THE INDIAN LAKE FAULT SYSTEM: TOWARDS CONSTRAINING A MAJOR GEOMORPHIC CONTROL ON PRECAMBRIAN BASEMENT STRUCTURE IN NEW YORK STATE

The Indian Lake Fault System consists of a high density of NNE lineaments, valleys, and elongate lakes which transect the Central Adirondack Highlands and define their dominant topographic features. A suspected conjugate set of lineaments (ESE to SE) is less well developed and has wider spacing. The origin of this fault system is unknown, but it is suspected to be related to the opening of the Iapetian Ocean and to have undergone modification during a long and complex reactivation history associated with Paleozoic orogenesis. Parallelism with the axis of the current topographic expression of the Adirondack Dome suggests the potential for Mesozoic to modern reactivation and seismicity. Substantial (>1300 m) normal displacement and westward-tilted strata define small graben (~10’s of km long) containing Lower Paleozoic rocks within the south-central Adirondacks. A component of strike-slip motion is indicated by off-set Mesoproterozoic lithologic units. Field work and new magnetic data suggests buried half graben structures may occur further north into the Adirondack Dome than previously recognized and expose progressively deeper structural levels of the faults. The roots of these faults are exposed within elongate valleys in the Central Adirondacks where they contain lower greenschist facies mineral assemblages, and display evidence for complex movement histories and brittle-ductile deformation. One such fault is exposed north of Wells, NY, where it attains a thickness of 100m and consists of breccia and a thick (>20m) and indurated medial gouge zone, flanked by breccia, developed within strongly foliated and lineated garnetiferous quartzofeldspathic gneiss. Along strike, the fault is spatially associated with zones of pervasive epidotization of anorthositic rocks of the Oregon Dome. To the south, in the Mohawk Valley and beyond, these basement faults likely control the topography of the Precambrian-Lower Paleozoic unconformity, delimit similar buried graben structures, influence the thicknesses of the overlying rock strata, serve as pathways for hydrothermal fluids, and exert control on key geomorphic features.