Structural models of faulted detachment folds

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Faulted detachment folds represent a common structural style in several hydrocarbon-producing fold and thrust belts, including the Zagros, Idaho-Wyoming-Utah, Carpathian, and Albanide belts, and several deep-water fold belts, including the Mississippi Fan and Per dido belts. These structures form in units with high competency contrasts and display a transition in deformational behavior from detachment folding to progressive fault propagation with increasing shortening. The structures first form by symmetric or asymmetric detachment folding, with the fold wavelength controlled by the thickness of the dominant units. Volumetric constraints require the movement of material from the synclines to the anticlines. Contin ued shortening results in tighter folds, with limb rotation by hinge migration, limb segment rotation, and internal shear. Rotation of limb segments between locked hinges results in thrust faults, which propagate through deformation zones on the frontlimb and possibly the backlimb. The faults may terminate both up and down section within brittle or ductile deformation zones. Eventually, a through going fault connects one of the major faults with the basal detach ment. Variations in the structural geometry are related to the rela tive thickness and competency of the units and the mechanisms of deformation. Because faulted detachment folds superficially resem ble fault-propagation folds, their geometry can be misinterpreted in areas of poor data quality. Characteristic features for distinguish ing these structures include (1) smaller variations in anticlinal length during fold evolution, (2) a more open and rounded fold geometry, (3) complex and nonuniform variations in fault slip, (4) a significant variation in deformational behavior among different units, and (5) fold-fault relationships indicating transition from folding to faulting, such as footwall synclines and decapitated fold geometries.