29 April 2013
Last week before GSW, I spent several pollen-choked hours in Rock Creek Park, GigaPanning some of the rocks of the Rock Creek Shear Zone. Here are some exposures in the bed of Broad Branch that show lovely kink banding. In at least one spot, you can see a conjugate pair, so these rocks were (1) sheared out in a ductile shear zone, producing the foliation, and then (2) compressed under brittle-ductile conditions parallel to the foliation, which kinked it.
Here’s the site (map modified from a detail of Tony Fleming, Avery Drake, and Lucy McCartan’s 1994 Geologic Map of the Washington West quadrangle:
A few screenshots to whet your appetite: