Polyphase Reactivation History of the Towaliga Fault, Central Georgia: Implications regarding the Amalgamation and Breakup of Pangea

Abstract

The Towaliga fault, southern Appalachians, contains fault rocks that formed under various P-T conditions, revealing a complex reactivation history. It trends 070 along the northwest flank of the Pine Mountain window, changes to 035 at the northeast end of the window, and continues northeastward into the Inner Piedmont. Isolated, kilometer-scale rhomboidal pods of silicified cataclasite along the Towaliga fault likely represent ancient dilational step-overs, which acted as conduits for hydrothermal fluid flow during faulting and, consequently, are sites of concentrated mineralization. Towaliga fault garnet-grade mylonite formed during large-displacement Alleghanian (∼295 Ma) dextral strike slip, while geometric and kinematic evidence suggests dilational step-overs formed in a small-displacement sinistral strike-slip system. Mutually overprinting crosscutting relationships with Mesozoic diabase dikes confirm cataclasis occurred ∼200 Ma. Silicified dilational step-overs are present along both fault orientations, although the absence of step-overs at the bend in the fault suggests two separate brittle faults reactivated discrete segments of the preexisting ductile fault. Ribbon quartz mylonite (∼400°C) is found locally along the Towaliga fault but is also widespread through this part of the Piedmont. This mylonite occurs along other brittle faults in the region, and brittle fabrics exclusively overprint plastic deformation where the two fabrics occur together. Formation during the late Alleghanian or early stages of Mesozoic rifting is indicated, although precise timing is not well delimited. This phase of deformation may provide insight regarding the character of the final amalgamation or early stages of Pangea breakup in the southern Appalachians. Recognition of the rhomboidal pods as dilational step-overs resolves confusion concerning timing and kinematics of Towaliga fault brittle deformation and has wider implications regarding the state of stress during initial stages of continental drift.