Fault tectonic analysis of Kii peninsula, Southwest Japan: Preliminary approach to Neogene paleostress sequence near the Nankai subduction zone

The southern part of the Outer Zone of Southwest Japan including the Kii peninsula belongs to the tectonic shadow zone, where fewer conspicuous active faults and less Quaternary sediments develop than in the Nankai subduction zone and Inner Zone of Southwest Japan. In order to study the paleostress sequence of the Kii peninsula, we analyzed fault-slip data and tension gashes at pilot sites of EarlyMiddle Miocene forearc sediments and Late Cretaceous accretionary complex. According to the results, six faulting events are reconstructed in sequence: (i) eastwest extension (normal faulting); (ii) eastwest compression and northsouth extension (strike-slip faulting); (iii) NNWSSE compression and ENEWSW extension (strike-slip faulting); (iv) northeastsouthwest compression and northwestsoutheast extension (strike-slip faulting); (v) WNWESE compression (strike-slip or reverse faulting); and (vi) NNESSW extension. The northsouth to NNWSSE trending dyke swarm of Middle Miocene age in the Kii peninsula is thought to be related to Event 3, implying that Event 3 was active at least during the Middle Miocene. Because Event 6 is recognized solely at a site, the overall latest faulting event seems to be Event 5. Assuming that the compression results from the motion of the crust or plate, the compression direction of Event 5 is in good accordance with the present-day WNW crustal velocity vectors of the Kii peninsula. The stress trajectory map of Southeast Korea and Southwest Japan reveals that the current compression directions of the Kii peninsula correspond to the combinatory stress fields of the Himalayan and Philippine Sea tectonic domains.