Friday fold: twice-folded turbidites at Black Pond

Today’s Friday fold comes to us courtesy of Gary Fleming, botanist extraordinaire and brother of Tony Fleming, geological Jack of All Trades. Together, the Fleming brothers led a field trip for the Geological Society of Washington. While I was on that field trip, the topic of polyphase deformation came up, which led a couple of weeks later to Gary sending me this photograph. He took this photo in the Black Pond area, on the Virginia side of the Potomac River near the property of Madeira School:

mathergorgefm_blackpond

That’s a set of twice-folded folds. The earlier generation of folds are quite tight enough that their limbs are parallel; we call this “isoclinal.” They display axial planes that run left-to-right across the photo. They are overprinted by a second generation of folds which are more open and broad. The second generation folds have axial planes which run top-to-bottom across the photo. Here’s an annotated copy showing the undulating form of the folds:

mathergorgefm_blackpond_anno

And here I’ve tacked on some color-coded axial plane traces: the first generation of folding (F1) is in yellow; the second generation (F2) is in blue:

mathergorgefm_blackpond_axes

The rocks in question are turbidites of the Mather Gorge Formation, folded up during the late-Ordovician episode of mountain building called the Taconian Orogeny. Relative to the orientation of this photograph, the F1 folds would have resulted from top-to-bottom compression, while the F2 folds would have resulted from a later episode of side-to-side compression.

It’s also worth noting the collection of small parasitic F2 folds in the schisty section at the top of the photo (greenish-gray, and partially obscured by mud).

Happy Friday! If your week has left you as contorted as these rocks, I hope you have a relaxing weekend…

Thanks to Gary Fleming for sharing this image and letting me publish it here.

Friday fold: wavelength contrast

I scored this photo off the Internet more than five years ago, the first time I taught Structural Geology at George Mason University. I failed to note the website I got it from, and now that website has apparently disappeared, at least as far as the view from Google is concerned. If anyone knows the provenance of this image, please let me know so that I can properly attribute it.

I hesitate to post something like this without knowing who took it, but I did note to myself that it came from the Point Lake Greenstone Belt in the Northwestern Territories of Canada. This image and its implications follow so nicely on to our discussion last week about fold wavelength and the Ramberg-Biot equation that I can’t resist it. Ready? Brace yourself…

point_lake_viscosity

I think that this is one of the coolest structural geology photos ever taken. Here it is graced with some annotations:

point_lake_viscosity_anno

Maximum compressive stress was in this case from the back to the front. The same vein, oriented ~parallel to σ1, is folded in two very different ways, depending on which rock type it is cutting across. As with a week ago, we can explain this behavior using the Ramberg-Biot equation:

L = 2 π t (η / 6ηo)

where L is the wavelength of the fold (in other words, the distance from one fold hinge to the next fold hinge); t is the thickness of the folded layer; η is the viscosity (resistance to flow) of the quartz vein (or, in general, the more competent of the two layers); and ηo is the viscosity of the rock unit (sandstone or shale) that the quartz vein cuts across.

If you keep t and η constant (for say, the rightmost of the two quartz veins), then the only thing left to vary would be ηo. So sandstone will have one ηo, while shale will have another ηo. The sandstone is more resistant to flowing than the shale is. The viscosity contrast between the quartz vein and the sandstone is less (they’re both made of quartz) than the viscosity contrast between the quartz vein and the shale (which have very different material properties).

The high viscosity contrast with the shale makes for a very big number, which raised to the ⅓ power (i.e., you take the cube root) makes for a very small number. This small number, multiplied by the constants of 2, π, and t, gives you L, which will also be a small number: hence the wavelength is small, and as a result, the folds are crunkled up next to one another like sardines in a can.

On the other hand, the low contrast between the viscosities of the quartz vein and the quartz sandstone means that you get a rather small number. Say η = 3. If ηo is also about 3, then you have: (3/(6*3)), or the fraction 1/6. Expressed as a decimal instead of a fraction, this is 0.167. Take the cube root of that, and you end up with a bigger number, in this case 0.55. Multiply that by 2, π, and t, and you get your new wavelength, L. Because you have a larger number in the (η / 6ηo) part of the equation, and everything else is the same, you end up with a larger wavelength. The result is only one fold antiform in the sandstone. In the neighboring shale, ~23 antiforms are packed into the same distance along strike of the vein.

Wild stuff, right? Happy Friday. Let’s hope your weekend is of sufficiently high contrast to the sludge of the week that you get all loose and wiggly, like the top part of the photo… : )

Friday fold: multilayer buckle folding demo

Check out this video I found online whilst uploading last week’s Friday fold:

This video was produced and published on YouTube by Markus Beckers, Michael Ketterman, Dennis Laux and Janos Urai.

It’s a nice demonstration of how multiple layers of material of different properties and different thicknesses can yield up different flavors of folds. In the movie, there are two materials present: white silicone and gray foam. The silicone layers are stronger (“more competent”) than the foam. But the two silicone layers are different thicknesses. It turns out that this ends up being a decisive factor in determining the way they fold.

We can explain this behavior using the Ramberg-Biot equation:

L = 2 π t (η / 6ηo)

where L is the wavelength of the fold (in other words, the distance from one antiform fold hinge to the next antiform fold hinge); t is the thickness of the folded layer; η is the viscosity (resistance to flow) of the silicone layer (or, in general, the more competent of the two layers); and ηo is the viscosity of the foam layers.

In other words, the (η / 6ηo) part of the equation reflects the viscosity contrast between the affected layers. In the video, this viscosity contrast is a constant, since we’re looking at two layers of the same stuff surrounded by the same matrix of other stuff. The only difference is the thickness of the two silicone layers.

So as far as our video up top is concerned, pay attention to the t value and the L value: the thicker the layer is, the larger the wavelength of the resulting fold. The thin layer has a lower t value, and so it ends up with a shorter wavelength: i.e., there are more folds packed into the same amount of vertical space as its stouter neighbor. The thick layer’s higher t value means it wıll have a proportıonately higher L value. It will have a longer wavelength, and fewer undulations will fit into the available vertical space.

Happy Friday, everyone! I’m heading back to DC tomorrow (from Turkey), so more regular posting wıll resume next week.

Friday fold: Archean gneiss from Montana

Friday fold: Siccar Point, Scotland

As with last week, I’m going to show you someone else’s fold today. This one should have strong resonance with most geologists, because it’s a fold in the tilted (and contorted) older strata exposed below the famous unconformity at Siccar Point, Scotland:

fold_siccar

I found this image on the British Geological Survey’s online repository of images, which are available for public use with attribution. I found out about the BGS photo repository via a post on StructuralGeology.org.

The photo was taken by T.S. Bain in 1979. Rock hammer (lower left) for scale.

The specific rock type here is shale, and their age is Silurian. Note the thinning of the limbs of the fold, and the relatively thick hinge area.

Happy Friday – may your workday rapidly thin (like the limbs of this “similar” fold), and your weekend be as thick as this fold hinge!

Friday fold: granite dikes, Barberton greenstone belt

FF5

Folded & boudinaged granite dikes in tonalitic gneiss, Barberton granite-greenstone belt, South Africa. From Passchier, CW, Myers, JS, and Kroner, A., (1990). FIELD GEOLOGY OF HIGH GRADE GNEISS TERRANES.

Very crudely annotated:
FF5_anno

This is a sweet example of how you can get different structures developing in different orientations relative to the principal stress directions. In this particular part of the Barberton Greenstone Belt, compression (orange arrows) operated from the top of the photo towards the bottom, and the rock stretched out from left to right (green arrows). Folds formed where granite dikes were compressed, but the same rock in a different orientation was boudinaged… Cool, eh?

So that’s your Friday fold! The boudinage is just a little bonus for you, because, hey, it’s Friday.

Friday fold: Kinky metagraywacke from DC

Fourth edition of the “Friday fold;” second one via video. Happy Friday!

Follow

Get every new post delivered to your Inbox.