Distribution of gravitational slope deformation and deep-seated landslides controlled by thrust faults in the Shimanto accretionary complex

The Cretaceous Shimanto accretionary complex is a typical accretionary complex uplifted without significant deformation after accretion and in which many rain-induced deep-seated landslides (DLs) have occurred. We undertook a thorough geological and geomorphological investigation of the middle Kii Peninsula, where many rain-induced DLs occurred in 1889 and 2011 during intense and prolonged rainfall associated with typhoons. Geomorphological investigations using 1 m digital terrain models (DTMs) obtained from airborne light detection and ranging (LiDAR) before and after the 2011 typhoon allowed us to examine the microtopography (e.g., small scarps and linear depressions) and to locate deep-seated gravitational slope deformations (DGSDs) and DLs. Low-angle thrust faults with > 1 m thick incohesive fault rocks were found to be aligned subparallel to each other with spacings of a few kilometers and control the distribution of DLs and DGSDs. When a thrust fault forms a cataclinal slope, river incision removes the overburden and undercuts and destabilizes the slope. This process causes gravitational deformation, which forms downhill-facing scarps and linear depressions at higher elevations on the slope. Tephrochronology applied to the deposits in the DGSD-induced depressions revealed that such deformation began at least 30,000-50,000 years ago and is ongoing. When a high-angle fault strikes along the line of maximum slope, it separates a volume of the rock mass from the juxtaposing slope and facilitates gravitational slope deformation and ultimately catastrophic failure. A rock mass on a thrust fault that forms a downslope-plunging syncline can also be gravitationally deformed and fail. A crush zone of a large-scale thrust fault is a continuous weak layer that also forms a barrier to groundwater flow, thus leading to a rapid increase in pore-pressure during rainstorms, potentially triggering catastrophic slope failure. Accretionary complexes, which typically contain thrust faults with incohesive fault rocks, thus provide a location for DGSDs and DLs.