Thin-Skinned Models for Undergraduate Teaching Labs – "Flour Structures"Dan M. Davis Dept. of Geosciences, Stony Brook University

IntroductionAnalog modeling of thin-skinned tectonics can be useful in teaching as well as in research.  Because ‘sandbox’ models can be difficult and time-consuming to set up and exhume, they are typically not well suited for use in undergraduate teaching laboratories.

I present here a simple, inexpensive way for an undergraduate lab class to create model thin-skinned wedges (Figure 1).  These experiments, in which students can create a wide range of model wedges, require only easily obtained materials: flour, sandpaper, graphite, transparency sheets, kitchen sifters, painting brushes, and athletic field marker chalk.

A photocopier transparency sheet makes a good tool to use in excavating the experiment after it is completed (Figure 8).

Summary A simple model, easily created and run in an undergraduate lab class, can demonstrate many aspects of the growth of thin-skinned mountain belts. Depending upon the strength of the décollement, the cross-sectional taper of the deforming wedge will be thin or broad, internal deformation will be mild or intense, and the structures either close to symmetric or strongly forward-vergent, just as at the analogous natural thrust belts. Other pertinent models can be generated equally easily:- By including a horizontal sheet of wood or Plexiglas (forearc basement

rocks) in front of the pushing block allows generation of a model accretionary wedge, outer-are high, and forearc basin.

- Adding a fixed wooden block in the foreland allows exploration of the effects of basement topography.

- A model that uses graphite in some parts of the box and sandpaper in others allows the generation of oblique compensation structures in response to along-strike strength contrasts.

Fig. 1

taper

Figure 15a

forearc basin

retro-wedge

forearc high

accretionary wedge

oceanic plate

deformation front

to

foreland

Fig. 2

Fig. 3

Experimental Set-upAny large rigid box will suffice for these experiments: we use drawers from geologic sample cases (60cm x 75cm, 5 cm deep). Each box is shared by 2-4 students.

Each group of students sets up their box with a unique set of boundary conditions. After all boxes are ready, the models are run one at a time, with the entire class observing the running and excavation of each of the experiments.

A key parameter is the strength of basal friction. To study its effects, separate groups can set up experiments with the base as either: 1) the bare base of the box, 2) sheets of ≈60 grit sandpaper duct taped together (Figure 2) and then taped along its edges, face up, onto the base of the box, or3) a minimal (<<1mm) layer of graphite that has been rained in from above using a sifter (Figure 3).

Before introducing flour into the box, a wooden block the width of the box and about 10 cm tall (see Figure 1) should be placed at one far end (the hinterland) of the box.

Flour preserves the internal structure of the model mountain belt far better than the granular materials like sand that are better from the standpoint of strength scaling. The total thickness of all the layers of flour (typically 2 to 6 cm) is not critical - it can vary, depending upon the depth of the box and the amount of flour available. Each layer of the flour (typically < 1cm) is added using a sieve (Figure 4). The scaled cohesion of the flour (with a model length scale factor of ≈105) is high, as evident by the fact that after the experiment one can cut stable cross-sectional slices a few cm high. Thus, at each stage the flour should be smoothed very gently, using a soft brush (Figure 5), so as to avoid packing it and increasing its cohesion even further.

At two or three stages in constructing the experiment, a colored layer (athletic field marker chalk) is sieved across the central 3/4 of the box (Figure 6) to create a marker layer to be viewed when the experiment is eventually excavated. This colored layer should be as thin as possible to insure that it has no significant effect on the deformation. The intervening flour layers are typically ≈1 cm thick.

Friction from the side walls distorts the stress field within a couple of cm from the walls, so the structures there are of less interest and it is not necessary to have marker layers there.

Any colored granular material will suffice. We sometimes use a green play sand, which can be emplaced using a sieve or pastry bag - anything that makes it possible to place the colored material on the surface of the flour in a controlled manner.

Fig. 4

Fig. 5

Fig. 6

Fig. 7

Fig. 8

It is useful to place thin colored lines at a uniform spacing on the the top surface of the experiment (Figure 7), because changes in that spacing during the experiment can be used as a direct measure of shortening in the model orogenic belt. We use a 10 cm spacing between these hash marks.

Running the Experiment

It is now possible to start the experiment. A thin wooden sheet the width of the box and about 10 cm high is used to provide a push from the hinterland (see Figure 1). It should be pushed slowly and carefully, avoiding sharp accelerations – and despite the image shown, two hands are recommended.

It is best to stop the experiment well before the frontal thrust reaches the far end of the box, so as to avoid having the deformation affected by that rigid boundary. Once the experiment is complete, it becomes possible to excavate the model and view the internal structure that developed as part of the deformation. Photocopier transparency sheets are very useful for this purpose (Figure 8).

There are several things to note: - A series of thrust scarps appear at the surface, with the youngest thrust toward the foreland. - Any given location remains essentially undeformed (with hash marks remaining 10 cm apart) until

the front of the orogen approaches it. - There is typically a small amount of shortening (compaction) even out in front of the frontal thrust. This can be thought of as analogous to compaction and layer-parallel shortening in front of a natural thrust belt. - The most rapid deformation is around the frontal thrust, though some deformation continues throughout the orogen. - That deformation produces a wedge-shaped orogen above the décollement, as in the cross-sections in Figures 9 and 10. The taper of a wedge formed over sandpaper (Figure 9) is quite large and the the wedge is tall, narrow, and intensely deformed. A wedge formed over a weak base in graphite (Figure 10) has a narrower taper and a less pronounced preference for

foreland-vergent thrusting. Thrusts in it are more widely spaced, the orogen is wider, and strain (measured using the surface hash marks) is smaller.

taper

Fig. 9

Fig. 10

Fig. 11

Fig. 12a

Fig. 12b

Fig. 13a

Fig. 13b

With a sufficiently large group of students, it is effective to have multiple stations, each set up by a team of 2-4 students. One interesting alternative is a model with a mixed strong/weak base. The sandpaper and graphite may be arranged in any way, but I find it useful to divide them along strike. In this case, the experiment produces (for example) a steep wedge on the left and a broader but less steep weak-basal one along strike to the right - separated by oblique accommodation structures, as imaged in an along-strike cross-section in Figure 11.

It is best if the students assembling the model don’t tell their classmates what their basal boundary conditions are and if a piece of cloth, behind the wooden push-block, covers the newly exposed basal sandpaper and graphite from view. The other students can then try to predict the nature of the detachment on the basis of surface structures and strains.

There are other interesting options. One is a ‘forearc’ model, in which a thin, rigid (e.g., Plexiglas) sheet the width of the box is placed on the base, directly in front of the wooden block (Figure 12a). This sheet then acts as a strong, low-angle ‘backstop’, leading to the formation of a two-sided orogen (Figure 12b)

Alternatively, the rigid sheet can be placed in the foreland, to mimic basement relief (Figure 13a). Combined with a graphite base and a pre-existing taper, this produces a model orogen reminiscent of the Salt Range and Potwar Plateau (Figure 13b).

The options are nearly limitless, and students enjoy exploring them. It is, however, a good idea to make clean-up a required part of the assignment!

In this case, the cross-section has been cut normal to convergence, in order to see along-strike structural variability.