Comparisons of geometric relations, microtextures, and the nature of fluid-rock interaction for deformed basement rocks in parts of the Rocky Mountain foreland, Wyoming, and the Sevier orogenic belt, northern Utah, reveal regional variations in structural style. Basement within the Wind River Range, Wyoming, is deformed by large-scale reverse faults and by intervening major brittle deformation zones (BDZs). Slip along faults and BDZs produced 30% regional shortening by bulk pure shear. Basement away from BDZs is cut by widely spaced fractures, but fracture intensity increases toward BDZs, and complex fracture networks occur along BDZ margins. BDZs contain anastomosing zones of breccia...