EXHUMATIONAL HISTORY OF THE TALLULAH FALLS DOME IN NE GEORGIA ESTIMATED USING LOW TEMPERATURE THERMOCHRONOMETRY

Domes are structural features found within mountain ranges throughout the world; the context and conditions of dome formation and exhumation provides valuable insight into the tectonic history of orogenic events. We use low-medium temperature thermochronology to determine the age of exhumation and the tectonic affinity of the Tallulah Falls Dome (TFD) in the Southern Appalachians.

The TFD, located in northeast Georgia, is defined by foliation dipping outward from the center, which interrupts the regional fabric of the Eastern Blue Ridge. The TFD is surrounded by metasedimentary rocks of the Tallulah Falls Formation and basement gneisses, which are separated from the metasedimentary rocks in the core of the dome by an Alleghanian age fault.

Previous authors, using ⁴⁰Ar/³⁹Ar dating of muscovite and hornblende, determined that the TFD is associated with early Alleghanian mountain building. We complement these previously published ages with new muscovite ⁴⁰Ar/³⁹Ar and zircon fission-track (ZFT) ages. Previously published hornblende and muscovite ⁴⁰Ar/³⁹Ar ages, with closure temperatures of ~550^oC and ~ 340^oC respectively, record relatively rapid cooling between 321-317 Ma. Moreover, muscovite ages immediately across the dome boundary record a 6.5 m.y. gap in ages, suggesting differential cooling and missing section. ZFT ages from within and around the dome record cooling to below 240^oC between 229-165 Ma, with no discernable spatial trend in cooling at the resolution of our sample distribution.

The relatively large gap between the ⁴⁰Ar/³⁹Ar and ZFT cooling ages suggests that the TFD had at least two different phases of cooling, including an initial phase of rapid cooling, ~210 ^oC between 321-317 Ma, followed by relatively slow cooling 0.5-1^oC/m.y. rate throughout Alleghanian time. The cooling rate between hornblende and muscovite (~30-50 C/m.y.) is similar to rates of other extensionally exhumed domes. ZFT reveals a second cooling phase at a much slower rate that is more consistent erosional exhumation throughout late- and post-Alleghanian time.