TECTONIC EVOLUTION OF THE TACONIC ALLOCHTONS, QUEBEC APPALACHIANS: INSIGHTS FROM DETAILED STRUCTURAL ANALYSIS AND K-AR DATING

In Northern Appalachians, Taconian Allochtons constitute the external Humber Zone (EHZ). Allochtons consist of siliciclastic rocks, limestones and mafic volcanic rocks deformed into imbricated, northwest-directed nappes. In Québec, these rocks vary from diagenetic to sub-greenschist facies of metamorphism. Thrusting from Taconian Allochtons and related low-grade metamorphism is currently constrained by faunal control, which suggests Middle Ordovician to Late Ordovician age (Taconian). In contrast to Newfoundland, there is yet no evidence for Acadian-related deformation in the EHZ of southern Quebec. We revisited cinematic evolution of EHZ of southern Quebec by mapping key outcrops in the Quebec City and Rivière-du-Loup areas. Structural analysis from these outcrops provided a collection of fault rock and hosting lithologies samples, each characterizing various types of structures. Clay material of 1-2µm- and <0,2µm-size fractions were extracted from these samples for XRD phases characterization and K-Ar isotopic dating. SEM and TEM analyses were performed to support geological significance of K-Ar data interpretations.

The analysis of K-Ar data leaded to the following interpretations: (1) a westward-directed tectonic imbrication of rock units at ca. 470 Ma resulted in the emplacement of thrust sheets (the Allochtons) currently exposing epizone-grade rocks in easternmost part of the EHZ; (2) at ca. 450 Ma, thrust sheets were re-imbricated during out-of-sequence faulting in response to northwest-directed compression; (3) a series of younger structures, formed ca. 412 Ma and occur as foreland- and hinterland-directed superposed folds/faults; and (4) a series of (commonly discrete) normal faults locally developed ca. 360 Ma. Our study emphasizes the fact that structural history of the EHZ is characterized by polyphased fault imbrications and younger faulting, each associated to distinctive types of fault rocks. These new structural and K-Ar data are critical to understand the kinematics of the EHZ from southern Quebec to Newfoundland. These data also represent essential clues in the elaboration from future oil and gas exploration program into the Canadian Appalachians.