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Geological Society, London, Special Publications; 1990; v. 54; p. 183-192;
DOI: 10.1144/GSL.SP.1990.054.01.18
© 1990 Geological Society of London

Instabilities and Localization

Mechanical controls on dilatant shear zones

A. Ord

CSIRO Division of Geomechanics, PO Box 54, Mt Waverley, Victoria 3149, Australia

The finite difference code FLAC is used to examine the distribution of regions of high and low mean normal stress (or pressure) and of maximum dilation around deforming, periodic shear zones. The assumption is made here that the fluid pressure is equal to the mean normal stress. Fluid flow is favoured by large pressure gradients, and is enabled by regions of dilatancy. It is commonly assumed that regions of high dilation are necessarily associated with regions of low pressure. However, it is shown here that this need not be the situation. Cases in which maximum dilation is associated with the maximum pressure may be useful for understanding the presence of periodic melt segregations whereas cases in which maximum dilation is associated with minimum pressure may be useful for understanding metamorphic differentiation during crenulation cleavage development.