FOOTWALL GEOMETRY AND THE RHEOLOGY OF THRUST SHEETS

The inter-relationships between the exact footwall geometry and the rheology of thrust sheets are investigated. Deviations in the thrust fault surface from an ideal plane will induce a local heterogeneous deformation. The resulting deformation processes depend upon the rate of thrust sheet displacement, the geometry of the feature causing heterogeneous flow, the deformation conditions and the lithologies involved. Two classes of features are particularly important in causing heterogeneous deformation in thrust sheets. The first features are small perturbations on bedding planes which may be inherited sedimentary structures or produced during layer-parallel shortening; the second class of features are ramps, where the thrust sheet climbs up the stratigraphic section. Particular attention is given to the range of fault rocks and deformation microstructures preserved after movement over a footwall with a complex geometry. Analysis of the influence of the exact geometry of ramps on deformation processes during displacement leads to two important conclusions which the paper discusses. Refs.