Lithospheric Shear Zones and Their Role in the Endogenic Activity of the Earth

摘要

Regional shear zones accumulate lithospheric fluid flows owing to the structural arrangement of the rock material therein expressed in physical and mechanical anisotropy, orientation of crystals and rock fragments, schistosity, crenulation, and ductile flow of rocks. The fluid filtration through shear zones is determined by properties of flow cleavage fractures with tortuous or rough walls. The roughness coefficient is a measure of cavity volume increment during shear deformation that determines the pressure drop within the shear zone. Domains of dilatancy arise in shear zones even under high stress and confining pressure. The mantle material consisting of polycrystalline rocks is inhomogeneous; therefore, the shear deformation leads to its rupture. As a result shear zones serve as the most probable conduits and concentrators of fluids and melts in the lithosphere that provide mass exchange between the Earth's crust and mantle. Investigations of processes that occurred in the regional shear zones of the western South Tien Shan and the Yenisei Range, as well as an analysis of published data, have demonstrated a significant role of shear zones in crustal hydrothermal and magmatic processes and differences in the effects of fluid flows through the upper and lower crust. If magma generation and fluid transport in the lower crust are accompanied by only subordinate ore deposition, in the upper crust the ore load is discharged and hydrothermal processes become widespread. The presented data testify to the close relationships between the ductile deformation in the process of shearing and the high-temperature metasomatism in ore fields. Thus, the shear deformation initiates the hydrothermal activity. The evolution of regional shear zones is a process consisting of recurrent cycles of exhaustion and replenishment of mantle fluid sources. These oscillations give rise to the cyclicity in magmatic and hydrothermal activities, including epochs of ore deposition.