ABSTRACT The Archean Wyoming Province formed and subsequently grew through a combination of magmatic and tectonic processes from ca. 4.0 to 2.5 Ga. Turning points in crustal evolution are recorded in four distinct phases of magmatism: (1) Early mafic magmatism formed a primordial crust between 4.0 and 3.6 Ga and began the formation of a lithospheric keel below the Wyoming Province in response to active plume-like mantle upwelling in a “stagnant lid”–type tectonic environment; (2) earliest sialic crust formed in the Paleoarchean by melting of hydrated mafic crust to produce rocks of the tonalite-trondhjemite-granodiorite (TTG) suite from ca. 3.6 to 2.9 Ga, with a major crust-forming event at 3.3–3.2 Ga that was probably associated with a transition to plate tectonics by ca. 3.5 Ga; (3) extensive calc-alkalic magmatism occurred during the Mesoarchean and Neoarchean (ca. 2.85–2.6 Ga), forming plutons that are compositionally equivalent to modern-day continental arc plutons; and (4) a late stage of crustal differentiation occurred through intracrustal melting processes ca. 2.6–2.4 Ga. Periods of tectonic quiescence are recognized in the development of stable platform supracrustal sequences (e.g., orthoquartzites, pelitic schists, banded iron formation, metabasites, and marbles) between ca. 3.0 and 2.80 Ga. Evidence for late Archean tectonic thickening of the Wyoming Province through horizontal tectonics and lateral accretion was likely associated with processes similar to modern-style convergent-margin plate tectonics. Although the province is surrounded by Paleoproterozoic orogenic zones, no post-Archean penetrative deformation or calc-alkalic magmatism affected the Wyoming Province prior to the Laramide orogeny. Its Archean crustal evolution produced a strong cratonic continental nucleus prior to incorporation within Laurentia. Distinct lithologic suites, isotopic compositions, and ages provide essential reference markers for models of assembly and breakup of the long-lived Laurentian supercontinent.

ABSTRACT We report the results of 167 calcite twinning strain analyses (131 limestones and 36 calcite veins, n = 7368 twin measurements) from the Teton–Gros Ventre (west; n = 21), Wind River ( n = 43), Beartooth ( n = 32), Bighorn ( n = 32), and Black Hills (east; n = 11) Laramide uplifts. Country rock limestones record only a layer-parallel shortening (LPS) strain fabric in many orientations across the region. Synorogenic veins record both vein-parallel shortening (VPS) and vein-normal shortening (VNS) fabrics in many orientations. Twinning strain overprints were not observed in the limestone or vein samples in the supracrustal sedimentary veneer (i.e., drape folds), thereby suggesting that the deformation and uplift of Archean crystalline rocks that form Laramide structures were dominated by offset on faults in the Archean crystalline basement and associated shortening in the midcrust. The twinning strains in the pre-Sevier Jurassic Sundance Formation, in the frontal Prospect thrust of the Sevier belt, and in the distal (eastern) foreland preserve an LPS oriented approximately E-W. This LPS fabric is rotated in unique orientations in Laramide uplifts, suggesting that all but the Bighorn Mountains were uplifted by oblique-slip faults. Detailed field and twinning strain studies of drape folds identified second-order complexities, including: layer-parallel slip through the fold axis (Clarks Fork anticline), attenuation of the sedimentary section and fold axis rotation (Rattlesnake Mountain), rotation of the fold axis and LPS fabric (Derby Dome), and vertical rotations of the LPS fabric about a horizontal axis with 35% attenuation of the sedimentary section (eastern Bighorns). Regional cross sections (E-W) across the Laramide province have an excess of sedimentary veneer rocks that balance with displacement on a detachment at 30 km depth and perhaps along the Moho discontinuity at 40 km depth. Crustal volumes in the Wyoming Province balance when deformation in the western hinterland is included.

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.