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Tectonic and Magmatic Controls on Extension and Crustal Accretion in Backarc Basins, Insights from the Lau Basin and Southern Mariana Trough

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Tectonic and Magmatic Controls on Extension and Crustal Accretion in Backarc Basins, Insights from the Lau Basin and Southern Mariana Trough TECTONIC AND MAGMATIC CONTROLS ON EXTENSION AND CRUSTAL ACCRETION IN BACKARC BASINS, INSIGHTS FROM THE LAU BASIN AND SOUTHERN MARIANA TROUGH A DISSERTATION SUBMITTED TO THE GRADUATE DIVISION OF THE UNIVERSITY OF HAWAI‘I AT MĀNOA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY IN GEOLOGY AND GEOPHYSICS MAY 2017 By Jonathan D. Sleeper Dissertation Committee: Fernando Martinez, Chairperson Patricia Fryer Richard Hey Michael J. Mottl John Sinton Keywords: Lau Basin; Mariana Trough; backarc basin; seafloor spreading; subduction zone ACKNOWLEDGEMENTS I would like to express my gratitude and appreciation to all those who have supported, helped, and encouraged me throughout this long journey. First and foremost, I would like to thank my advisor, Fernando, for his constant support, advice, and patience throughout the nearly nine years that we worked together on both my Master's and Doctoral projects. I learned more from my conversations with him over the years than all of the classes I've taken or papers that I have read, and no one else has had a greater influence on the scientist that I have become. Thank you also to my committee members, Drs. Patricia Fryer, Richard Hey, Michael J. Mottl, and John Sinton, for their support and advice to improve this dissertation. I also would like to thank the teachers that I have had throughout my career as a geologist, both from San Jose State University, where my educational foundation in geology was laid, and at UH Mānoa, where my horizons expanded beyond the realm of continental geology to the depths of the oceans and the large-scale processes of plate tectonics. I would also like to thank all of the administrative staff at UH who have helped me with the sometimes complicated logistical aspects of grad school: Lily, Grace, Susan, Evelyn, Alison, Leona, Marcie, and others who I'm sure I missed. On a personal level, I would like to thank my family for their constant support and advice to help me get through this challenging period of my life. Thank you to all of my friends that I have made throughout my time here at UH for the support and the good memories that will last a lifetime, and to my old friends who have been there since before I ever considered going to graduate school. Mahalo to all! Page i ABSTRACT This dissertation examines magmatic and tectonic processes in backarc basins, and how they are modulated by plate- and mantle-driven mechanisms. Backarc basins initiate by tectonic rifting near the arc volcanic front and transition to magmatic seafloor spreading. As at mid-ocean ridges (MORs), spreading can be focused in narrow plate boundary zones, but we also describe a diffuse spreading mode particular to backarc basins. At typical MORs away from hot spots and other melting anomalies, spreading rate is the primary control on the rate of mantle upwelling and decompression melting. At backarc spreading centers, water derived from the subducting slab creates an additional mantle-driven source of melt and buoyant upwelling. Furthermore, because basins open primarily in response to trench rollback, which is inherently a non-rigid process, backarc extensional systems often have to respond to a constantly evolving stress regime, generating complex tectonics and unusual plate boundaries not typically found at MORs. The interplay between these plate- and mantle-driven processes gives rise to the variety of tectonic and volcanic morphologies peculiar to backarc basins. Chapter 2 is focused on the Fonualei Rift and Spreading Center in the Lau Basin. The southern portion of the axis is spreading at ultraslow (<20 mm/yr) opening rates in close proximity to the arc volcanic front and axial morphology abruptly changes from a volcanic ridge to spaced volcanic cones resembling arc volcanoes. Spreading rate and arc proximity appear to control transitions between two-dimensional and three-dimensional mantle upwelling and volcanism. In the second study (Chapter 3), I develop a new model for the rollback-driven kinematic and tectonic evolution of the Lau Basin, where microplate tectonics creates rapidly changing plate boundary configurations. The third study (Chapter 4) focuses on the southern Mariana Trough and the transitions between arc rifting, seafloor spreading, and a new mode of "diffuse spreading," where new crust is accreted in broad zones rather than along a narrow spreading axis, apparently controlled by a balance between slab water addition and its extraction due to melting and crustal accretion. Page ii TABLE OF CONTENTS Acknowledgements ……………………………………………………………………………... i Abstract ………………………………………………………………………………………… ii List of tables ..………………………………………………………………………………….. vi List of figures …………………………………………………………………………………. vii List of abbreviations and symbols……………………………………………………………… ix Chapter 1. Introduction ………………………………………………………………………... 1 1.1 Overview …………………………………………………………………………… 1 1.2 Background ………………………………………………………………………… 1 1.3 Dissertation structure and chapter summaries ……………………………………… 5 Chapter 2. The Fonualei Rift and Spreading Center: Effects of ultraslow spreading and arc proximity on backarc crustal accretion ……………………………………………………….. 10 2.1 Introduction ……………………………………………………………………….. 11 2.2 Geologic setting and background …………………………………………………. 13 2.2.1 Northeast Lau Basin ………………………………………………………. 13 2.2.2 The Fonualei Rift and Spreading Center …………………………………. 13 2.3 Methodology ……………………………………………………………………… 15 2.3.1 Geophysical data sets and processing …………………………………….. 15 2.3.2 FRSC axial segmentation …………………………………………………. 17 2.3.3 Gravity data ……………………………………………………………….. 19 2.4 Observations and results …………………………………………………………... 19 2.4.1 FRSC morphology: Segment-by-segment descriptions …………………... 19 2.4.1.1 Arc-distal segments: MTJ-S1 and MTJ-S2 ……………………….. 21 2.4.1.2 Transitional segments: FRSC1 and FRSC2 ………………………. 21 2.4.1.3 Arc-proximal segments: FRSC3 and FRSC4 …………………….. 23 2.4.1.4 Arc-like segments: FRSC5 and FRSC6 …………………………... 25 2.4.2 Gravity anomalies ………………………………………………………… 25 2.5 Discussion ………………………………………………………………………… 27 2.5.1 General trends with arc proximity, comparison with ELSC/VFR ………... 27 2.5.1.1 FRSC-S morphology: Along-axis melt focusing due to ultraslow spreading rates? …………………………………………………… 29 Page iii 2.5.1.2 FRSC-S morphology: Rifting versus spreading? …………………. 31 2.5.1.3 Morphologic correlations with the arc ……………………………. 31 2.5.1.4 Geochemical correlations with the arc ……………………………. 33 2.5.1.5 Arc-distal segments ……………………………………………….. 33 2.5.1.6 Morphologic correlations with hot spot-proximal ridges ………… 36 2.5.2 Model of mantle wedge structure ………………………………………… 36 2.5.3 Opening rate effects on the volcanic expression of backarc crustal accretion ……………………………………………………………………………… 37 2.6 Conclusions ……………………………………………………………………….. 40 Chapter 3. Geology and kinematics of the Niuafo’ou microplate in the northern Lau Basin .. 42 3.1 Introduction ……………………………………………………………………….. 43 3.2 Tectonic setting and kinematics …………………………………………………... 44 3.2.1 Present-day tectonics in the Lau Basin …………………………………… 44 3.2.2 Lau Basin kinematics ……………………………………………………... 46 3.3 Geophysical data sets and processing …………………………………………….. 48 3.4 Plate boundary observations and interpretations …………………………………. 50 3.4.1 Niuafo'ou-Tonga plate boundary: Morphologic evidence for slower spreading rates ……………………………………………………………………….. 51 3.4.1.1 Niuafo'ou-Tonga plate boundary: Magnetic anomaly modeling and a new Euler pole ……………………………………………………. 51 3.4.1.2 Model setup and Euler pole location ……………………………… 53 3.4.1.3 Magnetic modeling results: Slow to ultraslow spreading along the FRSC/MTJ-S ……………………………………………………... 54 3.4.2 Central Lau Spreading Center, Lau Extensional Transform Zone, Peggy Ridge, and the nature of the Australia-Niuafo'ou plate boundary ………... 57 3.4.2.1 Central Lau Spreading Center morphology ………………………. 58 3.4.2.2 Central Lau Spreading Center opening rates ……………………... 60 3.4.2.3 Lau Extensional Transform Zone: Extensional structures ………... 63 3.4.2.4 Lau Extensional Transform Zone: Strike-slip structures …………. 65 3.4.3 Australia-Tonga plate boundary: Evidence for increasing opening rates over time ……………………………………………………………………….. 67 Page iv 3.5 Lau Basin kinematic model ………………………………………………………. 68 3.5.1 New Lau Basin three-plate kinematic model ……………………………... 68 3.5.2 Niuafo'ou-Tonga southern boundary and the Niuatoputapu plate ………... 75 3.5.3 The nature of Peggy Ridge and insights into A-N boundary evolution …... 76 3.5.4 Implications for rigid plate models and the dynamic nature of backarc basins ……………………………………………………………………………… 77 3.6 Conclusions ……………………………………………………………………….. 78 Chapter 4. Diffuse spreading, a new mode of crustal accretion in the southern Mariana Trough backarc basin ...................……………………………………………………………………... 80 4.1 Introduction ……………………………………………………………………….. 81 4.2 Geologic background: southern Mariana Trough ………………………………… 82 4.2.1 Mariana Trough opening and tectonics …………………………………… 82 4.2.2 Volcanism in the southern Mariana Trough ……………………………… 86 4.3 Methodology: Geophysical data sets and processing …………………………….. 89 4.4 Observations and results …………………………………………………………... 90 4.4.1 Southwest Mariana Rift morphology ……………………………………... 91 4.4.2 Diffuse volcanic zone morphology ……………………………………….. 93 4.4.3 MGR flank “organized spreading” morphology ………………………….. 98 4.4.4 Seismicity in the southern Mariana Trough ……………………………... 100 4.5 Discussion ……………………………………………………………………….. 101 4.5.1 Summary of observations on the mode of opening in the southern Mariana
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