Modelling hanging wall accommodation above rigid thrust ramps

Experimental models are used to study the role of material rheology in hang ing wall accommodation above rigid flat-ramp-flat thrust footwalls. The def ormation in the hanging wall was accomplished by forwards sliding along a r igid basal staircase trajectory with a variable ramp angle, cc, ranging fro m 15 degrees to 60 degrees. We model different ramp angles to examine hangi ng wall accommodation styles above thrust ramps of overthrust faults (alpha ranging from 15 degrees to 30 degrees), as well as above pre-existing norm al faults (alpha ranging from 45 degrees to 60 degrees). For the hanging wa lls we used stratified frictional (sand) and viscous (silicone putty) mater ials. In this paper we study three types of models. Type I models represent purely frictional hanging walls where accommodation above thrust ramps was by layer-parallel thickening and by generating a series of back thrusts. T ype 2 and 3 models represent stratified frictional/viscous hanging wails. I n these models, accommodation was by a complex association of reverse and n ormal faults, mainly controlled by the rheological anisotropy as well as by the ramp inclination angle cc. In Type 2 models the silicone covered only the lower flat, while in Type 3 models it also covered the rigid ramp. For cc less than or equal to 30 degrees in Type 2 models and alpha less than or equal to 45 degrees in Type 3 models, the Viscous layer inhibited the deve lopment of back thrusts in the frictional hanging wall, instead the silicon e thickened to develop a 'ductile ramp'. For alpha-values higher than 30 de grees in Type 2 models and alpha = 45 degrees in Type 3 models, back thrust s develop in response to the bulk compression. The experiments simulate man y structures observed above natural thrust ramps with alpha less than or eq ual to 30 degrees and pre-existing normal faults with alpha greater than or equal to 45 degrees. The models emphasise the importance of a basal ductil e layer, which allows the hanging wail to step-up over the rigid ramp by bu ilding up its own ductile ramp. The models also emphasise that foreland-dir ected normal faulting can develop at a thrust front in the case that the ve rtical stress due to gravity exceeds the horizontal stress due to end-loadi ng within a thrust wedge. (C) 2000 Elsevier Science Ltd. All rights reserve d.