A melting anomaly in Northwest Kyushu, Southwest Japan: A consequence of the tectonic evolution of NW Kyushu and the origin of a pseudo hot spot in a convergent zone

The origin of hot spot-type basaltic volcanism in Northwest Kyushu, Southwest Japan that occurred after the opening of the Sea of Japan is examined based on previous geological and petrological observations. Prior to the start of the basaltic volcanism, NW Kyushu had subsided to form the Karatsu–Sasebo coalfield. Swarms of NW–SE and NE–SW faults formed during the subsidence, suggesting that the NW Kyushu lithosphere weakened during pre and syn opening of the Sea of Japan. Uneven distribution of relative subsidence and uplift centers along the NE–SW Sazagawa Fault in the western part of the coalfield indicates that left-lateral transtensional strain essentially controlled the coalfield subsidence. The transtensional tectonics increased the geotherm of NW Kyushu prior to the basalt volcanism. Geothermometries of primitive basalts and mantle xenoliths included in the basalts do not indicate a significant thermal anomaly in the mantle below NW Kyushu at that time. High TiO2, Fe2O3⁎, K2O and Ce in the basalts indicate that dehydration melting of pargasite peridotites formed primitive magmas during the transtensional tectonics. Folds oriented in a NE–SW direction developed to the north of Kyushu from the late Miocene to Pliocene, which suggests that the stress field changed from left-lateral transtensional to NW–SE compressive after the opening of the Sea of Japan. The NW–SE compressive stress reactivated pre-existing NW–SE faults in the coalfield, which enabled basalt magmas to erupt to the surface. The compressive stress field was due to strong mechanical coupling between the Eurasian and the Philippine Sea Plates after the opening of the Sea of Japan. The volcanism, therefore, was not related to plume induced thermal anomaly, but rather a consequence of tectonic evolution of NW Kyushu.