Paleomagnetic Rotation Study of Woodlark-Australia plate motions in the Woodlark Rift, SE Papua New Guinea by Elizabeth Ann Cairns A thesis submitted to Victoria University of Wellington In fulfilment of the requirements of Master of Science In Geology Victoria University of Wellington 2014 Page 1 of 152 Page 2 of 152 Acknowledgements I am grateful for the opportunity to be involved in such an exciting place as SE Papua New Guinea, so first thanks go to Tim Little, Laura Wallace, and Susan Ellis for being involved in research on PNG tectonics, and for having me join them on an unforgettable journey. This project has been both a challenging, and extremely fulfilling Introduction to an amazing place to study and visit – best trip of my life!! I am especially grateful to my primary supervisor Prof. T.A. Little who has been integral to the progression of this study, not least for being half of the paleomagnetic sampling team in PNG. For being committed and encouraging; for useful critiques; and for being patient and sweet every time I cried in his office, my sincere thanks go to Tim. I also would like to thank my secondary supervisor Dr Gillian Turner for teaching me the principles of Paleomagnetism and lending her expertise. I am very grateful to have the benefit of Gillian’s experience, in terms of understanding the principles of the discipline, in the practical foundation that are the systems in the Paleomagnetism lab at VUW, and for direct assistance in reaching the results presented in this thesis. Additional thanks go to Laura Wallace for providing me with GPS solutions that have informed the interpretation of my paleomagnetic results, and for enthusiastic and encouraging response to the outcomes of this research. Susan Ellis also helped with paleomagnetic sampling in PNG, and her thoughts regarding the implications of certain results were very constructive to the final interpretations presented in this thesis. I would like to thank Ian Smith for lending me his personal copies of out of print geological maps that have been very useful to me through the course of this thesis, as well as for taking the time to meet with me in the planning stages to share his personal experience from working in PNG, towards identifying suitable paleomagnetic sampling targets for my project. I owe great thanks to Martin Chadima for help with Remasoft software, in rewriting/altering the program especially to read in the specific data file format of my corrected LongCore output files. Claudio Tapia provided me with warm introduction to and assistance in the paleomagnetic facilities at Otago University. To the many friends and family who have provided support through the course of my study, thanks to you too! Last but not least, I would like to acknowledge the many people in Papua New Guinea who helped us to find our way around the bush, and who carried heavy sampling gear all the while helping me over slippery rocks, and made the Papuan experience a joyful one. Page 3 of 152 Page 4 of 152 Abstract The Woodlark Rift in SE Papua New Guinea is a continental rift to the west of active oceanic spreading in the Woodlark Basin, which separates the Australian Plate to the south from the relatively anticlockwise rotating Woodlark Plate to the north. During Pliocene to Recent times the Woodlark Rift has been the setting for rapid exhumation of the world’s youngest UHP rocks (Baldwin et al., 2004, 2008; Gordon et al, 2012; Little et al., 2011), and is currently one of few places on the globe where active continental breakup is occurring ahead of a propagating oceanic spreading centre. While the Woodlark Basin contains a record of oceanic spreading since ~6 Ma (Taylor et al., 1999), and GPS data describe present-day crustal motions (Wallace et al., manuscript in review), the Neogene temporal and kinematic evolution of continental extension in the Woodlark Rift is less well constrained. We compare Characteristic magnetization directions for six formations, Early Miocene (~20 Ma) to Late Pliocene (3 ± 0.5), with contemporaneous expected field directions corresponding to Australian Plate paleomagnetic pole locations. We interpret declination anomalies (at 95% confidence) to estimate finite vertical-axis rotations of crustal blocks with respect to a fixed Australian Plate. Temporal and spatial relationships between declination anomalies for six formation mean directions, across four paleomagnetic localities, provide new evidence to constrain aspects of the Miocene to Recent history of the Woodlark Rift. We obtained 250 oriented core samples from Miocene to Pliocene aged rocks at four localities in the Woodlark Rift. Components of Characteristic Remanent Magnetization (ChRM) have been determined from step-wise thermal and alternating field demagnetization profiles of >300 individual specimens. A total of 157 ChRM components contribute to the calculation of representative paleomagnetic directions for six formations, which have undergone vertical-axis rotations with respect to the Australian Plate associated with development of the Woodlark Rift. Pliocene volcanic rocks at two key localities near the northern extent of the rift record that: 1) The Amphlett Islands has experienced 10.1 ± 7.6° of anticlockwise rotation since 3 ± 0.5 Ma; 2) NW Normanby Island has undergone a 16.3 ± 9.5° clockwise rotation during the same time interval. Sedimentary rocks at Cape Vogel Peninsula on the northern coast of the mainland Papuan Peninsula, record variable anticlockwise finite rotations of 28.4 ± 10.9° and 12.4 ± 5.5° for Early and Middle Miocene rocks respectively, in contrast to a younger clockwise rotation of 6.5 ± 11.2° for Late Miocene rocks. At the Suau Coast locality, on the south eastern coast of the Papuan Peninsula, Late Miocene dikes record 22.7 ± 13.3° of anticlockwise rotation. At the Amphlett Islands and NW Normanby localities paleomagnetic data are consistent with current GPS plate motions, suggesting the current kinematics in the rift were established by at least ~3 Ma. The Amphlett Islands result is consistent with the rate of Pliocene sea floor spreading in the Woodlark Basin, suggesting that locality can be considered as fully on the Woodlark Plate. The clockwise rotation indicated at NW Normanby Island may record development of an incipient dextral transfer fault within an active part of the Woodlark Rift. Time-varying declination anomalies from the Cape Vogel Peninsula suggest that rifting began there by ~15 Ma, 7 Ma earlier than previously inferred based on stratigraphic evidence. Furthermore, paleomagnetic data from the south coast of the Papuan Peninsula suggests that early rifting extended further south, and has since contracted to where continental extension is currently accommodated north of the Papuan Peninsula. Page 5 of 152 Contents Acknowledgements ........................................................................................................................... 3 Abstract ............................................................................................................................................ 5 List of Figures .................................................................................................................................... 9 List of Tables ................................................................................................................................... 12 1. Introduction ................................................................................................................................ 13 1.1 The Woodlark Rift .................................................................................................................. 13 1.2 Research Objectives ............................................................................................................... 15 1.3 Thesis Outline ........................................................................................................................ 15 1.4 Regional Geological Setting, and a Cenozoic Tectonic History of the SE Papua New Guinea Region from the Literature .......................................................................................................... 19 1.4.1 Paleogene Arc-Continent Collision................................................................................... 19 1.4.2 Miocene Basin Stratigraphy and Tectonic Implications Thereof ....................................... 19 1.4.3 Pliocene to Recent Plate Motions, Rift Development, MCC exhumation, and Volcanism. 21 2. Methodology ............................................................................................................................... 24 2.1 Fieldwork and Sampling ......................................................................................................... 24 2.1.1 Locality Selection and Rationale ...................................................................................... 24 2.1.2 Sampling and Field Annotation ........................................................................................ 25 2.1.3 Sample Preparation ......................................................................................................... 27 2.2 Natural Magnetization Processes and Magnetic Mineralogy .................................................. 28 2.2.1 Thin Section Petrology and Petrography .......................................................................... 28 2.2.2 Temperature Dependent Susceptibility ........................................................................... 29 2.2.3