Differences in the Tectonic Evolution of Thrust Sheets in the Blue Ridge Thrust Complex, Western North Carolina and Northeast Tennessee

The Blue Ridge thrust complex is composed of an imbricate stack of thrust sheets, each with a distinct tectonic and metamorphic history. In general, the predominant grade of metamorphism increases from the structurally lowest to structurally highest thrust sheet. The structurally highest sheet, the Fries thrust sheet exhibits deformation and metamorphism at grades of upper amphibolite facies, granulite facies, and eclogite facies. Isotopic ages suggest that these rocks record deformation and thermal events initiated during the Taconian orogeny and continuing through a prolonged Acadian orogeny. The localized greenschist facies overprint in these rocks records limited, retrograde metamorphism during the later Alleghanian orogeny.The structurally lower thrust sheets (Linville Falls-Stone Mountain-Unaka Mountain and the Little Pond Mountain thrust sheets) are dominated by anchimetamorphic to greenschist facies metamorphic rocks (disregarding the Grenville-age metamorphism exhibited by the basement rocks). These pressure-temperature conditions are evidenced by the predominant low-grade metamorphic mineral assemblages in the rocks, as well as deformation mechanisms exhibited in the shear zones. Isotopic ages from these thrust sheets do not record any Paleozoic ages older than the Alleghanian orogeny.The earlier Paleozoic higher grade conditions recognized in the upper thrust sheets are attributed to subduction and crustal thickening during the Ordovician and extending into the Devonian. The absence of a strong Alleghanian overprint on these rocks is due to the fact that these rocks were transported to higher structural levels and essentially rode passively during movement of the thrust complex to its present day location.The absence of a record of earlier Paleozoic metamorphism in the structurally lower thrust sheets is explained by evidence in the stratigraphic record. Rocks from these thrust sheets were uplifted during early stages of the Ordovician Taconian orogeny and provided detritus that was shed into the Blountian wedge.