FAULTED DETACHMENT FOLDS: AN ALTERNATIVE MODEL FOR FAULT-TIP FOLDS

Fault-tip folds represent a common structural style in a number of fold-thrust belts, and involve blind thrusts that lose slip and die out up section in folds forming at their tips. These structures are commonly interpreted using one of several fault-propagation fold models, which apply to thin-bedded units with a homogeneous lithology. Units characterized by strong competency contrasts form by a different mechanism, involving a transition in deformational behavior from detachment folding to progressive fault propagation with increasing shortening. The major forelimb and back limb thrusts are secondary to the fold, and accommodate strain resulting from variations in structural position and lithology. The units first deform by detachment folding, with the fold wavelength controlled by the thickness of the dominant unit. Volumetric constraints require the movement of material from the synclinal to the anticlinal hinges. Continued shortening results in a tighter fold and the propagation of a thrust fault through competent units on the steep front limb. The fault may initially terminate both up and down section within deformation zones in the incompetent units. With continuing deformation, it connects with the basal detachment to form a through going thrust fault. The relative importance of limb rotation and hinge migration depends on the thickness and competency of the basal unit, and the flexural-slip efficiency of the more competent upper units. Although asymmetric faulted detachment folds are more common, symmetric folds may also form, especially above horizontal detachments with a weak basal layer. Faulted detachment folds occur in a number of fold-thrust belts, including the Wyoming-Idaho, Central Appalachian, Zagros, Albanide, and Papua-New Guinea belts, as well as in the deepwater Mississippi fan and Perdido fold belts. Because these structures superficially mimic fault-propagation folds, their geometry can be misinterpreted in areas of poor data quality. Characteristic features for distinguishing faulted detachment folds include (1) smaller variations in restored wavelength with shortening, (2) complex variations in fault slip, (3) the development of footwall synclines, and (4) ductile deformation and anomalous fold-fault relationships within the basal unit.