March 10, 2020
By Philip S. Prince, Virginia Tech Active Tectonics and Geomorphology Lab
No, the model in question actually has more faults than usual. They each accommodate tiny amounts of displacement, so they are not obvious in cross section.
Fold-thrust belts (both real and model, like this one) develop fault and fold patterns that reflect the properties of the rock (or sand-like materials) being deformed. The model section shown below is interesting because it results from shortening a granular layer sequence by 50% and does not show any major thrust fault structures that cut through all of the layers (compare to section 1 further down). In reality, there are dozens of tiny thrust faults in this section, but the overall pattern is unusual for a model made of brittle, frictional materials.
The “faultless” section above is section 3 in the images below. The entire model shown here contains a proportionally large amount of glass microbeads, which are the gray/white layers. This material would represent shale or shale-rich interbed sequences in an actual sedimentary basin. The beads have little or no cohesion, making them mechanically different from the uppermost yellow layer and the thin blue, red, and pink layers. Note that the yellow layer is indeed thicker towards the left side of the model. The model was made on a slight incline, tilted backward toward the thick backstop at left. The uppermost yellow and light blue layers are thickest against the backstop to produce a nearly flat-topped layer pack surface despite the tilt.
Only sections 3 and 4 show the “faultless” geometry; sections 1 and 2 still develop imbricate thrust faults that are expected from brittle sand models.
I thought the range of styles produced in the model resembled the numerical predictions of fold-thrust belt structure shown below. The paper (Morgan, 2015) is found here. The numerical predictions use differences in cohesion between the simulated particles. In the sand model, cohesion was the same throughout, but basal friction was different, resulting in the variety of structural styles. The top two low-cohesion numerical models resemble the “faultless” style, while the higher cohesion trials at the bottom produce regular imbricate thrusts more typical of brittle media.
The video link below shoes the deformation sequence of the sand model. You can see the different fault and fold patterns develop, with lots of tiny wrinkles towards the bottom and larger folds related to significant thrusts near the top of the model.
Unlike the numerical simulations, the differences in style in the sand model result from differences in friction between the sand pack and moving base plate of the model. Towards the end of the movement sequence, the top of the model encounters high friction against the moving base plate. This forces the typical thrust faulting style expected of a brittle sand model, where larger thrusts develop to thicken and steepen the model wedge to allow it to continue sliding across the high friction base. Where the high friction zone is not present, the “faultless” style results.
The elliptical outermost anticline at the top of the model is developed above the outermost thrust fault, one of a “train” of anticlines that form in conjunction with the high friction zone.
Because the entire model is shortened the same amount, the different structural styles each accommodate the same amount of shortening. This means that the major thrust faults near the top of the model split into several thrusts towards the bottom of the model. Each of these thrusts accommodates less movement, but there are so many of them that they accomplish the same purpose as one larger, high displacement fault.
Light refraction in the microbead layers make the model nearly impossible to photograph, and I don’t exactly know how the numerous tiny faults in the upper yellow layer project downward. The model would have to be reproduced on a much larger scale to see the details of the faults and folds within the microbead section (the gray/white middle layers), as these structures are tiny compared to the overall size of the deformed section.
Another interesting aspect of the model is where the outer edge of layer deformation actually is. While the leading edge of folding/faulting appears to be obvious on the surface of the model while it is deforming, the sand grains at the surface actually begin to move slightly well beyond the outer edge of obvious surface folds. This won’t be visible in the video link due to resolution, but playing the raw video file in forward and reverse clearly shows that the layer pack starts to deform well ahead of the more eye-catching surface folds. Aspects of this concept are discussed in the numerical model paper linked above. The part of the model affected by the high friction zone also appears to “feel” the high friction area before it has actually reached the obvious surface folds.
Using a lot of minimally cohesive material in an extensional model produces a conceptually similar result. The asymmetric graben shown below contains a large amount of the white microbeads, allowing the pre-extension blue and yellow layer to fold during deformation without significant internal faulting. The overlying layers added during extension are also able to accommodate continued extensional movement by folding instead of faulting. Extensional models containing only brittle dry sand typically develop numerous steep internal normal faults and with little or no dip of layering during extension.
I don’t know if any real-world fold thrust belts develop similar styles due to large amounts of weak stratigraphy. The Chittagong-Tripura fold belt, mentioned in an earlier post, is interpreted to have a significant thickness of weak strata at depth and, as a result, limited structural relief, but its upper, stronger section is proportionally thicker and creates more widely spaced folds and faults. Some cross sections of the region can be found on Thomas Davis’ site here.
This post was originally published on The Geo Models blog.