Modeling a fold-thrust belt with a sandbox.

This is a photo of a simple device that can mimic some aspects of foreland fold-thrust development. A layer of sorted fine sand a bit more than a inch thick overlies a sheet of mylar which is to be pulled underneath the back stop board. The sand is thus put in compression and contractional features form. The upper portion of the sand is sprayed with water from the spritzer, and is wet for several grain diameters depth. This gives an upper layer with some cohesive strength that can act brittlely, underlain by a weaker dry layer. The spritzer is used to develop as even wetting as possible. The arrows show the direction the sand will move relative to the back stop. This device was built with a small grant for the Center of Improvement of Instruction at the University of Nebraska Omaha that was written by Valerie Murray and Donna Erickson.


This is a high angle oblique view of the structures that developed on the right side of the sand box after only several inches of movement. The heavy dark arrows show the relative motion of the sand relative to the backstop. The first structure to form was the asymmetric anticline, which in this case is cored by the relatively mobile dry sand which has obviously thickened and produced relative uplift. It is common that folds are cored by faults or mobile material. In this model tensional fractures often develop along the fold hinge area. The marked thrust front developed later. Note how it passes into a symmetric anticlinal structure to the right. Without significant thickening of the underlying sand (no uplift of the upper wetter surface) there must be a detachment (a flat) between the symmetric and asymmetric anticlines. These structures continue to develop into the geometry depicted in inset A of the next photo. Note dime for scale.


This is a view of the length of fold-thrust wedge that developed after 4 inches of motion of the underlying mylar. The white is where the dry sand has been exposed in 'breached' anticlines. A close up of the area in inset B is shown below. Note the significant along strike changes that occur. These are likely due to differences in overall sand thickness and in the upper wet sand layer, and the effect of the walls. A dominant transport direction has been established, so that the foreland is at the bottom and the hinterland is represented near the backstop. A wedge of deformed sediments is beginning to form in front of the backstop. These wedge dynamics are described by a variety of sources, including many structural geology textbooks.


Inset B: A map view of an irregular thrust front with the hanging wall at the top and the footwall at the bottom. Given the general lack of hanging wall deformation or uplift a long flat exists underneath the thrust sheet. Again, the arrows indicate the direction of motion of the underlying mylar (so the transport direction of the majority of the thrusts is in the opposite direction). As the thrust is traced to the left it changes via a tear fault into a backthrust. Given the geometry, the backthrust must be connected underneath the uplifted flap of sand to the forethrust, and the tear fault length is thus a measure of overall hanging wall transport (at a bit less than the diameter of the dime).


In this oblique view 4 thrust fronts are noted. 1 is the youngest (most recent) and 4 is the oldest. This model nicely duplicates the pattern of foreland propagation evident in real fold-thrust belts, where as the wedge thickens it transmits more stress into the foreland and the fault propogates beneath the existing structures towards the foreland where it emerges. The light colored sand seen here is exposed in overturned, faulted- out anticlines. The dry sand moves out and over overturned bits of the wet sand brittle crust. Note how thrust front 2 joins thrust front 3 to form an eye-shaped thrust sheet. Similar patterns are found in real nature.


In this high angle oblique view the thrust front is highly segmented . Abrupt along strike changes defining structural segments is an important structural/tectonic phenomena. The short black lines define the different segments. The arrow again shows the direction of movement of the sand relative to the backstop, so that the dominant thrust direction is from top to bottom. On the right side are tear faults linking fore and back thrusts. The segment boundary just left of center is actually a tensional feature within the forethrust. Evidence of limited extension parallel to the arcuate trace of thrust fronts is common in real life, but is out of proportion here. Inset C on the photo below shows the approximate position of this area in the overall thrust stack complex.


In this oblique view of the thrust stack, after some 7 inches of motion of the underlying mylar, closely spaced thrusts separate undeformed foreland sand from a broad uplifted hinterland area. In the photo below it can be seen that the lower center area is subsequently incorporated into a large thrust sheet as movement continues.


This is an overall, oblique view of the wedge of folds and thrusts that developed in this experimental run (we were running out of sand by this point). The large thrust sheet mentioned above is identified. Note its overall eye shape. Note also the tear fault that connects it with the main complex on the right. Along strike variations in fold-thrust geometry are again due to variations in the sand. Since stress was transmitted further into the foreland in the middle this material was in aggregate a bit stronger. This is likely due to a greater thickness of the wet sand layer in the middle.


In summary , features or behavior we have seen in this simple device that have larger counterparts in mountain belts are: a foreland propagation pattern, a dominant direction of thrusting, backthrusts, tear faults, segmentation, extensive flats, anticlines and overturned anticlines, and an overall wedge geometry. Pretty good for a pile of sand in the class room. Numerous structure textbooks describe the dynamics of such wedges and provide references.


If you have questions or comments please contact Harmon D. Maher Jr. (harmon_maher@mail.unomaha.edu.), Department of Geography &Geology, University of Nebraska, Omaha, NE 68182