Imaging fossil magma pluming system by broad-band magnetotelluric observation: root of the Okueyama volcano-plutonic complex

2022 Fiscal Year Final Research Report

• Project/Area Number: 20K04132
• Research Category: Grant-in-Aid for Scientific Research (C)
• Allocation Type: Multi-year Fund
• Section: 一般
• Review Section: Basic Section 17040:Solid earth sciences-related
• Research Institution: Kyushu University
• Principal Investigator: Aizawa Koki 九州大学, 理学研究院, 准教授 (50526689)
• Project Period (FY): 2020-04-01 – 2023-03-31
• Keywords: バソリス / 比抵抗 / 花崗岩 / 大崩山 / MT観測 / マグマだまり / マグマ供給系

Outline of Final Research Achievements

In the area around Mt. Okue, Japan, a granitic batholith and surrounding ring dikes, which intruded 14 Ma and later, are exposed on the surface (Takahashi, 1984). The granitic batholith (33 x 23 km), which is a fossil of a magma reservoir, has little pore space and fracture, and therefore, show electrical high resistivity. We conducted broadband MT observations around Mt. Okue in 2015, 2017 and 2020, 2021, and estimated the 3-D resistivity structure. A high resistivity (>3000 Ωm) body, which exceeds 3000 km3, is imaged beneath the ring dike. We interpreted the high resistivity body as solidified magma. The volume of the erupted magma is only 10 % of that of the magma solidified underground. The correspondence between the high resistivity body and the ring dyke is not perfect. There are regions without high resistivity zone beneath the ring dike. This finding is not consistent with the simple idea that the ring dyke intrusion uniformly occurred at the margin of the magma chamber.

Free Research Field

地球電磁気学

Academic Significance and Societal Importance of the Research Achievements

大崩山地下で固結したマグマの総体積は3000 km3以上と推定したが、過去の比抵抗構造調査からは、活火山の下にこれほどの大量のマグマが存在している証拠は提出されていない。火山の一生を通じて、低比抵抗である一つの巨大なマグマ供給系が成長していき、深部からのマグマの供給が途絶えると徐々に固結して高比抵抗になっていくというよりも、固結したマグマだまりの周辺に新たなマグマだまりが次々と形成されていく、このプロセスが繰り返されるのがマグマ供給系進化の実体である可能性がある。