THE TACONIAN OROGENY IN THE QUÉBEC APPALACHIANS: WHAT HAVE WE LEARNED SINCE ST-JULIEN & HUBERT (1975)?

Following the acceptance of plate tectonics in the early 70’s, St-Julien & Hubert (1975) published a tectonic synthesis for the Taconian orogeny in the Québec Appalachians, a contribution that has been instrumental for all structural and tectonic studies that followed. The main conclusions were that (1) the external continental domain of nappes was formed by NW- and SE-directed gravity sliding and folded by the end of the Middle Ordovician, (2) regional metamorphism/folding in the internal domain of mixed continental and oceanic lithologies was completed by Late Ordovician-early Silurian times, and (3) the Taconian tectonic setting was marked by Cordilleran-type subduction and arc-continent collision (ACC).

This talk is a synthesis of knowledge development on the tectonic evolution of the Québec Appalachians since that milestone paper. In the Northern Appalachians, ~50% of continental rocks of Laurentia (Humber zone) and adjacent peri-Laurentian oceanic rocks (Dunnage zone) occur in Québec where they are overlain by the Silurian-Devonian rocks of the Gaspé Belt. A similar Taconian history of accretion occurred along the whole length of the orogen but is best preserved in southern Québec. The acquisition of isotopic age data has been critical for a better comprehension of the Taconian orogeny. In pre-Silurian rocks, it highlights thermal pulses of Middle-Late Ordovician and Silurian/Early Devonian ages. Ordovician ages belong to typical Taconian burial metamorphism and are consistent with the stacking of thrust nappes and foreland-directed deformation of Laurentia during Taïwan-type subduction and ACC, from ophiolite obduction (475-460 Ma) to the emplacement of the Taconic allochthons (455-440 Ma). Obduction was followed by delamination, accounting for «arc-like» volcanic rocks interbedded with synorogenic deposits of the Dunnage zone. Late stage of the Taconian orogeny extends well into the Silurian. It is marked by the diachronic formation of basement slices and thin-skin - thick-skin tectonic transition, and is associated with Silurian/Early Devonian ages which are related to SE-verging solid-state deformation instead of gravity sliding. This latter event provides genetic mechanisms and timing constraint compatible with the exhumation of the Taconian orogen and formation of the Gaspé Belt.