Dual-layered mantle lithosphere beneath southeastern Canadian Cordillera.

The craton edge beneath southwestern Canada has been characterized as a 'lithospheric step' involving the transition from the warm Cordillera to an ancient, mechanically strong North American craton. Using converted shear waves that are sensitive to this boundary zone, this study shows a snapshot of this transition and reveals the existence of westward-dipping interfaces west of the Rocky Mountain Trench (RMT). These interfaces correlate well, both in location and conversion amplitude, with horizontal shear-velocity gradient zones within the craton core. In a span of 400-600 km west of the RMT, the presence of two distinctive horizontal structural gradients (at ~75 km and ~180 km) suggests a dual-lithosphere architecture, where the Precambrian craton core (Laurentia) now underlies the southeastern Cordilleran lithosphere. The morphology of the craton edge suggests influences from uplift and convective erosion over the past 60 million years.