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Book Chapter

General or Comparative Structural Studies of Interaction and Overlap

January 01, 1988

The Cordilleran thrust belt and Rocky Mountain foreland overlap in a zone 120 to 160 km wide, extending at least from the central Montana salient to the northern edge of the Colorado Plateau, along the area of transition from the North American craton to the Cordilleran miogeocline. Current literature describes the Cordilleran thrust belt and Rocky Mountain foreland structural provinces as a paired belt with two distinct styles of deformation. Eastward-directed, décollement-soled thrusts are described as typical of the thrust belt, whereas fault-bounded, basement-cored uplifts with various trends and intervening deep basins have been identified with the Rocky Mountain foreland.

The overlap zone is characterized by the overlap in space and in time of structures of the thrust belt and foreland provinces, by the interaction between thrust belt and foreland structures, and by superposed deformation at two crustal levels.

Timing of thrust sheet movement and foreland uplift employs chiefly paleontologic, and especially palynologic, dating of associated synorogenic deposits, stratigraphic bracketing based on paleontology, and radiometric dating of syntectonic igneous rocks. The results of numerous studies of this kind in the overlap province define an overlap between late Campanian (Late Cretaceous) and early Eocene time, a span of approximately 25 m.y.

Interaction of structures of the Rocky Mountain foreland and those of the Cordilleran thrust belt has been described from many local areas within the overlap province. The styles of interaction include ramping of thrust faults over fault-bounded foreland uplifts and folding of thrust sheets by postthrust foreland uplift.

The position of the overlap zone is controlled in part by the presence of the transitional margin between the Proterozoic cratonic shelf and the Cordilleran miogeocline. Thrusting in the miogeoclinal wedge is assumed to have been initiated at a zone of brittle-ductile transition and to have been controlled principally by the layering in the sedimentary section. We believe detachment in the region from the hingeline eastward occurred at two crustal levels: the upper one controlled by mechanical decoupling in the sedimentary overburden, and the lower one controlled by the zone of transition from brittle to ductile behavior in the basement. Deformation in the foreland also may have been controlled by compressional reactivation of zones of crustal weakness inherited from earlier orogenic events, particularly Middle and Late Proterozoic normal faults along the cratonic margin.

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GSA Memoirs

Interaction of the Rocky Mountain Foreland and the Cordilleran Thrust Belt

Christopher J. Schmidt
Christopher J. Schmidt
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William J. Perry, Jr.
William J. Perry, Jr.
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Geological Society of America
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January 01, 1988




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