Happy Friday – it’s the end of a very busy week for me, and I hope you too are looking forward to a fun and rejuvenating weekend.
Here’s your Friday fold – like last week, a guest submission from Joe Kopera:
Wowzers; that’s a looker!
What are we looking at here? Joe writes:
This photo shows geologic mapper and structural geologist Greg Walsh (USGS) explaining disharmonic folding to assembled geologists on a field trip at the 2012 Northeastern Section meeting of the Geological Society of America. The outcrop is of the migmatitic Old Lyme Gneiss in south central Connecticut. Although the rock itself is latest NeoProterozoic, the prominent folds here in outcrop (that Greg is standing on) formed synchronously with the migmatization ~285 million years ago during the Permian. The folds are related to the formation of the larger Lyme Dome, during the Alleghanian orogeny in New England– the latest of the “Too Many Orogenies” (Bentley et al., 2015) cycle that culminated in assembly of the supercontinent of Pangea from collision of Gondwana and Peri-Gondwanan microcontinents with Laurentia.
I see my buddy Bill Burton (gray raincoat) was on that trip. Looks like a great exposure – wish I had been there to see it in person.
TGIF!
I have an ongoing fascination with gneiss. I’m intrigued by the parallel banding of minerals and I’ve never read a satisfying explanation for why it occurs. Is it a sort of geochemical precipitation where they accrete to like minerals nearby and form atomic bonds with them?
You see a similar process in schists where there is cyclical or rhythmic banding of mineral types. And what determines the wavelength of the bands?
BTW, that is beautiful rock. It reminds me of the psychedelic, swirling art from the late 60’s. I bet piece of that cut and polished would be spectacular.