It is increasingly well recognized that high altitude mountains and continental plateaus, which owe their high elevations to crustal shortening, commonly undergo extension during plate convergence. Despite widespread recognition of this process, the causes of synconvergent extension are currently incompletely understood and remain an outstanding problem in continental tectonics. This study evaluates the processes that led to synconvergent extension in the Cretaceous Sevier orogen in the western United States, with application to other mountain belts. Three frequently cited mechanisms for synconvergent extension are being evaluated: (1) thermal relaxation of tectonically thickened crust, leading to partial melting and decoupling of crust from mantle; (2) localized crustal thickening resulting from duplex faulting leading to supercritical orogenic taper, and (3) the delamination of lithospheric mantle. Each of these processes has predictable effects that are being tested through studies of two localities within the hinterland of the Sevier orogen: the Raft River-Albion-Grouse Creek ranges in northwestern Utah and southern Idaho and the Funeral Mountains in southeastern California. These are ideal areas to evaluate processes related to synconvergent extension. As prior studies indicate, protracted metamorphic and deformational records of Mesozoic orogenesis are well preserved due to minimal or absent thermal overprints and preservation of Mesozoic fabrics between and beneath well-defined Cenozoic extensional shear zones and detachment faults. As part of this research project we are conducting detailed field mapping, structural and kinematic analysis, quantitative metamorphic thermobarometry and PT path determination on garnet, in situ ion microprobe Thorium-Uranium-Lead dating of monazite, Luticium-Hafnium and Samarium-Neodymium garnet geochronology, and Argon-Argon thermochronology. This integrated approach is necessary to (1) determine pressure-temperature-time paths, (2) evaluate the kinematics of early thrust burial and subsequent synconvergent extension, (3) test hypotheses for synconvergent extension by comparing the pressure-tempearture-time paths to those predicted by numerical simulations, and (4) correlate hinterland and foreland evolutions. The multiple geochronologic methods employed in this study provide a basis for refining the calibrations of temperature dependence of the Luticium-Hafnium and Samarium-Neodymium methods of dating garnet. This integrated study represents a research effort between three universities and principal investigors with expertise in structural geology, metamorphic petrology, numerical thermal modeling, isotope geochemistry and geochronology. The proposed work is fully collaborative and integrated at all levels. Information and techniques developed in this research is being disseminated to tectonic, petrologic, and geochemical communities. This project provides research training for four graduate students and four undergraduates, including members of underrepsented groups. The student participants conduct important components of this research as graduate and undergraduate theses, and gain experience in making field observations, data acquisition, and data analysis. They contribute to the dissemination of the findings of this study through presentations at university research expositions and geologic conferences, and contribute to scientific papers in refereed journals. Students benefit from the collaborative nature of the project through exposure to the investigative methods of this study, and the need to understand and integrate data and analysis from collaborators. The research complements two University of Nevada-Las Vegas geology summer field camps in the Marble Peak and Mill Creek areas of the Grouse Creek and Albion Mountains, and contributes to the scientific knowledge base for Death Valley National Park, where the Funeral Mountains are located.
