Deformation of basement in basement-involved, compressive structures

Understanding the kinematic development of basement-involved compressive structures that form at low temperature is dependent, in part, on gaining a better factual understanding of their deformational behavior. Results presented here show that the response of crystalline basement to deformation is incongruous among different structures in Colorado and Wyoming. At Big Thompson anticline and Rattlesnake Mountain anticline, Precambrian basement was not rotated in the anticlinal hinge during Tertiary folding. At both Banner Mountain and at a minor fold on Casper Mountain the basement has been rotated near the anticlinal hinge by as much as 26 degrees . In the steep limb of all four of these monoclinal structures the basement is in fault contact with the stratified cover. At two sites, the Five Springs Creek area of the Bighorn Mountains and Casper Mountain, evidence for greater rotation of basement is clear. Precambrian dikes at Five Springs exhibit rotation of as much as 85 degrees across the Laramide anticlinal hinge. Although the anticline at Casper Mountain shows minor folding, in the footwall a depositional contact on basement is rotated 50 degrees from regional dip. These structures (Big Thompson anticline, Casper Mountain, and Five Springs) show minor fractures in the basement that apparently do not control the kinematics of basement deformation, but do indicate the stress field orientation. Typically sigma (sub 1) is nearly horizontal and normal to the fold axis, sigma (sub 2) is parallel to the fold axis, and sigma (sub 3) is very steep. This is the stress field normally associated with thrust faulting.