Lithotectonic elements and tectonic evolution of Torngat Orogen, Saglek Fiord, northern Labrador

A cross section of the eastern part of the Early Proterozoic Torngat Orogen is exposed along Saglek Fiord in northern Labrador. It shows the transition from a foreland, through a frontal zone to the internal zone of the orogen, the latter being composed of two lithologically distinct terranes that are part of Abloviak shear zone, a crustal-scale transcurrent shear zone with a sinistral sense of displacement. One of these terranes is parautochthonous with respect to the foreland, the other is allochthonous and interpreted as being separated from the parautochthon by a cryptic suture.Ramah Group, an Early Proterozoic supracrustal sequence that lies unconformably on Archean gneisses in the foreland, can be traced across the frontal zone, but does not occur in Abloviak shear zone in the Saglek area.Tectonic evolution of Torngat Orogen involved crustal thickening to approximately double normal thickness, with transcurrent shearing of the thickened crust taking place during peak metamorphic conditions. Synmetamorphic uplift occurred synchronously with transcurrent shearing, and was followed by imbrication of orogenic zones along late thrust–reverse faults. Available age determinations suggest that peak metamorphism predated uplift by about 50 Ma. 39 Ar/ 40 Ar dating of hornblende from Abloviak shear zone has yielded ages of 1790–1750 Ma, some 15–65 Ma younger than recent estimates of the age of late faults, thus indicating slow and irregular uplift throughout the orogen.The orogenic history of Torngat Orogen is compatible with evolution in an oblique collision zone between two Archean cratonic blocks, and their subsequent welding into a single continental fragment that itself became incorporated into proto-Laurentia.