Cretaceous igneous rocks are widely distributed in the East Asian margin, and their formation is due to the large-scale migration of igneous activity areas from the continental interior to near the subduction margin. Detrital zircon U-Pb ages of trench-fill sandstones of the Shimanto accretionary complex in the Akaishi Mountains of southwest Japan can provide information on eroded igneous rocks, and their location at the eastern edge of East Asia makes them essential for better understanding the intensity and migration of the igneous activity. As a result of the dating, the Cretaceous zircons in the trench-fill sandstones were found to increase dramatically in the middle Cretaceous and to be abundant in the Late Cretaceous. In addition, these large amounts of Cretaceous zircons were found to be derived from igneous rocks such as the Abukuma Granites and Ryoke Granites distributed in the proto-Japan arc. These results indicate that migration of igneous activity from the continental side reached the eastern edge of East Asia in the middle Cretaceous and that large-scale igneous activity continued to occur in the eastern edge of East Asia until the Late Cretaceous. Thus, the detrital zircon age spectra in trench-fill sediments provide a clear picture of the degree of igneous activity in the source area and migration of the igneous activity on active continental margins. (c) 2024 Elsevier Ltd. All rights are reserved, including those for text and data mining, AI training, and similar technologies.

The Yokokawagawa metamorphic rocks (YMR), located east of the Itoigawa-Shizuoka Tectonic Line (ISTL) are considered part of the Sanbagawa metamorphic rocks (SMR) based on zircon U-Pb dating and Raman carbonaceous material (CM) geothermometry. Although previous thermal analyses suggested that the eastern YMR was affected by contact metamorphism related to an andesitic intrusion along the eastern margin, clear mineralogical evidence had not been reported. In this study, we discovered a biotite-bearing sample-characterized by randomly oriented biotite grains cross-cutting schistosity-at the eastern margin and conducted continuous thermal profiling using Raman CM geothermometry. Estimated peak temperatures range from 350 to 420 degrees C and show a V-shaped trend, decreasing from west to east and then increasing. The biotite-bearing sample records 400 degrees C but lacks garnet and plagioclase porphyroblasts, in contrast to the SMR, where these minerals are typically observed at similar temperatures. These findings support the interpretation that the eastern part of the YMR was affected by contact metamorphism caused by an andesitic intrusive body.

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.

We conducted detrital zircon U-Pb dating and Raman spectral analysis of carbonaceous material (CM) around the Takihara thrust, which is thought to be the boundary between the Sanbagawa metamorphic complex and Chichibu accretionary complex in the central Kii Peninsula, southwest Japan. The Mikabu greenstones, which are normally used to define the boundary, are missing from this area. U-Pb dating of psammitic rocks yields a youngest age of 118 Ma on the northern side of the Takihara thrust and 158 Ma on the southern side. The youngest age and age distribution obtained for the sample from the northern side is consistent with data for the Sanbagawa metamorphic complex in other regions, and the age data obtained for the sample from the southern side are consistent with the Chichibu accretionary complex, demonstrating that the Takihara thrust is the geological boundary between these complexes. We examined two parameters of the Raman spectra from CM: the temperature estimated from the full width at half maximum of the D1 band (TD1) and the ratio of the intensities of the D4 and D1 bands (D4/D1int). We obtained TD1 values of 275-290 degrees C on the northern side of the Takihara thrust and 275-280 degrees C on the southern side, with no clear difference across the thrust. In contrast, there is a clear difference in D4/D1int values across the thrust: 0.30-0.33 on the northern side and 0.47-0.50 on the southern side. This suggests that the D4/D1int value can not only be used to distinguish between the Sanbagawa and Chichibu complexes in the central Kii Peninsula, but it is also useful for detecting differences in metamorphic grade in low-grade metamorphosed rocks that cannot be detected by the geothermometer that uses the full width at half maximum of the D1 band.

ABSTRACT There are limited examples of incremental emplacement models of pluton that can be demonstrated through field observations. Comprehensive field investigations identified the existence of magmatic structures of alternating layers of multiple magmatic enclave‐rich and enclave‐free granite sheets in the Kinpusan pluton in central Japan, which geologically constrains the geometry and duration of the intrusion time interval of the incremental magmatic unit. Most layer boundaries are inferred by the presence or absence of enclaves, and the microstructure of the granites comprising the different layers is hardly distinguishable. However, several outcrops that show clear subhorizontal structures due to grain size contrast in the host granite exist. The emplacement history consists of repetitive downward accretion of 20–200 m thick subhorizontal magma sheets with relatively high magma flux, and is a good candidate for comparison with thermal models used by many researchers in recent years. The observed grain size, aplite, and miarolitic cavities show a tendency to increase in the lower center of the body, indicating a longer period of high‐temperature preservation than the pluton margin. Spatial patterns at this map scale were consistent with the predictions of the intrusion thermal model and the results of the zircon U–Pb dating together with a zircon saturation modeling, supporting the validity of the modeling.

Drone photogrammetry is carried out with an SfM-MVS analysis to obtain detailed topographic data and to make digital elevation models (DEMs) on a peninsula on the eastern shore of Lake Miwa, Ina City, Nagano Prefecture, where the topographic features and geological boundaries of the Median Tectonic Line are well observed. In the area covering the entire peninsula with several hundred-meter scales, the resolution of the DEMs produced by drone photogrammetry in this study is higher than that of 510 m mesh DEMs available free of charge. At the Mizoguchi outcrop on the southern edge of the peninsula, which is exposed on a several tens of meters scale with the clear lithological boundaries of the Median Tectonic Line, DEMs obtained by drone photogrammetry show a higher resolution than a 0.5 m mesh DEM constructed from airborne laser scanning. These results demonstrate the high utility of drone photogrammetry for a topographic analysis of an area within a few hundred meters of scale. In addition, the orientation of the lithological boundary at the Mizoguchi outcrop, estimated based on DEM images, agrees with that measured at the outcrop using a hand-held clinometer. This indicates that an analysis of geological structure using drone photogrammetry is practically feasible. Considering the advantages of cost and portability, it is suggested that drone photogrammetry may be a suitable approach for structural analyses in geological field surveys that require investigations in harsh environments such as steep mountain areas.

We estimated the protolith age and peak metamorphic temperature of the Yokokawagawa metamorphic rocks (YMR) east of the Itoigawa-Shizuoka Tectonic Line using detrital zircon U-Pb dating and Raman carbonaceous material geothermometry, respectively. U-Pb dating of a psammitic rock yielded a youngest age of similar to 100 Ma, which corresponds to the protolith age. Raman carbonaceous material geothermometry results for five pelitic rock samples give metamorphic temperatures of similar to 350-380 degrees C. The protolith age is consistent with those of the San-bagawa metamorphic rocks (SMR), strongly indicating that the YMR are an extension of the SMR. The increase in peak temperature toward intrusive rocks along the eastern margin of the YMR may indicate that the relatively young ages yielded by previous K-Ar dating of the YMR reflect thermal resetting due to contact metamorphism.

Low-grade metamorphic temperature conditions associated with the Sanbagawa metamorphic event were estimated by the Raman spectroscopy of carbonaceous material (RSCM) in pelitic rocks and an electron backscatter diffraction (EBSD) analysis of the quartz in siliceous rocks. Analytical samples were collected from the Sanbagawa metamorphic complex, the Mikabu greenstones, and the Chichibu accretionary complex in the eastern Kanto Mountains, central Japan. Previously, low-grade Sanbagawa metamorphism was only broadly recognized as pumpellyite–actinolite facies assigned to the chlorite zone. The RSCM results indicate metamorphic temperatures of 358 °C and 368 °C for the chlorite zone and 387 °C for the garnet zone of the Sanbagawa metamorphic complex, 315 °C for the Mikabu greenstones, and 234–266 °C for the Chichibu accretionary complex. From the EBSD analyses, the diameter of the quartz grains calculated by the root mean square (RMS) approximation ranges from 55.9 to 69.0 μm for the Sanbagawa metamorphic complex, 9.5 to 23.5 μm for the Mikabu greenstones, and 2.9 to 7.3 μm for the Chichibu accretionary complex. The opening angles of the c-axis fabric approximate 40–50°, presenting temperatures of 324–393 °C for the Sanbagawa metamorphic complex and the Mikabu greenstones. The temperature conditions show a continuous increase with no apparent gaps from these low-grade metamorphosed rocks. In addition, there exists an empirical exponential relationship between the estimated metamorphic temperatures and the RMS values of the quartz grains. In this study, integrated analyses of multiple rock types provided valuable information on progressive low-grade metamorphism and a similar approach may be applied to study other metamorphic complexes.

Raman spectral analyses were conducted of carbonaceous materials (CM) in meta-sedimentary rocks to estimate the metamorphic thermal structure of the Outer Zone of Southwest Japan in the Akaishi Mountains area, central Japan. Twenty samples of pelitic and psammitic rocks were collected along the Koshibu River and by traversing the Sanbagawa, Chichibu, and Shimanto belts. The datasets of the full width at half maximum of the D1-band (FWHMD1) -which is a sensitive thermal parameter of the Raman spectra of CM-imply that each rock records a single metamorphic thermal event. The peak metamorphic temperatures estimated with Raman CM geothermometry show 286 339 degrees C in the Sanbagawa Belt, 284-350 degrees C in the Chichibu Belt, and 328-347 degrees C in the Shimanto Belt with a continuous spatial change including at the boundaries between belts. From northwest to southeast, a clear temperature decrease is observed in the Sanbagawa Belt, but an increase in the Chichibu Belt with a constant high-T condition-similar to those recognized in southeast part of the Chichibu Belt-in the Shimanto Belt. A comparison of geological structures suggests that the present thermal structures in the Chichibu and Shimanto belts were formed with the formation of a "twist-bend" structure related to the collision of the Izu-Ogasawara (Izu-Bonin) Arc with respect to the Honshu Arc at middle Miocene. In contrast, the thermal structure in the Sanbagawa Belt was mainly formed before about 20 Ma but the above middle Miocene tectonics may involve the formation of the apparent steep geothermal gradient.

Structural analyses based on field survey and peak-temperature estimations using Raman spectra of carbonaceous-material thermometry were conducted for the Cretaceous subduction complexes on the central Kii Peninsula, southwest Japan, where there is a direct contact region between the Sanbagawa high-P/low-T Metamorphic Complex (MC) and the Shimanto Accretionary Complex (AC). In the study area, the Kosoku, Iro, and Mugitani complexes are arranged in descending order of structural position. The Kosoku and Iro complexes record exhumation-related deformation, which produced schistosity, stretching lineation, and folds, and peak temperatures of 280-440 degrees C. In contrast, the Mugitani Complex records both earlier accretion-related deformation which produced block-in-matrix structures and later exhumation-related deformation, as well as peak temperatures of 280-290 degrees C. The Mugitani Complex shares deformation characteristics with both the Shimanto AC and the Sanbagawa MC, and covers the interval from the upper temperatures of the Shimanto AC to the lower temperatures of the Sanbagawa MC. In addition, the two types of deformation differ in their kinematic patterns, suggesting that they took place diachronously in different tectonic settings in the subduction zone. We propose that exhumation-related Sanbagawa deformation occurred in a warm tectonic environment, such as ridge approach to the subduction zone.

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 lower‐temperature 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 higher‐temperature 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.

The Cretaceous Shimanto accretionary complex in southwestern Japan consists mainly of trench-fill deposits and formed in an active continental margin setting along the East Asian margin. Here, we present U-Pb ages of detrital zircons from the complex and compare youngest detrital zircon ages obtained from trench-fill sandstones with true depositional ages inferred from radiolarian fossils to assess the accuracy of zircon ages for estimating the timing of deposition of the trench-fill sediments. Four different methods were used to calculate the youngest detrital zircon ages, giving four different measures: youngest single-grain age (YSG), weighted mean age of the youngest cluster overlapping in age at 1 sigma (YC1 sigma) or 2 sigma (YC2 sigma), and youngest cluster age estimated using the Mixture Models (auto) of the DensityPlotter program (YCM). The YSG and YC1 sigma show good agreement with true depositional ages for Albian-Cenomanian strata, which were deposited during weak magmatic activity, and we find no age gap between YC2 sigma and the true depositional age, but YCM is clearly older. For Turonian-Maastrichtian strata, which were deposited during a period of strong magmatism, YC2 sigma and YCM correspond to the true depositional ages, and there is no age gap between YC1 sigma and the true depositional age. However, YSG is clearly younger than the true depositional age for Turonian-Maastrichtian strata. These results indicate that YC1 sigma and YC2 sigma show the closest correspondence to the true depositional ages, which suggests that these two measures best represent the true depositional age of trench-fill deposits in an active continental margin.

The Precambrian and lower Paleozoic units of the Japanese basement such as the Hida Oki and South Kitakami terranes have geological affinities with the eastern Asia continent and particularly strong correlation with units of the South China block. There are also indications from units such as the Hitachi metamorphics of the Abukuma terrane and blocks in the Maizuru terrane that some material may have been derived from the North China block. In addition to magmatism, the Japanese region has seen substantial growth due to tectonic accretion. The accreted units dominantly consist of mudstone and sandstone derived from the continental margin with lesser amounts of basaltic rocks associated with siliceous deep ocean sediments and local limestone. Two main phases of accretionary activity and related metamorphism are recorded in the Jurassic Mino-Tanba-Ashio, Chichibu, and North Kitakami terranes and in the Cretaceous to Neogene Shimanto and Sanbagawa terranes. Other accreted material includes ophiolitic sequences, e.g. the Yakuno ophiolite of the Maizuru terrane, the Oeyama ophiolite of the Sangun terrane, and the Hayachine-Miyamori ophiolite of the South Kitakami terrane, and limestone-capped ocean plateaus such as the Akiyoshi terrane. The ophiolitic units are likely derived from arc and back-arc basin settings. There has been no continental collision in Japan, meaning the oceanic subduction record is more complete than in convergent orogens seen in intracontinental settings making this a good place to study the geological record of accretion. Hokkaido lacks most of the Paleozoic history recognized in Honshu, Shikoku, Kyushu, and the Ryukyu Islands to the south and its geology reflects the Cenozoic development of two convergent domains with volcanic arcs, their approach, and eventual collision. The Hidaka terrane reveals a cross section through a volcanic arc and the main accretionary complex of the convergent system is represented by the Sorachi-Yezo terrane.

It is essential to clarify the lithological, structural, and chronological relationships between the Sanbagawa Metamorphic Complex (MC) and the Cretaceous Shimanto Accretionary Complex (AC) for understanding the tectonic evolution of SW Japan. To this end, we carried out a detailed field survey of the Sanbagawa MC and the Cretaceous Shimanto AC on the central Kii Peninsula, where they are in direct contact with each other. We also conducted U–Pb dating of detrital zircons from these complexes. The field survey showed that the boundary between the Iro Complex of the Sanbagawa MC and the Mugitani Complex of the Shimanto AC, Narai Fault, shows a sinistral sense of shear with a reverse dip‐slip component, and there are significant differences in the strain intensity and the degree of recrystallization between the two complexes across this fault. Detrital zircon U–Pb dating indicates that the Iro Complex in the hanging wall of the Narai Fault shows a significantly younger maximum depositional age than the Mugitani Complex in the footwall of the fault, and an apparently large gap in the MDA of ca. 35 Myr exists across this fault. This large age gap across the Narai Fault suggests that this fault is an essential tectonic boundary fault within the Cretaceous accretionary–metamorphic complexes on the Kii Peninsula, and is considered to be an out‐of‐sequence thrust. In addition, a similar shear direction and a large age gap have been identified across the Ui Thrust, which marks the boundary between the Kouyasan and Hidakagawa belts of the Cretaceous Shimanto AC. The Cretaceous accretionary–metamorphic complexes record the large‐scale tectonic juxtapositions of complexes, and these juxtaposed structures had been caused by sinistral–reverse movements on the tectonic boundary faults such as the Narai Fault and the Ui Thrust.

It is essential to clarify the lithological, structural, and chronological relationships between the Sanbagawa Metamorphic Complex (MC) and the Cretaceous Shimanto Accretionary Complex (AC) for understanding the tectonic evolution of SW Japan. To this end, we carried out a detailed field survey of the Sanbagawa MC and the Cretaceous Shimanto AC on the central Kii Peninsula, where they are in direct contact with each other. We also conducted U–Pb dating of detrital zircons from these complexes. The field survey showed that the boundary between the Iro Complex of the Sanbagawa MC and the Mugitani Complex of the Shimanto AC, Narai Fault, shows a sinistral sense of shear with a reverse dip-slip component, and there are significant differences in the strain intensity and the degree of recrystallization between the two complexes across this fault. Detrital zircon U–Pb dating indicates that the Iro Complex in the hanging wall of the Narai Fault shows a significantly younger maximum depositional age than the Mugitani Complex in the footwall of the fault, and an apparently large gap in the MDA of ca . 35 Myr exists across this fault. This large age gap across the Narai Fault suggests that this fault is an essential tectonic boundary fault within the Cretaceous accretionary–metamorphic complexes on the Kii Peninsula, and is considered to be an out-of-sequence thrust. In addition, a similar shear direction and a large age gap have been identified across the Ui Thrust, which marks the boundary between the Kouyasan and Hidakagawa belts of the Cretaceous Shimanto AC. The Cretaceous accretionary–metamorphic complexes record the large-scale tectonic juxtapositions of complexes, and these juxtaposed structures had been caused by sinistral–reverse movements on the tectonic boundary faults such as the Narai Fault and the Ui Thrust.