Thermal regime of the Southwest Japan subduction zone: effects of age history of the subducting plate

Abstract

During a mid-Miocene (at about 15 Ma) tectonic reorganization, an ocean spreading ridge perpendicular to the strike of the Nankai Trough subduction zone stopped spreading. Since that time there has been subduction of the cooling fossil spreading ridge. In the present study, we have developed a time-dependent thermal subduction model for this region using the finite-element method. There are good seismological and geological constraints for the plate geometry and for the subduction history of the past 15 Ma. Boundary conditions are specified so that the oceanic plate subducting at the Nankai Trough becomes cooler as it gets older. The model results agree with the present heat-flow trend that decreases landward, and explain the paleothermal regime of high mid-Miocene temperatures inferred from land geological studies. The tectonics of the region prior to 15 Ma has some uncertainties. Assuming subduction of an active spreading ridge or an 80-Ma-old lithosphere prior to 15 Ma give the same results for the present thermal regime of the seaward portion of the forearc but different results for the most landward region (> 250 km). The thermal history of the forearc since 15 Ma can be summarized as a rapid warming period as a consequence of ridge subduction, followed by a cooling trend to the present as a result of the aging of the subducting plate. The results illustrate the thermal consequences of one type of ridge subduction. They also demonstrate that the thermal regime of a subduction zone depends critically on the age history of the subducting oceanic lithosphere, especially if it is young, as well as such parameters as the subducting plate dip angle and thickness of insulating sediments on the incoming oceanic crust. This dependence is especially important when the thermal regime is used to constrain the seismogenic behaviour of the subduction thrust fault.