ABSTRACT Rattlesnake Mountain is a Laramide uplift cored by Archean gneiss that formed by offset along two reverse faults with opposing dips, the result being an asymmetric anticline with a drape fold of Cambrian–Cretaceous sediments. Rattlesnake Mountain was uplifted ca. 57 Ma and was a structural buttress that impeded motion of upper-plate blocks of the catastrophic Heart Mountain slide (49.19 Ma). North of Pat O’Hara Mountain anticline, Rattlesnake Mountain anticline has a central graben that formed ca. 52 Ma (U-Pb age on vein calcite in normal faults) into which O- and C-depleted fluids propagated upward with hydrocarbons. The graben is defined by down-dropped Triassic Chugwater shales atop the anticline that facilitated motion of Heart Mountain slide blocks of Paleozoic limestones dolomite (i.e., the Ordovician Bighorn Dolomite and Mississippian Madison Limestone) onto, and over, Rattlesnake Mountain into the Bighorn Basin. Heart Mountain fault gouge was also injected downward into the bounding Rattlesnake Mountain graben normal faults (U-Pb age ca. 48.8 ± 5 Ma), based on O and C isotopes; there is no anisotropy of magnetic susceptibility fabric present. Calcite veins parallel to graben normal faults precipitated from meteoric waters (recorded by O and C isotopes) heated by the uplifting Rattlesnake Mountain anticline and crystallized at 57 °C (fluid inclusions) in the presence of oil. Calcite twinning strain results from graben injectites and calcite veins are different; we also documented a random layer-parallel shortening strain pattern for the Heart Mountain slide blocks in the ramp region ( n = 4; west) and on the land surface ( n = 5; atop Rattlesnake Mountain). We observed an absence of any twinning strain overprint (low negative expected values) in the allochthonous upper-plate blocks and in autochthonous carbonates directly below the Heart Mountain slide surface, again indicating rapid motion including horizontal rotation about vertical axes of the upper-plate Heart Mountain slide blocks during the Eocene.

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.

The geometry of Laramide foreland uplifts near the basement-cover interface provides a critical test of models for basement-involved foreland structures. Limits and controls on basement folding are well defined by deflections of the basement-cover unconformity in the northeastern Front Range of Colorado, Rattlesnake Mountain anticline of northern Wyoming, and northern Teton Range of northwestern Wyoming. Where basement overlies sedimentary strata on a single reverse fault, basement folding is largely limited to the area within 1 km of the fault. In this area, distributed high-angle faulting bends the unconformity in the hanging-wall basement tip up to 20° toward the fault. Folding of the hanging-wall tip increases with progressive fault slip and decreased fault dip. Smaller structures commonly show little basement folding, contradicting the fold then fault sequence predicted by the fold-thrust structural model. The footwall basement is either undeformed or bent downward by hanging-wall loading, with upward folding limited to the fault’s narrow zone of cataclastic flow. Where fault splays break the basement into one or more fault-bounded wedges, overlying strata form more open folds without necessitating penetrative basement folding. The hanging-wall basement commonly shows little folding because the weak basement tip, defined by the main fault and the hanging-wall unconformity, is often left behind in the footwall as a subthrust wedge, filling the gap between the flexed sedimentary strata and the footwall basement block. The downward-narrowing zones of deformation with their variably curved fold surfaces and heterogeneous strain are consistent with fault-propagation folding by distributed shear in a triangular shear zone (trishear). Heterogeneous trishear models predict either extensive basement folding in the hanging wall (footwall-fixed trishear) or no basement folding in the hanging wall (hanging-wall–fixed trishear). These trishear modes form a continuum of basement behavior, with the strength of the hanging-wall basement tip controlling the shear-zone location and the extent of basement deformation. Structures with lower angle thrusts show more basement folding because their basement tips are narrowly tapered and weak, fixing the triangular shear zones with respect to the footwall. Structures with higher angle reverse faults show less basement folding because their basement tips are blunt and stronger, fixing the triangular shear zones with respect to the hanging wall.