Fission track (FT) annealing has been examined in apatites from the MITI-Tomikura well, which was drilled in the axis of the Tomikura anticline in the Kubiki district of NE Japan. Fission track densities were too low to measure confined fission tracks, and consequently direct measurements of semi-track lengths were made instead. Mean lengths, frequency distributions and measurable numbers of semi-tracks decrease below 3,300m depth. The apatite FT annealing zone in the MITI-Tomikura borehole is shallower than that previously reported from the Shin Yahiko-1 well, 4,600 m, which suggests a relatively simple subsidence on its burial history. Large scale uplift and denudation associated with formation of the Tomikura anticline probably caused the apatite FT annealing in the shallower levels of the MITI-Tomikura borehole. Sudden decrease in the numbers of semi-tracks observed at around 3,400m depth may be correlated with complex denudation events and overprinting of present-day and fossil annealing. Clay mineral transitions and homogenization temperature of fluid inclusions suggest an approximate paleo-geothermal gradient of 3.0 °C/100 m and 1,300m of denudation. This result is concordant with the uplift inferred from apatite FT annealing.
