Fluid Overpressuring in the Lower Seismogenic Zone - How Widespread?

Microseismic activity typically occupies the top 10-20 km of actively deforming continental crust but shallows in areas of active plutonism and geothermal activity and deepens in areas of rapid crustal convergence. Depending on composition, the base of this crustal microseismic zone is defined by low- to mid-greenschist facies metamorphic conditions at temperatures between 350 and 450 degrees. Larger ruptures (M>6) tend to nucleate in the lower half of this microseismic zone, or at its base. Fault frictional strength within the seismogenic zone likely represents a significant component of integrated lithospheric strength but is critically affected by variations in the ratio of fluid to overburden pressure (λv = Pf/σv). If fracture permeability in seismogenic crust is too high to sustain fluid overpressures (λv > 0.4), bulk strength is determined by optimally oriented faults with 'Byerlee' friction under hydrostatic fluid-pressure (λv ~ 0.4). However, high solute levels in hydrothermal fluids may 'self-seal' fractures in active flow systems. In addition, tectonic stress exerts strong controls on structural permeability and the containment of fluid overpressure in the crust, overpressures being more easily generated and sustained in compressional as opposed to extensional settings. These concepts are explored by comparing inferred fluid-pressure states within the actively extending Taupo Rift system, New Zealand with the compressional regime hosting the volcanic arc in NE Honshu, Japan. Both regions are magmatically active and occupy the hangingwalls of active subduction zones where magmatic fluids are entering the crust. Convective circulation of predominantly meteoric water under hot/cold hydrostatic pressure appears to extend throughout the 7-8 km deep seismogenic layer within the Taupo Rift system. By contrast, in the compressional setting of NE Honshu where crustal shortening and thickening is likely accompanied by prograde metamorphism and dewatering, a combination of seismological and electrical anomalies, together with the observed reactivation of poorly oriented reverse faults suggest that overpressuring of aqueous fluids to near-lithostatic values (λv ~ 1.0) is widespread in the lower seismogenic zone, consistent with the geological record of fault-related vein systems exhumed from sub-greenschist to mid-greenschist environments in former compressional regimes. Under such circumstances, some degree of fluid-pressure cycling through fault-valve action becomes probable with important consequences for rupture nucleation and recurrence.