GOING AGAINST THE GRAIN: LINKING BRITTLE CROSS-STRUCTURES WITH LANDSLIDES, HYDROGEOLOGY, AND EARTHQUAKES IN THE NORTH CAROLINA BLUE RIDGE AND PIEDMONT

Studies from the 1990’s to 2022 identified post-orogenic zones of brittle faults and fracture (joint) systems that crosscut the overall SW-NE structural pattern of the NC Blue Ridge and Piedmont. These zones postdate ductile fabrics and coincide with ESE-WNW, WSW-ENE and E-W trending topographic lineaments. Detailed investigations show that some of these cross-structures are linked to landslide occurrence, fractured rock hydrogeology, and seismicity.

Studies in the Piedmont for a proposed LLRW disposal site in the 1990’s identified ESE-WNW and WSW-ENE trending extensional faults that crosscut Triassic sedimentary rocks. Aquifer testing along an intersecting NE-striking normal fault indicates that it affected groundwater flow. Landslide mapping the Blue Ridge identified an ESE-WNW trending zone of rock slope instability and brittle faulting that overprints a segment of the Linville Falls ductile thrust fault in Watauga County. Later work identified the brittle zone as the Boone fault, located in an area of 2013-14 seismicity. Topographic lineaments parallel to the Boone fault project into the Deep Gap reentrant in the Blue Ridge Escarpment (BRE) where a 1940 tropical cyclone triggered over 600 landslides. Investigations for an EPA Superfund site identified the ESE-WNW trending Mills Gap fault zone that crosscuts rocks of the Ashe Metamorphic Suite (AMS) in Buncombe County. Hydrogeologic studies identified the fault zone as a pathway for groundwater flow to transmit contaminants. A fault strand juxtaposes bedrock with deformed colluvium indicating possible Cenozoic movement and is within the epicentral area of the 1916 M5.2 Skyland earthquake. Topographic lineaments project ESE through the Mills Gap fault zone ~21km ESE into the Hickory Nut Gorge (HNG) reentrant in the BRE, an area of recurring landslides. Rockfalls reported during 1848-1874 earthquakes there suggest a link between seismicity and past rock slope failures in HNG. Polk County mapping identified E-W trending fractures that control the North Pacolet River reentrant in the BRE, a focused area of debris flows triggered by a May 18, 2018 storm. The August 9, 2021 M_w 5.1 Sparta earthquake occurred along the newly found, ESE-striking Little River fault that crosscuts AMS rocks and produced the first documented seismic surface rupture in the eastern US.