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New Structural Interpretation of the Elk Range Thrust System, Southwest Colorado

Tully, Justin E.1, David R. Lageson1, and James C. Coogan2
1Montana State University, Bozeman, MT
2Western State College of Colorado, Gunnison,

     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 SWfacing, 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 basementrooted 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.

 

AAPG Search and Discovery Article #90071 © 2007 AAPG Rocky Mountain Meeting, Snowbird, Utah