The Structural Character of the Smith River Allochthon and the Lynchburg Group and Implications for the Nature of the Ridgeway Fault, near Pittsville in Southwest Virginia
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Date
2006-08-10
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Abstract
The Smith River allochthon (SRA) is a regional thrust sheet of metaclastic rocks situated in the southern Appalachians of southwest Virginia. The Ridgeway fault is the boundary between the SRA and the Lynchburg Group, a drift-facies sedimentary cover sequence deposited on the eastern Laurentian (ancient North American) passive margin. The rocks of the SRA and Lynchburg Group differ in biotite and muscovite content, and structural, metamorphic, and intrusive histories. There are conflicting interpretations of the origin of the SRA. A reported Early Cambrian monazite age could represent a tectonothermal event responsible for the metamorphism and structural record that is unique to the SRA. This date, in conjunction with the contrasts between the SRA and Lynchburg Group, has been interpreted as evidence for an exotic origin for the SRA. The SRA may be peri-Gondwanan because an Early Cambrian tectonothermal event at the eastern coast of Laurentia conflicts with the documented drift environment and because similar ages are found in other peri-Gondwanan accreted terranes in the southern Appalachians. However, the SRA and the Lynchburg Group have similar detrital zircon signatures, which suggests that the SRA is peri-Laurentian.
This study strips away the record of each sequential orogenic event to have affected the SRA and the Lynchburg Group, and suggests that Ridgeway fault activity post-dates a main deformation that is shared between the SRA and the Lynchburg Group, and that the foliation resulting from the main deformation on each side of the fault has a common origin. This evidence leads to the conclusion that a tectonothermal event, revealed in the orientation of planar inclusion trails in garnets and upper amphibolite metamorphic conditions, pre-dates the main deformation in the SRA. This is circumstantial evidence that the Early Cambrian monazite age, the early metamorphic assemblage, and an early foliation all precede the main deformation and therefore may represent the same Early Cambrian event. While detrital zircon data suggest that the SRA is related to peri-Laurentia rather than Gondwana, this study suggests that the SRA does not originate from the eastern passive margin of Laurentia.
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taconic orogeny, structural geology, garnet inclusion trails, Lynchburg Group, Smith River Allochthon, metamorphic geology, appalachian, Bassett Formation, Fork Mountain Formation, Piedmont, Ridgeway fault, monazite
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Degree
MS
Discipline
Marine, Earth and Atmospheric Sciences
