Radiometric Dating of the Rocky Mountain Foreland Thrust Belt

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Whereas direct dating of ductile shear zones is increasingly available through theapplication of U/Pb, Ar mineral dating and other isotopic techniques, radiometric datingof shallow faulting in the crust's brittle regime has remained difficult, because the lowtemperatures typical of these faults prevent complete syntectonic recrystallization orresetting achieved in deeper faults. Clay-bearing fault rocks are common in many faultzones, but these rocks typically preserve both primary (detrital) and newly-grown(authigenic), fine-grained phyllosilicates. Radiometric ages of these fault zonestherefore reflect a mixture of both mineral populations, while we are only interested inthe age of the newly-grown phase to date faulting. A second problem associated withdating clay minerals involves the loss of 39Ar during neutron irradiation, which canproduce erroneously old ages. Using characterization of the detrital clay fraction insamples using X-ray modeling of grain size populations and sample encapsulation fordating, the PI's have developed and tested a reliable geochronologic method to date near-surface faulting. The feasibility of their approach was illustrated by dating ~52 Ma fault gouge from the most frontal thrust (Lewis Thrust) of the southern Canadian Rockies. The Pi's propose a application of their dating method to a geologically important region of western North America, the Rocky Mountains, that has both regional tectonic significance as well as fundamental thrust belt implications. Preliminary results from other faults demonstrate the feasibility of the project and indicate several exciting implications for regional geology and basic structural geology. The proposed research will be conducted by two PIs who combine the needed structural, mineralogic and isotopic expertise, and a PhD student who will complete the work as part of his dissertation. The X-ray, electron microscopy and Ar facilities are all available at the University of Michigan.