EarthRef.org Reference Database (ERR) -- Honda & Yoshida 2005

We compare the results of 2-D/3-D time-dependent models of small-scale convection under the island arc with the geophysical and geologic data observed in the NE Honshu subduction zone, northeast Japan. Assuming that the temperature anomalies have a close connection with the seismic anomalies, we may constrain the geometry of the low-viscosity wedge (LVW) overlying the slab, which may be produced by the water dehydrated from the subducting slab. Our preferred model predicts step-like low-velocity anomalies above the subducting slab rather than smooth anomalies subparallel to the subducting slab since the shallow nature of the LVW is required for the small-scale convection to occur at the back-arc end of the LVW. A movement of cold plumes generated at the back-arc end of the LVW may be related to a possible migration of volcanism from back-arc to volcanic front side. To be consistent with the observation, which may suggest the migration rate of ~2 cm/yr, models require weak viscous couplings between the mantle wedge and the underlying subducting slab, whose speed of subduction is ~10 cm/yr. Three-dimensional models showing 3-D temperature variation consist of fairly continuous and strong temperature anomalies (several hundreds of degrees) under the volcanic front and weak finger-like temperature anomalies (several tens of degrees) behind them, which are similar to the pattern of the seismic tomography. The time-dependent behavior of the temperature field shows that the pattern of fingers flip-flops with a timescale equal to the ratio of the horizontal extent of the LVW to the migration speed of cold plumes. This change of pattern of fingers may have an important implication for understanding the past distribution of volcanism. Further studies related to (1) the detailed morphology of the low-velocity zone above the subducting slab, (2) the spatial and temporal evolution of volcanism, and (3) the dynamic 3-D model studies which self-consistently determine the distribution of water/volatiles and the resultant weak rheology are important for checking the validity of our model or developing alternative models. We compare the results of 2-D/3-D time-dependent models of small-scale convection under the island arc with the geophysical and geologic data observed in the NE Honshu subduction zone, northeast Japan. Assuming that the temperature anomalies have a close connection with the seismic anomalies, we may constrain the geometry of the low-viscosity wedge (LVW) overlying the slab, which may be produced by the water dehydrated from the subducting slab. Our preferred model predicts step-like low-velocity anomalies above the subducting slab rather than smooth anomalies subparallel to the subducting slab since the shallow nature of the LVW is required for the small-scale convection to occur at the back-arc end of the LVW. A movement of cold plumes generated at the back-arc end of the LVW may be related to a possible migration of volcanism from back-arc to volcanic front side. To be consistent with the observation, which may suggest the migration rate of ~2 cm/yr, models require weak viscous couplings between the mantle wedge and the underlying subducting slab, whose speed of subduction is ~10 cm/yr. Three-dimensional models showing 3-D temperature variation consist of fairly continuous and strong temperature anomalies (several hundreds of degrees) under the volcanic front and weak finger-like temperature anomalies (several tens of degrees) behind them, which are similar to the pattern of the seismic tomography. The time-dependent behavior of the temperature field shows that the pattern of fingers flip-flops with a timescale equal to the ratio of the horizontal extent of the LVW to the migration speed of cold plumes. This change of pattern of fingers may have an important implication for understanding the past distribution of volcanism. Further studies related to (1) the detailed morphology of the low-velocity zone above the subducting slab, (2) the spatial and temporal evolution of volcanism, and (3) the dynamic 3-D model studies which self-consistently determine the distribution of water/volatiles and the resultant weak rheology are important for checking the validity of our model or developing alternative models.