Identification of Wrench Faults Using Subsurface Structural Data: Criteria and Pitfalls (1)

Abstract

Many structures imaged with reflection seismic can resemble a wrench fault system because of the great structural diversity of strike-slip faults and the wide range of contractional and extensional features associated with these faults. Because of these resemblances, numerous faults are misidentified as wrench faults by workers using subsurface structural data. Credible identification requires combinations of profile and map criteria: (1) narrow, long, straight, throughgoing, solitary master fault or zone of deformation, (2) steep to moderate dip of the master fault at depth, (3) offset of the top of basement, (4) changes in relative upthrown side, separation sense, and/or fault dip direction at depth or along the strike of the master fault, (5) narrow fault slices within the master fault zone that steepen and join at depth (negative and positive flower structures), (6) different separation sense and orientation of relative upthrown side at these fault slices, and (7) coeval, en echelon flanking structures. The identification should be confirmed with other criteria that refute alternative interpretations. The main pitfalls to a reliable identification are not considering alternative structural styles and the misinterpretation of features or patterns that superficially resemble wrench systems. The profile geometries of basement-involved reverse or normal faults, particularly those with structural inversion, can resemble the single strand of convergent or divergent wrench faults. Profile geometries of flower structures can be mimicked by narrow contractional horsts or narrow extensional grabens, complexly faulted anticlines or synclines, splay faults, fault zones intruded by salt or shale, and overprinted structures. Pitfalls in interpreting map relationships are possible when structural patterns combine several tectonic influences, are controlled by rock anisotropy, or include elements hat are not coeval, and if the patterns characteristic of other styles are misinterpreted. Many erroneous identifications of wrench faults can be eliminated by demonstrating that the investigated fault or feature lacks a straight trace, lateral length, or throughgoing continuity.