Internal Stress Fields of a Large-Scale Submarine Debris Flow

Numerical experiments on subaqueous mass transport processes were conducted to understand the internal stress field associate with a natural example of a submarine debris flow, which was revealed by detailed analysis of a deposit exposed as a nearly 1.6 km continuous outcrop. Deposits of gravelly mudstone containing large deformed sedimentary blocks occur in the Upper Cretaceous to Paleogene Akkeshi Formation distributed in the Hokkaido Island, northern Japan. Application of the multiple inverse method to meso-scale faults observed in the blocks reveals possible internal paleostress fields that existed prior to deposition. This analysis suggested two different stress fields: (I) a uni-axial compressional stress field, where the maximum principal compression axis is normal to the bedding surface, and (2) a tri-axial compressional stress field, where the orientation of maximum principal compression axis is parallel to the paleocurrent direction. The results of numerical experiments imply that the first of these stress fields is generated by radial spreading of the flow during its downcurrent movement, while the second stress field results from compression during deposition on the basin plain. A horizontal compression paleo-stress field can be an indicator of the paleocurrent direction of the debris-flow. In addition, it is also suggested that existence of a horizontal compression paleostress field can provide a clue for the initial conditions of the submarine landslide.