Hanging wall accommodation styles in ramp- flat thrust models

Abstract: In this paper we study the dynamic and theologic ontrol of hanging wall accommo-dation in ramp-flat thrust models. In particular we vary the dimensionless ratio of shear strength to gravity stress to model hanging wall accommodation styles in different materials. In all models we require that the flat-ramp-flat footwall provides a surface of low frictional resistance. In viscous materials hanging wall accommodation progresses by wedge flow. In Bingham materials, wedge flow is also the preferred mode in cases where the gravity stress exceeds the yield limit of materials. Such models simulate the flow of salt or snow glaciers above ramp obstructions. At high ratios of shear strength to gravity stress the hanging wall blocks translate forward without bending and unbending to the form of the rigid footwall. In elastic-plastic strain-hardening materials ramp-flat accommodation progresses by fault-bend folding in case there is a near balance between theyield stress and gravity stress. In frictional materials hanging wall accommo-dation progresses by shear or kink-band nucleation above fault-bends. The shear or kink-bands which initially nucleate at the lower fault-bend change shape and reactivate by normal faulting or tensile failure at the upper fault-bend, depending on the ratio of shear strength to gravity stress. In nature, hanging wall accommodation by thrust nucleation above ramps and their subsequent reactivation may be anticipated infrictional sediments atupper crustal levels, where temperatures and pressures are low. Material models are used to study the dynamic and rheologic control of hanging wall accommodation above undeformed footwalls, the purpose being to gain insight into how materials flow past ramps as a function of rheologic and dynamic constraints. Since the initial work of Rich (1934), it has been suggested by a number of investigators (e.g. Serr