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 anal