Ages and structural relationships in the Ganderian central Cape Breton Highlands, Nova Scotia, Canada

Structural complexity of the Cape Breton Highlands is a key problem in reconstructing tectonic events in the northern Appalachian orogen. A new U-Pb thermal ionization mass spectrometry age of 428.53 +/- 0.16 Ma for metarhyolite in the Calumruadh Brook Formation shows that volcanic and sedimentary rocks were deposited before collision of the Aspy and Bras d'Or terranes along the Eastern Highlands shear zone. A new U-Pb laser ablation zircon age of 394 +6/-4 Ma confirms that peak metamorphism in the Middle River complex continued during convergence linked to late stages of the Acadian orogeny. The compressive tectonic environment evolved into a transpressional system after initial collision in the late Silurian and caused a repeated pattern of imbrication of units in the Aspy terrane in the hanging wall in the collision. The shear zones bounding the geological units are curvilinear and have south-directed kinematics, imbricating units and transporting higher grade rocks over lower grade rocks, and moving plutons upward relative to their host rocks during and shortly after intrusion. The vergence of imbrication is parallel to the direction of transpressional movement on the main Eastern Highlands shear zone. This geometry is present in Ordovician-Silurian rocks and repeated in Devonian plutonic rocks, indicating that the overall transpressional tectonic setting was a long-lived feature of the orogen. The shear zones localized late syn- to postdeformational plutons that intruded at ca. 375-370 Ma. By the latest Devonian, emplacement of the ca. 363 Ma Margaree and related plutons marked the beginning of extension in the central Cape Breton Highlands.