By Philip S. Prince, Virginia Division of Geology and Mineral Resources

The two models shown below are made from the same materials, but the materials are arranged differently and the models were treated differently during shortening. The focus of this post is the top model, which developed a passive roof fault (separating the stacked white stuff from the light yellow and red areas) during growth of the duplex stack. This tightly-packed mass of thrust horses would be extremely difficult to accurately interpret from surface data. A YouTube viewer asked if I could make this style, which is strongly associated with parts of the sub-Andes fold-thrust belts in Colombia and southern Peru. I looked at some papers and gave it a shot! To do it, I had to consider both stratigraphy and erosion and sedimentation patterns.

Both models are made form the same materials in the same rig, but the ordering of layers is slightly different and the pattern of erosion and sedimentation applied during shortening is very different. The top model shows the effects of focused erosion and heavy sedimentation (light red area above yellow layer); the bottom model experiences uniform, widespread erosion and minimal sedimentation.

The passive roof faults, which are backthrusts (see below) effectively disconnect the white layer from everything above it, allowing the thin white interval to produce a tightly-packed “stack” of thrust slices whose structure cannot be inferred from surface data.

The passive roof backthrusts isolate the white layer from the material above and below. As a result, it stacks on top of itself repeatedly, and this fault pattern is very isolated. Deposition during shortening (gray and red stuff at right) enhances this focused stacking. If you stood on the surface above the stack, no field data would give you a reliable picture of what is underneath you!

Check out the video to watch it happen. It includes two more experiments, and a few references:

 

Significant sediment deposition occurred ahead of the stack during its growth, keeping deformation focused within the stack zone. Deposition was paired with focused erosion above the growing stack, and the result shows good resemblance to interpretations of the Llanos Foothills in the vicinity of Yopal, Colombia.

Section is from Martinez (2006). The scale bar is 5 kilometers. The well bore targeting the deepest anticlinal horse is a deep one! Note that this horse is directly underneath the backlimb of the broad surface syncline…confusing!

The reason for showing the two models with very different geometries is to emphasize the fact that an analog result can be controlled by adjusting boundary conditions. Without appropriate decollements, spatially focused erosion, and foreland deposition, the tight duplex stack won’t form.

Heavy rain? Mora et al. (2014) is an interesting paper relating climatic and tectonic boundary conditions to duplex stacks in the Llanos Foothills and Madre de Dios regions of Colombia and Peru, respectively. In these areas, occurrence of the structural style matches up with zones of intense precipitation and other geologic conditions favoring duplex stacking. The parts of the sub-Andes that show this style receive over 4 meters of annual rainfall, and presumably have experienced a similar climate pattern for long enough to impact fold-thrust belt evolution.

Tight duplex stacks accommodate by passive roofs are interpreted in the areas circled in red. The northern area is the Llanos Foothills of Colombia (check out Yopal, Colombia on Google Earth), and the southern area is the western Madre de Dios basin in Peru, at the western edge of Manu National Park.

Precipitation (left) and relief maps from Mora et al. (2014). Rainfall in the vicinity of the duplex stacks exceeds 4 meters per year. Presumably, such a pattern has existed long enough to impact development of the stacks.

I think this structural style is very interesting because of its reliance on erosion and deposition during shortening and its lack of surface expression of deep structure. A geologist standing on back limb of the broad surface syncline (red dot below) would not see any surface expression of the small thrust anticline over 5 kilometers beneath the surface. Deep interpretation from surface data is always perilous, but a setting like this one would render it nearly impossible. Here, seismic and drilling data provide a fascinating look at deep structure.

Yopal, Colombia is the city at right. The outer edge of the deepest anticlinal horse would be directly beneath the red dot, which is located on the back limb of the surface syncline.

I took the Martinez (2006) section and chopped it out to match with Google Earth for a block diagram. The section is somewhat vertically distorted due to the perspective effect, but it provides some concept of structure size relative to relief in the Llanos Foothills area. The red dot in the top image would be located at the yellow-orange contact on the backlimb of the broad surface syncline.

A similar structural style is seen at the deformed western edge of the Madre de Dios basin in Peru. Here again, surface geology does not provide much evidence of the pile of thrust slices a few kilometers deep. As with the Llanos Foothills example, stacked anticlinal slices are actually found beneath the back limb of a surface syncline.

The precipitation gradient is easy to see in this large-scale image across the Madre de Dios region. Red dot is Macchu Picchu. The brown mountaintops just above the center of the image are 3,800 m or 12,000 ft above sea level. Area of detail below is just right of center.

For an Appalachian Valley and Ridge guy, looking at this structural style is really interesting. Even with deeper exhumation, the tightly-packed duplex piles are very different from what I deal with in a much broader thrust belt that has plenty of available decollements, but presumably lacked other forcings necessary to keep all of the wedge growth packed into one spot.

This post was originally published on The Geo Models blog.