The role and extent of dextral transpression and lateral escape on the post-Acadian tectonic evolution of south-central New England

Abstract

Bedrock mapping, structural analysis, documentation of kinematics and associated petrofabrics, and detailed zircon and monazite geochronology are the basis for a new interpretation of the post-Acadian tectonic evolution of southern New England. Early fabrics in the Central Maine Zone and eastern half of the Bronson Hill Zone are characterized by planar foliations, subhorizontal lineations, and dextral kinematic indicators. As transpression progressed, structures and fabrics reflected an increased component of shortening, marked by tightening of foliations, development of closed to isoclinal folds, localization of reverse high strain zones, steep dip-parallel lineations, and reverse top-to-east kinematic indicators. Syntectonic mineral assemblages show that cooling attended deformation, consistent with a component of vertical extrusion. In contrast, the western half of the Bronson Hill Zone is characterized by sinistral top-to-south deformation from Connecticut to New Hampshire, including two regional sinistral shear zones, and comprise the Western Bronson Hill Shear System. New zircon and monazite geochronology, along with previously published results, show that dextral transpression progressed continuously from ∼350 Ma to ∼295 Ma. Dextral transpression was initiated following ∼360 Ma to ∼355 Ma dioritic, tonalitic, and granitic magmatism. Sillimanite-grade reverse high strain zones developed by ∼330 Ma, while the presence of lower-grade fabrics, metamorphic zircon rims, and published hornblende cooling ages show deformation continued to at least ∼295 Ma in the eastern Bronson Hill and Central Maine. Sinistral top-to-south deformation in the Western Bronson Hill Shear System preceded ∼330 Ma to ∼300 Ma staurolite grade metamorphism, and syntectonic monazite and an abundance of published data show that deformation continued to ∼285 Ma. The conjugate shear zones were spatially and temporally linked, indicating that the dominant fabrics, structures, and mineral assemblages in bedrock exposures in this area of New England were driven by progressive dextral transpression and lateral escape in the Carboniferous, much younger than any previous tectonic reconstruction. This model can explain an assortment of enigmatic features in New England, including contrasting kinematics, differential exhumation of lithologies and lithotectonic zones, and the Pelham dome. These features are attributed to oblique convergence between Laurentian margin and Avalon/Meguma beginning in the Mississippian, rather than the Early to Middle Devonian Acadian orogeny as has long been proposed. The location between the New York promontory and the Quebec embayment could have provided the necessary geometry to focus deformation and lateral escape in New England, and shorten the orogen in this area by almost an order of magnitude.