Radiometric Dating of the Rocky Mountain Foreland Thrust Belt

Regents Of The University Of Michigan - Ann Arbor, Ann Arbor MI

Investigators

Abstract

Whereas direct dating of ductile shear zones is increasingly available through the application of U/Pb, Ar mineral dating and other isotopic techniques, radiometric dating of shallow faulting in the crust's brittle regime has remained difficult, because the low temperatures typical of these faults prevent complete syntectonic recrystallization or resetting achieved in deeper faults. Clay-bearing fault rocks are common in many fault zones, but these rocks typically preserve both primary (detrital) and newly-grown (authigenic), fine-grained phyllosilicates. Radiometric ages of these fault zones therefore reflect a mixture of both mineral populations, while we are only interested in the age of the newly-grown phase to date faulting. A second problem associated with dating clay minerals involves the loss of 39Ar during neutron irradiation, which can produce erroneously old ages. Using characterization of the detrital clay fraction in samples using X-ray modeling of grain size populations and sample encapsulation for dating, 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.