VARIATION IN PENETRATIVE STRAIN DISTRIBUTION WITH FOLDING ALGORITHM: A CASE STUDY FROM RATTLESNAKE MOUNTAIN, WY

Penetrative strain, or distributed deformation and volume loss, occurs as beds are folded, despite the common assumption that area is preserved in a 2D section during deformation. However, the magnitude and distribution of the penetrative strain component is poorly constrained and can be shown to vary with algorithm chosen to unfold or restore the section. This study takes a published cross-section across the basement-cored Rattlesnake Anticline in Wyoming and restores it using three different publically available algorithms (line length unfold, flexural slip and simple shear). The results are then compared to the results of a scaled analog model of a Laramide-style uplift made using potters clay and sand, in which the cumulative amount of penetrative strain can be tracked accurately because the initial conditions are precisely known, boundary conditions are carefully scaled and tectonic and penetrative deformation can be deconvolved.

Results from the analog model indicate that penetrative strain increases with depth in the sand pack, and then decreases again in layers which have been cut by faults; consistent with the typical trishear model for Laramide deformation. Unfolding the published cross-section with the line length unfold algorithm results in constant penetrative strain in the folded cover, and significantly less penetrative strain in the faulted layers. Unfolding using the flexural slip algorithm results in an increase in penetrative strain with depth in the folded cover, and again significantly less penetrative strain in the faulted layers. Using the simple shear algorithm results in variation in strain along the length of the beds, not necessarily with depth, and varies with the shear angle chosen. The flexural slip results most closely match the results of the analog model, and are therefore taken to most closely match the natural situation. Thus, for Laramide-style structures, unfolding using the flexural slip algorithm for folded cover accurately replicates the distribution of penetrative strain in the system.