4:30 PM - 4:45 PM
[SCG49-11] Contrasting volcano spacing along SW Japan arc caused by difference in age of subducting lithosphere
★Invited Papers
Keywords:volcano, 2D thermal modeling, dehydration, heat flow
The SW Japan arc built by subduction of the Philippine Sea (PHS) plate exhibits uneven distribution of volcanoes: thirteen Quaternary composite volcanoes form in the western half of this arc, Kyushu Island, while only two in the eastern half, Chugoku district.
In the non-volcanic forearc of the eastern half of this arc spring out characteristic deep-seated fluids referred to as the Arima-type brines possessing high Cl contents, high Li/Cl ratios. Such geochemical characteristics of these brines may be attributed to dehydration of the downgoing PHS oceanic crust. In addition, characteristic tectonic tremors and low-frequency earthquakes(LFEs) are observed in the Nankai subduction zone.
Here will be discussed based on the age of the subducted PHS plate inferred by reconstruction of plate motion, and calculation of temperature distribution along the sinking PHS plate beneath the Chugoku and Kyushu regions, and the behavior of water during subduction of the PHS plate. The calculation of 2D thermal structures in this study follows the previous models. We also used heat flow data from high-quality high-density Hi-net borehole and BSRs in addition to previously-used land boreholes and marine heat probes, resulting in the highdensity heat flow data to constrain the thermal structures.
Reconstruction of the PHS plate back to 14 Ma, together with examinations based on thermal structure models constrained by highdensity heat flow data and a petrological model for dehydration suggest that fluids are discharged actively at depths of 90–100 km in the hydrous layer at the top of the old (> 50 Ma), hence, cold lithosphere sinking beneath Kyushu Island. In contrast, the young (15–25 Ma) oceanic crust downgoing beneath Chugoku district releases fluids largely at shallower depths, i.e. beneath the nonvolcanic forearc, to cause characteristic tectonic tremors and LFEs and be the source of specific brine springs. Much larger amounts of fluids supplied to the magma source region in the western SW Japan arc could build more densely-distributed volcanoes.
In the non-volcanic forearc of the eastern half of this arc spring out characteristic deep-seated fluids referred to as the Arima-type brines possessing high Cl contents, high Li/Cl ratios. Such geochemical characteristics of these brines may be attributed to dehydration of the downgoing PHS oceanic crust. In addition, characteristic tectonic tremors and low-frequency earthquakes(LFEs) are observed in the Nankai subduction zone.
Here will be discussed based on the age of the subducted PHS plate inferred by reconstruction of plate motion, and calculation of temperature distribution along the sinking PHS plate beneath the Chugoku and Kyushu regions, and the behavior of water during subduction of the PHS plate. The calculation of 2D thermal structures in this study follows the previous models. We also used heat flow data from high-quality high-density Hi-net borehole and BSRs in addition to previously-used land boreholes and marine heat probes, resulting in the highdensity heat flow data to constrain the thermal structures.
Reconstruction of the PHS plate back to 14 Ma, together with examinations based on thermal structure models constrained by highdensity heat flow data and a petrological model for dehydration suggest that fluids are discharged actively at depths of 90–100 km in the hydrous layer at the top of the old (> 50 Ma), hence, cold lithosphere sinking beneath Kyushu Island. In contrast, the young (15–25 Ma) oceanic crust downgoing beneath Chugoku district releases fluids largely at shallower depths, i.e. beneath the nonvolcanic forearc, to cause characteristic tectonic tremors and LFEs and be the source of specific brine springs. Much larger amounts of fluids supplied to the magma source region in the western SW Japan arc could build more densely-distributed volcanoes.