Date of this Version
Hatfield, M.H., 2018, Quantifying the distribution and mechanisms accommodating penetrative strain along the western margin of the Laramide Denver-Julesburg Basin [Masters thesis]: University of Nebraska-Lincoln, 105 p.
Grain-scale volume loss is an important process in the deformation of a fold-thrust belt. Prior to activation of slip on fault surfaces, and initiation of layer buckling in folds, instability is recorded by thickening of parallel bedding surfaces, grain boundary migration, and stress-induced solution transfer of mineral grains. Volume loss recorded by these mechanisms is significant but is an afterthought in any type of cross-section restoration.
This research has three main objectives. Firstly, to estimate the amount of penetrative strain (PS) accommodated in weakly deformed sandstones along four E-W transects, from the thick-skinned Colorado Front Range into the Denver-Julesburg Basin. The methods employed to accomplish this were petrographic analysis, analog modeling, and cross-section restoration. Next, the distribution and mechanisms by which PS is accommodated in thick-skinned versus thin-skinned deformation belts were compared. Analog models simulating Laramide deformation helped to gain insight into strain partitioning by mimicking the change in crustal architecture from the Sevier belt into Laramide foreland basins. Finally, the study measured how changes in PS correlate with changes in physical parameters such as porosity and rebound strength. Additional factors, such as distance from major fault structures, were considered.
Results from samples within the Denver-Julesburg Basin record PS between 8% and 12%. An early hypothesis suggested PS would correlate strongly with porosity and rebound strength and would decay with increasing distance from faults. This research indicates a weak correlation between these parameters when all samples are incorporated into the analysis. A robust correlation is noted when values for these parameters are averaged by formation or transect.
PS values are limited by saturation of pressure solution shortening from grain impingement alone, while porosity values fluctuate with surface weathering. Analog models record decreased PS with increasing depth in the sedimentary section due to the presence of a rigid basement. Finally, cross-section restorations along Horsetooth Reservoir, the northernmost transect in the field area, and Rt. 34, near Loveland, record tectonic shortening values of 10% and 12.2%. Integration of tectonic shortening values, and PS values derived from analog models, estimate 0.79 and 0.36 km of PS related volume loss across the transects.
Advisor: Caroline M. Burberry