This project is designed to provide a much improved understanding and characterization of the process of fission-track annealing in the mineral apatite. Fission tracks form when uranium decays by spontaneous fission, and the damage caused by the fissioning particles anneals (is repaired) at a rate determined by the ambient temperature. Fission-track analysis has thus proven to be a powerful tool for determining the thermal history of rock bodies, and is widely used for tectonic studies, landscape evolution, and petroleum exploration, among other fields. The advances created by this research will be used to discern the nature of faulting that caused the final stages of uplift of mountain ranges in southern and west-central Arizona.
Although apatite fission-track thermochronology is a well-established and robust technique, our understanding of how annealing occurs, and how it is affected by apatite composition, remains extremely vague. Experimental studies have shown that impurities such as chlorine, iron and manganese can increase temperatures required for annealing by several tens of degrees, but the experimental database is currently far too sparse to determine how impurities work individually and/or in concert to bring about these changes. This study will vastly expand the annealing database by utilizing an efficient procedure to determine the relative annealing properties of 30 apatites with widely varying compositions and unit cell dimensions. These data will provide insight not only into compositional effects, but into the annealing mechanism itself. The resulting new model of fission-track annealing will be used to examine the relative roles of high-angle and low-angle normal faulting in the final stages of core-complex exhumation in the Arizona Basin and Range. Recent work has suggested that low-angle faulting may have accelerated to near-plate-tectonic rates. However, the data may also be explained by a transition from low-angle to high-angle faulting. These two scenarios make different predictions about how cooling rates vary in time and space depending on proximity to faults. The improvements in the fission-track method will be used to test these predictions, either confirming a dramatic transition in movement rates possibly caused by the migration of the slab window off of western North America, or illuminating how the nature of unroofing can change during a continuous episode of exhumation.
