AN EVALUATION OF THE PALEOZOIC TECTONIC HISTORY OF BLUE RIDGE ROCKS IN WESTERN NORTH CAROLINA AND EASTERN TENNESSEE: IMPLICATIONS FOR UNDERSTANDING THE GRENVILLE OROGENY IN THE SOUTHERN APPALACHIANS

The western Blue Ridge province of the southern Appalachians contains a rich record of the Mesoproterozoic Grenville orogeny, but subsequent Paleozoic metamorphic events have variably overprinted Grenville rocks and Paleozoic thrusting has telescoped Grenville rock units. To decipher the Grenville record, one must be able to accurately strip away the effects of Paleozoic orogenesis. Our recent work in northwestern North Carolina and eastern Tennessee has resulted in a revised model for the structural geometry and Paleozoic history of basement thrust sheets of the Blue Ridge. Grenville basement rocks in the Blue Ridge reside in a stack of Alleghanian thrust sheets that lie above the Grandfather Mountain and Mountain City windows. Within these thrust sheets Neoproterozoic igneous rocks crosscut Grenville fabrics and assemblages, and were subsequently metamorphosed during the Paleozoic. Field relationships, metamorphic grade, and isotopic ages in the igneous rocks are thus very useful in clarifying the post-Grenville history of the thrust complex. The structurally highest unit in the stack is the composite Fries thrust sheet, which contains metasedimentary and metavolcanic rocks of the eastern Blue Ridge (Ashe Metamorphic Suite) juxtaposed against Grenville basement rocks (Pumpkin Patch Metamorphic Suite) along the Devonian Burnsville fault. The Burnsville fault is an amphibolite-facies dextral strike-slip shear zone and is the only identifiable Acadian structure in the thrust stack. Structurally lower thrust sheets are juxtaposed along greenschist-facies Alleghanian thrust faults. West of the Grandfather Mountain window, the Sams Gap–Pigeonroost thrust splays off the Fries fault. Only the Fries and Sams Gap-Pigeonroost sheets appear to have been affected by Ordovician Taconic metamorphism. Below the Fries and Sams Gap-Pigeonroost sheets, Grenville basement rocks in the Fork Ridge and Linville Falls-Stone Mountain thrust sheets display widespread greenschist-facies metamorphism and deformation associated with Alleghanian thrusting. The lowest basement sheet is the Little Pond Mountain thrust sheet, which experienced only Late Paleozoic chlorite-grade metamorphism due to burial beneath thrust sheets during Alleghanian assembly of the Blue Ridge thrust complex.