Normal fault orientations in the Kumano forearc basin, Nankai Trough, from coherency data and automatic fault extraction

The Kumano forearc basin of the Nankai Trough accretionary prism contains more than 2.5 km of upper Miocene and younger strata. The basin fill has been tilted landward by uplift along the seaward edge of the basin. The section is cut by more than one generation of normal faults. The faults generally have offsets of less than 20m, dips of 60-70° and show no growth structures. We used coherency cube processing to extract the fault locations at each 5 m sample within our 3D data cube, to which we then applied automatic fault extraction (AFE) to determine the strikes and dips of the faults. The oldest generation of faults does not cut an unconformity dated at ~0.9 Ma at IODP Site C0009 and strikes ~050-060°. These faults are restricted to the NW portion of the 3D survey box. A younger generation of normal faults cuts the seafloor and has two fault populations, one that strikes ~100° and a second that strikes ~175°. Shallow faults near the SE margin of the basin curve from ~100° in the middle of the survey area to ~145° at the SE corner of the 3D box. Overall the normal fault pattern is consistent with the regional stress pattern determined from borehole breakouts at IODP Sites C0002 and C0009. Second-order fault oreintations are due to deformation caused by uplift of portions of the underlying accretionary prism.