In the resent years fragmentary crustal low velocity zones were revealed by DSS profiles at depths of 3-25 km around the Earth. In most cases they are considered as thermodynamical phenomena rather than a result of changing composition when mineral material is transformed by pressure and temperature at the depth of their occurrence. Multimethod laboratory studies of sample show that under PT conditions at 5-15 km depths rocks are subjected to cataclastic and dilatational changes. A major mechanism responsible for this behaviour is a resultant effect of irregular and differently oriented tensions in the sample. In contacts between grains they reach values which exceed the strength limit of individual minerals that destructs integrity medium at a microlevel. Here the rocks are characterized by low Young and shear modules, high brittleness (low Poissons’s ratio), high discompaction (high dilatancy), low thermal conductivity (λ). The inversion zones in most cases well coincide with low velocity zones in the Earth’s crust from DSS profiles. As low velocity zones result from temperature destruction of rocks uncompensated by pressure at 5-18 km depths, іnstability of the crustal thermobaric zones of low velocity result in their episodic occurrence in the crust and their vertical and horizontal migration depending on temperature fluctuations in the crust.