SYNOROGENIC BASIN DEVELOPMENT IN THE QUÉBEC APPALACHIANS: INSIGHTS FROM 40AR/39AR DATING AND STRATIGRAPHIC ANALYSIS OF THE ST-DANIEL MÉLANGE

In the southern Québec Appalachians, major backthrust faults define an upper plate-lower plate (UP-LP) boundary, with very low-grade metamorphic rocks in the hanging wall and medium-grade rocks in the footwall. Silurian-Early Devonian 40Ar/39Ar ages (430-415 Ma) characterize the lower plate. The St-Joseph fault and the Baie Verte-Brompton line (BBL) are southeast-dipping normal faults that crosscut this UP-LP boundary. Metamorphic rocks with Middle Ordovician 40Ar/39Ar ages (469-461 Ma), ophiolites and synorogenic oceanic deposits occur in the downthrown side of the normal faults, and are related to obduction and crustal thickening during the Taconian orogeny. The Saint-Daniel Mélange is part of the Ordovician oceanic sequence cropping out in the hanging wall of the St-Joseph-BBL fault system. It is an orogen-scale sedimentary basin currently interpreted as a subduction complex formed during the Taconian orogeny. Structural and stratigraphical data from the Laurentian continental margin and adjacent ophiolites indicate rather that the Saint-Daniel Mélange is a synorogenic basin representing the base of the Magog Group. The internal stratigraphy and regional framework of the mélange and its relationships with adjacent rock units have been established on the basis of six stratigraphic sections and implies that: (1) the Saint-Daniel Mélange is a sedimentary sequence that rests in depositional contact on different levels of the underlying ophiolites; (2) it is made up of four distinct and discontinuous units that record the obduction of ophiolites onto the Laurentian margin and the exhumation of basement rocks, followed by regional subsidence and deposition of the Magog Group; (3) the mélange record variations between ophiolite- and continental margin-derived sources, indicating progressive exhumation of both the ophiolite and the Laurentian margin upon which it was deposited. 40Ar/39Ar analysis of muscovite from metamorphic rock fragments in the mélange yielded 467+/-2 Ma, indicating that the uplift of metamorphosed margin rocks occurred shortly after ophiolite emplacement; (4) the Saint-Daniel Mélange is stratigraphically overlain by the Magog Group, and represents the base of a synorogenic basin developed in a forearc setting during the Taconian orogeny.