Fold-accommodation faults

Abstract

Fold-accommodation faults are secondary faults that accommodate strain variations related to structural and stratigraphic position during fold evolution. Four main types of fold-accommodation faults are commonly found. Out-of-syncline and into-anticline thrusts form primarily because of an increase in bed curvature within fold cores, although differential layer-parallel strain at different scales also contributes to fault slip. Depending on the kinematic evolution of the major fold, the thrusts may propagate along the steep or gentle limb of an asymmetric fold or along the hinge of symmetric folds. Wedge thrusts are primarily formed in competent units because of variations in penetrative layer-parallel strain between adjacent units. Limb wedges occur as hanging-wall and/or footwall fault-bend and fault-tip folds, whereas hinge wedges occur as multiple nested faults that tend to thicken the more competent units. Forelimb and backlimb thrusts form by a variety of mechanisms. Forelimb space-accommodation thrusts are low-displacement thrusts that resolve strain discontinuities resulting from increased curvature in fold cores. Forelimb shear thrusts form in the late stages of folding because of rotation and layer-parallel extension on the steep forelimbs of folds. Most backlimb thrusts originate as out-of-syncline thrusts. They may eventually link with forelimb thrusts to form forelimb-backlimb thrusts. Back thrusts accommodate hanging-wall strain during the formation of fault-related folds. They either form selectively in competent units or propagate through the section at the same rate as the main thrust. Although fold-accommodation faults are secondary features, they are important elements that define the geometry and size of structural traps in fold-thrust structures. Accurate mapping of these structures is therefore critical in interpreting the structural geometry of fold and thrust belts. Shankar Mitra holds the Monnett Chair and Professorship of 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 at ARCO Research and Exploration as senior and principal research geologist, director of Structural Geology, manager of Geological Research, and senior exploration research advisor. His primary research interests are in the development of structural concepts and methods and their application to exploration and production problems. He is a recipient of AAPG's Wallace Pratt and Cam Sproule awards.