[M-IS13] New frontiers in geology

[MIS13-10] Geothermal overprint of Neogene accretionary complex and forearc sediments associated with emplacement of the Ashizuri granite, SW Japan

*Shiho Nishizawa¹, Yuzuru Yamamoto¹ (1.Kobe University)

Keywords:Subduction zone, Igneous intrusion, 1D thermal modeling

Granite intrusions is one of the most important geologic phenomena for the Earth evolution. Middle-Miocene Ashizuri granite Southwest Japan is a good example to study their style of intrusion processes to the shallow part of the plate subduction zone. To examine the thermal effects of the intrusion, geologic structure and thermal properties around the granitic body were examined. In addition, one dimensional thermal modeling was performed to restrict the initial conditions and possible geologic events occurred during the intrusion. Thermal effects deduced from the vitrinite reflectance, Rock-Eval, and Raman spectroscopy shows unique two-independent rapid-temperature decreases. One is near the granite and the other is at the lithologic boundary between the middle and upper part of the Misaki Group, 9 km from the contact. The former represented rapid decreases from 489.7ºC to 358.5ºC within ~1 km, whereas the latter denoted 240.0ºC to 157.0ºC within 1.2 km, respectively. The unique second temperature decreasing occurred at the interval where thermal diffusivity of sandstone were decreasing (1.70 mm²s^-1 to 1.20 mm²s^-1), while porosity of sandstone were increasing (4.4 % to 16.6 %). Initial temperatures of the granite and the host rock, 700ºC and 100ºC, respectively, were determined by the thermal modeling based on explicit finite difference model. Time-variant model with >100ºC of temperature difference is also required to describe the second rapid-temperature decreases. To describe this characteristic paleo-temperature distribution by the explicit FDM, faults movement and/or hot-fluid migration 1.1 m.y. after the granite intrusion, at the interval between 6-9 km west from the contact, was predicted. These structures correspond to the Matsuzaki and Tobinosu faults that represent >1 km displacement with large population of quartz veins. The thermal effects deduced from field- and numerical-based analyses indicate that concentric thermal overprinting occurred in association with of the granite intrusion.