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

Understanding thrust fault geometries beneath active compressional mountain ranges is vital to evaluating seismic hazard as well as understanding the overall evolution of the mountain belt. In particular, trying to identify ramp step-ups between lower angle fault segments is important to understanding fault rupture potential. Gaining information about faults kilometers beneath the land surface is difficult, and geoscientists have to use creative approaches to gather this important information. Because fault segments of interest may be 10-15 km deep beneath extreme mountain topography (see scale on figure below), studying them can be quite a challenge.

Image sourced from this paper. This study used aftershock locations to image ramp structures and their potential influence on where fault rupture occurred and could occur again beneath the Himalaya.

A particularly interesting method of attempting to understanding deep fault geometry is using patterns of surface landscape evolution to identify the moving zone of uplift above a deep fault ramp.

Image sourced here. As the ramp is driven under the overriding tectonic wedge, a zone of uplift develops as the wedge moves up the ramp. This zone of rock uplift steepens and energizes rivers to incise into the landscape. Even after the ramp passes underneath a given area, the landscape retains signs of the uplift event and how rivers and hillslopes responded (and continue to respond) to it. It is thus possible to track relative movement of the ramp beneath the wedge and evaluate its present position.

A useful analogy for this concept is to visualize sliding a spatula underneath a cooking egg. Even though you can’t see the edge of the spatula, you can tell where it is because the thin, flat egg steps up onto the spatula from the base of the pan. The edge of the spatula is like a ramp, and as the spatula is pushed further under the egg, the bulge in the overlying egg moves along with it. In the case of a real mountain range, the dimensions of the edge of the spatula (the ramp) would be at the scale of kilometers, but the idea is essentially the same.

I guess a real egg photo would have been better.

The basic concept of uplift above a deeper ramp can be visualized in an analog model using strong, cohesive layers separated by very weak layers of microbeads. This setup creates ramp-flat-ramp faults, and surface uplift can be seen in the growing tectonic wedge as it warps over a ramp. Uplift occurs at the structural front, or leading edge of the deformed area, as well as above the deeper ramp that is underneath the interior of the deformed area.

There is uplift beneath the two white pointer lines, and at the leading edge of the wedge. Between these two areas, the model surface does not rise much at all. Note that there is actually slip on several stacked fault zones above the deep ramp. This typical in these models, particularly when microbead layers are dipping into the wedge interior.

The model ramp can be compared to the spatula edge (see below), and it produces a comparable effect in the overlying wedge, which is of course proportionally thicker than an egg. These models aren’t really set up to have precise lithotectonic analogies in a real system; they are simply intended to show the surface effects of ramp-flat fault geometries. The overarching pattern to look for, as mentioned above, is minor uplift at the structural front, and a zone of uplift above the much thicker part of the wedge well away from the structural front. Between these two areas, there is a zone where little or no surface uplift occurs despite ongoing movement in the model.

Material movement is up and back above the ramp. Hopefully the spatula edge analogy is apparent here.

Check out 00:38 in this video. The zone of little or no uplift between actively uplifting zones can be readily seen. The surface rises particularly quickly above the stack of purple duplexes above “Deeper ramp.”

In a real mountain belt, this uplift above a deep ramp would force accelerated river incision, which could be identified by large-scale studies of channel steepness and topographic relief or the identification of an older landscape into which rivers have recently incised (check out this paper). The models shown here greatly exaggerate the scale of the bulge and contain many other simplifications and assumptions, but the general spatula-under-egg concept is the same.

The amount of uplift depends on the height of the ramp and the structures within the overlying wedge, but in the models linked in the video below, you can see it happen (it is admittedly subtle in the third model).

 

This pattern is fairly easily reproduced in a model with appropriate materials; check out 00:38 in this video. While the papers linked here address mid-crustal ramps beneath orogenic interiors, I presume this concept might be observed in thinner, external sedimentary fold-thrust belts. The third model in the video might have more analogy to such settings. The second model in the video (see gif below) was constructed for the Bane Dome post here; I think the dome would have generated focused surface uplift during its formation, but no one was around in the Permian to take a look.

Image sourced here. Topographic steepness, high relief, and surface uplift above a ramp go hand in hand.

I think the concept of using landscape patterns to look at fault geometries kilometers below rugged mountains is a really cool practical application of geomorphology concepts. It also demonstrates the integration of geomorphology with other branches of solid earth geoscience. It requires understanding of “background” stream morphologies so that patterns of accelerated incision can be identified, and it also makes extensive use of processing digital topography to apply techniques to large and very rugged areas.

Image sourced here. In this case, large arrows show relative motion of overriding wedge, so the underthrust ramp is moving against the arrow direction. This paper is cool because it looks at real settings (Bolivia/Peru, Nepal, and Taiwan) as well as numerical models of the process.

Links to papers:

Eizenhofer et al. (2019)

Harvey et al. (2015)

Wang et al. (2017)

This post was originally published on The Geo Models blog.