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The Acadian and Neoacadian orogenies are widely recognized, yet poorly understood, tectono-thermal events in the New England Appalachian Mountains (USA). We quantified two phases of Paleozoic crustal thickening using geochemical proxies. Acadian (425–400 Ma) crustal thickening to 40 km progressed from southeast to northwest. Neoacadian (400–380 Ma) crustal thickening was widely distributed and varied by 30 km (40–70 km) from north to south. Doubly thickened crust and paleoelevations of 5 km or more support the presence of an orogenic plateau at ca. 380–330 Ma in southern New England. Neoacadian crustal thicknesses show a strong correlation with metamorphic isograds, where higher metamorphic grade corresponds to greater paleo-crustal thickness. We suggest that the present metamorphic field gradient was exposed through erosion and orogenic collapse influenced by thermal, isostatic, and gravitational properties related to Neoacadian crustal thickness. Geobarometry in southern New England underestimates crustal thickness and exhumation, suggesting the crust was thinned by tectonic as well as erosional processes. 

Abstract Gneiss domes are an integral element of many orogenic belts and commonly provide tectonic windows into deep crustal levels. Gneiss domes in the New England segment of the Appalachian orogen have been classically associated with diapirism and fold interference, but alternative models involving ductile flow have been proposed. We evaluate these models in the Gneiss Dome belt of western New England with U‐Th‐Pb monazite, xenotime, zircon, and titanite petrochronology and major and trace element thermobarometry. These data constrain distinct pressure–temperature–time (P‐T‐t) paths for each unit in the gneiss dome belt tectono‐stratigraphy. The structurally lowest units, Laurentia‐derived migmatitic gneisses of the Waterbury dome, document two stages of metamorphism (455–435 and 400–370 Ma) with peak Acadian metamorphic conditions of ~1.0–1.2 GPa at 750–780°C at 391 ± 7 to 386 ± 4 Ma. The next structurally higher unit, the Gondwana‐derived Taine Mountain Formation, records Taconic (peak conditions: 0.6 GPa, 600°C at 441 ± 4 Ma) and Acadian (peak: 0.8–1.0 GPa, 650°C at 377 ± 4 Ma) metamorphism. The overlying Collinsville Formation yielded a 473 ± 5 Ma crystallization age and evidence for metamorphic conditions of 650°C at 436 ± 4 Ma and 1.2–1.0 GPa, 750–775°C at 397 ± 4 to 385 ± 6 Ma. The structurally higher Sweetheart Mountain Member of the Collinsville Formation yielded only Acadian zircon, monazite, and xenotime dates and evidence for high‐pressure granulite facies metamorphism (1.8 GPa, 815°C) at circa 380–375 Ma. Cover rocks of the dome‐mantling The Straits Schist records peak conditions of ~1 GPa, 700°C at 386 ± 6 to 380 ± 4 Ma. Garnet breakdown to monazite and/or xenotime occurred in all units at circa 375–360 and 345–330 Ma. Peak Acadian metamorphic pressures increase systematically from the structurally lowest to highest units (from 1.0 to 1.8 GPa). This inverted metamorphic sequence is incompatible with the diapiric and fold interference models, which predict the highest pressures at the structurally lowest levels. Based upon P‐T‐t and structural data, we prefer a model involving, first, circa 380 Ma thrust stacking followed by syn‐collisional orogen parallel extension, ductile flow, and rise of the domes between 380 and 365 Ma. Garnet breakdown at circa 345–330 Ma is interpreted to reflect further exhumation during collapse of the Acadian orogenic plateau. These results highlight the power of integrating petrologic constraints with paired geochemical and geochronologic data from multiple chronometers to test structural and tectonic models and show that syn‐convergent orogen parallel ductile flow dramatically modified earlier accretion‐related structures in New England. Further, the Gneiss Dome belt documents gneiss dome development in a syn‐collisional, thick crust setting, providing an ancient example of middle to lower crustal processes that may be occurring today in the modern Himalaya and Pamir Range.