STRUCTURAL AND GEOCHRONOLOGIC EVIDENCE FOR MESOPROTEROZOIC (CA. 1570 MA) REACTIVATION OF THE CHEYENNE BELT SUTURE ZONE, SOUTHEASTERN WYOMING

The Cheyenne belt of SE Wyoming, the primary suture between the Archean Wyoming craton and accreted Paleoproterozoic terranes, is exposed in the Medicine Bow Mountains and Sierra Madre as NE-striking, reverse-sense, amphibolite-grade mylonite zones. In the Medicine Bow Mountains, the mylonite zones are overprinted by a network of non-penetrative, subvertivcal slip surfaces with subhorizontal, epidote-chlorite-tremolite lineations. Although these greenschist-grade fabrics appear randomly oriented in the field, detailed kinematic analysis has revealed a NW-SE shortening direction that was accommodated by complex strike-slip movement. E-NE striking surfaces typically have a dextral sense of shear and N-NW striking surfaces typically have a sinistral sense shear, which we interpret as conjugate slip orientations. These fabrics exhibit both ductile and brittle features, and kinematic indicators range from S/C fabrics in tremolite microshears to cataclastic imbrication of feldspar clasts in a fine-grained epidote-cholrite matrix. In contrast, the NE-striking fabrics of the Cheyenne belt in the Sierra Madre are truncated by a major thrust-tear fault system, and although epidote-chlorite fabrics are present, this fault is generally cataclastic. Geochronologic data from synkinematic samples taken from both mountain ranges yield Mesoproterozoic U-Pb (sphene and epidote) and Ar-Ar (muscovite and biotite) ages indicating that this event occurred at ca. 1570 Ma, and that the Cheyenne belt was reactivated in a NW-SE compressional stress regime despite lying nearly 1000 km from any known plate boundary. This event is neither related to ca. 1.4 Ga magmatism nor to 1.78-1.65 Ga accretion, and is a previously unrecognized tectonic event that punctuates a time in which the western United States was thought to be tectonically quiescent. Similar aged events in North America include 1.5 Ga island arc formation in NE Canada (Rivers, 1997) and 1565 Ma magmatism in Wisconsin (Van Wyck et al., 1994). If related, these events may represent reactivation of the Archean/Proterozoic tectonic boundary as an inboard manifestation of an active margin along southern Laurentia. Alternatively, the deformation observed in this study may be related to widespread deformation and magmatism in E. Australia between 1.6-1.5 Ga (AUSWUS).