SYNTHESIS AND INTERPRETATION OF 40AR/39AR DATA ON REGIONAL METAMORPHISM AND DEFORMATION ALONG THE NOTRE-DAME AND SUTTON MOUNTAINS ANTICLINORIA, SOUTHERN QUEBEC APPALACHIANS

In the southern Quebec Appalachians, metamorphic rocks of the Laurentian margin crop out along the Notre-Dame and Sutton Mountains anticlinoria, which represent up-plunge equivalents of the Green Mountain anticlinorium in Vermont. Metamorphism and deformation of these rocks have been traditionally attributed to the superposed Taconian (Middle-Late Ordovician) and Acadian (Middle Devonian) orogenies. In that scheme, the Silurian-Early Devonian was regarded as a period of relative tectonic quiescence. A comprehensive ⁴⁰Ar/³⁹Ar geochronological study comprising more than 150 analyses has revealed the imprint of four events; (1) Middle Devonian, (2) Silurian-Early Devonian, (3) Middle Ordovician, and (4) Precambrian. Group 1 (380-370 Ma) is recorded as a disturbing event in Taconian metamorphic rocks. It registers Middle Devonian Acadian metamorphism. Group 2 (431-405 Ma) occurs in the footwall of backthrust and normal faults bordering the anticlinoria, and is attributed to the tectonic exhumation of metamorphic terranes. This event has been coeval with crustal extension and the formation of major sedimentary basins. Group 3 (465-456 Ma) is provided by metamorphic rocks occurring in structural windows southeast of the St-Joseph normal fault, and is also locally preserved within the anticlinoria. This group records regional metamorphism and crustal thickening attributed to the Taconian orogeny. Group 4 (> 945 Ma) is preserved in detrital micas taken from faintly metamorphosed rock units in the immediate hanging wall of the St-Joseph fault. They probably have lied in shallow structural level during the Taconian orogeny and were subsequently juxtaposed to metamorphic rocks by extensional faulting during the Silurian-Early Devonian event. Two regional tendencies are observed: an increasing Acadian imprint to the southwest, and a southeastward statistical age decrease of Silurian-Lower Devonian data across the anticlinoria. The Silurian-Early Devonian age data characterizes hinterland-directed and extensional deformation in southern Quebec, which most probably extend southward into Vermont.