STRUCTURAL AND METAMORPHIC EVOLUTION OF THE CONNECTICUT VALLEY-GASPÉ TROUGH – INSIGHTS FROM SOUTHERN QUEBEC AND NORTHERN VERMONT

In Québec and New England, the Connecticut Valley-Gaspé (CVG) trough is an orogen-scale Silurian-Devonian basin of the Northern Appalachians. From Gaspé peninsula to southern New England, the CVG trough has experienced contrasting metamorphic and structural evolution during the Acadian orogeny. From north to south, it is characterized by increasing (1) deformation and polyphased structures, (2) intensity of regional metamorphism, i.e. from very low-grade to upper amphibolite facies, and (3) abundance of crosscutting ~ 390 to 370 Ma granitic intrusions. In southern Quebec and northern Vermont, a series of NW-SE transects in the CVG trough have been studied to better understand the along-strike structural and metamorphic variations. Detailed structural analyses, combined with isochemical phase diagram sections (IPDS), Raman spectrometry and muscovite ⁴⁰Ar/³⁹Ar dating have been performed.

In southern Québec, regional deformation is characterized by D₁-related NW-verging folds. Towards the Québec-Vermont border, D₂ fabrics progressively appears as evidenced by SE-verging F₂ folds associated with a S₂ crenulation cleavage that wraps around granitic intrusions. D₂ structures are locally overprinted by a late-stage crenulation cleavage (S₃). Temperature of regional metamorphism (M₁) has been determined from Raman spectrometry. It gradually increases toward the south, from ~ 370°C to 420°C, whereas IPDS suggest that M₁ pressure reached ~ 4 Kb. At the Quebec-Vermont border, the M1 temperature gradient was partially obliterated by the heat released from granitic intrusions. In Vermont, D₂ and D₃ structures are more pervasive and IPDS suggest that P-T conditions reached at least ~ 5 Kbar and 500°C. In Southern Québec, ⁴⁰Ar/³⁹Ar ages of metamorphic muscovites indicate that the D₁ -M₁ greenschist-facies event, peaked at ~ 380–375 Ma. However, in Québec-Vermont border and northern Vermont, ~ 360-365 Ma and ~ 335-340 Ma metamorphic muscovite from upper greenschist/amphibolite facies rocks reflects cooling, and thus minimum ages for the D₂ and D₃ events, respectively. Alleghanian metamorphic ages (~ 300 Ma) are found in easternmost part of the Vermont, in the footwall of the Monroe Fault. These results can be attributed to spatial and temporal partitioning of compressive deformation during the Acadian orogeny.