Permo-Carboniferous crustal structure and state of strain in the New England Appalachians, USA

The Permo-Carboniferous Alleghanian orogeny is thought to represent the final collision of Laurasia and Gondwana in the Late Carboniferous and early Permian. Yet, in comparison to the prior Acadian (Devonian) and Taconic (Ordovician) orogenies, the spatio-temporal distribution of deformation and metamorphism in the Appalachian Mountains (eastern USA and Canada) relating to the Alleghanian orogeny remains enigmatic. Notably, there are very few direct constraints on fabric formation related to proposed Permo-Carboniferous shear zones outside of the Honey Hill−Lake Char fault system. Here, we present a compilation of macrostructural observations, new microstructural observations, and new in situ geochronology and geochemical data of monazite and titanite to constrain the distribution of Permo-Carboniferous deformation and metamorphism across the entire New England Appalachians (USA) while placing explicit constraints on fabric formation ages. Our findings suggest that Permo-Carboniferous deformation and metamorphism in the northern New England Appalachians (Vermont, New Hampshire, and Maine) are characteristic of upper crustal conditions (<15 km depth). Deformation in northern New England was defined by localized steep strike-slip shear zones bounding a semi-rigid to rigid medial block, the Central Maine terrane. In southern New England (south-central Massachusetts and Connecticut), the context and grade of Permo-Carboniferous metamorphism transitions from retrograde greenschist facies to prograde amphibolite facies. Corresponding to this change in metamorphic grade, the deformation style also changes. Northward extrusion in the northernmost New England Appalachians at greenschist facies conditions during retrograde metamorphism transitions to southeastward extrusion in the southernmost New England Appalachians at amphibolite facies conditions during prograde metamorphism. These observations suggest that one of two regimes controls Permo-Carboniferous structural and metamorphic style in the northern Appalachians: (1) zipper tectonics and extrusion in the Permo-Carboniferous due to continental collision or (2) gravitational collapse and upper crustal extrusion of an overthickened orogenic plateau such as the “Acadian altiplano”.