AN EARLY AND HOT ALLEGHANIAN OROGENY IN THE EASTERN BLUE RIDGE OF THE SOUTHERN APPALACHIANS

The three distinct post Grenville Appalachian Paleozoic orogenies have been characterized by their peak metamorphic grade, and discrete periods of orogenesis separated by stretches of quiescence. Here we report a set of observations that conflict with this traditional view. Monazite (U+Th-Pb) and ⁴⁰Ar/³⁹Ar ages in conjunction with textural observations from across the Eastern Blue Ridge (EBR) of NC, SC, and GA indicate that the time interval between the traditionally defined Acadian-Neoacadian and Alleghanian orogenies may not record a period of tectonic quiescence nor a reduction in deformation temperatures.

Mylonitic shear zones located along the Tallulah Falls Dome (TFD) in northeast GA, the western margin of the Toxaway Dome (TD) along the NC-SC border, and the Gossan Lead fault in northwest NC display well developed S-C-C’ fabrics. In TFD and TD rocks, deformation conditions reached at least amphibolite facies based on analysis of dynamic recrystallization in quartz and feldspar, and EBSD data from quartz; the quartz and feldspar define the C-fabric in these shear zones, thus, they record conditions of mylonitization. Garnet-biotite-plagioclase-muscovite thermobarometry from the TD yields pressures of 0.67-0.78 ± 0.1 GPa and temperatures of 665-690 ± 25° C. New monazite geochronology from mylonitic samples from both the TFD and TD yield ages that range from 335-300 Ma, supporting amphibolite facies mylonitization at the onset of the Alleghanian orogeny. Hornblende ⁴⁰Ar/³⁹Ar dates of 329-322 Ma and muscovite ⁴⁰Ar/³⁹Ar dates of 319-308 Ma from the TFD support amphibolite-facies conditions during earliest Alleghanian times, followed by rapid cooling. Age constraints from the Gossan Lead fault show a similar pattern with mylonitic amphibolite samples recording ⁴⁰Ar/³⁹Ar amphibole ages of 347-345 Ma, and a mylonitic metagraywacke yielding a muscovite ⁴⁰Ar/³⁹Ar date of 336 Ma. Deformation conditions recorded by quartz and feldspar in these mylonites are slightly lower than those associated with the domes, and record amphibolite to upper greenschist facies conditions. Three locations across the EBR provide compelling evidence for an early onset of the Alleghanian orogeny at high temperature conditions and suggest a possible thermal continuum between the Acadian-Neoacadian and Alleghanian orogenies.