Geometrical and Mechanical Constraints on Basement Involved Thrusts in the Rocky Mountain Foreland Province

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Contribution to Book

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Interaction of Foreland and Thrust Belt, Western United States




Geological Society of America

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A review of the kinematics, mechanics, and mechanisms of deformation in Precambrian basement rocks and in the Paleozoic and younger sedimentary rocks is combined with recent data on foreland structures for the purpose of developing models of basement-involved structures. This chapter concentrates on the deformation of isotropic, crystalline basement rocks since other foreland basement rock types (i.e., foliated metamorphic rocks and low-grade metasedimentary rocks) can fold on shorter wavelengths. Precambrian crystalline rocks of the foreland and overlap provinces deformed at low to moderate temperatures and confining pressures; the deformation mechanisms consisted of brittle and semi-brittle fracture and cataclasis. Sedimentary rocks were highly anisotropic during the Laramide deformation, and the mechanical properties of the sedimentary rocks may have changed as the loading conditions and the dip of the rocks changed over time. The kinematic models of foreland structures presented here consist of monoclinal flexures that formed ahead of a wide zone of faults, ramp anticlines in the Precambrian rocks that develop above a dip change in the underlying fault, wedge-shaped shear zones in the Precambrian rocks, and structures above curved faults.

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