In 1986, 1181?km of marine seismic reflection data was collected to
18?20?s of two-way traveltime in the Gulf of St. Lawrence area. The
seismic profiles sample all major surface tectono-stratigraphic zones
of the Canadian Appalachians. They complement the 1984 deep reflection
survey northeast of Newfoundland. Together, the seismic profiles
reveal the regional three-dimensional geometry of the orogen.Three
lower crustal blocks are distinguished on the seismic data. They
are referred to as the Grenville, Central, and Avalon blocks, from
west to east. The Grenville block is wedge shaped in section, and
its subsurface edge follows the form of the Appalachian structural
front. The Grenville block abuts the Central block at mid-crustal
to mantle depths. The Avalon block meets the Central block at a steep
junction that penetrates the entire crust.Consistent differences
in the seismic character of the Moho help identify boundaries of
the deep crustal blocks. The Moho signature varies from uniform over
extended distances to irregular with abrupt depth changes. In places
the Moho is offset by steep reflections that cut the lower crust
and upper mantle. In other places, the change in Moho elevation is
gradual, with lower crustal reflections following its form. In all
three blocks the crust is generally highly reflective, with no distinction
between a transparent upper crust and reflective lower crust.In general,
Carboniferous and Mesozoic basins crossed by the seismic profiles
overlie thinner crust. However, a deep Moho is found at some places
beneath the Carboniferous Magdalen Basin.The Grenville block belongs
to the Grenville Craton; the Humber Zone is thrust over its dipping
southwestern edge. The Dunnage Zone is allochthonous above the opposing
Grenville and Central blocks. The Gander Zone may be the surface
expression of the Central block or may be allochthonous itself. There
is a spatial analogy between the Avalon block and the Avalon Zone.
Our profile across the Meguma Zone is too short to seismically distinguish
this zone from the Avalon Zone.