The Box Ankle and Ocmulgee shear zones of central Georgia: a study of geochemical response to Southern Appalachian deformation events

Abstract

The Pine Mountain window of Georgia and Alabama hosts the southernmost exposed Grenville aged basement terrane in the Appalachians. The window Is bounded on the east by the Box Ankle thrust fault which juxtaposes basement lithologies from hanging wall paragneiss, schist, and metavolcanic rocks of the Piedmont terrane. The Ocmulgee strike-slip fault separates Piedmont Terrane rocks from Avalon Terrane lithologies to the south and east of the Pine Mountain window. U-Pb ages of zircons constrain the timing of deformation along the Box Ankle and Ocmulgee faults at 304 ± 144 and 335 ± 7 Ma respectively. A contrast in zircon response to high grade deformation from both fault zones is observed. The response of zircon U-Pb systematics in these fault zones provides data on the effects of Pb loss versus U gain models, dissolution processes, and overgrowth binary mixing models from within selected mylonitized bulk rock chemistries.

In the Ocmulgee fault, zircon overgrowth associated with deformation dominates U/Pb age discordancy. Isotopic re-equilibration of Sr Isotopes did not occur on a cm whole rock scale during deformation. Porphyroclasts In the Ocmulgee shear zone retained partial Sr Isotopic signatures of the shear zone protolith. In contrast, Rb-Sr Isotope systems In the Box Ankle fault were re-equilibrated during ductile deformation. Zircons from the Box Ankle fault show evidence of dissolution with no apparent overgrowth. A regional tectonic model proposed from ages obtained in this study Include transpression and doming of the basement and Piedmont cover as seen In the Box Ankle fault trace. Dextral strike-slip with right stepover displacement between the Ocmulgee-Goat Rock fault system and the Towaliga system provide a transpressional environment at the eastern end of the Pine Mountain window.