Detection of Shear Heating from the Sanbagawa Belt nearby the Median Tectonic Line by using Raman Carbonaceous-Material Geothermometer

Mechanical work during fault movement is largely converted into heat energy, therefore, quantification of shear heating has potential to help in estimating shear stresses that operate on faults when they move. Surface heat-flow and thermochronology in the vicinity of the major San Andreas Fault (SAF) show no clear evidence for major shear heating. These results are commonly used to infer the SAF is a weak fault that supports lower shear stress than that expected based on rock deformation experiments. The cause of this discrepancy between observation and experiment remains unresolved. The Median Tectonic Line (MTL) separates the subduction-type Sanbagawa metamorphic belt to the south from the continental Ryoke high-temperature metamorphic belt to the north. It is the largest on-land fault of the Japanese Islands with a long movement history, and is a suitable candidate for a comparative study with the SAF. A progressive younging of apatite fission track ages towards the MTL within the Ryoke belt is indicative of shear heating. However, the detailed thermal structure around the fault and, in particular, in the Sanbagawa belt has not been determined. A semi-continuous core passing through the MTL was recently drilled in the Kii peninsula. The availability of this core enables us to conduct detailed analyses in key samples close to the fault. Raman carbonaceous-material geothermometry can be used to estimate peak temperatures in metamorphosed pelitic rock, which is the main rock facies of the Sanbagawa belt in the study area. Our study aims to investigate whether or not a heat-anomaly is present associated with the MTL by clarifying the peak temperature attained in the Sanbagawa belt. Results show a consistent temperature of ~340 ^oC at distances between 350 m - 4 km from the MTL. There is a significant rise of up to ~60 ^oC within 150 m from the MTL showing a significant amount of shear heating close to the fault. This detection of shear heating suggests that despite its similarities with the SAF, the MTL of Japan cannot be described as a weak fault. Although a clear zone of high temperature is recognized close to the MTL, the recorded profile is not compatible with the results of thermal modeling assuming heat transport by conduction alone. One-dimensional heat conduction calculations for single fault movements show a clear temperature increase only within 50 m, and the duration times at high temperature nearby the fault are short (< 10 months). Results for similar calculations using constant slip rates show that high temperatures can be achieved near the fault if sufficient time has elapsed (> 2 m.y. since the onset of fault movements), but in this case the associated thermal anomalies are broad (> ~10 km from the fault). One explanation for the discrepancy between the conduction modeling and the results is that advection may also have been an important heat-transport mechanism. Studies of the MTL therefore not only reveal the presence of shear heating characteristic of relatively strong faults, but also suggest operation of heat transfer processes that could explain why similar shear heating affects are difficult to observe in some other major fault locations such as the SAF.