Estimation of Curie point temperature and geothermal structure of island arcs of Japan

Abstract

Curie depth analysis from aeromagnetic data was performed in a nationwide project to assess the geothermal resources of Japan. Creation of a Curie depth map of Japan was completed in 1984 and it agrees well with thermal structures such as the high-temperature gradient regions of volcanic chains, low-temperature regions of the forearc basins and the intermediate-temperature regions of the backarc basins. Measured temperature-gradient ( ▽T) data were gathered in order to estimate the Curie temperature from the ▽T and the Curie depths. When ▽T is assumed to be constant with respect to depth, a remarkable difference is found in the estimated Curie temperature ( T e c ) of the backarc and forearc side of the basins. In the backarc side an average T e c of 450 °C is reasonable for both the Curie depth estimated from aeromagnetic data (average depth of about 10 km in the backarc side) and the temperature gradients (average gradient of 4.5° C/100 m). The average T e c on the forearc side was found to be low, at about 300 °C. The difference in the Curie temperatures between the two areas may be due to a regional difference in rock type (e.g., granite as opposed to basalt). In the case of the Northeast Japan arc, as suggested by some authors, the isotherms may suddenly fall between the volcanic front and the trench. Based on model studies, we recognized that a Curie depth map obtained by the algorithm we developed should include an averaging effect. This suggests that the inferred Curie depth curve across the line marking the sudden fall in the Curie isotherm should gradually increase towards the trench. Therefore, the low Curie temperature that we estimated may indicate an apparent shallow Curie depth caused by the averaging effect against an area bearing deep Curie depths. Hence, the reasons for the inconsistency of the estimated Curie temperature between the forearc side and backarc side may be as follows: (1) the averaging effect of the Curie depth estimate, which makes the Curie depth apparently shallower in the forearc side and (2) the lower Curie temperature of basaltic rocks which are assumed to be distributed along the east side of the aseismic front.