Conditions for fault-valve behaviour

Abstract

Evidence for fluid pressures well in excess of hydrostatic during crustal deformation is provided by direct measurements of pressure levels in active tectonic regions and by hydrothermal vein systems in ancient orogenic belts. Faults that act as impermeable seals except immediately postfailure, when they become highly permeable channelways for fluid discharge, may behave as fluid-pressure-activated valves wherever they transect a suprahydrostatic fluid pressure gradient. Such fault-valve behaviour, causing abrupt fluctuations in fluid pressure linked to the earthquake cycle, is particularly likely for faults that remain active while unfavourably oriented for frictional reactivation in prevailing stress fields. The most extreme fault-valve action is likely to be associated with high-angle reverse faults. Valve activity may be especially prevalent near the base of the seismogenic zone when unstable frictional faulting gives way to aseismic shearing with increasing depth. At such structural levels, cyclic variations in the style of deformation may accompany fluid pressure cycling. Fault-valve activity plays an important role in the development of much fault-hosted mineralization, and may also assist the migration of hydrocarbon fluids in some structural settings.