Physical
analog models show that structural geometry of fold-thrust belts is
influenced by lateral and vertical differences in elastic mechanical
layering that reflect facies changes in sedimentary strata. Three
physical models were designed, each composed of horizontal layers of
dry silica sand (σ0 ~102 Pa) and powdered kaolinite (σ0 ~104 Pa)
with a total stratigraphic thickness of 5cm, representing
approximately 5km of sedimentary rock. Half of each model provided a
control composed entirely of layers of dry sand, while the other
half consisted of kaolinite layers interspersed with layers of sand.
Models were constructed in a 60cm x 60cm compression box with one
moving wall. Overlapping plastic sheets constituted the base of each
model and acted as a detachment to localize deformation across the
center of the box rather than along the moving wall. Each model
experienced 8cm of horizontal shortening at a rate of 4cm/hr.
The resulting fold-thrust belt models reveal that the number of
surface terraces, the width of individual surface terraces, and the
width of the entire thrust belt depend on two factors: 1) proximity
of kaolinite layers to the upper surface and 2) total thickness of
kaolinite. Within each model, the sand control side possesses fewer
faults and terraces than the side containing both sand and
kaolinite. Models with thicker kaolinite strata develop more faults
and surface terraces. Cross sections confirm that surface terraces
correspond with thrust faults that propagate upward from the basal
detachment. In each model, the lateral facies change from sand only
to sand plus kaolinite marks the location of a distinct change in
overall thrust belt width. These findings imply that the presence
and amount of stronger elastic material, as well as its location in
the stratigraphic section, affect structural development of
fold-thrust belts. Results may be applicable to understanding the
roots of orogenic curvature as found in the Pennsylvania salient of
the Appalachian Mountains, or the development of surface terraces in
active tectonic regions like Wheeler Ridge in the San Joaquin
Valley, California.