3 July 2012

Granite dikes that have been folded and boudinaged

Posted by Callan

It seems like a good morning to return to Ontario, and to the Archean shear zones exposed at the Quetico/Wabigoon subprovince boundary of the Superior Craton. Readers will hopefully recall that I spent several days absorbing structural goodness from these rocks on a field trip before the Minneapolis GSA meeting last fall. The trip was led by Basil Tikoff, Bob Bauer, Dyanna Czeck, and Peter Hudleston. Our final Canadian stop on the trip was to examine some granite dikes intruding metasedimentary rocks. The whole package was then subjected to right-lateral transpressive shearing. Depending on the dikes’ initial orientation, they would then experience (a) no deformation, (b) shortening [folding], or (c) extension [boudinage]. Let’s take a look at a few photos to get a sense of what we’re dealing with here…

Boudinage, to start with:

Lonesome boudin:

Folding:

How can these granite dikes be both folded and boudinaged? It all depends on their initial orientation. Imagine a bunch of granite dikes overlaid onto a central point, according to the compass rose. Then subject that asterisk-like shape to right-lateral shearing, and see what happens…

The strain ellipse comes into focus as an oval shape that bounds these deformed dikes. Note there are “slice-of-pie” shaped zones within that oval wherein the dikes are boudinaged (elongated) and other zones where the dikes are buckled up (shortened). Now take another look at the outcrop and think about the stress orientations that would be necessary to produce this suite of structures on (what we assume were) identical dikes:

For the rest of the photos, I’m going to shift perspective, and look along strike of the shear zone rather than across. The foliation will be oriented parallel to the vertical dimension of the this blog, as opposed to the horizontal parallelism we have enjoyed thus far.

Here, with people instead of pencils for scale, you can get a sense of the vertical orientation of the foliation:

This one seems to have experienced some small scale faulting as a lead-in to incipient (but incompleted) boudinage:

This looks to be some relict bedding in the host rock:

After this stop, we ate lunch and then crossed back into the States for the next phase of the field trip…