ILLITE AR-AR THERMOCHRONOLOGY APPLIED TO THE TIME OF FORMATION OF THE FRONT RANGE MONOCLINE, COLORADO SPRINGS, CO

The Front Range Monocline (FRM) in Colorado Springs is considered a Laramide tectonic feature formed in Cretaceous-Paleogene times on the basis of stratigraphic constraints, which are based on the age of deposition of syn-tectonic clastic deposits in the foreland. Considering that deformation took place in the shallow upper crust, we took advantage of the of the Ar-Ar illite dating technique, which targets the age of authigenic illite precipitated during faulting/shear of fine grained rocks, to try to improve the resolution of the age of the FRM. For this purpose, we analyzed one fault gouge sample from the Hidden Inn fault (reverse faults affecting the steep limb of the FRM, sample 512HIF), and two shale layers interbedded with limestone (Fort Hayes Fm-sample 612Kn6) or between limestone and arenite (Leadville Fm-sample 612BC2). Significant shear parallel to bedding is evidenced by pervasive cleavage within the shale layers and the presence of lustrous anastomosed surfaces known as nanocoatings. Sample 612Kn6 was collected in the steep limb and sample 612BC2 in the sub-horizontal upper limb of the FRM.

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.