Heterogenous sediment input along the Nankai Trough subduction zone

Abstract:

The Nankai Trough is arguably the most extensively studied subduction zone. However, previous studies have only concentrated on three main transects which have significantly different lithostratigraphic and deformation characteristics. We investigated the incoming sediment along the entire margin to understand the role of sediment variations in producing the along-strike changes in deformation. We used two existing 3D seismic surveys, three drilling transects, more than 100 legacy 2D seismic lines and 19 new high-resolution seismic lines. We found that normal faulting in the incoming sediment is far more extensive than previously documented. Normal faults penetrate all the sediment packages along the entire margin and at least 100km seaward of the frontal thrust. However, the faults are not uniformly distributed. Clusters of normal faults with seafloor displacement are concentrated above even small changes in basement topography (<50m). The sediment hosting these clusters is extensively deformed and in many places forms anticlines. The sediment also exhibits strong amplitude variations and, in some fault clusters, the youngest sediment is acoustically blank. The hemipelagic and pyroclastic-hemipelagic facies of the Shikoku Basin sediment package are the most extensively fractured units, but many faults extend up and down-dip into turbidite packages. The décollement propagates within the most extensively fractured portion of the hemipelagic unit. Thin packages within the pyroclastic-hemipelagic unit are also more intensely fractured and are lower amplitude than the surrounding sediment. We also find evidence for channels within the pyroclastic-hemipelagic unit which show complex onlap patterns and strong amplitude variations. The fractured packages and channel deposits are important because the Shikoku Basin units are considered by many to be relatively homogenous with low-permeability. The heterogeneities may control fluid flow and modulate pore pressure as the sediment is subducted. Pore pressure plays an important role in deformation and seismicity at subduction zones. Therefore, mapping these heterogeneities along-strike enables us to extrapolate the sediment packages between the well-studied sites and helps us understand the variability of deformation along the entire margin.