The origin of reflections beneath the Blue Ridge‐Piedmont allochthon: A view through the grandfather mountain window

Abstract

A perplexing variety of crustal models have been proposed for the region beneath the southern Appalachian overthrust. Determining the true nature of crustal composition and structure in this region carries important scientific and economic implications. To evaluate the numerous models, compressional wave velocities and densities were measured on 30 rock samples collected within the Grandfather Mountain window in the Blue Ridge of North Carolina. The window breaches high‐grade crystalline rocks, exposing Grenville gneisses, Late Proterozoic metasedimentary and metavolcanic rock, and Cambrian quartzite, phyllite, and dolostone that are likely to underlie the regional Blue Ridge‐Piedmont thrust. Measured velocities range widely from 4.21 km s−1 for Cambrian phyllite to 7.47 km s−1 for tectonized Cambrian Shady Dolomite at 200 MPa. Velocity anisotropy is as great as 47% in the phyllite. The samples exhibit a broad range in densities as well: from 2500 kg m−3 in fractured quartzite to 3000 kg m−3 in metabasalts. Synthetic reflection seismograms were generated using these physical properties to consider the origin of subthrust reflection events. In addition to the much‐publicized interpretation of Paleozoic sedimentary rocks residing beneath the crystalline sheet, modeling reveals that the reflections could also be explained by metamorphosed Paleozoic strata, by the regional Late Proterozoic metavolcanic and metaclastic rift sequence, and by pervasive shear zones (i.e., mylonites) within compositionally homogeneous gneisses. We interpret field data to strongly favor mylonitic gneisses as the subthrust reflectors. Still, surface data remain sufficiently ambiguous that the question will not be definitively settled without continental drilling.