Here’s a horse of a different color; this duplex is as interesting as it is rare
In the Mojave Desert of southern Nevada, Arrow Canyon incises the eastern margin of the Arrow Canyon anticline, on the NE side of the Arrow Canyon mountain range. Where the canyon crosses Mississippian Monte Cristo Group carbonates it is steep-walled and narrow, forming a slot canyon that sees little of the region’s harsh sunlight. This is where the duplex structure in this image occurs.
The duplex crosses near the center of the image, in the midst of these otherwise gently east-dipping strata. The duplex is a zone in which multiple thrust imbrications of a layer, usually bounded below and above by detachment layers. This structure is especially evident near the center of the image, just left of a small bush. The thickness of the duplex zone is about 0.7 m.
The structure within this duplex zone is distinctly different from that of the plane-bedded strata that encase it. The overall fabric defined by the internal thrusts dips about 70 degrees to the left. Internally, the individual thrust-bound slices, called horses, are about 20 cm thick. They formed as slip from the lower detachment horizon transferred to the upper detachment. The transport direction of the hanging wall in this system was from left to right (west-directed).
Dark brown-gray chert lenses are common in these rocks. They are elongate parallel to bedding, and may be seen in the gently dipping strata as well as in the duplex horses. In the duplex the chert lenses have been deformed along with the individual horses, so where they are visible in the center of the scene thin chert lenses may be seen dipping parallel to the thrust horses. This shows, without doubt, that the strata internal to the duplex have been rotated during the deformation.
We can make a crude estimate of the amount of overall thrust slip within this zone based on this panorama. I count a minimum of 16 horses with 0.5m slip on each, suggesting at least 8m of slip. This represents the magnitude of slip transferred from the lower to the upper detachment across this zone. This is a minimum estimate; I believe more horses are present along this zone but the exposure limits the number that can be definitely seen in the photograph.
This duplex represents west-directed thrust transport within a larger-scale, east-vergent fold-thrust system. This duplex could be a result of back-thrust wedging, or possibly just an adjustment of strain as the anticline folded by flexural slip (like the slip between pages of a book that is folded).
Finally, we arrive at the issue of the fractures in the duplex. Most of the sparse vegetation obscuring this canyon wall is rooted in the duplex. The only vegetation outside of the duplex is growing in fractures on the left side of the image. All these vegetated spots almost certainly emerge from areas of increased water flow or retention due to fractures. In a subsurface reservoir a feature like this duplex, with its fractures, many spaced 20 cm apart, would likely act as a conduit for fluid flow. And being bedding-parallel it would be likely to encounter a wide network of more-common bed-normal fractures. Would you be able to recognize a duplex in an image log from a well that penetrated such a structure?