4D quantification of stress and strain tensors at Sheep Mountain Anticline (Wyoming, USA) using calcite twin analysis

Abstract

We use the calcite twin analysis to investigate the relationship between fold development, stress and strain distribution. We chose for this study the Sheep Mountain Anticline (Wyoming, USA) as a natural laboratory. Because it's asymmetric and basement-cored fold, this anticline was formed during the Laramide orogeny in the Early Tertiary.The calcite twin have been measured in folded Lower Carboniferous to Permian age carbonates and sandstones. Calcite twin recorded both in the matrix and in the veins, highlight three different tectonic stages: the first phase is a pre-folding compression parallel to fold axis, a second one is also pre- folding compression but it's perpendicular to the future fold axis and the third stage is also perpendicular to the fold axis but it's a post-folding compression. Furthermore, calcite twin data provide information about the evolution of stress (Etchecopar's method) and strain (Groshong's method) through time and space. Both pre- folding and post-folding NE-directed compressional stress and/or shortening were recorded within pre-folding veins (set I) as well as in fold-related veins (sets II and III). Set III veins also recorded outer rim extension along the fold hinge line.Besides, calcite twin analysis allow us to quantify stress and strain. Our results point out both temporal and spatial evolution of stress and strain tensors. Spatially, we notice that both strain and particularly differential-stress in the backlimb are higher than in the forelimb. We are also able to show that differential-stress drops both in the backlimb and in the forelimb between pre-folding and post-folding stages. Our new dataset should putting better constrains on numerical models in order to increase our knowledge on fold mechanics.