Collaborative Research: Paleogeographic Record of Contractional to Extensional Tectonics in the Cordilleran Hinterland, Nevada

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Orogen topography - elevation and relief - is a critical component in geodynamic models of the growth, evolution, and collapse of Earth's major mountain belts such as the Himalayas, Andes, and North American Cordillera. Although estimates of surface uplift are common, we lack precise records of past orogen topography: quantified changes in elevations, exhumation patterns, and drainage system evolution. The Cordilleran hinterland from Nevada to western Utah is interpreted as a Paleogene orogenic plateau, supported by compressional boundary forces, with mean elevations of 3-4 kilometers prior to Neogene extensional collapse. The Paleogene transition to extensional tectonics, however, and the presumed dynamic crustal response to flat slab removal remain poorly understood, particularly in portions of the hinterland overprinted by Neogene extension. Eocene fluvial and lacustrine sedimentary rocks in eastern Nevada - near the proposed paleo-divide - interbedded with volcanic units, span the time of this tectonic transition. Preliminary data show that these rocks provide crucial insights into the tectonic and surface processes during the transition from flat-slab subduction and contraction to extensional tectonics. This multi-disciplinary study will reconstruct the topography and morphology of the region, constrain the timing and magnitude of initial extension from ~50 to 30 million years ago, and build a tectonic model for the crust-mantle dynamics of the transition to extension. This includes the following: (1) fluvial and lacustrine basin sedimentology and stratigraphy to reconstruct drainages and basin morphology, (2) Argon geochronology of interbedded tuffs to reveal depositional history, sediment accumulation rates, and changes in deposition style over time; (3) stable isotope analyses of hydrated volcanic glasses and lacustrine carbonates to constrain paleoelevations and lake water chemistry over time, (4) detrital zircon U-Pb geochronology to reconstruct the fluvial drainage network and identify sediment provenance patterns, and (5) (U-Th)/He double dating of detrital zircon grains to pinpoint sources of similar crystallization age and quantify exhumation rates. The integration of multiple disciplines to quantify geodynamics is at the forefront of tectonics research. The proposed research will differentiate between proposed mechanisms for basin formation, so as to create a reproducible tectonic model of the collapse of the Cordilleran hinterland by tracking deep mantle processes through the surface record,. Our findings will help quantify the crustal response to heating, destabilization, and delamination in thrust belt hinterland regions, and have the potential to document a new mechanism for walled basin formation and basin hydrology on orogenic plateaus. Our final model of the surface expression of orogenic plateau collapse will improve our understanding of the crustal and mantle dynamics that drive the evolution and eventual degradation of regions of high elevation worldwide.Approximately 45 million years ago, the state of Nevada resembled the Andes of western South America, the Earth's second highest mountain range. Since then, this high area has collapsed and extended into a series of smaller ranges separated by low elevation basins. During the initial phases of this collapse, a large lake (or series of smaller lakes) formed in what is now a very dry desert, similar to Lake Titicaca in Bolivia and Peru. The deposits of this lake and the rivers that flowed into it contain important clues about the evolution of the area. This project is an interdisciplinary study of how and why this ancient mountain range was destroyed, and how this affected the climate and environments of the region. Knowing past topography and geography is critical to understanding: (1) the construction, evolution, and collapse of mountains through plate tectonic movements, (2) the effect of changing topography on climate, precipitation, and surface/ground water transport, (3) the weathering, erosion, and shaping of Earth's surface, (4) the relationship between extension and the occurrence of super-volcanoes, and (5) the formation and development of economically-important oil, gas, and gold deposits. We will study layered sedimentary deposits (strata) that accumulated in Nevada during the initial phase of mountain collapse using several cutting-edge physical and chemical techniques. In the field, we will measure the thickness and composition of lake strata, which can tell us whether the lake was deep or shallow and salty or freshwater. We will also collect multiple volcanic ash beds that accumulated in the lake, and use crystals within those beds to determine high-precision age determines by measuring the radioactive decay of potassium within the crystal. Using water trapped within glass shards in these volcanic ash beds, we will determine the isotopic composition of ancient precipitation in order to estimate past elevations of the region. Finally, we will collect sandstone that was deposited by ancient rivers, and separate zircon crystals from them for a number of analyses. By measuring the respective amounts of Uranium, Thorium, and Lead from zircon mineral grains, we will determine the age of individual grain and when it was eroded from the rock they formed in. Combining all of the techniques outlined above, we will be able to answer the following questions: (1) What was the size and extent of this ancient lake and the corresponding drainage system? (2) What types of rocks were exposed and eroding at the surface 30-50 million years ago, and how quickly did they erode? (3) What was the past topography and relief of Nevada? and (4) When and how quickly did Nevada extend into the isolated desert basins and ranges that exist today? In addition to the research objectives of this project, the award is contributing to support of two early career researchers; broadening of participation of underrepresented groups in an STEM discipline; involvement of graduate and undergraduates in research; contributions to research infrastructure; and contributions to geologic mapping and quantification of past extension, which is critical to understanding mineralization trends across the region, the formation of gold deposits, and the occurrence of geohazards such as earthquakes and volcanoes.