Generation and maintenance of pore pressure excess in a dehydrating system 2. Theoretical analysis

Abstract

Fluid is released by dehydration reactions during prograde metamorphism. If the dilation of the pore space is insufficient to provide storage for all the released fluid, then pore pressure excess is generated. Whether the excess can be maintained over long duration hinges on the hydraulic transport properties of the rock. Motivated by recent experimental and microstructural observations, we developed a theoretical model which incorporates dehydration and porosity production rates as source terms in the hydraulic diffusion equation. The permeability was assumed to be sensitively dependent on the porosity. The finite difference technique was used to analyze the generation and maintenance of pore pressure excess for several types of boundary conditions of importance in laboratory and crustal scales. Analytic estimates of the pore pressure anomaly were also obtained. The model is in reasonable agreement with experimental observations on dehydration‐induced weakening and transient buildup of pore pressure in a nominally drained sample. It provides hydrogeological constraints on the development of pore pressure excess in metamorphic and tectonic settings. The maintenance of a nearly lithostatic pore pressure requires the permeability to be below a critical value which increases with increasing dehydration rate and thickness of the dehydrating layer, and with decreasing porosity production rate. If these constraints are not met, the pore pressure excess can only occur as a transient pulse, the amplitude of which may approach lithostatic for sufficiently large dehydration rate and layer thickness, or sufficiently low permeability.