TECTONIC IMPLICATIONS OF GEOLOGIC MAPPING IN THE WESTERN NEWTON WINDOW, INNER PIEDMONT, NC

The composite Inner Piedmont (IP) consists of two different terranes; the eastern Tugaloo (western IP) and the Cat Square (eastern IP). The Newton window occurs in the eastern IP and exposes Tugaloo terrane rocks below the Brindle Creek thrust sheet (Cat Square terrane, CSt). The Brindle Creek fault (BCf) is a terrane boundary between the Laurentian Tugaloo and the mixed Laurentian/Peri-Gondwanan CSt (based on detrital zircon ages). The CSt, a Siluro-Devonian remnant ocean basin, is composed of polydeformed metapelite and metapsammite intruded by Late-Devonian to Mississippian anatectic meso-to-catazonal plutons, e.g. the Walker Top and Toluca Granites (407 to 350 Ma). In contrast, Tugaloo terrane rocks consist mostly of Cambrian (?) Tallulah Falls Formation (TFF), deposited on oceanic basement (with continental basement fragments), and intruded by Ordovician plutons (Henderson Gneiss and Toccoa Granite; 480 to 466 Ma).

EDMAP-supported 1:12,000 and 1:24,000 geologic mapping of a 155 sq. km. area south of Hickory, NC, has revealed distinct plutonic and tectonostratigraphic characteristics that delimit the evolution of the western Newton window. Early ductile events (D1 and D2) transposed primary structures and formed most meso-and-macroscale structures. The mylonitic, low-angle, SW-dipping BCf is a sharp contact where observed, truncating CSt plutons and migmatitic paragneisses against migmatitic TFF rocks, suggesting BCf is a post-350 Ma fault. Foliation (S2) is sub-parallel on both sides of the BCf, suggesting deformation persisted after BCf emplacement. Early (D2 and D3) folds axes verge W-to-SW, are upright to overturned, and vary from open to isoclinal. Coaxial mineral lineations that parallel mesoscopic fold axes indicate these features formed during or after BCf emplacement. Metamorphic conditions for Tugaloo terrane rocks inside the window are 575 to 725°C and 7 to 9 kB. These data suggest plutons were emplaced in the lower middle crust as the CSt was being subducted beneath the Carolina superterrane prior or during movement along the BCf. A high-angle, SW-NE-trending, silicified cataclasite zone represents a Mesozoic brittle event coeval with Mesozoic dikes.