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Structural Models of Faulted Detachment Folds

¹ School of Geology and Geophysics, University of Oklahoma, Norman, Oklahoma, 73019; smitra{at}ou.edu

Shankar Mitra holds the Victor E. Monnett Chair in Energy Resources at the University of Oklahoma. He received his Ph.D. in structural geology from Johns Hopkins University in 1977. He spent 19 years in research and exploration at ARCO Exploration and Production Company and ARCO International Oil and Gas Company. His research interests are in developing structural concepts and methods for interpretation of complex structures. He is a recipient of AAPG's Wallace Pratt and Cam Sproule awards and served as an associate editor for the AAPG Bulletin for nine years.

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 Perdido 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. Continued 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 throughgoing fault connects one of the major faults with the basal detachment. Variations in the structural geometry are related to the relative thickness and competency of the units and the mechanisms of deformation. Because faulted detachment folds superficially resemble fault-propagation folds, their geometry can be misinterpreted in areas of poor data quality. Characteristic features for distinguishing 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.

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