EFFECTS OF A LATERAL FACIES CHANGE ON STRUCTURAL PATTERNS IN PHYSICAL MODELS

We hypothesize that the presence and stratigraphic position of a lateral facies change from weaker to stronger material can cause differences in structural geometry of thin-skinned thrust belts.

We designed two physical analog models, each containing a lateral facies change from dry silica sand to glass microbeads at different stratigraphic positions. Each model consisted of six, 0.5 cm layers of colored sand in a 60 cm x 90 cm Plexiglas box. In an arbitrary coordinate system the 60 cm western wall translated eastward causing 36 cm of horizontal shortening at a rate of 4 cm/hr. Photographs of a surface sand grid taken at 15-minute intervals allowed tracking of progressive deformation. After deformation we sliced the models in 3 cm increments to expose serial cross sections. In the Model 1 design a 30 cm x 90 cm x 0.5 cm basal layer of glass microbeads transitioned northward to an equivalent layer of sand, representing a facies change. In Model 2 the 30 cm x 90 cm x 0.5 cm layer of microbeads was located 0.5 cm above the basal layer of sand to represent a facies change from south to north at a shallower stratigraphic level.

After deformation, both models displayed 50% horizontal shortening and developed no orogenic curvature. Thrust fault dips decreased from hinterland to foreland. The southern side of Model 1 with the microbead detachment layer produced fewer faults, greater fault displacement and horse length, and more back-thrusts than the northern side. Locating the layer of microbeads 0.5 cm above the base in Model 2 significantly reduced the structural differences across the northward transition from weaker microbeads to stronger sand.

Results suggest that lateral facies changes in the stratigraphy of mountain systems may be important drivers of along-strike variations in structural style.