Field Area 1: Garth Run

Garth Run

Class by the Garth Run outcrop.  Photo courtesy of Laura Snyder.

Garth Run is a mesoproterozoic  aged basement complex.  It's a high shear zone that experiences ductile shear.  It's comprised of many different rock types such as granite, meta granite, granite gneiss, and blue quartz.  Blue quartz is indicative of basement rock in this area.

Blue quartz seen in rocks.

There were many different structures that could be seen in the area.  There was asymmetric porphyroclasts that ranged in size from 2-3 millimeters. It looked like they were comprised of feldspar.  It also looked like they had been stretched out.  This could have been the process of turned into mylonite.  The picture below is what our professor, Callan Bentley, drew to describe the different stages of mylonite.

Stages of mylonite as illustrated by Callan Bentley.

There were many good examples of mylonite at the outcrop.  There was a particularly good example across from the outcrop by the river.  Unfortunately, there was poison ivy everywhere so only those brave enough were able to see it well.  The photo below shows an example of ultra-mylonite.  You can see the folialtion on the rock below.

Ultra-mylonite. Photo courtesy of Laura Snyder.

When measuring the foliations, I found that there were two different sets of foliations-284°, 74° and 030°, 70°.  An explanation for this is that it could be an anticline.  Stress would be coming from both directions- σ_1.

Stereonet of the two joint sets.

Garth Run experienced two orogenies, the Grenville and the Alleghenian.  The two joint sets would have been formed during the two different orogenic events.