Collaborative Research: Dating complex polyphase shear zones within metamorphic core complexes via novel crystallographic vorticity-enhanced apatite U-Pb petrochronology

About This Grant

Studying the history of deformation on ancient faults and shear zones can advance understanding of modern earthquakes, past motions of tectonic plates, mountain building processes, and the evolution of the Earth’s crust. This project will develop and validate a new method for directly dating shear zones. The method involves isotopic dating combined with characterization of deformation within grains of the mineral apatite which is found in many shear zones. This project will focus on shear zones within two case-study locations in the western United States, which expose large extensional shear zones that experienced multiple deformational events over tens of millions of years. Research at these sites will be used to refine the dating technique, learn about crystal-scale deformation, and recover a detailed history of shear-zone activity. The goal of this project is to produce a validated method to directly date rock deformation within shear zones. The new tool will enable Earth Scientists to link the timing of deformation and with information on temperature and rock strength to derive an integrated history of shear zone deformation. The project supports national and societal interests by training two PhD students, advancing the research programs of early- and mid-career faculty, and building a pipeline from community colleges and four-year institutions to engage undergraduate students in STEM through field and laboratory research. This work will test and validate a novel analytical technique of integrating crystallographic vorticity axis (CVA) analyses using electron backscatter diffraction (EBSD) with apatite U-Pb petrochronology to directly date deformation. The method will be developed and validated using samples from the Albion-Raft River-Grouse Creek (UT) and Chemehuevi (CA) Mountains, which expose major extensional shear zones that exhume mid-crustal rocks. Deformation in distinct shear zones at different structural levels is anticipated to yield predictably differing ages and deformation patterns that can be uniquely related to temporally and tectonically disconnected deformational events. Comparing these deformation ages to previous interpretations, low temperature thermochronology, and basin sedimentation records will confirm that the obtained chronology aligns with expectations, thus validating the method. Combining thermometry with this approach will allow the reconstruction of integrated temperature, time, and rheologic history of shear zone deformation. This new method will have broad applications to a wide range of geologic questions. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.