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In the pre-Silurian lithotectonic units of the northern Vermont Appalachian...
In the pre-Silurian lithotectonic units of the northern Vermont Appalachians, the timing of orogenesis and tectonometamorphism has traditionally been ascribed to the combined effects of the Middle Ordovician Taconian orogeny and Middle to Late Devonian Acadian orogeny. However, numerous geo chronological studies throughout the Northern Appalachians, including neighboring southern Quebec, have obtained Silurian and Early Devonian age data that document more or less continuous tectonometamorphic activity throughout the Ordovician - Devonian. The structural and metamorphic evolution of northern Vermont can be separated into three regional phases, which are characterized by distinct structures, fabrics, and metamorphic parageneses. The fi rst phase (D1), associated with westward emplacement of various thrust slices leading to crustal thickening and regional metamorphism, and the second phase (D2), characterized by bivergent structures and metamorphic overprint, have both been considered to be Taconian. The third phase, the structure and fabric of which are also observed in the Silurian - Devonian rocks to the east, is considered to be Acadian. We present new step-heating and spot fusion 40Ar/39Ar geochronological data on amphibole and fabric-forming muscovite from samples taken across the Green Mountain anticlinorium, which, coupled with published data, provide improved age constraints on tectonometamorphism of D1 (latest Cambrian to Middle Ordovician), D2 (Silurian - Early Devonian), and D3 (Middle Devonian) events. By comparing structural and metamorphic characteristics, and now timing, these phases are interpreted to be correlative to the tripartite tectonometamorphic evolution documented in southern Quebec, and they further exemplify the along-strike diachronism of tectonism induced by the inherited irregular geometry of the Laurentian margin.
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