Midcrust

Midcrust is a geological term used to describe the portion of Earth's continental crust that lies between the upper crust and the lower crust. Its depth range is not fixed and varies with tectonic setting, but it is commonly placed roughly from about 15 to 40 kilometers below the surface. The midcrust records processes related to crustal growth, thickening, and metamorphism that occur during mountain building and other tectonic events.

The midcrust typically contains metamorphic and igneous rocks such as granites and high-grade metamorphic rocks, including amphibolite and granulite facies lithologies. It often exhibits ductile deformation and hosts shear zones, intrusions, and mineral assemblages that reflect prolonged heating and deformation. The composition and rheology of the midcrust influence how continents respond to tectonic stresses and how rocks are exhumed from depth.

Geophysically, the midcrust is identified by contrasts in seismic properties relative to the upper and lower crust. It generally shows intermediate seismic velocities and different anisotropy patterns, aiding in the delineation of crustal structure through methods such as seismic refraction, reflection, and magnetotelluric studies. Discontinuities that separate midcrust from the adjacent layers can mark changes in lithology, fabric, or metamorphic grade.

In tectonic and geodynamic models, the midcrust plays a key role in phenomena such as crustal thickening, rheological stratification, and midcrustal flow, which can contribute to the exhumation of high-grade rocks and influence surface topography. Because crustal composition and structure vary widely, the exact location and characteristics of the midcrust are regionally dependent.