Reactivation involves the accommodation of geologically separable displacement events (intervals >1 Ma) along pre-existing structures. The definition of a significant period of quiescence is central to this phenomenological definition and the duration of the interval chosen represents the resolution limit of reactivation criteria found in most ancient settings. In neotectonic environments, reactivation can be further defined as the accommodation of displacements along structures that formed prior to the onset of the current tectonic regime. This mechanistic definition cannot always be applied to ancient settings due to the uncertainties in constraining relative plate motion vectors. Four sets of criteria may be used to recognize reactivation in the geological record: stratigraphic, structural, geochronological and neotectonic. Some structural criteria may not be reliable if used in isolation to identify reactivated structures. Much of the previously published evidence cited to invoke structural inheritance is equivocal as it uses similarities in trend, dip or three-dimensional shape of structures. Numerous fault and shear zone processes can cause significant weakening both synchronously with deformation and in the long-term and may be invoked to explain reactivation. The collage of fault-bounded blocks forming most continents therefore carries a long-term architecture of inheritance which can explain much of the observed complexity of continental deformation zones.

the onset of fluid-assisted, grain size-sensitive diffusional creep in the most highly deformed and altered parts of the fault zone. Phyllonitic fault rocks also occur in older, more deeply exhumed parts of the fault zone, implying that phyllonitization had previously occurred at an earlier stage and that this process is possible over a wide temperature (depth) range within crustal-scale faults. Our data provide an observational basis for recent theoretical and experimental studies which suggest that crustal-scale faults containing interconnected networks of phyllosilicate-bearing fault rocks will be characterized by long-term relative weakness and shallow (-5 km)

The Outer Hebrides Fault Zone is a major reactivated structure cutting amphibolite-grade Lewisian basement gneisses in NW Scotland. During a regionally important phase of sinistral strike-slip movements, the influx of chemically active hydrous fluids along the fault zone was associated with the formation of a network of greenschist-facies phyllonitic shear zones. Later ESE-directed extensional strain was preferentially focused into these pre-existing zones of weakness. The syn-tectonic alteration of a relatively strong, feldspar/hornblende-dominated load-bearing framework microstructure to an interconnected weak layer microstructure of fine-grained, strongly aligned phyllosilicate aggregates leads to the long-term weakening in the fault zone. Comparison with experimental data suggests that this produces a shallowing of the frictional-viscous creep ('brittle-ductile') transition and a substantial reduction in total crustal strength. Similar processes may account for the apparent weakness of many long-lived fault zones.

The Outer Hebrides Fault Zone is a major ESE-dipping reactivated fault within the Lewisian gneisses of the Laurentian craton in NW Scotland. Early thrust structures are overprinted by a network of retrograde ductile shear zones in which fluid channelling has hydrated the pre-existing basement rocks at low temperatures, forming chlorite-white mica phyllonites in regions of highest strain. Strike-parallel mineral lineations and shear-sense indicators suggest sinistral displacements that are thought to be late Caledonian based on isotopic data and regional considerations. The phyllonites have focused extensional movements that overprint strike-slip fabrics and may control the location of the Mesozoic Sea of Hebrides and Minch basins.

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