POLYPHASE MOVEMENT ON THE ROSMAN FAULT, BREVARD ZONE, SOUTH CAROLINA

The Rosman fault (RF) marks the NW-margin of the Brevard Zone in the southern Appalachians. In this study, mapping primarily focused on the RF in Tamassee quadrangle, Oconee County, SC, and was guided by earlier mapping of Roper and Dunn (1970). They defined the RF as a NE-striking thrust between S-C’-deformed Chauga River phyllonite (SE) and Tallulah Falls gneiss (NW). Criteria that we used to locate the RF along strike are: chert-like breccia float, SE-verging folds, and abrupt changes from Chauga River to Tallulah Falls rocks. Younger NW-striking faults displace the RF up to 200 m of its older mapped position and juxtapose different structural levels as a result of oblique-slip movement. Observations of these various structural levels identify a polyphase history of deformation.

Identified polyphase deformation is: 1) initial SE-vergent, inclined open folds and oscillating thrusting; 2) subsequent localized, sub-vertical strike-slip faulting; 3) followed by NW-vergent thrusting. Commonly, initial deformation is marked by SE-vergent folds with sub-vertical forelimbs. However, depending on structure level exposed, a complicated pattern of folding and an oscillating sequence of backthrusting, to forward thrusting, to backthrusting is present (cf. Smit et al., 2003). Sub-vertical strike-slip faulting is localized in the steeper forelimbs of the SE-vergent folds and offsets older, first-phase deformation. An asymmetric damage zone of brecciated Chauga River rock up to 30 m wide can be present SE of the strike-slip contact with the Tallulah Falls gneiss. Positive flower structures at several locations also indicate transpressive strike-slip movements. Subsequent NW-vergent thrusts offset both older deformation phases and are marked by a narrow, shallow-dipping chaotic zone of breccia. The SE-vergent folds and damage zone are carried in the hanging wall of the younger thrusts. This relation suggests that the older structures were too steep for this younger period of reactivation to climb, resulting in footwall shortcut thrusting. In one location, thrust imbrication was seen, and fault-bend folding is developed on the third-phase thrusts. The younger, NW oblique-slip faults further complicate this polyphase pattern by offsetting and juxtaposing different structural levels of the RF.