STRUCTURAL AND GEOCHRONOLOGIC EVIDENCE FOR REACTIVATION OF THE CHEYENNE BELT SUTURE ZONE, SOUTHEASTERN WYOMING AT 1750 MA AND 1600 MA

The Cheyenne belt of SE Wyoming is exposed in the Laramie Mountains, Medicine Bow Mountains and Sierra Madre. It is the primary suture between the Archean Wyoming craton and accreted Paleoproterozoic terranes, and has been reactivated multiple times since its inception at ca. 1780 Ma (Premo and Van Schmus, 1990). The first phase of reactivation was a period of extension after the initial collision (Jones et al., 2010), and was manifested by intrusion of the 1763 Ma Sierra Madre granite (A-type chemistry. The second period of reactivation formed the dominant fabric along the Cheyenne Belt, an amphibolite-facies, sillimanite-bearing, dip-slip fabric that represents renewed collision after the period of extension. Syn-deformational sphene dates of 1748 ± 10 from an amphibolite in the Barber Lake Block and 1748 ± 6 from high-grade, hornblende-bearing fabrics from the French Creek shear zone represent this period of deformation in the Medicine Bow Mtns. These support U-Pb zircon ages of leucosomes in the Sierra Madre that indicate penetrative deformation ca. 1750 Ma (Jones et al, 2010). The third period of reactivation along the Cheyenne belt is manifested by non-penetrative, greenschist facies, subvertical epidote-chlorite veins with subhorizontal slickenfibers that crosscut the amphibolite facies fabrics at high angles in the Medicine Bow Mtns.Kinematic analysis of hundreds of slickenfibers has revealed a NW/SE shortening direction accommodated by complex strike-slip movement dated by three samples of deformationally-linked epidote veins with Pb/Pb ages of 1602 ± 27, 1568 ± 31 Ma and 1533 ± 68 Ma. In contrast, the amphibolite facies fabrics along the Cheyenne belt in the Sierra Madre are truncated by NW-directed thrust-tear fault system, directed dated by Ar-Ar data from synkinematic muscovite ca. 1.6 Ga. Although epidote-chlorite fabrics are present, this fault is generally cataclastic in the Sierra Madre. While the epidote-bearing, greenschist fabrics represent NW/SE directed shortening in the Medicine Bow Mtns, the degree of shortening is much less than in the Sierra Madre. These ages imply that the Cheyenne belt was reactivated by NW/SE compressional stresses despite lying nearly 1000 km from the plate boundary at the time.