Posted by: Alan Pitts | May 12, 2010

Structural Geology Trip Part I : Garth Run

During the weekend of April 17th I had the opportunity to take a great trip with my structural geology class from George Mason University to the Blue Ridge and Valley and Ridge provinces.  Our convoy of minivans pushed off from the GMU parking lot early on a Friday morning,  carrying GMU’s most advanced team of structural geology students. For me this trip was about the pinnacle of my education in Earth science and represented a “bringing it all together” moment where I felt that I was really synthesizing all that i had learned.  The next several entries will document our experience in the field.

Our first stop on the trip was the Garth Run Shear Zone, north of Standardsville VA. Here we saw rocks associated with the basement complex of the Blue Ridge province and would be the oldest rocks we would see  the whole trip  (about 1.2 billion years old). These rocks formed at depth as plutons of charnokite (which of course is a pyroxene-bearing granite), and then experienced deformation during the Grenville orogeny and Alleghanian orogenies, respectively.

Here is an overall picture taken by Stephanie S of our first field area with the crew eagerly making observations and taking notes:

Next is a picture of a sketch I made in my field notebook. I’m sort of hesitant to include it , as I feel that blogging a picture of a sketch is maybe a bit like sending Morse code through a phone call  or watching a beta max  on Blu-Ray.  I’ve always felt like making a quick sketch brings me closer to the study area

This is aerial sketch  of the outcrop. Certain areas have been slightly exaggerated in order to illustrate the more notable features,  such as the large downward facing V-shaped marks, which open towards the north.  The road at the bottom is for scale. This quick sketch provides a little map of several of the structures we discovered.

Here is another quick sketch I made which is more of a profile view of the outcrop, but illustrates the same features:

The V shaped cuts in the next photo by Nik D. are from two distinctly different joint sets present along two foliations in the outcrop. This is a smaller example of the larger V’s  drawn in my sketch but they were visible on several scales through out the unit. The two different foliations indicate two separate events of deformation. One  foliations strikes roughly northwest and the other strikes roughly northeast.  We established that the foliations striking roughly northeast were older by examining the cross cutting relationships. We hypothesized that the older set were a result of the Grenville orogeny approximately  1.1 billion years ago, and the younger set a result of the Alleghanian orogeny approximately 250-300 million years ago.

Another great structure we found was this stunning blue quartz vein, which is noted on the eastern side of my hand-drawn “treasure map”. This vein is a response to a fracture or void which formed due to deformation of the rock and then filled with precipitated silica-rich fluids. This vein is sub-parallel to the orientation of the older foliations, which indicates its association with the earlier deformation event.

I also found this delta porphyroclasts hanging out amongst some other boudinaged feldspar grains. The porphyroclast of interest is just above and to the left of the lighter:

A close up picture to get a better look:

Here is my interpretation of whats happening to our poor little feldspar:

The structures present here tell a rich and detailed story of this outcrop, which correlate important phases in the geological history of Virginia and the Appalachian mountains. The rocks at Garth Run began as plutons at depth, where they cooled very slowly and experienced (or as my structure professor would say “enjoyed”) several phases of deformation.  Then were exposed by erosion and mass wasting so we could come and enjoy them in the field.

Next time … Part II: Swift Run

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Responses

  1. Great stuff! Photos very well done. Maybe some time talk about how you rig your trips photo-wise and otherwise. Keep blogging!


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