STRUCTURE AND METAMORPHISM OF THE KEENE-BRATTLE-BORO AREA, NEW HAMPSHIRE-VERMONT

Abstract

Metamorphosed sedimentary and volcanic rocks, ranging in age from middle Ordovician (?) to lower Devonian, occupy about 55 per cent of the Keene-Brattleboro area of southwestern New Hampshire and southeastern Vermont. Rocks of the low-grade zone of metamorphism are present in a north-south band near the western edge of the area; rocks of the middle-grade zone occur on either side of the low-grade zone; and rocks of the high-grade zone occupy the central and eastern parts of the area. The lithology of the stratified rocks is similar to that farther north. The Bernardston formation of north-central Massachusetts is correlated with the Littleton formation. Foliated igneous rocks of middle Devonian (?) and late Devonian (?) age occupy about 45 per cent of the area. These include three plutons of the Oliverian magma series and five members of the New Hampshire magma series. The stratified rocks were severely folded, broken by a major thrust fault, and metamorphosed during the late Devonian (Acadian) orogeny. The resulting structural features in this area, trend almost north. The Bronson Hill anticline occupies most of the area. Three elongate domes, each with a plutonic core of the Oliverian magma series and a surrounding envelope of stratified rock, are present on this anticline. A pluton of Kinsman quartz monzonite occurs in a structural basin on the anticline, near the center of the area. The steeply dipping western limb of the Brattleboro syncline occupies the western one-eighth of the area. The Northey Hill thrust, a major fault of western New Hampshire, separates the Bronson Hill anticline from the Brattleboro syncline. This thrust, which cuts out the eastern limb of the Brattleboro syncline, has a stratigraphic separation of about 10,000 feet. Foliation and bedding are mutually parallel in most of the stratified rocks east of the Northey Hill thrust, whereas foliation is parallel to the axial planes of folds in most of the rocks west of the thrust. Minor folds of two ages are present west of the thrust and in a few places just east of thrust. The older minor folds are isoclinal, or nearly so, and have a well-developed axial-plane flow cleavage. The younger minor folds are more open and have an incipient to strong axial-plane slip cleavage. It is believed that the Oliverian magma series was forcefully injected as a thick composite sheet during the very early stages of the orogeny, while the enclosing rocks were essentially flat. The New Hampshire magma series was emplaced during the late stages of the orogeny. Initial movements along the Northey Hill thrust may have occurred during early stages of the orogeny, but a considerable part of the movement took place after the metamorphic intensity in the rocks along the fault had reached its maximum. The Triassic border fault, which forms the eastern border of the Triassic rocks of the Connecticut valley in Massachusetts, enters the area 1 mile east of the Connecticut River and trends northeasterly to the vicinity of Keene, where it apparently ends. The maximum stratigraphic displacement along the fault is about 15,000 feet. Numerous small high-angle normal faults are present. In three localities, graben, in which the roof of one of the plutons of the Oliverian magma series is preserved, form very striking structural features. The normal faults which bound the graben have a maximum stratigraphic separation of 3000 to 4000 feet. The age of the faulting associated with the graben, and also much of the normal faulting of western New Hampshire, is believed to be Triassic. Fluorite deposits of the Keene-Brattleboro area are also believed to be Triassic.