We have determined crustal thickness in the Gulf of St. Lawrence,
an area that corresponds to an offset of the main northern Appalachians
units. A "complete" Bouguer anomaly was calculated from recent depth-to-basement
compilations and sediment densities from well data. The Moho surface
was obtained by inverting the Bouguer anomaly, assuming a single
density contrast at depth, and using an average depth provided by
deep reflection seismic data. The resulting crustal model shows a
Moho depth of 42?44?km beneath the Grenville Craton, north of the
Appalachian deformation front. South of this front, the depth to
Moho displays a pronounced thinning of the crust beneath the Carboniferous
Magdalen Basin. This is in striking contrast to the deep seismic
data, which give a Moho depth of about 43?km. The modelling of the
Bouguer anomaly in the Magdalen Basin, taking into account the seismic
reflection and refraction data, reconciles these different results
and suggests that a 43?km deep Moho beneath the basin is associated
with a lower crustal layer about 13?km thick, with high velocity
(7.35?km/s) and density (3.05?g/cm3). The Bouguer anomaly suggests
that the lateral extent of this high-density layer is confined roughly
to the Magdalen Basin. We suggest that this layer is due to mantle
underplating of the crust as a result of the Carboniferous-age formation
of the Magdalen Basin, and that it is not a feature related to the
early to middle Paleozoic development of the Appalachian Orogen.