The effects of lithology and initial fault angle in physical models of fault-propagation folds

Abstract

Experimentally deformed physical rock models are used to examine the effects of changing mechanical stratigraphy and initial fault angle on the development of fault-propagation folds over a flat-ramp-flat thrust geometry. This study also investigates the scaling properties of the models to that of natural structures and processes. The model configurations used consist of a stratigraphic package that overlies a thrust fault that ramps upward from a basal detachment through a competent sandstone layer along a 20 or 30 degree precut ramp. Two different stratigraphic packages are used to show how the overlying stratigraphic sequence can affect the final geometry of the fault-propagation fold. One stratigraphic sequence contains a thick weak ductile layer that can undergo large thickness changes by shearing (lead is used to simulate a shale) and an overlying limestone layer that maintains constant thickness during deformation. The second stratigraphic sequence replaces the lead layer with a weak brittle layer that deforms by faulting and fracturing (dried pottery clay simulates an interbedded siliciclastic unit). The models were deformed in a triaxial deformation rig at confining pressure of 50 Mpa at room temperature. Each model formed a fault-propagation fold with an associated footwall syncline in the footwall of the fault and hanging wall anticline. The layers within the footwall syncline exhibit steeply dipping to overturned beds. The hanging wall anticline has an asymmetric geometry with a steeply dipping forelimb and a gently dipping backlimb. Although the preceding description applies to both the lead and clay models, the mechanical responses and resulting fault-propagation fold geometries between the two stratigraphies are strikingly different. In the lead models, the thick ductile lead unit absorbs a large amount of fault displacement and promotes folding in the overlying competent unit. In the clay models, the clay absorbs limited amounts of fault displacement and has a tendency to localize strain, which results in macroscopic faulting within the clay layer. The clay transmits the associated stresses and faulting to the overlying limestone unit, which develops a forelimb thrust. The models are then compared/contrasted to natural examples.