Abstract
A 12 km wide, 56 km long, three‐dimensional (3‐D) seismic volume acquired over the Nankai Trough offshore the Kii Peninsula, Japan, images the accretionary prism, fore‐arc basin, and subducting Philippine Sea Plate. We have analyzed an unusual, trench‐parallel depression (a “notch”) along the seaward edge of the fore‐arc Kumano Basin, just landward of the megasplay fault system. This bathymetric feature varies along strike, from a single, steep‐walled, ∼3.5 km wide notch in the northeast to a broader, ∼5 km wide zone with several shallower linear depressions in the southwest. Below the notch we found both vertical faults and faults which dip toward the central axis of the depression. Dipping faults appear to have normal offset, consistent with the extension required to form a bathymetric low. Some of these dipping faults may join the central vertical fault(s) at depth, creating apparent flower structures. Offset on the vertical faults is difficult to determine, but the along‐strike geometry of these faults makes predominantly normal or thrust motion unlikely. We conclude, therefore, that the notch feature is the bathymetric expression of a transtensional fault system. By considering only the along‐strike variability of the megasplay fault, we could not explain a transform feature at the scale of the notch. Strike‐slip faulting at the seaward edge of fore‐arc basins is also observed in Sumatra and is there attributed to strain partitioning due to oblique convergence. The wedge and décollement strength variations which control the location of the fore‐arc basins may therefore play a role in the position where an along‐strike component of strain is localized. While the obliquity of convergence in the Nankai Trough is comparatively small (∼15°), we believe it generated the Kumano Basin Edge Fault Zone, which has implications for interpreting local measured stress orientations and suggests potential locations for strain‐partitioning‐related deformation in other subduction zones.
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