TECTONIC EVOLUTION OF THE QUéBEC APPALACHIANS – A LAURENTIAN PERSPECTIVE AND IMPLICATIONS FOR THE TACONIAN-GRAMPIAN OROGENY

The Québec Appalachians form a 1000 km-long orogenic segment that escaped Late Paleozoic deformation, and preserve evidence of the early geological history. It is made up of four lithotectonic units, (1) Neoproterozoic to Upper Ordovician rocks of the Humber Zone (Laurentian margin), (2) Ordovician rocks of the Dunnage Zone that consists of supra-subduction zone (SSZ) ophiolites and volcanic arc rocks, all being unconformably overlain by piggyback basin deposits made up of «mélanges» and conglomerates grading upward into Cr-bearing sandstone, Fe-rich black shales and tufs, and (3 and 4) two unconformable sequences of Late Ordovician-lower Silurian and Silurian-Devonian rocks, the latter known as the Gaspé Belt. The Humber and Dunnage zones are separated by an originally shallow-dipping fault now folded and faulted. The Humber Zone is made up of an external subzone of NW-verging nappes stacking of low-grade rocks formed during the Ordovician Taconian orogeny, and an internal subzone of greenschist- to amphibolite-facies metamorphic rocks with complex polyphased structures spanning Ordovician to Devonian ages. The Dunnage Zone forms a mobile belt and occurs in the hanging wall of normal faults (locally transcurrent) that separate crustal domains of contrasting structures and metamorphic ages.

The ophiolites were obducted onto Laurentia in Early-Middle Ordovician times, the obduction lasted for c. 15 m.y. and was completed by c. 460 Ma. Taconian deformation was then transferred into foreland- and hinterland-thrust propagation and crustal thickening, coeval with exhumation of the obduction-related collisional wedge. Except for slightly older ages (i.e. 10-to-20 m.y.) for ophiolite obduction, exhumation, and metamorphism of the Laurentian margin in the British/Irish Caledonides, the structural model envisionned for the Taconian phase is consistent with the one proposed for the Grampian orogeny, indicating that the Taconian-Grampian plate collision has been diachronic along-strike and dominated by SSZ ophiolite emplacement. The number, age, and geometry of oceanic domains involved in the early stages of mountain building may however vary along-strike in a way comparable with present-day SE Asia.