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Structural and thermochronologic studies in the Sierra Madre, southern Wyoming, indicate that parts of the 1.78–1.75‐Ga Cheyenne belt, an Archean‐Proterozoic suture, were reactivated at ca. 1.6 Ga. New shear zones, notably the brittle Battle Lake thrust‐tear fault zone, also formed at this time. The Battle Lake fault zone (BLFZ), which displaced parts of the Cheyenne belt at least 30 km to the north, corresponds to the surface expression of a south‐dipping reflector imaged by regional seismic studies (Continental Dynamics of the Rocky Mountains experiment). 40Ar/39Ar and U‐Pb data define age patterns in the Sierra Madre that are delineated by discrete structures. North of the 1.78‐Ga Divide Peak mylonite zone, a foreland structure associated with the Medicine Bow orogeny, 40Ar/39Ar biotite dates suggest cooling below 300°C by 2.0 Ga. South of the Divide Peak mylonite zone but north of the BLFZ, hornblendes yield apparent 40Ar/39Ar ages of ca. 1.80–1.78 Ga that may record tectonic burial and cooli...

The transcontinental belt of ca. 1.4‐Ga granite plutons that extends from southern California to Labrador has conventionally been interpreted as either “anorogenic” or emplaced in an extensional tectonic setting. Recent studies in the western United States, however, demonstrate that several ca. 1.4‐Ga plutons were emplaced in a broadly northwest‐southeast shortening strain field. Our study of the \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage[OT2,OT1]{fontenc} \newcommand\cyr{ \renewcommand\rmdefault{wncyr} \renewcommand\sfdefault{wncyss} \renewcommand\encodingdefault{OT2} \normalfont \selectfont} \DeclareTextFontCommand{\textcyr}{\cyr} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document} \landscape $1404+1.5$ \end{document} ‐Ma Boriana Canyon pluton, northwestern Arizona, est...

The age and inferred tectonic setting of the 1.76 Ga Horse Creek anorthosite complex (HCAC) in the Laramie Mountains of southeastern Wyoming place important constraints on the origin of middle Proterozoic anorthosite complexes. The 100 km2 HCAC consists of strongly recrystallized anorthosite and two small intrusions of monzonite and granite. U‐Pb crystallization ages from euhedral zircons in anorthosite and monzonite are 1761.5 ± 2 Ma and 1754.5 ± 2.2 Ma, respectively. An additional period of zircon growth in the anorthosite occurred at 1753.4 ± 2 Ma, as represented by a small population of anhedral zircon. We attribute the origin of this second morphological variety of zircon in the anorthosite to the loss of Zr from ilmenite during reaction with plagioclase to form sphene. This reaction took place in response to heat and fluid influx during intrusion of the adjacent monzonite. The HCAC and the younger 1.43 Ga Laramie anorthosite complex to the north were intruded along or near a Paleoproterozoic suture ...