FAULT DATING – HISTORY AND EVOLUTION

Fault dating was first attempted almost 25 years ago after the initial development and application of Illite Age Analysis (IAA) as part of a low temperature thermochronometer in sedimentary rocks. IAA was adapted in 1991 at Exxon to a new technical business problem – how to identify traps in the Canadian Rockies that formed after gas generation had ceased. Although the K/Ar dating method available to us at the time limited the precision of our analyses, we recognized distinctive mineralogies, mineral forms, and age distributions in fault samples compared with diagenetically evolved samples. Our early attempts to evaluate method accuracy were limited by the difficulty in deriving independent, stratigraphic age constraints.

In the ensuing years, Ben van der Pluijm and his students and collaborators have pursued numerous studies that have enlarged on these initial topics and developed new ones. The first and most important development is the introduction of the encapsulated 40/39Ar dating method which produced large improvements in age precision and sample requirements. Furthermore, many studies repeatedly tested and confirmed the premise that clay mineral reactions are directly related to the deformation history, although the reasons why ages sometimes reflect early or later parts of the displacement history requires further work. Various studies have documented these clay mineral transformations both as prograde reactions in sedimentary sequences and retrograde reactions in crystalline igneous and metamorphic rocks. These observations have led to ancillary studies that consider the effect of these reactions in light of the mechanical properties of faults.

Fault dating has been applied to all fault types but has found the greatest application in thrust faults, perhaps because independent stratigraphic age constraints are eroded in this setting. These studies have led to tectonic insights into punctuated versus continuous deformation and the evolution of fold-and-thrust belts. Lastly, an underappreciated aspect of this work is that the work in the Canadian Rockies and Moab Fault represent rare instances of independently reproduced results, a satisfying application of the scientific method.