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Instabilities and Localization |
Center for Tectonophysics, Departments of Geology and Geophysics, Texas A&M University, College Station, TX 77843, USA
To model the regular spacing of the thrust faults in duplexes we subject a single layer embedded in an overlying viscous fluid, and an underlying rigid medium, to combined layer-parallel shortening and shear. The layer is a rigid-plastic solid at yield. A cusp introduced as a small perturbation on the upper layer interface perturbs the flow. The lower layer interface is weak and remains flat throughout the deformation, whereas the upper interface is deformable and can assimilate both weak and strong conditions. If the upper interface is strong it folds into a broad open fold. If the interface is weak the flow localizes into narrow shear bands that cut into the layer from the cusp, and reflect from the layer interfaces at a constant spacing. Material is displaced across the shear bands in a fault-like manner. If the layer is subjected solely to layer-parallel shortening these shear bands are symmetric. Superposing a shear component on the layer interface reorients the shear bands. The shear band dipping opposite to the sense of shear is more strongly developed and intersects the interface at a lower angle than the other. This is in agreement with the observations of duplex configurations.
