Nature of Ottawan Deformation in the Adirondack Lowlands

The nature and timing of deformation within the Adirondack Lowlands is important with respect to understanding the tectonic history of the southern Grenville Province. Thermal history curves for the Adirondack Highlands and Lowlands suggest they shared a common early history but diverged prior to the Ottawan Orogeny. Therefore, the regional structure of the Adirondack Lowlands has been described as resulting from multiple folding events related to the Elzevirian and Shawinigan Orogenies or developed by sheath folding associated with regional ductile shear. A series of NE-trending ductile shear zones and brittle faults cut the Lowlands into segments such that the general lithologic trend is NE-SW.

This study focuses on an area around the Black Lake Ductile Shear Zone (BLDSZ) in order to understand Lowlands deformation in this area. The dominant foliation, S1, strikes NE, has a steep to vertical dip, is formed by the transposition of isoclinal folds (F1) of compositional layering and increases in intensity toward the BLDSZ. F1 axes have a moderate plunge to the SW. A stretching lineation, L1, plunges steeply to the north. Preliminary dating of a syn-kinematic tourmaline-bearing granite (1062+/-45 Ma) indicates that S1 is Ottawan in age. A weak schistosity defines S2 and is axial planar to F2 folds of S1. S2 appears to fan and cross-cuts a late-Ottawan (1035+/-77 Ma) granite. Southeast of the BLDSZ the interference of F1 and F2 produces Type III fold interference patterns. Fabric within the BLDSZ strikes north-south to northeast-southwest, dips steeply, contains strong down-dip stretching lineation and is cut by S2. Kinematic indicators show conflicting senses of shear suggesting multiple motions or transpression. S1 and BLDSZ fabrics may be associated; however there is evidence that BLDSZ fabric overprints an earlier fabric (S1?). Evidence for pre-Ottawan deformation has yet to be identified.