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°. 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° 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° 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 σ 1 is nearly horizontal and normal to the fold axis, σ 2 is parallel to the fold axis, and σ 3 is very steep. This is the stress field normally associated with thrust faulting.

Abstract Paleoproterozoic supracrustal rocks in the region near Big Thompson Canyon, northern Colorado, have long been recognized as a spectacularly exposed example of regionally zoned metamorphism, preserving an apparently complete sequence from biotite- to migmatite-zones. Due to its location and relatively easy access, the Big Thompson Metamorphic Suite has also provided a valuable field-based educational experience for universities and colleges all along the Front Range and from elsewhere. In addition to a number of other studies, the pioneering work of William Braddock and graduate students from the University of Colorado resulted in more than a dozen M.Sc. and Ph.D. theses from the 1960s to the 1990s. Despite the volume of ground-breaking science conducted on these rocks in the past, there remain a number of fundamental questions regarding the metamorphic history and overall tectonic significance of many of the observable features. Several lines of evidence suggest there is potential for a complex tectonometamorphic history that likely spans from ~1.8 to 1.4 Ga. These include: thermochronologic and geochronologic data supporting multiple thermal and magmatic episodes, structural evidence for multiple deformation events, multiple generations of typical Barrovian minerals (e.g., staurolite), and the widespread occurrence of minerals not commonly associated with a classic Barrovian sequence (e.g., andalusite, cordierite). One purpose of this fieldtrip is to foster new ideas and stimulate new research directions that will utilize the Big Thompson Metamorphic Suite, and the Colorado Rockies in general, as field laboratories for better understanding fundamental orogenic processes.