MAGMATIC PROCESSES UNDER ARCS AND FORMATION OF THE VOLCANIC FRONT

Temperature structure and stress under arcs are simulated in a two-dimensional cross section taking into account the flow induced by the subducting slab in the mantle wedge. Results of the calculations show three important features with respect to magmatic processes under arcs: (1) Temperature structure in the crust and the mantle wedge under arcs is insensitive to the angle and velocity of slab subduction, the temperature structure of the slab, and that of the back-arc region. This indicates that physical conditions such as temperature and pressure are similar under various arcs. It is thus inferred that primary magmas generated under various arcs should have similar chemical compositions, if chemical composition and the flux rate of fluid from the slab are similar and the chemical compositions of mantle wedge materials are the same. (2) Calculated deviatoric stress magnitude is relatively large (more than a few tens of mega-pascals) in the partially molten mantle. Cracks may open under high differential stress, and magma can easily segregate and accumulate through interconnected cracks while the buoyancy driven compaction of partially molten mantle proceeds. (3) The deviatoric stress values in the region over the partially molten mantle are relatively large, and the direction of the principal stress changes horizontally; the direction of the maximum compressional stress is nearly vertical under the volcanic zone and is nearly horizontal under the fore-arc region. It is considered that magma segregated in partially molten mantle migrates upward through the brittle mantle and crust by the magma fracturing mechanism. The propagation direction of magma-filled fissures is controlled by the stress field in the crust and mantle and is parallel to the maximum principal stress. The calculated stress is highly compressive horizontally on the trench side, while it becomes tensile on the back-arc side. The location of this stress transition coincides with that of the volcanic front. The location of this transition indicates that the volcanic front marks a change in the ease of upward migration of the magma-filled cracks under relatively high differential stress field.