May 14, 2019
By Philip S. Prince, Virginia Division of Geology and Mineral Resources
This post steps back to the Rocky Mountain Front Range models from a few weeks back (linked at the end), in which I used a model that took a large-scale perspective on the Front Range for comparison to some published work. This time around, another model does a better job of showing certain specific features at the smaller scale, including an unusual (and subtle) younger-over-older thrust fault system. Like last time, central to this model is the concept of a wedge of deeper rock “splitting” the overlying sedimentary section. Forward movement of the wedge (gray-blue-white in the sand model) 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 came up due to the elegant cross section by Siddoway and Fitz Diaz (2013) shown below. It illustrates the wedge tip concept very well, along with interesting structural features associated with it:
At the larger scale, the wedge tip geometry and its effect on the sedimentary cover is still apparent…note the intended scale at upper left. 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.
The video linked here shows how all of these features form. 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. This is a cool model because there are SO many faults collectively accommodating the shortening. If you watch closely, you will find a very complex array of fore- and backthrusts active at various points during deformation. It would be an interesting exercise to try to mark all of them.
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 whole 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 slip, the white layers near the basement-involved thrust faults would be deformed in very different ways. The resulting structures would have affects on surface outcrop as well as the type of subsurface structures that might interest exploration geologists.
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.
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 the backthrust would 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.
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.
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.
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!
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.
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.