Mechanical models of fault propagation folds and comparison to the trishear kinematic model

Faults propagating through the Earth generate a wave of deformation ahead of their tip lines. We have modeled this process to understand the relationship between fold geometry and fault propagation. Using finite element modeling (FEM), we investigate the response of incompressible frictionless and frictional materials, and a compressible frictional material with associated flow, to vertical and dipping faults whose tip lines propagate at rates 3–3.5 times their slip rates. The fold geometries, finite strain, and velocity fields in models with incompressible materials are very similar to those produced by the trishear kinematic model, even though the latter uses a purely ad hoc linear velocity field. Furthermore, when the trishear grid search is applied to the final geometry of the mechanical folds, the best fit kinematic models have approximately the same propagation-to-slip ratio as was used in the FEM experiments. However, when the compressible frictional material is used, the mechanical models exhibit a main triangular shear band in front of the tip line and a conjugate shear band in the fold backlimb, both migrating with the propagating tip line. The conjugate shear band, antithetic to the fault, produces a gentle anticlinal back limb, even though there is not a bend in the fault.