The relationship between displacement and thickness of faults in the Shimanto accretionary complex

Abstract

The correlation between the displacement and thickness of faults is an important parameter that enables an increased understanding of the processes involved in fault growth and earthquake generation. We determined this correlation by measuring the thicknesses of outcropping faults and using vitrinite reflectance (R_o) to determine paleotemperature estimates for the hangingwall and footwall units of 18 faults exposed in the Upper Cretaceous Shimanto accretionary complex of southeast Kochi prefecture, Japan. Fault displacement was estimated from the temperature difference across the faults, assuming a paleothermal gradient and the dip of the fault plane. The ratio of fault thickness to displacement ranges from ~10⁻⁵ to ~10⁻³, far lower than compared with previously identified thickness-displacement scaling relationships (10⁻³ to 10⁻¹; e.g., Shipton et al., 2006). Permeability measurements within fault zones in the study area indicate that both hangingwall and footwall units have low permeability (less than 10⁻²¹ m² at an effective pressure of 100 MPa), although fault zones have permeabilities that are four orders of magnitude higher than these units. This type of fault zone permeability structure means that similar fault zones in accretionary wedges could sustain high fluid pressures, promoting fault slips without significant generation of fault gouge and resulting in the formation of fault zones with lower ratios of thickness to displacement, as observed in the study area.