Deep crustal structure along the Niigata-Kobe Tectonic Zone, Japan: Its origin and segmentation

A seismic tomography study has revealed a detailed three-dimensional seismic velocity structure along a high-strain-rate zone, the Niigata-Kobe Tectonic Zone (NKTZ), located in Japan. The results show that the depth extent of the low-velocity zone varies along the NKTZ. We divided the NKTZ into three regions on the basis of the velocity structure. A low-velocity anomaly observed in the lower crust beneath the Southwestern part of the NKTZ is probably attributable to the fluids derived from the Philippine Sea slab, while a prominent low-velocity anomaly extending from the upper crust to the uppermost mantle in the volcanic region, the middle part of the NKTZ, may be caused by the existence of melts and a higher-temperature condition that results from magmatic activity. The northeastern part exhibits low-velocity anomalies in the upper crust and the uppermost mantle, which are probably due to the thick sediment and fluids related to the back-are volcanism, respectively. The strength of the crust and uppermost mantle along the NKTZ may have been weakened by the concentration of the fluids, which in turn facilitates the large contraction there. The heterogeneous structures revealed in this study suggest that the origin of the high-strain-rate zone varies along the NKTZ.