ABSTRACT

Frictional instability leading to fault rupture may be driven by increasing differential stress or by increases in pore‐fluid pressure within the rock mass. Geological evidence (from hydrothermal vein systems in exhumed faults) together with geophysical information around active faults support the localized invasion of near lithostatically overpressured hydrothermal fluids, derived from prograde metamorphism at greater depths, into lower portions of the crustal seismogenic zone at depths of about 10–15 km (250°C<T<350°C). This is especially true of compressional–transpressional tectonic regimes that lead to crustal thickening and dewatering and are better at containing overpressure. Extreme examples are associated with areas undergoing active compressional inversion where existing faults, originally formed as normal faults during crustal extension, undergo reverse‐slip reactivation during subsequent shortening though poorly oriented for reactivation. Extreme fault‐valve action is likely widespread in such settings with failure driven by a combination of rising fluid pressure in the lower seismogenic zone lowering fault frictional strength, as well as by rising tectonic shear stress—dual‐driven fault failure. Localized overpressure affects rupture nucleation sites, but dynamic rupturing may extend well beyond the regions of intense overpressuring. Postfailure, enhanced fracture permeability along fault rupture zones promotes fault‐valve discharge throughout the aftershock period, increasing fault frictional strength before hydrothermal sealing occurs and overpressures begin to reaccumulate. The association of rupture nucleation sites with concentrated fluid overpressure is consistent with selective invasion of overpressured fluid into the roots of major fault zones and with nonuniform spacing of major vein systems along exhumed brittle–ductile shear zones.

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