Friday fold(s): More kinky phyllite, but this time from the field

Last week, the Friday fold featured a kinked phyllite of unknown provenance that is currently resident in David King Hall 2074 on the campus of George Mason University. However, on Tuesday of last week, I found another kinked phyllite, this one out in the real world, at Thoroughfare Gap, in the Harpers Formation of the Bull Run Mountains.

So, a couple things to note: this phyllite is verging on schist, just as Justin Bieber is verging on puberty. Second, the dark stripes that you see oriented laterally in these photos are the kink bands; they are made of the exact same stuff as the shiny stuff — it’s just in a different orientation, so it’s not in a position to reflect as much light at the camera lens. Third, note that the rock’s foliation comes in only two orientations: either “shiny” or “tilted down” — it’s a very regular pattern, like this if you looked at it from the side:

In a foliated rock with a well-developed mechanical anisotropy, compressed perfectly parallel to the foliation plane, these kinks will form in a conjugate pair of orientations, each at the same angle relative to the maximum compressional stress. But here, we see only one angle, and its conjugate twin is missing. This implies that the angle between the maximum stress and the foliation was oblique to some degree, with the foliation plane itself accommodating the strain in one orientation (slip along the foliation plane), and then kinking only occurring in the other orientation.

See this study that modeled kink banding with a computer simulation to learn more about kink banding.