Geometry and Tectonics of the Laramide Front Range, Colorado

Abstract Detailed E-W structural cross sections based on published geologic maps, oil well, and related data extending across the Colorado Front Range between 38° 30′ and 41° 00′ N latitude at 7.5′ intervals show the following: • North of Boulder (~40° N) the east flank of the Front Range is bounded by an east-dipping monocline cut at intervals by relatively small E- to NE-dipping, high-angle reverse faults and fault-propagation folds. From Boulder south, the monocline is locally modified by the east-directed Boulder and Golden thrusts, and the Jarre Canyon, Perry Park, Rampart Range, and Cheyenne Mountain faults whose net slips are less than ~3 km. These faults are not continuous along the mountain front and are located where the dip of the sediments exceeds ~60°E. • The west flank is bounded by west-directed thrusts that include, from north to south, thrust of Canadian River, Cameron Pass thrust, Never Summer thrust, Williams Range thrust, and Elkhorn thrust. They are associated with large-scale, commonly overturned folds that apparently developed prior to propagation of discrete thrust faults. The net slip on the thrusts is ~10–15 km, based on exposures in the Never Summer Mountains and adjacent to the Montezuma Stock in the Roberts Tunnel near Dillon. Additional west-directed thrusts are found in the North, Middle, and South Park basins. These observations indicate that the Laramide Front Range was produced by dominantly west-directed thrusting associated with shortening of the crust beneath the range. Tectonic uplift of the range relative to the Denver Basin exceeds 6 km. West-dipping thrusts along the east flank are interpreted as back-thrusts. Tectonic models that involve vertical uplift on faults whose dip increases with depth on both flanks of the range are not compatible with the geologic data, nor are models that treat the range as a large flower structure on a wrench fault system. If the thrusts are assumed to dip ~35°E beneath the range, >6 km of tectonic uplift would require >10.5 km of tectonic transport, similar to the actual slips on the Never Summer and William Range thrusts. Given an average width of ~60–65 km, the depth at which the thrusts must flatten to produce the homocline on the east flank of the range is ~42–45 km, which is the approximate depth of the Moho. These observations argue that the Laramide Front Range uplift was produced by west-directed crustal-scale thrusts with 10–15 km of net movement. It is suggested that localization of the Front Range and related Laramide uplifts in the foreland basin was a result of foundering of the flat-subducted Farallon Plate well east of the Sevier fold/thrust belt and related tectonic activity along the active margin of the continent.

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