March 25, 2020

Cracked mountaintops Part 2: Sinking summits?

Posted by larryohanlon

By Philip S. Prince, Virginia Tech Active Tectonics and Geomorphology Lab

Some upper Devonian sandstone mountains in the Virginia Valley and Ridge show evidence of deep-seated landsliding, resulting in the formation of a downthrown block (graben) along the summit ridge. The scarps bounding the downthrown blocks are visible in hillshades derived from a new USGS LiDAR dataset (also used in this post). Movement of the downthrown blocks has indeed resulted in lowering of the summit ridge, although the movements appear to be very minor.

The scarps in the image above define the boundaries of the downthrown block, on which the highest point of the ridge crest is still located despite its downward movement. The oblique views below provide a bit more context.

This perspective is looking right-to-left in the first image. Note undercutting of the slope by the river bend at lower left.

Projection of fault planes that bound the graben, or downthrown block.

The red shapes are upward projections of the fault planes which produce the surface scarps. The orange arrow suggests general outward movement of the left flank of the ridge into the valley (see blue sand model below). The faults are a conjugate set that is rooted in a deep, very gently dipping glide plane beneath the ridge. This glide plane is undoubtedly hundreds of feet beneath the ridge crest, which rises about 880 ft (268 m) above the river that is visible at lower left. The sketch below shows a conceptual cross section.

The dashed purple lines are folded sandstone layers that support the ridge. The fold axis is indeed on the left (south) flank of the mountain, towards the river, and can be seen just below the upper left corner of the left fault plane projection. Shale and interbeds below the strong sandstone layers are intensely folded, and probably just create an overall weak zone instead of offering a single layer for the flat glide plane. A sand model ridge can produce this deformation and scarp pattern by moving one flank of the ridge outward.

Another ridge crest graben complex occurs in the same geologic interval to the northwest of the first example.

This one is a bit more complex, with two sets of conjugate faults creating a “master” graben with a smaller inner graben. The graben complex is about 630 ft (195 m) above the stream valley at the far right of the image (southeast on the map).

The structural context here is similar to the first example, with a synclinal fold axis to the right of the ridge crest. The scarps at left are developed roughly on bedding planes, while the scarps at right would cut downward across bedding planes. The ridge-forming sandstones would again be underlain by interbeds and shale at greater depth.

This complex is on a broad zone of high, rugged topography between two deeper and well-developed river valleys. In addition to the graben, there are numerous other slide features visible in the landscape. Below, they are marked by small orange “x’s” to keep their outlines visible in the hillshade. The summit scarps are marked with yellow lines, and the large yellow arrow indicates probable direction of ridge flank movement.

A final example is located above a small reservoir. The scarps here are quite subtle, but they are cut by a road and the fault planes could probably be observed as offsets in outcrop. The lower image marks the scarps with yellow lines for comparison to the unmarked top image. In this case, sandstone layering appears to be nearly flat lying, and both of the scarp-forming faults would cut downward through the layers at a high angle.

These features reflect minor movement on the deep glide planes, and (as usual) I have no idea of their age or activity, past, present, or future. I suppose these would qualify as “sackung” features, which I have always associated with larger-scale mountain topography, typically in post-glacial landscapes. A google search for “sackung” will provide plenty of examples from the Rockies, European Alps, and elsewhere.

Studies of very non-mountain landscapes around the New Madrid Fault Zone provide an interesting comparison to the Valley and Ridge features shown above. The image below turned up in a Google Search–it can be found on the OpenTopography account on Twitter here. The ridge crests in this beautiful DEM show numerous scarps associated with ridge spreading and surface lowering, and in this case, the deformation is associated with seismic shaking.

Numerous scarps can be seen running along the crest of the ridge at left…they are less obvious but very much present on the neighboring ridges as well.

Some great USGS work associated with these “co-seismic sackungen” is summarized here. A conceptual cross section through the left ridge in the colored DEM is shown below. I took this figure from one a paper graciously provided by Ryan D. Gold of the USGS. This setup is mechanically similar to the Valley and Ridge examples, although it involves different materials.

It’s possible to make a model sackung by shaking a ridge of granular material. The model shown below has a weak microbead core covered in a cohesive layer of sand mixed with corn starch.

The New Madrid comparison is certainly not meant to imply that the Valley and Ridge examples are associated with seismic events–there is no evidence for this. The history of relief production in the Valley and Ridge, combined with changes in slope behavior during glacial-interglacial cycles, probably offer a better explanation for the summit grabens. The main reason I like the New Madrid example is that it shows sackung-style deformation in a less steep and lower relief setting than the typical example. I think it would be interesting to look for stored sediments along the summit graben scarps to try to extract material for radiocarbon age dating.

This post was originally published on The Geo Models blog