Extensional fault-propagation folds: mechanical models and observations from the Modoc Plateau, northeastern California

Extensional fault-propagation folds are produced by warping of material ahead of a growing normal fault. We use field observations of surface-breaking normal faults in the Modoc Plateau, California, in conjunction with three-dimensional linear elastic boundary element computer models, to address the mechanical controls on extensional fault propagation folds. Detailed topographic and structural maps of the near-tip regions of several kilometer-long faults reveal: throw decreases rapidly toward the tip, suggesting elliptical slip distributions; fault-parallel monoclinal folds are present beyond the tip, with axes offset into the hanging wall; and open fissures are common in the tip region. Parametric tests with the model show that the amplitude and form of folds at the fault terminations are influenced by slip distribution, fault dip, elastic properties of the rock, and the presence or absence of a blind portion of the fault. Models reproduce the form of observed folds to first order and suggest that the faults, both isolated and in relay zones, are probably blind for some lateral extent beyond the surface break.