Regional thermal model of forearc area of the Japanese Islands
Abstract
With increase of many drillhole temperature data over the Japanese Islands, regional thermal structure has been revealed. However, owing to local hydrodynamic perturbation and heterogeneity of thermal conductivity, measurements of thermal gradient and heat flow are often scattered and not good enough for the firm regional thermal regime. Thus we obtain the average model to understand the regional thermal structure of the forearc area of the Japanese Islands. The average thermal models by statistical approach show a significant regional change of a high value in the western part of forearc inner zone and a low value in the Kanto area. The Kanto area marks 18 K/km for mean geothermal gradient, 44 mW/m2 for mean heat flow, while the western forearc area shows 27 K/km for mean geothermal gradient, 63 mW/m2 for mean heat flow. Besides, the geothermal gradients of the Osaka Plain and Nobi Plain in the western forearc area are 29 and 36 K/km, while the value of the Kanto Plain is 21 K/km. Heat flux in the crust depends on the age and the volume of granite rich in radioactive elements. There are few granitic rocks in the Kanto area, while granitic rocks are dominant in the western forearc area. The heat flow of 20 mW/m2 is attributed to the granitic rocks of about 8 km in thickness. There are two oceanic plate subductions of the Pacific plate and the Philippine Sea plate under the Kanto area, while only the Philippine Sea plate has been subducting under the western forearc area. The heat flux from the cooling oceanic lithosphere depends on the age of plate. The Shikoku Basin, a part of the Philippine Sea plate, off the western forearc area is 15-30 Ma, while the Pacific plate off the Kanto area is 122-132 Ma. Theoretically, heat flux values of 15 Ma and 50 Ma oceanic plate range 120 60 mW/m2. When the two subducting plates under the Kanto area are much older than the plate under the western forearc area, there could be a thermal regime difference in order of several tens of mW/m2.
- Publication:
-
EGS - AGU - EUG Joint Assembly
- Pub Date:
- April 2003
- Bibcode:
- 2003EAEJA.....1273O