LOWER CRUSTAL EROSION INDUCED BY MANTLE DIAPIRIC UPWELLING - CONSTRAINTS FROM SEDIMENTARY BASIN FORMATION FOLLOWED BY VOLUMINOUS BASALT VOLCANISM IN NORTHWEST KYUSHU, JAPAN

A sequence of geological events, beginning with basement subsidence to form a shallow-water sedimentary basin and subsequent voluminous basa lt volcanism and uplift of land, has been observed in the back-are reg ion of northwestern Kyushu, Japan. The basin consists of a succession of marine and non-marine sediments with a total thickness of 1000-1500 m which range in age from 43 to 10 Ma. The basalt volcanism commenced at 10 Ma and continued until 1 Ma. Uplift started at around 30 Ma and continued after the cessation of the basalt volcanism. These geologic al phenomena may be explained by the convective coupling between the d uctile lower crust and upper mantle induced by mantle diapiric upwelli ng. Thus, surface subsidence leading to sedimentary basin formation is attributed to lower crustal erosion by mantle diapiric upwelling. For an earth model with lower crustal and upper mantle viscosities of 10( 19)-10(20) Pa s, 5 km of lower crust can be eroded 10-20 Myr after the start of convective coupling between the ductile lower crust and uppe r mantle, compatible with the period estimated by observations. In thi s process, the melt due to adiabatic mantle diapiric upwelling accumul ates beneath the lower crust. The accumulation of low density melt in the space originally occupied by mantle material causes crustal uplift . When the stress state became extensional, as inferred from the exten sion of Okinawa Trough during the middle to late Miocene [1], the melt filling the eroded lower crustal area may have reached the surface, l eading to voluminous basalt volcanism.