Sharp mantle transition from cratons to Cordillera in southwestern Canada

Abstract

AbstractThe Western Canada Sedimentary Basin marks the transition from the old North American continental lithosphere to young accreted terranes. 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 the Canadian Rockies and Alberta Network, a regional broadband seismic array based in Alberta, Canada. The depths of the 410 and 660 km seismic discontinuities are correlated and, on average, are 9 km and 7 km greater than their respective global estimates. The largest depression is observed beneath the Rocky Mountain foreland belt in southern Alberta, which highlights a steep southward/westward structural gradient from the 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 410 km discontinuity. This result is corroborated by locally increased impedance contrast across the 410 km discontinuity and a strongly depressed 660 km discontinuity. A relic Mesozoic slab fragment may be partially responsible for the deep olivine phase boundary at the base of the upper mantle.