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.

Cross section at left is from Starck and Schulz (2017), accessible here. The match isn’ t perfect, but the model shows many elements of the real structure, particularly major thickening of the weak layer in the core of the anticline. The tall forelimb of the anticline and the “T”-shaped area near the surface are also interesting. The main difference between the actual and the model is the lack of a flat glide plan at depth in the model. This causes a thrust to break through the base of the forelimb. Note the 5 km scale bar on the real cross section. Model materials are discussed a few photos down.

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.

This structural style is all about mechanical contrast. The strong layers shorten and thicken by breaking (faulting) and stacking on themselves, while the weak layers experience a more flow-like process.

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.

A simple doodle with bad erasing. Compressing and shortening the layer pack moves points A and B together. The bottom layer faults on top of itself, but the white middle layer between A and B is “mashed” into the core of the fold and completely changes its shape during shortening. This isn’t drawn to scale. If it were, the white area mashed into the fold core in the bottom image would have to have the same area (and thus volume in 3-D) as the rectangular white area between A and B in the top sketch.

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 “normal thickness” of the lower white layer is minor within the model, but it becomes monstrously thick in the core of the fold. If you could see bedding planes in it, they would appear intensely deformed, buckled, and faulted at the small scale as suggested by the black form lines.

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.

In this part of the model, I think bedding in the thickened weak zone would show this pattern of buckling. If you look closely at the actual model, you can faintly see these patterns. They don’t come through in the photo. Note that minor faults within the pink and green part of the anticline don’t cross the white layer to break overlying yellow.

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.

Steep sides mean that the particles lock together well, resisting deformation and failure. Gently sloping sides (at left) mean that the particles slide and roll past each other freely, creating mechanical weakness. The red sand is mechanically the same as pink, green, dark blue, and orange in the model. The same microbeads are used with strongly cohesive material to produce landslide models.

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.

Don’t breathe too hard or the little vertical face at the top of the cone will collpase. If you can do this to your sand, it’s slightly cohesive. This is necessary to create the appropriate contrasts needed for the model, but is arguably unrealistic as an analog to sedimentary rock with depth-appropriate pore fluid pressure.

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 block diagram at top shows the Aguaragüe Anticline beneath the moderate-relief land surface. Tartagal and Mosconi, Argentina are shown. Topography offers few details to the subsurface structure. Black lines indicate the paths of hydrocarbon-seeking wells, which target different possible traps in these structure. The large-scale image below shows the block diagrams location (yellow box).

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.

https://www.sciencedirect.com/science/article/pii/S0191814112000557

https://www.sciencedirect.com/science/article/pii/B9780128160091000162

https://www.researchgate.net/publication/260136361_Analisis_de_algunos_aspectos_geometricos_

y_evolutivos_de_las_estructuras_de_la_faja_plegada_subandina_del_Norte_de_Argentina_y_Sur_de_Bolivia

This post was originally published on The Geo Models blog.