Is the central Piedmont suture a low-angle normal fault?

Abstract

In the crystalline southern Appalachians, the Carolina are terrane is in fault contact with the Piedmont terrane along a seismically reflective surface dipping toward the hinterland and called the central Piedmont suture. The central Piedmont suture may be interpreted as a thrust, but existing data also support a Silurian-Devonian, normal-slip origin: (1) There are lower-grade rocks in the hanging wall than in the footwall. (2) A normal-fault solution allows simultaneous metamorphism of the Piedmont terrane and Carolina terrane, prior to their juxtaposition along the central Piedmont suture. (3) Mineral ages in the Piedmont terrane are older in the west than in the east, consistent with an eastward-progressive unroofing. (4) Along the western edge of the Carolina terrane, a linear belt of Devonian subalkalic to alkalic granitoids and gabbro-norites with low initial 87Sr/86Sr ratios may represent mantle-derived magmas along the axis of rifting that are contemporary with major crustal extension. The westernmost Piedmont terrane includes the Chauga belt. The Chauga belt comprises metavolcanic and metaplutonic units similar in rock type and age to those of the western Carolina terrane. Chauga belt rocks are interpreted to be the westernmost exposures of the Carolina terrane, translated west on the lower plate by extension. The Piedmont and Carolina terranes may thus compose a single lithotectonic element. The Piedmont terrane would represent the basement on which the arc was constructed; the terrane was uplifted during extension along a major low-angle normal fault, recognized today as the central Piedmont suture.