Over 1600 km of deep seismic data were recently acquired to probe
the crust and the upper mantle of the Appalachians in Canada. Although
it is usually difficult to relate the surface expression of tectonic
provinces to characteristic seismic signatures in the crust, our
data show consistent seismic patterns in the lower crust, over distances
of several hundreds of kilometres. These patterns enable us to identify
three lower crustal blocks which underlie the orogen. In some places,
boundaries between crustal blocks are identified by consistent differences
in the seismic character of the Moho reflections. We suggest that
one of these block boundaries (between the Central and Avalon crustal
blocks) corresponds to a fault at the surface which has been identified
as the Gander-Avalon terrane boundary.
High reflectivity of the lower crust and/or bright elongated Moho
reflections are observed along some profiles where velocities higher
than 7.0 km/s have been measured by earlier refraction studies. Although
similar observations have been associated with extensional terranes
by many authors, our data indicate that they can also be found in
convergent environments.