NEW STRUCTURAL INTERPRETATION OF THE ELK RANGE THRUST SYSTEM, SOUTHWEST COLORADO

The Elk Mountains of southwest Colorado expose a thick Pennsylvanian-Permian succession that was displaced southwestward in Late Cretaceous-Early Paleogene time along the NE-dipping Elk Range Thrust System (ERTS). The ERTS trends southeast from Redstone to Taylor Canyon and includes the low-angle, en-echelon Elk Range and Brush Creek thrust faults. This thrust system represents the deeply eroded up-plunge core of a major Laramide tectonic feature in western Colorado, the Grand Hogback monocline. The ERTS forms the thrust-faulted southwestern margin of the Sawatch arch and is separated from the core of the uplift by a SW-facing, NNW-trending, reverse-faulted monocline of Precambrian through Mississippian rocks. The geometrically awkward arrangement of this basement-cored monocline immediately adjacent to the low-angle ERTS led previous investigators to propose gravitational sliding from the crest of the Sawatch arch. New mapping, balanced cross-sections and kinematic analyses demonstrate that the ERTS is basement-rooted and that the region evolved through two stages and two different styles of Laramide shortening: 1) southwestward displacement of the ERTS on low-angle basement-rooted thrust faults, followed by 2) high-angle reverse faulting along the western flank of the Sawatch arch that truncated the ERTS. As a result, the Precambrian core of the Sawatch Range uplift was juxtaposed against Paleozoic rocks of the ERTS hanging wall.