Evolution of Cordilleran Lithosphere: Transition From Mesozoic Shortening to Cenozoic Extension, East-Central Nevada

Stanford University, Stanford CA

Investigators

Abstract

The geologic history of the western U. S. Cordilleran is complex and an increasing number of questions have been raised about its deep crustal architecture. The most debated question is how its geology was shaped during the transition from plate convergence and crustal shortening in the Mesozoic to divergence and extension in the Cenozoic. Across today's Basin and Range Province, this transition period is characterized by shallow-slab subduction and the shut-off of arc magmatism ~ 70-50 m.y. ago followed by one of the biggest volcanic eruption events in its history, ~ 45-20 m.y. ago, setting the stage for Miocene Basin and Range faulting. This research focuses on placing better absolute ages on events occurring during this transition period across the central part of the Basin and Range province (east-central Nevada and adjacent Utah) in order to address current controversies about this history and produce a more holistic whole-crust model for this time-span. A multidisciplinary and comprehensive approach will mesh: 1. A pioneering effort to detail the U-Pb radiometric ages, trace element and isotopic geochemistry of xenocrystic/inherited zircon populations in magmatic rocks that span the transition from shortening to extension, revealing the temperature-time history of the deep crust 2. Structural/ microstructural studies coupled with petrology, geochronology and thermochronology of key rock transects that link inferred deep levels of the crust to near surface levels in the Snake Range metamorphic core complex- a locality where deformation and cooling histories figure prominently in the debate between very thick versus thinner crust and its role in crustal flow and collapse during the transition period. 3. The time-space evolution of sedimentary basins whose history provides a robust, definitive set of constraints for evolving landforms, their relation to volcanic eruptins, and the onset of fault uplift of the deeper rocks of core complexes. This comprehensive approach will allow us to better link deep lithospheric/crustal processes such as magmatism and deformation/flow to surface processes and thus provide data to test a range of ideas and hypotheses about the lithospheric evolution of the Great Basin as it transitioned from shortening to extension. Our geologic knowledge of the Cordillera and Rocky Mountains of the western U.S. forms the foundation for understanding its broad spectrum of natural resources, yet there are many aspects of the history of this mountain belt that are still highly debated and controversial. One of these is our conceptual understanding of fault systems formed during stretching or extension of continental crust (such as is happening today across the Basin and Range province). Despite the importance of such fault systems to the exploration of natural resources such as groundwater and hydrocarbons in continental and offshore basins, there is no consensus on how their geometries evolve through time. We also don't fully understand how surface-breaking extensional faults are linked to processes in the deep earth such as magmatic activity, heating and solid-state ductile flow of the crust, despite the importance of these questions to mineral resource exploration, hydrothermal energy sources, earthquake and volcanic hazards. Our research goals are to provide a more comprehensive data set that provides radiometric dating of events from the deep crust to surface breaking faults, volcanic eruption and sedimentary basins in order to understand the time-constrained links between deep earth processes and surface events during a time span where the Cordillera transitioned from plate tectonic convergence and shortening to divergence and extension from ~ 80 Ma to 20 Ma ago. This project will provide partial support for field and laboratory work of 3 Ph.D. and one M.S. student(s) contributing to professional training of future earth scientists. The project provides leverage to obtain funding for undergraduate research from Stanford. Compilation of geologic map data at 1:62,500 spanning the Great Basin National Park to the Kern Mountains will be made available to the general public. This map will stimulate new research and benefit teaching of geology field classes and field trips in this region.