Sharp Mantle Tansition from Cratons to Cordillera in Southwestern Canada

English Abstract:

The Western Canada Sedimentary Basin (WCSB) marks the transition from the old North American continental lithosphere to young accreted terranes and Cordillera. Earlier studies in this region have suggested a large number of intricate basement domains as well as major seismic velocity gradients in the mantle. To investigate the effect of the accretion and subduction on the mantle structure beneath the western margin of the North American craton, we analyze P-to-S converted waves from upper mantle discontinuities from Canadian Rockies and Alberta Network (CRANE). The depths of the 410 and 660 km seismic discontinuities are correlated and are, on average, 9 km and 7 km greater than the respective global estimates. The largest depression is observed beneath the Rocky Mountain foreland belt in southern Alberta, which highlights a steep south/westward structural gradient from cratons to Cordillera at lithospheric mantle depths. The severity of the depressions, especially in the southernmost Alberta, may be triggered by diffuse partial melt or increased water content above the mantle transition zone. This result is jointly corroborated by locally increased impedance contrast across the 410-km discontinuity and a strongly depressed 660-km discontinuity. The existence of, and the mantle processes associated with, a Mesozoic slab fragment may be responsible. Overall, the availability of local arrays, particularly the CRANE network, offers a new window into the history of the lithosphere and upper mantle beneath the southern WCSB.