Mechanical models of trishear-like folds

Previous workers have formulated velocity descriptions of the trishear kinematic model of fault propagation-folds, which are inherently non-unique. We present two complete mechanical models of fault-related folding and assess the validity of the assumptions used in the assignment of velocities in the trishear description and to eliminate the untenable situation of an infinite number of possible solutions for velocity fields. The mechanical model of forced-folding, Forced Fold, based on viscous folding theory, is used to derive the velocity fields in an anisotropic sedimentary cover overlying faulted and displaced rigid basement blocks. The solution of displacements around a stress-free fault in an elastic body is used to model fault-arrest folds that form around a fault imbedded in a deformed medium. The results indicate that the velocity fields assumed in the trishear model more closely resemble the velocities derived in the mechanical Forced Fold model than the mechanical model of fault-arrest folding. The Forced Fold model produces a triangular region of concentrated deformation similar to the trishear region assumed in the kinematic models, while the deformation produced by the fault-arrest model is not concentrated within a triangular zone.