Postcollisional exhumation history of the Tanzawa Tonalite Complex, inferred from (U-Th)/He thermochronology and fission track analysis

摘要

The Tanzawa Mountains in the South Fossa Magna region, in the middle of the Japan Arc, were formed due to collision of the Izu-Bonin Arc against the Japan Arc during the last several million years. The Tanzawa Tonalite Complex is exposed in the Tanzawa Mountains and is inferred to represent middle crust of the Izu-Bonin Arc. We performed new thermochronometry, consisting of (U-Th)/He dating, fission track dating, and fission track length analysis for the complex to investigate the exhumation history of the Tanzawa Tonalite Complex caused by arc-arc collision. Typical (U-Th)/He ages were 2.0 ± 0.2 Ma on apatite, 3.3 ± 0.2 Ma on zircon, and 4.5 to 6.9 Ma on zircon using the fission track method. The mean fission track length was 9.7 ± 0.6 μm (2 SE). The cooling rate of the tonalite has been calculated to be in the range of 20°–60°C Ma?1 from ～7 to 3.3 Ma, ～80°C Ma?1 from 3.3 to 2.0 Ma, and ～30°C Ma?1 from 2.0 to 0 Ma assuming a surface temperature of 10°C. The exhumation rate has been calculated to be 0.5 to 1.5 mm a?1 from ～7 to 3.3 Ma, ～2 mm a?1 from 3.3 to 2.0 Ma, and ～0.8 mm a?1 from 2.0 to 0 Ma with an assumed geothermal gradient of 40°C km?1. The exhumation rate has not changed significantly despite the simultaneous tectonic changes around the Tanzawa Mountains after ～7 Ma.