Abstract

Determining the relationship among crustal blocks within an orogen is a key factor in understanding the architecture and construction of that orogen. Within the large, mid-Proterozoic Grenville Province, the relationship between the Adirondack Lowlands and the adjacent Frontenac terrane is ambiguous. Review of previous work demonstrates that the Adirondack Lowlands have different plutonic suites, a lower grade of metamorphism, and a different geochemical signature. However, the timing and kinematics of deformation in the Lowlands, and their relation to major orogenic events, have not previously been well constrained, making comparisons with the Frontenac terrane difficult.

On the northwestern edge of the Adirondack Lowlands, detailed structural analysis of upper-amphibolite grade migmatites and marbles reveals two penetrative deformation phases. Interference of F1 and F2 folds results in Type 3 fold interference patterns and is sufficient to produce the regional map patterns. The Noname ductile shear zone, a 0.5–2-km-wide northeast-striking steep ductile shear zone with subvertical lineation, developed during D2. The steep geometry of the Noname ductile shear zone, paired with consistent sinistral kinematic indicators only found in subhorizontal surfaces, indicate that kinematics for D2 was sinistral transpression.

Sensitive high-resolution ion microprobe–reverse geometry (SHRIMP-RG) U-Pb zircon geochronology from three granitic samples that have well-defined relationships with D1 and D2 indicates that both deformation phases developed through continuous or progressive deformation during ca. 1185–1145 Ma. Zircon geochronology from a quartzite, and the presence of melt during all deformation phases, demonstrate that metamorphism was synchronous with deformation.

This work reveals that the Shawinigan orogeny (1190–1140 Ma) developed the dominant structural features observed in the northwest Adirondack Lowlands. These structures are the result of the northward collision of a rifted slice of the Laurentian margin (Adirondis) into previously accreted terranes on the margin of Laurentia. Shawinigan deformation of the Adirondack Lowlands may have outlasted that of the Frontenac terrane across any potential terrane-bounding shear zone. While Frontenac terrane and Adirondack Lowlands geology are sufficiently distinct to warrant separate terrane designation, evidence is lacking to indicate that a suture exists between them.

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