Metamorphic, thermal, and tectonic evolution of central New England

A new, detailed tectonic model is presented for the Acadian orogenic belt of central New England (Vermont and New Hampshire) that accounts for a wide range of petrological and structural observations. Three belts are considered: the Eastern Vermont, Merrimack, and intervening Bronson Hill belts. Specific observations in eastern Vermont that are accounted for in the model include the following. P-T paths are clockwise with maximum pressures near the Athens, Chester, and Strafford domes of 8-11 kbar, but with maximum pressures decreasing to 3-5 kbar at the boundary with the Bronson Hill belt. Differential exhumation of the Vermont domes relative to the rocks in eastermost Vermont is required by the recorded differences in maximum pressure (5-6 kbar; 15-20 km) and the present-day geographical separation (7-10 km). Specific observations in New Hampshire that are explained include the following. P-T paths in the Merrimack belt are counter-clockwise with maximum pressures of 4-5 kbar and are related to high regional heat flow and heat transfer by early Acadian plutons. P-T paths in the Bronson Hill belt are intimately associated with structural position. An early contact metamorphism is evidenced in the Skitchewang and Fall Mountain nappes near contacts with the early Acadian Bethlehem gneiss (∼ 400-410 Ma). Peak metamorphic temperature rises upwards in the nappe sequence (an inverted metamorphic sequence) whereas peak pressures decrease. Near-simultaneous intrusion of the Bethlehem gneiss and Kīnsman quartz monzonite is required to account for the low-P, high-T metamorphism observed in the Chesham Pond and Fall Mountain nappes. The dominant schistosity, which is related to isoclinal folding, postdates early contact metamorphism in the Fall Mountain and Skitchewaug nappes, and pre-dates peak metamorphism and isothermal loading in the Fall Mountain, Skitchewaug and Big Staurolite nappes. Reactivation of this fabric during thrusting is recorded in some rocks of the Big Staurolite nappe by rotated garnets that grew during near-isothermal loading. Only the sillimanite isograd crosses the Fall Mountain-Skitchewaug nappe boundary. Metamorphic breaks across the Skitchewaug-Big Staurolite nappe, and at the margin of the Keene and Alstead domes require post-metamorphic thrusting when P-T conditions were in the greenschist facies. These observations can be explained by a relatively simple model involving in-sequence thrusting from east to west commencing in central New Hampshire at 400-410 Ma. Preservation of the low-grade belt along the Vermont-New Hampshire border requires that crustal thickening in Vermont was not caused by emplacement of New Hampshire nappes onto eastern Vermont and that the nappes of western New Hampshire had time to cool before final juxtaposition against the low-grade belt. Cooling ages constrain this final juxtaposition to have occured in the Carboniferous, suggesting that the Acadian was a prolonged event spanning as much as 100 Myr.