KINEMATIC ANALYSIS OF THE HOMESTAKE AND SLIDE LAKE SHEAR ZONES, CENTRAL COLORADO

Mesoproterozoic deformation across southwestern North America is characterized by magmatism and the reactivation of older, northeast-striking structures. Kinematic analysis and field mapping of the Homestake shear zone (HSZ) and Slide Lake shear zone (SLSZ) in the Sawatch Range of central Colorado provide new evidence for strain partitioning in the mid-crust at ~1.4 Ga. The NE-striking, subvertical HSZ comprises a ~10-km-wide set of anastomosing mylonite-ultramylonite ductile shear zones and pseudotachylyte-bearing brittle fault zones that reactivated a NE-SW-trending, high-temperature fabric formed during Paleoproterozoic upright folding (1.71-1.68 Ga). ~3-km south of the HSZ, the NNE-striking, subhorizontal mylonites of the SLSZ form three ~1-10-m-thick shear zone splays. Shear sense within the two lower splays of the SLSZ is dominantly top-to-the-SE, with the upper splay exhibiting both top-to-the-NW and top-to-the-SE motion. Orientation of ~1.7 Ga fabric remains fairly consistent from the HSZ into the SLSZ where the steeply dipping high-temperature fabric is truncated by the low-angle, moderate temperature SLSZ mylonite. We use these field relationships to infer that Mesoproterozoic deformation within the SLSZ coincides with ~1.4 Ga mylonite and ultramylonite development in the HSZ. Quartz grain boundaries within both the SLSZ and HSZ mylonite and ultramylonite are dominated by subgrain rotation and grain boundary migration, suggesting similar deformation temperatures. Based on the relative timing of fabric development and similar deformation temperature between the SLSZ and the HSZ we propose that they formed during strain localization within the mid-crust at ~1.4 Ga. Our new data from the low-angle SLSZ is compatible with previous interpretations of the HSZ as being part of a transpressional shear zone system, where subvertical shuffling accommodated an evolving ~1.4 Ga strain field that resulted in localized extension and shortening on the SLSZ.