Abstract

The Saint-Daniel Mélange is an orogen-scale sedimentary basin of the Québec Appalachians that is commonly interpreted as the remnant of a subduction complex formed during the Taconian orogeny. However, geochronological, structural, and stratigraphical data from Laurentian continental margin rocks and adjacent ophiolitic rocks in southern Québec indicate that the Saint-Daniel Mélange is an olistostromal, syncollisional piggyback basin that represents the base of the Magog Group forearc basin. The regional stratigraphic framework of the Saint-Daniel Mélange and its relationships with underlying and overlying rock units have been established on the basis of six stratigraphic sections from the Thetford-Mines, Asbestos, and Mont-Orford ophiolitic complexes. Our results imply that (1) the Saint-Daniel Mélange is a sedimentary sequence that unconformably overlies different structural and pseudostratigraphic levels of the southern Québec ophiolites; (2) it is made up of four distinct and laterally discontinuous units that record the obduction of ophiolites onto Laurentia and exhumation of basement rocks, followed by subsidence and deposition of the overlying Saint-Victor Formation of the Magog Group; (3) the sedimentary rocks of the Saint-Daniel record a transition from ophiolite-dominated to continental sources, indicating progressive exhumation of both the ophiolite and the continental margin upon which it was obducted; 40Ar/39Ar analysis of muscovite from metamorphic rock fragments in debris flows of the mélange yield an age of 467 ± 2 Ma, which is within the range of Ar ages measured in metamorphic rocks that underlie the ophiolites, and indicates the uplift of metamorphosed continental deposits during or shortly after obduction; (4) the Saint-Daniel Mélange is stratigraphically overlain by the Magog Group and represents the base of a syncollisional basin developed in a forearc setting during the Taconian orogeny.

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