Fault Dating in Rosendale, New York Using Clay Polytype Quantification

The techniques of fault dating, though still underdeveloped on a wide scale, have been increasingly studied over the past decade. This study draws on these recent observations in relative literature and relates them to a fault in Rosendale, New York, the location of significant tectonic events during the mid to late Paleozoic. Direct dating of clay-rich brittle fault gouges involves two components: clay polytype quantification and 40Ar/39Ar thermochronological dating. This study will focus on the polytype quantification aspect of the dating process. Polytypes are variations in mineral structure which can be related to the conditions of formation of the clay mineral. The two polytypes of illite formed in a fault are 2M1, reflecting the detrital material from the wall rock of the fault, and 1M/1Md which are authigenic clays that form during movement on the fault. By splitting the fault gouge sample into three grain size fractions and determining the percentages of each polytype in each fraction, we can project the age of the fault. We used X-ray diffraction analysis to measure polytype peak intensities and WILDFIRE© modeling software to determine percentages. Our results showed that the largest size fraction's comparative polytype percentage was 33.33% 1M and 66.66% 2M1, the intermediate size fraction's comparative percentage was 50% 1M and 50% 2M1, and the smallest size fraction's comparative percentage was 77.77% 1M and 22.22% 2M1. The percentages of the three size fractions can be graphed against the 40Ar/39Ar age and extrapolated until pure authigenic age is determined. This step will be carried out by our colleagues at the University of Michigan.