Velocity structure of reflective lower crust beneath the Grand Banks of Newfoundland

Abstract

A seismic refraction experiment using air guns and ocean bottom seismometers was carried out on the southern Grand Banks, in order to study the velocity structure of highly reflective lower crust observed on a previous deep multichannel reflection profile. The data were analyzed by interactive forward modeling of travel times and amplitudes, wide‐angle normal moveout analysis and downward continuation of slant stacks. Laterally varying structure was used in the final stage of modeling. The results indicate an upper crust with a P velocity of 6.0–6.2 km/s, a zone of high velocity gradient from about 19 to 24 km depth, and a lower crustal velocity of 6.6–6.8 km/s. Moho is at 29 km. The depth of the transition zone is uncertain by 2 or 3 km, and its detailed velocity structure is not well resolved. However, the top of the zone corresponds approximately to the onset of reflectivity, while the lower crustal layer is characterized by more coherent and continuous reflections. A thin (<1 km) localized high‐velocity (around 7.2 km/s) layer is tentatively correlated with one of the reflectors in the lower crust. Bulk anisotropy of the lower crust is less than 5%. The presence of well‐defined high‐velocity layers and the lack of anisotropy tend to support mafic intrusion as the mechanism for reflectivity in the lowest part of the crust. However, there is no evidence of a thick high‐velocity underplated layer at the base of the crust.