The extension of Grenville Basement beneath the northern Appalachians: Results from the Quebec‐Maine seismic reflection and refraction surveys

Abstract

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.