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1 Hans Ramberg Tectonic Laboratory, Institute of Geology, University of Uppsala, Box 555, S-751 22 Uppsala, Sweden
2 The Netherlands Center for Integrated Solid Earth Sciences, Faculty of Earth and Life Sciences, Vrije Universiteit, De Boelelaan 1085 HV, Amsterdam, The Netherlands
In this paper we study the dynamic and rheologic control of hanging wall accommodation 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 the yield stress and gravity stress. In frictional materials hanging wall accommodation 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 in frictional sediments at upper crustal levels, where temperatures and pressures are low.
