Mechanics of deformation in thrust fault zones

Fabrics in fault rocks along the Lewis thrust fault at Marias Pass, northwest Montana, depend on lithology. Fault rocks derived from hanging-wall dolomite are random-fabric cataclasites, whereas fault rocks that include both footwall shale and hanging-wall carbonate display microscopically ductile fabric. Dedolomitization of rocks at the base of the hanging wall and subsequent pressure solution of calcite contributes to the development of this ductile fabric. Progressive development of cataclasite from hanging-wall dolomite involves widening of cataclastic zones, whereas shale deformation becomes more localized.;Deformation at the base of the Lewis thrust sheet is spatially heterogeneous, with large regions dominated by either contraction or extension faults. This spatial heterogeneity of large-scale contractional and extensional features may be the result of spatially heterogeneous fault-zone strength. Analytical models of deformation within a thrust sheet overlying a fault zone of spatially heterogeneous strength predict spatially variable compression directions in the overlying thrust sheet, which lead to the development of both thrust and normal faults. The spatial heterogeneity of pressure solution and cataclasis in response to variable mean stress within a fault zone with spatially heterogeneous strength may ease thrust sheet movement relative to movement over a fault zone with spatially uniform strength.;Deformation in thrust fault zones is modelled using a form of plasticity theory that allows strain hardening and strain softening. Modelling of an infinitely thick strain-hardening fault zone predicts the spread of active deformation through the fault zone. The amount of displacement accommodated by a strain-hardening fault zone of infinite thickness is unlimited, whereas that accommodated by a strain-hardening fault zone of finite thickness is limited. The strength of thrust fault zones is affected by the position, thickness and strength of strong layers in the fault zone. Strain softening causes localization of deformation within the fault zone and increases the amount of displacement accommodated by the fault zone. These models may explain the distribution of deformation within the Lewis fault zone.