Precise timing constraints on the kinematic development of the Hyco shear zone: Implications for the central Piedmont shear zone, southern Appalachian orogen

The recently discovered Hyco shear zone is a major tectonic boundary that separates terranes (Milton and Carolina) with sharply contrasting rock types, structural and metamorphic histories, and Nd isotopic compositions. Precise U-Pb zircon dates for an orthogneiss, a granitic gneiss, and synkinematic granites within the Hyco shear zone constrain the timing of motion, deformation, and metamorphism along this fundamental southern Appalachian structure. The synkinematic Yanceyville granite gneiss yields an upper intercept date of 335.4 ± 2.2 Ma. The syn-to late-synkinematic Kilgore granite crystallized at 327 ± 1.5 Ma. A Neoproterozoic hornblende gneiss from the northern portion of the zone yields a lower intercept date of 322.5 ± 2.7 Ma, which is interpreted to reflect the timing of deformation and metamorphism. The crystallization age of the Farmers Lake granite, a late-synkinematic intrusion, is 319.6 ± 0.7 Ma. These new dates document that the Hyco shear zone was active by mid-Mississippian time and that deformation and metamorphism were in their final stages by latest Mississippian time. The Hyco shear zone is considered the northernmost segment thus far recognized of the central Piedmont shear zone, an orogen-scale structure within the southern Appalachians that separates rocks of uncertain crustal affinity in the Piedmont zone from exotic, arc-related rocks of the Carolina zone. Comparison of the kinematic history of the Hyco shear zone with kinematic histories of more southerly segments of tne central Piedmont shear zone indicates that Alleghanian deformation and metamorphism probably occurred contemporaneously along the entire shear zone. These observations are permissive of a model for the central Piedmont shear zone as an Alleghanian ductile shear zone that may extend from Georgia into the central Appalachians.