February 17, 2020

Outcrop patterns in a fold-thrust belt analog model, round 1

Posted by larryohanlon

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.

Looking south along the active Chittagong-Tripura fold belt in India, Bangladesh, and Myanmar. This is a really cool area to look at on Google Earth or Google Maps. Bedding can easily be seen along the numerous narrow anticlines if you zoom in.

Looking south along the inactive, much older, and more eroded Appalachian fold-thrust belt in Pennsylvania, Maryland, West Virginia, and Virginia. Chemical weathering and physical erosion have etched out deformed sandstone layers into complex ridge patterns, while reducing limestone/dolomite and shale outcrop zones to valleys. Several synclines enter the image from lower left; they are plunging towards lower left.

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.

The uneroded surface pattern, produced by fault-related fold growth.

Deeper erosion after deformation of the model has ended.

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.

Variations in the mechanical stratigraphy of the model, among other variables, will impact the final outcrop pattern. Here, the pink sand is comparatively strong due to its fine grain size, and where the pink layer is thick, deeper layers tend to experience less deformation and less vertical movement along thrust faults.

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.

Towards the left side of the cross section cut, the thick portion of the pink “lid” has produced a broad synclinorium, beneath which the blue layer is not significantly deformed. The blue layer has been thrust upward to shallower depth where the pink layer is thinner, towards the center and right of the cut edge of the 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.

Three sections from the center portion of the model, where overshortening really crunched the structure and produced lots of steeply dipping faults. The goal for the model was to keep fault dips shallower and limit upward movement of the blue layer.

Three cross section views. The top and middle sections lacked the pink “lid,” which is present in the bottom section. Note that the model’s baseplate was tilted back towards the backstop. The surface slope seen on these model slices does not represent the surface taper of the wedge during deformation, which would have been a bit lower.

Small-scale buckling of the pink/green layer in the center is one interesting by-product of overshortening the model.

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.

The lack of the thick, strong lid produced a weak zone, which localized deformation and extruded stronger blocks outward towards the edges of the model. While not my desired result, the process is interesting to watch in the video. It appears to proceed with minimal obvious movement along any existing faults.

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.

Everything looks better with faults drawn on it.

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.

These side views are from the portions of the model squeezed outward as the center of the model was compressed. The duplex patterns seen in the deep blue layer here are nice, and are what I had hoped for in the entire model (oops).

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!

Structure in the model is complex in all directions due to the presence of the weak glass beads (white/gray) and variations in the thickness of the lid.

This post was originally published on The Geo Models blog.