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

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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
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