9 September 2016

Friday folds: A study in contrasts at the Walls Boundary Fault, Shetland

Posted by Callan

Today, in search of folds, we head for a fault!

This is the Walls Boundary Fault, looking north along its trace near Ollaberry in mainland Shetland, U.K.:

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Can’t see it? Here, let me help:

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Still not obvious? Okay, let’s try this, the view from the same spot but looking in the opposite direction (south):

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…Annotated with the trace of the fault:

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Still not clear? Okay, I guess I’ll have to bring out the big guns. Northward view again, but closer to the north side of the peninsula:

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Slightly oblique view, with local geologist and guide Allen Fraser standing on the opposite side for scale:

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Peering across the fault from Allen’s vantage, at the opposite side:

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…And the reciprocal view, from the beach below, looking back the way that I came:

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Here’s the Google Maps view of this neat place. The trace of the fault strikes about 010° here, and the Ollaberry Peninusla just barely includes the fault. The eastern tip of the peninsula is a hard pink granite. West of the Walls Boundary Fault are folded Dalradian metasedimentary rocks – and if we want to find a Friday fold today, that’s where we are going to have to turn our attention!

I spent about two hours here. I put the GigaPan to work, documenting the vertical fault plane:

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Here is the result:
Link 2.12 Gpx GigaPan by Callan Bentley

The Walls Boundary Fault is interpreted as the northernmost strand of the Great Glen Fault – the one that’s so famous in Scotland for being etched out into Moray Firth, Loch Linnhe, Loch Lochy, Loch Oich, and (of course) Loch Ness. It runs from Inverness down to Fort William, and beyond — in both directions!

Here, as noted, it has juxtaposed granite and metasedimentary rocks. But a big mass of granite doesn’t fold very well, and today’s Friday so if we want a fold, we’re going to have to look at the western side of the fault, where Dalradian quartzites (metaarenites) and phyllites (metapelites) may be found.

And:

They. Are. Folded.

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You may have noticed that I just showed you two very different styles of folding – tight kink folds in the strongly layered pelites, and broader, loopy tight folds in the thicker mechanical units of the quartzites.

Presumably, these two different lithologies were subjected, broadly speaking, to the same stresses as the rocks of the west side of the fault were dragged along the rocks on the east side of the fault. But they reacted differently. They strained differently.

Compare and contrast:

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Here is another GigaPan, of one of the folds in quartzite, that I shot at the site:

Link 1.16 Gpx GigaPan by Callan Bentley

Now try getting a 3D perspective on each kind of fold:


3D model by Marissa Dudek (using a photo set by Callan Bentley)


3D model by Callan Bentley

Pretty nice folds, amiright?

Here’s the view I had as I hiked out of this extraordinary place, back up the trace of the fault:

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