DOUBLE DATING DETRITAL ZIRCONS IN TILL FROM THE ROSS EMBAYMENT, ANTARCTICA Bethany Marie Welke Submitted to the faculty of the University Graduate School in partial fulfillment of the requirements for the degree Master of Science in the Department of Earth Science, Indiana University September 2013 i Accepted by the Faculty of Indiana University, in partial fulfillment of the requirements for the degree of Master of Science. Kathy J. Licht, PhD Sidney R. Hemming, PhD Master’s Thesis Committee Pamela A. Martin, PhD ii ACKNOWLEDGEMENTS I’d like to express my gratitude to my advisor, Dr. Kathy Licht for this opportunity to work on this wonderful project. She was abundantly helpful and offered excellent guidance, encouragement, enthusiasm, and support throughout this research and thesis. I could have not wished for a better or friendlier advisor. Thank you, Kathy, for introducing me to the Antarctic world, and geochronology. I am also thankful to my committee members, Dr. Sidney Hemming and Dr. Pamela Martin for their useful comments, conversations, and engagement throughout this research. I would like to thank my fellow lab mates and friends, Nicole Bader and Theresa Dits, for helping me out the past two years, for their assistance and support, for helping me throughout all the good times, bad times, humorous times, and keeping me sane. Thank you for the wonderful memories. In my daily work at IUPUI, I’m thankful for a friendly group of fellow graduate and undergraduate students including Samapriya Roy, Anna Samuels, Shiva Laden, Fotios Kafantaris, Justin Hodgson and Alex Schnur and others. Without their friendship, support, and numerous coffee trips I would have never pulled through. I would also like to thank the staff in the Earth Sciences Department at IUPUI for their help in the past two years. In addition, a thank you to Mark Pecha, Clay Loehn and other staff at the University of Arizona Laserchron Center for their assistance in zircon imaging and U/Pb analyses and Erin Abel and Uttam Chowdhury at the Radiogenic Helium Laboratory for their assistance in zircon picking and (U-Th)/He analysis. I’d also like to thank Dr. Stuart iii Thomson and Dr. Paul Fitzgerald for their helpful conversations and insightful comments and Dr. Leigh Stearns for constructing an ice flow map of the Ross Embayment. I would also like to thank my parents, grandparents, and the rest of my family. They have always encouraged me with their constant support. This research was supported by the National Science Foundation Office of Polar Programs grant NSF OPP-1043572, a 2012 Geological Society of America Graduate Student Research Grant, IUPUI Educational Enhancement Grant and IUPUI School of Science Graduate Student Council Grant. iv ABSTRACT Bethany Marie Welke DOUBLE DATING DETRITAL ZIRCONS IN TILL FROM THE ROSS EMBAYMENT, ANTARCTICA U/Pb and (U-Th)/He (ZHe) dating of detrital zircons from glacial till samples in the Ross Embayment, Antarctica records cooling after the Ross/Pan-African orogeny (450-625 Ma) followed by a mid-Jurassic to mid-Cretaceous heating event in the Beacon basin. Zircons were extracted from till samples from heads of major outlet glaciers in East Antarctica, one sample at the mouth of Scott Glacier, and from beneath three West Antarctic ice streams. The Ross/Pan-African U/Pb population is ubiquitous in these Antarctic tills and many Beacon Supergroup sandstones, thus 83 grains were analyzed for ZHe to subdivide this population. Two ZHe age populations are evident in East Antarctic tills, with 64% of grains 115-200 Ma and 35% between 200-650 Ma. The older population is interpreted to be associated with the Ross/Pan-African orogeny including cooling of the Granite Harbour Intrusives and/or exhumation of the older basement rocks to form the Kukri Peneplain. The lag time between zircon U/Pb, ZHe and 40Ar/39Ar ages from K-bearing minerals show cooling over 200 My. Grains in East Antarctic tills with a ZHe age of 115-200 Ma likely reflects regional heating following the breakup of Gondwana from the Ferrar dolerite intrusions, subsidence within the rift basin, and a higher geothermal gradient. Subsequent cooling and/or exhumation of the Transantarctic Mountains brought grains below the closure temperature over a span of 80 My. This population may also provide a Beacon Supergroup signature as most of the tills with this age are adjacent to nunataks mapped as Beacon Supergroup and contain an abundance of v Beacon pebbles within the moraine. Nine zircons grains from three Beacon Supergroup sandstones collected from moraines across the Transantarctic Mountains yield ages from 125-180 Ma. West Antarctic tills contain a range of ZHe ages from 75-450 Ma reflecting the diverse provenance of basin fill from East Antarctica and Marie Byrd Land. ZHe and U/Pb ages <105 Ma appear to be distinctive of West Antarctic tills. The combination of U/Pb, ZHe and 40Ar/39Ar analyses demonstrates that these techniques can be used to better constrain the tectonic evolution and cooling of the inaccessible subglacial source terrains beneath the Antarctic Ice Sheet. Kathy J. Licht, PhD vi Table of Contents 1. Introduction ..................................................................................................................... 1 1.1 Provenance Tools ...................................................................................................... 3 1.1.1 U/Pb dating of detrital zircons ............................................................................ 3 1.1.2 (U-Th)/He dating of detrital zircon ..................................................................... 4 1.1.3 40Ar/39Ar dating of detrital K-bearing minerals .................................................. 5 2. Geologic Setting.............................................................................................................. 6 2.1 Antarctic Ice Sheet ..................................................................................................... 6 2.2 Sediment erosion and transport ................................................................................. 7 2.3 Geologic history ........................................................................................................ 8 2.3.1 West Antarctic Rift System and the exhumation of the Transantarctic Mountains ................................................................................................................... 10 3. Materials and Methods .................................................................................................. 12 3.1 Sample Acquisition.................................................................................................. 12 3.2 Geology of site locations ......................................................................................... 12 3.2.1 West Antarctica ................................................................................................. 12 3.2.2 East Antarctica .................................................................................................. 12 3.3 Zircon Preparation and Imaging .............................................................................. 14 3.4 U/Pb analysis ........................................................................................................... 15 3.5 (U-Th)/He analysis .................................................................................................. 16 3.6 40Ar/39Ar Analysis ................................................................................................... 18 4. Results ........................................................................................................................... 19 vii 4.1 U/Pb ......................................................................................................................... 19 4.1.1 Statistical Analysis for U/Pb data ..................................................................... 20 4.2 (U-Th)/He ................................................................................................................ 20 4.3 40Ar/39Ar .................................................................................................................. 22 5. Discussion ..................................................................................................................... 23 5.1 East Antarctica ......................................................................................................... 23 5.1.1 Ross/Pan-African Orogeny ............................................................................... 23 5.1.2 Break-up of Gondwanaland .............................................................................. 24 5.1.3 Exhumation of the Transantarctic Mountains ................................................... 28 5.2 West Antarctica ....................................................................................................... 30 5.3. Glacial till as a provenance tool ............................................................................. 31 6. Conclusion .................................................................................................................... 32 Appendix A ....................................................................................................................... 51 Appendix B ....................................................................................................................... 90 Appendix C ......................................................................................................................