Transect Trip 22: S-fold breached by thrust fault

Oriskany sandstone, folded into an S-fold, then snapped down the middle!

Transect Trip 21: hanging wall anticline

Hoo-hoo! An anticline in the hanging wall of a thrust fault in the Valley & Ridge. This is the redbeds of the latest-Ordovician Juniata Formation. Lynn Fichter for scale.

Transect Trip 19: Germany Valley

Looking north along the Germany Valley, which lies in the core of a breached plunging anticline. The topography is defined by the erosion-resistant ridge of Tuscarora Sandstone. This is the Wills Mountain Anticline. The Tuscarora is Silurian; at the bottom of the valley (core of the anticline), you find Ordovician carbonates.

Folds of New York

Thursday is ‘fold day’ here at Mountain Beltway.

Let’s take a look at some folds I saw last weekend in New York City. We’ll start with a bunch seen in the Manhattan Schist in Central Park. Here’s an example of the foliation in the schist. It’s got finer-grained regions and coarser, schistier regions with big honking muscovite flakes. Metamorphic petrologists: Does this correspond to paleo-bedding? (i.e. quartz-rich regions that metamorphose less spectacularly, and mud-rich regions that converted more totally to muscovite during metamorphism?)

Anyhow, here’s what it looks like when it’s folded (accented with a small granite dike):

And another, with some boudinage thrown in for flavor:

This was one of the best outcrops I saw that weekend (on the edge of the ‘lake’), but it was inaccessible to closer photography. Sorry about all the branches in the image. What you’re looking at here is a series of folds with axes plunging at ~45° towards the lake:

Crudely annotated version:

Granite dike:

Boudinaged granite dike:

Folded and boudinaged granite dike #1:

Folded and boudinaged granite dike #2:

Lastly, here’s a couple of folds from inside the American Museum of Natural History. A metaconglomerate:

A little model mountain belt made out of compressed sand layers:

The thing that really struck me about this sand model is the folds visible in the green and yellow central part of the mountain belt: There are refolded folds there. The lower-central antiform with dark green atop yellow is the best example. I had the idea in my head that two generations of folds meant two generations of deformation, but here you’ve got two generations of folds resulting (presumably) from a single episode of ‘mountain building.’

Such beautiful complexity! I want a sand model like this for my lab.

Snowy décollement

Earlier in the month, during the big snowstorms, my window got plastered with snow. This snow formed a vertical layer which then deformed under the influence of gravity. Looking at it through the glass, I was struck by how it could serve as a miniature analogue for the deformation typical of a mountain belt.

Let’s start our discussion by taking a look at an iPhone photograph of the snow:

So here’s what I notice about this (vertically-oriented) photo:

The big sheet of snow is sliding downward over the face of the glass. This surface of slip is thus analogous to a low-angle thrust fault. Here, the maximum principal stress (known as σ₁ to structural geologists) is gravity. The minimum principal stress (σ₃) is perpendicular to the window, and the intermediate principal stress (σ₂) is horizontal, parallel to the bottom edge of the window (i.e., left-to-right). As deformation proceeds, the snow slab folds up on itself and pooches outward in the area of least stress (σ₃); away from the surface of the window.

As the snow layer moves downward, it creates a major fold which thickens the snow in a big line perpendicular to gravity, parallel to σ₂. Along the vertical part of the window frame, the snow sheet has detached in a vertically-oriented fracture (i.e., parallel to σ₁). Oblique to both σ₁ and σ₂ is a series of smaller folds with diagonal axes.

We can see a similar pattern in this map of the Himalayan mountain belt:

Note that the map* is oriented with north at the bottom, and south at the top, so as to be able to better compared it to my window. Note the broad arc of the Himalayan mountain front (~parallel to the Nepali border) which is perpendicular to the motion of India relative to Eurasia. The minimum principal stress direction (σ₃) is vertical, which is why the mountains grow upwards (and the crust thickens downwards into the mantle, too, making the Himalayan mountain belt the site of the thickest crust on the planet). Along the edge of the impactor (analogous to our snow sheet), for instance in northern Burma, we see the same “splay” of folds with axes perpendicular to the the India-Eurasia convergence vector. The crust there is not as thickened.

Though a gooey slab of snow on my window isn’t a perfect analogue for Himalayan mountain-building, we can see some similarities in gross morphology — structural similarities that are fundamentally tied to the orientation of the principal stress directions.

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* Modified by me from a Google Maps “terrain” view.