There are some striking similarities and some differences between
the seismic reflection sections recorded across the fold and thrust
belts of the southeast Canadian Cordillera, Quebec-Maine Appalachians
and Swiss Alps. In the fold and thrust belts of all three mountain
ranges, seismic reflection surveys have yielded high-quality images
of. (1) nappes (thin thrust sheets) stacked on top of ancient continental
margins; (2) ramp anticlines in the hanging walls of faults that
have ramp-flat or listric geometries; (3) back thrusts and back folds
that developed during the terminal phases of orogeny; and (4) tectonic
wedges and regional decollements. A principal result of the Cordilleran
and Appalachian deep crustal studies has been the recognition of
master decollements along which continental margin strata have been
transported long distances, whereas a principal result of the Swiss
Alpine deep crustal program has been the identification of the Adriatic
indenter, a crustal-scale wedge that caused delamination of the European
lithosphere. Significant crustal roots are observed beneath the fold
and thrust belts of the Alps, southeast Canadian Cordillera and parts
of the southern Appalachians, but such structures beneath the northern
Appalachians have probably been removed by post-orogenic collapse
and/or crustal attenuation associated with the Mesozoic opening of
the Atlantic Ocean.