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Knowledge of the relationships between porosity, permeability and fluid flow is crucial to our understanding of the mechanical behaviour of the crust. The presence or absence of a fluid, as well as the magnitude of the pore pressure, strongly influences the mechanisms by which crustal rocks deform. In addition, movement of fluid through the crust has a significant influence on heat flow, on the distribution of oxygen and hydrogen isotopes, on chemical dissolution and precipitation. Porosity and/or permeability may decrease with time and the resultant behaviour involves two interrelated processes; the gradual build up of pore pressure followed by expulsion of water out of the decreasing pore space. Elevated pore pressures may lead to brittle failure and episodic loss of fluid from the overpressured regions. But porosity and/or permeability may increase also with time. This is the likely result of deformation in enhanced fluid flow.
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