Structural interpretation of the Elk Range thrust system, Western Colorado, USA
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Date
2009
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Montana State University - Bozeman, College of Letters & Science
Abstract
The Elk Mountains of western Colorado expose Pennsylvanian-Permian strata that were deposited along the western margin of the Ancestral Central Colorado Trough. These rocks were displaced southwestward in Late Cretaceous-Early Paleogene time along the northeast-dipping Elk Range thrust system. The thrust system trends southeast from Redstone, CO to the Fossil Ridge wilderness and includes the en echelon Elk Range and Brush Creek thrust faults. This thrust system represents the deeply eroded up-plunge core of a major Laramide basement-cored fold in western Colorado, the Grand Hogback monocline. The emergence of the thrust system from the fold's core is well documented at all scales of geologic mapping over the northwest end of the system. This surface relationship is undemonstrated in previous structural interpretations, which invoke a mechanism of gravity sliding within the sedimentary package, induced by vertical basement uplift. To the southeast a critical portion of the system had remained unmapped in any contiguous detail. This critical area exposes the basement roots of the thrust system, as it merges with the reverse-faulted southwestern margin of the Laramide Sawatch Range basement arch. This thesis presents a new structural architecture for the Elk Range thrust system through: 1) new 1:24,000 scale mapping of the emergent root zone, 2) regional balanced cross-section analysis 3) demonstration of a genetic relationship with the Grand Hogback monocline, and 4) consideration of contemporary basement-involved foreland contraction models. The fault system is a basement-rooted, right-stepping, en echelon thrust front. The Elk Range thrust sheet is truncated by high-angle reverse faults to the east and the Brush Creek thrust becomes steeper and merges with reverse faults to the southeast. The western Sawatch front shows evidence for late-stage, north-south directed contraction. Thus, the Elk Range thrust system represents an inverted segment of the western Ancestral Colorado Trough. Structurally, it represents a transitional deformation regime between fold-shortening (Grand Hogback monocline) and high-angle reverse-faulting (Sawatch arch). Together, this tectonic continuum marks Colorado's westernmost Laramide deformation front against the Colorado Plateau. Younger deformation is observed and discussed with respect to the region's dynamic transition from Laramide contraction to Rio Grande rifting.