February 17, 2020
By Philip S. Prince, Virginia Tech Active Tectonics and Geomorphology Lab
Earth’s sedimentary fold-thrust belts make very interesting landscape patterns when fold structures interact with erosion and weathering. These patterns may reflect active fault movement and fold growth or progressive uplift and erosion of older, tectonically inactive thrust belts. Good examples of both can be observed with Google Earth or Google Maps.
Sand analog models can be used to create similar patterns by controlling characteristics of the layer pack. The model shown here did not work out as planned because I shortened it too much, but the overall appearance is still cool and reflects local variations in the layer pack. In real fold-thrust belts, the local or regional variations in folding and faulting style also reflect the details of the layer sequence being folded and faulted, among many other conditions.
This model featured a strong “lid” of very fine grained sand (light pink) tapering towards the right far right corner of the model, away from the backstop (see below). Where the thick pink layer is absent, its thickness is replaced by colorless glass microbeads, which are its mechanical opposite…very weak.
The “lid” strongly impacts the vertical growth of fault-related folds, and generally reduces structural relief, meaning deep layers don’t move upward very far and the upper layers (like the pink stuff) don’t get pulled down into deep synclines in the deformed model.
The goal here was to have the thick pink “lid” terminate in cool zig-zags angling across the surface of the model. It didn’t quite work out for a variety of reasons, one of which was shortening the model too much. If movement stopped around 0:38 in the video below, deeper erosion into the layer pack would have made a nice pattern.
I kept deforming the model, which ended up completely squashing the portion of the model near the backstop. A good bit of this “squashing” was accommodated by buckling the weak layers and stacking the more rigid layers against themselves, so the excessive shortening did not result in the deepest blue layers being squeezed to the surface.
The over-shortening did produce one interesting effect due to the layout of the “lid” and the open sides of the model. Once erosion had almost removed the already thin pink layer near the center of the model, deformation focused in this area due to the loss of its main strengthening element. This resulted in pushing more rigid portions of the model adjacent to the weak area outward, particularly the side of the model near the bottom of the screen where the lid was still intact. The overall appearance is of a wedge-shaped zone driving towards the backstop.
This is the opposite of the typical behavior of an open-sided model, which would normally produce a wedge with its point away from the backstop.
I plan to try this again using some more localized variations in the layer pack. Hopefully something better will result! Portions of this model do look more “defined” in cross section with faults drawn in the deeper blue section. Adding defining stratigraphy in this interval changes the mechanical behavior because the sand I use varies mechanically with color, so leaving the deepest horizon as a single color produces a specific fault pattern.
Outside of the overshortened zone, the differences in fault patterns between areas of lid and no lid are apparent. In both areas, though, the deep blue layer creates an undulating duplex pattern.
One of the sections broke, showing what the model looks like from a head-on orientation, looking in the direction of shortening. Structural complexity exists here as well!
This post was originally published on The Geo Models blog.