Thermal structure in subducted units from continental Moho depths in a palaeo subduction zone, the Asemigawa region of the Sanbagawa metamorphic belt, SW Japan

Abstract

Raman CM geothermometry applied to 126 samples of pelitic schists collected over an area of 11 km × 7 km reveals the thermal structure of the Asemigawa region of the Sanbagawa metamorphic belt, southwest Japan in unprecedented detail. In general, the estimated temperatures gradually increase from south to north in the range of 288-553°C. However, a temperature gap from ~380 to ~440°C is identified near the boundary between the chlorite and garnet zones. This temperature region matches the depth of the continental Moho of the Sanbagawa subduction zone. The temperature gradient in the higher-temperature domain is higher than that in the lowertemperature domain, and large-scale tight folds that affect the thermal structure are developed in the high-grade units and in the vicinity of the temperature discontinuity.These geological structures probably reflect that the exhumed slab units was dammed at the Moho depth due to the upward movement being impeded by increase in the coupling strength of the overlying rocks associated with exhumation from beneath serpentinite rocks to a shallower domain overlain by crustal rocks. Changes in the coupling strength along the subduction boundary led the strong folding at the highertemperature domain and the pre-formed foliation developed at the Moho depth may have acted as the tectonic boundary, resulting in a temperature discontinuity. These results will contribute to elucidating various geological phenomena occurring in the forearc regions of modern subduction zones. K E Y W O R D Scontinental Moho, Raman carbonaceous material geothermometer, Sanbagawa (Sambagawa) metamorphic belt, slow earthquake, thermal structure

An automatic peak deconvolution method for Raman spectra of terrestrial carbonaceous material for application to the geothermometers of Kouketsu et al. (2014)

“…In general, CM of lower maturity exhibits a stronger influence of fluorescence on its Raman spectrum (e.g., Schito et al, 2017). This trend is also identified in the analyzed spectra (Figure S5), whereby the slope Nakamura et al (2019) and Kouketsu et al (2021). Error bars in FWHM, temperature of the previous studies, and temperature of the present study represent standard deviation, uncertainty of Equation ( 11), and 95% prediction intervals of Equation ( 13), respectively.…”

“…Error bars in FWHM, temperature of the previous studies, and temperature of the present study represent standard deviation, uncertainty of Equation ( 11), and 95% prediction intervals of Equation ( 13), respectively. N2019, Nakamura et al (2019); K2021, Kouketsu et al (2021) Among the six samples fully or partially analyzed with Fitting C, Γ D2 of the three less mature samples (TK15, N30, and N29) are almost consistent with those of K2014 (Table 2). These samples show small variation in the Γ D2 histogram, with a localized distribution around the mean value (Figure S6a-c).…”

“…On the basis of the above discussion, we conclude that the subjective selection of the fitting method affects the predicted temperature only modestly so long as (i) the FWHM values are calculated by employing Fitting E , F , or G (our recommendation is Fitting E ), and (ii) the temperature is estimated from $$$ {\Gamma}_{\mathrm{D}1} $$$. The second requirement may be popular among users of the geothermometers of K2014, as some previous studies calculated metamorphic temperatures only from $$$ {\Gamma}_{\mathrm{D}1} $$$ (e.g., Kouketsu et al, 2021; Mori et al, 2015, 2017; Nakamura et al, 2019).…”

“… TABLE S1 : Summary of the FWHM values of the D1‐band and the estimated metamorphic temperatures for samples analyzed by Nakamura et al (2019) and Kouketsu et al (2021). …”

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

“…The chlorite zone is further divided into lower‐ and higher‐grade subzones by the presence or absence of pumpellyite in mafic lithologies, respectively (Nakajima, 1982; Figure 3). The metamorphic conditions vary from those of the pumpellyite‐actinolite facies in the lower‐grade chlorite zone, through the blueschist‐greenschist facies boundary, to the epidote amphibolite facies in the albite‐ and oligoclase‐biotite zones (e.g., Enami et al, 1994; Kouketsu et al, 2021). The metamorphic zonation map of the Hibihara district (Figure 3) shows that the higher‐grade metamorphic zones are only observed in the northeastern part, and the greater part of the Hibihara district belongs to the chlorite zone.…”