ABSTRACT This three-day field trip focuses on the stratigraphy and the structural characteristics of the late- and post-Taconian sedimentary basins of the southern Québec Appalachians, with a particular emphasis on N-to-S and W-to-E structural and lithological variations. In order to discuss various aspects of the regional structural evolution of these basins, we will visit a series of key outcrops following three sections, the Beauce/Thetford-Mines sections, the Sherbrooke section, and the Coaticook section. RÉSUMÉ Cette excursion de trois jours se concentre sur la stratigraphie et les caractéristiques structurales des bassins sédimentaires tardi- et post-Taconien des Appalaches du sud du Québec, en mettant l’accent sur les variations structurales et lithologiques du nord au sud et d’ouest en est. Afin de discuter des divers aspects de l’évolution structurale régionale de ces bassins sédimentaires, nous visiterons une série d’affleure ments clés en suivant trois sections, soient les sections de Beauce/Thetford-Mines, de Sherbrooke, et de Coaticook.

The Rivière-des-Plante ultramafic Complex lies along the Baie Verte–Brompton line in southern Québec and has previously been interpreted as an ophiolitic mélange. It is bounded on the northwest by a northwest-dipping thrust fault and unconformably overlain by conglomerates belonging to the Saint-Daniel Mélange to the southeast. It consists of harzburgite, serpentinite, ophicalcite, gabbro, granite, and granofelsic to mylonitic fragmental rocks. The latter have been interpreted as “exotic” metasedimentary rocks correlative with those of the Chain Lakes massif of western Maine. Our mapping suggests that the Rivière-des-Plante ultramafic Complex is not a mélange, but rather a deeply eroded ophiolitic remnant mostly represented by mantle peridotites that correlate with those of the Thetford-Mines ophiolite. The granofelsic to mylonitic rocks represent xenolith-bearing granitoids crosscutting the peridotites rather than “exotic” blocks derived from the Chain Lakes massif. These granites are similar to ca. 470 Ma peridotite-hosted granitoids of the Thetford-Mines ophiolite, which were generated by anatexis of the Laurentian margin during ophiolite obduction. A comparison of metasedimentary rocks of the Chain Lakes massif with those of the southern Québec Laurentian margin, as well as stratigraphic and geochronological data for both the southern Québec and western Maine Appalachians, suggests that the Chain Lakes likely represents more or less in situ Laurentian margin, and that metamorphism and anatexis dated at 469 Ma may have been caused by the obduction of the southern Québec ophiolites.

Abstract The Ordovician Thetford Mines Ophiolite Complex (TMOC) is an oceanic terrane accreted to the Laurentian margin during the Taconic Orogeny and is affected by syn-obduction (syn-emplacement) deformation and two post-obduction events (Silurian backthrusting and normal faulting, and Acadian folding and reverse faulting). The southern part of the TMOC was tilted to the vertical during post-obduction deformation and preserves a nearly complete cross-section through the crust. From base to top we distinguish cumulate Dunitic, Pyroxenitic and Gabbroic Zones, a hypabyssal unit (either sheeted dykes or a subvolcanic breccia facies), and an ophiolitic extrusive-sedimentary sequence, upon which were deposited sedimentary rocks constituting the base of a piggy-back basin. Our mapping has revealed the presence of subvertically dipping, north-south- to 20°-striking faults, spaced c. 1 km apart on average. The faults are manifested as sheared or mylonitic dunites and synmagmatic breccias, and correspond to breaks in lithology. The fault breccias are cut by undeformed websteritic to peridotitic intrusions, demonstrating the pre- to synmagmatic nature of the faulting. Assuming that rhythmic cumulate bedding was originally palaeo-horizontal, kinematic analysis indicates that these are normal faults separating a series of tilted blocks. In the upper part of the crust, the north-south-striking fault blocks contain north-south-striking dykes that locally constitute a sheeted complex. The faults correspond to marked lateral changes in the thickness and facies assemblages seen in supracrustal rocks, are locally marked by prominent subvolcanic breccias, and have upward decreasing throws suggesting that they are growth faults. The base of the volcano-sedimentary sequence is a major erosional surface in places, which can penetrate down to the Dunitic Zone. The evidence for coeval extension and magmatism, and the discovery of a locally well-developed sheeted dyke complex, suggest that the TMOC formed by sea-floor spreading. The dominance of a boninitic signature in cumulate and volcanic rocks suggests that spreading occurred in a subduction zone environment, possibly in a forearc setting.

Regional deformation encountered in Ordovician rocks of the Ascot Complex and the Magog Group of the Dunnage zone is synchronous with the development of the La Guadeloupe fault. Structural analysis indicates that along most of the length of this fault, Silurian and Devonian rocks of the Saint-Francis Group were thrust over the Dunnage zone and its post-Ordovician cover sequence. Structural fabrics of these units are thus related to the Acadian Orogeny, and pre-Acadian deformations are of much less importance. Major Acadian faults in the northeastern part of the Québec Appalachians are dextral strike-slip faults, whereas they are largely thrust faults in its southwestern parts. The northern part of the Beauce area lies within the transition zone between these two fault regimes. Transected folds on both sides of the La Guadeloupe fault suggest a dextral oblique-slip tectonic transport. Northwest of the Magog Group, Upper Silurian rocks of the Cranbourne Formation unconformably overlie the Baie Verte-Brompton Line. The Cranbourne Formation is folded and cut by the same regional cleavage as the rocks below the unconformity. This demonstrates that the Acadian Orogeny is the major regional phase of deformation of all these units.