by Philip S. Prince, Virginia Tech Active Tectonics and Geomorphology Lab
In addition to having different chemical compositions, origins, and appearances, Earth’s different rock types can behave differently when they are subjected to tectonic stress. Varying mechanical behaviors can be particularly pronounced in sedimentary fold-thrust belts, where weak rock types, particularly shale, separate stronger, more brittle rock types such as sandstone or limestone. Shale can appear to “flow” within thrust belt structures (though not to the extent of salt/evaporites), thickening considerably in the hinges of folds and disconnecting the brittle layers above and below it. This effect can be reproduced in a physical model using granular materials with different particle shapes.
The anticline on the left of the image above is the Aguaragüe Anticline, a structure in the sub-Andes fold-thrust belt in northwest Argentina. The deep pink layers are mechanically strong and brittle, and the upper yellow, orange, and gray sequence is also reasonably strong. The brown layer in the middle of the section, however, represents a shale-rich sequence and is quite weak. As a result, it slides freely between the stronger rock sequences and thickens considerably in the hinge of the anticline. The model at right replicates this effect, with the white layer representing the weak brown layer in the section at left. While the match is not exact, the model does capture the overall style of folding which results from the specific layer combination.
The brittle layers at the bottom of the sequence respond to shortening by thrust faulting and stacking, and the weak layers experience focused internal deformation to produce a flow-like behavior to fill in the tight but vertically extensive core of the fold. Both styles of deformation record essentially the same amount of shortening in this position in the model, but they do so through different styles of deformation.
Unfortunately, it is not possible to color the weak layer in the model to see the details of its small-scale internal folding and faulting. The addition of any other material would alter the mechanical behavior to the point that the model would not work. If you could add some marker layers, you would likely see some interesting patterns and structures.
The black lines in the thickened zone above are intended to show the concept of internal folding within the weak white layer. The image below is taken from another part of the model on the same structure. If you look closely, it is possible to see some dark blue sand squeezed down into the top of the thickened white zone.
Something is lost in these photos, but when I look at the actual slice of the model in detail, I would draw form lines in the white area like what are shown below.
I actually tried to make the model do this–it was not a chance occurrence. By controlling the mechanical properties of different parts of the layer pack, a wide variety of compressional thrust (or fold) structures can be formed if a good variety of granular materials is available. It is possible to make a quick comparison of how the granular materials will behave by simply looking at the cone that forms when the material is poured onto a flat, level surface. Shown below are two colored sands and white glass microbeads.
The yellow sand at right is the most fine-grained, and it is the strongest and most brittle material. Its particles are angular and very small, and they lock together very effectively to resist failure and support the steep sides of the cone. The red sand in the middle has slightly larger particles. It is the same material as the pink, green, dark blue, and orange sand in the model. It is still strong and brittle, but its cone is slighlty less steep-sided because its particles produce fewer and slightly weaker frictional connections. The microbeads on the left are spherical and have a friction-reducing coating so that they flow efficiently in abrasive blasting use (their intended purpose!). As a result, the particles do not lock together very well at all. The beads tend to flow outward to form more of a “puddle” than a cone. They make a nice analog for shale when encased between the stronger sands. This is a simple visual comparison, but it works for the same reason that the model behaved as it did.
The yellow sand is interesting because it could be considered “too strong” for use in the model. It is slightly cohesive, meaning the strength of connection between its particles does not only result from the weight of overlying material pushing the particles together. They also stick together slightly, allowing a vertical face to be carved in the cone. This takes a steady hand and will quickly collapse at the slightest bump or air movement, but it does show the effect of cohesion. Modeling of sedimentary rock deformation in the upper part of the Earth’s crust should, in theory, be done only with non-cohesive materials, but exaggerating contrasts a bit is necessary to create the style of model shown here.
I like the Aguaragüe anticline and this model because they emphasize the fact that not all rocks are the same. This seems obvious, but I guess I mean it provides a reason to be able to identify different rock types and understand how their mineral composition can impact their mechanical behavior. A geologist who knows that strongly contrasting layers are present in the section can be ready to interpret unusual or unexpected subsurface structure resulting from layer contrast. The real Aguaragüe anticline (source of the cross section here), has been extensively explored by drilling, so there is good understanding of its subsurface structure. I think it would be very tough to predict this subsurface geometry from surface data alone!
The Aguaragüe Anticline shows that structures observed on the surface don’t always project downward very far, particularly when shale detachments are available in the rock column. Elements of this structural style are seen throughout this portion of the sub-Andes. A Google search for “Aguaragüe anticline” will produce a large number of results, with plenty of cool imagery to check out. This is a very well illustrated portion of the sub-Andes thrust belt, with emphasis on both single structures and the thrust belt as a whole.