Regional Tectonic Synthesis of Northwestern New England and Adjacent Quebec

Publisher Website
Google Scholar
Abstract
The northwestern New England and adjacent Quebec region is an area of 30,000 square miles on the northwest side of the northeastern Appalachian Mountains belt, and extends into the adjacent Hudson, Champlain, and St. Lawrence Valleys to the northwest. It is athwart a major change in trend of this belt from northerly to northeasterly. The synthesis is a rationale of the tectonic relations of this region, discussed in chronological order. The Precambrian basement, exposed at the core of a Paleozoic anticlinorium in the mountain belt, is made up of complexly deformed diaphthoritic miogeosynclinal rocks intruded by granitic plutons and pegmatite dikes. The exposed rocks are part of a northeast-trending Precambrian mobile shelf at least 300 miles wide that includes a wide belt to the northwest in the North American craton. The lower (Cambrian and Ordovician) and middle (Silurian and Devonian) Paleozoic orthogeosyncline, which coincides mainly with the Appalachian belt, includes a broad eugeosynclinal zone, a miogeosynclinal zone to the northwest and probably also to the southeast of the eugeosynclinal zone, several geanticlines, and a quasi-cratonic belt; it thus contrasts with the broadly miogeosynclinal Precambrian rocks of the basement. The eugeosynclinal deposits, whose maximum thickness is more than 50,000 feet, average at least three times the thickness of those in the miogeosynclinal zone. Pelitic and semipelitic rocks dominate the upper deposits and lap over geanticlines, a quasi-cratonic belt, and the margin of the craton. The sources of sediments include cratonal areas, geanticlines that formed tectonic islands, and volcanic islands. The transition between the miogeosynclinal and eugeosynclinal zones is one of sedimentary facies and thickness change, and of stratigraphic convergence and unconformity. In the lowest Paleozoic rocks the transition is west and north-west, respectively, of the Green and Sutton Mountains. The miogeosynclinal zone is missing in Quebec northwest of the northern Sutton Mountains and the eugeosynclinal zone extends to the northwestern margin of the orthogeosyncline. The belt of transition, however, moved southeast in younger rocks, so that in the middle Paleozoic rocks it lies between the Green, Sutton, and Notre Dame Mountains and the Connecticut and St. Johns Rivers. Unconformities indicate stillstand in the miogeosynclinal zone, and general uplift of the northwestern part of the orthogeosyncline, followed by subaerial denudation of the geanticlines in both the eugeosynclinal and the miogeosynclinal zones and by repeated geosynclinal folding. The unconformities are within the lower Paleozoic (especially beneath the Middle Ordovician), are the most extensive between the lower and middle Paleozoic, and are within the middle Paleozoic (especially beneath the Lower Devonian). Geanticlines, two of which coincide with gravity highs, are recognized by unconformable overlap and convergence of bedded rock units toward their axes. The lower Paleozoic Vermont-Quebec geanticline is northwest of the Green and Sutton Mountains in northwestern Vermont and neighboring parts of Quebec, but to the south and northeast it swings more into line with the mountains. It coincides with the lower Paleozoic belt of northwest-southeast transition from the miogeosynclinal to the eugeosynclinal zone, except near the north end of the Sutton Mountains where it trends into the eugeosynclinal zone. The lower Paleozoic Stoke Mountain geanticline coincides with the Stoke Mountains in Quebec, contains eugeosynclinal lower Paleozoic rocks, and is a little north-west of the belt of transition southeastward between the middle Paleozoic miogeosynclinal and eugeosynclinal zones. The lower and middle Paleozoic Somerset geanticline, which nearly coincides with the upper Connecticut River valley and the Boundary Mountains between Quebec and Maine, is cored by rocks of the lower Paleozoic eugeosynclinal zone and is truncated by the unconformity beneath middle Paleozoic rocks. The distribution of the preorogenic igneous rocks of the eugeosynclinal zone reflects the southeastward retreat of this zone during the lower and middle Paleozoic. These rocks include mafic to intermediate metavolcanic and hypabyssal bodies, prevailingly of oceanic theoleiitic composition, and mafic and ultramafic plutons. The plutons reach Lower Cambrian to possibly Middle Ordovician stratigraphic levels. The eugeosynclinal zone, and to a lesser extent the miogeosynclinal zone, are sites of regional metamorphism, shown most universally by the foliation that began to form with compaction of the shales. Isograds climb from low stratigraphic levels in the geanticlines to higher stratigraphic levels in the intervening geosynclinal troughs, showing a direct correlation between the thickness of bedded rocks and metamorphic intensity. Zones of highest grade metamorphism coincide with uplifts, but were probably originally deepest in the geosynclines. Undeformed garnet and staurolite-kyanite coincide with domes and arches, and deformed garnet and chloritoid-kyanite zones with anticlines. Some staurolite and sillimanite zones adjoin granitic plutons, but others are not so associated. Retrograde metamorphic effects in the Precambrian basement include replacement of garnet and hornblende by biotite and chlorite, and sillimanite by muscovite; these effects are caused by folding of the dry basement with the wet Paleozoic. In wet Paleozoic rocks, midway between the dry terranes of the basement and the domes and arches, garnet was replaced by chlorite and kyanite was replaced by muscovite as a result of uplift, denudation and cooling. In Quebec, an exogeosyncline containing about 5000 feet of rocks overlies the northwestern part of the orthogeosyncline and the adjoining craton north-west of the Sutton Mountains. This is a secondary geosyncline northwest of the Vermont-Quebec geanticline. It contains Upper Ordovician sandstone, shale, and...

This publication has 0 references indexed in Scilit: