Paper No. 4
Presentation Time: 9:00 AM
40AR/39AR GEOCHRONOLOGICAL DATA FROM THE SUTTON/GREEN MOUNTAINS ANTICLINORIUM, SOUTHERN QUEBEC-NORTHERN VERMONT APPALACHIANS: EPISODIC AND DIACHRONOUS TECTONISM FROM MIDDLE ORDOVICIAN TO MIDDLE DEVONIAN
In the southern Quebec-northern Vermont Appalachians, metamorphic rocks of the Laurentian margin crop out along a mega-structure, the Notre-Dame-Sutton-Green Mountains anticlinoria, which progressively exposes deeper tectonometamorphic levels from North (southern Quebec) to South (northern Vermont). Regional deformation and metamorphism of these rocks are currently attributed to the combined effects of Taconian (Middle-Late Ordovician) and Acadian (Middle Devonian) orogenies, and consist of three generations of superimposed folds and faults and at least one (Taconian) metamorphic paragenesis. Recently-acquired 40Ar/39Ar isotopic data from several sections across the mega-structure has revealed age groups; (1) Middle Devonian, (2) Silurian-Early Devonian, (3) Middle Ordovician, and (4) Precambrian. The second 40Ar/39Ar age group is of special interest as it spans the apparent period of relative tectonic quiescence between the Taconian and Acadian orogenies. In the Sutton Mountains anticlinorium transect, this group of ages (430-405 Ma), occurs throughout the anticlinorium structure and an overall west-to-east age decrease is observed. In the northern Green Mountain anticlinorium transect, preliminary 40Ar/39Ar data of that age group appears to be slightly older (441-430 Ma). These latter ages were obtained from white micas in rock samples where the second regional foliation (S2) is penetrative and interpreted to be associated with SE-verging structures. In southern Quebec, 40Ar/39Ar age data from similar fabrics are interpreted as deformation ages that characterize Silurian-Early Devonian, hinterland-directed thrusting and extensional faulting. Preliminary age data of northern Vermont and along-strike structural correlations of regional fabrics between Vermont and Quebec suggest that their overall tectonic evolution has been similar. In summary, structural analyses and geochronological data from both transects suggest episodic and diachronous tectonism characterized by contrasting structural vergences and metamorphic conditions.