Observational and theoretical evidence for frictional-viscous flow at shallow crustal levels

Along the Hikurangi Subduction Margin, accretionary prism uplift has exposed Hungaroa fault zone, an inactive thrust developed within Middle to Late Eocene Wanstead Formation. Within ~33 m-wide core, deformation of smectitic, calcareous mudstone matrix produced a penetrative foliation that locally wraps around clasts. Deformation occurred at temperatures constrained by syntectonic calcite vein clumped isotope thermometry, which yielded narrow range Δ47 values between 0.445 ± 0.024‰ and 0.482 0.013‰, corresponding mean precipitation temperature 82−12+13 °C. Optical scanning electron microscopy analyses reveal underwent: dissolution along stylolites clast, vein, microlithon margins; in foliation-parallel foliation-perpendicular extension veins; hybrid veins strain fringes. Maximum differential stress estimates obtained from twin densities (44.1 13.9 96.6 20.8 MPa) are consistent with those sustainable cohesionless ~3 km depth friction coefficient measured for two mudstones (μ = 0.38 0.50) micrite clast 0.61 0.64). Marlstone clasts foliated contain mutually cross-cutting shear fractures crack-seal textures, providing evidence temporal fluctuations strength resulting pore fluid overpressure transients. At rates imposed during laboratory experiments, frictional sliding involves granular flow processes. Yet, microstructures indicate diffusive mass transfer played important role accommodating deformation. We model zone rheology assuming diffusion-controlled frictional-viscous flow, γ˙≤ 10−9 s−1 able have taken place very low stresses (τ < 10 given sufficiently short diffusion distances (d 0.1 mm), even absence overpressures. However, if grain-scale fracture-scale processes change distance, zones deforming via can exhibit temporally variable rates. Thus, our results suggest shallow (up-dip) limit seismogenic is not simple function governed rheology.