Gravity in North-Central New England
Twelve hundred and sixty-six gravity stations were established in 15,000 square miles of New Hampshire, Maine, Massachusetts, and Vermont. Within that area three detailed surveys were made in places of particular interest. To aid in the interpretation density determinations were made on rock specimens. Two broad features stand out on the Bouguer map, a coastal belt of positive anomalies and a gravity low over the northern half of the area. The low could be explained by assuming that the exposed granite bodies enlarge with depth and extend beneath the metamorphic rocks, forming a sheet 15,000 feet thick, or it could be ascribed to an increase of about 2 kms in the thickness of the crust. The relationship between the local gravity features and the surface geology is described. Most of the local anomalies are due to the density contrast between the plutonic rocks and the metamorphic rocks. In general, the plutons are associated with lows, the most prominent of which is found over the White Mountain batholith. Analysis of the gravity data shows that the density contrast between the granite of this batholith and the surrounding metamorphic rocks extends downward at least 5000 feet. The detailed survey near Lake Winnipesaukee disclosed a gravity high over the Conway Granite of the Merrymeeting stock. There is also a magnetic high over the stock. But the Conway, which belongs to the White Mountain plutonic series, has the lowest density of all the rocks exposed in the area and contains no magnetite. Beneath the surface of the stock there must be a high-density body which contains a magnetic mineral. This body is probably an older member of the White Mountain series, a dionte or gabbro, and the Conway Granite was probably emplaced along the contact of the older body, which subsided to make room for the granite. The same detailed survey revealed a gravity low over the Pine Mountain ring-dike complex, which also belongs to the White Mountain plutonic series. Analysis of this anomaly shows that the complex forms the apex of a much larger stock below and that the density contrast extends to a depth of at least 5000 feet. A detailed survey in northeastern Massachusetts showed a sharp high at Salem Harbor. This feature is probably caused by the Salem Gabbro-Diorite exposed there. Other features on the Bouguer anomaly map of northeastern Massachusetts are also discussed.