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

Tilted sedimentary layers along the edges of mountain belts offer a good opportunity to visualize geologic movement. One of my earliest recollections as a geology student was a discussion of the upturned sedimentary rocks in The Garden of the Gods at the eastern foot of the Rocky Mountain Front Range in Colorado. I distinctly remember hearing about the vertical movement of a large block of crust, which would ultimately produce the high mountains, tilting the overlying sedimentary layers out of its way as it rose. This basic narrative seems to match up fairly well with the layout of rock types and structures along the Front Range, as seen in the view towards distant Pikes Peak (underlain by rock from greater depth) across The Garden of the Gods (steeply tilted shallow sedimentary layers) below.

Image from colorado.com. Upward movement of deeper rock units in the background tilted the fins of orange sandstone in the foreground towards the observer to vertical before they were uncovered and etched out by erosion…they didn’t rise out of the ground looking like this. The high mountains in the background are underlain by continental crustal rock that was once deeper in the earth than the sandstone prior to being tectonically “pushed up.” This post discusses the details of the upward movement of this once deeply buried rock and how it moved the sedimentary layers out of the way.

In my mind, I visualized a geologically impossible scenario like the one illustrated in the block diagram below. Vertical movement of a block of crust meant just that to me–vertical movement–and sedimentary layers (tan) were pushed out of the way of the rising block to end up leaning against its edges. Erosion during and after movement etched out the peaks and gorges of the modern landscape, illustrated by the yellow line. The upturned sedimentary layers at the foot of the mountains would thus appear to be backed by a “wall” of deeper rock (the gray stuff) that was pushed upward.

Don’t fall in love with this diagram…it can’t work for a variety of reasons. It shows the simplest way to visualize the deep igneous and metamorphic crustal rock (gray) moving upward and pushing the overlying sediment layers at its margins out of the way. Erosion during and after uplift would sculpt the high Rockies out of the gray rock and leave upturned sedimentary stubs at the foot of the mountains. This model puts all the parts in the right places, but big blocks of crust just don’t move in this way.

My understanding is that models invoking this type of motion were once put forth for the Front Range, but they cannot be rectified with rock mechanics and various other aspects of tectonic movement as it is understood today, along with details of field observations. A 2019 Google search about Front Range geology will produce numerous newer models using predominately horizontal compression of the crust and angled thrust faults (in red, below) to accomplish upward movement of deep rock and tilting of the overlying sedimentary layers. Development of the “wall” of deep rock behind the upturned sedimentary layers is a bit tougher to visualize in this setup, but the right combination of angled faults makes it possible.

Angled thrust faults resulting from compression are a geologically possible way to move big blocks upward and tilt overlying sediment layers. In the case of the Front Range, compression and a component of strike-slip motion are now regarded as the driver for uplift; the strike-slip component is not illustrated here for simplicity. With the right combination and orientation of angled thrust faults, the “wall of rock” geometry and tilted sedimentary layers can still develop. The Garden of the Gods is located has formed within the tilted sedimentary section. This sketch is based largely on Sterne (2006), which is an outstanding read. Focus on the forelimb of the pop-up…I didn’t take the time to re-draw the back limb.

The sandbox model below provides a cross-section view of the general idea of the angled thrust diagram. The shallow red and white layers have indeed been pushed out of the way of the “wall” of deeper gray and white material, but this has occurred via movement on angled thrust faults (see the video link a few pictures down). Erosion down to the jagged black line would produce high mountains exposing rock from great depth and a zone of steeply tilted sedimentary layers at the foot of the mountains. Following the “wall” downward, it is easy to see it has been pushed over the green/white layer from left to right and not has not undergone purely vertical motion . Noteworthy is that the “wall” has not continued to push up and over the shallower layers. Instead, they have been able to slide up along the front of the wall and out of its way, compensating for its forward movement without being heavily faulted themselves. The wall itself has been able to rotate forward, tilting the lowest sedimentary layers (green and white) to vertical as well. This independent movement of the upper portion of the section relative to the deeper rock is key to many modern interpretations of structure at the eastern edge of the Front Range.

This sand model developed angled thrusts from horizontal compression and produced a geometry like what is interpreted at the eastern foot of the Front Range. Note that “deep rock” means rock that originated at greater depth than the shallowest sedimentary layers. The gray material represents igneous and metamorphic continental crust, or basement; the white layer immediately atop it is the deepest sedimentary unit. The dark black line represents the modern mountainous land surface after long term uplift and erosion; individual peaks are greatly exaggerated for illustrative effect.

Compare these two models. Upper sedimentary layers are pushed out of the way of the deeper rock at left, and they are run over by the deeper rock at right. The zone of vertically-tilted layers between the gray material and upper layers doesn’t exist in the model at right. Everything tilts in the direction of thrusting (to the right), and the layers are badly damaged and “smeared” by the overriding thrust sheet.

The relative movements of the different layer horizons is not the easiest concept to capture in a still image, but the diagram below, presented by Christine Siddoway of Colorado College, does an excellent job (https://www.coloradocollege.edu/other/senseofplace/geology.html). This cross section, based on Sterne (2006), effectively communicates the idea of shallow layers (green) sliding up the front of a wedge of deeper rock (blue and pink) along a “roof thrust”. The green layers can thus be tilted all the way to vertical without actually being run over by the blue/pink wedge. They are indeed pushed out of the way as suggested by the faulty first block diagram, but the movement is accommodated on angled fault planes.

This beautiful diagram drafted by Christine Siddoway at Colorado College really captures the motion style needed to push the shallow sedimentary layers out of the way of deeper rock moving up on angled thrust faults. Note the physical scale. When comparing this to the sand model above, keep in mind that this diagram shows the modern land surface after prolonged erosion. It should match up reasonably well with what is underneath the jagged black land surface line on the sand model.

I think comparison to the “cow catcher” on an old steam locomotive is an odd but potentially useful way to visualize the movement in this diagram. The angled cow catcher wedges under the cow, driving it up the front of the locomotive and (dynamically) pushing it out of the way. This still ends badly for the cow, but presumably leaves it much less messed up than being completely run over! Visualizing an inanimate obstacle on the track may be preferable…

The front of the locomotive is the “wall,” and the tilted cow represents upturned sedimentary strata.

Seeing this style of motion in a sandbox model is admittedly much more nuanced than a train hitting things, but keep the above visuals in mind and watch for the red and white layers moving independently of deeper layers in the video:

In addition to providing a geologic context for iconic landforms, the roof thrust model is a good example of how the right combination and orientation of faults can explain seemingly impossible structural arrangements (Sterne (2006) is highly recommended). The model shown below illustrates how a very unusual geometry is explained using a floor thrust-roof thrust combination. The deeper gray and white material has been thrust up and to the right a considerable distance. The overlying red and white layers don’t show the same movement, but they must have somehow responded to the motion of gray/white. They did so by moving on a roof thrust above the gray/white wedge. The cow catcher perspective works well here; the red and white layers have slid up along the front of gray/white as it moved left to right.

Yellow thrust faults are ugly, but black lines are invisible in the wedge zone. As gray/white thrusts towards the right, the red/white layers slide up and in front of it on a roof thrust. The cow catcher is at work again…

These sand models have a number of shortcomings, particularly their inability to accommodate for crustal flexure due to the thrust loading. Even so, they do a reasonable job of illustrating the roof thrust concept with some attention to geometry of the overall system. The figure below is from Sterne (2006), and shows just how small the roof thrust system and overall Front Range topography is in terms of subsurface structures. The model displays a comparable overall shape, although the main thrust should continue to greater depth and warp downward due to loading. It is also important to remember that the the rocks seen at the surface today were still buried under kilometers of additional rock when thrust faulting ended; subsequent regional uplift and erosion have “unburied” them to produce the landscape seen today (Pazzaglia and Kelley (1998) is a good read).

The large-scale cross section at the top matches the model fairly well. Note that the scale of the basement structures (gray stuff) completely dwarfs the upturned sedimentary zone at the right edge. The cross section offers some suggestion of how much rock has been eroded away in this system to ultimately expose the modern landscape…up to 4 or 5 km along the crest of the Front Range (see Pazzaglia and Kelley (1998) among others).

I ran several trials of this model setup and ended up with some results that resemble interpretations presented in Sterne (2006). Note in these sections that both the layers above the roof thrust and the deepest sedimentary units on the “wall” can both produce fins or hogbacks at the foot of the higher mountains. In the sand models, this means that the blue and white layers as well as the lowest red and white layers could both conceivably be producing dramatic topographic fins.

Sedimentary layers are tilted vertically. Note in the top cross section that forethrusts have offset an earlier roof thrust. Here, the white layer just above blue as well as the red and white layers could all appear as upturned fins of sedimentary rock in the landscape.

In this model, the cow catcher wedge consists of two thrust “slices.”

In this model, the heavy black line shows how deeply the model should be eroded to match the Sterne (2006) section above.

All of these models used the same setup and produced subtly different results, each of which involves a roof thrust to separate shallow sedimentary layers from deeper crustal rock during movement. Roof thrusts also develop in models using a more “thin-skin” type of setup when glide horizons step upward to produce a ramp anticline. If the forelimb of the anticline tilts and bends sufficiently, the tightening is accommodated by a roof thrust. Collectively, these features can be regarded as “out-of-syncline” thrusts and represent an interesting way to explain how everything fits together in certain thrust systems.

https://www.academia.edu/9192480/Stacked_Evolved_Triangle_Zones_Along_the_Southeastern_Flank_of_the_Colorado_Front_Range1

http://geomorphology.sese.asu.edu/Papers/Pazzaglia_Kelley_1998.pdf

This post was originally published on The Geo Model blog.