Thermal structure and paleo-heat flow in the Shimanto accretionary prism, Southwest Japan

Thermal structural analysis and paleo-heat flow estimation provide clues to understanding the thermal evolution of the accretionary complex. The therm al structure and heat flow in the Jurassic Chichibu and Cretaceous to Terti ary Shimanto accretionary complex, Southwest Japan, have been investigated by vitrinite reflectance measurement and fluid inclusion analysis. As a res ult, the local and multistage metamorphisms were recognized as follows. Fil st, the Tertiary complex around the Miocene Ashizuri granite underwent exp osure to extra-high temperatures. Second, the Okitsu Melange underwent expo sure to higher temperatures than the surrounding strata and was formed conc urrently with the Kula-Pacific ridge subduction beneath the Japanese Island s in the Eocene. Finally, the thermal structure of most of the Cretaceous a nd southern Jurassic complexes is independent of the geologic structure, in dicating that these areas suffered ther mal overprint. Regional radiometric dating studies show that most of the Cretaceous Shimanto complex was heate d in the Eocene; the ther mal overprint might have occurred as a result of ridge subduction. The heat flow during peak heating was estimated to be 95- 120 mW/m(2) except for the Cretaceous Okitsu melange and the Cretaceous Non okawa formation, north of the Okitsu Melange; a much higher value of heat f low of similar to 200 mW/m(2) was estimated in the Okitsu Melange. An estim ation of heat flow failed for the non-okawa formation because thermal equil ibrium between the fluid and rocks has not yet been reached. It is probable that the southern str ata underwent a higher heat flow. Such a trenchward increase in heat flow resembles the present situation of the Nankai Trough, although the heat flow in the Eocene was much higher.