EVIDENCE OF LATE PALEOZOIC LATERAL-SLIP FAULTING, FRONT RANGE, COLORADO

A complex lateral-slip system with associated thrusting and extension tectonics during late Paleozoic is evident west of Grape Creek on the outskirts of Cañon City, Colorado. Reactivation of this fault and uplift during the Laramide resulted in the stripping of much of the sedimentary section from Proterozoic basement, but along some sections, evidence of pre-Mesozoic, lateral-slip tectonics remains as slivers of brecciated Ordovician Fremont Dolomite and Harding Sandstone, juxtaposition of Harding Sandstone and lower sections of the Fremont Dolomite against Pennsylvanian Fountain Formation, differential erosion of uplifted and down-dropped blocks containing lower Paleozoic units, and exposed faults and folds with structural attitudes that are distinctly different from the eastward dipping Pennsylvanian-Permian and Mesozoic section whose structural attitude reflects Laramide tectonism.

Thrust faults and a compensating right-lateral strike-slip fault are evidence for significant left-lateral slip along the Grape Creek fault. One fault juxtaposes a Proterozoic basement hanging wall over Ordovician Harding Sandstone in the footwall, whereas a second fault has Harding Sandstone in both hanging wall and footwall. Thrusting is attributed to compression in a restraining bend of the Paleozoic Grape Creek fault. To the south, thicker Harding Sandstone and Fremont Dolomite occurring in topographically high positions are fault offset to lower areas with thin or absent Ordovician section below the pre-Fountain unconformity. These thicker sections likely were down-dropped blocks within releasing bends along the Paleozoic Grape Creek fault and elevated to their present position by Laramide tectonics.