The deep structure of continents

The Lehmann discontinuity at 220-km depth is an important global feature which occurs under both oceans and continents. It is a barrier to penetration by young lithosphere and marks the base of seismicity in regions of continent-continent collision. The strong lateral variation in upper mantle velocities occurs mainly above this depth. Continental roots extend no deeper than about 150-200 km. The basalteclogite transformation and eclogite-harzburgite separation may be responsible for the geometry of intermediate depth earthquakes. Oceanic and continental geotherms converge above about 200 km and become less steep than the melting gradient at greater depth. This implies a low viscosity channel near 250 km. This would give a decoupling zone of maximum shear beneath continental shields. The Lehmann discontinuity may be the interface between two distinct geochemical reservoirs. The velocity jump, and the inferred density jump, at 220 km are consistent with an increase in garnet content. The mantle may be garnet lherzolite above and eclogite immediately below the Lehmann discontinuity. The transition region may be mainly eclogite and be the source region for oceanic tholeiites.