We have determined the seismic velocity structure of the crust in
the vicinity of the Appalachian deformation front off western Newfoundland
and the adjacent Gulf of St Lawrence. These measurements were made
from two perpendicular wide-angle seismic refraction profiles, one
of which is collinear with a previously recorded deep seismic reflection
profile. The Grenvile foreland crust, about 45 km thick, is characterized
by velocities of 6.35 km/s in its upper part and 6.7 km/s in its
lower part. Close to the coast of Newfoundland, a deep crustal reflective
wedge is bounded by a northwest-dipping reflector and by the crust-mantle
boundary, which is at only 39 km depth beneath the wedge. In the
wedge, velocities of 7.2-7.3 km/s may indicate the presence of mafic
and ultramafic rocks. We speculate that several processes could have
caused the high velocities and the high reflectivity. The most attractive
is perhaps crustal extension with consequent underplating during
the formation of the Iapetus Ocean or during later reactivation by
Carboniferous strike-slip movements.