FOLD ANALYSIS AT THE ELK RANGE THRUST FAULT & CROOKTON REVERSE FAULT, GUNNISON COUNTY & SAGUACHE COUNTY, COLORADO

We compare two folds associated with west-vergent, reverse and thrust faults, located in Gunnison and Saguache Counties of Colorado. The Strand Hill syncline is in the footwall of Elk Range Thrust Fault 36 km north northeast of the town of Gunnison, CO. The Needle Creek anticline, 37 km to the east southeast of Gunnison is in the footwall of the Crookton Reverse Fault. Both these folds provide an opportunity to better understand fold kinematics and variations in penetrative strain associated with footwall deformation at a reverse and thrust fault with. The results presented here are a compilation of work done in the undergraduate Research in Structure and Tectonics course at Western State Colorado University.

We compare percent shortening and fold mechanisms associated with each fault type. We also characterize the fracture distributions and mineralization associated with dip variations at the non-cylindrical Needle Creek anticline. To compare percent shortening field mapping and stratigraphic thickness data were used to balance cross-sections and create fold restoration models. Cross-sections were balanced using a flexural-slip model which estimates 14% shortening at each fault. Use of the flexural-slip model is based on analysis of slickenline data collected in the Dakota Sandstone, which are oriented perpendicular to the fold axes. At Needle Creek the Dakota Sandstone is cut by through-going mineralized fractures. Maximum fracture densities occur where dips range from 50 to 60 degrees. These two folds occur in the footwall of west-vergent faults and record a similar shortening and footwall deformation mechanisms associated with Laramide compression.