K-AR DATING AND STRUCTURAL ANALYSIS OF THE TACONIAN ALLOCHTONS, QUÉBEC APPALACHIANS, CANADA

The Taconian Allochtons consist of siliciclastic rocks, limestones and mafic volcanics that are deformed into a series of imbricated NW-directed thrust nappes. In the Québec Appalachians, these rocks vary from unmetamorphosed to sub-greenschist facies in the external zone, to medium-grade metamorphic rocks in the internal zone. Structural and 40Ar/39Ar data from the internal zone suggest that Silurian-Devonian (ca. 430-415 Ma) backthrusts associated with exhumation by normal faulting overprints the Middle Ordovician (Taconian) thrust system (ca. 460 Ma). Mapping of key outcrops in the external zone in the Québec city and Rivière-du-Loup areas shows that the backthrust system crosscuts the earlier thrust system, indicating that these structures also occur in the external zone. The emplacement of the Taconian Allochtons and related low-grade metamorphism in the external zone is mainly constrained by faunal control of synorogenic flysch units, suggesting emplacement in Middle to Late Ordovician times.

Fault rocks from the external zone thrusts (NW-verging) and backthrust (SE-verging) faults were sampled for K-Ar dating in the Rivière-du-Loup and Quebec city areas. In the Québec city area, Cambrian-Ordovician shales and phyllites of the Taconian allochtons were sampled along the Chaudière River Transect for determination of the K-Ar values of their <2 um-size fractions in order to obtain age data for the nappe stacking. Among these samples, K-Ar dates of 491-467 Ma were obtained and most likely represent mixed values of detrital material affected by diagenetic or tectonic processes. The thermal grade increases, however, to epizonal conditions towards the SE and along major thrust faults. K-Ar analysis of the <0.2 um-size fraction of fault gouges marking thrust faults in the Rivière-du-Loup and Québec city areas yield ages of 451±8 and 458±7 Ma respectively. Such ages are consistent with 40Ar/39Ar data obtained in the Appalachian internides and are considered as the age of thrust faults characterizing the Taconian Allochtons. Along the Chaudière River Transect, K-Ar analysis of the <0.2 um-size fractions from a phyllite sample affected by backthrusting fabrics yield 402±7 Ma, which is younger than 40Ar/39Ar age data (430-415 Ma) from the adjacent metamorphic rocks of the internal zone.