Crustal Structure of Eastern North Carolina: Piedmont and Coastal Plain

We present the results from an onshore seismic refraction and wide-angle reflection profile, conducted in 2015, across the coastal plain and eastern Piedmont provinces of North Carolina. We use forward modeling to create 1D synthetic seismogram models and then invert first break picks to create 2D P- and S-wave velocity models. The crustal thickness is 38 km beneath the Piedmont and central coastal plain, but i t thins to 32 km at the coastline. The average thickness of the upper crust is 11 km with an average P-wave velocity (V-P) of 6.0 km/s and S-wave velocity (V-S) of 3.5 km/s. A prominent seismic low-velocity zone (LVZ) (V-P < 6.0 and V-S <3.6 km/s) exists between the depths of 6 and 11 km, beneath the western third of the seismic profile. The middle crust varies greatly in thickness, increasing from 3 km in the west (eastern Piedmont) to 13 km in the east (coastal plain), with seismic velocities of 6.5 km/s for V-P and 3.8 km/s for V-S. The lower crust thins significantly toward the rifted Atlantic margin, decreasing from 24 km thick in the west (Piedmont) to 8 km at the coastline, with velocities of approximately 6.9 km/s for V-P and 3.9 km/s for V-S. We estimate the composition of the crust by comparing the measured values of V-P and Poisson's ratio with laboratory measurements. The upper and middle crusts are in agreement with a felsic composition, while the lower crustal composition is predominately felsic to intermediate. The LVZ in the upper crust is associated with thin layers of the mylonitic rocks involved in the top and the bottom of thrusting, and the top of the lower crust could be the master detachment fault during the thin-skinned Alleghanian orogeny. The eastward thinning of the lower crust is consistent with crustal extension during the Mesozoic rifting of the Atlantic margin.