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You are browsing the archive for shear zones Archives - Mountain Beltway.

16 January 2017

Shear zones in Scottish gabbros

A quest to visit the “first shear zones” described in the scientific literature leads to an alternate location, and some GIGAmacro images of samples from the real, original spot.

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25 November 2016

Friday fold: intrafolial folds in Eriboll mylonite

At the birthplace of the term “mylonite,” we can find Friday folds hidden in the foliation.

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4 September 2013

Granite balls, boudins, and tadpoles in Archean schist, Laramie Range, Wyoming

A final post from my visit to the Laramie Range with Bob Bauer and Fred McLaughlin earlier in the summer -Recall that there is schist and granite at this site, and much of the granite shows cross-cutting or inclusive relationships indicating it is younger than the schist, while still younger mafic dikes cross-cut than the granite. Both exhibited folding. However, in other places, we observed the opposite: granite inclusions within …

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7 June 2013

Friday fold: South Pass City, Wyoming

Two summers ago, my wife (then my very new wife) Lily and I participated in a two-week workshop for teachers on the energy resources of Wyoming. We also indulged in a visit to a mineral resource site: South Pass City, a site in the southern Wind River Uplift that was the second oldest incorporated town in Wyoming (after Cheyenne). It was a gold mining site, and today it is preserved …

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8 May 2013

A comparative look at the Klingle Valley outcrop

A week ago, I featured six new GigaPans here, of the extraordinary rocks in Klingle Valley, DC. I hadn’t been able to get the next phase of imaging that site ready in time for the post, but here it is: an annotated view of the outcrop. Annotation color code: PINK = Granite contact BLUE = Sericite after staurolite pseudomorphs YELLOW = Outlines of stretched clasts within the metaconglomerate GREEN = …

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30 April 2013

Strained metaconglomerate in Klingle Valley, DC

Following on yesterday’s post about the kink bands within the strained metagraywacke of the Laurel Formation in DC, let’s take the opportunity today to go to Klingle Valley, site of a different facies within the Laurel Formation: a strained metaconglomerate. Though the exposure isn’t as great as the Purgatory Conglomerate, I think you’ll find plenty to hold your attention in these rocks. Close looks will reveal sericite-after-staurolite pseudomorphs (evidence of …

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29 April 2013

Kink bands in highly strained Laurel Formation, Rock Creek Shear Zone, DC

Last week before GSW, I spent several pollen-choked hours in Rock Creek Park, GigaPanning some of the rocks of the Rock Creek Shear Zone. Here are some exposures in the bed of Broad Branch that show lovely kink banding. In at least one spot, you can see a conjugate pair, so these rocks were (1) sheared out in a ductile shear zone, producing the foliation, and then (2) compressed under …

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26 April 2013

Friday folds: Simple shear and the unfolding of folds

The Friday fold combines a moment of insight on a field trip in the Archean Superior Province and a new paper published this month in the journal GEOLOGY.

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1 April 2013

“Outcrops” on the barn at Peirce Mill, DC

The Friday before last, I was in DC for a fun geology/botany field trip, and I got the opportunity to stroll around the barn at historic Peirce Mill, a historical grain mill along Rock Creek in Rock Creek Park, Washington, DC. The barn is immediately south of Tilden Street NW. It appears to have been constructed from local stone: metamorphic rocks of the Rock Creek Shear Zone, a ductile fault …

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4 December 2012

Mystery mylonite

I went into work on the weekend recently, and found two lovely new specimens of mylonite in the lab. Not sure whose they are, or where they came from, but I knew I had to photograph them so I could share them here with you:

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