Yoshitsugu Furukawa

Depth of the decoupling plate interface and thermal structure under arcs

Subduction proceeds as thrust movements at the boundary between the subducting plate and the rigid overlying plate. At depth below this mechanically decoupling plate interface, the subducting plate couples with the overlying mantle, and flow is induced in the mantle wedge. This induced flow is the main factor that controls thermal structure under arcs. Depth of this decoupling to coupling transition at the plate interface is estimated from observed surface heat flow in the northeastern Japan arc using a two-dimensional numerical simulation of thermal structure. The estimated depth is about 70 km. The termination depth of thrust earthquakes agrees well with the depth of the coupling transition, which indicates that these earthquakes occur at the decoupling plate interface. The mechanism of this decoupling plate interface is discussed in this paper. It is believed that water associated with the subducting oceanic crust reduces the frictional stress at the plate interface and thrust movement thus becomes possible. Petrological studies show that dehydration reaction in the subducting oceanic crust terminate at about 2.0 GPa and have little temperature dependence. Termination depths of thrust earthquakes estimated from seismic studies are nearly the same (60 ∼ 80 km) for various arcs. This indicates that depth of the decoupling to coupling transition is controlled by the termination depth of dehydration reaction in the subducting oceanic crust. Across-arc heat flow profiles observed in various subduction zones show similar features; heat flow is very low in the forearc region and increases steeply at the volcanic front. Considering that the depth of the slab is about 100 km at the volcanic front for various arcs, this heat flow feature is closely related to the depth of the slab. Thus it is concluded that thermal structure under arcs is mainly controlled by the depth of the pressure dependent dehydration reactions in the subducting oceanic crust.

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 megapascals) 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.

Melting of a subducting slab and production of high‐mg andesite magmas: Unusual magmatism in SW Japan at 13∼15 Ma

Characteristic high-Mg andesite magmas were produced in the SW Japan arc at 13∼15 Ma that was synchronous with the commencement of subduction of a very young (<11 m.y.) lithosphere of the Shikoku Basin. Numerical simulation suggests that temperature at the surface of such a young subducting plate is high enough for partial melting both of the subducting sediments and oceanic crust at the beginning of the subduction. High-Mg andesite magmas were likely to be produced by interaction between silicic slab melts and the overlying mantle wedge. HMA magmas may be commonly produced in the Archean subduction zones under relatively high mantle temperature conditions, contributing to making continental crusts.

Double Seismic Zone for Deep Earthquakes in the Izu-Bonin Subduction Zone

A double seismic zone for deep earthquakes was found in the Izu-Bonin region. An analysis of SP-converted phases confirms that the deep seismic zone consists of two layers separated by ∼20 kilometers. Numerical modeling of the thermal structure implies that the hypocenters are located along isotherms of 500� to 550�C, which is consistent with the hypothesis that deep earthquakes result from the phase transition of metastable olivine to a high-pressure phase in the subducting slab.

Thermal and geochemical evolution of the mantle wedge in the northeast Japan arc: 1. Contribution from experimental petrology

Melting phase relations at high pressures have been examined for four Mg-rich basalts from both the trench and the backarc sides of the Miocene NE Japan arc. All these basalts crystallize olivine and orthopyroxene on the liquidus at pressures lower and higher than the point of multiple saturation with those two phases, respectively. The pressure of multiple saturation is rather constant for all samples (1.0–1.4 GPa), suggesting that (1) there was no across-arc variation in the depth of magma segregation in the Miocene NE Japan arc, (2) the depth of magma segregation remains constant since 20 Ma in the trench side of the volcanic arc, whereas it has changed from 30–40 km to >60 km beneath the backarc side of the arc. This temporal variation may be caused by increasing thickness of the lithosphere beneath the backarc region associated with cooling of the mantle wedge after the opening of the Japan Sea backarc basin in the early Miocene.

A third volcanic chain in Kamchatka : thermal anomaly at transform/convergence plate boundary

The Kamchatka volcanic arc, which is located at the northern edge of the Kurile arc, consists of three volcanic chains, all parallel to the trench axis. In contrast, most subduction zones have only two subparallel volcanic chains. The third chain in Kamchatka, which is farthest from the trench, is characterized by the occurrence of voluminous plateau lavas; volcanoes in the two chains closer to the trench are stratovolcanoes typical in arc magmatism. The third chain magmatism is also unusual in that lavas show concentrations of incompatible elements intermediate between those in the two trenchward chains. Both the unusual occurrence of the third volcanic chain and the unusual lava chemistry could be caused by partial melting of K-amphibole bearing peridotites in the downdragged hydrous layer at the base of the mantle wedge under anomalously high-temperature conditions associated with the characteristic tectonic setting of transform/convergence transition in the region.

Two types of deep seismicity in subducting slabs

The relation between variation of seismic activity with depth and temperature structure in the subducting slabs in five subduction zones (Kuril, NE-Japan, Izu-Bonin, Mariana, and Tonga) is investigated. When the temperature of the core of the slab at depths of 300∼350 km is higher than the kinetic cut-off temperature of the olivine-spinel phase transformation, there is no seismicity maximum in the slab below that depth range. When the temperature of the core of the slab at 300∼350 km depths is lower than the kinetic cut-off temperature, there is a depth range of relatively high seismic activity in the slabs. Moreover, estimated temperature in the depth range of relatively high seismicity is near the kinetic cut-off temperature. It is inferred that the phase transformation of meta-stable olivine triggers deep earthquakes and causes high seismic activity below the depths of 300∼350 km in relatively low temperature slabs, when the phase transformation is the cause of deep earthquakes. In relatively high temperature slabs, however, it is considered that the phase transformation occurs in nearly equilibrium conditions and meta-stable olivine does not exist. Other mechanisms in addition to the phase transformation are required to explain observed deep seismicity especially in relatively high temperature slabs.

Heat flow in the Southwest Japan Arc and its implication for thermal processes under arcs

We present 35 new heat flow measurements for the Kinki district in the eastern part of the southwest Japan arc. Heat flow distribution in this region shows zonal structure parallel to the trench. In the across-arc heat flow profile there are two high heat flow peaks: one is located at the volcanic front and the other is between the south coast of this arc and the trench. The high heat flow at the volcanic front is probably caused by the induced flow in the mantle wedge. The high heat flow off the south coast is located in the zone where slab depth is 10∼20 km, and is immediately to the ocean side of the boundary at which P-wave velocity increases landward in the upper crust. We thus consider the high heat flow is caused by the uplift of accreted materials along the backstop in the accretionary prism, assuming that seismic velocity is an indicator of rigidity of rocks. The upward flow velocity is estimated to be ∼1 mm/yr using a simple one-dimensional erosion model, which is consistent with that estimated from the present heights of marine terraces.

A new technique of radiation thermometry using a consumer digital camcorder: Observations of red glow at Aso volcano, Japan

We newly developed a technique of radiation thermometry using a Sony’s consumer digital camcorder. Our system is not only convenience and cost effective but with a better performance than previous infrared thermometers, particularly in the place like a crater of volcano where is abundant in gas. This is because our system uses the submicron wavelength band, in which radiation is less influenced by absorption of gas than in the thermal infrared wavelength (>3 μm). We carried out observations of red glow at Aso volcano and succeeded in measuring the temperature of about 800°C, which is much more acceptable than previously reported values of 200–400°C. When we measure the temperature of about 300–700°C and 600–900°C in the place where is abundant in gas, using the camcorder with the near-infrared and with the visible wavelength mode is better than the thermal infrared region, respectively.

Interplate coupling and deformation in the accretionary prism in the Southwest Japan Subduction Zone

Frictional coupling at the plate interface will control seismicity and tectonics in subduction zones. In this study temperature structure and deformation in the accretionary prism in the Southwest Japan subduction zone are simulated using a two-dimensional isoviscous corner flow model. Using surface heat flow data the average interplate frictional stress and the effective viscosity of the prism are estimated to be 2∼30MPa and 1020∼21Pa·s, respectively. The low stress value is probably due to high pore pressures caused by dehydration reactions in the subducting crust. Estimated temperature of the landward limit of the seismogenic zone is 350∼500°C; when the frictional stress is considered, the temperature becomes ∼100°C higher than that of the onset of plasticity of granitic rocks (350°C). The brittle-plastic transition of gabbroic rocks might control the depth extent of the seismogenic zone in lower crustal depths. Flow velocity at the bottom of the prism is estimated to be ∼0.01m/yr, indicating that ∼25% of the conversion has been consumed by the deformation of the accretionary prism in this subduction zone.