The dating of shallow faults in the Earth's crust

Direct dating of ductile shear zones and calculation of uplift/exhumation r ates can be done using various radiometric dating techniques. But radiometr ic dating of shallow crustal faulting, which occurs in the crust's brittle regime, has remained difficult(1-4) because the low temperatures typical of shallow crusted faults prevent the complete syntectonic mineral recrystall ization that occurs in deeper faults. Both old (detrital) and newly grown ( authigenic) fine-grained phyllosilicates are thus preserved in shallow faul t zones and therefore their radiometric ages reflect a mixture of both mine ral populations. Also, the loss of Ar-39 during neutron irradiation in dati ng of clay minerals can produce erroneously old ages. Here we present a met hod of characterizing the clay populations in fault gouge, using X-ray mode lling, combined with sample encapsulation, and show how it can be used to d ate near-surface fault activity reliably. We examine fault gouge from the L ewis thrust of the southern Canadian Rockies, which we determine to be simi lar to 52 Myr old. This result requires the western North America stress re gime to have changed from contraction to extension in only a few million ye ars during the Eocene. We also estimate the uplift/exhumation age and sedim entary source of these rocks to be similar to 172 Myr.