AR DATING OF CLAY GOUGE IN SHALLOW CRUSTAL FAULTS; CANADIAN ROCKIES

Major advances in understanding crustal evolution have been made through the application of modern geochronology to fault zones. Direct dating of ductile shear zones and rates of uplift/exhumation have become increasingly available through the application of U/Pb, Ar mineral dating, and other isotopic techniques. However, radiometric dating of shallow crustal faulting, which occurs in the crust’s brittle regime, has remained a major challenge, because the low temperatures typical of these faults prevent complete syntectonic recrystallization achieved in deeper faults. Many shallow fault zones preserve both primary (detrital) and newly-grown (authigenic), fine-grained phyllosilicates. Radiometric ages of these fault zones reflect a mixture of both mineral populations, whereas we are interested in the age of the newly-grown phase formed as a result of faulting. Rather than (erroneously) assuming that little or no detrital material is left in very fine grain size fractions, we analyze the variable ratios of authigenic and detrital mica in different clay size fractions using quantitative X-ray analysis and modeling of spectra. The detrital mica component is characterized by 2M1 polytype, whereas the authigenic form is 1M/1Md polytype (typically mixed-layer illite/smectite, I/S). A second problem associated with dating clay minerals involves the loss of 39Ar during neutron irradiation, which produces erroneously old ages. In the vacuum-encapsulation methods, the sample is in a fused silica vial that is evacuated to high vacuum and sealed. The capsule is then irradiated and any recoiled 39Ar is trapped within the capsule. If the whole capsule is fused the measurement is functionally equivalent to a K-Ar analysis. Using an integrated mineralogic/geochronologic method on clay-bearing samples we can devise a reliable Ar dating approach for near-surface faulting, as illustrated by study of foreland thrusts in the southern Canadian Rockies. In addition to timing of fault activity, the method provides an estimate of the uplift/exhumation age of detrital micas in the source region.