PETROGRAPHICALLY-LINKED, GEOCHRONOLOGIC EVIDENCE FOR WIDESPREAD CA. 1.6 GA DEFORMATION IN THE NORTHERN COLORADO PALEOPROTEROZOIC CRUST: METHODS AND EXAMPLES

This poster presents the results of detailed field mapping, petrology, and kinematically-linked geochronology from five shear zones in Colorado with emphasis on the Soda Creek-Fish Creek shear zone (SCFCSZ) in Routt National Forest, Colorado, and the Kings Canyon mylonite near Walden, CO. The SCFCSZ records at least two periods of deformation: 1) a higher temperature ductile phase up to feldspar plasticity at ~1.68 Ga, and 2) a lower temperature brittle phase with epidote growth and formation of cataclasites at ~1.60 Ga (e.g., Hamlin, 2013; Duebendorfer et al., 2015; Cubrich et al., 2016; Cubrich, 2017). The Kings Canyon mylonite records an upper greenschist grade in the form of Epidote + Quartz + Chlorite + Actinolite pinstripe mylonites with epidote and titanite deformational ages that range from ~1.65-1.55 Ga. This study presents U-Pb data from multiple periods of metamorphic growth of titanite and monazite that directly date SCFCSZ activity/deformation from 1.77-1.57 Ga, but focuses on textural relations and the reaction mechanisms that led to the formation of these mylonites/shear zones. Our data suggest at least 5 periods of tectonism for the region: 1.77 Ga regional accretionary fabrics (tightly constrained at 1774±2 Ma in SCFCSZ); an event at 1.72-1.70 Ga, with unknown tectonic significance (SCFCSZ); at least two periods of high strain reactivation, ca. 1.66 and 1.60 Ga (SCFCSZ and Kings Canyon); and 1.42 Ga plutonism. The compilation of new geochronology and structural data from these shear zones add to a growing body of data that document a major period of regional tectonism at 1.65-1.55 Ga, well inboard from the active accretionary margin, with breakout, foreland thrusts concentrated in the lithosphere west of a N-S 1.68-1.60 Ga transform fault.