Organochemical characteristics of carbonaceous materials as indicators of heat recorded on an ancient plate‐subduction fault

Coseismic shear stress and slip distance affect subduction‐related earthquake processes. They need to be understood to evaluate the earthquake's mechanism and the tsunami generation potential near trenches. The amount of frictional heat generated depends on the shear stress and slip distance, which are therefore able to be derived from the temperature recorded in the fault. Here we developed a new temperature proxy for carbonaceous materials by performing spectroscopic, thermogravimetric, and organic elemental analyses in conjunction with heating experiments. We found marked anomalies in the infrared and Raman spectra and atomic compositions of carbonaceous materials retrieved from the slip zone of an ancient megasplay fault in the Cretaceous Shimanto accretionary complex, Japan: the infrared spectra show extinction of aliphatic C–H bonding and very weak aromatic C=C bonding, and the Raman spectra show a slightly elevated ratio of disordered band intensity to graphitic band intensity and relatively low H/C and O/C ratios. These correlate well with the spectral and elemental features of host‐rock carbonaceous materials after heating to 600°C. Thus, we conclude that the slip zone experienced a temperature of 600°C during a past earthquake event, indicating coseismic slip of 2–9 m, which could have generated a large tsunami if the ruptures propagated to the seafloor. Key Points Organochemical characteristics of carbonaceous materials may be indicators of past heating Multiple analytical approaches are required for accurate detection of the heat record of CMs An ancient megasplay fault around the Kure Mélange experienced a temperature of 600°C