Fault displacement-gradient folds and the structure at Lost Hills, California (U.S.A.)

Geometric models of fault-propagation folds require that the underlying fault tip propagate upward across layering. However, related folds can be generated if the tip remains stationary, or even moves back along the ramp as a pre-existing fault is reactivated. Fault-propagation folds, detachment folds, as well as those produced when there is no propagation are generated by a displacement gradient along the underlying fault, and, as a general class, can be called fault displacement-gradient folds. New geometric equations are derived which relate fold shape to displacement gradient, and fault-propagation folds are a special case of these more general equations. Growth sequences associated with these structures have been used to distinguish fault-propagation folds from fault-bend folds when the underlying structure is obscured. A displacement-gradient fold in which the fault tip retreats back down the ramp generates a growth sequence that in some cases could be confused with that of a fault-bend fold. The structure at Lost Hills, in the San Joaquin Valley of California (U.S.A.) has been interpreted as a fault-bend fold, but it may be a displacement-gradient fold which formed as a pre-existing fault was reactivated during a Pliocene change in plate motion.