On the History and Integrity of the Cratonic Lithosphere in Western Canada

English Abstract:

The crystalline basement beneath the Western Canada Sedimentary Basin (WCSB) exemplifies a complex tectonic assembly of welded lithospheric fragments during the Paleoproterozoic era. Earlier studies have suggested extensive subduction, magmatism and accretion, especially along the boundaries of major tectonic blocks such as the Hearne and Rae provinces, Wabamun domain, Medicine Hat Block (MHB) and Trans-Hudson orogen. The existence and the state of the lithosphere beneath southern WCSB remain largely speculative, however, due to the thick Phanerozoic sedimentary cover and relatively few regional broadband seismic stations prior to 2006. In this study we combine teleseismic travel time data from the USArray, the Canadian National Seismographic Network (CNSN) and a recently established Canadian Rockies and Alberta Network (CRANE) to invert for a new P-wave velocity model of the mantle lithosphere beneath the craton-Cordillera transition region. We adopt finite-frequency kernels during the inversions to account for 3D finite-volume sensitivity to model and improve data coverage surrounding the geometrical ray paths. The P wave velocity decreases by more than 1 percent from Alberta Basin to the foothills of the Rocky Mountains at lithospheric depths, which suggests a sharp mantle gradient along the Cordillera deformation front. The stable cratonic lithosphere appears to extend down to a maximum depth of ~180 km beneath the Archean Loverna Block ---- the core of the Hearne craton. High mantle velocities are further observed beneath the controversial Medicine Hat Block (MHB) in southern Alberta. The complex shape of the lithospheric root beneath the MHB could be an indication of considerable erosion. Distinct high velocity anomalies beneath the Loverna Block and MHB, which are separated by lower crustal and average mantle velocities beneath the Vulcan structure (VS), suggest different Archean formation and collision histories between these two tectonic regimes. Overall, the mantle lithosphere beneath the craton-Cordillera transition region in southwestern Canada appears to be relatively intact down to 120-150 km depths.