Metamorphic, Thermal, and Tectonic Evolution of Central New England

Abstract

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