LONG-LIVED CRUSTAL DEFORMATION AND REACTIVATION IN THE EASTERN BLUE RIDGE NEAR SPARTA, NC

The 9 August 2020, Mw 5.1 earthquake near Sparta, North Carolina provides a new opportunity to investigate long-lived crustal deformation and landscape evolution in the Blue Ridge. The earthquake generated 4 km of co-seismic surface rupture, the Little River thrust fault (110/45), and uplifted a surface area of ~11 km². Bedrock and surficial geologic mapping in the area identifies important connections between Paleozoic and Mesozoic bedrock geology, the Little River fault, and landscape evolution.

The epicenter is in polydeformed rocks of the Ashe and Alligator Back Metamorphic Suites in the eastern Blue Ridge. Bedrock structure formed during multiple Paleozoic orogenies; regional structures strike NE-SW and dip SE (mean foliation 063/57). ⁴⁰Ar/³⁹Ar muscovite ages suggest the fabric formed 340–330 Ma, although earlier deformation events may be preserved. Mapping has identified several sets of veins (quartz and quartz+epidote), late ductile shear zones, and later brittle faults that cut the Paleozoic foliation and strike roughly parallel to the Little River fault. The veins and late shear zones are interpreted to be associated with late Paleozoic deformation. Dominant joint sets strike 0–10°, 130-150° and 310–330° and produce several linear valleys occupied by third to fifth order streams. Brittle faults oriented similar to the Little River fault are mapped up to 4 km along strike from the co-seismic rupture, and locally appear to reactivate late Paleozoic shear zones and veins. Trenches across the Little River fault document two prior events and the reactivation of an older fault by the 2020 earthquake. Surficial mapping identified several terrace deposits and soft-sediment deformation in alluvium. Terrace deposits along Bledsoe Creek in Sparta yielded ²⁶Al/¹⁰Be isochron burial ages of 1.85 ± 1.47 and 0.49 ± 0.14 Ma, the latter overlies a brittle fault. An additional terrace deposit 30 km W is covered by 9 m of colluvium and yields a ²⁶Al/¹⁰Be isochron burial age of 1.01 ± 0.09 Ma and suggests saprolization of bedrock over a similar period of time. Collectively, the 9 August 2020 earthquake reactivated an older brittle fault system that may have localized along late Paleozoic ductile structures and/or a Mesozoic fracture set. Past movement of some fault segments is bracketed between 1.0 to 0.5 Ma and three events < 0.5 Ma.