INFLUENCE OF MECHANICAL STRATIGRAPHY ON THRUST BELT MORPHOLOGY IN PHYSICAL MODELS

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.