GENERATION AND MAINTENANCE OF PORE PRESSURE EXCESS IN A DEHYDRATING SYSTEM .2. THEORETICAL-ANALYSIS

Fluid is released by dehydration reactions during prograde metamorphis m. If the dilation of the pore space is insufficient to provide storag e 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 experi mental and microstructural observations, we developed a theoretical mo del which incorporates dehydration and porosity production rates as so urce terms in the hydraulic diffusion equation. The permeability was a ssumed to be sensitively dependent on the porosity. The finite differe nce technique was used to analyze the generation and maintenance of po re pressure excess for several types of boundary conditions of importa nce In laboratory and crustal scales. Analytic estimates of the pore p ressure anomaly were also obtained. The model is in reasonable agreeme nt 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 pressu re excess in metamorphic and tectonic settings. The maintenance of a n early 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 produ ction rate. If these constraints are not met, the pore pressure excess can only occur as a transient pulse, the amplitude of which may appro ach lithostatic for sufficiently large dehydration rate and layer thic kness, or sufficiently low permeability.