy along the Chatham Rise, and the Hikurangi Plateau (Davy et al., 2008). Additionally, several

16 bottom samples (Figure 3.1) were acquired within the Pegasus Basin and are used in a variety of interpretations to control ages of major ridges in the area.

The highly dynamic tectonism and concurrent deposition that characterize the accretionary prism of the East Coast Basin make it very diicult to tie wells on the shelf to the Pegasus Basin 2D survey. Structures change over short distances, and are easily eroded or thinly covered by sediments. Sub-basins between anticlinal highs may have diferent stratigraphic architecture than others nearby. The seismic relectors are often no help for correlating horizons across faults as the cyclic deposition styles results in repetitive relection patterns (Uruski & Bland, 2011). This lack of well data requires careful consideration of tectonostratigraphic package geometries and model based interpretation for continuity.

Figure 3.1 Location of seismic data within Pegasus Basin. The PEG09 survey traces are in red. The closest wells to Pegasus Basin, Titihaoa-1 and Tawatawa-1 are in the northeast corner of the map. Bottom samples and ages are shown by the black dots. Other figures are shown in yellow.

17 Figure 3.2 Welltie to PEG09 survey for Titihaoa-1 from Uruski & Bland 2011. Uninterpreted seismic profile (bottom) is shown in time. Interpretation is shown on the top, also displayed in time. Note the well location on the shelf, and the various piggyback basins separated by thrust fault related ridges. Location of seismic profiles shown in Figure 3.1.

18 3.3 Earthquake Data

Earthquake data (Figure 3.3 and Figure 3.4) were acquired through online databases that release the hypocenter, magnitude, depth, dip, ofset, and various other variables related to each seismic event. A variety of diferent ilters were applied to acquire the most recent and most accurate data for this study including region, date of occurrence, magnitude, depth, and ofset. For example, while this research was being conducted, a 7.8 magnitude earthquake occurred on November 13, 2016, and produced several aftershocks of high magnitude that were captured and combined with depth cross sections to understand the structural variation along the margin, as well as understand which structures accommodate the current shortening across the margin. The hypocenters of these events were combined with the depth converted seismic cross sections to provide insight to the geometries of the faults within the Pegasus Basin and the East Coast Basin

(Chapter 4). When looking in map view, the earthquake hypocenters can be used to locate the top of the Paciic Plate as it dips steeply below the South Island. The concentration and spread of

Figure 3.3 Earthquake data from the past ten years with a magnitude of 3+. Additional data is available with filters for date of occurrence, depth, and magnitude on the EQC & GNS website.

19 hypocenters are closer together in the south, and spread out in the north, indicating a shallower dip below the North Island (Figure 3.3). The hypocenters can also be overlain onto velocity models such as those by Eberhart-Phillips et al., (2010) (Figure 3.4) to understand plate interactions far below the seismic cross sections in this study. Plate interactions below 15km are beyond the scope of this study, but the existence of seismic data at shallow depths is helpful for interpretation of shallow seismically active faults.

Figure 3.4 Taken from Eberhart-Phillips et al., (2010). Section location shown in Figure 2.2. Seismic events from 2001-2009 shown in depth. Seismicity is observed to follow the plate boundary interface of the Pacific Plate down into the Moho. Color scheme is the result of the Eberhart-Phillips 3D seismic velocity model for the New Zealand region. The white line repre- sents a resolution SF=3.5 (spread function).

20 3.4 Alternate Data

Datasets that measure motion through time can contribute signiicantly to understanding current plate boundaries. Paleomagnetic data can span millions of years, active fault data thousands, and GPS data tens. Understanding the contribution and limits of each dataset, and identifying consistencies across various ages of data helps restore long-term plate boundary zone deformation processes, provided that short term processes (elastic strain) are accounted for (Wallace et al., 2012). An example of compiled datasets working in harmony is shown in Figure 3.5, where displacement vectors created from a block rotation model show patterns that are consistent across all ages of datasets (Lamb et al., 2011).

Paleomagnetic measurements in Cretaceous and Cenozoic sedimentary and volcanic rocks record rotation about vertical axes of crustal blocks along the Hikurangi margin (Figure 3.6) (Lamb et al., 2011). The paleomagnetic data are utilized for longer term restorations rather than the short period of deformation recorded in the accretionary wedge of the PEG09 survey. However, incorporation and consideration of the ancient tectonic evolution of the margin is essential for restoration of the margin over shorter periods of time. Factors that cannot be

Figure 3.5 Taken from Lamb et al., (2011). Displacement of the Hikurangi margin over the past 4Ma to present day setting. Constrained by data from finite plate motion, major fault displace- ment measurements, and assuming rigid body rotation of blocks. A. Displacements relative to Australian Plate. B. Displacements relative to the Pacific Plate. Both demonstrate significant changes in motion vectors at a hinge near the northern South Island, at the apex of the Pegasus Basin, indicating the change in plate motion vectors, and in strain accommodation.

21 accounted for, such as non-rigid deformation result in inconsistencies in long term restorations, and cause various problems with reconciliation of plate boundaries, and realignment of basement

markers in restorations that extent tens of millions of years (Figure 3.6). Utilizing the variety of datasets, and consideration of their limits are essential for restoration of a plate margin.

Figure 3.6 Taken from Mortimer (2014). Reconstructions demonstrating the evolution of the oroclinal bend in the South Island and the basement markers that allow measurement of offset at their current setting. a) 105Ma with assumed colinear basement markers. b) 85Ma after rift- ing and initial bending and stretching of basement markers. c) 45Ma shows three options for the restoration of the Fiordland part of the Median Batholith. d) Present day alignment of basement markers.

22 CHAPTER 4 SEISMIC INTERPRETATION Interpretation of the seismic cross sections relied on both horizon placement and fault geometries. Where the data lacked in quality, alternate methods including previously published fault maps, and model-based fault geometries were incorporated into the interpretations. The basic worklow, justiication, and description of the horizons, faults, and models are discussed below.

4.1 Horizon Interpretation

A set of basic horizon picking parameters were created based on models of obliquely convergent margins, and other research and interpretations involving the PEG09 survey. Each of the horizons (Figure 4.1) used for structural interpretation, depth conversion, and restoration were picked in accordance with a study by Ghisetti et al., 2016 (Barnes and Mercier de Lépinay, 1997; Barnes et al., 2010; and Plaza-Faverola et al., 2012) that is focused approximately 100km northeast of the Pegasus Basin, but still within the accretionary wedge. This previous research

Figure 4.1 Horizons that were used to interpret seismic time profiles are listed with their inferred age, and velocity used for depth conversion (from Barnes et al., 2010 and Plaza-Faverola et al., 2012). Tectonostratigraphic packages in this section were used for both restoration and depth conversion. This seismic profile is located at the deformation front on seismic profile PEG023, the second northernmost line in the PEG09 dataset (Figure 3.1).

23 provides velocities for the stratigraphic units used for the time-depth conversion of the data used in this project. Because stratigraphy in this active margin is closely associated with tectonic events, these horizons are also used in restorations to track the shortening and deformation periods through the Pegasus Basin and East Coast Basin. The conidence and consistency in each of the horizons picks varies across the basin, but generally become far less reliant on seismic relector signatures and become more model driven towards the west.

Interpretations of the horizons in this study are signiicantly more model guided than the Ghisetti et al., (2016) study and other restorations further north of the study area. The complexity of the structures within the Pegasus Basin washes out the seismic relectors needed to positively correlate horizons into the continental shelf sediments. Therefore, model based interpretation combined with restoration iterations and required structural geometries are the primary controls for the horizons in the areas where the seismic relectors become discontinuous and non-unique.

4.1.1 R8 (Basement)

The basement horizon represents the deepest relatively consistent seismic relector interpreted in the seismic cross sections (Figure 4.1) and is estimated to represent the top of the oceanic volcanics of the Hikurangi Plateau (Figure 2.1). This horizon placement is often estimated because the relector it is picked on is discontinuous when there are obvious deformation structures above which obscure its continuity. The Paciic Plate is known to dip at around 3° until it is well under the North Island of New Zealand, after which if plunges into a steeply subducting slab (Figure 3.3) (Davey et al., 1986a; Henrys et al., 2006; Barker et al., 2009; Barnes et al., 2010). This previous research allows the basement horizon to dip at a consistent angle (3°) where the seismic relectors are no longer unique.

4.1.2 R7

This horizon is estimated to be about 30Ma (+/- 5Ma) (Barnes et al., 2010; Ghisetti et al., 2016) and is observed continuously along the Paciic Plate. The seismic relector that deines the R7 horizon then disappears to the west as the structural complexity of the East Coast Basin

24 obscures the continuity of the relector. It is thought that in the southern part of the East Coast Basin, and into the Pegasus Basin that this horizon is the major décollement surface that deines the interface between the Paciic Plate and the Australian Plate as the subduction deepens to the west.

Between horizon R7, and R8, there are two signiicant stratigraphic packages that are not necessarily present in all of the seismic proiles. Sequence Y, a late Cretaceous early Paleogene condensed sequence published by Davy et al., (2008) is only certainly present in the northern PEG09 survey seismic lines. It is a strongly relective unit that is composed of a condensed sequence of chalks and shales. (Ghisetti et al., 2016). The remainder of the stratigraphy between R7 and R8 belongs to the sequence MES, a late Cretaceous sequence, equivalent with Davy et al. (2008)’s MES (Plaza-Faverola et al., 2012). Both of these sequences have the possibility of being sandstone rich, but the lack of well control leaves their lithology unknown (Ghisetti et al., 2016). Horizon R7 and the underlying units are all contained within the Paciic Plate, and are being subducted underneath the North Island.

4.1.3 R6

The unit between R7 and R6 is very weakly relective. It is inferred to be a unit comprised of nanofossils chalks interbedded with tephras and clays. (Ghisetti et al., 2016, Barnes et al., 2010). This horizon is used in the northern cross section of the study, but becomes un- important to the south. This horizon is not utilized in restorations.

4.1.4 R5C

Barnes et al., (2010) used this horizon as a control for the lower part of the accretionary wedge geometries. In this study, it is used to control the spatial relationship and the timing of various events between the R5B horizon and the R5 horizon as the wedge thickens to the southwest. It is not represented in Figure 4.1, but is used for restoration purposes. It is also incorporated in the Ghisetti et al., (2016) interpretations as marker R5a.

25 4.1.5 R5B This horizon marks a regional erosional unconformity that is associated with the western tilting of the Hikurangi Plateau, overlain by trench turbidites and slope basin sediments (Figure 4.1) (Lewis and Pettinga, 1993, Barnes and Mercier de Lépinay 1997; Lewis et al., 1998; Barnes et al., 2010). The onlaps onto this horizon are obvious, especially where the wedge thickens to the north. Restorations past this horizon are purely speculative and are not conidently age constrained.

4.1.6 R5 The accretionary wedge has a variety of onlap surfaces, indicating tilting of the margin, and potential time for non-activity or even erosion. This horizon is interpreted to be a paleo- sealoor from the early Pleistocene (Plaza-Faverola et al., 2012).

4.1.7 R3 and R4

Horizons R3 and R4 (Figure 4.1) are easily correlated within the accretionary wedge, though the continuity and certainty become increasingly unclear to the east through the deformation. Once the seismic quality and the seismic signature are lost, these horizons are primarily controlled by model based wedge and thrust belt geometries and restoration iterations. These horizons are important to the restorations because they are placed on major unconformities that can be consistently picked based on onlap surfaces through the wedge, and in the isolated mini-basins.

4.1.8 R0

This horizon, shown in Figure 4.1, represents the sealoor as it is today. The bathymetry of the sealoor is extremely inconsistent and unpredictable in 2D cross sections. However, bathymetry maps indicate a series of high elevation troughs and ridges that are approximately parallel to the coast. This change in bathymetry means that the water depth above the seismic data varies signiicantly. For the time-depth conversion, the water column needs to have its own initial velocity, so the depth of the sealoor makes a big diference to the results below as

26 the water depth either shallows or deepens dramatically. The sediment below the R0 horizon can belong to a variety of ages, depending on the geometries of the structures observed so occasionally it acts as the erosional unconformity marker for older sediment packages.

4.1.9 Torlesse Supergroup

This horizon is not considered in the restorations, though it becomes a signiicant part of the interpretation of the onlap geometries and isopachs of the sediment packages over the

Chatham Rise to the south of the study area. The Torlesse Supergroup (Figure 2.4) overlies the basaltic oceanic crust of the Hikurangi Plateau (horizon R8) that is the ultimate oceanic basement of the Paciic Plate. This horizon is not shown in Figure 4.1, but can be seen in Figure 4.5, and Figure 4.6. This group represents the inactive Mesozoic accretionary wedge from the ancient Gondwana subduction zone. Occasional fault planes can be seen in the seismic cross sections, though the faults are not presently active, and the spacing in the seismic data limits their regional correlation.

4.2 Time Depth Conversion

The time-depth conversion has a signiicant impact on the results of this study. The geometry of the faults and stratigraphic horizons in cross section are dependent on the accuracy

of the time-depth conversion. For this study, the work by Ghisetti et al., (2016) and Barnes et al., (2010) was used to determine velocity intervals for the various horizons and stratigraphic packages shown in Figure 4.1. Within Midland Valley's Move (2016) software, there are a variety of diferent methods to convert seismic time proiles into depth correct seismic cross sections. The method selected for this project’s time depth conversion required several V0 values associated with the various stratigraphic packages in the area. These horizons are then interpreted and converted within the software. Because the depth conversion is dependent on

the interpretation provided by the user in time, there are an unlimited number of non-unique solutions for the depth conversion, and it is extremely sensitive to the initial interpretation of

the seismic time proile, and the geometries that are selected prior to the conversion. The depth

27 conversions in this study place the décollement surface at an acceptable depth, and faults retain

acceptable geometries in the converted depth proiles. The depth conversion for this study is suicient for interpretation and restoration purposes.

4.3 Fault Interpretation This margin is largely controlled by compressional forces, so thrust faulting geometries are expected, as well as some lower structures as strike-slip strain accommodation increases to the south. Interpretation of fault geometries vary within both time and depth. The primary control on placement and geometries of the faults was reliant on breaks in the stratigraphic packages, or steeply dipping relectors within the seismic cross sections. The second control of faulting was determined by the bathymetry of the ocean loor. Some faults are presently active, so an increase in topography of the ocean loor without sediment ill keeping up with deformation, indicates an active fault somewhere below. Recently published maps of the area were helpful in determining

Figure 4.2 Cook Strait faults (black) shown with annual movement in millimeters per year in red for currently active faults. Bathymetry is shown with 500m contours. The large red arrow is the Pacific-Australia relative plate motion vector. PEG09 survey lines PEG009 and PEG017 are shown for reference to interpretations. Taken from Wallace et al (2012).

28 the extent and magnitude of certain faults (Figure 4.2), and were conirmed by model guided interpretations discussed in section 4.4.

The faults that were used for the restoration were the most prominent and active structures along the margin and were picked with the highest conidence. There are several other faults and structures within each time proile that were depth converted, however for restoration purposes, they were not included in the shortening assessment. Fault interpretation was adjusted as restorations proved certain geometries to be inconsistent, or geologically impossible.

4.4 Model Based Interpretation

There are many variables that have an inluence on the geometries of structures that are accommodated within the afected accretionary wedge at obliquely convergent margins. Sandbox models examine the efects of convergence angle, backstop geometry, and backstop height on the accretionary wedge and illustrate the major diferences these variables have on the structures present (Figure 4.3) (Gravleau et al., 2012). For this research, the seismic data become scattered and inconsistent as the structural complexity increases from the evenly distributed turbidites of the Pegasus Basin to the fold and thrust belt of the East Coast Basin. Few structures or horizons can be conidently placed, so sandbox models and previously published area fault maps (Figure 4.2) are used to help determine geometries of faults as the obliquity of the margin increases. To the south there is less margin normal accommodation of shortening, and more margin parallel displacement so the sandbox models (Figure 4.3) that demonstrate higher angles of obliquity become more important to structural interpretations where the seismic resolution becomes impossible to decipher.

The Coulomb wedge dynamics with oblique slip have also been utilized in understanding the styles of deformation along this margin (Fagereng, 2011). The décollement angle, the wedge material strength, and the subduction megathrust strength are sensitive variables that control the geometry of the wedge and its associated deformation geometries along the extent of the Hikurangi margin. Strong interseismic locking is inferred to occur in the southern margin while

29 the northern part freely subducts and deforms. A smaller Coulomb wedge taper angle to the south is observed as expected from the along strike variations in the accretionary prism material and

subduction megathrust strength (Fagereng, 2011). These wedge dynamics inluence the various stress regimes and therefore have associated fault geometries both laterally and vertically within

Figure 4.3 Examples of obliquity influence on the Coulomb wedge, taken from Graveleau et al (2012). Models that illustrate the main components and structural features of oblique convergent margins. A. (Biagi, 1988) Map view of fault systems displaying two separate characteristics, one of frontal thrusts, and internal strike-slip faults. B. (Biagi, 1988) 3D view of internal fault structures and their crescent shaped contacts bounded by strike slip faults and a major thrust. C. (Calassou et al., 1993) Cross section view of internal fault geometry when comparing direct convergent wedges to obliquely convergent wedges (angle of obliquity 45-55°). There are fewer thrusts with steeper angles in the latter. D. (Malavielle et al., 1992) High obliquity faults display two geometries, major sub-vertical faults along the backstop, and minor faults between major thrusts. E. (Martinez et al 2002) backstop height and its effect on wrench zone location, thick- ness, and geometry. Compare to Figure 4.2 for similarities in map view fault geometries.

30 the wedge. Upper plate dynamics of strain accommodation combined with deeper structures along the plate interface use these models as a guide when placing seismic horizons and fault cutofs at areas below seismic resolution.

Modelled strike-slip faults observed in Figure 4.3B are observed as subvertical and are connected with deeper thrusts that detach along the plate interface. They can appear as normal faults in cross section, while the main sense of displacement is lateral. 2D modelling limits the ability to restore the shortening values of strike-slip dominant faults. The error in shortening values that are obtained from margin perpendicular restorations under-represent the amount of shortening as obliquity to the margin increases, and ofset is accommodated laterally rather than by compression. There is also the possibility that these strike-slip faults are not strictly limited to shortening accommodation on the overlying Australian Plate, and have some efect on the underlying Paciic Plate, though this is outside the scope of this study. Earthquake data (Figure 3.3) concentrated within the northern Marlborough Fault Array suggest that the plate boundary steepens until it is almost vertical underneath the western half of the Cook Strait and the South Island (Figure 3.4). The deeper seismic events that coincide with the top of the subducting place, and slow slip events (SSEs) increasing in frequency from the Wairarapa region to the Marlborough region (Wallace et al., 2007), while those that are not associated with the plate boundary are far less frequent, and are much lower in magnitude. In the area immediately south west of seismic survey PEG09, these seismic events are deeper and further east than the seismic data extent. Some shallow hypocenters of the earthquakes have been shown on seismic cross sections, and help with the placement of the plate boundary, and strike-slip faults in the southern Pegasus Basin. For modelling and estimates of shortening in this study the geometry and ofset of the strike-slip faults shown in the seismic cross sections detach on the subduction plate boundary at around 12-15km depth. The combination of earthquake data (Figure 3.3), geometry from obliquely convergent margins (Figure 4.3), and previously published fault maps (Figure 4.2) help place fault geometries within the seismic cross sections, though the timing of the activation of each fault is more subjective to horizon placement.

31

Figure 4.4 Interpretation of seismic cross section PEG023. The top figure shows the depth converted, uninterpreted seismic line. The lower figure shows the final interpretation of the cross section. Primary controls of this interpretation come from the clear reflectors in the undeformed section of the wedge in the eastern part of the wedge. The dramatic changes in bathymetry control placement of major thrust faults, and their associated anticlines in the hanging wall. Interpretation of strike-slip faults versus oblique thrusts are identified from the previously published fault maps. Geometries of the thrusts at depth are guided by sandbox models and previous interpreta- tions of the seismic data. Location of seismic line can be found in Figure 3.1.

32 Figure 4.5 Interpretation of seismic cross section PEG017. The top figure shows the uninterpreted, depth converted seismic line. The lower section shows the final interpretation of the cross section. Interpretation of the faults comes primarily from discontinuity in the seismic reflectors, and the deformation of the seafloor. Previously published fault maps show thrusts that dip towards the strike slip zone on either side of a major strike-slip wrench zone. Sandbox models show similar geometries at depth of this wrench zone. Earth- quake hypocenters are projected from 10km out onto the depth profile, and indicate the décollement surface at the plate interface. The location of seismic line can be found in Figure 3.1.

33 Figure 4.6 Interpretation of seismic cross section PEG009. The top figure shows the uninterpreted depth converted section of the seismic cross section. The lower figure shows the final interpretation of the cross section. Previous fault maps and research in the area of this line indicate steeply dipping thrust faults that connect onto thrusts at depth, that then detach onto the plate interface. Seismic reflectors, tectonostratigraphic packages and restorations of observed deformation controlled horizon placement. This line is in the southwestern Pegasus Basin where seismicity is far more frequent than in other cross section. Earthquake hypocenters are projected from 10km out onto the depth profile. High magnitude earthquake hypercenters align with placement of faults in the westernmost part of the cross section. Location of seismic line can be found in Figure 3.1.

34 4.5 Alternative Interpretations This study creates structurally consistent cross sections and restorations through the Pegasus Basin. Careful attention was made in interpretations to ensure slip values decreased up section along the planes of the thrust faults, and to ensure that faults were geometrically possible, and restorable.

These seismic data (PEG09 survey) have been used by other researchers, though the interpretations often lack structural integrity, and are geomechanically impossible. The interpretations are often in time, though a "depth-correct" interpretation is presented. This means that correction for velocity pull ups underneath fault planes, and changes in velocity due to rock type and thickness are not accounted for in the interpretations. Figure 4.7 shows a mixture of these interpretations for the seismic line PEG017 of the PEG09 survey, and should be compared to the interpretation of the same line from this study in Figure 4.5. The uninterpreted seismic time proile is shown in time in Figure 4.7A, and in depth in Figure 4.5A. In Figure 4.7B, there are a variety of inconsistencies in the amount of slip between horizons on the major thrusts verging to the east. There are also faults that are shown which have no ofset of the horizons. The décollement surface is interpreted on the time proile where it is expected to be in a depth cross section, though the seismic data are in time. The décollement surface in depth is dipping at around 3° below the Australian Plate, though in a time proile it should move up as sediment on top of it increases due to velocity changes in the thicker part of the wedge. Figure 4.7C shows limited structural analysis, and has focused more on the sediment packages within the wedge. There are indications that faults exist in the area, but suggest that the wedge sediments merely drape over these faults, and are not ofset by the faults at all. Figure 4.7D shows similar problems to Figure 4.7B, in that the faults show increasing slip values up section along the thrust faults. There are also several faults that do not show any ofset, and faults that do not appear to reach a common décollement surface. In comparison, the structural interpretation in Figure 4.5 is structurally possible, relects geometries expected on an obliquely convergent margin, and restores geomechanically.

35 Figure 4.7 Alternate interpretations of seismic line PEG017 from previous work.

36 CHAPTER 5 RESTORATIONS

Restorations were performed in Midland Valley's Move (2016) software. This program allows restoration of both 2D and 3D data, however, this study used only the 2D restoration methodology along seismic cross sections from the PEG09 survey. Restorations allow the measurement of shortening acquired along a particular cross section, while ensuring that the interpretation is structurally balanced and geologically viable. These restorations require careful consideration of fault geometries, fault timing, depositional timing, horizon ofset, erosional surfaces, lexural subsidence, changes in provenance, downwarping due to subduction, and many more conditions that afect the wedge geometry and tectonostratigraphic packages through time. For this particular dataset, fault geometries are the primary control of the interpretation, followed by the tectonostratigraphic horizons that are ofset by the faults.

5.1 Worklow The worklow for restoration of these cross sections is repetitive and is done in several iterations. Working backwards through geologic time, the top layer is irst restored, followed by the stratigraphy in reverse order down section. For each stratigraphic layer, multiple steps are required. The irst step is to match the hanging wall to the footwall on faults that show ofset for the topmost horizon. After this step, the horizons are often no longer straight. In this case, the stratigraphy needs to be unfolded following the restored fault ofset to relect its original depositional geometry. In most restorations, decompaction of sediments is the third step in the process of restoration of a horizon. For this study, decompaction of layers was not included, because horizon placement is so subjective to begin with, and lithologies are not well known within the basin. The results from adding decompaction would have a minimal impact on the shortening values, and are not incorporated into the shortening measurements. Once the stratigraphy has been both restored to a reasonable depositional geometry, the topmost horizon can be removed, and the process starts over on the next horizon. The complexity of growth stratigraphy that is simultaneous with faulting means that often strata are only deposited in a

37 piggyback basin rather than being deposited and ofset by faults. This means that the restoration of the horizons where there is no fault inluence depends on depositional geometries. It also means that multiple faults are restored for each horizon, as most of them have some degree of ofset for each depositional event.

Models that demonstrate fault geometries for convergent margins typically show forward breaking faults, moving deformation seaward within the wedge and increasing the wedge taper angle as deformation continues. For this wedge to maintain its taper angle, the wedge thickens and stretches laterally. Thickening of the wedge is attributed to internal deformation simultaneous with new material accumulating at the deformation front. For the wedge to enlarge both in width and in height, a combination of forward breaking thrusts must accommodate strain at the deformation front, while out of sequence thrusting occurs within the landward part of the wedge and reactivation of old thrusts build the wedge thickness (Davis et al., 1983; Morley, 1988; Barnes and Mercier 1997; Platt, 1998 [1990]). These out of sequence thrusts are observed in the seismic cross sections and restorations, and allow the deformation timing to be controlled by horizon placement, rather than by restricting fault activation to forward breaking thrusts only.

Sedimentation of this wedge is recent and occurred extremely quickly. It is estimated that over 6,000m of sediment has accumulated within the last 2Ma in the Pegasus Basin, equal to around 3mm/year of sediment deposition (Uruski & Bland, 2011). Deformation within this wedge has had to keep up and surpass the rapid sedimentation for the such large changes in bathymetry (1km+) to exist within the wedge. Frontal accretion within the wedge has occurred rapidly, and the wedge has widened by the seaward advance of the principal deformation front. Structures that lie in front of the underlying Cretaceous and Paleogene foundation are expected to have formed within the last 2Ma (Barnes and Mercier de Lépinay 1997). This short time interval of deposition and deformation is also supported by structural restorations focused on the strata at the deformation front by Ghisetti et al., (2016), and is honored by this study as well.

38 Restrictions in the fault timing, and fault geometry give a basic framework for the restoration, and force certain horizons to be placed accordingly. Due to the lack of well control,

there are endless non-unique solutions for the horizons placement, so previous interpretations of the PEG09 survey (Uruski & Bland, 2011; Bland et al., 2015; Kroeger et al., 2015) were used to help guide initial horizon placement. The ages and horizon signature in Figure 4.1 are continuous throughout the undeformed region in the eastern part of the wedge, and are the main source of control for the wedge deposition intervals. Initial interpretation of the 2D seismic

lines was performed on the seismic time proiles within Schlumberger's Petrel (2015), and then transferred into Midland Valley's Move (2016) for depth conversion and restoration. Adjustments and modiications to the geometries of the faults and placement of the horizons were made as restoration attempts proved previous horizon and fault placement to be geologically impossible.

Areas where seismic relectors are washed out due to structural complexity are simpliied, and guided by model based interpretations of fault geometries within oblique convergent margins, as discussed in Chapter 4.4.

5.2 Kinematics in Move

Within Midland Valley's Move (2016), there are six diferent algorithms designed to predict fault movement; simple shear, fault parallel low, fault bend fold, fault propagation, trishear, and detachment fold. The appropriate methods for restoration rather than forward modelling include simple shear, fault parallel low, and trishear. Ghisetti et al., (2016) conirmed that the trishear algorithm is the most suitable for the restoration of the kinematics within the fold and thrust belt of the east coast of New Zealand. It is the most suitable kinematic model for interpreting the geometry of layers folded above the fault tips, and for restoring fault separation that decreases up-section (discussed in detail in Chapter 5.2.1) (Ghisetti et al., 2016). Therefore, a combination of trishear and fault parallel low algorithms were used to restore the hanging wall to the footwall of each fault in these seismic cross sections. Fault parallel low was primarily used when there weren't signiicant growth strata above the fault tips, and the hanging wall to footwall needed to be restored.

39 5.2.1 Trishear The trishear algorithm, developed in part from collaboration with Colorado State

University (Erslev, 1991), is designed to model geological structures by deforming beds within a triangular zone of shear, emanating from the tip of a propagating fault. The algorithm deforms beds in a single or series of nested triangular zones of shear, where the magnitude of slip is varied from a user-deined value at the top of the zone to zero at the base of the zone. The direction of slip is varied from parallel to fault-dip at the top of the zone to parallel to the angle of the base of the zone at the base of the zone (Hardy and Ford, 1997). Most of the more recently active faults were restored using the trishear algorithm to account for the sedimentation, and to restore the triangular shear zone caused by these thrust faults. The triangular zone of shear is best observed in the blind thrusts at the deformation front within the accretionary wedge (Figure 4.4 and Figure 4.5).

Other kinematic models can be used to mimic these fold accommodating geometries at the tips of the propagating thrusts, however, the structures produced by each of these modules are end-members. The trishear module in Midland Valley's Move (2016) allows the user to change the P (fault propagation) to S (slip) ratio, as well as modifying the angular shear angle at the apex of the fault. The lexibility and control that the trishear algorithm gives the user allows a variety of diferent geometries associated with fault propagation folding to be restored (Ghisetti et al., 2016). In a forward model of the syndeformation deposition of sediments, the trishear model most accurately reproduced the geometry of the fault propagation fold in the hanging wall

(Ghisetti et al., 2016). It also reproduced the triangular zone of growth stratigraphy in front of the fault tip (Ghisetti et al., 2016). Outside of the triangular zone of deformation, the user also has the option for the remainder of movement to act in fault parallel low, or in simple shear. Due to the proximity of the Ghisetti et al., (2016) study, the trishear module has been used for the most seaward fault propagation folds and faults, with fault parallel movement outside the trishear zone of deformation.

40 5.2.2 Fault Parallel Flow The fault parallel low algorithm is based on the continuing work of Egan et al., (1997). and is based on the particulate laminar low over a fault ramp. It is best suited for modelling hanging wall movement on faults from fold and thrust belts. The fault parallel low module is also the module best suited for forward modelling and restoration of deformation in

compressional systems. The fault parallel low algorithm is best suited for systems that have angles less than 40°, and with low fault bend angles, which are common in fold and thrust belts. Faults in the Hikurangi accretionary wedge often ramp up past 40° as they connect to steeper protothrusts. This algorithm is used in restorations of thrust faults when the faults are low

angle (<40°), and do not have signiicant triangular stratigraphic package growth marking their initiation and growth.

5.2.3 Flexural Slip Unfolding

The layers in a restoration, once ofset has been restored, often need to be unfolded. The lexural slip algorithm within Midland Valley's Move (2016) allows unfolding of each layer while preserving horizon line length or package area. The algorithm works by rotating the limbs of a fold to a datum, or a template geometry provided by the user. Layer parallel shear is applied to the rotated fold limbs to remove the efects of lexural slip component of folding. This algorithm was used because the alternate solution, the simple shear module, does not preserve line length. Line length preservation is important for this study, as the placement of horizons and their relationship to the fault are the major source of control of the shortening values obtained by the restorations of the cross sections. Unfolding horizons was only used when deformation in the horizons was a major detriment for further deconstruction and when folding was clearly a result of fault growth, and was not resolved by the fault algorithm used. The faults that utilized the unfolding are primarily observed in folds and faults closest to the deformation front. Minor irregularities that were unassociated with folding within the horizons were not unfolded.

41

Figure 5.1 Restoration of seismic cross section PEG023. Internal deformation of the wedge oc- curs until the critical taper is reached, and faults then step foward into the wedge.

42

Figure 5.2 Restoration of seismic cross section PEG017. Faults deform internally until critical taper is obtained, then thrusts step forward into the wedge. There is a significant amount of dis- placement on the strike-slip faults that cannot be accounted for in 2 dimensional restorations.

43 Figure 5.3 Restoration of seismic cross section PEG009. Faults tend to step forward into the wedge through time, though out of sequence faults are required to maintain critical taper. Faults initiate as thrusts, but accommodate more strike-slip as the wedge grows.

44 5.3 Results For each of the steps in the restoration, line lengths were recorded at the various time intervals. In order to calculate the strain and shortening accommodation at each step, line lengths

are compared. Strain is deined as the change in length (Lf - Lo), divided by the original length

(Lo at time Tmax) (Figure 5.4). Therefore, the shortening value is dependent on the time interval of interest and the associated change in length. For these cross sections, shortening values can be calculated from an in initial point in time (Tmax), all the way through the present, with the shortening values increasing in value as time from initial deformation increases (Figure 5.4B).

In this case, maximum deformation will be in the most recent cross section (Tpresent) because the change in length (Lf - Lo) is the largest. Tmax represents the maximum restoration of each of the cross sections, but it is important to note that there may be shortening prior to Tmax however it is not represented in this study.

For comparison of shortening values between diferent studies, the length at the initiation of deformation (Lo) at time (Tmax) needs to alter according to the study in question. For example, when comparing results to Ghisetti et al., (2016), Lo needs to relect the length of each cross section at 2Ma (Tmax), while Lf remains at the present day length (See Figure 5.4A for visual aid, and Appendix B for calculated values). This changes both the value of the change in length, and especially changes the denominator, which has the largest impact on the shortening value. For calculating the individual shortening values between each time step, the method shown in

Figure 5.4C can be used, altering Lo and Lf accordingly. The values from each of these various combinations of Lo and Lf are represented in Appendix B, and are displayed in both kilometers of shortening, and percent shortening for all three proiles. These calculated results can then be compared to regional work, the plate motion vectors, and GPS monitored fault maps of the area. Comparison of these results is discussed in the following chapter.

45 Figure 5.4 Different methods to calculate strain based on placement of Lo and Lf. For compari- son to different ages, method A is used to calculate strain rates. For calculating total strain for each step of the reconstruction, method B is used. When determining the amount of strain be- tween individual time steps, method C is used. The method used to determine strain rates de- pends on what time period the strain rates are being compared to.

46 CHAPTER 6 DISCUSSION

Nicol et al., (2007) state that shortening values obtained from seismic restorations commonly have an error percentage of 20-50%, especially when performed on an obliquely convergent margin. Despite this assumption, the shortening values acquired from the restorations of seismic cross sections PEG023, PEG017 and PEG009 have been acquired up to 5Ma. The shortening values are compared to other studies, plate motion vectors, and fault movement maps to understand the distribution of shortening along the margin.

6.1 Comparison of Shortening Values To validate these values and legitimize the restorations and interpretations, the shortening values have been compared to two previous studies of the margin; Ghisetti et al., (2016) and Nicol et al., (2007). These provide comparisons of shortening accommodation within Pegasus Basin at 2Ma and at 5Ma accordingly. The current plate motion vectors are considered in the shortening accommodation comparisons as well.

6.1.1 Comparison at 2Ma

Ghisetti et al., (2016) utilized structural restorations (Figure 6.1) to the northeast of the PEG09 survey. The extent of their restorations is limited to the deformation within the recent wedge sediments 2Ma or younger, and is within the Neogene accretionary wedge sediments.

There is limited inluence of the Cretaceous/Paleogene backstop, and it is left uninterpreted in the eastern part of their cross sections where data become indistinguishable, though shortening values should still be acceptably compared to those of the PEG09 survey. Shortening values from the restoration of all three PEG09 cross sections at 2Ma are acceptably similar to those acquired from Ghisetti et al., (2016). There is a trend in decreasing shortening to the southwest through the values obtained from this study (Figure 6.1), but it is important to note the change in present plate motion vectors from north to south. In the north, the vector is almost perpendicular to the planes that were restored by the Ghisetti et al., (2016) study while the PEG09 survey is further

47 southwest where the motion vector is far more plate parallel. Shortening amounts decrease as expected to the south as the plate normal vector decreases, and more displacement is acquired on the plate parallel motion vector. High rates of shortening are expected in the north, while to the south, subduction is more recent, and shortening has been accommodated as rotation and plate parallel translation of displacement is more likely rather than compressional shortening on the subduction thrust. The similarity in values and decreasing trend from the Ghisetti et al., (2016) study compliment those collected at 2Ma for this study's restorations.

Figure 6.1 Shortening values obtained from this study in comparison to the plate motion vectors, and the Ghisetti et al (2016) study since 2Ma.

6.1.2 Comparison at 5Ma

The seismic cross section PEG023 coincides with Nicol et al. (2007)'s transect E (Figure 6.2). However, the PEG09 survey was shot at a slight angle to these transects. To compare results from the PEG09 survey to transect E, alignment of the shortening value from the restored cross section to the orientation of the Nicol et al. (2007) is done through trigonometry (Figure 6.3). After alignment to transect E and the plate normal shortening vector, the total amount of shortening normal to the margin from PEG023's restoration is ~34km. Before a comparison of these shortening values can be made, there must be clariication of what the values of each study

48 mean. The values from the Nicol et al. (2007) study include measurements acquired from the upper crust deformation based on seismic relection lines, gravity proiles, fault-displacement rates, and geologic mapping, and all of which exclude strain accommodated on the subduction thrust, and therefore only represent 10% of total shortening. A signiicant amount of shortening (~85%) is accommodated by detaching thrusts along the plate boundary within the subduction thrust system, while only around 10% of plate convergence was translated into the upper plate (Nicol et al., 2007) (Figure 6.4). Shortening values from this study's restorations represent the 85% shortening on the subduction thrust, while the values of shortening from the Nicol et al. (2007) study are related to the 10% of the shortening translated into the upper crust (Figure 6.4).

Figure 6.2 Shortening values obtained from this study in comparison to the plate motion vectors, and rates from Nicol et al., (2007). Shortening values across the North Island are summed across profiles A-E. Final shortening estimates are shown on the right of the transects. Estimates from the 3.5+ Ma cross section in this study were used to compare with the Nicol et al., (2007) study.

49 If the assumption that 85% of plate normal shortening (-34km) is being accommodated on the subduction thrust (shortening value obtained from PEG023), then the total amount of plate normal shortening at PEG023's location is -40km [-34/0.85]. The value of -3(±3)km of shortening from Nicol et al. (2007)'s observations in the most seaward transect accounts for 0-15% of the total shortening amount (-40km) in these restorations. This value its with the assumption that ~10% of shortening is translated into the upper crust. It also leaves ~7.5% shortening to be attributed to alternate shortening processes including layer parallel slip, minor unresolved faults not incorporated for restoration, ofset in the protothrusts in the accretionary wedge, and slight misinterpretation of horizon geometries within the structural restorations.

Figure 6.3 Shortening values parallel and perpendicular to the plate motion vector at 5Ma using angles of the plate motion vector within Pegasus Basin.

When compared to the easternmost portion of transect E (-3±3km), the shortening values align with calculated shortening values. However, when compared to the entirety of transect E, the total shortening along the transect is at least -120km [-12 ±3km/10%]. In order to understand why the shortening values do not account for the shortening across the entirety of transect E, the extent of the dataset must be considered. The subduction thrust extends far beneath the North

Island (Figure 6.4). The cross section of PEG023, only represents a small portion of the

50 subduction thrust that is accommodating shortening. The distribution of shortening along the subduction thrust is not equal across transect E. In the easternmost section of transect E, (Figure 6.2 and Figure 6.4) -3±3km of shortening is measured over the last 5Ma. This represents ~25% of the upper plate shortening measured across transect E (-12 ±3km). The cross section of PEG023 covers approximately the same area as the -3±3km section of transect E. If the assumption is made that PEG023 represents 25% of the subduction thrust shortening because -3± 3km represents 25% of the shallow crust shortening, then the calculated shortening for PEG023 is -25.5km (120km*85%*25%). -25.5km of shortening is only a minimum estimate of strain in the easternmost region of transect E. The value attained from the restorations, -34km, is slightly higher than the calculated values, though still falls within the reasonable range of uncertainty for the transect.

Figure 6.4 Schematic cross section of transect E (Nicol et al., 2007), and the location of the PEG023 seismic cross section, with their associated percentages of the total shortening from the plate motion vector.

Shortening values at 2Ma and up to the 3.5+ Ma step in restorations are acceptable when compared to the work of Ghisetti et al. (2016), and Nicol et al. (2007). Alternate studies that

51 are not graphically compared include Barnes and Mercier de Lépinay (1997), which calculated 12.5% shortening over 2Ma on diferent seismic cross sections in the same area as Ghisetti et al., (2016). In that same region, Nicol and Beavan (2003) calculated 15-33km of shortening over 5Ma (Ghisetti et al., 2016). There is little constraint on shortening rates past the 3.5Ma point,

though faults that still contained ofset during restoration at that time were restored for the Tmax shortening values. There is no exact timing correlation for horizons interpreted in the seismic cross sections. Accurate timing of each of these seismic horizons selected for restoration is required to absolutely quantify the amount of shortening acquired by these structures through time.

6.1.3 Plate Motion Vector Comparisons While shortening values attained from restorations of the PEG09 survey match previous shortening studies, the shortening values within the seismic cross sections do not match the values calculated from the current plate motion vectors. For the comparison to the Ghisetti et

al., (2016) study at 2Ma, the maximum value acquired for shortening was along cross section PEG023, and is approximately -19km (19.5% shortening). While the trend of these shortening values match the trend from the Ghisetti et al., (2016) study at 2Ma, the current plate motion vector for margin normal convergence in the PEG09 survey area is ~30-42mm/year. Over 2Ma, there should be a minimum of ~60-80km of shortening. -19km of shortening from cross section PEG023 at 2Ma) accounts for less than 30% of the total expected from the plate motion vector. For the values attained for the 5Ma comparison, there should be 150+ km (30mm/yr * 5Ma) of shortening, though restorations only estimate a minimum of ~34km of shortening (PEG023 at 3.5+Ma) that is margin normal (Figure 6.3). This accounts for only 22% of the total shortening per the plate motion vector.

The extent of the dataset again must be considered in order to accurately compare the calculated value of shortening, to that of the restorations. The subduction thrust extends far

beyond the cross sections that are restored (Figure 6.4). Based on the assumption made in the previous section, the PEG023 cross section represents approximately 25% of the shortening

52 accommodated on the subduction thrust. At 5Ma, the plate motion vector expects a total of -150km of shortening. Of that -150km, 85% is accommodated along the subduction thrust

(-150*85% = -127.5). The PEG023 cross section accounts for 25% of the subduction thrust shortening, which is -31.8km of shortening (-127.5*25% = -31.8). The expected value from the restorations is -34km of shortening, which is extremely close to the -31.8km of shortening expected from the plate motion vector.

6.1.4 Active Fault Motion Comparison Another comparison of shortening value accommodation on various structures can be made in the southern Pegasus Basin, near the Cook Strait. Measurement of active faults in the northern South Island by Wallace et al. (2012) (Figure 4.2) accounts for ~31-36mm/year of the Paciic Australia Plate motion budget of 42mm/yr. Wallace et al. (2012) suggest that "the remainder of this budget is likely to be taken up by a combination of clockwise rotation of northeast Marlborough (described in section 6.2) as well as the Wanganui and Manawatu blocks (2-3mm/yr) (Figure 6.5) and deformation ofshore of the northeastern South Island (5-6mm/yr)" (Wallace et al., 2012). The most highly active faults shown in Figure 4.2 are dominantly strike- slip faults, meaning that the accommodation of the plate motion vector in the southern Pegasus

Basin, is primarily perpendicular to the restorations in this study. The "deformation ofshore" of the South Island of 5-6mm/yr its the shortening values calculated along PEG009 and PEG017 in this study, and accommodate around 2.5-4mm/yr of convergence accordingly (-5 and -8km over the last 2Ma).

53 6.2 Rotation Accommodation The transition from the Marlborough Fault Array to the actively subducting deformation front requires signiicant translation of plate motion from margin normal to margin parallel at the apex of Pegasus Basin (Wallace et al., 2007). There is a signiicant lack of knowledge of how the crust accommodates the subduction to strike-slip change from minimal crustal block rotation within the South Island, to rapid vertical axis rotation in the North Island (Wallace et al., 2012). There are no through-going faults that connect the North Island and the South Island fault systems. There is a major change in fault strike from the NE-SW striking Marlborough Faults in the northern South Island to the E-W striking dextral faults in central Cook Strait. Most of these fault strike changes appear to align along a "hinge" line that separates the rotation of the northern blocks from the purely dextral southern blocks (Figure 6.5) (Pondard et al., 2010).

The east-west strike-slip faults through the Cook Strait and the Pegasus Basin play a key

Figure 6.5 Block model from Wallace et al., (2012) combined with rotation rates since 4Ma from Lamb et al., (2011) through paleomagnetic studies. Blocks in the south remain dominantly strike-slip in translation of strain, whereas to the north there is significant rotation, subduction, and transpression accommodating and dispersing strain within the Australian Plate.

54 role in this tectonic shift (Wallace et al., 2012). Slip rates from the strongly active faults from the Marlborough Fault Array (Figure 4.2), such as the Hope Fault, are distributed along strike-slip faults that deine block boundaries within the block model proposed by Wallace et al., (2012).

The Boo-Boo Fault (Figure 4.2) is the primary structure that accommodates the rapid rotation of the Wairarapa and Palliser blocks relative to the Paciic Plate (Figure 6.5) (Wallace et al., 2012) and is within 20° of the azimuth of Paciic/Australian Plate motion vector in this region (Pondard et al., 2010). Estimates from Townsend and Little (1998) suggest that the transition from rapid forearc rotation immediately next to the subduction margin, to block translation in the strike-slip zone is almost purely accommodated by the margin parallel strike-slip component on the Boo-Boo Fault. Other major strike-slip faults in the region that deine block boundaries align along a crustal scale hinge are responsible for translating motion from the Marlborough Fault

Array onto the subduction zone of the coast (Figure 6.5). These major faults and their associated block geometries coincide with the end of subduction at the intersection of the Chatham Rise, and the South Island (Pondard et al., 2010), suggesting that with the southward progression and counter-clockwise rotation of the Chatham Rise, will also lead to the migration of the rotation boundary (Wallace et al., 2007, 2009).

Rapid clockwise rotation of the forearc (60mm/yr near 38°S and ~25mm/yr near 41.5° S (Wallace et al., 2012)) is the driving factor for the increasing convergence rate to the north, and accommodates a large portion of margin parallel component of Paciic/Australian Plate motion. Up to 20% of margin-parallel plate motion occurs via rotation at the southern Hikurangi margin, and up to 60% in the north (Wallace et al., 2004 ; Wallace et al., 2012). Similar changes in plate motion accommodation can be measured along major faults within rotation blocks, and explains how certain faults can terminate suddenly (Figure 4.2), while remaining kinematically feasible (Wallace et al., 2012).

Rotation of individual crustal blocks (Figure 6.5) in this plate boundary zone contribute substantially to the northward decrease in strike-slip motion, and resulting subduction dominated

55 plate margin (Wallace et al., 2012). As fault blocks rotate, the deformation structures that deine the blocks rotate as well, which places the deformation front at a new orientation. Therefore, it is often diicult to restore early deformation as it is rotated and strained at various diferent orientations. Restorations in 2D do not have the ability to account for the amount of rotation or lateral ofset that occurs between these crustal blocks. The unaccounted-for displacement, and accommodation of shortening are likely distributed as lateral ofset along these strike-slip faults, rather than compression along the subduction interface.

6.3 Petroleum and Modelling Implications

Seismic data and both oil and gas seeps (Figure 2.7) conirm active petroleum systems within the basin, though modelling and predictions of oil and gas generation within Pegasus

Basin are very poorly constrained. Heat low, source rock analysis, rock properties, velocity models, and alternate essential petroleum system models are all based on highly variable assumptions and have no well data for calibration. Uruski & Bland (2011) have predicted around 100 billion barrels of oil and 400tcf of gas may have been expelled from mature source rocks since the early Cretaceous, though the timing of the structures of the area postdate this expulsion

(Uruski & Bland 2011). However, DHIs, gas chimneys, pockmarks, seeps, and slicks indicate that some of those expulsed hydrocarbons are still trapped in the basin.

Structural permeability is inferred to be important at all levels of the thrust system

(Barnes et al., 2010). There is a clear relationship between the seeps and the major seaward verging thrust faults near the outer edge of the deforming Cretaceous and Paleogene inner foundation rocks (Figure 6.6) (Barnes et al., 2010). Thrust faults are the suspected primary luid conduits for these seeps. The Cretaceous and Paleogene formations have extremely low permeability, and force luid low to the outer edge of their occurrence (Figure 6.6), and to permeable thrust faults. Source of luids may be the inner parts of the thrust wedge and subducting sediments below the décollement surface (Barnes et al., 2010). Therefore, the Cretaceous and Paleogene rocks that act as a deformable backstop are signiicant in the

56 petroleum system. They are estimated to have 1-2% TOC, and HIs of 300mg/g based on samples collected in outcrop (Barnes et al., 2010). The contact between the inner foundation of the wedge, and the accretionary wedge sediments is interpreted as a signiicant feature with respect to hydrogeology of the margin, as it appears to control the locations of presently active luid seeps (Barnes et al., 2010) that can be seen in map view in Figure 6.6. Presently there are no constraints on relative luid low between the frontal wedge and the active mid-slope luid seeps that are unassociated with the Cretaceous basement (Barnes et al., 2010).

Figure 6.6 Faults and seismic line locations shown with the yellow dashed line, representing the separation of wedge sediments underlain by Cretaceous and Paleogene basement (north of the yellow line) and the late Cenozoic frontal accretionary wedge, from Barnes et al., (2010). The orange dashed line is the suggested change in this boundary from this study. This change is based on seismic signatures observed in southern seismic time profiles PEG009, PEG007 and PEG005.

6.3.1 Gas Hydrate Modelling

An active petroleum system is present in the Pegasus Basin based on direct hydrocarbon indicators (DHIs) in the acquired seismic data, and by oil and gas seeps both on and ofshore of the Pegasus Basin (Bland et al., 2014). Most of the anticlines in the Pegasus Basin are transected by strong bottom simulating relectors (BSRs). Occasionally, the BSR has a break in it, and at the ocean loor a gas seep has been conirmed by Barnes et al., (2010). Structurally, the thrusts and their associated anticlines combined with the BSR are of most interest for exploration purposes. Source rocks for these accumulations are most likely Cretaceous and Paleogene in age, which are the primary backstop of the accretionary deformation in this region. Models suggest that gas is

57 being generated along the subduction margin, and then migrating up faults into their associated fault propagation folds. Questions regarding the origin of this gas that has created these gas hydrates, and the assumed free gas accumulations below are still not clear. Thermogenic gas is predicted to be the primary source of gas hydrates in the area although 0.9%-1.6% of gas is estimated to be biogenic in nature (Kroeger et al., 2010).

6.3.2 Pore Pressure Modelling Modelling petroleum systems is essential to the development and understanding of actively producing petroleum ields. An accurate structural understanding is fundamental for all basin modelling, especially when understanding paleostresses and low restraints (Burgreen- Chan et al., 2016). Understanding present day pore pressure, as well as how pore pressure and porosity evolved are essential for safe development and drilling. Pore pressure afects migration of luids and gases, as well as the kinetics of vitrinite and hydrocarbon maturation (Burgreen- Chan et al., 2016) making the structural interpretation the foundation of all other assumptions in the models applied to each basin. Pore pressure modelling is not within the scope of this research, however should be seriously considered before development of any type of drilling program in the area progresses.

6.4 Error The geologic restorations of the southern Hikurangi margin within the Pegasus Basin are based on a variety of interpretations, assumptions, and simpliications that all inluence the resulting shortening values acquired. The amount of error associated with each of these factors is varied. Some aspects of this process are far more sensitive than others, and have a higher impact on the results than others.

The 2D dataset has signiicant distances between seismic lines. This large spacing between transects leaves a huge margin of error for accurate correlation of structures observed in this study. Lines that run parallel to the coastline correlate stratigraphy within the undeformed wedge at a high level of conidence, though faulting and lack of seismic imaging decreases

58 that conidence further into the deformation front (landward). Based on published maps and alternate datasets, the cross sections selected for restoration do not contain faults that are shared by more than one cross section. If these restorations were to be performed on the entirety of the PEG09 study, this data gap would have a large impact on interpretation, and timing of faults and horizons, and the associated mini-basins in the anticlines for each thrust fault.

Interpretation of this sparse data has a signiicant impact on the results of this study. The geometry and angles of the faults combined with the horizon placements are extremely variable, and can be placed in many unique combinations. The solutions presented for these shortening values are highly reliant on errors based on parameters used for trishear and fault parallel low restorations, retro-deformation of units to a non-horizontal datum, and misplacement of horizons below the resolution of seismic data. Using a combination of seismic relectors, models, earthquake data, and previously published maps, this uncertainty and error can be minimized, though without striking seismic signatures and well control points, the interpretations and restoration processes allow signiicant error in shortening calculations.

Depth conversion variables have a strong efect on the results of the shortening values obtained from a restoration. Because geometries of the faults and horizons play such a large role in determining the shortening rates of these cross section restorations, the initial velocities obtained from previous research (Figure 4.1) for the depth conversion are extremely sensitive to skewing these results. The results of these depth conversions however, were compared to those in other studies. The décollement surface (plate interface) is at an acceptable depth within each cross section, and the geometry of the faults and horizons originally interpreted in time, are also acceptable after depth conversion.

Most of all, the vectors of motion in the Pegasus Basin move from being margin normal, to margin parallel, so the shortening values obtained via restorations are not only subject to interpretation, but signiicantly under-represent the amount of shortening accommodation within the region. The shortening that is not within the plane of the 2D transects on strongly

59 oblique faults is not accounted for, and requires a 3D seismic dataset to be properly evaluated. The shortening values obtained in this study, particularly in the southern lines (PEG009 and PEG017), have a large amount of error associated with their shortening values. It is also important to note that within the transition zone, where rotation is the highest, the faults that exhibit the highest rates of movement each year, often do not have the highest amount of ofset in the seismic cross sections (See Boo-Boo fault in map view in Figure 4.2 and in interpreted cross section in Figure 4.5). Though the faults at the deformation front are actively deforming the sealoor, and out of sequence thrusts are actively maintaining the wedge taper, the role of the strike-slip deformation are often higher than the subduction thrust shortening accommodation, and are essential to the distribution of strain from the convergent subduction zone into the strike- slip Marlborough Fault Array, and the Alpine Fault.

6.5 Megathrust Earthquakes Many people have described the subduction thrust at the apex of Pegasus Basin to be

"interseismically locked," or strongly coupled (Figure 6.7) (Walcott, 1987; Bibby, 1981; Reyners, 1998; Wallace et al., 2012) down to approximately 40km (Wallace et al., 2004, [2009]; Pondard et al., 2010). In this particular subduction zone, the term interseismically coupled refers to the relative motion between adjacent rocks on either side the plate interface over the time between large magnitude earthquakes along the interface. This means that over time, the majority of shortening and strain is accommodated on the upper plate structures shallower than 40km or so, and is no longer accommodated on the subduction thrust. This leads to a low slip rate along those shallow thrusts and high normal stress along them. The importance of the strike-slip faults increases in the shallow crust as they accommodate more of the plate normal displacement.

Within the rotating block model, the majority of the plate motion vector is transferred into the upper plate in the southern Pegasus Basin. From north to south, the relative plate motion is transferred from the subduction thrust along the plate boundary onto upper plate structures

(Boo-Boo and Kekerengu faults). This decreases the amount of seismic activity that subduction

60

Figure 6.7 Taken from Wallace et al. (2010). The Hikurangi margin and its transition from aseis- mic creep in the North Island, to interseismic locking zones in the South Island. Different aspects of the wedge dynamics are compared as the margin moves from subduction into the strike-slip Marlborough Fault Array on the South Island. Red and blue shading indicate coupling coeffi- cients. Red indicates areas that are currently locked and are likely to rupture in future subduction thrust earthquakes. Blue indicates smooth aseismic creep. Green contours show areas of slip in slow slip events since 2002 from Wallace and Beavan (2010). Convergence rates are shown in red (in mm/yr). The accompanying table shows along-strike variations in various subduction margin properties for the accretionary wedge. The inset shows the large scale tectonic setting. HT is Hikurangi Trench, KT is Kermadec Trench, TT is Tonga Trench, NI is North Island, and SI is South Island.

61 the plate interface accommodates at the apex of the Pegasus Basin even though there is a relatively high plate motion vector of 42mm/yr (Wallace et al., 2012). If the assumption is made that a shear zone or detachment surface at the plate boundary exists beneath the northern South Island, the Marlborough Fault Array faults could "sole-out" onto the detachment onto the top of the Paciic Plate. Magnetotelluric studies suggest high-conductivity zones related to the Hope and Clarence Faults to the top of the subducting Paciic Plate (Wannamaker et al., 2009; Wallace et al., 2012), though at deeper levels along this interface, the relative motion on the subduction interface occurs at full Paciic/Australia relative plate motion rates (Wallace et al., 2012).

If the normal strain accommodation on the upper plate faults exceeds the coupling, signiicant earthquakes could occur as the energy is released, and the plates slide past one another (Wallace et al., 2009). The impact of the seismic events would be devastating for those living within range of the hypocenter, but also could have serious aftermath consequences, such as tsunamis as a seismic hazard (Wallace et al., 2009). It is impossible to estimate from restorations where and when the next seismic event will take place. There is no evidence for timing and placement of where and when that stress will be released, and is far beyond the capabilities of this study.

6.6 Accretionary Wedges Worldwide Wedges have a variety of properties that afect the way they deform and the geometry of the wedge itself. These properties include; backstop height, frontal ramp friction, backstop rheology, along-strike backstop thickness changes, lateral changes in material properties, décollement strength, base décollement angle, lux of sedimentation, erosion, angle of obliquity, underplated sediment, luid pressures, internal friction, and many more (Graveleau et al., 2012). When the wedge material undergoes compression, the wedge will deform internally until a critical taper is attained. (Davis et al., 1983). The critical taper is the shape for which the wedge is on the verge of failure under horizontal compression everywhere including the basal décollement. A wedge of less than critical taper will not slip when strained, but will deform

62 internally, steepening its surface slope until the critical taper is attained (Davis et al., 1983). The critical taper, and the surface slope are controlled by luid pressures within the wedge, and the rock properties of the deforming units. Therefore, there is a unique relationship between the surface slope, luid pressures, and décollement angle that the deformation slides along (Davis et al., 1983), called the Coulomb wedge theory.

The Coulomb wedge theory identiies the relationship between the luid pressure ratios, décollement angles, and the surface slope of the accretionary prism. Wedges from around the world and their properties are compared in Figure 6.8. However, the Coulomb wedge theory is pressure-dependent. Therefore, when the wedge exceeds 15km in thickness, the brittle-plastic transition in quartz rich rocks for typical geothermal gradients breaks down. The décollement of the cross sections is around 10-12km, which allows the Coulomb wedge theory to still be applied to this wedge.

Figure 6.8 Taken from Davis et al. (1983). Theoretical linear relationships that compare the décollement dip (β), and the topographic slope (α), to predict the range of possible fluid pressure ratios. Boxes indicate observed geometries of active wedges, used to infer fluid pressure ratios within them. Heavy boxes indicate wedges with available pressure data. The theoretical linear relationship of the décollement to the topographic slope, α + Rβ = F, is controlled by the ratio of fluid pressure ratios, λ to λb assuming that basal friction (μb) = 0.85 and internal friction (μ) = 1.03 (consistent with Byerlee's empirical law of sliding friction). Hikurangi prism shown in red.

63 CHAPTER 7 CONCLUSIONS Shortening values were obtained from this study through the development of a cross sectional restoration driven model that explain how the shortening is accommodated at the southern Hikurangi margin. Through this process, several conclusions can be made for this complex structural transition:

1) Seismic data, sealoor bathymetry, earthquake epicenters, and previously published fault maps all support structural interpretations made by this study. Restorations of these lines relect similar shortening values compared to other studies that have been done on a variety of datasets near and within the Pegasus Basin.

2) Tectonic complexities within the accretionary wedge mean that sub-basins between anticlinal highs may difer in stratigraphic architecture than those nearby. Structural interpretations within this study imply that most tectonostratigraphic packages are present in each piggyback-basin, and continue onto the shelf. For restoration purposes this is an acceptable interpretation, but well control will allow exact correlation between troughs, and can further constrain timing on each piggyback-basin and its associated underlying thrust.

3) Plate motion vectors are not perpendicular or parallel to any of the seismic lines, so restoration of the seismic cross sections underestimates the margin normal component of shortening. Utilization of trigonometry helps to better understand the margin-normal and margin- parallel components of deformation, but the obliquity of the margin complicates acquisition of true shortening rates from 2D cross-sections.

4) Active out of sequence thrusting is widespread within the wedge behind the deformation front to maintain the wedge taper. (Barnes et al., 2010). This out of sequence thrusting can be mimicked through the restorations of the plate margin, and plays a signiicant role in the strain accommodation and activation sequence of fault propagation in the margin.

64 5) The structures accommodating the largest amount of shortening are not always the structures that have the largest amount of ofset in the seismic cross sections. The obliquity that each seismic cross section captures cannot account for rotation or plate parallel deformation that the strike-slip faults hold are often relected in lateral movement, which is not captured in a 2D cross section.

6) Shortening rates acquired from these restorations do not accommodate the entirety of the plate motion vector shortening rates. The subduction thrust continues much farther west than the cross sections cover. Restorations account for ~<25% of the shortening accommodation within the Hikurangi subduction thrust, because it likely only represents ~<25% of the extent of the subduction thrust. Remaining components of the plate motion vector derived shortening deicit within the cross section area are likely to be explained by the rotation of blocks within the basin, resulting in plate parallel accommodation, rather than by compression within the accretionary wedge.

7) The subduction zone has steadily rotated clockwise, and migrated south, so compression and accommodation along the current plate margin may not have been recorded or even existed past ~2-5Ma (Figure 2.5).

7.1 Future Work

A new, private, 3D seismic survey is being acquired by Schlumberger starting in November 2016, and collection should last through June of 2017. Analysis of these data will provide signiicant insight into the structures and horizons that have only been loosely correlated across the basin due to sparse data availability. The structural framework can be conirmed with the 3D survey, and will conirm or remap faults that are implied to be continuous along the oceanic ridges within the East Coast Basin and the Pegasus Basin. Correlation of horizons within the isolated piggyback basins with the new survey will be especially important because with a 2D dataset, there is no way to ensure horizon consistency within the sub-basins on the hanging walls of growing thrust faults. This dataset should be utilized to better understand the structural

65 framework of the basin, and to further evaluate the shortening and deformation occurring along the tectonically active margin. A 3D seismic survey will allow the mapping, not only of seismic structures and tectonostratigraphic packages, but could provide an avenue to create facies maps and depositional models from seismic data. This will help track the Hikurangi Trench back through time, and can give clues about the state of the margin with much more conidence than just 2D data can.

Even with the acquisition and access to 3D seismic data, it will be extremely diicult to correlate and conirm ages of stratigraphy deeper than the middle Miocene strata that are intercepted by the Titihaoa-1 and Tawatawa-1 wells without any further well control.

Tectonostratigraphic packages will be better deined in the new 3D survey, but will not have any age control. Conirming the ages of the strata within the basin will solidify correlation to the outcrops along the coasts of New Zealand. Well control will also provide a better understanding of lithologies and reservoir potential. It will also allow for more accurate constraints on subsidence and sediment compaction, which can have signiicant implications for maturity, and trap formation in the basin. Checkshots from drilled wells can help constrain the velocity models used to depth convert. My recommendation for data acquisition would be to drill a well within the accretionary wedge, to help identify the ages and lithologies of the stratigraphy, and to better reine the velocity model.

In December 2016, Anadarko relinquished their frontier exploration permits in the Pegasus Basin after holding them for just over four years. The block release for 2017 is underway, with bids due in September of 2017. Block awards will be announced in December of 2017. Data utilized by this study and other regional data are available to companies with interest in these block ofers. However, the active seismicity in the area combined with the 4000m+ water column with strong currents will most likely delay any drilling within the Pegasus Basin in the near future.

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72 APPENDIX A

Figure A.1 Map showing the extent of each horizon within the undeformed region of the wedge Younger units depositional areas increase in area to the south and east.

Figure A.2 Map showing the extent of horizon R3 throughout the Pegasus Basin. Horizon is not continued west through the deformation front.

73 Figure A.3 Map showing the extent of horizon R4 throughout the Pegasus Basin. Horizon is not continued west through the deformation front.

Figure A.4 Map showing the extent of horizon R5 throughout the Pegasus Basin. Horizon is not continued west through the deformation front.

74 Figure A.5 Map showing the extent of horizon R5B throughout the Pegasus Basin. Horizon is not continued west through the deformation front.

Figure A.6 Map showing the extent of horizon R5C throughout the Pegasus Basin. Horizon is not continued west through the deformation front.

75 Figure A.7 Map showing the extent of horizon R7 throughout the Pegasus Basin. The horizon is not continued past deformation, but must exist there, as it is the décollement surface for the subduction thrusts.

Figure A.8 Map showing the extent of horizon R8 throughout the Pegasus Basin. The horizon dips down underneath the North Island, and represents the top of the Hikurangi Plateau.

76 Figure A.9 Map showing the extent of the Torlesse supergroup in the Chatham Islands. The Tor- lesse extends much further south and east of this study area.

Figure A.10 Map showing the extent of horizon L. This is the surface observed in the southern lines that represents the Oligocene sediments that the Neogene sediments onlap onto on the Cha- tham Rise

77 APPENDIX B This appendix shows strain and shortening values for each age of each seismic restoration

The irst three tables (Tables B.1-B.3) correspond to strain percentages while Tables B.4-B.6

show values of Lo-Lf for reference to the change in section length in km.

Table B.1 Table showing the strain rates (in percentages) of the various Lo and Lf combinations for the restoration of seismic profile PEG023.

Table B.2 Table showing the strain rates (in percentages) of the various Lo and Lf combinations for the restoration of seismic profile PEG017.

Table B.3 Table showing the strain rates (in percentages) of the various Lo and Lf combinations for the restoration of seismic profile PEG009.

78 Table B.4 Table showing the changes in line length (km) of the various Lo and Lf combinations for the restoration of seismic profile PEG023.

Table B.5 Table showing the changes in line length (km) of the various Lo and Lf combinations for the restoration of seismic profile PEG017.

Table B.6 Table showing the changes in line length (km) of the various Lo and Lf combinations for the restoration of seismic profile PEG009.

79