STRUCTURAL AND METAMORPHIC CHARACTERIZATION OF THE CONNECTICUT VALLEY-GASPÉ TROUGH IN THE SOUTHERN QUÉBEC-NORTHERN VERMONT APPALACHIANS, CANADA AND USA

In the Québec and New England Appalachians, the Connecticut Valley-Gaspé trough hosts the sedimentary sequence of the Gaspé belt, a major post-Taconian Silurian-Devonian basin of the Northern Appalachians that extends for a thousand kilometers from New England to Gaspé Peninsula. In southern Quebec, these sedimentary rocks are exposed in the Connecticut Valley-Gaspé synclinorium, and located in the hanging wall of the La Guadeloupe Fault, an important reverse fault related to the Middle Devonian Acadian orogeny. From southern Québec to New England, these rocks experienced a contrasting metamorphic and structural historiy during the Acadian orogeny, expressed by a significant increase in the intensity of regional metamorphism and by the progressive development of increasingly penetrative and polyphased structures in northern Vermont as compared to southern Québec. Southern Québec and Vermont therefore represent a key region for a better understanding of North-South metamorphic and structural variations of the Gaspé Belt.

In the studied area, regional metamorphism varies along-strike from prehnite-pumpellyite to greenschist facies in southern Québec, to amphibolite facies in northern Vermont. Besides, we observe increasing polyphased regional deformation towards the Quebec-Vermont border. In southern Québec, regional deformation is characterized by NW-verging tight F1 folds evolving southward into F2/F1 refolded folds, F2 folds being SE-verging and associated with a well-developed crenulation cleavage. At the border, the D2 structures are cross-cut by a vertical NE-SW-trending late stage crenulation cleavage (S3) that is axial-planar to large-scale dome structure. The principal reason for these along-strike, metamorphic and structural variations in the Gaspé Belt can be attributed to differential thickening and exhumation, associated with oblique lithospheric convergence and partitioning of metamorphism/deformation during the Acadian orogeny.