CONSTRAINING THE STYLE AND TIMING OF SHAWINIGAN DEFORMATION IN THE NORTHWEST ADIRONDACK LOWLANDS

The Grenville Province is predominately located along the eastern margin of Canada but is also present in northern New York State as the Adirondack Mountains. It is the product of multiple generations of orogenic processes including subduction, igneous activity, and plate and island arc collisions. These events span from about 1300 Ma about 1000 Ma and most events are broken into various “orogenies.” In order, the orogenies and other events include the Elzevirian Orogeny (ca. 1245-1220 Ma), the Shawinigan Orogeny (ca. 1190-1140 Ma), anorthosite-mangerite-charnockite-granite (AMCG) intrusion (ca. 1155-1140 Ma), the Ottawan Orogeny (ca. 1090-1020 Ma), and the Rigolet Orogeny (ca. 1010-980 Ma).

The Adirondacks are divided into two regions, the Adirondack Highland and the Adirondack Lowlands. They have similar early tectonic histories being affected by the Elzevirian Orogeny, Shawinigan Orogeny, and AMCG instrusion, but the Adirondack Lowlands largely escaped any major tectonism following the Shawinigan Orogeny. However, the Adirondack Highlands experienced significant tectonism during the Ottawan Orogeny. Rigolet tectonism has not been demonstrated anywhere in the Adirondacks. An area on the northwest edge of the Adirondack Lowlands was studied to determine the kinematics, nature, and timing of deformation to confirm if indeed the Adirondack Lowlands escaped Ottawan Orogeny deformation.

Results show that two deformation phases exist in the study area. Fold interference patterns produced by D1 and D2 folding (F1 and F2 folds, respectively) are of type 3 geometry, which is consistent with the folding, and thus D1 and D2, being produced by progressive deformation. Kinematic analysis of D2 reveals the deformation style to be sinistral transpression. SHRIMP-RG U-Pb zircon dates from plutons with known relationships to D1 and D2 establishes that all deformation occurred from ca. 1185 to ca. 1145 Ma during the Shawinigan Orogeny, which presumably is a result of the collision of a crustal block that is now part of the south and east Adirondack Highlands. This fits well with the established temporal constraints of this orogeny in other parts of the Grenville Province and strengthens the argument that this part of the Adirondack Lowlands was not affected by Ottawan deformation.