underplating

Underplating is a geological process in which material is added to the base of the crust, increasing crustal thickness from below. It is commonly discussed in the context of convergent plate boundaries and continental collision, where magmatic and tectonic activity delivers material to the lower crust rather than to the surface.

Mechanisms involve magmatic underplating at subduction zones, where melts and cumulates derived from the subducted slab and mantle wedge crystallize at the crust–mantle boundary. The resulting lower-crustal units can accumulate to form a thicker, mafic to ultramafic crustal root. Underplating can accompany crustal thickening during collision, and in some settings may be complemented by intrusion or partial melting within the lower crust. Delamination of dense lower-crustal material can be followed by replacement with underplated material, contributing to crustal growth from below.

Geophysical and petrological evidence for underplating includes seismic velocity contrasts that indicate a more mafic, high-velocity lower crust, as well as crustal thickness anomalies. The presence of granulite- or basaltic-textured xenoliths, and isotopic or geochemical signatures pointing to mantle-derived sources, also support underplating interpretations in some regions.

Significance lies in its contribution to long-term crustal growth and to the rheology and evolution of orogenic belts. By increasing the thickness and altering the composition of the lower crust, underplating can influence uplift, crustal stability, and tectonic styles. It is debated in many regions and is considered alongside other processes such as surface accretion and delamination in shaping continental crust.