TIMING OF INNER PIEDMONT TERRANE ACCRETION: NEW STRUCTURAL AND GEOCHRONOLOGIC DATA PROVIDE IMPORTANT TIMING RELATIONSHIPS

High strain zones in exhumed metamorphic cores of orogens are often marked by the presence of high-temperature mylonites. Interpretations of map-scale crosscutting relationships combined with detailed meso- and microstructural analyses and high-resolution geochronology of northeastern Inner Piedmont (IP) orthogneiss mylonites have delimited the timing of crystallization, peak-metamorphism, deformation, mylonitization, and retrograde metamorphism. These reflect the timing and kinematics of terrane accretion and orogenesis in the southern Appalachian orogenic core.

Map relationships show the Brindle Creek fault zone truncates the Poplar Springs shear zone (~409 Ma) at the Cat Square-Tugaloo terrane boundary. Mylonitization along both of these boundaries, the northwestern Mill Spring fault zone, and the internal portions of the Ordovician Henderson Gneiss bodies occurred during the Neoacadian accretion of the Cat Square and Carolina superterrane 360-340 Ma.

Zircon geochronology data from IP mylonites provide evidence for three distinct tectonic pulses. Crystallization of eastern Inner Piedmont (EIP) anatectic melts record initial subduction of the EIP beneath the Carolina superterrane at ~407 Ma. WIP geochronology data from metavolcanic packages indicate ~449 Ma and ~446 Ma crystallization/deposition ages. Zircon rim data from both terranes reveal later metamorphic events at ~362 Ma, ~346 Ma, and ~322 Ma. Two granitic gneisses, which cut dominant IP fabrics, yield crystallization ages of ~321 Ma.

These data suggest initial subduction of the Siluro-Devonian Cat Square (EIP) terrane beneath the Carolina superterrane at ~407 Ma, which initiated sill-II grade metamorphism and SW-directed mid-crustal escape within the IP orogenic channel between 400-360 Ma before and during BCT emplacement culminating at ~346 Ma. Emplacement of the BCT facilitated evolution of the IP into a dextral type-F thrust stack, transposing and truncating earlier structures and producing the first shared thermal event between the EIP and WIP. Continued tectonism during the Alleghanian (~322 Ma) transformed the IP thrust stack into a type-C megathrust producing uplift, exhumation, and retrograde metamorphic overprint.