low velocity zones

As low velocity zones result from temperature destruction of rocks uncompensated by pressure at 5-20 km depths, changes in T at these depths can lead to change in intensity of the thickness of these zones and rate of decrease in Vp within them. Crustal thermobaric zones are shown to increase, decrease and disappear depending on ∂T/∂Н, ∂Р/∂Н, λ, Т. Instability 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.

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.