Feilfolding

Feilfolding refers to the formation of folds in rock layers as a result of fault movement. The term is used in structural geology to describe fault-induced folding that accompanies slip on faults, including dip-slip and strike-slip motion. Feilfolding can affect sedimentary strata, igneous rocks, and metamorphic units adjacent to faults and shear zones and is observed in many tectonic settings.

Mechanisms and forms include fault-propagation folding, fault-bend folding, and drag folding. Fault-propagation folds develop as a fault tip advances and bends enclosing rocks into a curved surface. Fault-bend folds form when beds are folded over a bend or jog in the fault plane. Drag folds arise from deformation in the hanging wall near the fault, producing curved beds adjacent to the fault. The resulting geometry depends on fault geometry, slip rate, rock rheology, and the orientation and preexisting stratification of the rocks.

Interpretation and significance: Feilfolding provides information about fault geometry, slip history, and tectonic stress regimes. It affects fracture networks, permeability, and mechanical behavior in the fault zone, which has practical implications for hydrocarbon reservoirs, groundwater flow, and mineral exploration. It is commonly identified in field outcrops, seismic-reflection profiles, and drill cores, and can range from microscopic to regional scales.

Terminology: In some literature, feilfolding is used to describe fault-related folding more broadly, with specific subtypes such as drag folds and fault-propagation folds. The term is more common in certain European or Scandinavian geological texts and may be encountered alongside broader descriptions of fault-related deformation.