Fault-propagation folding, a common folding mechanism in fold and thrust belts, occurs when a propagating thrust fault loses slip and terminates upsection by transferring its shortening to a fold developing at its tip. Area-balanced theoretical models that relate the footwall cutoff angle (θ) to the fold interlimb half-angles (γ* and γ) show that open folds (high γ* and γ) are characterized by relative thickening of stratigraphically higher units, whereas tight folds (low γ* and γ) are characterized by relative thinning of these units. The propagation of thrusts is commonly characterized by the progressive tightening of the fold hinge and steepening of the front limb. Thickening of stratigraphically higher units in the early stages of folding is followed by localized thinning of the front limb in the late stages.
The geometry of a fault-propagation fold can be modified by subsequent translation on propagating thrusts. The thrust fault may propagate through the undeformed units, along the synclinal axial plane, or through the forelimb of the anticline, depending on the tightness of the fold. Deeper thrusts commonly are abandoned upsection, and the slip transferred to steeper imbricates, resulting in the listric geometries of many thrust faults. The fold also can be transported over a ramp and onto an upper detachment, resulting in a transition to fault-bend folding. Fault-propagation folds with or without additional fold translation can be distinguished from translated detachment folds by the detailed geometries of the hanging wall and footwall structures, and by the characteristic differences in their relations between fault slip and depth to detachment. Some important characteristics of fault-propagation folds are that they require no slip transfer in or out of the structure, involve a minimum amount of shortening, and have a relatively large depth to detachment, compared to other types of fault-related folds. Fault-propagation folds form important hydrocarbon traps in fold and thrust belts. Some common trap types include fold traps in the crestal area, and fault traps in the footwall and along imbricates on the forelimb and the backlimb of major basement-detached and basement-involved anticlines. Secondary traps also occur in intraplate and leading-edge structures within major thrust sheets.