Tuesday, August 18, 2009

Parasitic folds & boudins: a series

Unannotated photo:
parasites_and_boudins_plain

Photo with quartz veins outlined, highlighting boudinage and parasitic folding:
parasites_and_boudins_outline

Photo with vein quartz boudins and folds highlighted in yellow:
parasites_and_boudins_yellow

Sketch interpreting stresses that produced these structures:
parasites_and_boudins_sketch

This nice example of ~horizontal shortening and ~vertical stretching is seen in metagraywacke muscovite schist with hydrothermal quartz veins, near Potomac, Maryland. It is located on the C&O Canal, just upstream from the bridge going to Olmstead Island and the Great Falls overlook.

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3 Comments:

Blogger Kim said...

Hmmm. I would read it as 1) vertical stretching (to give the veins), 2) vertical shortening (to give the boudins), and 3) horizontal shortening (to fold the boudins).

August 18, 2009 6:27 PM  
Blogger Erik said...

Good job on showing the undecorated picture first. :-) Seeing only a marked-up picture makes it impossible to interpret it for yourself.

August 18, 2009 10:59 PM  
Blogger Callan Bentley said...

Hey there Kim,

Yep - some extension first to give space to fill in with quartz. I had assumed the quartz veins to be pre-existing in my interpretation.

But... you don't think the boudinage and folding could be synchronous?

These rocks have experienced multiple generations of deformation and metamorphism, so you could be right. But as far as this outcrop goes, it seems simpler to me to fold and boudinage the veins in the same episode.

C

August 19, 2009 6:49 AM  

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