Geometry and Fracture Distribution in Fault-Propagation Folds in Nature and Experiments

Judith S. Chester

Fault-propagation folds in compressional terranes are attractive reservoir targets, particularly where deformation processes such as fracturing and faulting enhance reservoir and transport properties. Field examples and laboratory modeling suggest that fold geometry, fracture distribution, and propagation of the master fault vary with lithologic layering. Fault-propagation folds in the Absaroka thrust sheet, Salt River Range, Wyoming, are characterized by a complexly folded and faulted sequence overlying imbricate thrust ramps. The lower Paleozoic section consists of a mechanically anisotropic rock sequence that has variable bedding thickness and relative ductility. Above thrust ramps, thick-bedded brittle units shorten by isolated faulting and fracturing. The faulted and fractured zones are separated by disharmonic folds and zones of interlayer slip in the thinly bedded units. This geometry suggests that fault-propagation folds in mechanically layered rock grow by the linking of structurally higher isolated faulted segments rather than by the continuous propagation of the master thrust upsection.

A similar mechanism of fold growth has been observed in experimentally deformed rock models. Displacement on the master thrust is accommodated in the overlying sequence by fracturing and faulting in the brittle units and by disharmonic folding and interlayer slip in the intervening units. In sequences having a high degree of anisotropy and moderate ductility, a narrow zone of fracturing and faulting forms. This zone is localized in the fold hinge region ahead of the propagating master thrust. In sequences having a high degree of anisotropy and high ductility, intensely fractured zones form on the frontal and dorsal limbs of the fold, whereas the crest remains relatively undeformed. As each fault-propagation fold grows, the isolated fault segments propagate up and down section, eventua ly coalescing with the master thrust ramp.

AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.