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You are browsing the archive for shear zones.

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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13 November 2012

Neoacadian Inner Piedmont trip, part II: more Walker Top

Today, I’ve got a batch of additional photos of the Walker Top Granite to share, variably mylonitized (sheared out under ductile conditions)… Nice garnets in some spots: Delta porphyroclast: Lovely sheared-out rocks…

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8 November 2012

Neoacadian Inner Piedmont trip, part I: the Walker Top

Check out these lovely images (mouth-watering, even) of the protomylonitic and mylonitic Walker Top Granite of North Carolina’s Inner Piedmont. There’s also some views of the field trip participants and scenery.

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4 November 2012

Animated GIF of the Walker Top Granite

Here’s one of the cool rocks I saw on my pre-GSA-Charlotte field trip to the Neoacadian Inner Piedmont field trip: the protomylonitic Walker Top Granite (~366 Ma crystallization), which was deformed as the terrane it was intruded into got shoved up and to the west (and perhaps the southwest!). Here’s an animated GIF I made to show off one particularly striking bit of float: The rock is dominated by big …

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28 October 2012

Geology and wine in northern Virginia, part I: the Blue Ridge

Callan attends the Geological Society of Washington’s fall field trip, examining the relationship between grape-growing and the underlying geology of two provinces in northern Virginia: the Blue Ridge and the Valley & Ridge. With GSW compatriots, Callan visits Hume Vineyards in the Blue Ridge basement complex and North Mountain Winery in the Shenandoah Valley. This is part I.

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19 October 2012

Friday fold: Pofadder shear zone mylonites in 3D

Christie Rowe sent me these two images. They were taken by Ben Melosh and Louis Smit.  The folded layers are mylonites of the Pofadder Shear Zone in South Africa. I love it when folds are expressed not only in profile, but also in three dimensions. Nothing in life could possibly be better. We’ve featured the Pofadder Shear Zone’s folded mylonites before – and that time, too, they came from a …

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3 July 2012

Granite dikes that have been folded and boudinaged

It seems like a good morning to return to Ontario, and to the Archean shear zones exposed at the Quetico/Wabigoon subprovince boundary of the Superior Craton. Readers will hopefully recall that I spent several days absorbing structural goodness from these rocks on a field trip before the Minneapolis GSA meeting last fall. The trip was led by Basil Tikoff, Bob Bauer, Dyanna Czeck, and Peter Hudleston. Our final Canadian stop …

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25 May 2012

Friday fold: Pofadder shear zone

The Friday fold “guest fold” series continues with a folded ultramylonite from South Africa.

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30 April 2012

Exquisite boudinage in Ontario

Callan shares images of extraordinary boudinage outcrops in strained Archean meta-basalts on the Quetico-Wabigoon subprovince boundary within the southern Superior Craton. Ogle these gorgeous structures in awe and wonder.

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