30 April 2012

Exquisite boudinage in Ontario

Posted by Callan Bentley

Structural geologists admire the beautiful outcrops of boudinage-rich meta-basalt in the sheared rocks along a subprovince boundary (Quetico to the southeast, Wabigoon to the northwest) in the Superior Craton of southern Ontario, Canada. This is yet another stop on my pre-GSA Minneapolis field trip, which I’ve blogged about a fair bit (and plenty more to come, don’t you fret).

Here’s some looks at the extraordinary, exquisite boudinage exposed at this site… Note the mix of symmetrical and asymmetrical boudinage, the vertical foliation (and vertical boudin separation direction), and the contrast of how some boudin necks being filled with less competent surrounding rock while others have bright white veins of quartz in that same position.

Lastly, if you walk across the road, you can see this lovely set of tight to isoclinal folds in a granite dike that cuts horizontally across the vertical foliation.

The folding of this dike is consistent with horizontal compression and vertical extension: structurally, it tells the same story that the boudinage and cleavage do. Studies of the magmatic fabric support the idea that this is not a coincidence, but that this dike was intruded syntectonically — that is to say, at the same time as deformation.