Numerical modeling of fold initiation at thrust ramps

The movement of the hangingwall over the footwall at thrust ramps produces a variety of structures found commonly at different scales in fold-and-thrust belts. The nature of the structure depends on the relative rigidity of the hangingwall and footwall, friction along the fault, fault 'dip', fault displacement, and the boundary conditions of the deformation. Structures include fault-bend style folds, fault-propagation style folds and wedge folds. We investigate the initial stages of development of such structures using the unite-difference code FLAC. The rock layers are represented as continua with elastic-plastic Mohr-Coulomb constitutive relations, and the fault and bounding bedding planes are assigned normal and shear stiffnesses and coefficients of friction. Under conditions with all layers compressed, antisymmetric fault-propagation style folds develop in both the hangingwall and footwall. With rigid footwall and restricted far-field slip in the hangingwall, a single fault-propagation style fold develops. With far-field displacement of the hangingwall allowed, broader antisymmetric wedge folds develop if hangingwall and footwall are deformable, and a single fault-bend style fold develops if the footwall is rigid. All structures become accentuated with increasing slip on the fault. Where both hangingwall and footwall are deformable, the deformation reduces the ramp angle and tends to minimize distortion of the rock adjacent to the fault. Fault-propagation style folds, paired wedge folds and fault-bend style folds are common in nature, and small-scale examples can be found in various stages of development. Continued slip on thrust faults may lead to the mature structures commonly seen in fold-and-thrust belts.