Magnetotelluric imaging of the Miocene Kumano-Omine volcano-plutonic complex in the Kii Peninsula, southwest Japan

西南日本,中新世の熊野-大峯火山深成複合岩体の深部比抵抗構造

梅田 浩司

Umeda, Koji

紀伊半島に湧出している温泉の成因を解明する一環として、当該地域の地下構造を推定するため、地磁気・地電流の観測及び地下30kmまでの比抵抗構造の解析を行った。その結果、中期中新世の大峯花崗岩及び熊野酸性火成岩の下位に1000m以上の巨大な高比抵抗体が存在することが明らかになった。また、ほかの年代値,岩体の化学組成の特徴等から両者は従来から考えられてきたような独立した火成岩体ではなく、一連の火山・深成複合岩体であると考えられる。

A wide-band magnetotelluric survey was carried out with a 50 km long MT profile running across the Omine Mountains in order to image the deep crustal structure of the Kumano Acidic Rocks and the Omine Granitic Rocks. A body with high resistivity (more than 1000 ) is clearly visible in the subsurface shallower than 20 km, which corresponds to the distribution of the Omine Granitic Rocks and the western part of the Kumano Acidic Rocks. The resistive body extends to about 20 km depth and widens with increasing depth. The model suggests that these two igneous complexes are connected in the subsurface rather than discrete resistive body corresponds to either igneous complex. Radiometric ages, geochemical compositions and other geological data also support that the resistive body represents the fossil of large silicic magma system in relation to the Kumano-Omine volcano-plutonic complex.