Timing of tectonic uplift rate change at Araki, Vanuatu, derived from 230-Th dating of fossil corals A Thesis SUBMITTED TO THE FACULTY OF UNIVERSITY OF MINNESOTA BY Claire Rabine IN PARTIAL FULFULLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE Dr. Christina Gallup, Advisor January 2019 1 © Claire Rabine, 2018 i Acknowledgements This project was greatly improved by the knowledge and comments from my advisor, Christina Gallup, and my committee, Byron Steinman, Kathryn Schreiner, John Goodge, and Fred Taylor (University of Texas – Austin). Special thanks are owed to Larry Edwards for use of his laboratory, as well as Xianlei Li and Pu Zhang for their invaluable help (University of Minnesota, Twin Cities). Training and insight about x-ray diffraction by Tsutomu “Shimo” Shimotori is also greatly appreciated (UMD). Most of all, thanks to my fiancé, family, and friends for their continual support throughout this project. i Abstract New data from precisely leveled and 230Th-dated coral fossils allow for better constraints on the recent (~125 ka) tectonic history of Araki, Vanuatu. Subduction of two large underwater landforms have caused significant deformation to the intraoceanic arc on which Araki is centered and induced a variable and complex tectonic history. Due to the relatively few intraoceanic arcs undergoing similar collisions, an effort to better constrain and understand the tectonic history of Araki is valuable. This study presents an uplift model that builds on previous constraints of the tectonic history of Araki using fossil coral elevation and age data and a reference sea level curve. Data from fifty-nine samples was collected and used to provide chronological constraints for paleo shorelines that in turn offered insight into the tectonic history. The model suggests subsidence at a rate of 3.9 mm/yr from 125 ka to 106 ka, a shift to uplift at 106 ka continuing at a rate of 1.4 to 1.6 mm/yr until 25 – 30 ka, and then increasing to a rate of 4.6 mm/yr until the present. The abrupt changes in vertical tectonics implied by this modeling offers insight into the rapid tectonic variability and possible mechanisms controlling convergent margin tectonics. ii Table of Contents Acknowledgements..................................................................................................................i Abstract................................................................................................................................... ii List of Figures ....................................................................................................................... iv 1. Introduction ....................................................................................................................... 1 1.1 Tectonic history of New Hebrides ......................................................................................... 3 1.2 Determination of past sea levels using coral terraces ......................................................... 8 1.3 History of fossil coral studies on Araki............................................................................... 12 1.4 230Th Dating ............................................................................................................................. 13 1.5 Coral Sample Selection ......................................................................................................... 17 2. Methods ............................................................................................................................ 17 2.1 Surveying and Collection of Samples.................................................................................. 17 2.2 Process of U-Th Dating ......................................................................................................... 18 2.3 X-Ray Diffraction................................................................................................................... 21 3. Results ............................................................................................................................... 25 4. Discussion ......................................................................................................................... 27 4.1 Early subsidence from 125 to 106 ka .................................................................................. 28 4.2 Long-term uplift rate ............................................................................................................. 29 4.3 Uplift rate since ~20 ka.......................................................................................................... 29 4.4 Modeling uplift rate from 20-105 ka ................................................................................... 31 4.5 Errors encountered and future work.................................................................................. 39 5. Conclusions: ..................................................................................................................... 44 References: ........................................................................................................................... 46 Appendix A: 230Th Dataset ............................................................................................... 51 Appendix B: XRD Results ................................................................................................. 54 iii List of Figures Figure 1: Vanuatu overview map ................................................................................... .4 Figure 2: Espiritu Santo subduction cross-section ........................................................ .5 Figure 3: Papua New Guinea map .................................................................................. .9 Figure 4: Sea level curve by Lambeck et al. (2002) ........................................................ 12 Figure 5: Uranium decay chain ...................................................................................... 14 Figure 6: Calcite and aragonite diffractograms ............................................................. 23 Figure 7: Calcite/aragonite calibration curve ................................................................ 25 Figure 8: Drilling locations near Araki .......................................................................... 29 Figure 9: Simplified uplift history ................................................................................... 31 Figure 10: Example of model using Sample AF ............................................................. 34 Figure 11: Vertical uplift rates modeled ......................................................................... 37 Figure 12: Sea level estimates from model ...................................................................... 38 Figure 13: Error due to growth depth of corals ............................................................ 39 Figure 14: Error in uplift rates due to 106 ka data point ............................................... 41 Figure 15: Error in sea levels due to 106 ka data point ................................................. 42 Figure 16: Porites distribution histogram ....................................................................... 43 iv 1. Introduction Araki is a small island within a much larger chain of islands in the Pacific known as the Republic of Vanuatu. Vanuatu sits at the convergence of the Pacific and Australian plates (Figure 1), and is known to have an uncommonly variable tectonic history (Collot et al., 1985). The subduction of two massive underwater features, an extinct subduction zone known as the d’Entrecasteaux Zone (DEZ) and an underwater plateau known as the West Torres Massif (WTM), has had tremendous impact on the structure and tectonic development of the arc. As these features collide and subduct, the islands of Vanuatu uplift or subside in response. Other notable effects include the slowing of plate convergence rates closest to the DEZ and WTM, rapid thickening and uplift of the forearc, and a very shallow trench (Meffre and Crawford, 2001) (see Figure 2). Additionally, the forearc islands are unusually close to the trench, which means the seismogenic interplate thrust zone is directly beneath Araki at a depth of only 20 – 25 km (Baillard et al., 2015). It is unusual and fortuitous to have islands positioned to record vertical tectonic deformations directly over this zone, which is greatly influenced by tectonic deformation. It is possible to document this movement using coral fossils from Araki and other nearby islands, as many species of corals grow at or near sea level and are able to be precisely dated using uranium-thorium radiometric dating (Edwards, 1988). It has been previously estimated that the highest rate of uplift in Vanuatu is along the western coast of its largest island, Espiritu Santo, just north of Araki, at rates as high as 6 mm/yr (Taylor, 1992). High uplift rates (~ 4 – 6 mm/yr) have been occurring since 1 approximately the Last Glacial Maximum (~20 ka), which preceded slower rates estimated to be about 3.2 mm/yr (Taylor, 1992). There is evidence of subsidence as well, prior to about 170 to 100 ka (Taylor, 1992; Taylor, 1995). However, the timing and magnitude of these changes in rate are unclear. A collection of 59 coral fossils between the ages of 30 and 140 ka collected by Fred Taylor (UT Austin) and Nick Freiburger (UMD) in 2005 may provide additional constraints on the tectonic history. Eleven samples were dated by Freiburger using 230Th dating as part of