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.
Effects of a lateral facies change on
structural patterns in physical models