11 March 2010
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:
Boudinaged granite dike:
Folded and boudinaged granite dike #1:
Folded and boudinaged granite dike #2:
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.