Three-dimensional structure of the Torngat Orogen (NE Canada) from active seismic tomography

The crustal velocity structure and the Moho depth of the Proterozoic Torngat Orogen, NE Canada, is determined by active seismic tomography using travel times of crustal turning rays and Moho reflections. The orogen developed during oblique convergence of the Archean Superior and Nain Provinces, which trapped an interior belt of Archean crust (Core Zone) between them, with the Torngat Orogen evolving between the Core Zone and the Nain Province. Beneath the central orogen a crustal root is found with a preserved depth of >52 km and a width of ∼100 km. To the north, the root shallows to <44 km and narrows to a width of ∼45 km. The root correlates with a set of major, late orogenic shear zones that accommodated oblique convergence of the Superior and Nain Provinces. This correlation suggests that the transpressional shearing focused strain in the region of the root and contributed to the crustal thickening. Absence of postorogenic magmatic activity prevented reworking or thermal relaxation of the root. The lack of late magmatism is probably related to the depleted and refractory nature of the Archean lithosphere underlying the orogen. Upper crustal velocities are lowest in the Core Zone (∼5.7 km/s at the surface) and are compatible with laboratory measurements carried out on gneissic rock samples from that area. Higher velocities in the Nain Province (∼5.9 km/s) correlate with felsic gneiss and anorthosite rock samples. A high-velocity region immediately to the north of the crustal root is associated with a Moho uplift (∼34 km). This is explained by extension along the Ungava transform fault, and possibly in Hudson Strait, at ∼55 Ma when rifting in the Labrador Sea was transferred into Baffin Bay.