Ptygmatic folds

Ptygmatic folds (from πτύσσω, to buckle in ancient Greek) involve an irregularly folded, isolated “layer”, typically a quartzo-feldspathic vein in a much more ductile schistose or gneissic matrix. They occur in high-grade rocks, mostly migmatites as trains of rounded and near-parallel, commonly concentric folds in which the amplitude is large (>10) and the wavelength small with respect to the almost constant layer/vein thickness (meander-like pattern). They have a lobate, tortuous to squiggled appearance (for example, limbs fold back on themselves and the interlimb angle is negative) and tend to be polyclinal; however, they have no axial plane foliation

J.-P. Burg. http://www.files.ethz.ch/structuralgeology/jpb/files/english/8folds.pdf

You can also see in the photo below that the folds have multiple wavelengths: a short wavelength (couple cm) on top of a larger (couple meters). There is a second, thinner vein with shorter small wavelength and larger long wavelength (bottom left corner).

The image is a mosaic of several high resolution images. I must figure out how to publish an HD image. Location: Broken Hill, NSW

IMG_9264_stitch-s

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