Manifestations of upwelling mantle flow on the Earth's surface

Asamori, Koichi; Umeda, Koji; Ninomiya, Atsushi*; Negi, Tateyuki

Remarkable uplift of 1400 m during the Quaternary has been recognized in the Mesozoic crystalline mountains (Asahi Mountains) located on the back-arc side of the Northeast Japan Arc. Crustal and mantle structures beneath the mountains were imaged as a 2D resistivity model, using magnetotelluric survey. The resulting resistivity structure clearly indicates that an anomalous conductive body is present in the central part of the mountains. The conductor extends from the middle crust to the upper mantle. Also, helium isotope ratios were determined from free gas and groundwater samples collected in and around the mountains. The highest value is similar to those of MORB-type helium derived from mantle volatiles. These results provide strong evidence for the presence of a latent magma reservoir and related high-temperature aqueous fluids beneath the Asahi Mountains. The presence of a latent magma reservoir could lead to thinning of the brittle upper crust and the aqueous fluids could weaken the crustal rocks. Thus, contractive deformation could arise locally above the reservoir under compressive stress field. Although the uplift is considered to be controlled by active reverse faults on the west side of the mountains, the highest peak of the mountains is not located near the active faults, but rather is above the prominent conductive zone. It is concluded that the notable uplift of the mountains can mainly be attributed to locally anelastic deformation of the entire crust.