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.
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.
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.
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.
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.
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…
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.
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).
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.
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.