MAGNETIC MODELING OF THE SNOWY MOUNTAIN DOME AND THE INDIAN LAKE FAULT ZONE

The Snowy Mountain dome, located in the central Adirondacks, NY, was laterally cut by a NE striking fault zone located in the center of Indian Lake. The fault zone is manifest as meso-scale faults, fracture zones, and gouge zones in outcrops along the lake shore as well as on islands in the lake. The rocks along the eastern margin of Indian Lake appear to have been down dropped thousands of meters, in relation to the rocks of the eastern flank of Snowy Mountain. The Snowy Mountain dome consists of a core of anorthosite mantled by gabbroic and charnockitic gneisses. The rocks of the eastern Indian Lake area are primarily composed of calc-silicate metasediments and charnockitic gneiss. There are also minor amounts of granitic mylonite forming a roughly NNW striking shear zone, and minor amphibolite bodies that are interlayered with the metasediments.

Using a proton precession magnetometer, magnetic data was collected along an E-W line from the summit of Snowy Mt., across Indian Lake to Round Pond Brook, a total distance of about 11km. The magnetic data was collected to model the deep structure beneath the lake associated with the Indian Lake fault zone. Data was collected at an interval of about 400m. Along the survey line, there are a number of magnetic anomalies with a range of about 1000nT. Both sides of the fault zone are characterized by high magnetic anomalies with a substantial low that correlates roughly with the width of Indian Lake. Magnetic models were produced using a computer modeling program, taking into consideration the detailed surface data including the rock types and structures. The magnetic model that best solves the observed data, and is geologically reasonable, is one with two steeply dipping faults bounding a half-graben, with the eastern flank of the Snowy Mountain dome having been displaced downward about 2000 meters. This is consistent with cross sections produced exclusively from the detailed field data. These interpreted faults are most likely part of the NE striking fault system that developed during the Tertiary rise of the Adirondack massif.