Wrinkle Ridges? Sort of!

Recently, a user found an enigmatic feature (see forum thread here) and several suggestions were given for how it formed, including (1) impact melt flows, (2) lava flow fronts / thick lava flows, and (3) wrinkle ridges.

Figure 1. Enigmatic feature found by Moon Zoo user Tom128 and discussed on the Moon Zoo Image of the week here from LROC strip: M117732304RE

Let’s address each hypothesis individually to see if we can determine which is the most likely scenario for the formation of the structures.

(1)   Impact Melt Flows. A few things are important to note about impact melt flows. First, they are always associated with impact craters. Second, like water, the flows will always head downhill. Third, the flows often have lobate morphologies at their termini. Here are a few a beautiful examples of impact melt flows from the crater Giordano Bruno (Figs. 2). In these images, we see the characteristic lobate shape of flows. The features in the original post do not appear to either be near or closely associated with large impact craters (though the field of view afforded by NACs is admittedly small) or formed as the material flowed downhill. Evidence for this latter point comes from the observation that the structure has two steep scarps to it (one on either side) which would be difficult to get if there was a dominant flow direction.

Figure 2. Left: Image of impact melt flow on the southern flank of the crater Giordano Bruno. From Denevi et al. (2010). Portion of NAC frame M101476840LE. Illumination is to the west. The flow moves downhill to the lower right hand side of the image. North is up. Resolution 1.5 m/px. Right: Impact melt flows on the flanks of Giordano Bruno. Portion of NAC frame M110919730L (centered at 35.84°N, 102.72°E). Downhill is roughly towards the lower left corner of the image. Note the multiple different flows (if you are having trouble seeing these flows as positive relief features look at the white features – these are boulders – hopefully the impact melt flow should pop out into a flow!). Image resolution is 0.6 m/pixel.  See the full frame here.

(2)   Lava Flows. Similar morphological features are also characteristic of volcano lava flow fronts (flow downhill, lobate terminus), except lava flows do not typically emanate from impact craters. Lava flows on the Moon are typically much wider and longer (up to 800 km long and 20-40 km wide in places!) than the narrow impact melt flows seen in Fig. 2, as they are formed from highly effusive sources (vents) when there was lot of lava erupted to produce large-scale flows. However, there is a possibility that the feature could represent a localized, highly viscous flow unit, as suggested by Irene in the Image of the Week post. As much of the Moon has not been seen in such high-resolution before, we may well be viewing geological constructs that are completely new to lunar science!

(3)   Finally, we should address the final formation hypothesis: wrinkle ridges. (See also a previous blog entry for some background structural geology.) So, what is a wrinkle ridge and how does it form? Since their discovery on the Moon (and elsewhere), there has been much debate about these questions. In the last 10 years or so, there has been a convergence in the community toward the “right answer:” that these structures are blind thrust fault anticlines. That’s structural geology speak for ridges or convex-upward shapes formed by a thrust fault that does not yet reach the surface. They form in response to horizontal squeezing (or compression) of the material they form in and this compression is usually related to regional-scale processes, like sagging of the crust under large impact craters filled with mare. Wrinkle ridges seem to preferentially form in layered material, where there are mechanically weaker layers (like ash or regolith) separating the mechanically stronger layers (the lava flows). This mechanical juxtaposition means that when the material is compressed, three things happen: folding, sliding, and faulting. As the thrust fault propagates toward the surface, the material folds above it and slip occurs along weak interfaces. Sometimes a thrust fault will form which slopes opposite to the main thrust fault (the backthrust, like this y) and this is manifested at the surface as the wrinkle.

The structures shown in the Image of the Week thread do not necessarily have what I would call typical wrinkle ridge (WR) morphology (except those shown in Tranquilitatis). This is what is interesting! If these are wrinkle ridges, it tells us something different is going on here and that difference will tell us about the material in which these structures formed and the geologic evolution of the area where they are found. When I first looked at these images, I wasn’t sure what to make of them. In the NAC frame to the left of the first image (see Figure 3 for both NAC frames montaged together), I saw a structure with more typical WR morphology and that really clinched the identification for me. I also took a look at the “WR” in Aitken crater from the LROC site. So, are they wrinkle ridges? Well, the answer is…sort of.

Figure 3. Section of montage of NAC frames M117732304RE and M117732304LE showing wider field of view of the area of interest (the image width is ~5 km). Note that the original section of image is at top right.

If you allow me to get on my soapbox for a few sentences, I can explain what I mean. To structural geologists like me, there is a connotation associated with terms like wrinkle ridge or lobate scarp telling us about the mechanical properties of the material, the morphology of the structures, and how they formed. Since these structures do not have the typical morphology of a wrinkle ridge, I would not call them wrinkle ridges, though they likely formed in a similar manner: compression of layered lava flows separated by weak layers, forming thrust faults and folds. The narrowness of the sinuous ridges implies that the interbedded basalt is fairly thin (maybe less than a kilometer). The lack of a broad ridge is something for which I do not yet have an explanation.

As Drew Enns suggests in his blog post on the LROC Picture of the Day site, the size of the ridges implies that the thickness of the basalt in both Aitken crater and near Bessarion W is thin. It may also suggest that the faulted layer is thin. That is, there may be more basalt at depth, but the weak mechanical layer may only be a few tens to hundreds of meters below the surface. It is in this way that structures like wrinkles ridges can tell us about the physical properties of the materials they deform and the geologic history of the areas where they form.

Dr. Amanda Nahm is a tectonic-fault expert at the Lunar and Planetary Institute

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