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Fault zones control the location, architecture and evolution of a broad range of geological features, act as conduits for the focused migration of economically important fluids and, as most seismicity is associated with active faults, they also constitute one of the most important global geological hazards. In general, the repeated localization of displacements along faults and shear zones, often over very long time scales, strongly suggests that they are weak relative to their surrounding wall rocks. Geophysical observations from plate boundary faults such as the San Andreas fault additionally suggest that this fault zone is weak in an absolute sense, although this remains a controversial issue. Our understanding of fault-zone structure and mechanical behaviour derive from three main sources of information: (1) studies of natural fault zones and their deformation products (fault rocks); (2) seismological and neotectonic studies of currently active natural fault systems; (3) laboratory-based deformation experiments using rocks or rock-analogue materials. These provide us with a basic understanding of brittle faulting in the upper crust of the Earth where the stress state is limited by the frictional strength of networks of faults under the prevailing fluid-pressure conditions. Under the long-term loading conditions typical of geological fault zones, poorly understood phenomena such as subcritical crack growth in fracture process zones are likely to be of major importance in controlling both fault growth and strength. Grain-size reduction in highly strained fault rocks produced in the plastic-viscous and deeper parts of frictional regime can lead to changes in deformation mechanisms and relative weakening that can account for the localization of deformation and repeated reactivation of crustal faults. Our understanding the interactions between deformation mechanisms, metamorphic processes and the flow of chemically active fluids is a key area for future study. An improved understanding of how fault- or shear-zone linkages, strength and microstructure evolve over large changes in finite strain will ultimately lead to the development of geologically more realistic numerical models of lithosphere deformation that incorporate displacements concentrated into narrow, weaker fault zones.

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