The tectonic evolution of the Connecticut Valley Synclinorium: Constraints from argon/argon thermochronology, uranium-lead geochronology, thermobarometry and thermal modeling

The tectonic evolution of New England is explored by focusing on a single terrane, the Connecticut Valley Synclinorium (CVS), situated between Laurentia and all major outboard terranes. The Connecticut Valley Gaspe trough (CVGT) is a Silurian basin developed on Ordovician and older rocks of the CVS. Despite widespread exposures and numerous studies focusing on this basin, uncertainties still exist regarding the tectonic setting for the deposition of these sediments. U-Pb SHRIMP ages of detrital zircons obtained from quartzites of the CVGT constrain the provenance and age of these sediments and suggest tectonic models for the development of the basin. A lithologic boundary identified in sediments of the CVGT implies a change in the depositional setting of the basin from an intercontinental back-arc extensional setting to a foreland basin setting. Given the timing of the approaching Acadian wedge, this shift in basin style was likely caused by westward migrating thrust sheets during the accretion of the Avalon terrane to the evolving eastern margin of North America.;Consequently, early Devonian loading and subsequent metamorphism of the CVS took place during the Acadian orogeny. One-dimensional thermal modeling of Acadian metamorphism in southeastern Vermont constrained by new and existing 40Ar/39Ar thermochronology, U-Pb geochronology, and thermobarometry has revealed that the Devonian thermal evolution of the crust is strongly influenced by residual heat from Ordovician, Taconian metamorphism. Moreover, a comparison between models of upper and lower crustal rocks suggest final emplacement of gneissic domes was not entirely an Acadian event but was surprisingly delayed until sometime after the Mississippian and may represent Alleghanian deformation.;Additional Alleghanian deformation in southeastern Vermont is demonstrated using 40Ar/39Ar data obtained from chlorite-grade phyllonites. Detailed optical and SEM-petrography of phyllonites collected along the CVS-Bronson Hill terrane boundary shows that white mica and chlorite defining younger cleavages are recrystallized. White mica ages as young as ∼300 Ma obtained from these cleavages indicate the boundary was likely a site of significant strain localization in late Pennsylvanian to early Permian times. These results suggest that Alleghanian deformation, so well documented in coastal and southern New England, was transmitted across the relatively strong composite Avalon-Ganderia crustal block to the boundary with Laurentian crust (underlying the CVS) as far north as Vermont where strain was localized along a weak preexisting structure. Thus an integration of these tools has contributed to unraveling the tectonic history of a complicated convergent plate margin, where decades of mapping and petrographic work have proven insufficient to discriminate among the Taconic, Acadian, and Alleghanian orogenic events.