The origin of arc volcanism has not been clarified with a reasonable model of the physical and chemical processes. Seismological observations show that a hot mantle wedge is adjacent to the cool subducting slab. Observed volcanic material and heat flow are largely concentrated near the volcanic front and decrease to backarc. There is a systematic variation in the chemical composition of volcanic rocks across the arc. These observations do not seem to be well interpreted by the convection that is mechanically induced by the slab motion. The mantle diapirs containing magmas fail to have a sufficiently high ascending speed simultaneously with enough heat. Therefore another model with an ascending mantle flow beneath the volcanic front is proposed to explain the mechanism of magma generation as well as those observations. According to the model, the migration of mass in the ascending flow causes upwelling of hot material from deeper mantle, which supplies heat and buoyancy to preserve the flow itself. The subducting slab brings volatile and other components that facilitate partial melting in the ascending flow. The flow that has passed the top moves toward backarc, and it is gradually cooled so that magmas in it are partly solidified and have more incompatible elements.

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Thermal structure beneath subduction zones plays a key role in understanding their tectonic activities, such as seismicity, volcanism, arc formation and genesis of back-arc basins. In this study, thermal structure in the crust of Northeast Japan was investigated to account for the surface heat flow data. To estimate the contribution of the heat generation in the crust to the surface heat flow, heat production rate was obtained on granitic rocks collected from the Hidaka terrain, Hokkaido, where the crustal section of an island arc is supposed to be exposed. INNA method was adopted to analyze the concentration of heat producing elements. The obtained rate of heat generation and its decrease with increasing depth for the Hidaka terrain were found to be similar to those estimated for stable continental areas. The vertical distribution of heat production rate in the crust of the Northeastern Japan arc was estimated from the present results and previous estimates of heat production rates of different rocks, using crustal structure determined by explosion seismology. The total heat generation in the crust thus estimated is about 30 mW/m². With the surface heat flow data and estimated heat generation distribution in the crust, the thermal structure in the crust was simulated numerically. Modeling calculations were made on a cross section along the line from a Japan trench to the Japan Sea of which direction is parallel to the direction of convergence of the Pacific and Eurasian plates. The models were two-dimensional and included the frictional heating at the upper surface of the slab. Calculated thermal structure in the crust shows that the temperature at the Moho is about 850℃ and almost about 250℃ lower than previous estimates. The magnitude of the interplate stress was derived by considering the rate of frictional heating at the interface between the plates. In order to explain the surface heat flow between the trench and aseismic front, the interplate stress should be less than 200 bars.

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Through analyses of experimental simulations of the mantle vortex induced by downgoing slab, the following things are deduced. An effective vortex occurs in the low Q low V layer from 50 to 150m depth and a material at 130km level raises up to 70km level for 4 to 5 million years. In a turning zone where the flow of vortex turns to a direction of the downgoing slab, increment of shear strain per unit length of streamline is extremely large. When a material at lower level rises up, partial melt will increase. As the partial melt among crystal grains is connected in network within a certain extent, such network is called “magma network” in this paper. Until the material reaches the turning zone, the magma networks in it effectively collide each other to coalesce into larger ones. In the turning zone, each magma network is strongly sheared and its extent increases. Thus the turning zone itself may grow into a giant network of magma. Since flow quantity of the vortex across the turning zone is known by the above experiment, if the volume fraction of partial melt is 5%, a rate of the melt inflow in the turning zone is evaluated to be 5×10^-5 km³ /km/yr. Referring to that volcanoes in East Japan are arranged in a mean distance of 60 km along the volcanic front, the amount of 3 km³ melt should accumulate during 1000 years in the turning zone with the horizontal width 60 km. The melt squeezed out rises up easily through the turning zone to form a magma pool under the volcanic front.

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The subduction of cool oceanic lithosphere produces high temperature magmas at convergent plate boundaries. This paradox may be solved by introducing the concept of induced convection in the mantle wedge and of diapiric uprise of mantle wedge material.

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The two possible causes, crack alignment and olivine-crystal alignment, of the observed anisotropy in the wedge portion of the upper mantle between the earth's surface and a subducting plate are discussed on the basis of recent results from the analyses of shear-wave anisotropy and 3-D P-velocity inversion. At present, the observed anisotropy can be explained equally well by the two models. The former mechanism, possibly related to magma-filled cracks, seems plausible because the study area is located beneath active volcanoes landward of the volcanic front. A crack density of 0.05 and an aspect ratio of 0.1 are obtained for the anisotropic and low-velocity mass with vertical and horizontal extents of about 100 km at depths 50 to 150 km assuming lengths and aspect ratios of the cracks within the mass to be uniform.

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Two examples of mantle-derived alpine-type peridotites, one from Ronda, Spain and the other from Horoman, Japan are reviewed in the context of magma generation and migration in the upper mantle. The peridotite bodies are characterized by layered structure of variable scales and by the presence of mafic layers intercalated within the peridotite. Detailed investigations of the Horoman body revealed that the layering is wavy and oscillatory across the layer structure in terms of chemical and mineralogical composition. A new hypothesis recently proposed for the melt segregation processes is reviewed, in which the layering and the chemical variation of the peridotite are best explained in terms of the partial melting and melt segregation in an ascending partially molten mantle. Because permeable flow of partial melt is coupled with the compaction of the solid in a deformable matrix, magma waves may be generated in the gravity field due to the nonlinear effect of the flow. The layering observed in many alpine-type peridotites may be the “fossils” of such magma waves.

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