Back to the Rocky Mountains…another sandbox model of a thick-skinned structural front

by Philip S. Prince, Virginia Division of Geology and Mineral Resources

This post steps back to the Rocky Mountain Front Range post from a few weeks back (linked at the end), in which I used a thick-skin style sandbox model to try to offer context for some published work. This time around, I use another model that does a better job of showing some of the specific structural features that can develop on the leading edge of a thick-skin thrust system, including an unusual (and subtle) younger-over-older thrust fault system.  Central to this model is the concept of a wedge of deeper rock, which includes basement, “splitting” the overlying sedimentary section during thrust emplacement.

Hanna Basin 2
A seismic line across the southern edge of the Wyoming’s Hanna Basin, from Mount et al. (2011)…it appears again further down. The concept of a basement thrust sheet “wedging” into overlying sedimentary layers is clearly seen here.

Forward movement of the basement-involved wedge (gray-blue-white in the sand model below) within the sedimentary section is accommodated by relative backthrust movement on a fault on the upper surface of the front of the wedge. This is the “roof thrust,” due to its spatial position. This whole topic was inspired by the elegant Colorado Front Range cross section by Siddoway and Fitz Diaz (2013), after Sterne (2006), shown below. It illustrates the wedge tip concept very well, along with interesting structural features associated with it:

modified siddoway
Check out the young-over-old fault contact…Graneros-Carlisle (lt green) is younger than Lyons (lt blue), and stratigraphically separate from it, but the main backthrust brings them into contact. Pink is basement, and its involvement near the leading edge of thrust faulting defines the thick-skin style.
Siddoway comparison
This model shows the young-over-old thrust contact (green touching blue) and unit thickening above the main backthrust. This thickened zone contains the displaced material that allows for the young-over-old contact to develop. Gray is basement.  The upper fault is the roof thrust, a backthrust, and the deeper fault is the floor thrust. I use roof thrust and backthrust interchangeably in this post, but they don’t always mean the same thing! From https://www.coloradocollege.edu/other/senseofplace/geology.html, after Sterne (2006).

In the model used here, the wedge tip geometry and its effect on the sedimentary cover are quite apparent (see below). I don’t think it’s the best idea to visualize any part of the model as a completely rigid wedge; the mechanical properties of the layers don’t actually differ that much. The specific and carefully selected layer combination  and its interaction with the stress field makes the model deform as it does. Even so, the “splitting” concept helps in visualizing the relative movement on the linked faults, which ultimately produces unusual structural contacts.

rockies restore starter
The wedge-shaped mass of deeper layers (gray-blue-white) effectively “splits” the sedimentary section as it advances left to right on a thrust. The upper thrust fault with right-to-left arrow is the “roof thrust,” a backthrust. The deeper left-to-right fault is the floor thrust. Note intended scale at upper left…12 km is 40,000 ft.
wedge concept
It’s reasonable to visualize the entire mass above the floor thrust as an intact wedge splitting the sedimentary cover apart along the weak layer between blue and green. The weird young-over-old thrust fault contact is just down and right of the yellow arrow. Note intended scale in upper left inset.

The video linked here shows how this structural system develops. Towards the end of the video, the deformation sequence is played in reverse (the easiest section restoration ever!) and there is a zoomed-in clip of the area where the roof thrust/backthrust produces the young-over-old contact. The “retro-deforming” clip is really interesting to watch, and I find it makes the more nuanced features easier to see.

While not as obvious as the main forethrusts during deformation of the model, the roof thrust does produce notable results in the final geometry, particularly if you look closely.  Keeping in mind that the sedimentary section (the portion with white layers) is supposed to be ~4 km thick, the roof thrust increases separation between points B and C by something like 2 kilometers (6,600 ft) or so. This is definitely significant, and if the roof thrust did not develop, the white layers near the basement-involved thrust faults would be deformed in very different ways. The resulting structures would have effects on surface outcrop as well as the type of subsurface structures that might interest exploration geologists.

rockies restore concept dots
The dots at right show initial separation of A, B, and C. At the end of shortening, B and C have been wedged apart, while A and B see little change in separation. By the model’s scale, separation between B and C has increased by around 2 km (6,600 ft). This looks like a minor consideration in the grand scheme of the model…
offsetscale zoom
…but the offset by the backthrust look more significant when you focus on the frontal structures. Keep in mind ~4 km is ~13,100 ft. Yes, this is just a model, but this is an attempt to make it illustrate reasonable spatial relationships! 
Hanna Basin 2
The structural style of the model is worth comparing to this seismic line from the Sierra Madre Uplift and southern edge of the Hanna Basin in Wyoming. Here, the basement thrust sheet can be seen splitting the overlying cover sequence, with movement on its basal fault accommodated by a backthrust in a roof position. The basement sheet is internally faulted, like in the model. Note the large amount of sedimentary fill in the Hanna Basin at right; the deformed area further left resembles the model’s intended scale. From Mount et al. (2011), linked at the end of the post. 

The younger-over-older relationship produced along the roof thrust zone is a particularly interesting component of this structural style. It’s very clear that the green layer is effectively in contact with the blue layer; stratigraphically, they should be well separated by the weak gray layer. They were faulted into this position in an entirely compressional model, so the responsible fault is a thrust fault…but it places younger strata onto older strata.

rockies restore concept
If green and blue are touching in the model, it’s due to fault motion. In this case, green is thrust faulted onto blue despite being younger than blue. This is atypical in stratified rock, but can happen when a thrust cuts across layers that are already dipping.

This seems a violation of a principal rule of structural geology, but it can definitely happen if the fault cuts across already dipping strata.  This is significant to field interpretation because portions of the backthrust could look like a normal fault due to the layers it brings into contact and its dip. A careful geologist would be able to find kinematic indicators within exposed fault planes to decipher the actual direction of movement and place it into context. If you drew the eventual trajectory of this fault onto undeformed strata, it would cut down-section as shown here by the dashed black line at right.

rockies restore concept with fault
The younger-over-older thrust would actually look like a normal fault if its trajectory were marked on a restored section. The dashed line at right shows what this might look like. It would cut down-dip, just like a normal fault.

In a “geologic map view” of the eroded model, I think the younger-over-older fault relationship would be visible along both the marked backthrusts, with teeth pointing towards the right. The backthrust at left appears to cut across an earlier forethrust, faulting the green and white part of the section over the gray basement. In this location, I think the backthrust chopped off the leading edge of the basement thrust sheet and carried it upwards.

rockies restore surface geo
Teeth on thrust faults point to the hanging wall. Two backthrusts are seen; these are part of the roof thrust system. I think the backthrust closest to the gray basement outcrop actually cut off the leading edge of an earlier thrust sheet (see below). The outermost forethrust is drawn as a conjugate to the roof thrust system, not a continuation of the basement thrust (though they eventually link).

Assuming this forensic interpretation is correct, the fragment of basement thrust sheet is the little patch of blue and white visible during early erosion of the model.

clipped off
I think this is a reasonable interpretation, but I’m not completely certain. The little patch of blue-white indicated here appears to be “rootless” as erosion works deeper, suggesting it was clipped off and moved upward by the backthrust. This would be something like the little stub next to the yellow arrow being cut off by the backthrust instead of overridden by it.

So, even though the roof thrust doesn’t accommodate a huge amount of shortening, it still impacts structural relationships in an interesting way and exerts an influence at the kilometer scale. It’s important to keep in mind that this is still pretty dang big from a human or even a landform perspective. The image below shows the view northwest from Golden, Colorado in Google Earth, with the distant mountains rising about 1.2 km (4,000 ft) above the city in the foreground. These mountains would look big in person, and the amount of roof thrust offset produced in the model would significantly exceed the topographic relief between the mountaintops and Golden. The tectonic scale of orogenic systems is always MUCH bigger than the associated topography!

Golden perspective
The highest mountains on the horizon at left are about 1.2 km (4,000 ft) or so above Golden, CO. Compare this physical scale to the size of movements within the model, as well as the overall size of the tectonic system the model is intended to reflect (ultimately 10’s of kilometers thick, although the model only focuses on the upper ~12 km (40,000 ft) of crust).

This structural style is tough to produce with granular media, as it requires extreme mechanical contrasts within the layer pack that are almost beyond the capability of the canon of granular materials. The way in which the blue-white wedge tip folds back under itself in the model is evidence of this; the thin leading edge of the thrust sheet cannot glide without itself experiencing deformation. As a result, the “functional” tip of the wedge mass is actually a bunch of microbeads plastered against the nose of the blue-white fold.

folded tip

Even so, I think this is model is more representative of the style and provides a better visual of the rigid wedge mass “splitting” the sedimentary cover to produce relative backthrust motion. It also shows how a significant amount of  movement can be distributed onto several faults which collectively accommodate the total amount of shortening. None of the faults shown here have particularly impressive diplacement; it is their sum total that is important for a restoration of the system.

Previous post:

geomodelsvt.wordpress.com/2019/03/22/a-sandbox-model-perspective-on-the-tilted-sedimentary-layers-of-the-eastern-rocky-mountain-front-range/

Hanna Basin paper:

https://www.researchgate.net/publication/291821278_Basement-involved_Contractional_Wedge_Structural_Styles_Examples_from_the_Hanna_Basin_Wyoming

 

 

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