LARAMIDE PARTITIONING OF BASEMENT-INVOLVED DEFORMATION ALONG THE SOUTHERN MARGIN OF THE WHITE RIVER UPLIFT, COLORADO

The southern margin of the Laramide White River 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 km-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.