16 November 2012

Friday fold: Neoacadian Inner Piedmont trip, part IV: Migmatitic metagraywacke

Posted by Callan Bentley

For the Friday fold, let’s examine an outcrop we saw late in the day on my pre-GSA-Charlotte field trip to the Inner Piedmont of North Carolina.

Click to enlarge

In the field guide, this stop is described as “Migmatitic metagraywacke intruded by Muscovite-Biotite Granitoids (Toluca Granite?).”

It was the largest exposure of rock we saw on this field trip – and there were lots of fun details to examine.

These are rocks of the Cat Square Terrane, which was thrust over top and along side the Tugaloo Terrane to its west. These are the rocks of the hanging wall, in other words.

It was a great site to view tight and isoclinal folds in granitoid dikes and veins, and that’s why I saved it for Friday. It’s got folds!

Note the multiple scales of folding here – layers of different thicknesses buckled into folds of different wavelengths:

This is called ptygmatic folding – the buckled layer looks like intestine in cross-section.

I was even able to collect myself a small “pocket fold” containing these ptygmatically folded veins:

That one will go in my structural geology teaching collection…

Happy Friday!