DETAILED STRUCTURAL ANALYSIS OF THE DIANA COMPLEX IN CARTHAGE-COLTON MYLONITE ZONE, NEW YORK: IDENTIFICATION OF THE ADIRONDACK HIGHLANDS-LOWLANDS BOUNDARY

The Carthage-Colton Mylonite Zone (CCMZ) is an enigmatic, complex, and significant tectonic boundary within the southernmost Grenville Province. It separates the Adirondack Highlands and Adirondack Lowlands terranes in northern New York State. The Diana Complex is an intrusive body found along the southern portion of the CCMZ.

In the central Diana Complex, the CCMZ is defined by a northwest dipping mylonitic foliation with a northerly plunging lineation. Fabric across the CCMZ ranges from moderately developed to strongly in the zone’s interior. Microstructural analysis of the mylonitic fabric reveals two distinct microstructural types. The first type has relatively small quartz (~50-350 μm) and feldspar (~<10-130 μm) neoblast grain sizes and has limited evidence for recovery. The second type has larger quartz (~450->2000 μm) and feldspar (~130-400 μm) neoblast grain sizes and has extensive evidence for high temperature grain boundary migration. Different temperature of deformation and thermal history are thought to be the dominate factors controlling the development of the microstructures.

The transition between the microstructural types is abrupt and is delineated by a zone of high ductile strain overprinted by brittle deformation. Kinematic indicators of both microstructural types, and the high ductile strain zone between them, show that the Lowlands were obliquely thrust southward over the Highlands. Associated with this thrusting event are numerous meso-scale ductile shear zones. Other structures in the area include a ductile fault that post-dates the mylonitic fabric, but likely predates brittle deformation. Based on stereographic analysis, the ductile fault developed from oblique-dextral thrusting of the Highlands over the Lowlands.

Published CCMZ geochronology suggests the defining mylonitic fabric developed during the Ottawan Orogeny, interpreted to be the collision between Amazonia and Laurentia during formation of the Grenville Province. Cooling histories of the Highlands and Lowlands necessitate significant down-dropping of the Lowlands relative to the Highlands at the end of the Ottawan Orogeny. The identified brittle structure must accommodate this motion as it juxtaposes the different microstructural types, and thus, is taken to be the Adirondack Highlands-Lowlands boundary.