DETAILED GEOLOGIC MAP OF THE PINE MOUNTAIN WINDOW AND VICINITY, WEST-CENTRAL GEORGIA AND EAST-CENTRAL ALABAMA

A new geologic map compiled from 1:24,000—and smaller—scale geologic maps provides a better understanding of the tectonic evolution of the Pine Mountain window (PMW) in central GA and eastern AL. The PMW is framed by Alleghanian faults of different timing, rheology, and kinematics (dextral Towaliga, Goat Rock, and Dean Creek, and Box Ankle thrust), with the Towaliga reactivated (dextral and normal) during the Mesozoic. The dextral Alleghanian Bartletts Ferry and Mesozoic normal Shiloh faults occur in the internal parts of the PMW, and are important kinematic components. The new map and U-Pb dates reveal that the southwesternmost exposures of the central Piedmont suture are located farther NW, narrowing the PMW from 22 km in central GA to 5 km in AL. At its narrowest, the flanks of the PMW are marked by two thin normal faults (the Mesozoic Towaliga and Shiloh faults, NW and SE) that may have excised the wider Alleghanian mylonite zones. SHRIMP and single-grain U-Pb ages of igneous, metamorphic, and detrital zircons from Grenville basement rocks, associated metasedimentary units, and cover rocks help clarify the pre-Appalachian history to better delimit the distribution of Laurentian versus peri-Gondwanan units along the SE flank of the PMW. U-Pb results indicate some units that earlier had been correlated with Laurentian rift deposits (i.e., Sparks/ Halawaka Schist), actually are Grenvillian supracrustal rocks deposited prior to ~1100 Ma that were intruded and metamorphosed during the Ottawan phase of the Grenville orogeny. Phelps Creek Gneiss zircons are 425 ±7 Ma, and overlap in time with plutons that intruded the Carolina superterrane during the Silurian (e.g., Concord-Salisbury suite). Phelps Creek Gneiss host units had also previously been interpreted as Sparks/Halawaka Schist, but field relations plus the new Silurian intrusive age suggest they belong to the peri-Gondwanan Carolina superterrane, which helps refine the location of the central Piedmont suture in the PMW.