On the Lehmann discontinuity

The Lehmann discontinuity (∼200km) in the upper mantle is proposed to result from a change in the preferred orientation of olivine as a result of a change in deformation mechanism: in the shallow upper mantle, deformation is due to dislocation creep and results in an anisotropic structure, whereas in the deeper upper mantle, diffusion creep becomes dominant due to a smaller activation volume, resulting in an isotropic structure. For anisotropy due to dislocation‐glide corresponding to a horizontal flow, the resultant anisotropy will yield V SH >V SV and V PH >v PV . Thus, for waves propagating nearly vertically (e.g., ScS waves), there will be discontinuous jumps in the S‐ and P‐wave velocities at this discontinuity. In this model, the depth of the Lehmann discontinuity depends on the deformation conditions such as stress level, geotherm, water fugacity and grain‐size, giving a natural explanation for its observed variation with tectonic setting.