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.

Abstract The Laramie anorthosite complex (LAC) is the one of the best-studied and most accessible anorthosite complexes in the world. Its components, which range in age from ca. 1432 to1436 Ma, are exposed over ~800 km 2 in the southern Laramie range of southeastern Wyoming. Although the eastern margin of the LAC has been truncated by Laramide faulting, its western and northern margins expose local areas of contact with the adjacent Archean gneiss and supracrustal rocks that were reworked by Paleoproterozoic deformation and metamorphism. Three major compositional units are recognized in the LAC: (1) anorthositic rocks, which occur as three major domal structures, the Poe Mountain anorthosite, the Chugwater anorthosite, and the Snow Creek anorthosite; (2) leucogabbroic rocks, which occur as the Strong Creek Intrusion and as smaller intrusions into the anorthositic rocks; and (3) monzonitic rocks, which form three major intrusions—the Red Mountain, Sybille, and Maloin Ranch, rimming the anorthositic rocks. The first day of the field trip examines the anorthositic layered zone and leucogabbroic layered zones within the Poe Mountain anorthosite, which is the oldest and northernmost of the LAC anorthositic rocks. The stops illustrate the processes involved in the formation of anorthositic magma chambers at middle to upper crustal levels, and the magmatic structures that form in Proterozoic anorthosites and mafic layered intrusions where plagioclase is a major constituent. The first half of the second day of the trip examines the Sybille monzosyenite intrusion that rims the western margin of the Poe Mountain anorthosite and is in contact with the Archean granitic gneiss and supracrustal rocks to the west and northwest. The stops illustrate mineralogical variations in the Sybille monzosyenite intrusion, a pendant of ferrodiorite and anorthosite, and the role of fluids in stabilizing the mafic mineral assemblages in these rocks. The latter half of the second day will be a walking traverse across the contact aureole on the western margin of the LAC where temperatures in excess of 900 °C were reached. The last day of the trip examines contact metamorphic zonation and local structural relationships in the Archean country rocks adjacent to the Red Mountain pluton (RMP). The stops illustrate the contact effects along the western margin of the RMP where the country rocks contain interlayers of the RMP. The intrusion of the main RMP body reoriented pre-intrusion deformation features in the country rocks, deformed early intrusive veins of the RMP, and produced contact metamorphic zones that overprint assemblages from previous periods of Paleoproterozoic metamorphism. Thermometry and reactions relationships in the RMP thermal aureole indicate temperatures as high as 800 °C near the contact and within interlayered regions to ca. 600 °C 1.5 km from the contact. The trip stops include easily accessible outcrops along Wyoming Highway 34, and less accessible stops on a private ranch property.

Strain in sedimentary rocks is linked to deformation of underlying basement rocks during the formation of basement-involved folds. Strains are represented by an array of structures such as lift-off folds, thrust faults, heterogeneous thickness changes, extensional faults, and boudinage. Detachment surfaces define the boundaries of structural lithic units within sedimentary rocks that are characterized by different deformation styles. A kinematic model is presented to investigate how strain distribution in folds is controlled by basement deformation. The model examines folds that form above a basement block that is displaced on a single reverse fault. Rocks in the fold undergo layer-parallel shortening (and thickening) and/or extension (and thinning). Extensional strains increase as fault angle and fault slip increase. As the basement fault propagates upsection strains will vary in the hanging wall and footwall of the fault and in unfaulted beds upsection from the fault tip. Results predicted by the modeling compare favorably with folds in the Rocky Mountain foreland province.