Effects of subsidiary faults on the geometric construction of listric normal fault systems (Geologic Note)

Abstract

Listric normal fault systems are one of the most important sites for petroleum exploration in extension basins. The geometry of these systems at depth is commonly uncertain because of poor seismic data. Thus several techniques have been developed to construct the shape and position of the master fault at depth using one or more shallower horizons. Antithetic and synthetic faults commonly disrupt the continuity of bedding in the deformed hanging wall and contribute to the overall extension. Current models either neglect or do not adequately account for the effect of these faults in estimating the geometry of the master fault. We suggest that the extension of individual subsidiary faults should be added to the amount of extension on the master fault during the bed-fault construction. The extension on subsidiary faults is supposed to be transferred into the main fault, along the inclination of shear, although the linking mechanism between the small and main faults is unclear. The inclined shear angle can be estimated by orientation of the antithetic faults. Occurrence of synthetic faults may slightly distort the particle motion trajectories (inclined shear) relative to a hanging-wall reference frame but does not change the overall relative motion pattern. Application of our modification of the inclined shear model to unpublished and published seismic data, as well as an analog experiment model, agrees well with observed fault geometry. Negating the amount of heave on subsidiary faults results in an overestimation of depth to detachment.