Gravity signatures and geometric configurations of some Oliverian plutons: Their relation to Acadian structures

Abstract

Oliverian plutons in west-central New Hampshire consist of Upper Ordovician mantled gneiss domes along the axis of the Bronson Hill anticlinorium, as well as other stocks farther west. All are intrusive into Middle Ordovician volcanic and metasedimentary rocks, but have been remobilized and have acquired their gneissic foliations during the late stages of the Early-Middle Devonian Acadian orogeny. Gravity studies are used to demonstrate that the Moody Ledge, Owls Head, and Baker Pond domes are irregularly shaped flat plutons with maximum thicknesses of 2.5 km. The Croydon dome, 4.2 km thick, is probably of laccolithic shape. The largest dome surveyed, the Mascoma, is mushroom shaped, with a stem extending to a depth of at least 4.5 km. The coeval Lebanon pluton west of the domes is a discordant north-northwest-plunging stock intrusive into a higher stratigraphic level (the Partridge volcanics), and extends to a depth of 8.5 km. It is bordered on the southeast, south, and west by the Cornish nappe, which is overfolded toward the west in the area south of the pluton, but is backfolded toward the east in a large region west and north of the stock. An oval outcrop area in the Mount Cube quadrangle resembles the surface outcrop of a subjacent Oliverian dome, but cannot be, because it lacks the appropriate gravity field. The Devonian Fairlee quartz monzonite is shown to be rootless, in accordance with the fact that Ammonoosuc normal fault passes beneath it, slightly below ground level. A subsurface (Devonian?) granitic stock occurs a short distance to the south. The Devonian Indian Pond pluton of Bethlehem gneiss in the Mount Cube quadrangle is also shown to be rootless, in agreement with its inferred structural position inside a nappe which has been overfolded from east to west, and just touches the present Earth9s surface. Recent pressure-temperature-time ( P-T-t ) metamorphic studies in western New Hampshire have led to proposals that there is a series of nested thrust plates involving the Bronson Hill and adjacent terranes. The possibility arises that the Oliverian domes may be allochthonous. However, nothing in the mapped geology, the geophysics, or the geochronology supports this hypothesis. We conclude that the basement below the Oliverian granites in western New Hampshire is likely to consist of metamorphic formations of Cambrian-Ordovician age.