23 March 2011

Shear band in a granite

Posted by Callan Bentley

Walking back to my hotel after departing the Northeastern / North-central GSA section meeting on Sunday, I noticed this fine shear band cutting across a polished slab of granite on the exterior of an office building in downtown Pittsburgh:

You’ll notice some grain-size reduction along that shear zone, and what looks to be a decent S-C fabric developed, with a sense of motion of top-to-the-left.

If you look at the larger structure, it looks like this central band is a step-over from one more discrete fault to another, and here the strain is more distributed through a lens-shaped body of granite, though still localized within that lens along the dark, mica-rich domains. Note how the larger fault seems annealed in the lower left (of the first picture) with feldspar and quartz.

Anything else you notice here?