Kinematic evolution of the Homestake and Slide Lake shear zones, central Colorado: Implications for mid-crustal deformation during the Mesoproterozoic

Kinematic analysis and field mapping of the Homestake shear zone (HSZ) and Slide Lake shear zone (SLSZ) in central Colorado provide new evidence for strain partitioning in the mid-crust at ~1.4 Ga. The northeast-striking, steeply dipping HSZ comprises a ~10-km-wide set of anastomosing ductile shear zones and pseudotachylyte-bearing faults. Approximately 3-km south of the HSZ, the north-northeast-striking, shallowly dipping mylonites of the SLSZ form three 1-10-m-thick shear zone splays. Both top-up-to-the-northwest and top-down-to-the-southeast shear sense are recorded in the SLSZ and HSZ. Oblique stretching lineations in both shear zones show vertical (top-down-to-the-southeast and top-up-to-the-northwest) and dextral movement occurred during mylonite development. Quartz and feldspar deformation mechanisms and quartz [c] axis lattice preferred orientation (LPO) patterns are consistent with deformation temperatures ranging from ~280-500°C in the HSZ to ~280-600°C in the SLSZ. Mean kinematic vorticity and quartz [c] axis LPOs for parts of each shear zone suggest plane and non-plane strain general shear with contributions of 47-69% pure shear and 31-53% simple shear. Based on micro- and mesoscale kinematics along with mean kinematic vorticity values and deformation temperature estimates, we propose that HSZ and SLSZ formed during strain localization and partitioning within a mid-crustal transpressional shear zone system that involved subvertical shuffling at ~1.4 Ga.