Structural framework of the Fries fault zone south of Riner, Virginia
Whitmarsh, Richard Sawyer
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The Fries fault zone south of Riner, Virginia is marked by a ductile, greenschist-facies thrust that places Middle Proterozoic gneiss over deformed Late Proterozoic(?)—Early Paleozoic rocks of the western Blue Ridge province. This work presents an analysis of the field relationships and finite strain patterns within the fault zone, and further relates these features to an interpretation of its structural framework. A geologic map of the fault zone is provided, in addition to more detailed lithologic descriptions within the text. Noteworthy aspects of the field geology include: (1) the discovery of a reasonable protolith to the mylonitic Little River Gneiss, which could previously only be inferred; and (2) the recognition of a varied lithologic assemblage that is considered to be correlative with the Pilot Gneiss, which is exposed along strike within the Brush Creek anticlinorium. Kinematic analysis of tectonic fabrics within the Little River Gneiss, Pilot Gneiss, and Chilhowee Group suggest that the fault zone developed in response to southeast—northwest shortening, accommodated by general noncoaxial flow, which produced a top-to-the-northwest sense of shear at all scales of observation. However, it is evident that the original stratigraphic anisotropy within the Chilhowee Group effectively partitioned the coaxial and noncoaxial components of strain. Additional finite strain analyses within the Chilhowee Group, including the correlation of Rf/Ø data with quartz c-axis fabrics, indicate that there is considerable variation in the geometry of finite strain along the fault zone. Whereas flattening strains appear to predominate, it is evident that domains of constructional strain and plane strain are localized near the nose of the Brush Creek anticlinorium. These data are considered to support an interpretation in which the Pilot Gneiss and Chilhowee Group were metamorphosed and folded into a doubly-plunging antiform during the Taconic orogeny (ca. 480-435 Ma), and that progressive shortening of the Laurentian continental margin during this interval caused the Little River Gneiss to be uplifted along the Fries fault. The present structural framework of the Fries fault zone south of Riner, Virginia is thus considered to represent a northwest-vergent fold composed of Late Proterozoic—early Paleozoic strata, which is transected by the base of the Little River Gneiss.
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