Hisayoshi Yokose

Ice age as a trigger of active Quaternary volcanism and tectonism

Abstract The stress accumulation within the crust, caused by the surface mass redistribution associated with the glaciation-deglaciation cycle during the Quaternary, was numerically evaluated in order to examine the relationship between active Quaternary volcanism and tectonism in island-arc areas and ice age. The vertical gradient of horizontal stress difference in the lithosphere for a meltwater of 130 m in equivalent sea-level reaches a maximum value of 0.8 MPa/km, which is corresponding to the equivalent buoyancy of about 100 kg/m3 for magma-filled cracks, for an earth model with a lithospheric layer of 20–30 km thickness and with a viscosity greater than 1023 Pa s. The changes in stress difference during the stages of deglaciation of 10,000 years amount to 13 MPa for both the top and bottom of the thin lithosphere. Thus, the additional stress difference within the crust may be effective for island-arc areas with thin lithospheric thickness. We, therefore, speculate that the stress accumulation associated with ice age may be an important trigger and/or accelerator on the active Quaternary volcanism and tectonism for the areas along the circum-Pacific.

Significance of serpentinization of wedge mantle peridotites beneath Mariana forearc, western Pacific

In the Mariana forearc, horst and graben structures are well developed in the outer forearc basement, which is composed of both island arc and oceanic crust-mantle rocks. A zone of dome-shaped diapiric seamounts, which are composed mainly of serpentinized peridotites, formed on the basement in the outer forearc regions. Serpentine minerals in peridotites from both diapiric seamounts and basement are mostly chrysotile and/or lizardite. Antigorite, however, is rarely found in peridotites recovered from Conical, Big Blue, Celestial, and South Chamorro Seamounts. Antigorite-bearing peridotites always contain secondary iron-rich olivine and metamorphic clinopyroxene, and antigorite seems to coexist stably with them. Iron-rich secondary olivine (Fo 86–90 ) occurs as overgrowth on the rim or along the cleavage traces of primary olivine (Fo 90–92 ). The assemblage shows high-temperature conditions of serpentinization at ~450–550 °C, whereas chrysotile- and/or lizardite-bearing assemblages occur at ~200–300 °C. In antigorite-bearing samples, chrysotile and/or lizardite veins both predating and postdating antigorite formation are recognized. This may reflect a complex process of tectonic cycling of shallow mantle wedge serpentinized peridotites to depth and then back again to the surface.

Ups and downs on spreading flanks of ocean-island volcanoes: evidence from Mauna Loa and Kīlauea

Submarine-flank deposits of Hawaiian volcanoes are widely recognized to have formed largely by gravitationally driven volcano spreading and associated landsliding. Observations from submersibles show that prominent benches at middepths on flanks of Mauna Loa and Kīlauea consist of volcaniclastic debris derived by landsliding from nearby shallow submarine and subaerial flanks of the same edifice. Massive slide breccias from the mature subaerial tholeiitic shield of Mauna Loa underlie the frontal scarp of its South Kona bench. In contrast, coarse volcaniclastic sediments derived largely from submarine-erupted preshield alkalic and transitional basalts of ancestral Kīlauea underlie its Hilina bench. Both midslope benches record the same general processes of slope failure, followed by modest compression during continued volcano spreading, even though they record development during different stages of edifice growth. The dive results suggest that volcaniclastic rocks at the north end of the Kona bench, interpreted by others as distal sediments from older volcanoes that were offscraped, uplifted, and accreted to the island by far-traveled thrusts, alternatively are a largely coherent stratigraphic assemblage deposited in a basin behind the South Kona bench.

Helium and methane sources and fluxes of shallow submarine hydrothermal plumes near the Tokara Islands, Southern Japan

Shallow submarine volcanoes have been newly discovered near the Tokara Islands, which are situated at the volcanic front of the northern Ryukyu Arc in southern Japan. Here, we report for the first time the volatile geochemistry of shallow hydrothermal plumes, which were sampled using a CTD-RMS system after analyzing water column images collected by multi-beam echo sounder surveys. These surveys were performed during the research cruise KS-14-10 of the R/V Shinsei Maru in a region stretching from the Wakamiko Crater to the Tokara Islands. The 3He flux and methane flux in the investigated area are estimated to be (0.99–2.6) × 104 atoms/cm2/sec and 6–60 t/yr, respectively. The methane in the region of the Tokara Islands is a mix between abiotic methane similar to that found in the East Pacific Rise and thermogenic one. Methane at the Wakamiko Crater is of abiotic origin but affected by isotopic fractionation through rapid microbial oxidation. The helium isotopes suggest the presence of subduction-type mantle helium at the Wakamiko Crater, while a larger crustal component is found close to the Tokara Islands. This suggests that the Tokara Islands submarine volcanoes are a key feature of the transition zone between the volcanic front and the spreading back-arc basin.