Wide‐angle, three‐component seismic reflection profiling of the crust along the East Coast Gravity High, southern Appalachians, using quarry blasts

Abstract

Inversion of refraction/wide‐angle reflection travel times for two‐dimensional velocity structure along the axis of the East Coast Gravity High in the Carolina Terrane of Georgia indicates much higher crustal P wave velocities than previously modeled. Thirty‐nine timed quarry blasts recorded with a 20‐element array of three‐component recorders at 330‐m spacings provide reversed and overlapping coverage along the profile which crosses Consortium for Continental Reflection Profiling Georgia line 5. Ray paths are concentrated in high‐grade rocks of the Charlotte and Kiokee Belts. Shot gathers show strong first arrivals, Moho reflections, and numerous reflections from within the crust. Shear wave arrivals are particularly strong, with prominent direct arrivals (Sg) and reflections from the Moho (SmS) observed for all quarries. Three alternative models show average crustal P wave velocities of 6.5–6.6 km/s and Moho depths between 37 and 39 km. P wave velocities increase from 6.0–6.2 km/s at 1 km depth to 6.7–6.8 km/s at a first‐order discontinuity at a depth of 24–26 km. Alternative models for the lower crust feature both a slight increase in velocity and velocity reversals at the 25‐km discontinuity, with velocities steadily increasing to between 6.9 and 7.5 km/s at the base of the crust. Low average Vp/Vs ratios for the crust (1.70–1.75) estimated from travel time ratios for SmS and PmP reflections suggest that the high compressional‐wave velocities derived by inversion are associated with rocks of high metamorphic grade and granitic to intermediate, not mafic, average composition over most of the crust. The low average ratios can also be explained in part by the presence of a layer of low P wave velocity in the midcrust, possibly associated with a package of quartz‐rich platform and rift‐basin sediments overthrust late in the Alleghanian orogeny by the Carolina Terrane allochthon, as suggested by earlier workers. Crustal models incorporating the new estimate for crustal thickness beneath the Carolina Terrane show that the long‐wavelength component of the gravity gradient in Georgia can be explained in terms of crustal thickness variations; aeromagnetic data indicate that density variations responsible for local steepening of the gradient are confined largely to the lower half of the crust.