Frictional-viscous flow and the aseismic-seismic transition at shallow depths

Abstract

During the interseismic period of an earthquake cycle, creeping patches and locked asperities on crustal faults control the distribution of accumulated elastic strain and thus their seismic potential. Yet the frictional and frictional-viscous processes that facilitate creep on shallow crustal faults, such as near-trench subduction zone décollements, remain poorly understood. At mid-to-low latitudes, calcareous sediments are important subduction zone input materials. Compared with siliciclastic lithologies, calcareous rocks more readily accommodate strain aseismically via crystal plasticity and diffusive mass transfer processes at low temperatures and pressures in the upper crust. Along the Hikurangi Subduction Margin of New Zealand, accretionary prism uplift has exposed the Hungaroa fault zone, an inactive thrust fault developed within fine-grained, calcareous sedimentary rocks. In this research, we present observational and theoretical evidence that the Hungaroa fault zone accommodated deformation primarily by distributed aseismic creep within a ~33 m-wide fault core.  Syntectonic calcite vein clumped isotope thermometry and maximum differential stress estimates indicate that deformation took place at 2 to 4 km depth. We model the fault zone rheology assuming diffusion-controlled frictional-viscous flow, with deformation at strain rates ≤10-9 s-1 able to have taken place at low shear stresses (τ <10 MPa) given sufficiently short diffusion distances (d <0.1 mm), even in the absence of pore fluid overpressures. Critically, fault zones with diffusion-controlled frictional-viscous flow rheology can exhibit spatially and temporally variable strain rates if grain-scale and fracture-scale processes change the diffusion distance. Thus, the shallow (up-dip) limit of the seismogenic zone is not a simple function of temperature in fault zones governed by a frictional-viscous flow rheology.