Lower crustal reflections in central Virginia, USA

Summary. Vibroseis reflection data across the Blue Ridge and Piedmont provinces in Virginia, acquired in 1981 by the United States Geological Survey, have been reprocessed at Virginia Tech. The 12‐fold, 14–56 Hz vibrator data were originally 8 s in length, but were extended to 14 s during correlation. Interpretation of the data was further improved by using a well‐defined crustal velocity model derived by previous workers from earthquake and blast analyses in the area. Low reflectivity areas on the seismic section are interpreted to represent Grenville‐aged crust on the section, in contrast to highly reflective allochthonous units. These regions of reflectivity allow for mapping of the gross crustal structure. Strong, subhorizontal arrivals at 9 to 12 s in the reflection data are interpreted to be from lower crustal layering just above the Mohorovicic Discontinuity (Moho) defined by earlier refraction work. This layering may be reflective only beneath regions of deformed crust and not at the base of the undisturbed craton. The depth to these reflectors is approximately 35 km beneath Richmond, Virginia and increases gently westward until the reflections disappear beneath the Blue Ridge Mountains about 70 km west of Richmond. These thicknesses are in agreement with earlier refraction work which also indicated greater Moho depths to the west. A rethickening of the crust near the Atlantic coast is also interpreted from the refraction data but is not evident on the reflection line, probably due to an acquisition or energy penetration problem. The deep reflections consist of a subhorizontally layered package about 5 km in thickness, thinning slightly toward the craton. Their base coincides with the crust‐mantle boundary determined from refraction data, indicating that the reflections lie within the lower crust. Lower crustal reflections were not recorded west of the Piedmont on the multichannel data. PmP arrivals recorded by earlier workers from beneath the Blue Ridge, however, are consistent with a Moho reflector about 8 km shallower than indicated by the refraction data. If the crust‐mantle boundary is a second‐order discontinuity (smooth transition zone) there, the PmP arrivals may have been reflected from near the upper boundary of the transition zone and the refracted waves may have travelled along the base. The discrepancy in depths may therefore be a measure of the transition zone thickness.