Zircon OD-3 from the Paleogene Kawamoto Granodiorite (Mihara body) in Japan has been identified as a potential multi-grain secondary standard for U-Pb dating. We have carried out an inter-laboratory evaluation in order to evaluate possible heterogeneity amongst the OD-3 zircon grains. U-Pb ages were obtained using two analytical techniques (a sensitive high-resolution ion microprobe and a laser ablation-inductively coupled plasma-mass spectrometry) in eight laboratories. All the 238 U-206 Pb ages show good agreement, with an overall weighted average 238 U-206 Pb age of 33.0 Ϯ 0.1 Ma (2s). The U-Pb age results revealed no significant variation or heterogeneity in the U-Pb ages of the OD-3 grains. Twelve fission-track (FT) ages from three laboratories are also reported, and have a weighted average of 32.6 Ϯ 0.6 Ma (2s). Despite the different closure and annealing temperatures of the U-Pb and FT chronometers, respectively, the FT age is in good agreement with the U-Pb age. This suggests that the OD-3 zircon had a relatively fast cooling history and has not experienced later thermal annealing. The chronological dataset reported here clearly demonstrates that the OD-3 zircon could be a useful and reliable secondary standard for use during U-Pb dating studies of Cenozoic zircons.

Fission-track (FT) and (U-Th-Sm)/He (He) analyses are used to constrain the denudation pattern and history of the Kiso Range, a Japanese fault-block mountain range which has been uplifted since ca 0.8 Ma. We obtained nine zircon FT ages ranging 59.3-42.1 Ma, 18 apatite FT ages ranging 81.9-2.3 Ma, and 13 apatite He ages ranging 36.7-2.2 Ma. The apatite FT and He ages are divided into an older group comparable to the zircon FT age range and a younger group of <18 Ma. The younger ages are interpreted as a reflection of uplift of the Kiso Range because they were obtained only to the east of the Seinaiji-touge Fault, and the event age estimated from apatite FT data is consistent with the timing of the onset of the Kiso Range uplift. On the basis of the distribution of the younger ages, we propose westward tilting uplift of the Kiso Range between the boundary fault of the Inadani Fault Zone and Seinaiji-touge Fault, which implies a model of bedrock uplift that is intermediate between two existing models: a pop-up model in which the Kiso Range is squeezed upward between the two faults and a tilted uplift model which assumes that the Kiso Range is uplifted and tilted to the west by the Inadani Fault Zone. The original land surface before the onset of uplift/denudation of the Kiso Range is estimated to have been uplifted to an elevation of 2700-4900 m. We estimated denudation rates at 1.3-4.0 mm/y and maximum bedrock uplift rates at 3.4-6.1 mm/y since ca 0.8 Ma. The Seinaiji-touge fault is interpreted as a back thrust of the west-dipping Inadani Fault Zone. The older group of apatite FT and He ages is interpreted to reflect long-term peneplanation with a probable denudation rate of <0.1 mm/y.

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In this study, multi-system thermochronology, i.e., fission-track (FT) , K-Ar and U-Pb methods are used to identify the cooling and denudation history of the Tsuruga body of K ‾ ojaku granite, southwest Japan. Apatite FT age of 51.8±6.5 Ma, zircon FT age of 70.4±2.0 Ma, biotite K-Ar ages of 66.7-62.0 Ma, and zircon U-Pb age of 68.5±0.7 Ma were obtained for granitic samples, whereas plagioclase K-Ar ages of 19.1-18.8 Ma and whole-rock K-Ar age of 19.0±2.9 Ma were inferred for the basaltic dyke intruding into the granite. The zircon FT lengths are not significantly shorter than their initial lengths, implying rapid cooling at the zircon FT partial annealing zone (PAZ) . On the other hand, the apatite FT length distribution shows a typical pattern for granitic pluton without reheating, indicating a slow cooling history at the apatite FT PAZ. Based on the results of these thermochronometric analyses, inverse thermal calculations using the FT data, and simple thermal conduction modeling of the granitic body, the cooling and denudation histories of the Tsuruga body are reconstructed: (1) the Tsuruga body intruded at ca. 68 Ma, late Cretaceous, at a depth of several kilometers, (2) rapidly cooled to below the zircon FT PAZ by heat conduction within a few million years or less, and (3) slowly cooled due to peneplanation during the past 50-60 million years. On the other hand, the whole-rock Rb-Sr age previously reported for the K ‾ ojaku body is younger than when the cooling curve of the Tsuruga body obtained by this study intersects with the closure temperature of the whole-rock Rb-Sr system. This may imply a time lag between the formation ages of these bodies, but more thermochronometric studies are required to draw a definitive conclusion. The K-Ar ages of the basaltic dyke are interpreted as 202 - -its formation age, indicating that dyke intrusion was associated with the Green Tuff movement.

Fission track (FT), (U‐Th)/He (He), and U‐Pb data were used to identify the denudation history of the Akaishi Range, central Japan. The northern Akaishi Range is bounded on the east by the Itoigawa‐Shizuoka Tectonic Line Fault Zone (ISTL‐FZ). The thermochronometric ages progressively decrease with the decreasing distance to the ISTL‐FZ. Thermokinematic calculations suggest that the age pattern observed can be explained by 5–7.5 mm/yr reverse slip on the ISTL‐FZ that dips 34–45° west and soles onto detachment at 20–22.5 km depth. By assuming the same geometry and slip rate of the fault, the bedrock uplift rates and denudation rates are estimated at ~4 mm/yr. Thus, the uplift and denudation style of the northern Akaishi Range is well explained as a simple tilted thrust block that has been exhumed along the listric ISTL‐FZ. On the other hand, considering both the difference in apatite FT age and the active fault distribution, the southern Akaishi Range might be different in uplift origin and timing, although the difference in apatite FT ages between them may be attributable to the chlorine content variation in apatite. The inferred total denudation is larger than several kilometers and likely exceeds 10 km since the beginning of the northern Akaishi Range uplift, suggesting that the low‐relief surfaces on the ridges and the relatively constant elevations of the summits reflect postuplift denudation rather than preexisting low‐relief landforms.