Neogene Paleomagnetism and Geodynamics of the Hikurangi Margin, East Coast, New Zealand by Christopher James Rowan Msci (Hons.), MA (Hons.)
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When referring to this work, full bibliographic details including the author, title, awarding institution and date of the thesis must be given e.g. AUTHOR (year of submission) "Full thesis title", University of Southampton, name of the University School or Department, PhD Thesis, pagination http://eprints.soton.ac.uk UNIVERSITY OF SOUTHAMPTON FACULTY OF ENGINEERING, SCIENCE AND MATHEMATICS School of Ocean and Earth Science Neogene paleomagnetism and geodynamics of the Hikurangi margin, East Coast, New Zealand by Christopher James Rowan MSci (Hons.), MA (Hons.) Thesis for the degree of Doctor of Philosophy March 2006 UNIVERSITY OF SOUTHAMPTON ABSTRACT FACULTY OF ENGINEERING, SCIENCE AND MATHEMATICS SCHOOL OF OCEAN AND EARTH SCIENCE Doctor of Philosophy NEOGENE PALEOMAGNETISM AND GEODYNAMICS OF THE HIKURANGI MARGIN, EAST COAST, NEW ZEALAND by Christopher James Rowan Vertical-axis rotations are an important component of Neogene deformation in the New Zealand plate boundary region, and potentially offer fundamental in- sights into the rheology of continental crust. Extensive paleomagnetic sampling along the Hikurangi margin, on the East Coast of the North Island, has provided new insights into the patterns, rates and timings of tectonic rotation, and also an improved understanding of the magnetic signature of New Zealand Cenozoic mudstones. Rigorous field tests reveal numerous late remagnetizations, which have often formed several million years after deposition and can be irregularly distrib- uted within an outcrop. Scanning electron microscopy and rock magnetic analyses indicate that the remanence carrier is predominantly the ferrimagnetic iron sul- phide, greigite, which is present as a mixed population of single domain and super- paramagnetic grains that are characteristic of arrested authigenic growth. Strong viscous overprints are the result of later, usually recent, oxidation of these sul- phides. The recognition of late-forming magnetizations leads to a completely new view of the Neogene tectonic evolution of the Hikurangi margin, with no tectonic rotations being evident prior to 8–10 Ma; coherent rotation of most of the Hiku- rangi margin since that time refutes the existence of the independently rotating ‘domains’ that were inferred from earlier paleomagnetic data. This pattern is more consistent with the short-term velocity field, and allows all Neogene rotation to be more simply explained as a large-scale response to realignment of the subducting Pacific plate. Tectonic rotations have been accommodated by a variety of struc- tures since 10 Ma; in the Late Miocene and Pliocene, rates of tectonic rotation were 3–4 times faster than presently observed and possibly involved a much larger region, before initiation of the North Island Dextral Fault Belt and the Taupo Volcanic Zone at 1-2 Ma instigated the current tectonic regime. Collision of the Hikurangi Plateau in the Late Miocene is interpreted to have caused both the initiation of tectonic rotation, and the widespread remagnetization of sediments, making it a key event in the Neogene evolution of the plate boundary region. i Some thoughts on the nature of scientific research “It doesn’t matter how beautiful your theory is, it doesn’t matter how smart you are. If it doesn’t agree with experiment, it’s wrong.” Richard Feynman “Our work is like this doughnut. Sure, it’s all fluff, has no nutritional value, and there’s a gaping hole in the middle. but, if you sugar coat it, and add colourful little sprinkles, people will eat it up.” Mike Slackenerny (as immortalised by Jorge Cham) ii Contents Abstract i List of figures vii List of tables x Author’s declaration xi Acknowledgements xii 1 Introduction 1 1.1 Rationale ................................ 2 1.2 Geological development of the New Zealand region . ... 4 1.3 Paleomagnetic investigations of the Hikurangi margin . ..... 8 1.3.1 Raukumara ........................... 11 1.3.2 Wairoa.............................. 11 1.3.3 Wairarapa............................ 12 1.3.4 Marlborough .......................... 12 1.4 Active faulting and deformation in the plate boundary region . 13 1.5 Synthesis................................. 16 1.6 Aimsandobjectivesofthisstudy . 17 2 Geological background 19 2.1 StructureofthepresentHikurangimargin . 20 2.1.1 Subductingplate . 20 2.1.2 Plate interface and forearc . 22 2.2 The New Zealand geological timescale . 25 iii Contents 3 Paleomagnetic theory and experimental methods 27 3.1 Principles of paleomagnetism . 28 3.1.1 TheEarth’smagneticfield . 28 3.1.2 Types of magnetic behaviour . 29 3.1.3 Preservation of remanent magnetization . 30 3.1.4 Remanence acquisition in sedimentary rocks . 34 3.2 Paleomagnetic measurements . 35 3.2.1 Paleomagnetic sampling . 35 3.2.2 Measurement of remanent magnetization . 36 3.2.3 Stepwise demagnetization . 36 3.2.4 Display of stepwise demagnetization data . 38 3.2.5 Principal component analysis . 40 3.2.6 Calculation of mean directions . 41 3.2.7 Paleomagnetic field tests . 42 3.3 Rockmagneticmeasurements . 44 3.3.1 Hysteresisproperties . 44 3.3.2 First order reversal curve (FORC) diagrams . 46 3.4 Scanning electron microscopy . 47 3.4.1 Basicprinciples ......................... 47 3.4.2 Identification of magnetic iron sulphide minerals . 48 4 Relocation of the tectonic boundary between the Raukumara and Wairoa domains (East Coast, North Island, New Zealand): Implications for the rotation history of the Hikurangi margin 50 4.1 Introduction............................... 51 4.2 Geological background . 54 4.3 Samplingandmethods . 55 4.4 Results.................................. 56 4.4.1 Stepwise demagnetization . 56 4.4.2 Paleomagnetic directions . 61 4.5 Discussion................................ 62 4.6 Conclusions ............................... 69 5 Tectonic and geochronological implications of variably timed magnetizations carried by authigenic greigite in marine sediments from New Zealand 71 5.1 Introduction............................... 72 5.2 Samplingandmethods . 73 5.3 Results.................................. 75 iv Contents 5.4 Discussion and conclusions . 77 6 Magnetite dissolution, diachronous greigite formation, and sec- ondary magnetizations from pyrite oxidation: Unravelling complex magnetizations in Neogene marine sediments from New Zealand 80 6.1 Introduction............................... 81 6.2 Samplingandmethods . 85 6.3 Paleomagneticdata. 86 6.3.1 Differentially timed synfolding magnetizations, Mahia Peninsula ............................ 87 6.3.2 Early forming magnetization, Rakauroa region . 90 6.3.3 Early and late magnetizations, Waihau Beach . 93 6.3.4 Strong present-day field overprint, Wairoa Syncline . 93 6.3.5 Marlborough .......................... 93 6.4 RockmagneticandSEMobservations. 94 6.5 Discussion................................100 6.6 Conclusions ...............................106 7 Neogene tectonic rotations within the Australia-Pacific plate boundary zone, Hikurangi margin, New Zealand 107 7.1 Introduction...............................108 7.2 Sample collection and analysis . 113 7.3 Results..................................117 7.3.1 Demagnetization behaviour . 117 7.3.2 Mean paleomagnetic directions . 118 7.3.3 Constraints on the timing of remanence acquisition . 122 7.3.4 Inferred tectonic rotations . 128 7.4 Discussion................................131 7.4.1 Comparison with published paleomagnetic data . 131 7.4.2 Comparison of long-term rotation patterns with active deformation ...........................132 7.4.3 Structural accommodation of large rotations . 133 7.4.4 Revised Neogene reconstructions for the Hikurangi margin . 141 7.5 Conclusions ...............................145 8 Summary and conclusions 147 8.1 Summaryandconclusions . .148 v Contents 8.1.1 The ubiquity of greigite and the consequences of late remagnetizations . .148 8.1.2 The non-existence of paleomagnetic ‘domains’ . 149 8.1.3 The evolving tectonic regime . 149 8.1.4 Collision of the Hikurangi Plateau: a key tectonic event? . 150 8.1.5 Length scales of deformation . 151 8.2 Furtherwork ..............................151 References 153 A Summary of sampling localities 170 B Demagnetization and rock magnetic data 209 vi List of Figures 1.1 Bathymetry of the southwest Pacific in the New Zealand region . 3 1.2 Early Miocene reconstruction of the Hikurangi margin . ... 6 1.3 Rotational domains of the Hikurangi margin . 9 1.4 Published paleomagnetic data from the Hikurangi margin . 10 1.5 Distribution of active deformation in the New Zealand plate boundaryregion............................. 14 2.1 N–S cross-section along the Hikurangi margin . 21 2.2 Comparison of the New Zealand and international geochronological timescales ................................ 25 3.1 Geocentric axial dipole (GAD) field . 28 3.2 The three fundamental types of magnetic behaviour . 29 3.3 Forms of exchange coupling in ferromagnetic (s.l.) materials. 30 3.4 The internal demagnetizing field and shape anisotropy . 32 3.5 Variation in domain state and coercivity