Collaborative Research: Documenting the transition from contraction to extension in the Ruby-East Humboldt-Wood Hills Metamorphic Core Complex, Southwestern U.S. Cordillera

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The mountainous western U.S., commonly referred to as the Cordillera, has experienced near constant geologic deformation and reorganization over the past few hundred million years. Large mountain ranges have been created and destroyed, volcanism has flared up and died away, and an enormous section of the west, basically from Salt Lake City, Utah to Reno, Nevada, has roughly doubled in size due to extreme stretching and extension of Earth's upper crust. This extended region, the Basin and Range province, has been the focus of decades of research and exploration by geoscientists and prospectors, and now hosts some of the largest mineral deposits in North America. Despite years of investigation using a wide range of methods, fundamental questions persist about the mechanisms that drove extension. Central to these debates is the timing and rate of deformation. Fundamentally, evidence from different depths of the crust yield differing interpretations, with evidence from the deeper earth documenting older, Cretaceous (approximately 70 million years ago) deformation, whereas studies of the shallower crust highlight later, Miocene (approximately 15 million years ago) processes. This study proposes to bridge the gap between these two data sets and reconcile these seemingly disparate histories by focusing on an exceptionally well-exposed section of crust in the Ruby Mountains - East Humboldt Range - Wood Hills Metamorphic Core Complex, an area of northeastern Nevada that is also of great economic interest due to its situation in the heart of North America's richest gold-producing province. The rocks that make up these ranges represent a thick column of the crust that was brought to the surface and tilted on its side during extension, and offer an unprecedented view into the geologic processes operating across multiple crustal levels. A suite of relatively new isotopic dating techniques will document the time - temperature history of the rocks and thus the full history of tectonic uplift. The research conducted during this study will provide important societal outcomes through training of undergraduate students in an important STEM (science, technology, engineering, and mathematics) discipline, thus contributing to development of a globally competitive workforce. The project will also contribute to increased scientific literacy and public engagement with STEM. The project embeds substantial outreach and educational components. The world-class geology of the study area provides an ideal stage for direct community engagement through strategic and comprehensive outreach to local science educators and geoscience professionals. By forging an innovative collaboration between two R1 Research institutions and Great Basin College--the institution primarily responsible for training science educators across most of rural Nevada. The principal investigator's educational outreach will focus on three main goals; (1) creating and making widely available interactive field trip guides and tutorials focusing on both the research as well as numerous fundamental geoscience concepts that can be demonstrated in the proposed research area; (2) creating permanent and mobile displays for Great Basin College campuses across the state of Nevada; and (3) creating learning modules and tutorials for learners at a variety of levels, including dual-credit high school students enrolled in geology courses, traditional college students, and education students preparing to become K-12 educators. Outreach materials will highlight the central role that northeastern Nevada's tectonic history plays in its citizens' physical and economic well-being. Technical results of the research will be widely disseminated through presentations at professional geoscience meetings and the peer-reviewed scientific literature. One of the most dramatic shifts in the tectonic architecture of North American in the Phanerozoic is the Late Cretaceous to Cenozoic transition of the Cordillera from large-scale shortening and crustal thickening to widespread regional extensional collapse. While certain aspects of this geologic history are well understood, critical questions remain unresolved regarding the distribution, rates, style, and timing of the early extensional evolution. Key to addressing these crucial questions are the metamorphic core complexes--regions where extreme crustal extension has exposed thick crustal sections with protracted tectonic histories. However, each of the classic metamorphic core complexes of the northeastern Great Basin yield fundamentally disparate interpretations of the timing and tectonic significance of exhumation depending on whether the data derive from deeper or shallower structural levels. In each case, lines of evidence drawn primarily from higher temperature thermochronometry, integrated Pressure-Temperature-time paths, and structural analyses of mid- to deep-crustal rocks suggest older, more protracted and often more complex exhumational histories; in contrast, low-temperature thermochronometry and syntectonic sedimentation commonly record a simpler and more youthful record of widespread Miocene extensional unroofing. This study will directly address these problems and bridge the current gap in understanding by using low- and medium-temperature (Uranium-Thorium)/Helium and 40Argon/39Argon thermochronology within a detailed and well-understood structural framework to constrain a complete cooling and exhumational history for northeastern Nevada's Ruby Mountains - East Humboldt Range - Wood Hills metamorphic core complexes. By integrating a range of thermochronometers and field relationships this study will reconcile disparate exhumational histories derived from disconnected investigations of deep-crustal and upper-crustal processes. In doing so, it will directly test competing hypotheses for the onset, duration, and driving forces of Basin and Range extension and exhumation. Specifically, this proposal will address three key questions: (1) when did extension initiate in the Ruby Mountains - East Humboldt Range - Wood Hills metamorphic core complexes, (2) how did the crustal geotherm evolve during the Cretaceous to present, and (3) if early phases of extension occurred, why did they fail to produce a syntectonic stratigraphic record?