LARAMIDE-AGE STRUCTURE, GEOCHEMISTRY AND ORIGIN OF UNAWEEP CANYON IN NORTHERN UNCOMPAHGRE PLATEAU, COLORADO

Laramide-age deformation overprinted the classic late Paleozoic Ancestral Rockies Uncompahgre Plateau of western Colorado. Unaweep Canyon is a major NE-SW striking valley that cuts the modern Uncompahgre Plateau. The geomorphic origin of Unaweep Canyon is controversial. Was it formed in the Permian by glacial erosion and then exhumed by a major ancestral river? We consider the Permian glacial valley hypothesis implausible; instead, we have identified a pattern of oblique-slip, Laramide-age deformation suggesting that Unaweep Canyon originally formed as a series of down-to-the SE normal fault structures. These fault zones served as zones of weakness that were later incised in the late Cenozoic by a major ancestral river (the Gunnison or Colorado or both). The NE-SW striking Unaweep Canyon normal faults formed as a transtensional bends between controlling WNW-ESE striking, left-lateral oblique-slip faults that cross-cut Unaweep Canyon. These major controlling faults include the Nancy Hanks Gulch, Pitch Gulch – Big Dominguez, North Gill Creek fault zones. Based on slickenside striations along faults, these structures have left-lateral strike-slip and both normal and reverse-dip slip. These faults are the eastward continuation of similar WNW-ESE striking, left-lateral oblique-slip faults found in the Pinon Mesa area, northwest of Unaweep Canyon.

Country rocks adjacent to these faults are highly silicified and affected by synkinematic, epithermal mineralization. The sequence begins with synkinematic fluorite-amethyst-calcite-hematite-goethite, followed by post-kinematic barite, Cu-carbonates, and small concentrations of sulfides. The mineralization zones are continuous on both the northern and southern rims of Unaweep Canyon, evidenced by ICP-OES and ICP-MS lithogeochemistry data of samples taken from mineral vein sites on both sides of Unaweep Canyon at varied elevations. These samples revealed low concentrations of rare earth elements which can be used to date the mineralization and act as a signature in confirming continuity. Also, multiple types of brecciation have been observed along faults suggesting they formed by a combination of high fluid pressures and tectonic comminution in zones of transtensional faulting.