Laramide Folding Associated with Basement Block Faulting in the Western United States
Monocline fold pattern of the Colorado Plateau
Published:January 01, 1978
George H. Davis, 1978. "Monocline fold pattern of the Colorado Plateau", Laramide Folding Associated with Basement Block Faulting in the Western United States, Vincent Matthews, III
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Geologic relationships in the Grand Canyon region demonstrate a spatial, genetic association of monoclines with reactivated, ancient, steeply dipping faults. If one assumes that these relationships hold for monoclines throughout the Colorado Plateau, the monocline pattern, in total, records the upper-crustal expression of many elements of the regional, basement-fault system. The map pattern of monoclines in northern Arizona seems to bear this out, for the curvilinear and branching nature of individual monoclines within an overall angular, orthogonal pattern implicitly favors a relationship of monoclines to basement faults. Analysis of the monocline pattern of the Plateau, in total, demands the additional but less obvious interpretation that many of the basement faults extend well beyond areas where their traces have clear-cut monoclinal expression. Specifically, the monocline (that is, basement-fault) segments can be fit to a systematic, interdependent network of lineaments, regional in extent, inferred to represent traces of basement-fracture zones, the loci of steeply dipping planar elements such as shear zones, faults, jointing, and lithologic contacts in basement rocks.
Two sets of major basement fracture zones are recognized in the monocline pattern, and these trend N20°W and N55°E. The fracture zones serve to subdivide the basement of the Plateau into a mosaic of crustal blocks, some of which moved differentially during the time(s) of monoclinal folding. The distribution of uplifts in the Colorado Plateau is seen to be very systematic when viewed in the context of a basement partitioned by basement-fracture zones.
Most of the monoclinal folding took place in the Laramide, a time of northeast-southwest compression in the Plateau. As a response to the regional compression, individual basement blocks within the mosaic were uplifted by reverse movements along segments of the high-angle fracture zones. Modeling of the deformation serves to emphasize that a very small amount of horizontal crustal shortening can produce significant structural relief when the shortening is accomplished along relatively widely spaced, steeply dipping faults.