Stress, fluid pressure and structural permeability in seismogenic crust, North Island, New Zealand

Abstract

Stress and fluid-pressure conditions within seismogenic crust are compared for two subparallel belts of active deformation and fluid redistribution associated with the obliquely convergent Pacific-Australia plate boundary in the North Island of New Zealand. Whereas seismic activity on extensional normal faults in the arc-backarc Taupo volcanic zone is restricted to <8 km depth in a high heat-flow, near-hydrostatic fluid-pressure regime undergoing vigorous hydrothermal convection, rupturing along the thrust interface of the contractional Hikurangi subduction margin and in its hangingwall extends to ∼25 km depth in crust with fluids overpressured towards lithostatic values. The contrast in fluid-pressure levels stems partly from the abundance of low-permeability mudrocks in the forearc and partly from superior containment of overpressures by a compressional thrust-fault regime. Maximum supportable levels of differential stress and fluid pressure are critically interdependent in the overpressured regime of the Hikurangi subduction margin. Frictional instability leading to fault rupture in such settings may be triggered by increasing fluid pressure as well as by accumulating shear stress, so that nucleation and recurrence of earthquake ruptures are likely to be affected by cycling of fluid pressure through fault-valve action as well as by stress accumulation. Coupling across the subduction interface is also likely to be highly sensitive to the degree of overpressuring. Different factors are responsible for the localization of active deformation within the two crustal seismic belts. Within the magmatically active Taupo volcanic zone, thermal weakening is clearly responsible for concentrating seismicity and deformation with respect to the surrounding crust. However, in the hangingwall of the Hikurangi subduction margin, where heat flow has been reduced by subduction refrigeration and frictional interaction extends to ∼25 km depth, relative weakening arises principally from reduction of frictional resistance by fluid overpressuring.