Geometry and Kinematics of Thrust-Fold Structures in Central Rocky Mountain Foreland: ABSTRACT

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Seismic profiles, true-scale structural cross sections, and photographed clay-model analogs illustrate the two-dimensional geometry and kinematics of basement-thrusted oil field structures of the central Rocky Mountain foreland. A two-dimensional geometric model of an ideal thrust fold based on these data allows the construction of thrust-fold cross sections that extend to deeper levels with a minimum of surface or near-surface data. The thrust-fold structures of the central Rocky Mountain foreland are the ultimate product of Laramide compressive stress. They are produced by thrusting and rotation of a wedge of rigid, crystalline basement that acts as homogeneous, brittle material. The causal thrust fault is usually initiated in the basement at about 30/sup 0/ to the sediment-basement contact and becomes listric as it propagates upward into the sedimentary section, producing an asymmetric anticline. The thrust-induced folds in the Phanerozoic rocks are not truly concentric because ductile sedimentary layers are attenuated over the steep forelimb and along the causal fault zone. Since the basement surface remains essentially planar and continuous plateau uplift cannot occur, a hinge is developed behind the causal thrust zone along which backlimb rotation takes place. In this model, magnitudes of backlimb rotation and thrust dip, displacement on the basement, andmore » vertical uplift are directly proportional to horizontal shortening. In advanced stages of thrust-fold development, the leading edge of the basement thrust wedge may be rounded through fault imbrication under the constriction of the tightly folded, overlying sedimentary section. Crestal extensional faulting may occur at shallow levels.« less