25 November 2016

Friday fold: intrafolial folds in Eriboll mylonite

Posted by Callan Bentley

Loch Eriboll in the North-West Highlands of Scotland, east of Durness and west of Scrabster, is a beautiful place.


It’s also a place of immense importance to the development of geologic thinking. It was here that Charles Lapworth first demonstrated the bizarre stratigraphy of the North-West Highlands resulted from tectonic shuffling along faults that were more or less parallel to bedding (“thrust faults”) and these faults were often marked by sheared-out packages of rock he dubbed “mylonite.” Lapworth chose the neologism mylonite because he wanted to describe his proposed mechanism for their strange fine-grained, highly foliated texture. The grain size reduction relative to nearby undeformed versions of the same rock was because the grains were were “milled” out – in other words, crushed up like wheat in a grain mill. A few examples, showing the very well developed foliation overprinted on various flavors of local bedrock:




Today, mylonite gets applied a bit more broadly, as it’s also used to include rocks that have seen ductile deformation contemporaneous with their grain size reduction. This is often easiest to spot in multi-mineral assemblages, where the different components of the rock have different rheological responses to being sheared out. Some break and get pulverized; others smear out and flow. The combination of these two responses can produce some of the most beautiful textures in structural geology.


But the dynamic nature of the deformation along one of these bedding-parallel shear zones had another effect: (1) it developed a profoundly well-defined foliation, but then (2) it deformed it. The foliation in some cases was folded over on itself, producing folds with looooooooooooooooooooooooooong limbs, mostly obscured from view as the various components of the fold were transposed along the plane of foliation. In places, though, isolated hinges of these folds remain visible to the watchful structural geologists, like knots in otherwise smooth-grained wood.


These are called intrafolial folds, folds caught up in the foliation of the larger body of rock.



Here are two GIGAmacro images of mylonites that bear intrafolial folds. Check them out:

Link GIGAmacro by Callan Bentley

Link GIGAmacro by Callan Bentley

Happy Friday. May your weekend be restful and rejuvenative.