STRUCTURAL EVOLUTION OF THE THETFORD-MINES OPHIOLITIC COMPLEX, QUEBEC: FROM SYN-OCEANIC RIFTING TO OBDUCTION AND POST-OBDUCTION DEFORMATION

In the southern Québec Appalachians, the Thetford-Mines ophiolitic Complex (TMOC) and St-Daniel mélange occur in the hanging wall of a major SE-dipping normal fault, the St-Joseph fault, which bounds the eastern flank of the Notre-Dame Mountains anticlinorium (NDMA). The NDMA contains greenschist-grade metamorphic rocks of continental margin origin, characterized by ductile tectonic fabrics formed during Ordovician emplacement of the ophiolite, and Late Silurian-Early Devonian backthrusting and exhumation of the Laurentian margin. Except for pre-obduction structures, the structural history of the TMOC is similar to that of the Laurentian continental margin. From youngest to oldest structures, the tectonic evolution of oceanic rocks is characterized by (1) upright folding related to Type 1 and 2 fold interference patterns and high-angle reverse faults attributed to the Acadian orogeny, (2) SE-verging folds and shear zones related to stratigraphic inversion of the TMOC, attributed to Late Silurian-Early Devonian backthrusting and normal faulting (i.e. the St-Joseph fault) (3) NW-verging shear zones related to the Ordovician ophiolite obduction, recorded by amphibolite- to greenschist-grade metamorphic fabrics developed at the base of the TMOC, and (4) NW- to N-trending, ductile to brittle faults there are interpreted as syn-oceanic extension structures, associated to « late » ultramafic intrusions, stratigraphic excision, formation of fault-scarp breccias (?) and debris flows in the crustal sequence of the TMOC. A pre-obduction palinspastic reconstruction of the TMOC suggests that it forms a half-graben, and that the transition between the volcano-sedimentary sequence of the TMOC and the overlying St-Daniel Mélange represents lateral and vertical facies variations. The occurrence of ophiolite-derived mafic/ultramafic fragments and metamorphic rock fragments of continental origin, characterizes both the ophiolitic sedimentary sequence and the St-Daniel mélange, suggesting that the contact between the oceanic crust and the overlying mélange is an erosional unconformity rather than a fault, and that the St-Daniel probably forms a syn-obduction piggy-back basin.