Thermal state under the Tohoko arc with consideration of crustal heat generation

Abstract

The thermal structure beneath subduction zones plays a key role in aiding understanding of tectonic activities such as seismicity, volcanism, arc formation and genesis of back-arc basins around subduction zones. Many simulations have been made to estimate the thermal process in the mantle wedge which causes the surface activities. In the Tohoku Arc, many heat flow measurements have been made and the heat flow profile from the trench to the back-arc has been constructed. In this study, the thermal structure in the crust of Northeast Japan is investigated to account for the surface heat flow data. To estimate the contribution of the heat generation in the crust to the surface heat flow, the heat production rate was obtained on granitic and metamorphic rocks collected from the Hidaka metamorphic zone (Hokkaido, Japan) where the vertical crustal section of an island arc is understood to be exposed at the surface. Instrumentation neutron activation analysis and gamma-ray spectrometry were adopted to analyze the concentration of heat-producing elements. The obtained rate of heat generation and its decrease with increasing depth for the upper part of the crust were found to be similar to those estimated for stable continental areas but the heat generation rate was almost constant in the lower part of the crust. The vertical distribution of the heat production rate in the crust of the Northeastern Japan arc was estimated from the present results and previous estimates of heat production rates in various rocks, using the crustal structure determined by explosion seismology. The total heat generation in the crust thus estimated is about 30 mW/m 2 . With the surface heat flow data and estimated heat generation distribution in the crust, the thermal structure in the crust was simulated numerically. Modeling calculations were made on a cross section along the line from the Japan Trench to the Japan Sea, a direction parallel to the direction of convergence of the Pacific and Eurasian plates. The models were two-dimensional and included frictional heating at the upper surface of the slab. The calculated thermal structure in the crust shows that the temperature at the Moho is about 850°C and almost about 250°C lower than previous estimates. The magnitude of the interplate stress was derived by considering the rate of frictional heating at the interface between the plates. In order to explain the surface heat flow between the trench and aseismic front, the interplate stress should be less than 20 MPa at depth in the crust.