Recognition of broad thermal anomaly around the median tectonic line in central Kii peninsula, southwest Japan: Possible heat sources

Abstract

Application of Raman spectroscopy of carbonaceous material thermometry to samples of metasedimentary rock from the low‐grade Sanbagawa belt in the central Kii peninsula reveals a progressive decrease in temperature from ~390 °C close to the northern boundary, a major continental shear zone—the median tectonic line (MTL)—to ~270 °C and ~7 km to the south and roughly constant temperature distribution thereafter. Within the Sanbagawa belt, the thermal structure is not significantly modified by slip‐on fault boundaries between different geological units or folding. Meso‐ and microstructural observations combined with strain analysis using detrital grains in meta‐mudstone indicate a similar deformation history throughout the area and no correlation between ductile strain and temperature gradients. These observations suggest the observed thermal structure was developed after the main stages of ductile deformation of the Sanbagawa belt were complete and are not due to localized preferential exhumation along with the MTL. The observations also require a heat source along with the MTL. Order of magnitude estimates suggest the influx of warm fluid along the MTL are viable causes of the observed thermal anomaly. Although shear heating would be another possible explanation, thermal calculations require anomalous fast slip rates along the MTL and much greater frictional strength than generally considered reasonable. For these reasons, fluid infiltration is our preferred model.

Protolith age and metamorphic temperature of the Yokokawagawa metamorphic rocks in Nagano Prefecture, central Japan, and comparison with the Sanbagawa metamorphic rocks

“…) falls within the common temperature range of the chlorite zone recognized in most types of metamorphism. In addition, estimates of temperature from the chlorite zone of the SMR obtained using Raman CM geothermometry are ~360-370 °C for the Kanto Mountains(Hara et al, 2021), ~290-390 °C for central Japan(Mori et al, 2021; Nakamura et al, 2022), ~260-400 °C for the Kii Peninsula(Mori et al, 2015;Yoshida et al, 2016; Shimura et al, 2021;Yamaoka and Wallis, 2022), and ~290-450 °C for the Shikoku region(Endo and Wallis, 2017;Kouketsu et al, 2021), which are also in agreement with the results of this study. These findings imply that the protolith ages obtained from the chlorite zone of the SMR are the best candidates for comparison with that of the YMR (~100 Ma).…”

Overprint of secondary Du folding in the Sambagawa metamorphic belt, SW Japan: Implications for strain ellipsoids and Paleogene tectonics of the east‐Eurasian margin

“…At the same time, the subduction of the Izanagi plate is orogenoblique and is considered to have caused left-lateral displacement on the MTL, the land-side major fault bounding a fore-arc sliver (Figure 16b; e.g., Ichikawa, 1980;Miyata, 1990;Mori et al, 2015;Takagi, 1986;Yamaoka & Wallis, 2022). After the burial by this subduction, the Sambagawa metamorphic rocks accreted to the hanging wall at different depths and subsequently underwent the early stage of exhumation associated with E-W stretching during Ds (Figure 16b).…”