Lithospheric layering in the North American craton

Area of Science:

• Geophysics
• Geochemistry
• Tectonics

Background:

• Cratons are ancient, stable continental crustal areas, with their formation and preservation over billions of years being a significant geological debate.
• Seismic receiver function studies have identified a shallow structural boundary beneath cratons, inconsistent with the traditional lithosphere-asthenosphere boundary (LAB).

Purpose of the Study:

• To investigate the structure of the North American continent's lithosphere using seismic anisotropy.
• To reconcile conflicting geophysical and geochemical data regarding cratonic lithosphere structure.

Main Methods:

• Analysis of seismic-wave azimuthal anisotropy changes with depth.
• Comparison of seismic data with geochemical inferences from xenolith thermo-barometric analysis.
• Integration of seismological, geochemical, and geodynamical data.

Main Results:

• Evidence for two distinct lithospheric layers in the North American continent, identified by azimuthal anisotropy.
• The upper layer's thickness correlates with a highly depleted chemical layer.
• A relatively flat lithosphere-asthenosphere boundary (180-240 km depth) exists, likely a thermal root around the depleted layer.

Conclusions:

• The detected shallow seismic boundary likely represents a mid-lithospheric boundary, not the LAB.
• The findings integrate diverse datasets, providing a unified model for North American cratonic lithosphere.
• This study clarifies the complex structure and evolution of ancient continental lithosphere.