Spatial variations in 3He/4He ratios along a high strain rate zone, central Japan

Abstract

A linear zone with high strain rates along the Japan Sea coast, the Niigata-Kobe Tectonic Zone (NKTZ), is considered to be associated with rheological heterogeneities in the lower crust and/or upper mantle. Helium isotope variations along the NKTZ reveal a close association with the geophysical evidence for rheological heterogeneities in the crust and mantle. In the southern NKTZ, the 3He/4He ratios lower than 3.4Ra (Ra denotes the atmospheric 3He/4He ratio of 1.4×10−6) could be interpreted as a two-component mixture of helium stored in aqueous fluids driven off the subducting oceanic crust and radiogenic crustal helium. Higher 3He/4He ratios are observed in the central NKTZ where Quaternary volcanoes and high-temperature hot springs are concentrated, suggesting that the 3He emanation manifest in the central NKTZ results from the effective transfer of mantle helium by intrusion and degassing of mantle-derived magma in the crust. In the northern NKTZ where two large inland earthquakes occurred recently, there appears to be many samples with 3He/4He ratios significantly higher than those observed in the fore-arc side of northeast Japan. A plausible source of mantle helium could be attributed to upward mobilization of aqueous fluids generated by dehydration of the subducting Pacific Plate slab.