MAGNETIC ANOMALY MAP OF LONG LAKE, ADIRONDACK MOUNTAINS, NEW YORK

Long Lake, located in the northern Adirondack highlands, occurs in a long narrow valley along the course of the Raquette River. Long Lake is part of one of the most prominent topographic lineaments in the crystalline basement, and it traces 035 degree for a distance exceeding 120 km. Like several linear valleys in the Adirondacks, Long Lake is suspected to conceal a fault zone, and some earlier workers described the Long Lake fault as having zero displacement. During June 2008, a detailed magnetic anomaly data set was collected on Long Lake to produce a detailed magnetic anomaly map with the objective of identifying possible displaced anomalies. The magnetic anomaly map is based on more than 9300 measurements collected with a walking magnetic gradiometer lashed to bow of an inflatable motorboat, over a period of approximately 4 hours. The data was corrected for diurnal variation and anthropogenic objects in the lake that produced unusually high magnetic readings. The magnetic anomalies associated with the geology on Long Lake have a range of ~1000 nT. The northeastern end of the lake contains a series of high-low linear anomalies forming magnetic stripes with a trend of 075-080 degree. These magnetic stripes may reflect variations in lithology, but they do not correlate with available detailed geologic maps. The central region of the lake is characterized by a large region (~5.0 km long) with a high- and low- magnetic anomalies. These anomalies may correlate with a belt of pyroxene-hornblende gneiss and quartz-rich gneisses that appear on geologic map of Fallon (1990). The southwestern reaches of the lake is very narrow, and the magnetic anomaly map continues to show km-scale high-low anomalies. From the preliminary analysis and continuity of the magnetic anomalies across the width of Long Lake, there does not appear to be evidence for substantial displacement. Further 2D and 3D modeling of the magnetic data with susceptibility measurement made in the field will further constrain the geometry of rock bodies and the source of the Long Lake lineament.