Abstract

A total of 1,111 gravity stations were occupied in a rectangular area approximately 137 miles long and 23 miles wide in eastern New York, central Vermont, and central New Hampshire. The survey was carried out for the primary purpose of supplementing geologic data and contributing toward the solution of local and regional structural problems. Where possible, an areal coverage method of surveying was used, and stations were located at intervals ranging from 1 to 1½ miles. The relative accuracy of the gravity measurements is approximately 1 milligal. Simple Bouguer anomalies were obtained by the usual method of reduction. Although the stations were not corrected for terrain effects for the regional analysis, this factor was taken into consideration for local analyses.

Several hypotheses are examined in an attempt to relate a large negative Bouguer anomaly field with the tectonic history of the Middlebury synclinorium, and an associated relative gravity high with the Green Mountain anticlinorium. The theory of warping of the crustal layers is applied to this region. The possibility of high-angle thrust faulting of the crust is also considered. Calculations show that the maximum amplitude of the downwarp at the base of the granitic and intermediate layers beneath the Champlain lowland is approximately 16,000 feet. This amplitude is of the same order of magnitude as the depth to the Precambrian basement in the Middlebury synclinorium.

A gravity high over the Taconic allochthone indicates that the high density slates of the Taconic sequence attain their maximum thickness in the eastern part of the thrust sheet. Because the structure of the Taconic klippe is similar to that of the Middlebury synclinorium, the rocks of the thrust sheet were probably folded concurrently with those of the synclinorium. Consequently, the Taconic thrust appears to have been emplaced prior to the close of the orogeny in the Green Mountain region.

The north-south trending regional anomalies in the Green Mountains decrease eastward to a broad gravity low in the Connecticut Valley. In the latter area, the anomaly trends swing around to the northeast before attaining east-west trends in central and eastern New Hampshire.

Differences in residual anomalies as high as −5 milligals are associated with a series of domal uplifts in eastern Vermont. These anomalies must result from a mass of low density rock which occurs beneath the high density metamorphic rocks at the surface. Calculations show that the depth to the core rocks probably ranges from 1000 to 2500 feet. An analysis of gravity minima associated with the granite core of the Lebanon dome (−13 milligals) and the core rocks of the Mascoma dome (−10 milligals) yields information as to the shape of the plutons at depth. Calculations also show that the core rocks of these domes persist to depths which are the same order of magnitude as those of the domes of eastern Vermont (12,000 to 16,000 feet). Consequently, the core rocks may be either intrusions or uplifts of a low density rock which occurs at depth throughout the region.

Because local gravity anomalies in central and eastern New Hampshire are obscured by a regional gravity low centered over the White Mountains, a residual anomaly map was prepared for this region. Computations indicate that the schists of the Littleton formation are cut off at relatively shallow depths (3500 to 8700 feet) by one or more of the magma series which occur in central New Hampshire.

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