TECTONIC HISTORY OF THE EASTERN PIEDMONT IN SOUTH CAROLINA

Recent map compilation and data interpretations have resulted in a new and different understanding of the eastern Piedmont in SC. New maps of Irmo NE, Richtex, and Winnsboro Mills 7.5-min quadrangles provide the basis for understanding deformation in the Carolina (CA), Charlotte (CT), and Dreher Shoals (DS) terranes, and the nature of their contacts. One outcome of structural analysis is the discarding of numerical structural nomenclature. Deformation ages across terranes do not coincide, and with the possibility of defining new deformations in the future, we abandoned numerical notation. Instead, we name deformations using geographic localities, which reinforce the spatial limitations of regional correlations.

We recognize 8 separate deformations. Some deformations are restricted to particular terranes, others occur in more than one terrane, and terrane boundaries are products of plate tectonic deformations. Felsic gneisses and amphibolites in CT are deformed by the oldest deformation, Horse Creek, >>551Ma. The Chappells shear zone separates CT from CA. These terranes are stitched by the Longtown metagranite and similar intrusives that are dated at 551Ma. The oldest deformation in CA is named Delmar; it consists of transposed layering, recumbent isoclines, and scattered fold remnants. Traces of Delmar are recognized in CT. The Clarks Hill deformation is the next oldest in CA, consisting of mostly upright, tight to open folds plunging NE and SW. The Irmo shear zone is the youngest deformation in CA. It is a dextral shear zone that has a map pattern of a strike-slip duplex system. The remaining deformations are found in DS or between DS and CA. Recent zircon age dates from pelitic schist (~421Ma, David Barbeau, personal communication) in the spillway at Lake Murray dam have wrought confusion on previous geologic interpretations. New models are currently being considered.

Mapping and structural analyses indicate that the architecture of Delmar deformation is part of a recumbent fold system. Younger deformations create superposed structures that are recognized in map patterns and outcrops.