The Piedmont province of Southern Appalachians is comprised of exotic and North American terranes. Many of the bounding faults between these terranes are wide, and well developed ductile shear zones that exhibit clear evidence of Alleghanian dextral motion. Together they define the Eastern Piedmont Fault Zone (EPFZ).
The Modoc Zone is one example of a ductile fault in the EPFZ, and has been mapped from near Columbia, S.C. to near Macon, Georgia. The map below is old, but shows the position of the Modoc zone. Along the South Carolina border it separates the high grade and older Savannah River Terrane (formerly the Kiokee belt) from the low-grade metavolcanics and metasediments of the Carolina slate belt to the northwest.
Various components and features are briefly described below:
One of the signature rocks of the MFZ are button schists, so called because they weather into small button shaped bits. The rock is a phyllonite and shows well developed normal slip crenulations that indicate dextral motion as seen in this small subhorizontal outcrop face.
Another distictinve and common feature in the highly deformed metavolcanic and metasedimentary rocks of the MFZ are asymmetric boudins within the steeply dipping layering also indicative of dextral shear, as seen in this subhorizontal lake shore outcrop.
This is another example of such a boudin, in this case of a more amphibolitic geniss. Note the small quartz veins where the boudins are separting indicating that fluid migration occurred during deformation. These boudins could be isolated for at least meters along the layer strike length, indicating extreme dismemberment.
Intrafolial folds with a dextral vergence, such as seen here in this subhorizontal outcrop surface, were also common. These were interpreted to represent reverse slip crenulations. Interestingly these had a consistent southwest plunging fold axis, suggestive of a combined dextral and northwest-side down motion.
A very crucial component of the MFZ were orthogneiss sheets - lenticular bodies of mylonitized granitic bodies. Porphyroclasts were small, but a careful inspection of the photo here shows an asymmetry consistent with dextral shear. Mappable orthogneiss sheets were up to one km wide, and 10 km long. These orthogneiss sheets were dated as roughly 300 Ma; i.e. Alleghanian, and are part of the evidence that the MFZ and EPFZ were Alleghanian fault zones in the hinterland of the Appalachians. They also suggest that magmatic lubrication of the fault zone may have been a very important part of the story.
This outcrop is from the southeast border of the fault zone (the "bottom"), and shows coarser-grained, and higher metamorphic grade rocks involved, and a much more ductile deformation style.
This photo of the outcrop right beneath the Lake Murray dam, shows rocks of the migmatitic core of Savannah River Terrane being cut by post-kinematic, fine-grained granitic rocks that are also Alleghanian in age. U-Pb dates suggest the migmatitic rocks are roughly 600 Ma old.
This is a northwest to southeast cross section across the Kiokee belt (Savannah River Terrane) with the MFZ on the northwest flank of a large anticlinorium structure. Several lines of evidence suggest that these structures are thin-skinned and sit on top of a detachment. The interesting question then arises of how the dextral MFZ interacted with the Alleghanian detachment.
Pictured here on an outcrop of MFZ dextrally folded ampibolitic gneisses along Gundy Creek are Don Secor and Allen Dennis. I had the distinct pleasure of working with Don Secor, Allen Dennis and Paul Sacks on these rocks in the 1980s.
These are mylonitic rocks from along the Goat Rock Fault Zone close to the Alabama border. What is impressive about these rocks is that the protoliths were coarse grained migmatic rocks. Thanks to Mark Steltenpohl for showing me these rocks.
When the GRFZ really got going it produced these ultramylonites out of the high grade basement gneisses. That is significant grain size reduction!
Some larger scale considerations and questions: