ANALOG MODELING OF PENETRATIVE STRAIN AROUND LARAMIDE STRUCTURES AND IN THE LARAMIDE FORELAND

Penetrative strain or distributed deformation is that component of the deformation within a system that is not accommodated by mesoscale folds and thrusts. The magnitude and distribution of penetrative strain within a system is typically unknown and the amounts recorded vary with mechanism measured, with position in the system with respect to the hinterland, and with depth within the sedimentary cover. It is therefore important to have a theoretical framework to understand the magnitude and distribution of penetrative strain in thick-skinned settings. Analog models can give such a theoretical framework because the initial model conditions are constrained, allowing both the tectonic shortening and the shortening due to penetrative strain to be deconvolved. Two series of analog models simulating thick-skinned (Laramide) structures were run in this study. The rigid basement was approximated by potters’ clay and the overlying sedimentary cover by fine grained sand. A preexisting fault was cut into the basement before the model was shortened and the fault was lubricated with gear oil to allow slip to occur. In one model series, the fault was cut with a dip of 60 degrees and successive models were shortened to higher bulk shortening to analyze the progressive accommodation of penetrative strain. In the second model series, the angle of the fault dip was varied, with fault dips ranging from 40 to 80 degrees. Results indicate that penetrative strain decreases with depth in a Laramide setting, but increases in relative importance as deformation progresses. The amount of deformation accommodated in each model varies with the dip of the fault plane. Finally, a zone of deformation featuring significant penetrative strain is observed in the models which stretches into the foreland for a distance equal to the width of the Laramide uplift. The amount of strain in this zone decreases in a linear fashion away from the uplift. Thus, penetrative strain is a crucial component of all Laramide thick-skinned deformation systems and should be taken into account when creating and restoring cross-sections.