Structural style and tectonostratigraphy of the external-internal Humber zone boundary in the Sainte-Marie - Saint-Sylvestre area, Quebec Appalachians

The geology of the Sainte-Marie-de-Beauce–Saint-Sylvestre area depicts a tectonostratigraphic, structural, and metamorphic transition that characterizes the boundary between the external and internal Humber Zone. It also corresponds to a region of reversal in structural vergence and tectonic transport, i.e. from foreland-directed structures in the northwestern part to hinterland-directed structures in the southeastern part. A series of thrust nappes make up tectonostratigraphic packages that are correlated to various stratigraphic units: the Sainte-Hénédine, Rivière Filkars, Richardson, and Oak Hill nappes comprise rocks units of the Île d’Orléans, Saint-Roch, Armagh, and Oak Hill groups, respectively. To the southeast, the Bennett Fault separates these nappes from 2001-D13 S. Castonguay et al. 1 CURRENT RESEARCH RECHERCHES EN COURS 2001 Article Article Titles Titres Purchase Information Information pour acheter 1 Contribution to the Appalachian Foreland and St. Lawrence Platform NATMAP Project 2 INRS-Géoressources, Quebec Geoscience Centre 880, chemin Sainte-Foy, C.P. 7500, Sainte-Foy (Québec) G1V 4C7 metamorphosed and polydeformed lithologies of the Rosaire Group and Bennett Schists. The Bennett Fault is a southeast-directed backthrust fault, which best corresponds to the dominant structure representing the boundary between the external and internal Humber Zone. INTRODUCTION T transition between external and internal (i.e. foreland-hinterland) domains represents key areas for a better understanding of orogenic belt architectures. In such regions, sudden or major breaks in tectonostratigraphic assemblages, structural style, and metamorphic grade often occur, thus creating an apparent discontinuity in tectonic style and evolution. In the Quebec Appalachians, such a transition zone occurs 50 km south of Québec, where the northwest-directed Richardson Fault has been classically CURRENT RESEARCH RECHERCHES EN COURS 2001 Article Article Titles Titres Purchase Information Information pour acheter 2001-D13 S. Castonguay et al. 2 Résumé La géologie de la région de Sainte-Marie-de-Beauce–Saint-Sylvestre représente une transition tectonostratigraphique, structurale et métamorphique qui caractérise la limite entre la zone de Humber externe et interne. Elle correspond aussi à une région d’inversion de la vergence structurale et du transport tectonique dominant, c.-à-d. où les structures dirigées vers l’avant-pays dans le nord-ouest passent à des structures dirigées vers l’arrière-pays dans le sud-est. Une série de nappes de chevauchement forment des assemblages tectonostratigraphiques qui sont mis en corrélation avec diverses unités stratigraphiques : les nappes de Sainte-Hénédine, de Rivière Filkars, de Richardson et d’Oak Hill comprennent des unités lithologiques des groupes de l’Île d’Orléans, de Saint-Roch, d’Armagh et d’Oak Hill, respectivement. Vers le sud-est, la Faille de Bennett sépare ces nappes des lithologies métamorphisées et polydéformées du Groupe de Rosaire et des Schistes de Bennett. La Faille de Bennett est une faille de rétrochevauchement dirigée vers le sud-est qui correspond le mieux à la structure dominante représentant la limite entre la zone de Humber externe et interne. identified as the boundary between the external and internal domains of the Humber Zone (St-Julien and Hubert, 1975; St-Julien et al., 1983); however, recent studies in southern Quebec (Tremblay and Pinet, 1994; Pinet et al., 1996a, b; Castonguay, 2000) have shown that the structural style and metamorphic characteristics of the internal Humber Zone have been greatly influenced by a major hinterland-directed tectonic event that includes southeast-verging backfolding, backthrusting, and normal faulting. This contrasts strongly with the classical foreland-directed structural evolution of the fold and imbricated thrust belt constituting the Humber Zone. In terms of tectonostratigraphy, the study area lies in a region where lithostratigraphic facies and nomenclatures of the Lower St. Lawrence area and southern Quebec Appalachians converge, and where metamorphic and tectonic overprint has obscured original stratigraphic relationships. In addition, rock assemblages vary significantly between each of the thrust nappes (e.g. Slivitsky and St-Julien, 1987; Lebel and Hubert, 1995a, b), and offer only sparse age constraints, thus resulting into a complex regional tectonostratigraphic framework that hinders regional (acrossand along-strike) stratigraphic correlations. This ongoing study aims to clarify and better understand the structure, stratigraphy, and tectonic evolution of the external and internal Humber zone boundary along transect #2 (Québec–Chaudière) of the Geological Bridges of Eastern Canada Project. The study area lies in the eastern half of the NTS 21 L/6 map area (Saint-Sylvestre) in the Quebec Appalachians. It occurs at the northeastern periclinal termination of the Notre-Dame anticlinorium, a prominent feature of the internal Humber Zone, and encompasses the transition zone between the external and internal domains of the Humber Zone (Fig. 1). 2001-D13 S. Castonguay et al. 3 CURRENT RESEARCH RECHERCHES EN COURS 2001 Article Article Titles Titres Purchase Information Information pour acheter