Folded granitic veins

Rangeley Fm Gilead, Maine

Folded granitic veins, Gilead, Maine. Photo by J. Dykstra Eusden.

Dyk Eusden (Prof. at Bates College, Maine) sent me this nice picture of buckled granitic veins. Note how the wavelength is smaller for the thin veins than for the thick vein; the good old wavelength-thickness relationship still works. The thicker vein (oriented NW-SE in this picture) seems to be the youngest and has recorded higher shortening strain than the thinner ones that run parallel to the foliation (NE-SW). So what happened?

Theory 1: All the deformation happened after the vein formation was completed. This implies shortening in two perpendicular directions, which means constriction (stretching in the direction more or less perpendicular to the picture). How to test this? Sections perpendicular to the image should show elongation parallel to hinge lines. A L>S fabric (strong lineation) may be expected, and perhaps boudins.

Theory 2: The thin veins were folded prior to the formation of the thick vein, then a second deformation folded the thick vein. The challenge is that the very strong shortening parallel to the thick vein must have affected the thin vein folds equally much. The shape of the thin vein folds looks fine as is. They don’t seem to have experienced any extreme post folding strain. Hence I personally like the first theory better. But I have no information about the third dimension, and the truth may be a much more complex deformation history.

The fold is from the early Silurian Rangeley Formation that was regionally deformed and metamorphosed during both the Silurian Salinic and early Devonian Acadian orogenies. The veins probably intruded and were then folded during the Acadian.

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About Haakon Fossen

Professor of structural geology, University of Bergen. Author of book Structural Geology, published at Cambridge University Press
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