Geometry of the Hanging Wall Above a System of Listric Normal Faults--A Numerical Solution

Abstract

The process of hanging-wall deformation above a system of listric faults is controlled by the local stress field, rock properties, and fault geometry. Although fault geometry provides a boundary condition, the strained hanging wall can be adequately described by a process of simple shear during hanging-wall collapse. A numerical solution of hanging-wall deformation above a set of listric normal faults clearly demonstrates the importance of inclined simple shear. The true amount of horizontal extension and sense of inclined shear is estimated based on both rollover and fault geometry. This forward modeling technique treats listric faults as continuously curved surfaces, and may incorporate any number of faults, even or unevenly spaced, with variable fault geometry and near surface dip. Basin width is quite sensitive to sense of shear within the hanging wall and depth of detachment, as well as the apparent horizontal extension calculated solely upon the basis of bed-length balance. The model also indicates that antithetic shear is likely to occur near steeply dipping normal faults, whereas geologically reasonable basin geometries are generated under synthetic shear for low-angle normal faults. Ramp and flat listric-fault geometries can be modeled with inclined simple shear to generate basins. Care must be taken when assessing the amount of horizontal extension in extended terranes because the percentage of horizontal extension is quite sensitive to fault geometry and angle of internal shear. Arrays of listric normal faults may develop extensional geometries that resemble domino-style extension. If an apparent planar fault geometry observed near the surface assumes a listric shape downward and distributed deformation occurs within the hanging wall, the resulting estimation of extension based on the angular relationship between faults and beds is likely to be wrong. The model successfully describes the overall geometry of half-graben-type basins, and indicates an important link between fault geometry and sense of shear within the hanging wall of listric normal faults. Near-vertical and synthetic shear are determined near low-angle normal faults, whereas antithetic shear is likely to be observed near steeply dipping normal faults.