MECHANICAL MODELS OF FAULT PROPAGATION FOLDS AND COMPARISON TO THE TRISHEAR KINEMATIC MODEL

In modeling folding ahead of a propagating fault tip, one is compromised between the versatility of kinematic models and the physical soundness of mechanical models. Bridging this gap, we have implemented 2D, large deformation finite element models (FEM) of faults propagating in elasto-plastic materials, and compared these mechanical simulations to the trishear kinematic model that successfully replicates many natural and analog examples of fault propagation folding. Faults propagating in incompressible -frictionless and frictional- materials generate yielding in a triangular zone symmetric to the fault that migrates with the fault tip. The resultant folds have forelimbs that become more open up-section and triangular zones of plastic strain symmetric to the faults. The trishear grid search applied to the final geometry of the mechanical folds gives best-fit kinematic models that have approximately the same propagation-to-slip ratio (P/S) as was prescribed in the FEM experiments (3-3.5), and trishear angles of 20 to 30°. These best-fit trishear models match the fold geometry, the finite strain, and the velocity fields of the mechanical experiments. However, faults propagating in compressible frictional materials generate yielding in two conjugate zones that migrate with the fault tip, one symmetric to the fault and the other towards the backlimb. Besides forelimbs that taper up-section and triangular zones of plastic strain symmetric to the fault, the resultant folds have an anticlinal backlimb and a backlimb zone of plastic strain. These models show that anticlinal backlimbs can form without any change in fault dip, a concept almost heretical to the kinematic modeler. Backlimb rotation diminishes as the experiments evolve and increases with material strain softening and dilation. Our experiments confirm the basic soundness of the trishear model but also point out the limitations of it and all kinematic models. More realistic mechanical models with natural fault tip propagation and mechanical anisotropy are needed to explore the mechanical controls on P/S and trishear angle.