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 cooling associated with Cheyenne belt tectonism. South of the BLFZ, a hornblende sample has a 40Ar/39Ar age of 1618 ± 3 Ma. Replicate analysis of synkinematic muscovite from the BLFZ a yielded three 40Ar/39Ar dates between 1597 and 1579 Ma, with a preferred crystal growth age of 1590 ± 10 Ma. We interpret the young ages to record a thermal and deformational event in the southern Sierra Madre between 1.62 and 1.59 Ga. This event may represent a previously unrecognized period of contractional or transpressional deformation in southwestern Laurentia. The 1.62-1.59-Ga time interval is not widely represented in the geologic record worldwide. Of the proposed conjugate margins of western Laurentia, only Australia records significant tectonism at this time. A more thorough understanding of the timing and extent of this deformation may have implications for Rodinia configurations, particularly with regard to the AUSWUS and AUSMEX reconstructions.
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