--> Restoration of a Crustal-Scale Trans-Colorado Rocky Mountains Cross Section in the Interstate-70 Corridor

AAPG ACE 2018

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Restoration of a Crustal-Scale Trans-Colorado Rocky Mountains Cross Section in the Interstate-70 Corridor

Abstract

A crustal-scale cross section across the Colorado Rocky Mountains was constructed from a variety of data sets described in a companion abstract by Sterne et al., including isopachs and low-temperature thermochronology used to restore the cover section to the top Cretaceous, and magnetotelluric data indicating a mid-crustal low resistivity zone adopted here as the basal thrust detachment . The section parallels a mean Laramide shortening direction of 065. It was restored and balanced using fault-slip and transform functionality in LithoTectTM software from Landmark Graphics Corp., thus simulating rigid block rotation/translation and flexural slip deformation. Basement blocks were allowed to deform under the assumption that the heterogeneous crystalline basement of Colorado (with numerous strength anisotropies among multiple lithologies and numerous fracture orientations) permits small levels of plane strain in the line of the section, something < 10%. Bed-length preservation (flexural slip assumption) has been used to restore the sedimentary cover. No decompaction was applied to the stratigraphy. Only the Laramide components of reactivated Ancestral Rocky Mountain structures, such as the Gore fault, were restored, thereby preserving elements of the earlier deformation in the central part of the section. The Rocky Mountains at this latitude were built by thrusts that ramp from the mid-crustal detachment upward through basement into detachments in the sedimentary section, thereby creating a variety of fault-bend geometries. Most of the uplift was accomplished by displacement on the west-directed Williams Range-Elkhorn and Grand Hogback thrust systems, and on an east-directed thrust system bounding the eastern flank of the Front Range. Where the level of erosion permits their preservation, stacked triangle zones, often with internal backthrusts, characterize all of the thrust systems. The restoration implies that the mid-crustal detachment was deformed as the brittle upper crust thickened and displaced the ductile lower crust. Although the lower crust was not explicitly treated in this model, it was assumed to independently shorten during the Laramide and thus accommodate deformation of the mid-crustal detachment. The section implies 79 km (50 miles) of shortening between the Grand Hogback and the Front Range.