THE ULTIMATE FATE OF CONTINENTAL CRUST IS TO MELT – GEOLOGIC RECORD AT HIGH AND ULTRAHIGH PRESSURE

Continental collision results in crustal thickening, thermal maturation, and partial melting of a significant fraction of the crust. Migmatite terrains, which represent the layer of crust that was once partially molten, typically contain inclusions of refractory material ranging from meter to kilometer scale. These inclusions preserve a record of high and in some cases ultrahigh pressures attained prior to or coeval with incorporation in the partially molten crust that developed at granulite-eclogite conditions near the base of the thickened crust (~1.0-1.5 GPa ) or at greater depths (~2.5-3 GPa) if the continental crust was subducted. These pressures are typically not exceeded because partial melting dramatically modifies the viscosity and buoyancy of the crust, forcing the partially molten layer to flow upward with high-pressure refractory rocks within it. Geodynamic modeling results indicate that low viscosity (partially molten) crust spreads laterally and moves upward if solicited by pressure gradients that arise from geometry (uneven Moho, residual relief of intracrustal interfaces between various density layers, etc.) or tectonic forcing (normal/oblique extension of brittle upper layer). In all these cases the low-viscosity crust is opportunistic and moves laterally to minimize Moho relief during plateau development, and flows both laterally and vertically during subsequent orogenic collapse; the net effect is transfer of material and heat and ultimately stabilization of the orogenic crust. Migmatite terrains and their inclusions are evidence that the continental crust melts during orogeny at crustal levels if the geotherm is high and at mantle levels if the continental crust is subducted. Partial melting limits the depth at which continental crust is buried because the generation of low-viscosity crust favors lateral and upward flow of partially molten crust. Some unmelted crust could be buried beyond the depth of partial melting, in which case it would be mixed in the mantle. However, partial melting of continental crust stands as a major process on the crustal preservation side of the equation for continental recycling and growth.