2 December 2011

Friday folds (and boudinage): Superior Craton day 3, stop 1

Posted by Callan Bentley

Today the Friday fold comes with copious bonus structures. It’s the first stop we hit on Day 3 of the pre-GSA Minneapolis field trip to examine the structural geology of the subprovince boundaries within the Superior Craton. This particular site showed granitoid dikes that had been deformed during dextral transpression into a variety of structures depending on their initial orientation, pre-deformation. Those with orientations parallel to the maximum extension direction were thinned and boudinaged; those with orientations parallel to the maximum shortening direction were folded.

The host rock here alternate between pelitic (mud-protolith) and psammitic (sand-protolith) schists. The competence contrast between the dikes and the pelitic schist is greater than that between the psammitic schist and the dikes, so the tightness of the folding in the pelites is greater than in the psammites.

Thick hinge, thin limb:

Dancing boudins:

Boudin train in the extensional realm:

Zooming in on a nice dextrally asymmetric boudin:

Big asymmetric quartz boudin (with one of field trip leader Dyanna Czeck’s grad students for scale):

Isoclinal folds with boudinage along the limbs:

Nice example of folding top-to-bottom (parallel to the pure shear component of transpression) and incipient boudinage parallel to foliation (left-to-right):

Late brittle overprints:

And, to finish things off, here’s some plumose structure developed on joint faces:

Lastly, consider this plumose structure (with smaller plumes on each of the undulating major hackles), then re-adjust your focus to see the cross-sectioned boudins exposed on the joint surface (both top and bottom):

Double your pleasure, eh? It doesn’t get much better than that.

Happy Friday.