Geological reconstruction based on metamorphic thermal analysis and utility in active fault research

Reconstructing the geological configuration around a major fault prior to its movement is essential for understanding regional tectonics and estimating total fault displacement. We aimed to reconstruct and validate the geological distribution across the Itoigawa–Shizuoka Tectonic Line (ISTL), central Japan, based on a metamorphic thermal analysis of the surrounding basement metamorphic rocks. The thermal structure of metamorphic rocks south of the ISTL shows a temperature increase in two directions: from east to west toward the western margin of the geologic body, and from south to north toward the intruded plutonic body. The east–west thermal structure closely resembles that of the metamorphic rocks north of the ISTL. Based on this regional-scale correspondence in metamorphic thermal structure across the ISTL, the reconstructed pre-faulting configuration suggests that the now discontinuously exposed plutonic bodies to the south and north of the ISTL were originally a single plutonic body before faulting. The extent of the intrusive magma body, inferred from the estimated thermal structure and modeled through intrusion thermal analysis, is consistent with the north–south span of the reconstructed plutonic body. This consistency supports the validity of the reconstruction based on regional thermal structure. Furthermore, our results are consistent with the strike–slip faulting along the ISTL having occurred after the intrusion of magma at approximately 10 Ma and caused a total horizontal displacement of approximately 13 km. Thus, analyzing the metamorphic temperatures of basement rocks offers a novel and effective approach for reconstructing pre-faulting geological relationships and for investigating fault history over geological timescales.