THE PATTERN OF TECTONIC OVERPRINTING IN THE WESTERN-CENTRAL BLUE RIDGE BY DEFORMATION PROPAGATING FROM EASTERN BLUE RIDGE-INNER PIEDMONT CONVERGENCE

The pattern of southeasterly younging of metamorphism (Taconian to Acadian to Alleghanian) in the Blue Ridge, Inner Piedmont, and Carolina terranes of TN-NC-SC-GA results in regions of variable overprinting of fabrics and mineral assemblages, and variable disturbance of isotopic systematics, by successive orogenies. Although overprinting often obscures critical features, and leads to erroneous to confusing tectonic reconstructions, the pattern may be used to link histories of widely separated regions of the Blue Ridge. For example, the regional Barrovian metamorphic sequence developed in Ocoee rocks in the Great Smoky Mtns (GSM) region attains Sil to Sil-Kfs grade with development of migmatites at approx. the WBR-CBR boundary. Numerous isotopic systems applied to metamorphic minerals are most consistent with Taconian (450-460 Ma) peak metamorphism in the GSM and late to Neo-Acadian (370-350 Ma) cooling through Mu closure to Ar diffusion. However, Bt through Sil-grade rocks are overprinted by retrograde (relative to highest grade index mineral at each grade) fabrics that vary from a steep, spaced shear band cleavage (‘slip cleavage') to open-tight folding with development of an axial planar crenulation cleavage to retrograde ductile high strain zones (HSZ), which have been shown to continue to the NE into the Blue Ridge thrust complex (BRTC). Geochronology for the BRTC is most consistent with Taconian peak metamorphism, ‘Neo-Acadian' (360-350 Ma) disturbance of isotopic systematics by regionally non-penetrative deformation, and early Alleghanian (~330 Ma) thrusting, uplift and cooling through closure of Mu to Ar diffusion. The tectonic histories of the two regions are linked by retrigrade ductile shearing, recrystallization, and mineral growth, suggesting retrograde overprinting in the GSM is Neo-Acadian. The driving force for Neo-Acadian through early Alleghanian tectonism in the GSM/BRTC is proposed to be deformation propagating from the Eastern Blue Ridge/Inner Piedmont where: (1) Neo-Acadian zircon growth corresponds to the time of Sil-grade metamorphism; (2) Hbl Ar plateau ages are broadly Neo-Acadian to early Alleghanian (350-320 Ma), consistent with cooling through Ar closure at that time; (3) Mu plateau ages young to the SE from 320 to 305 Ma.