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Five undergraduate students participated in geologic mapping and other field research for this project from May to August, 2009. One student presented work from 2008-09 at the Rocky Mountain section meeting of the Geological Society of America in May 2009. Three students participated in presentation of the results described below at the 2010 annual meeting of the Geological Society of America. One former undergraduate student directly supported by PRF funds during the first year of the project has now completed an M.S. degree in geology at West Virginia University focusing on petroleum geology. An undergraduate student who worked on this project immediately prior to the funding award has also completed an M.S. focusing on petroleum geology from the same institution; his work was instrumental in collecting preliminary data that led to the preparation of the grant proposal. Both students with recent graduate degrees are employed in the energy sector and work in the Appalachian basin.

The goal of this research is to examine the deep structure of an exhumed fault zone and document the geometry and dynamics of early fault development as compared to: (1) the pre-existing structure of mid- to lower-crustal metamorphic fabrics, and (2) the geometry of overprinting brittle faults formed during younger reactivation. Research over the past 12 months has focused on goal (2), the geometry of overprinting faults and the reactivation history of the Grizzly Creek shear zone.

The study area is within the southern margin of the Laramide White River uplift in the Rocky Mountain foreland of Colorado. The uplift is a south- to southeast-dipping monocline associated with the east- to northeast-striking Grizzly Creek fault (GCF). The GCF cuts Proterozoic basement and Upper Cambrian through Lower Pennsylvanian strata, and was generated by reactivation of the Proterozoic Grizzly Creek shear zone (GCSZ); a kilometer-thick zone of north-dipping metamorphic foliation, mylonite, and pseudotachylyte.

Our new geologic mapping in the Glenwood Springs and Shoshone 7.5 minute quadrangles redefines the attitude of the GCF and identifies two precursor faults and fault-related folds. In the canyon of Grizzly Creek, a deeply incised tributary of the Colorado River at Glenwood Canyon, the GCF is a south-vergent reverse fault oriented 260/46˚ N with ~200 m of stratigraphic separation. Separation diminishes 2 km to the west of Grizzly Creek where the GCF steepens and dies out into a monocline. The GCF generated an overturned footwall syncline in cover strata, and a gentle hanging wall anticline that is locally broken by a steep splay rooted in the GCF. Along the eastern wall of Grizzly Creek canyon south of the GCF, a west-dipping monocline related to an east-dipping, basement-involved reverse fault strikes ~185˚. The structure is truncated by the GCF and refolded into a large-scale, type 2 superimposed fold in the GCF footwall syncline. East of Grizzly Creek canyon, the GCF cuts a south-dipping, east- to northeast-striking reverse fault preserved in Cambrian through Mississippian strata. This older fault includes an overturned footwall syncline with an upright limb that has been refolded into a syncline within the footwall of the GCF. We interpret the southern margin of the White River uplift to be a product of Laramide strain partitioning in a compressive regime. In this interpretation, deformation initially generated the north-striking, west-directed reverse fault and monocline. During compression, the Proterozoic GCSZ provided a zone of weakness that caused deformation to localize along an east-northeast trend. Initially, the footwall of the GCSZ failed along south-dipping lithologic fabrics creating a local north-vergent reverse fault, which was subsequently cut as the GCF developed.