Copyright By Nick Andrew Moreno 2016 U-Pb and 40Ar/39Ar geochronology of rhyolitic tuffs and lavas of the Sierra Madre Occidental silicic large igneous province: Insights into Eocene-Oligocene silicic volcanism from a new sample transect, Chihuahua and Durango border region, northern Mexico By Nick Andrew Moreno A Thesis Submitted to the Department of Geological Sciences California State University, Bakersfield In Partial Fulfillment for the Degree of Masters of Science in Geology Spring 2016 U-Pb and 40 Arf39Ar geochronology of rhyolitic tuffs and lavas of the Sierra Madre Occidental silicic large igneous province: Insights into Eocene- Oligocene silicic volcanism from a new sample transect, Chihuahua and Durango border region, northern Mexico By Nick Andrew Moreno This thesis or project has been accepted on behalf of the Department of Geological Sciences by their supervisory committee: Dr. Graham D.M. Andrews Committee Chair Dr. Sarah R. Brown Dr. William C. Krugh For my mother Jeannette— The light of my life. Acknowledgements I would first like to thank Dr. Graham Andrews for inviting me on to this project -- I never anticipated a simple rock-cutting gig would ultimately evolve into the focus of my graduate career. I would also like to thank him for his excellent mentorship and support in the laboratory, classroom, and field -- our time in Mexico was magical, when I was not dying of illness. His insightful input improved this thesis by many orders of magnitude, and also made me a better geoscientist. I do not think I could thank him enough for the impact he has had on my academic pursuits. I extend my appreciation to Dr. Sarah Brown for her assistance with sample preparation, tutelage on the various analytical instruments, data analysis, and for her thesis feedback. I would also like to acknowledge Dr. William Krugh for his valuable input on this thesis – how some typos and omissions made it through all the drafts I will never know. To Dr. Pablo Davila- Harris: thank you for assisting with the field work in Mexico and for the creation of the beautiful composite stratigraphic column used in this thesis. The hospitality you and your department extended to us in Mexico was as divine as the bottle of Mezcal you suggested I buy. To Linda Anderson, for her meticulous point counting work, I am eternally grateful. The data proved to be crucial for the sample correlations. For their assistance with puck imaging, I thank Alana Crown, Alyssa Kaess, and Dr. Sarah Brown. Their help came at a time when the SEM was required to run nearly 24 hours per day in order to meet a strict deadline. I also express my appreciation to the folks at the UCLA SIMS Lab, Matt Wielicki and Ming-Chang Liu: they ensured the ims 1270 was up and running smoothly, even after it died several times. To fellow Bruin Brenda Pack: thank you for your help with mount polishing and gold coating, and for being my travel buddy during our geochronology trip to Alabama. Dr. Bill Hames of the Auburn Noble Isotope Mass Analysis Laboratory (ANIMAL) at Auburn University, Alabama: thank you for assistance with the argon data processing and for welcoming me and Brenda into your wonderful lab. I would also like to thank the CREST program (NSF award HRD1137774) for fully supporting me financially, and making it possible for me to attend school and conduct this research. Likewise, thank you to the Student Research Scholars program for also providing much-appreciated funding. Andrea Medina – I honestly can’t fathom what the current state of the CREST program would be without your tireless efforts to ensure the ship was always running smoothly. To Elizabeth Powers: I should have kept a notepad around so I could document all the ways in which you helped me during my time at CSUB. You opened locked doors, dealt with the IT department to get much-needed software packages installed, took time out of your day to assist me in locating documents and samples. You taught me how to service the SEM, how to earn my defensive drivers training certificate, even how to turn in this thesis. The geological sciences department would collapse without you. Likewise, thank you Sue Holt for assisting with class enrollment, door access, and for your absolutely refreshing positive attitude. To Michelle (Casterline) Anderson: thank you for your support and understanding nature during my time as an intern at the Kern County Water Agency. My time there was my first exposure to the science of hydrogeology. I would also like to give very special thanks to Alana Crown, with whom I have worked closely throughout my time as a graduate student at CSUB. She was always there for me when I needed to vent, work through a problem, crush rocks, analyze data, build a map, whine about class, nearly anything and everything. Alana is insightful, witty, and a wonderful human being. Once she even gave me a dollar for knowing a Strange Brew reference. I am truly a better individual for having her in my life. To all my friends in the geology department: thank you for the laughs, Lengthwise brews, and late nights that truly made my time at CSUB special. Finally, I thank my mother Jeannette and stepfather John for encouraging me to return to school to pursue a master’s degree in the geosciences. This choice has irrevocably altered my life in so many positive ways I could never do it justice by describing it here. ABSTRACT I present seventeen new U-Pb and eight 40Ar/39Ar ages from a previously unstudied geological traverse across the central Sierra Madre Occidental silicic large igneous province along the Chihuahua-Durango state border in Mexico. These data indicate progressive younging of volcanism to the southwest, with a succession of exclusively Eocene rhyolites in the northeast (c. 40 – 37 Ma), Eocene rhyolites overlain by Oligocene rhyolites in the center (c. 39 – 33 Ma), and primarily Oligocene rhyolites in the southwest (c. 34 – 30 Ma). A 39.6 Ma ignimbrite overlies Mesozoic metasedimentary basement at the northeastern end of the transect, approximately 20 km west of Hidalgo del Parral. A 29.5 Ma ignimbrite overlies a Cretaceous ignimbrite (67.0 Ma) at the western end of the transect at Guadalupe y Calvo. Southwestward younging of the volcanic stratigraphy agrees with data from elsewhere in the SMO, and all are inferred to record southwestward slab-rollback of the Farallon slab and arc migration during the waning stages of Sierra Madre Occidental volcanism. The non-uniform rate of Farallon slab retreat was approximated using available age and spatial data. These rates were 15.1 km/Ma through the northern sector, 11.7 km/Ma through the central sector, and 13.7 km/Ma through the southern sector. These values are in close agreement with reported rates of arc-trench retreat in modern arcs. In addition, I used these geochronological data to constrain the longevity of these silicic magmas to ≤2.2 Myrs and to attempt to correlate between ignimbrites. One correlation between three deposits is observed: the informally-identified 38.5 Ma, ≥26 km3 Ojito tuff. Contents List of Tables .................................................................................................................................. 4 List of Figures ................................................................................................................................. 5 Highlights ...................................................................................................................................... 10 1. Introduction ............................................................................................................................... 11 1.1 Geological history of the Sierra Madre Occidental and adjacent areas .............................. 11 1.1.1 The Proterozoic basement and accreted terranes .......................................................... 11 1.1.2 The Laramide magmatic belt and the Lower Volcanic Complex (LVC) ..................... 13 1.1.3 The Upper Volcanic Supergroup (UVS) and Southern Cordillera Basaltic Andesite (SCORBA)............................................................................................................................. 17 1.1.4 Cenozoic extensional domains in northern Mexico ..................................................... 19 1.2 Project Motivation ............................................................................................................... 19 2. Methods..................................................................................................................................... 23 2.1 Sample Collection and Preparation ..................................................................................... 23 2.2 Zircon and U-Pb Geochronology ........................................................................................ 23 2.2.1 Sample Preparation ....................................................................................................... 31 2.2.2 Analytical Methods....................................................................................................... 31 2.2.3 Statistical Methodology ................................................................................................ 35 2.3 40Ar/39Ar Geochronology .................................................................................................... 37 2.3.1 Sample Preparation ....................................................................................................... 37 2.3.2 Analytical Methods......................................................................................................