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Inversion of Proterozoic Extensional Faults

The Rocky Mountains, monoclines of Colorado Plateau and fault zones of U. S Midcontinent differ structural relief but resemble in structural style and regional orientation. The cratonic platform in the United States includes both the Midcontinent and the Rocky Mountains-Colorado Plateau province. Two Phanerozoic deformation events affected the Rocky Mountain-Colorado Plateau province. The late Paleozoic Ancestral Rockies event created basins and basement-core uplifts.

The Laramide orogeny occurred in Cretaceous-Tertiary time in response to plate convergence along the West coast. The Laramide uplifts are basement cored and their formation results from reverse-sense to transpressional displacement on underlying basement-penetrating faults. This displacement is 15-20 km to 1-2 km in some areas. These monoclines are fault-propagation folds, the steep limb of which faces the downdropped block. The intracratonic folds of the Rocky Mountains-Colorado Plateau province do not display regional parallelism.

Strata locally developed strain by growth of mesoscopic fault arrays and development of cataclastic deformation bands. The origin of Rocky Mountains-Colorado Plateau faults has two explanations. Either they were formed during the Ancestral Rockies and Laramide events in response to compressive stress or they were formed before this event their movement during the above-mentioned time represents fault reactivation. The weak faults were formed during Proterozoic rifting, thus Phanerozoic movement represents intracratonic rift inversion.

The presence of Proterozoic rifts, that the inversion has affected documented rifts, that uplift in the region parallel documented rifts and that the geometry resembles inverted all justify the proposal of rift inversion. Thus these were formed as normal faults and were later reactivated as reverse or transpressional faults. Works Cited Marshak, Stephen, Karlstorm, Karl, Timmons, J. Michael. “Inversion of Proterozoic extensional faults: An explanation for the pattern of Laramide and Ancestral Rockies intracratonic deformation, United States. ” Geology. August, 2000; v. 28; no. 8; p. 735-738.

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