POSSIBILE CYCLIC POST-SEISMIC NORMAL FAULTING DURING OFF-SCRAPING IN ACCRETIONARY PRISM DEVELOPMENT: EXAMPLES FROM NANKAI AND BOSO COMPLEXES, JAPAN

Strata within active and ancient accretionary prism complexes exposed in western Japan are cut by numerous normal faults that likely formed following seismic events. Normal faults observed cutting strata in submarine exposures of the prism toe of the active Nankai Trough accretionary complex likely formed in the deeper, internal portions of the prism, and are not associated with submarine sliding at shallow burial depths. Similar normal faults are also exposed within the Boso Peninsula accretionary complex (8 Ma) of east-central Japan. Cross-cutting relations among structures observed in the Boso Peninsula accretionary complex suggest that normal faulting was synchronous with reverse, thrust, and layer parallel-faulting. In addition, evidence for soft-sediment deformation across several of the normal faults observed in exposures of the Boso Peninsula accretionary complex suggests that strata were unconsolidated during deformation. These field observations provide a basis for a new scenario for the off-scraping accretionary processes within a prism complex. In the conventional view of accretionary prism development, subduction-related subhorizontal maximum principle stresses lead to progressive lateral shortening that is accommodated by arrays of reverse, thrust, and layer-parallel faults and fault-related folds. However, the orientation and magnitudes of stresses in an accretionary prism must change drastically before and after large earthquakes. Our observations suggest that transient periods of extensional strain occur within developing accretionary prisms. Specifically, shortening strain is generally accumulated within the prism during pre-seismic and inter-seismic periods and extensional strain occurs during post-seismic periods.