18 November 2011

Friday fold: mafic metavolcanics

Posted by Callan Bentley

Okay – in spite of numerous distractions (see every other post so far this week), it’s time to return to the pre-GSA Minneapolis structural geology field trip. Our final stop of the second day in the field was a series of folded up mafic metavolcanics. I’ve got some photographs of them.

These mafic volcanics were strained in a relatively incompetent (weak) fashion between two more competent (stiff) gneiss domes in the sub-province boundary. The foliation (which strikes ~northeast in this area) is one planar structure that is seen having been folded, and so are a series of leucocratic veins. Check ’em out…

Isoclinal fold in foliation:

A close fold in mafic metavolcanics’ foliation:

A “Z” fold in a leucocratic vein:

…and another “Z” fold in another leucocratic vein:

Here’s some bonus structures for you, since they were at the same stop…

Boudinaged leucocratic vein:

And in the purely brittle realm, here’s a granitic intrusion bearing thin, angular mafic xenoliths, and cross-cut by a small fault (sorry, no scale in this one):

Happy Friday. I’m in the field today (doing a Billy Goat Trail geology hike with H-B Woodlawn students), and planning on going into the field again tomorrow (to see this), so it’s looking to be an excellent weekend. Hope yours is the same.