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

Abstract

[1] 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 mm (2 SE). The cooling rate of the tonalite has been calculated to be in the range of 20°-60°C Ma À1 from $7 to 3.3 Ma, $80°C Ma À1 from 3.3 to 2.0 Ma, and $30°C Ma À1 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 À1 from $7 to 3.3 Ma, $2 mm a À1 from 3.3 to 2.0 Ma, and $0.8 mm a À1 from 2.0 to 0 Ma with an assumed geothermal gradient of 40°C km À1 . The exhumation rate has not changed significantly despite the simultaneous tectonic changes around the Tanzawa Mountains after $7 Ma.

A procedure for calculating a consistent set of closure temperatures and cooling rate using Dodson’s formula

“…Diffusion parameters of He in zircon are from Reiners et al (2004). Effective radii of zircons of the dated samples, 60 µm, are from Yamada and Tagami (2008). Our results shown in Table 2 indicate that slightly higher T C of (U-Th)/He ages and slightly slower cooling compared to those given by Tani et al (2010).…”

“…Tani et al (2010) calculated cooling rates of the Tanzawa pluton using U-Pb and (U-Th)/He ages of zircon. They used Ti-in-zircon temperatures instead of T C for U-Pb ages, and assumed that T C for (UTh)/He ages is 180°C according to Yamada and Tagami (2008). Using the procedure presented in this paper, we calculated consistent sets of T C of zircon (U-Th)/He age and cooling rate for two samples.…”

“…the Izu-Bonin-Mariana (IBM) arc has been colliding against the Honshu arc from Middle Miocene (e.g., Saito et al, 2007;Yamada and Tagami, 2008). Tani et al (2010) concluded that the Tanzawa complex was not denuded middle crust of the IBM arc but formed during arc-arc collision.…”

Rock-magnetic properties of single zircon crystals sampled from the Tanzawa tonalitic pluton, central Japan

“…4-5 Ma using a sensitive high resolution ion micro probe and laser ablation-inductively coupled plasma mass spectrometry (Tani et al 2010;Suzuki et al 2014). Yamada and Tagami (2008) also reported (U-Th)/He zircon and apatite ages. On the basis of these results, post emplacement thermal history of the Azegamaru pluton can be estimated.…”

“…a Geological map and b cooling path of the Azegamaru pluton. K1037 indicates the point where the temperature versus age data were obtained (Yamada and Tagami 2008;Tani et al 2010) The Nakagawa River flows along bodies of the Tanzawa plutonic complex. Since the fluvial sands were collected just downstream of the Azegamaru pluton (Fig.…”