Reconciling Seismic and Thermo‐Chemical Models of Cratonic Lithosphere

Most published global and regional shear‐wave (VS) velocity models of cratons include a VS increase with depth below the Moho, with a maximum at 100-150 km depth. This feature has long been debated, as it appears to require either (implausible) temperature decreases with depth, or substantial, layered components of non‐peridotitic material such as eclogite, diamonds, or metasomatic compositions. Here, we assemble a representative data set of phase‐velocity curves of Rayleigh and Love surface waves for cratons globally, including the all‐craton averages, averages over regions in southern Africa, and interstation measurements elsewhere. We perform both thermodynamic and purely seismic inversions and show that the sub‐Moho VS increase is not required by the data. Models with equilibrium, conductive lithospheric geotherms and ordinary, depleted‐peridotite compositions fit the surface‐wave data fully. A model‐space mapping quantifies the strong trade-off between seismic velocities just below the Moho and at 100–150 km depth, which is the cause of the ambiguity. The main reason why most seismic models contain a VS increase with depth belowthe Moho is regularization that penalizes deviations from global average reference models, which are much slower than cratonic VS profiles Unaccounted‐for radial anisotropy may also cause similar biases, due to the tradeoff between radial anisotropy and Rayleigh‐wave velocity.