September 21, 2018
Written by Philip S. Prince, Virginia Division of Geology and Mineral Resources
If you are looking at this post and haven’t read Part 2, it’s worth a look (also, here is Part 1). Some of the ideas here will be a bit more familiar with a quick read of the earlier Cove Mountain posts…
After finishing some 7.5-minute quadrangle mapping around the “2,” a bit more information about its architecture and structural context is available. Exposure is not the best in the area due to vegetation and colluvium, but I feel good about most of the interpretations I will share here. They are undoubtedly over-simplified due to the number of possible detachment levels in this area of the Appalachian Valley and Ridge, but everything presented here fits into the constraints provided by the land surface, outcrop data, and seismically-supported depth-to-basement information. As discussed in the previous Cove Mountain discussions, the plunging anticline-syncline pair that forms the “2” when eroded (shown here by the sophisticated Play-Doh model) is key to the whole story.
Expression of this paired anticline-syncline structural pattern is clearly visible in available outcrop around the “2,” supporting the overall idea of the folds shown in the Play-Doh model. Look for this anticline-syncline pair in the following images of preliminary cross sections of the area. What the “2” structure looks like now (eroded) and its probable geometry at the end of thrust belt assembly make for an interesting comparison!
Even though the overall structural concept in this area can be generalized from inspection of the high-quality surface imagery above, field mapping provides critical information about dip of bedding, unit thicknesses, and how much section is actually involved in the folding beneath the Cove Mountain “2” and on neighboring thrust sheets. These data allow the structures to be projected into the subsurface and placed into context with other regional geologic structures.
These cross sections were generated from surface outcrop, which is adequate within the area, particularly when combined with the LiDAR digital topography shown in the earlier posts. The vertical axes are 5,000 feet (1524 m), so about 2,500-4,500 feet (760-1370 m) of section are shown. This is more than anyone will need for engineering or water well purposes, but it’s not enough to relate the Cove Mountain “2” to neighboring geologic features. I have tried to project this section to about 16,500 feet (5030 m) thickness, which is somewhere in the neighborhood of the estimated thickness of the sedimentary fold belt in this part of Appalachia (check out Kulander and Dean (1986) and Woodward and Gray (1985), among others).
I started drafting this out by hand, and so far I haven’t put a finely graduated ruler to it or attempted restoration, but the area is small enough that it’s not a big deal with respect to the “2” structure. One thing that is immediately apparent is that the “2” is small with respect to the neighboring thrust sheets, but it is still absolutely enormous in comparison to modern topography. Present-day relief on Cove Mountain is about 1,500 feet (457 m), but the underlying “2” structure likely extends about 8,000 feet (2438 m) into the subsurface, and would have been a bit over 10,000 feet (3048 m) from bottom to top prior to erosion! Even with these proportions, it is still quite small compared to the wedge-scale folds that dominate regional topography.
Without interpretive projection (which is all conjecture at this point), the scale of topographic relief relative to structure size is clear. Sinking Creek Mountain, the highest point near the left side of the thin cross section, is one of the largest mountains in the area in terms of relief. When you’re on foot and mapping, it’s definitely an imposing feature, but it’s completely dwarfed by the scale of the fault-related folding out of which it has been eroded.
If the wedge is projected upwards to about 28,000 feet thick (8.5 km), scale becomes even more impressive. There is no way to know if the external thrust belt was this thick, thinner, or even thicker here at the end of its assembly, but 8.5 km kilometers thick near the outer limit of the crystalline thrust sheets is reasonable within a global survey of younger, less eroded external thrust belt systems. The comparatively small horse block that is the “2” has been lodged against the large, rigid upper Devonian-Mississippian ramp (pink and green), which effectively peeled the “2” off of the bottom of what is now the overlying thrust sheet. Being lodged against this ramp provides a reasonable model for the tightness of the “2”‘s folding, as well as a reason for its plunge to the northeast. The Devonian-Mississippian ramp appears to have been absent to the northeast at the time of emplacement; it was already thrust away above the weak dark blue detachment zone deeper in the Devonian.
So is there an analog model connection here? Sort of…Granular media models don’t really like to form classic ramp-flat-ramp geometries, which are necessary in the interpretation presented here. The internal friction angles of microbeads and sand aren’t sufficiently different, and faults that form in granular substrates are not as weak as faults in actual rock masses appear to be, possibly due to the lack of a pore fluid pressure component. Even so, a model utilizing very fine-grained, angular sand and several microbead horizons will produce sufficiently abrupt ramps to disrupt overriding thrust sheets.
When model thrust sheets are locally rotated to steeper dips on ramps, microbead layers become particularly active as their dips approach their internal friction angle with respect to the orientation of sigma 1 in the wedge. The result is a complicated mess of slivers, but some aspects of the model structures may be useful in visualizing possible geometries. More on this next time…
This post was originally published on the GeoModels blog.