Abstract

Syn- to postorogenic collapse is increasingly being recognized as a fundamental process in most orogens, although the mechanism of collapse at mid- to lower-crustal levels is not well understood. The Adirondack Highlands of the Grenville Province in northern New York are an exhumed granulite terrane and represent an ideal setting in which to investigate mid- to lower-crustal orogenic processes. Although the highlands experienced high-grade metamorphism and crustal thickening during the Ottawan orogeny, few extensional shear zones have been recognized in this region. We document a new shear zone in the eastern Adirondack Highlands that accommodated postorogenic extensional collapse during the late Ottawan. The shear zone accommodated extension via noncoaxial top-to-the-southeast shear, although we cannot rule out a significant coaxial vertical thinning component. Some earlier thrust motion may also have occurred in the zone. U-Pb sensitive high-resolution ion microprobe zircon and in situ electron probe U-Th-Pb monazite ages demonstrate that extension occurred from ca. 1050 to 1025 Ma, following peak metamorphism during the Ottawan orogeny. Our results suggest that late Ottawan extension in the Adirondack Highlands was more widespread than previously recognized and was likely driven by gravitational collapse. Extension along this eastern Adirondack Highlands shear zone may connect with the Tawachiche shear zone to the north in Quebec, indicating that tectonic exhumation during the Ottawan occurred on a system of interconnected shear zones. Top-to-the-northeast extension in the eastern Adirondack Highlands was synchronous with top-to-the-northwest extension on the Carthage Colton shear zone in the western Adirondack Highlands, suggesting that the highlands granulite terrane may have been partially exhumed as a large-scale symmetrical gneiss dome.

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