FAULTING, FLUID REDISTRIBUTION, AND SEISMIC STYLE: CASE STUDIES FROM AN ACTIVE ARC

Levels of shear stress and fluid-pressure driving faulting, and processes of fluid redistribution resulting from abrupt coseismic changes in stress state and fault permeability are considered for subparallel belts of extensional and compressional faulting within the arc setting of North Island, New Zealand. In the back-arc, rhyolite-andesite dominant Taupo Volcanic Zone (TVZ), NW-SE extension occurs by distributed normal faulting. Along the Hikurangi Subduction Margin (HSM), NW-SE contraction involves low-angle thrust rupturing along the subduction interface, reverse-dextral faulting in its immediate hanging wall, and a largely aseismic accretionary prism offshore. In both belts, fluid redistribution is inferred throughout the seismogenic zone (T < 350°C) which deepens from 6-8 km in the high heat-flow back-arc, where hydrothermal convection is widespread, to c.25 km along the subduction interface and in its hanging-wall, where near-lithostatic but variable fluid overpressuring likely affects the degree of seismic coupling.

Stress controls on fault-fracture permeability lead to relatively high and low vertical permeability, respectively, in the extensional and compressional regimes; with directional permeability along strike in both settings. This, coupled with contrasting loading behaviour (variation of mean stress with shear stress), helps to account for varying seismic style. In contrast to the load-strengthening HSM environment where reverse fault mainshocks range up to M8, swarm activity is widespread in the TVZ with normal fault ruptures (M<6.5) often preceded by foreshocks and with aftershock sequences extending along strike, consistent with load-weakening. Fine cataclastic detritus enhances chemical reactivity along faults, affecting permeability through solution and redeposition with interseismic 'healing' of cohesive strength. Instability results from two driving forces, rising shear stress and fluid-pressure; both affect rupture nucleation and recurrence. Flow regimes are perturbed chiefly in the postseismic aftershock period; massive redistribution is likely in the hanging wall of the subduction interface following near-total shear stress release.