A driving potential for metamorphic differentiation in the development of a crenulation cleavage arises from the difference in the mean stresses acting on the mobile solid phase in the hinge and limb regions of a crenulation. The rate of metamorphic differentiation is proportional to the resulting difference in the mean chemical potential of the solid in fluid-filled pores in the two regions. A comparison of the magnitude of the rate of dilation, representing the addition of the mobile phase (quartz) to the hinge region of a crenulation fold, with the total strain rate indicates when differentiation will be important in natural and experimental settings. Dilation rates comparable to tectonic strain rates of 10−1 to 10−14 sec−1 are achieved in pelitic rocks at stress differences of 10 to 40 b.

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