Late Neoproterozoic to Permian tectonic evolution of the Quebec Appalachians, Canada

Abstract Due to the unique location of the Quebec Appalachians in a re-entrant of the paleo-North American margin and to the fact that the area largely escaped late Paleozoic deformation, this segment of the orogen preserves evidence of the early orogenic stages that are partly obscured elsewhere. Despite diachronicity both along- and across-strike of the orogen, five tectonic events of regional significance are recognized: (1) the earliest event is mainly recorded by Neoproterozoic to Early Ordovician normal faults associated with extension of the Laurentian craton and the establishment of a passive continental margin; (2) the second event, (the Ordovician Taconian orogeny), is marked by foreland-directed deformation and corresponds to the principal deformational and metamorphic events recorded by the Laurentian margin; (3) the third event (early Salinic) is early Silurian. It is restricted to the southern part of the Quebec Appalachians and consists of hinterland (SE)-directed thrusts/folds and retrograde metamorphism; (4) the fourth event (late Salinic) is recorded by regional-scale, late Silurian to Early Devonian normal (or transtensional) faults and is associated with the formation of the Connecticut Valley-Gaspe Trough; and (5) the last event (the Devonian Acadian orogeny) affects, with variable intensity, all the pre-Carboniferous lithologies. This tectonic synthesis implies that the subdivision of the peri-Laurentian oceanic tracts proposed in other segments of the Appalachians should be applied with caution in Quebec and that the structural style, the number of oceanic domains and possibly the vergence of subduction change along the strike of the orogen. In particular, the similarities of the Thetford-Mines and Boil Mountain ophiolitic complexes and sedimentary covers and the lack of evidence for a major intervening suture are consistent with ‘the maximum allochtony’ hypothesis, i.e., the assumption that the southern Quebec ophiolites are rooted east of the Chain Lake massif. A geodynamic model involving the Taconian obduction of a large ophiolitic nappe(s), the docking of a peri-Gondwanan (Gander/Medial New England) terrane in early Salinic, the formation of large sedimentary basins (Connecticut Valley-Gaspe and Merrimack troughs) due to trench rollback and/or delamination during the late Salinic event and collision at the outer margin of composite Laurentia during the Acadian orogeny, can account for the protracted structural and metamorphic history of the Quebec Appalachians.