Abstract

A major problem in the controversial Laramide orogeny is understanding the mechanism whereby seemingly brittle gneisses of the Precambrian basement take almost the exact configuration of folds developed in overlying sedimentary cover, well exposed in the Medicine Bow Mts. Detailed study of Coad Mountain suggests that basement folding under relatively low load pressures might take place through passive folding along microfractures parallel to joints when the basement is once again placed in a stress field. The folding mechanism by which sedimentary veneer and basement mutually form a congruent fold surface remains a mystery. One consistent pattern is the reflection of Precambrian faults in younger fault systems; folds seem to be independent of Precambrian structures. Possibly the Laramide fold axis is perpendicular to a Precambrian lineation.

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