Abstract

An anticline three miles west of Loveland, Colorado, exposes Precambrian basement rock, and permits a study of fold mechanisms within the en echelon fold belt of the Colorado Front Range. The contrast in mechanical properties between basement rock and superjacent sediments has resulted in two entirely different mechanisms of folding in this area. Studies of fold geometry, jointing, and stratigraphic thickness indicate that the sedimentary rocks yielded to Laramide stresses by concentric folding, up to a limiting curvature. Thereafter, additional displacements took place by faulting across the steeper limb of the anticline. The relationships of Precambrian structures to the fold geometry indicate that basement adjustments to Laramide stresses took place by differential movement along pre-existing fractures, within a zone adjacent to the unconformity that separates the basement from the sedimentary cover. In the lower basement zone, displacement along high angle reverse faults is virtually the only recognizable feature of Laramide deformation.

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