Tensile Overpressure: How to Trap a Liquid-Filled Fracture?

[Nick]

Hello,

I'm working on how liquid-filled fractures can propagates or be trapped.

I understand that the driving pressure of liquid : Pd = Pm - sigma3
sigma 3 = minimum compressional stress
Pm = (rho_fluid*g*z) + Po = fluid pressure inside the fracture
rho_fluid*g*z = weight of the fluid column
Po = overpressure

But I don't understand how can you trap a fluid-filled fracture in the case of example 4 in the attached document. Since I will need negative overpressure.. and there is no void inside the earth..

thank you

 

[Baluncore]

Consider the fluid as a magma derived from gravitational separation of the local melt. That fluid has a density that is less than the surrounding rock. Where there is a significant vertical connection for the fluid there is a differential pressure between the fluid and the rock that forces the fluid to move upwards creating a fracture that may close behind. Fluid can only rise while the fluid is less dense than the surrounding rock. When it reaches a density transition such as the Moho where there is a step change in rock density, the density differential is removed and so stops the upward migration.

In diagram 4 the fluid density is intermediate between crust and mantle. The bubble of fluid therefore stops opening a fracture and halts at the Moho.