Spatiotemporal Characterization of Smectite-to-Illite Diagenesis in the Nankai Trough Accretionary Prism Revealed by Samples From 3km Below Seafloor

The up-dip limit of seismogenesis in subduction zone forearcs depends on the lithological composition of the incoming sediment and its subsequent modification during compaction and diagenesis. Here we present results of a multimethodological approach to characterize the smectite-to-illite diagenesis in the accretionary prism of the Nankai Trough subduction zone offshore SW Japan. Our X-ray diffraction analysis, scanning electron microscopy, and transmission electron microscopy clay mineralogical analysis reveal that advanced states of smectite-to-illlite diagenesis occur in samples recovered from down to 3km subseafloor at Integrated Ocean Drilling Program Site C0002, which sampled the inner accretionary prism. Our temperature- and time-dependent reaction kinetics models require elevated temperatures in the prism to explain the illitization, which is consistent with revised thermal models based on recent plate reconstructions. Biostratigraphic data suggest that the inner prism sediment was deposited during a period of slow or inactive subduction and buried in the accretionary prism after the Philippine Sea plate resumed subduction. Rapid burial in the past 6Ma led to the formation of two authigenic smectite-illite phases consistent with a broad 1-9.3Ma age determined by K-Ar dating. The low K+ smectite-illite reflects the long-term burial history, whereas the K+-rich smectite-illite was inherited from the younger accretion event. Our study predicts the illitization of smectite approaches completion at 5km below the seafloor in the hanging wall of the plate boundary and the megasplay fault zone, respectively, which coincides with the proposed seaward extent of coseismic slip in the 1944 M-w=8.1 Tonankai earthquake along those faults.