IMPLICATIONS OF LITHOSTRUCTURAL DOMAINS IN THE ADIRONDACK MOUNTAINS

The lithostructural pattern of the Adirondack Lowlands is consistent with the early stacking of isoclinal thrust nappes to the southeast. A similar pattern extends across the CCMZ into the Highlands and involves meta-igneous rocks of the ca. 1150 Ma Diana Complex. Outcrop patterns of granitic rocks (ca. 1100 and 1050 Ma) with lesser fabric development, including undeformed fayalite granites, southwest of CCMZ suggest juxtaposition of the Highlands and Lowlands sometime between 1100-1150 Ma; coinciding with a recognized break in igneous activity. The lack of Ottawan metamorphic ages in the Lowlands, and distinct geochemical trends in plutonic rocks from both sides of the CCMZ, support this contention. Reactivation of the CCMZ during late transpressional and/or exhumation events is consistent with reported field relations.

Map patterns and rheological differences suggest that Ottawan deformation was focused along the margins of the anorthosite massifs. To the north and west of the Marcy massif, irregular lithologic contacts suggest lesser deformation despite high-grade regional metamorphism at ca. 1050 Ma throughout the Highlands. The southern and eastern margins of the Marcy Massif served as a rigid buttress against which rocks in the arcuate Old-Forge-Schroon Lake Belt and Eastern Adirondacks were compressed. The Central Adirondacks consist of a crustal-scale lozenge cored by massif anorthosite domes, displaying counter-clockwise rotation, and bounded by mylonitic shear zones. These relations, and existing geochronological constraints, document intrusion of the massif anorthosite prior to Ottawan deformation.

In the Southern Adirondacks, the Piseco Lake Shear Zone displays intense parallelism of lithologic contacts, fabric elements, and pervasive subhorizontal ribbon lineation, and is interpreted as a consequence of late sinistral transpression superimposed on earlier fabrics. Within the Adirondacks, the timing of shear zone initiation appears to young to the south perhaps reflecting the progressive migration and changing character of deformation. Documented reactivation along late, dextral mylonite zones in the Hudson Highlands and new metamorphic ages (ca. 1005 Ma) are consistent with this trend and long-lived tectonism in the core of Ottawan Orogeny.