In southern Canada and the northern United States, the present-day western edge of the North American craton (Laurentia) is marked by a profound change in lithospheric thickness, from <100 km within a hot backarc setting beneath the Cordillera to >200 km beneath the craton. A nearly uniformly flat Moho in the backarc region of southern Canada has been interpreted as a young feature that was produced by lower crustal channel flow. The tectonic environment of the southern Canadian Cordillera extends southward to the Lewis and Clark shear zone in the northern United States, where previously deployed EarthScope Transportable Array stations provide coverage on a ≈70 km grid. Here, data from Transportable Array stations are used to investigate teleseismic P to S receiver functions of crustal structure across the craton-backarc transition, coupled with lithospheric structure derived from a recent global tomographic model. These images suggest that, north of the Lewis and Clark shear zone, the shallower Moho beneath the Cordillera has encroached on the craton, creating a narrow complex transition zone. To the south, a step-like change in crustal thickness is observed, and the complex Moho transition zone is absent. Taken together with other geological and geophysical data, these observations suggest that a young Moho has progressed eastward accompanied by thermal and/or mechanical modification of the Laurentian lithosphere. Plain Language Summary We observe an interesting deep crustal and lithospheric feature in the north western United States. In the crust it appears like a complex or double layer at the crust and mantle interface. Deeper in the mantle lithosphere, there appears to be a lobate west dipping anomaly. Since this feature aligns with heat flow indicators suggesting it occurs at the edge of a Cordilleran wide high heat flow, we propose the formation of this feature is related to the high heat flow. We propose this crust and lithospheric feature is relatively young and that it is the relict North American lithosphere being eroded back by a combination of mantle flow and heat diffusion at the edge of the ancient Northern American margin.
