A sharp cratonic lithosphere–asthenosphere boundary beneath the American Midwest and its relation to mantle flow

• Image of lithosphere–asthenosphere boundary at base of thick cratonic lithosphere.
• Mantle flow under lithospheric root causes change in seismic velocity gradient.
• Enhancement of azimuthal anisotropy from mantle flow under cratonic root.
• Imaging of mid-lithospheric discontinuity beneath the American Midwest.

Beneath the American Midwest, S-to-P (Sp) converted wave imaging and multi-mode surface wave tomography identify a north-trending transition in seismic structure at 150–250 km depth. To the east of this American Midwest transition (AMT), the lithosphere–asthenosphere boundary (LAB) is imaged as a 1–2% Sp/Sv amplitude arrival at 200–240 km depth, consistent with the depth of negative shear velocity and azimuthal anisotropy gradients imaged by surface wave tomography. To the west of the AMT, Sp conversions are much shallower at 150–190 km depth and are much weaker (<0.7%<0.7%) or absent. Azimuthal anisotropy constrained by surface wave tomography also changes across the AMT, with stronger anisotropy to the east of the transition beneath the thicker lithospheric root. We suggest that the seismic changes across the AMT can be explained by considering the effects of asthenospheric flow beneath the leading edge of the thick lithospheric root. The mantle flow is dominantly driven by the drift of the North America plate. Locally higher flow velocities are expected where the asthenosphere is forced to flow beneath the thicker root. This mantle underflow could create a sharper seismic LAB east of the AMT via two effects. First, the local increase in flow velocities could steepen the thermal gradient at the base of the lithosphere, and hence the isotropic velocity contrast. Second, the increased strain rate along edge of the lithosphere could enhance the magnitude of azimuthal anisotropy. Our results suggest that seismically detectable LAB sharpness variations could be used to constrain geographic variations in coupling between plates and mantle convection. A secondary result is the image of a Mid-Lithospheric Discontinuity arrival at 80–110 km depth that is found primarily to the east of the AMT. This arrival is interpreted as produced by a layer of low-velocity metasomatic minerals that have accumulated since the >1.8 Ga>1.8 Ga creation of the lithosphere.