Pleistocene Shorelines and Coastal Rivers: Sensitive Indicators of Quaternary Faults, Atlantic Coastal Plain, USA

Eight possible active faults beneath Quaternary Coastal Plain sediments are indicated by models showing expected patterns of shoreline regression and river deflections along tectonically inactive and active (where subsurface faults may affect surface processes) parts of the passive margin. Non-tectonic processes produce consistently spaced parallel shorelines and an absence of river deflections or seaward deflections of shorelines by major deltas with spacing dependent upon sediment influx rates. Differential regional uplift will cause divergence of shorelines toward, and deflection of rivers away from the end with greater uplift. Tectonic models show shoreline regression with a shoreline-parallel fault that is either seaward-side-up (SSU) or seaward-side-down (SSD) or with a shoreline-perpendicular fault (SPF). SSU interrupts non-tectonic patterns with convex-seaward deflections and wider spacing of older shorelines across uplifts and river deflections toward uplift-margins. SSD produces a convex-landward deflection and wider spacing of younger shorelines on the down-dropped side of faults and river deflections toward uplift-margins. SPF produces: convex-seaward deflections with wider spacing of older shorelines across uplifts and river deflections toward uplift-margins; convex-landward deflection and wider spacing of younger shorelines on down-dropped sides of faults where river deflections merge toward the lowest area; shorelines are discontinuous and may be difficult to correlate across faults. SPF patterns in the vicinity of the 1886 Charleston, SC earthquake indicate a NW-trending, 50-km long, NE-side-up fault active since Early Pleistocene near the Ashley River and suggest a similar 30-km long fault near the North Edisto River and a NW-trending, 50-km long, SW-side-up fault near the Broad River. Variations of SPF patterns in NC suggest E-W-trending, 150-km long, N-side-up faults extending westward from both Cape Fear and Cape Lookout and possibly a shorter one near Cape Hatteras. Previous workers invoked SSU to explain zones of river anomalies in the Carolinas, but those anomalies do not exhibit SSU patterns. SSD patterns both east and southwest of the Okefenokee Swamp suggest a N-trending, 100-km long, W-side-up fault near the St Marys and Satilla rivers in GA and a NW-trending, 75-km long, NE-side-up fault near the Suwannee River in FL. Possible Quaternary faults at these locations may indicate significantly greater risk from seismic hazards for east-coast communities