DEVELOPMENT OF THE PISECO LAKE GRABEN IN A SINISTRAL PULL-APART STRUCTURE, SOUTHERN ADIRONDACKS, NEW YORK

An integrated structural geology and magnetic gradiometry analysis was conducted in the Piseco Lake region of the southern Adirondacks, NY in order to develop a geometric and kinematic model for concealed brittle faults that transect the Proterozoic basement structures. Geologic mapping resulted in a detailed structure contour map of the Piseco structural dome, the trace of the dome hinge axis and its off-set reveals the locations of multiple concealed faults that experienced several kilometers of apparent sinistral displacement, and at least one of the faults transects Piseco Lake. Two steeply dipping fracture sets were documented that strike NW and NNE. Although fracture faces are not well preserved, plumose features and steep to shallow plunging lineations were observed on both sets suggesting a complex strain history. The attitude, distribution and length of lineaments were collected from a DEM. The NNE striking fractures may correlate with prominent NE trending lineaments. Systematic variation of 20 degrees between fracture strike and lineament trend suggests a sinistral pinnate relationship. A magnetic gradiometry survey of Piseco Lake produced a series of linear magnetic anomalies that parallel the topographic lineaments. Susceptibility information was collected for the local bedrock and glacial sediment bodies to construct four structural models based on the magnetic survey. Model solutions required the addition of a rock body with very low negative susceptibility, suggesting that a remnant of Paleozoic carbonate may exist beneath Piseco Lake in a graben structure, similar to other small graben along the southern fringe of the Adirondack dome. Additionally, the model’s suggest the locations of a series of steeply dipping, NNE striking normal faults. Correlation of the two dimensional magnetic models resulted in a sinistral releasing-bend fault geometry beneath Piseco Lake. Perhaps, Piseco Lake resides over a sinistral pull-apart basin with sufficient throw to preserve the lowermost Paleozoic strata that once covered much of the Adirondack massif. This would account for both sinistral strike-slip and normal displacement on the interpreted faults.