Modeling of deep temperatures and heat flow in Central Honshu, Japan

Abstract

This paper presents the results of a numerical study of the distribution of deep temperatures and of the heat flow at depth over an area of almost 100,000 km 2 in Central Honshu, Japan. Due to the complicated geological-structural characteristics of the crust and specific plate-tectonic features of the lithosphere, this area has been the target of different geophysical investigations for decades. From the quite abundant number of seismic-refraction profiles available in the region, we have carried out temperature calculations along six transects with a total length of about 1500 km. The calculation of deep temperatures and the extrapolation of heat flow to depth were carried out on the basis of a numerical solution of the two-dimensional, steady-state equation of thermal conduction in a heterogeneous medium. To assess the distribution of radiogenic heat production (A) along a profile with a specified seismic velocity ( ν p ) pattern, an empirical A-ν p relationship was used. Thermal conductivity was assumed to depend on temperature. In zones of low surface heat flow, temperature at the Moho amounts to 350–500°C, whereas it may reach or even excedd some 1000°C in areas of very heat flow. Regional variations of the Moho heat flow range from 15–20 to 60–70 mWm −2. The results definitely suggest the possible existence of the asthenosphere at depths as shallow as 50 km in zones of pronouncedly high heat-flow anomalies. A comparison between the features of the distribution of deep temperatures and those of the variations of certain other geophysical characteristics (crustal structure, seismic velocity perturbations, seismic-wave attenuation. Curie-point depths, electrical conductivity) has also been made. In most cases fairly good correlations were found.