Tracking Lower-to-Mid-to-Upper Crustal Deformation Processes through Time and Space through Three Paleozoic Orogenies in the Southern Appalachians Using Dated Metamorphic Assemblages and Faults

The southern Appalachians (SA) consist of crustal elements accreted by arc obduction, transpression, and head-on continent-continent collision during three Paleozoic orogenies, converting sediments and volcanics formed at the surface into new crust in the mid- to lower crust. The central Blue Ridge (BR) recorded burial depths of 33 km (9.8 kb, pyx granulite) during the Taconian orogeny (480–450 Ma); the Inner Piedmont (IP) 23 km (7.1 kb, sill II) during the Acadian–Neoacadian orogeny (407–350 Ma); and the IP and Kiokee belt 15 km (4.5 kb, sill I) during the Alleghanian orogeny (325–260 Ma). Eclogite occurs along the Burnsville fault (eastern BR) (52 km, 15 kb) and in the western Carolina superterrane (CS) (42 km, 12 kb). Retrograde assemblages (to middle amphibolite and greenschist, gs, facies) in all high grade terranes indicate hydration and reequilibration during later unroofing.

Large faults provide greater insight into the burial and unroofing history of the SA. The multiply reactivated Brevard fault (BF) (VA to AL), and Alleghanian Box Ankle (BA, thrust), Towaliga (T, dextral), and Dean Creek (DC, dextral) faults framing the Pine Mountain window in GA and AL record P–T conditions reflecting movement at mid- to upper crustal conditions at different times. The BF moved >400 km during high temperature conditions as it buttressed mid-crustal, tectonically forced, SW-directed channel flow during Acadian-Neoacadian A-subduction of the eastern BR and IP beneath CS. This fault was reactivated dextrally ~280 Ma (Alleghanian) under gs conditions, and again shortly thereafter as a brittle thrust during BR-Piedmont megathrust sheetemplacement. This sequence of BF reactivation highlights deformation as the crust was uplifted and BR and Piedmont elements were progressively unroofed. A parallel scenario exists along the BA, T, and DC faults that moved only once, but under sill I, garnet (retrograding sill I), and gs facies conditions, respectively.