Structure and deformation of the northern Canadian Cordillera: A broadband seismic reconnaissance study

The northern Canadian Cordillera is one of the most seismically active regions in North America, yet lithospheric structure is poorly constrained due to a historical lack of infrastructure. Increased coverage in broadband seismic stations in the past few years now allows detailed models of the crust and mantle. We analyse data from all available broadband stations in northwestern Canada to investigate the structure and anisotropy of the crust and upper mantle using a suite of complementary techniques that are sensitive to scale lengths of 10 to 1000 km: Rayleigh-wave tomography, receiver functions and shear-wave splitting. Surface-wave velocity models show lower bulk crustal velocities of metasedimentary packages making up the crust in the central Cordillera, with higher crustal velocities on the adjacent Canadian Shield to the east and accreted oceanic terranes to the west of the Tintina Fault. Upper mantle is characterized by low velocities beneath the Cordillera and high velocities beneath the Canadian Shield. The Cordillera-Craton boundary appears to extend into the Cordillera at upper mantle depths. Receiver function data reveal a flat Moho at 32 +/- 4 km, and a sub-Moho layer characterized by 10% anisotropy that may be associated with eclogitized material. Seismic anisotropy data from all three data sets show large lateral and vertical variations in the amplitude and orientation of anisotropy. Upper mantle seismic fast axes are associated with the large shear zones, indicating a clear distinction in lithosphere and sub-lithosphere fabric between ancestral North America and that of allochtonous terranes. Crustal seismic anisotropy results are more ambiguous and may be related with pervasive cracks and/or mica-rich mylonites. Our results support the orogenic float model where thin crust of the Cordillera is supported by elevated mantle temperature, although the relation between crustal structure and stress transfer across the Cordillera remains unresolved.