GRAVITY SIGNATURES AND GEOMETRIC CONFIGURATIONS OF SOME OLIVERIAN PLUTONS - THEIR RELATION TO ACADIAN STRUCTURES

Oliverian plutons in west-central New Hampshire consist of Upper Ordov ician mantled gneiss domes along the axis of the Bronson Hill anticlin orium, as well as other stocks farther west. All are intrusive into Mi ddle Ordovician volcanic and metasedimentary rocks, but have been remo bilized and have acquired their gneissic foliations during the late st ages of the Early-Middle Devonian Acadian orogeny, Gravity studies are used to demonstrate that the Moody Ledge, Owls Head, and Baker Pond d omes are irregularly shaped flat plutons with maximum thicknesses of 2 .5 km, The Croydon dome, 4.2 km thick, is probably of laccolithic shap e, 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 pluto n west of the domes is a discordant north-northwest-plunging stock int rusive into a higher stratigraphic level (the Partridge volcanics), an d 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 i n 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 Ol iverian 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 beneat h it, slightly below ground level, A subsurface (Devonian?) granitic s tock occurs a short distance to the south, The Devonian Indian Pond pl uton of Bethlehem gneiss in the Mount Cube quadrangle is also shown to be rootless, in agreement with its inferred structural position insid e a nappe which has been overfolded from east to west, and just touche s the present Earth's surface, Recent pressure-temperature-time (P-T-t ) metamorphic studies in western New Hampshire have led to proposals t hat there is a series of nested thrust plates involving the Bronson Hi ll and adjacent terranes, The possibility arises that the Oliverian do mes 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 Hampshi re is likely to consist of metamorphic formations of Cambrian-Ordovici an age.