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From 1983 to 1984 the United States Geological Survey and the Geological Survey of Canada undertook a series of seismic experiments to determine the crustal structure of the Appalachian mountain belt in Maine and southeast Québec. These experiments included four deep crustal seismic reflection profiles perpendicular to the strike of regional geological trends and several high‐resolution refraction/wide‐angle reflection profiles both parallel and perpendicular to strike. Here the data recorded in Québec and the northwest part of Maine are interpreted in conjunction with existing reflection profiles from Québec. Synthetic seismogram calculations in laterally varying media are used to model the refraction/wide‐angle reflection data, while reprocessing, migration, and attribute plotting are used in the interpretation of the reflection data. A major zone of reflections extending over a distance exceeding 200 km can be traced from shallow depths beneath the St. Lawrence Lowlands southeast to about 25 km depth beneath the southeast edge of the Chain Lakes Massif. This zone of reflections is interpreted as a master décollement separating autochthonous Grenville basement from overlying allochthonous rocks of the Appalachian Orogen, including foreshortened miogeoclinal rocks, the remnants of one or more magmatic arcs, the Connecticut Valley ‐ Gaspé Synclinorium, and the Chain Lakes Massif. In the vicinity of the Chain Lakes Massif, extension has caused Grenville basement to be thinned by as much as 40%. Reflections from the décollement show strong layering and evidence for basement faulting associated with the Guadeloupe and other faults. Normal incidence and wide‐angle reflections are observed to be almost continuous beneath the Baie Verte ‐ Brompton Line and Connecticut Valley ‐ Gaspé Synclinorium, indicating that the Baie Verte ‐ Brompton Line is confined to the upper crust. The Connecticut Valley ‐ Gaspé Synclinorium is characterized by P‐wave velocities of about 5.3 km s−1 and is shown to be a shallow structure with a maximum depth of about 3 km. Rock velocities immediately below the Connecticut Valley ‐ Gaspé Synclinorium are typical of Chain Lakes material (6.1 km s−1). A new model for the tectonic development of the region is proposed in which all pre‐Silurian units between the Baie Verte ‐ Brompton Line and the southeastern edge of the Chain Lakes Massif are allochthonous and confined to the upper crust. We infer that the Chain Lakes Massif, volcanics of the Ascot‐Weedon Formation, and associated deeper crustal rocks were thrust over Grenvillian basement as a composite unit and may now underlie parts of the Connecticut Valley ‐ Gaspé Synclinorium.

Early Paleozoic amalgamation of composite terranes was contemporaneous at widely separated regions that were later accreted to either ancestral North America or to Gondwana as those two continents approached each other. Terranes closer to Laurentia amalgamated in the Late Cambrian to Middle Ordovician Penobscottian orogeny and were accreted to ancestral North America in the Middle and to Late Orodovician Taconic orogeny. Peri-Gondwanan terranes formed from Late Cambrian and Early Ordovician rocks were amalgamated in the Late Ordovician and Early Silurian to form the Salinic orogenic belt, now exposed from western Europe to southern New England. Salinic orogenic activity involved extensive thrust faulting and metamorphism, large strike-slip faults, and plutonism, all of which are represented in coastal Maine. In the Penobscot Bay region, Maine, the peri-Gondwanan St. Croix terrane was thrust northwest in the Silurian(?) upon middle amphibolite facies Ordovician and Early Silurian rocks of the Fredericton trough. Seismic reflection profiles show that the thrust fault dips southeasterly at -30° and becomes listric at about 13 ± 2 km. The thrust sheet was broken initially by the Late Silurian Penobscot Bay-Smith Cove-North Blue Hill dextral strike-slip fault which juxtaposed the peri-Gondwanan Ellsworth terrane, followed by emplacement of the South Penobscot Intrusive Suite and by a sequence of strike-slip fault zones each with up to 20 km of dextral Silurian and Early Devonian(?) movement. The strike-slip faults are interpreted to either remain steep until they reach the sole of the thrust sheet or to become listric within the thrust sheet. In the Devonian Acadian orogeny, more outboard terranes with Gondwanan affinities, like the Avalonian terranes in southern New Brunswick and in eastern Massachusetts, were amalgamated with the Silurian orogenic belt, and the resulting composite terrane was accreted to ancestral North America. Acadian deformation, mcta-morphism, and plutonism are superimposed on the Silurian orogen, blurring or obliterating the evidence of Silurian orogeny. RÉSUMÉ Le fusionnement du Paléozoique inférieur des formations géologiques composites a été simultané dans des régions très separées qui ont ultérieurement été accolées, soit à l'Amérique du Nord ancestrale, soit au Gondwana, au fur et à mesure que les deux continents se sont rapprochés l'un de l'autre. Les formations géologiques plus proches du bouclier laurentien se sont fusionnées à l'Amérique du Nord au cours de l'orogénèse Taconique de l'Ordovicien moyen à supérieur. Les formations géologiques périgondwaniennes formées de roches du Cambrien supérieur à l'Ordovicien inférieur ont été fusionnées au cours de l'Ordovicien supérieur et du Silurien inférieur pour former la ceinture orogénique Salinique, maintenant découverte à partir de l'Europe occidentale jusqu'au sud de la Nouvclle-Angleterre. L'activité orogénique salinique a comporté un métamorphisme et une dislocation de compression importants, des décrochcments étendus et le plutonisme, des phénomènes qui sont tous représentés sur la côte du Maine. Dans la région de Penobsquot Bay, au Maine, la formation géologique périgondwanienne de St. Croix a été poussée vers le nord-ouest pendant le Silurien (?) sur les roches de l'Ordovicien et du Silurien inférieur du facies intermédiate à amphibolite de la dépression de Fredericton. Les profits de réflexion sismique montrent que la faille de compression s'incline vers le sud-est selon un angle de 30° et qu'elle devient courbe après environ 13 ± 2 km. La nappe avait initialement été fracturée par le décrochement dextre du Silurien supérieur de Penobsquot Bay-Smith Cove-North Blue Hill qui avait juxtaposé la formation périgondwanienne d'Ellsworth, événement suivi par le positionnement du cortege intrusif de South Penobsquot et par une séquence de zones de décrochements accusant chacun jusqu'à 20 km de déplacement dextre au cours du Silurien et du Dévonien inférieur (?). On interprète les décrochements comme des accidents qui restent abrupts jusqu'à ce qu'ils atteignent la surface inférieure de la nappe ou comme des failles devenant courbes à l'intérieur de la nappe. Au cours de l'orogénèse Acadienne du Dévonien, d'autres formations géologiques extérieures ayant des affinites avec le Gondwana, comme la formation géologique d'Avalon, dans le sud du Nouveau-Brunswick, et dans Test du Massachusetts, ont été fusionnées avec la ceinture orogénique Silurienne, et la formation composite qui en est résultée a été accoiée à l'Amérique du Nord ancestrale. La déformation, le métamorphisme et le plutonisme Acadiens sont superposed par-dessus l'orogène du Silurien, ce qui efface et brouille la preuve de l'orogénèse Silurienne. [Traduit par la rédaction]

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For nearly 40 years, the former Soviet Union has carried out an extensive program of seismic studies of the Earth's crust and upper mantle, known as “Deep Seismic Sounding” or DSS [Piwinskii, 1979; Zverev and Kosminskaya, 1980; Egorkin and Pavlenkova, 1981; Egorkin and Chernyshov, 1983; Scheimer and Borg, 1985]. Beginning in 1939–1940 with a series of small‐scale seismic experiments near Moscow, DSS profiling has broadened into a national multiinstitutional exploration effort that has completed almost 150,000 km of profiles covering all major geological provinces of northern Eurasia [Ryaboy, 1989].