[M-IS17] New frontiers in geology

[MIS17-P02] Geothermal structure of the western part of the Izu-Bonin Island Arc Collision zone, central Japan: Implication for tectonic history

* Yuino Kobayashi¹, Yuzuru Yamamoto¹ (1.Kobe University)

Keywords:Vitrinite reflectance, Rock-Eval, Raman spectroscopy, Fossa Magna

The geothermal structure of the western part of the Izu-Bonin-Mariana (IBM) Island Arc Collision zone, in central Japan, Fossa Magna, was examined by means of combination of three different geothermometers: vitrinite reflectance, T_max of Rock-Eval pyrolysis, and Raman spectroscopy. Because the collision-related geologic bodies in the Fossa Magna are mainly composed of volcaniclastics of IBM Arc origin, which contain limited amount of vitrinite, this study tried to use T_max as a new thermal indicator. Based on the comparison between the traditional vitrinite reflectance and T_max sampled from the same locations, a strong correlation (T_max (℃) = 62.36Ro (%) + 394.7, R² = 0.9398) was established. Using the equation, we successively revealed the geothermal structure showing significant temperature variations even within the collision zone. Regional thermal difference between outside and inside the collision zone was identified: In the collision zone, maximum temperature was generally high (78-259℃) and some Late Miocene Plutonic intrusion affected thermal overprint (>200℃). On the other hand, outside the collision zone, the forearc-basin sediments (Sagara Group: 12-5 Ma), contemporaneous heterotopic facies of the Fossa Magna sediments, represented the constant and relatively lower temperatures (<70℃) without any igneous intrusion. The IBM arc collision played a crucial role making the regional complex geothermal structure including the effect of igneous activity, an increased geothermal gradient, and rapid sedimentation followed by uplift. Excluding contact metamorphism, high-temperature zones (~160-180℃) within the Fossa Magna, were notably localized within1-2 km zone west of the ISTL. This localization was controlled by north-south trending major faults, leading to formation of major thermal gaps across faults: 117℃ (Mogura Fault), 92℃(Akebono Fault), 88℃ (Otoshita Fault) and 48℃ (Minobu Fault). These data indicate that the uplift along faults associated with the IBM collision is restricted along the western margin of the Fossa Magna. Furthermore, the thermal gap along the Minobu Fault suggests that the fault was formed after the cooling of the Aimata plutonic body. Given the thermochronology of the Aimata plutonic body, the timing of the fault movement should be later than 2.5 Ma.