POLYDEFORMED PROTEROZOIC BASEMENT IN THE BLUE RIDGE THRUST COMPLEX OF NORTHEAST TENNESSEE-NORTHWEST NORTH CAROLINA PART II: INSIGHTS GAINED FROM MICRO- AND MESOSTRUCTURAL ANALYSIS OF A GRENVILLIAN SHEAR ZONE

Mesoproterozoic (~1.1 Ga?) Grenville basement gneisses and amphibolite intruded by foliated mafic dikes and sills of the Bakersville suite (~735 Ma), exposed in a new roadcut on I-26 approximately 5 km north of the TN-NC border, reveal evidence of a polydeformational history. A dominant foliation (S₁), defined by gneissic banding and alignment of biotite and quartz-feldspar aggregates, is overprinted by a later weak to moderately developed foliation (S₂). Relative timing of S₁ and S₂ is constrained by crosscutting relationships, mineral assemblages defining the foliations, and relationship to mesoscale structures. Approximate metamorphic conditions were estimated from observed microstructures associated with active deformation mechanisms.

The earliest event, D₁, involved shearing and development of isoclinal (F₁) folds and sheath folds parallel to S₁. The common limb between isoclinal antiform-synform pairs is often sheared, suggesting progressive non-coaxial deformation. Mantled porphyroclasts reveal top-to-the-northwest sense of shear, and pervasive core-and-mantle structure suggests subgrain rotation and grain boundary migration recrystallization were the dominant deformation mechanisms. D₁ is interpreted to be Grenvillian since this high temperature fabric is not observed in the mafic intrusives. High temperature-high strain rate conditions are inferred to have prevailed during D₁.

D₂ involved northwest-vergent folding (F₂) of S₁ and development of a second axial-planar foliation (S₂) in the biotite gneiss and mafic intrusives. Development of this foliation in the Bakersville suite constrains timing to the Paleozoic. Superficially, the presence of two foliations in the Bakersville gabbro suggests two episodes of Paleozoic deformation, but petrographic evidence reveals that the dominant foliation, consisting of aligned euhedral to subhedral pyroxene, amphibole, and plagioclase crystals, is magmatic. Ubiquitous epidote and chlorite alteration of feldspar in pegmatite veins, and pyroxene altering to amphibole in the Bakersville gabbro indicates upper greenschist to lower amphibolite facies retrograde metamorphism associated with Paleozoic (Alleghanian?) deformation.