Field studies of jointing, faulting, and folding in the San Juan-Grand Canyon region, Arizona, and the Bighorn-North Laramie region, Wyoming, show that tectonic movements are primarily vertical. Maximum deformation of the rocks occur along narrow, linear zones which appear to follow elements of a primordial fracture pattern in the Precambrian basement. Sedimentary rocks play a passive role in the formation of folds and structural lineaments.

Fold geometries are determined by the developing geometry of the basement surface. The basic causes and mechanisms of deformation are generated in basement rocks below the level of the sedimentary strata. Structures in the sedimentary strata are of secondary origin, generated as a result of folding following basement deformation.

Prominent fracture trends of Precambrian age in the basement rocks of the Grand Canyon region are present as major elements of the joint patterns in overlying sedimentary rocks.

Large-order structural lineaments in the Bighorn region are long, narrow zones of deformation in sedimentary rocks which reflect deformations in the underlying basement rocks. Areally, structural lineaments follow some element of the regional fracture pattern.

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