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Eagle collapse center: Interpretation of evidence for late Cenozoic evaporite-related deformation in the Eagle River basin, Colorado

January 3, 2002

Evaporite tectonism resulted in deformation and collapse over an area of ~2500 km2 that is referred to as the Eagle collapse center. The collapse center includes much of the Eagle and Colorado River drainage basins between Vail, Dotsero, and McCoy, Colorado. The volume loss of evaporitic rocks by dissolution in the collapse center is estimated to be nearly 1700 km33 . Before ca. 10 Ma, Miocene basaltic flows partly covered an extensive, nearly horizontal, low-relief surface. Parts of this surface collapsed 1.3 km near the present-day Eagle and Colorado Rivers. Remnants of this surface outside the area of collapse, such as highlands of the White River uplift, the flank of the Gore Range, and Basalt Mountain, stand at elevations of 2.9–3.6 km. The high-standing Castle Peak basaltic cap, situated near the center of the Eagle collapse center, may not have collapsed, or collapsed little. The areas of collapse lie within or nearby known and inferred limits of the Pennsylvanian Eagle Valley Evaporite (mostly halite, gypsum, and anhydrite) that was deposited in the Central Colorado trough. Our geologic mapping and research in the Eagle collapse center delineate synclinal sags in the basaltic flows with amplitudes of 0.5–1 km, sinuous and discontinuous high-angle faults that cut basaltic flows, elongate grabens, evaporite-cored anticlines, and an ellipsoidal fault system that drops a 30 km X 10 km mountain block of younger strata into evaporite. Collapse as far as 20 km from the Colorado and Eagle Rivers suggests that the greater load on evaporite beneath surrounding highlands causes lateral flow of evaporite toward anticlinal crests in river valleys. Thus, gravity-driven evaporite flow and removal of evaporite by dissolution in groundwater and by subsequent discharge to surface waters combine to produce large-scale collapse. Although most evaporite tectonism post dates the basaltic flow capped surface, local angular unconformities under this surface record earlier, possibly Laramide evaporite tectonism, and overthickened post-evaporite red beds record some late Paleozoic evaporite deformation