RADIOMETRIC DATING OF BRITTLE FAULTS – ADVANCES AND APPLICATIONS TO YOUNG FAULTS

Until recently, the direct dating of brittle faults has been a major obstacle in understanding the kinematic and temporal evolution of recently active areas. Our first efforts of fault dating work focused on Sevier-age thrust faults, but we are now able to extend this approach to the dating of considerably younger faults in Cordilleran metamorphic core complexes..

We date authigenic illite in fault gouge by the Ar-Ar method after identifying and quantifying the percentage of authigenic and detrital illite in various size fractions of gouge by iterative quantification of XRD patterns. Percentages of the 2M1 polytype (detrital) and 1Md polytype (authigenic) illite are quantified using WILDFIRE calculated XRD patterns Accuracy of the approach, reducing the age uncertainty, was significantly improved by calibrating calculated patterns with mixtures of known composition, revealing that a strict lowest-variance approach to determining the ‘best match' can yield anomalously authigenic-rich compositions.

Illite Age Analysis on gouge has assumed the gouge to be a binary mixture of authigenic and detrital illite, but an independent check on the age of the detrital component has frequently proved elusive. New work on gouge from the Sierra Mazatán metamorphic core complex in NW Mexico, in conjunction with robust K-feldspar Ar-Ar ages (Wong & Gans, 2003) demonstrates the validity of our approach. The 18.4 Ma age of the 2M1 (detrital) component derived from illite age analysis of fault gouge compares remarkably well with the cooling history obtained from K-feldspar spectra. Moreover, the age of the 1Md (authigenic) component (14.9 Ma) matches well with independent age constraints from field relationships for the cessation of fault movement. This narrow range in ages of the detrital and authigenic components of matches well with independent results, indicating that a binary mixing model of detrital and authigenic illite is valid for natural gouges and that Miocene faults can also be reliably dated with Illite Age Analysis.