Kinematic and Vorticity Analysis of the Slide Lake and Homestake Shear Zones, Central Colorado

Kinematic and quantitative vorticity analysis of the Homestake shear zone (HSZ) and the Slide Lake shear zone (SLSZ) in central Colorado provide further insight into crustal processes that controlled deformation within the Colorado Mineral Belt (CMB) shear zone system throughout the Proterozoic. Evaluating vorticity, that is, the relative contributions of pure to simple shear in sheared rocks, is crucial to understanding lithospheric tectonic evolution and allows for kinematic modeling of shear zone development at a variety of scales. Moreover, novel vorticity analyses involve determining the orientation of the vorticity vector and vorticity normal surface which can give insight into shear zone geometry independent of fabric elements. The CMB is a system of Laramide age plutons that lie within a Proterozoic zone of weakness defined by northeast-striking shear zones that have been repeatedly activated since ~1.7 Ga. The steeply dipping HSZ consists of a set of anastomosing shear stands that outcrop as mylonite and ultramylonite zones in Homestake valley. The SLSZ outcrops as three gently dipping mylonite zones with variable thickness above tree line on Homestake peak. Kinematic indicators within mylonite from the HSZ and SLSZ record top-down-to-the-southeast, top-up-to-the-northeast, and dextral movement suggesting the shear zones are kinematically linked and were active at similar mid-crustal levels. Further detailed microstructural analysis and crystallographic vorticity axis analysis, utilizing newly acquired electron backscatter diffraction data, were implemented on all phases to produce the orientation of the bulk vorticity vector and vorticity normal surface in the HSZ and SLSZ. We apply these results to reevaluate the model of shear zone deformation proposed by previous research. Additionally, quartz slip system analysis was conducted on samples from the field area to estimate temperatures associated with mylonite development. We conclude that strain localization within the HSZ involved general shear within a monoclinic, wrench-dominated transpressional setting. Results from this analysis demonstrate that the SLSZ likely exhibits a triclinic component of deformation. This research demonstrates the utility of modern vorticity analysis as applied to mono- and polyphase lithologies.