The Piseco Lake Structure: Arc Plutonism, Generation of the AMCG Suite, and Escape Tectonics, Southern Adirondacks, New York?

Originally recognized and mapped by McLelland, the Piseco Lake Shear Zone (PLSZ) is a trans-Adirondack E-W structure, up to 20 km wide, defined by the regional fabric including subhorizontal, penetrative lineation and variable foliation development. Lineations are subparallel to lithologic contacts and regional lineation trends. The PSLZ is characterized by highly deformed granitic rocks with arc-like geochemistry. Mapping supported by the SUNY Oswego Geology Field Program has identified two domains: a steeply dipping (5-7 km wide) mylonite zone (southern domain) that merges across strike with a broad, open foliation arch (Piseco Antiform, northern domain). Sinistral kinematic indicators are common in the southern domain. Locally, the trend of the antiform axis is asymmetric to the adjacent mylonite zone and consistent with sinistral shear. To the south across strike of the southern domain, the dip of the zone abruptly shallows (1-2 km), and the granitic lithologies are in contact with mylonitized pelitic metatexite migmatites. Steeply dipping mylonite with transcurrent strain history and adjacent antiform suggests development as a positive flower structure during sinistral transpression. The timing of anatexis in the migmatites is ca. 1180 Ma, similar to preliminary ages from granitic lithologies. Metamorphic minerals from hypersthene to chlorite define fabrics, indicating development over a wide temperature gradient or reactivation at lower temperature conditions. Small (dcm scale) cross-cutting, north-striking extensional ductile shear zones occur throughout the area. A tectonic model is proposed which includes: 1) development of Shawinigian arc complex (ca. 1190-1180 Ma); 2) closure of the intervening basin and collision with an outboard arc (ca. 1350 Ma tonalitic suite); 3) subsequent lithospheric delamination and emplacement of the AMCG suite immediately to the north (1160-1150 Ma); and 4) reactivation of the suture as a strike-slip escape structure as part of a Himalayan-type syntaxis during the Ottawan orogeny, followed by orogenic collapse and extension.