Abstract

About 2000 gravity stations were established 3-5 miles apart in the Adirondack Mountains and surrounding area. The data show an excellent correlation with geology: the anorthosite bodies are conspicuously outlined by gravity lows; the Northwest Adirondacks, a belt of Grenville rocks along the St. Lawrence River, is associated with a gravity high; and centered over Tug Hill is a small gravity low. A few of the 23 residual anomalies in the area have no apparent correlation with geology.

The residual anomaly limits the possible three dimensional shapes of the anorthosite massif; it cannot be a batholith or a large tilted lens. It is essentially a slab having two roots that extend downward about 10 km. Because the residual anomaly is a gravity low, the anorthosite is less dense than the surrounding rocks. The density of the anorthosite is 2.72 gm/cm3, and the density contrast is −0.10, implying that the average density of the surrounding Precambrian rocks is 2.82.

Variations in the regional anomaly are attributable to small warps of the M discontinuity. A north-trending ridge with 2 degree dip 36 km beneath the mountains satisfies the gravity data and conforms with previous seismic work.

The regional isostatic anomaly, near zero in the immediate vicinity of the mountains, suggests that the region is near isostatic balance.

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