INTEGRATED MAGNETIC AND STRUCTURAL INVESTIGATION OF CONCEALED BRITTLE FAULTS, ADIRONDACK MOUNTAINS, NEW YORK

A network of pronounced NE-SW and NW-SE trending topographic lineaments are well documented in the Adirondack Mountains, NY. Many of the lineaments correspond with brittle faults that developed during Late Proterozoic rifting and remained active during the development of the Paleozoic passive margin. It’s also been proposed that they developed during the post-Paleozoic rise of the Adirondack dome, in addition to a statistical link between NW trending lineaments and modern earthquakes. However, within the Adirondacks, glacial cover and mature forest, compounded by intense weathering, limits the direct observation of these brittle faults in most places. Since most of the inferred faults occur within some of the deepest valleys, the faults occur beneath elongate lakes. To understand the geometry and kinematic history of some faults, we carried out water-based high-resolution magnetic gradiometry surveys integrated with structural mapping of adjacent regions for several lakes (Eaton, Hinkley, Indian, Long and Piseco). Magnetic anomaly maps were produced for each lake. For modeling, magnetic susceptibility data was obtained for bedrock, glacial till and lake sediments. The models were constrained by structures that occur adjacent to the lakes. At Indian and Long lakes, magnetic anomalies appear to be displaced parallel to fracture, breccia and gouge zones that were observed on bedrock islands and shore exposures. Linear magnetic anomalies most likely correspond to the trace of faults under Piseco and Hinkley lakes. Faults at Piseco and Indian lakes truncate Proterozic structural domes, locally constraining the heave and throw. Additionally, magnetic models suggest that Piseco lake is underlain by Paleozoic carbonate rocks, while models for Hinkley lake supports the presence of a Proterozoic antiform that is truncated by the concealed E-W striking Prospect fault. Based on the integration of on-the-ground structural information, digital elevation model analysis, magnetic anomaly maps and magnetic models, we conclude that that brittle faults associated with the previously mentioned areas in the Adirondacks experienced components of both normal and strike-parallel displacement.