S-C fabric in meta-ignimbrite

Here’s a sample from my 2004 geology M.S. thesis work in the Sierra Crest Shear Zone of eastern California. The rock is a sheared ignimbrite (ashflow tuff) tuff bearing a porphyritic texture and a nicely-developed “S-C” fabric.

With annotations, showing the S- and C-surfaces, and my kinematic interpretation:

S-C fabrics develop in transpressional shear zones: ~tabular zones of rock that are subjected both to compression and lateral shear (“transform” motion). The S-surfaces (foliation) initially form at about 45° to the shear zone boundary, and then progressively tilt over in the direction of shearing as deformation proceeds. This gives this sample (when viewed from this angle) a dextral (top to the right) sense of shear. (previous examples on Mountain Beltway) The C-surfaces are shear bands, where a large amount of shear strain (parallel to the shear zone boundary) is accommodated.

You should be able to click through (twice) for big versions of these images.

I polished up this little slab and made a refrigerator magnet out of it. I think it’s a lovely rock.

3 Responses

  1. I must confess that I’m having a hard time visualizing the c-surfaces, even using the embiggened versions. Based on the lowest arrow in your second image, I am under the impression that I should be looking for features that run parallel to the text in the post (as opposed to the very clear features running about 20 degrees off the text, which I take to be the s-surfaces). The lowest 0-degree line jumps off the screen at me, but I can’t see the rest.

    Am I misunderstanding?

    (P.S. — I love your blog).

    • Hi Ezra,
      Thanks for the note, and the kind comment. I’ll endeavor to cook up a post that explains it better. Want to annotate a copy yourself so I know specifically what you’re referring to? Then shoot it to me via e-mail attachment (or post it online and link to it here), and I can figure out how best to respond.

  2. […] time this afternoon playing with my ultra-cool Nikon microcamera, and decided to photograph the S-C fabric in one of the rock samples I collected in […]

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