THE EASTATOEE FAULT: RECOGNITION OF A REGIONAL THRUST-SHEET BOUNDING FAULT IN THE INNER PIEDMONT THRUST STACK OF NORTHWESTERN SOUTH CAROLINA AND WESTERN NORTH CAROLINA

The structural style of Inner Piedmont, thrust-stack bounding faults is poorly described. The Seneca fault at the base of the Six Mile thrust sheet is the best defined geographically and structurally. This ductile fault is marked by a sharp contact and by pronounced grain-size reduction in the footwall. Moreover, disparate rock units occur in the footwall and in the hanging wall across the contact.

Field observations in northwestern South Carolina and western North Carolina show that the contact between the Chauga River Formation and the Henderson Gneiss has a similar deformation style to that of the Seneca fault, which lies structurally higher in the thrust stack. Deformation also is defined by an abrupt change in rocks across a sharp contact and by pronounced grain-size reduction in footwall Henderson Gneiss upward to that contact. Other types of contacts exist between the Chauga River Formation and the Henderson Gneiss; e.g. brittle fault repetition and intrusion. However, those contacts are not associated with grain-size reduction. Localization of ductile shearing and recrystallization of the biotite augen Henderson Gneiss to ultramylonite below a knife-edge contact indicate that the surface is a ductile, thrust-sheet bounding fault within the thrust stack. We refer to this fault as the “Eastatoee fault.”

The recognition of this bounding fault establishes the lithostratigraphic units within and the geographic limits of the Walhalla nappe. Chauga River Formation rocks, Poor Mountain Amphibolite, and the Table Rock (Caesars Head) suite of granitic gneisses make up the rock units. The Henderson Gneiss is part of an underlying thrust sheet, which we informally term the “Jocassee thrust sheet”. Our mapping in northwestern South Carolina and adjacent western North Carolina indicates that, northwest to southeast, the thrust stack order upward is Jocassee thrust sheet, Walhalla nappe, and Six Mile thrust sheet.