The tectonic evolution of the Altavista area, Southwest Virginia Piedmont
Gates, Alexander E.
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The Altavista area lies at the north end of a large area of continuous detailed mapping in the proposed westward thrusted Smith River Allochthon of the Southwest Virginia Piedmont. It also lies at the south end of an area of continuous mapping in the central Virginia Piedmont. The stratigraphy of the Smith River Allochthon has not been related to any other in the Southern Appalachians. The units defined to the north of Altavista are Late Precambrian to Early Paleozoic in age and correlated to many other areas in the Central and Southern Applalchians. At Altavista the two stratigraphies merge and are correlatable. The Bowens Creek Fault, which bounds the west side of the Smith River Allochthon, separates blocks that contain the same stratigraphy. If allochthonous at all, the Smith River Allochthon has therefore not been thrusted any great distance. The rocks of the Smith River Allochthon have been metamorphosed to midde to upper amphibolite facies conditions during the Taconic Orogeny whereas those of Central Virginia only achieved upper greenschist conditions during this event. The Evington Group pelitic schists and gneisses in Altavista exhibit an inverted prograde metamorphism and subsequent retrogression. The Pressure·temperature path for these rocks forms the lower part of a loop from high pressure to lower pressure and higher temperature followed by a nearly isobaric retrogression. Paths of this type are characteristic of terranes that have experienced nappe emplacement. The Altavista area represents the footwall beneath a nappe that has been eroded away because the metamorphic gradient is inverted yet the stratigraphy is upright. Two phases of deformation in this event formed isoclinal folds and refolded isoclinal folds and a pervasive S2 foliation. The formation of large domes along the Bowens Creek Fault postdates the high grade metamorphism. The structures were formed in a three·stage dextral transpressional event. This Carboniferous dextral transcurrent event is Appalachian wide and well documented in the Brookneal zone to the east. The Bowens Creek Fault is therefore unrelated to the high grade metamorphism, further disproving the existence of the Smith River Allochthon.
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