Christopher T. McLain¹, John H. Spang¹

(1) Texas A&M University, College Station, TX

ABSTRACT: The Effects of Mechanical Stratigraphy on the Development of Fault-Propagation Folds in Physical Models

Experimentally deformed physical rock models are used to examine the effects of changing mechanical stratigraphy on the development of fault-propagation folds over a ramp-flat thrust. The model configuration consists of a stratigraphic package that overlies a thrust fault. The thrust fault ramps upward through a competent layer (sandstone) from a basal detachment. 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 ductile layer that can undergo large thickness changes by shearing (lead simulates a shale) and an overlying limestone layer that maintains constant thickness during deformation. The second stratigraphic sequence replaces the weak ductile layer from the first sequence 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 rig at 50 Mpa and room temperature.

Each model formed a fault-propagation fold with an associated footwall syncline 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. 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 transmits the associated stresses and faulting to the overlying limestone unit, which develops a forelimb thrust.

AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado