CRUSTAL SHORTENING DURING THE PROGRESSIVE ASSEMBLY OF NORTH AMERICA; NEW EVIDENCE FROM SLIDE LAKE SHEAR ZONE, CENTRAL COLORADO

The northeast striking, shallowly dipping Slide Lake shear zone (SLSZ) occurs along a major trend of NE-striking, sub-vertical shear zones in central Colorado that were active during the Paleo- to Mesoproterozoic. The 1-km-wide SLSZ is exposed 3 km south of the sub-vertical Homestake shear zone in the Sawatch Range. The low-angle SLSZ potentially records a component of crustal shortening associated with transpressional movement. SLSZ contains mylonites with a well-developed, southeast-dipping foliation and a southeast-plunging stretching lineation. SLSZ samples display micro-scale shear bands, asymmetric tails on porphyroclasts, mica fish, and oblique grain shape fabric that consistently record top-to-the-NW shear sense. Mylonite, ultramylonite, and pseudotachylyte from the Homestake shear zone record top-to-the-NW (reverse) and top-to-the-SE (normal) shear sense, both with a dextral component. Quartz grain boundaries in SLSZ samples display evidence for subgrain rotation while feldspars remain rigid. The Homestake shear zone is characterized by early melt present fabric overprinted by lower temperature and higher strain mylonites. Both macro- and micro- kinematic shear sense indicators suggest that the SLSZ accommodated a component of NW-directed crustal shortening that was associated with the assembly of Laurentia. Continued field-based mapping and microstructural analysis will test the potential interplay between the SLSZ and Homestake shear zone.