Distribution of radioactive heat production around an active fault and in accretionary prisms of southwest Japan

Abstract

Contents of radioactive heat-producing elements (U, Th and K) were measured on core samples from a borehole drilled into the shear zone of the active Nojima fault, southwest Japan. The heat production rate is rather uniform in a granite layer below 560 m with an average of 1.6 μW/m 3. In the lower part of the granite layer, which is close to the fault zone, the U/Th and U/K ratios seem to be lower than those in the upper part. A possible cause of this anomaly is redistribution of U by groundwater flows in highly fractured layers. Through monitoring of the temperature profile in the borehole, a local anomaly, which may be associated with groundwater flow along a fracture zone, was detected in the uppermost part of the hole. The conductive heat flow was found to increase with depth, which is possibly due to downward fluid flow in the surrounding formations. Measurements of heat production were also made on rock samples from Permian to Cretaceous accretionary prisms in southwest Japan. The heat production of sandstone and mudstone, major components of the accretionary prisms, is about 1.5 μW/m 3 on average, which is similar to the average value for granitic rocks in southwest Japan. These values are significantly higher than those reported in accreted terranes in the Cascadia subduction zone.