Collaborative Research: Utilizing Cooling Histories to Determine the Sequence and Rates of Thrusting

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This proposal is aimed at understanding the structural, tectonic, and exhumation of a part of the Himalayan orogen in central and western Nepal. The principal investigators are investigating how fault magnitude, geometry and rate are related to uplift and exhumation in convergent margin plate tectonic systems where two continents are colliding. This research will allow the principal investigators to constrain both magnitude and age of faulting and gain insight into the geometry of these faults along a fundamental plate boundary between India and Asia in the Nepalese Himalaya, which will provide fundamental insights into how convergent plate margins evolve through time. This region is the site of large, destructive earthquakes that are a function of both geometry of the faults and the rates at which these faults move; thus, the research has the potential to further understanding in a region of active seismicity. This proposal combines thermochronometry (the age at which minerals cool) with numerical modeling to determine fault magnitude, geometry and rate. Geologic mapping and associated cross sections provide an estimate of fault geometry. This geometry provides a testable path along which rocks were displaced and cooled from the subsurface to the Earth's surface. The principal investigators will test the hypothesis that the geometry of faults controls the first-order pattern of cooling ages that are recorded in rocks, which can then be tested by comparing measured cooling ages to modeled cooling ages using different fault geometries. This approach will provide workflows, methodologies, and examples of how fault geometries, as determined from geologic cross sections, and rates impact predicted cooling ages. In addition to the scientific objectives of the study, the project is contributing to the national well-being and other socially relevant outcomes by providing for training of graduate and undergraduate students in an important STEM discipline, as well a contributing to the broadening of underrepresented groups in the earth sciences. The project it is contributing to the development of research infrastructure at two U.S. university systems, and is promoting international collaboration between U.S., German, and Nepalese scientists. Results from this research will be incorporated into classroom curricula. As part of this project, the principal investigators are developing a series of research and teaching modules that will provide interested graduate students and researchers from other institutions the skills needed to use this research approach for their own field areas and datasets, and allow educators to assign advanced undergraduates and graduate students assignments that teach the systematics of how compressional fault systems form and the relationships between deformation, erosion and deposition. The results of the research will be disseminated through peer-reviewed scientific publications literature and by presentations at professional society meetings; data obtained from the project will be archived in appropriate community supported data repositories. Combining thermochronometry with numerical modeling has an enormous potential to quantify the rates, magnitudes, and timing of deformation and erosion in active, contractional plate tectonic settings. However, the interpretations of thermochronometric data are critically dependent on determining the correct thermal, kinematic, and erosion models. Understanding how fault magnitude, geometry and rate are related to exhumation in compressional systems requires quantitatively linking the geometry and magnitude of fault slip to the distribution and amount of erosion. In this project, the principal investigators suggest that the geometry of fold-thrust belts is best delineated through balanced geologic cross-sections, and they hypothesize that the geometry of a fold-thrust belt, particularly the location and magnitude of ramps in the decollement, control the first-order pattern of cooling ages. To address this hypothesis they will apply a 2 dimensional thermo-kinematic and erosion model to forward modeled balanced cross sections to quantify the cooling history in a thrust belt setting. Balanced cross-sections provide the kinematic sequence of rocks and structures necessary to reproduce the mapped surface geology. The principal investigators will test the validity of this kinematic sequence by assigning ages over which displacement occurs, and use the range of potential velocity vectors to calculate heat transport, erosion, and rock cooling. Matching the measured cooling histories recorded by a suite of thermochronometers to that predicted by the kinematics of a balanced cross-section is an additional support for the validation of the cross section. This work will examine proposed structural geometries, the cooling ages of rocks and their interdependence in central Nepal and far western Nepal. In central Nepal, there is an abundance of geochronologic/thermochronologic data and recent maps and cross section interpretations. In far western Nepal, there are detailed maps and cross section interpretations with a growing body of geochronologic/thermochronologic data. The researchers will capitalize on these established and growing datasets to test our hypothesis by 1) evaluating a range of permissible geometries for balanced-sections (both prior to and following field work), 2) collecting critical field observations (bedding, foliation, cleavage) and necessary thermochronologic samples, 3) establishing the cooling history of duplexes and thrust sheets via a suite of thermochronometers, and 4) modeling the dependence of chronometers on structural geometries, displacement paths and rates. The intellectual merit of this project involves investigating the sensitivity and utility of thermochronometer and kinematic data for calculating the age and rate of thrust motion and associated exhumation. Using this approach in Nepal, the principal investigators can evaluate the range of shortening rates in space and time and determine if long-term shortening rates though the Himalaya are constant or variable. These methods/processes will be transportable to other contractional systems worldwide and will lead to a reevaluation of the kinematics, geometry and rates in orogenic systems.