ILLITE AR-AR THERMOCHRONOLOGY APPLIED TO THE TIME OF FORMATION OF THE FRONT RANGE MONOCLINE, COLORADO SPRINGS, CO
Illite Age Analysis, which considers the total gas ages and illite polytype quantification (% of 2M1illite, presumed as the detrital illite component) of at least three grain-size fractions of clay particles and York regression analysis, allowed estimation of age of authigenic illite in each sample as follows: 512HIF=57.5±5, 6132-Kn6=55.7±1.6 and preliminary results from 612BC2 indicate authigenic illite in this sample is older than 120 Ma. The closeness in age obtained in samples 512HIF and 612Kn6 indicates that both shear-parallel to bedding and fault activity occurred about the same time, presumably during the FRM steep limb rotation.
The older age of authigenic illite in the sub horizontal upper limb implies that shear occurred prior to the Laramide Orogeny and the limb was inactive during FRM development. The most insightful finding through this exercise was the correspondence in timing of monocline formation (58±4Ma) with the Paleocene-Eocene Thermal Maximum at ca. 55 Ma, and event that was marked by changes in precipitation, weathering and massive sediment mobilization that could cause gravitational unloading and potentially trigger bedrock uplift and fault activation.