High fluid pressures have promoted the coupled operation of grain-scale dilatancy and solution-precipitation processes as dominant deformation mechanisms during cleavage development in a sequence of Cambrian silicic volcanics in the Mount Lyell area, Tasmania. Episodic opening of pervasive microcracks has localized the precipitation of material removed from dissolution sites, and has led to the growth of oriented crack-seal microstructures which are a major element of the overall deformation microfabric. It is argued that transient, dilatancy-driven fluid pressure gradients, and consequent fluid migration within the fluid-containing grain-boundary and microcrack network, can play an important role in contributing to mass transfer between dissolution sites and dilatant microfracture sites. The enhancement of grain-scale microfracture processes and coupled solution-precipitation processes, which accompanies the development of near-lithostatic fluid pressures, is expected to lead to high fluid pressure crustal regimes becoming substantially weaker than otherwise similar low fluid pressure regimes. The onset of such weakening processes in response to rising fluid pressures is probably a significant factor triggering pervasive regional deformation of the upper-crust.