TECTONIC EVOLUTION OF THE NORTHERN LAURENTIAN MARGIN WITH EMPHASIS ON THE SILURIAN-EARLY DEVONIAN: ARE THE SOUTHERN QUEBEC APPALACHIANS DISTINCT?

In the Northern Appalachians, the Laurentian margin is made up of allochthonous continental rocks that have been metamorphosed during and after the closure/accretion of Iapetus (sensu largo) in Ordovician to Silurian time. The metamorphic wedge is subdivided into a NW external zone of low-grade rocks and a SE internal zone of higher-grade rocks (up to amphibolite). To the SE, these metamorphic rocks are in tectonic contact with remnants of Iapetus along the Baie Verte-Brompton line (BBL). In southern Quebec, the external zone is characterized by NW-verging folds and thrust nappes whereas both NW- and SE-directed structures occur in the internal zone. From NW to SE, structures of the external zone are increasingly overprinted by NW-dipping shear fabrics related to back folds and back thrust faults. In the internal zone, NW- and SE-verging structures commonly form a composite fabric. Eastward, SE-dipping normal fault zones, including the BBL, crosscut earlier structures. ⁴⁰Ar/³⁹Ar and U/Pb metamorphic ages from the Quebec and Newfoundland Appalachians yield similar results. Metamorphic rocks occurring as structural windows in the hanging wall of the BBL and related normal faults in southern Quebec, and those that make the internal zone in Gaspé Peninsula, give ages that vary between 469-456 Ma, consistent with consequences of Taconian crustal thickening. In southern Quebec and Newfoundland, metamorphic rocks of the footwall of the BBL, which form the bulk of the internal zone, yield Silurian to Early Devonian ages (435-415 Ma). In parts of Newfoundland, they have been attributed to crustal thickening during the Salinian orogeny, a continental collision. In Quebec and northwestern New England, there is no evidence for major compressive deformation along the Laurentian margin in the Silurian-Early Devonian. Age data of southern Quebec are rather interpreted as the timing of recrystallization and deformation related to the progressive exhumation of the internal zone following either, a period of back thrusting induced by basement wedging or a major event of crustal extension. Several structural, metamorphic features of the northwestern New England Appalachians yield analogous characteristics with those of southern Quebec, and may have shared a similar Late Silurian-Early Devonian tectonic evolution.