ИСТИНА

Rocks formed at the pressure P ∼ 0.5-1.0 GPa are widespread on the Precambrian cratons. This indicates that during their lifetime a layer of rocks ∼ 15-30 km thick has been removed from the crustal surface by denudation. As a result rocks which were initially located in the lower crust emerged to the middle or upper crust. Due to metamorphic reactions under the new P-T conditions the density of rocks would become considerably lower. The analysis of phase diagrams of typical crustal rocks shows that the associated expansion of rocks could produce the crustal uplift up to 2000 m. However no metamorphism occurred in rocks as long as they remained dry. Rapid uplift of the Precambrian cratons during the last several million years can indicate infiltration into the crust of fluids from the mantle. The volume of fluid infiltrated into the crust should be comparable with the cumulate volume of the Neotectonic crustal uplift. Under such circumstances it can be expected that in some Precambrian regions a fluid rich layer now exists within the crust. It should be characterized by a high electrical conductivity and decreased S-wave velocities.