Abstract

Calculations of the decline in fH2O in equilibrium with typical metamorphic assemblages during cooling indicate that there will be a large drop in pore-water pressure within 100° of the metamorphic maximum. The loss of pore fluid and the generation of large effective pressures across grain boundaries will inhibit rock deformation by those mechanisms that depend on the presence of an aqueous fluid phase. In many metamorphic belts, extensive deformation occurs below the metamorphic maximum during heating (that is, while fluid is being generated), whereas rocks do not deform in the same ductile manner on cooling, even while temperature is high. This may reflect changing mechanical properties of the rocks rather than merely changes in regional stresses; mobile belts could be made rigid in this way in about 20 m.y.

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