Inner Zone Of Southwest Japan Research Articles

How and when did Paleozoic Maizuru back-arc basin close? Implications on the East Asian continental margin tectonics

The paleo-continental reconstruction of Asia during the Permian–Triassic boundary indicates major tectonic readjustments between multiple continental blocks in the north of North China Craton (NCC) due to complex plate subduction system(s). One of the major consequences was the closure of the Paleo-Asian Ocean. In a similar time frame, Proto-Japan was also evolving through plate-subduction with the opening of the Maizuru back-arc basin. The spectacular linear near-continuous belt of the Maizuru Terrain in the Inner Zone of Southwest Japan holds the rock record of the opening of the back-arc basin, sediment-fill in this basin culminating into closure-related pulsative debris flow deposits (i.e. the Tonoshiki Formation). However, the exact deformation mechanism(s), paleo-stress regime, their mesoscopic to microscopic rock record, and exact timing hold the keys to connecting the closure event of the Maizuru back-arc basin and the possible tectonics, in particular the closure of the PAO in Asia. With this aim, we report here the micro- to meso-structural geological records of rock units of the Maizuru Terrane which indicate evidence of (1) pre-depositional hydrofracturing, (2) extensive syn-lithification fracture-vein formation due to (3) broad WNW-ESE to E-W compressional paleostress regime using the fracture-vein analysis, (4) deformational e-twin development with the dominance of Type II twin indicating a maximum temperature of 300 °C with no evidence of overprinting. This brittle deformation affected all the lithounits of the Maizuru back-arc basin but not the overlying units deposited after the basin closure. New U-Pb detrital zircon age data of the rocks belonging to the Maizuru Group and also the overlying Fukumoto Formation fixes the age of the above-mentioned deformation event at ∼ 250 Ma. All these data help to assess the possible paleo-position of Proto-Japan and a clear tectonic scenario between the NCC, the Central Asian Orogenic Belt, and the Khanka-Jiamusi Massif in East Asia.

Permian tectonic evolution of the proto-Japan and East Asia: Insights from detrital zircon geochronology and crystal morphology

Combination analyses of detrital zircon ages and morphology from Permian sandstones in Japan provide evidence of orogenic evolution caused by collision in East Asia and reveal their paleogeographic setting. This study compares published and newly measured data from Permian fore-arc accretionary complexes (Akiyoshi Belt) and back-arc basin deposits (Maizuru Belt) in the Inner Zone of Southwest Japan. The fore-arc deposits and lower part of the back-arc deposits consist mainly of approximately 300–250 Ma zircon grains that are close to the depositional ages and are generally angular to subangular. These sediments formed around the Late Paleozoic volcanic arc isolated from the older continental crust, and their source rocks were exposed immediately prior to their erosion and transport and were transported directly to the depositional site in a relatively short time. In contrast, upper part of the back-arc deposits generally contains approximately 2,700–1,800 and 500–250 Ma zircon grains, which are commonly older than their depositional ages and are rounded. These grains were supplied from multiple sources, such as the Paleozoic volcanic arc and Precambrian continental block, and were affected by long-distance transport and a strong abrasion effect during transport. Comprehensive detrital zircon data emphasize that the oceanic arc–back-arc basin–continental margin system formed along the eastern margin of East Asia, and that the volcanic arc collided with the eastern margin of the Sino-Korean Block during the Permian. The variation in detrital zircon ages and shapes of back-arc deposits provides a constraint for modeling the East Asian Permian collision. Detritus derived from continental crust was captured by a back-arc basin and blocked by the uplift zone of the volcanic arc. However, a small amount of detritus originating from the continental crust was supplied to the fore-arc.

Clockwise rotation of SW Japan and timing of Izanagi–Pacific ridge subduction revealed by arc migration

Igneous rocks associated with the Cretaceous to Paleogene volcanic arc in SW Japan show ages that young from west to east in a direction parallel to the Median Tectonic Line suggesting corresponding translation of a heat source traditionally interpreted in terms of oblique subduction of a spreading ridge. However, recent oceanic plate reconstructions suggest ridge subduction may be younger than the main arc activity. Age compilations of 1227 points of felsic to intermediate Cretaceous and Cenozoic igneous rocks from the Japan arc show arc magmatism that can be separated into an early active period 130–60 Ma (stage 1), a subsequent period of quiescence 60–46 Ma (stage 2), which is followed by a resumption of igneous activity from 46 Ma onward (stage 3). In southwest Japan, the orientations of the magmatic arcs of stages 1 and 3 show and angular discordance of about 20°. The lack of active arc magmatism and the occurrence patterns of adakitic and high-Mg andesitic magmas indicate that ridge subduction occurred during stage 2. The arc age distribution pattern of stage 1 is explained by the slab shallowing related to a younging of the subducting slab as the ridge approaches. Furthermore, the obliquity of the arcs formed at stages 1 and 3 is explained by a 20° clockwise rotation of the inner zone of southwest Japan during the ridge-subduction phase. Oceanic plate reconstructions show counterclockwise rotation in the subduction direction after the ridge subduction phase, and coupling of the subducting oceanic plate with the upper plate would support microplate rotation in the inner zone. The new proposed tectonic reconstructions provide a framework to related Paleogene subduction of an active spreading ridge along the east Asia margin not only to the distribution of granitic bodies but also to rift-related basin formation on the eastern margin of the Eurasian continent and to rotation of crustal blocks indicated by paleomagnetic data of Cretaceous terranes.

Guadalupian–Lopingian (Middle–Late Permian) radiolarians from clastic rocks and zircon U–Pb ages of intercalated tuff and tuffaceous sandstone on Sado Island, central Japan

We investigated Permian strata in the Ota area of the Kosado Hills, Sado Island, central Japan, and their depositional ages were determined based on radiolarian biostratigraphy and zircon U–Pb dating. We also focused on the geology of the region and its lithologic correlation with other geological units in the Inner Zone of Southwest Japan. The radiolarian analysis identified the presence of Follicucullus porrectus and F. dilatatus in mudstone exposed on the coastline of Ota, indicating a middle–late Capitanian to early Changhsingian age. Mudstone exposed along the forest road in the Ota area contained Cariver charveti, suggesting a late Capitanian–early Changhsingian age. Zircon U–Pb dating of the tuff layer intercalated within the Follicucullus-bearing mudstone yielded a weighted-mean age of 259.9 ± 2.0 Ma, corresponding to a Capitanian–Wuchiapingian age. The tuffaceous sandstone interbedded with mudstone containing C. charveti yielded a weighted-mean age of 264.1 ± 6.3 Ma, which covers the Roadian to Wuchiapingian ages. The youngest model peak age (256.6 ± 2.6 Ma; Wuchiapingian) provides the most reliable estimate of the maximum depositional age. The Paleozoic strata in the Kosado Hills have previously been correlated with the Ultra-Tamba and Maizuru belts in Southwest Japan based on fossil occurrences. Our findings indicate that the Permian rocks in the Kosado Hills exhibit differences in lithology from those in the Ultra-Tamba belt but share similarities with the upper Guadalupian–Lopingian strata in the Maizuru belt.

Metamorphic P–T evolution of the Gotsu blueschists from the Suo metamorphic belt in SW Japan: Implications for tectonic correlation with the Heilongjiang Complex, NE China

In the Gotsu area of the c. 200 Ma high-P/T Suo metamorphic belt in the Inner Zone of southwest Japan, blueschists occur as lenses or layers within pelitic schists. Prograde, peak, and retrograde stages are distinguished in the blueschists, and the prograde and the peak metamorphic conditions are determined using pseudosection modelling in the NCKFMASHO system. The prograde metamorphic stage is defined by inclusions in porphyroblastic epidote and glaucophane, such as phengite, chlorite, albite, epidote and glaucophane/winchite, and the estimated metamorphic conditions are <325 °C and < 4–5 kbar at the boundary between the glaucophane schist facies and the greenschist facies. The peak metamorphic stage is well-defined by the schistosity-forming minerals, i.e. epidote, glaucophanic amphibole, phengite, and chlorite, suggesting the glaucophane schist facies conditions of 475–500 °C and 14–16 kbar. Actinolite/magnesiohornblende, chlorite, and albite replacing the peak stage minerals suggest the retrograde metamorphism into the greenschist facies. The metamorphic facies series of the Suo belt is defined by pumpellyite–actinolite facies to epidote–blueschist facies, and it has a relatively lower-P/T compared with the c. 300 Ma Renge belt in the Inner Zone of southwest Japan, which is defined by a sequence of lawsonite–blueschist facies to glaucophane–eclogite facies. The P– $$ {M}_{{\mathrm{H}}_2\mathrm{O}} $$ pseudosection and water isopleth show that the rocks were dehydrated during the initial stage of the exhumation and remained in water-saturated conditions. Similarities of the detrital zircon and peak metamorphic ages of the blueschists from the Suo metamorphic belt in southwest Japan and the Heilongjiang Complex in northeast China suggest that both metamorphic belts were probably formed in the same Paleo-Pacific subduction system in the Late Triassic to Jurassic period.

Provenance of a Permian accretionary complex (Nishiki Group) of the Akiyoshi Belt in Southwest Japan and Paleogeographic implications

The Akiyoshi Belt in the Inner Zone of Southwest Japan is made up of a Permian accretionary complex, the main component of which is the Late Permian Nishiki Group that mainly consists of sandstone, mudstone, felsic tuff and a minor amount of chert and conglomerate. We employ multiple methods, which includes sandstone petrography, detrital garnet composition and detrital zircon U-Pb dating to investigate the likely sources of these terrigenous deposits and to reconstruct the paleogeographic link between the proto-Japan and the East Asian continent. The highly immature Late Permian sandstones are interpreted to derive from multi-type source rocks that include felsic igneous rocks, basalts, sedimentary rocks and low to medium-grade metamorphic rocks in proximal locations. The detrital zircon U-Pb results show that all samples contain a dominant Early to Late Permian zircon population (294–254 Ma) and these zircons are interpreted to be derived from an active volcanic arc, which was most likely caused by subduction of the Paleo-Pacific plate beneath the South China Block. We found that spessartine-rich almandine garnets and almandine garnets dominated assemblages in the lower unit changed to grossular-andradite garnets dominated assemblage in the upper unit, which was caused by a progressive uplifting and denudation of the Permian volcanic arc.

The Ultra-Tamba Zone, A New Unit in the Inner Zone of Southwest Japan

The Ultra-Tamba Zone, A New Unit in the Inner Zone of Southwest Japan

A high‐Tmetamorphic complex derived from the high‐PSuo metamorphic complex in the Omuta district, northern Kyushu, southwest Japan

AbstractNew U–Pbages of zircons from migmatitic pelitic gneisses in the Omuta district, northern Kyushu, southwest Japan are presented. Metamorphic zonation from the Suo metamorphic complex to the gneisses suggests that the protolith of the gneisses was the Suo metamorphic complex. The zircon ages reveal the following: (i) a transformation took place from the high‐PSuo metamorphic complex to a high‐Tmetamorphic complex that includes the migmatitic pelitic gneisses; (ii) the detrital zircon cores in the Suo pelitic rocks have two main age components (ca1900–1800 Ma and 250 Ma), with some of the detrital zircon cores being supplied (being reworked) from a high‐grade metamorphic source; and (iii) one metamorphic zircon rim yields 105.1 ±5.3 Ma concordant age that represents the age of the high‐Tmetamorphism. The high‐Pto high‐Ttransformation of metamorphic complexes implies the seaward shift of a volcanic arc or a landward shift of the metamorphic complex from a trench to the sides of a volcanic arc in an arc–trench system during the Early Cretaceous. The Omuta district is located on the same geographical trend as the Ryoke plutono‐metamorphic complex, and our estimated age of the high‐Tmetamorphism is similar to that of the Ryoke plutono‐metamorphism in the Yanai district of western Chugoku. Therefore, the high‐Tmetamorphic complex possibly represents the western extension of the Ryoke plutono‐metamorphic complex. The protolith of the metamorphic rocks of the Ryoke plutono‐metamorphic complex was the Jurassic accretionary complex of the inner zone of southwest Japan. The high‐Pto high‐Ttransformation in the Omuta district also suggests that the geographic trend of the Jurassic accretionary complex was oblique to that of the mid‐Cretaceous high‐Tmetamorphic field.

Timing of the Late Cretaceous ignimbrite flare-up at the eastern margin of the Eurasian Plate: New zircon U–Pb ages from the Aioi–Arima–Koto region of SW Japan

The Kinki district in the Inner Zone of southwest Japan is characterized by Late Cretaceous volcanic rocks of the Aioi and Arima groups and the Koto Rhyolites (from west to east). These rocks are dominated by pyroclastic flow deposits related to caldera-forming events. Here, we present new laser ablation–inductively coupled plasma–mass spectrometry (LA–ICP–MS) zircon U–Pb ages for the Aioi Group (four isolated caldera-filling deposits); the lower, middle, and upper parts of the Arima Group and one intrusive rock (the Kashihara Quartz Gabbro); and the lower and upper parts of the Koto Rhyolites, including the oldest volcanic unit within these rhyolites. These analyses yielded ages of ca. 86–82Ma for the Aioi Group, ca. 83–81Ma for the Arima Group, 78.6Ma for the Kashihara Quartz Gabbro, and ca. 74–73Ma for the Koto Rhyolites. The Aioi Group represents a cluster of calderas and the ages obtained for individual units in this group differ from the ages of adjacent units. The Arima Group and the Koto Rhyolites both consist of pyroclastic flow deposits associated with caldera-forming events, and the ages of these rocks are all the same within error. This suggests that the Aioi Group represents a series of individual caldera-forming eruptions that are distinct from the Arima Group and the Koto Rhyolites, which formed during a single stage of caldera formation. The U–Pb ages presented here indicate that the Late Cretaceous caldera-forming eruptions in the study area occurred at intervals of >1Myr and represent individual events that lasted for <1Myr. The oldest volcanic unit within the Kinki district is similar in age to the oldest volcanic unit within the Chubu district, suggesting that caldera-forming eruptions in southwest Japan commenced at ca. 90Ma.

Cretaceous granitoids and their zircon U–Pb ages across the south‐central part of the Abukuma Highland, Japan

AbstractZircons separated from Cretaceous granitoids are dated from a south‐central transect of the Abukuma metamorphic and granitic terrane. The zircon ages do not follow ‘older’ and ‘younger’ granitoid ages that are used conventionally. In the western part of the study area (Zones I, II and III) where the Takanuki and Gosaisho metamorphic rocks are exposed, the Iritono quartz dioritic stock intruding the greenschist facies rocks in Zone III exhibits the oldest age of 121 Ma in the studied region. Quartz diorite located northward shows 112 Ma, but the other four granitoids intruding into the Takanuki and Gosaisho metamorphic rocks are younger and 103–99 Ma. Two‐mica and biotite granites belong to the youngest age group of 99 Ma. The granitic activities of both the Abukuma and Ryoke belts were initiated by intrusion of quartz dioritic magmas and were ended by two‐mica granite activity. The ages of the eastern two batholiths vary from 110 to 106 Ma (four samples), and show no age common to the Kitakami granitoids farther to the north. Throughout the Japanese Islands arc, Cretaceous granitic activities became younger toward the marginal sea side from the Kitakami Mountains, to the Abukuma Highland, and the Ryoke Belt, then to the Sanin belt of the Inner Zone of Southwest Japan.

Behavior of rare elements in Late Cretaceous pegmatites from the Setouchi Province, Inner Zone of Southwest Japan

Behavior of rare elements in Late Cretaceous pegmatites from the Setouchi Province, Inner Zone of Southwest Japan

Fault tectonic analysis of Kii peninsula, Southwest Japan: Preliminary approach to Neogene paleostress sequence near the Nankai subduction zone

AbstractThe southern part of the Outer Zone of Southwest Japan including the Kii peninsula belongs to the tectonic ‘shadow zone’, where fewer conspicuous active faults and less Quaternary sediments develop than in the Nankai subduction zone and Inner Zone of Southwest Japan. In order to study the paleostress sequence of the Kii peninsula, we analyzed fault‐slip data and tension gashes at pilot sites of Early–Middle Miocene forearc sediments and Late Cretaceous accretionary complex. According to the results, six faulting events are reconstructed in sequence: (i) east–west extension (normal faulting); (ii) east–west compression and north–south extension (strike‐slip faulting); (iii) NNW–SSE compression and ENE–WSW extension (strike‐slip faulting); (iv) northeast–southwest compression and northwest–southeast extension (strike‐slip faulting); (v) WNW–ESE compression (strike‐slip or reverse faulting); and (vi) NNE–SSW extension. The north–south to NNW–SSE trending dyke swarm of Middle Miocene age in the Kii peninsula is thought to be related to Event 3, implying that Event 3 was active at least during the Middle Miocene. Because Event 6 is recognized solely at a site, the overall latest faulting event seems to be Event 5. Assuming that the compression results from the motion of the crust or plate, the compression direction of Event 5 is in good accordance with the present‐day WNW crustal velocity vectors of the Kii peninsula. The stress trajectory map of Southeast Korea and Southwest Japan reveals that the current compression directions of the Kii peninsula correspond to the combinatory stress fields of the Himalayan and Philippine Sea tectonic domains.

物理定数から見た白亜紀―古第三紀花崗岩類― その4．西南日本内帯

西南日本内帯の花崗岩類の密度，孔隙率，磁化率の測定を行った． 密度についてみると，中部地域，東近畿地域のK1-2（古期領家）および北九州地域のK2（新期領家）の平均密度が2.72～2.74（g/cm3＝103 kg/m3以下同様）と最も大きく，北陸地域の古第三紀（暁新世）及び中央中国地域の古第三紀（始新世）花崗岩が2.62で最も小さい．孔隙率についてみると，平均孔隙率は東近畿地域（新期領家）が0.45 %で最も低く中央中国地域，古第三紀（始新世）の1.27 %で最も大きい．その最頻値はほぼ0.22から0.79 %であっておおざっぱにみて平均密度に逆比例している． 磁化率についてみると白亜紀後期に分類される試料（K1-2，K2）は中部地域，東近畿地域そして中央中国地域では10-4（SI 単位，以下同様）前半の試料が非常に多く，北陸地域，西近畿地域では10-3 前半の試料が加わる．東中国地域と北九州西部地域は更に10-2 を示す試料がある程度存在する．古第三紀に分類される試料（PG1，PG2）は10-4 の試料が一部存在するものの，10-3 の試料に加え10-2 の強度を示す試料が主流を占める． 残留磁化と密度の相関は全く見られないが，残留磁化と磁化率の相関（Qn 比）は時代区分に関係なく0.2～0.3付近に見られる．これは火山岩に比べ花崗岩の強磁性鉱物の磁区構造がかなり大きいことを意味しているものと考えられる．

中部日本土岐花崗岩体の岩相と化学組成の累帯変化

The Toki granite, a single pluton, is located in the Cretaceous Sanyo Belt in the Inner Zone of Southwest Japan. 483 samples collected from 19 borehole sites in the Toki granite display spatial variations in mineral assemblage, mode, bulk chemical composition and mineral composition, indicative of a zoned pluton. Based on the mode and the mineral assemblages with or without hornblende, the Toki granite can be divided into three rock facies from the margin to the interior: muscovite-biotite granite (MBG), hornblende-biotite granite (HBG) and biotite granite (BG). The ASI values [molar Al2O3/(CaO+Na2O+K2O)], Al2O3, K2O and Na2O change systematically from MBG through HBG to BG, corresponding to a systematic variation from peraluminous in the margin to metaluminous granite in the interior. Fe3+/Fe2+ ratios gradually increase from MBG through HBG to BG, corresponding to a systematic change from ilmenite-series in the margin to magnetite-series in the interior. The sedimentary rocks of the Mino terrane (Kamiasou unit) were intruded and metamorphosed to hornfels by the Toki granitic magma. Na2O/K2O and Fe3+/Fe2+ ratios become closer to those of the hornfels towards the margin of the body, suggesting the assimilation of the crustal host rock by the Toki granitic magma. However, the same ranges in SiO2 among three rock facies leave the possibility that each rock facies derived from a different magma. Both petrography and bulk chemistry imply that 1) the chemical variation of the Toki granite is not solely due to fractional crystallization, and 2) assimilation of crustal host rocks or simultaneous intrusion of different magmas has played a significant role during the emplacement process.

Sulfur isotopic study of vein deposits in the Kinki district, Inner Zone of Southwest Japan

Sulfur isotopic study of vein deposits in the Kinki district, Inner Zone of Southwest Japan

REE-bearing minerals of the Late Cretaceous ilmenite-series granites of the Inner Zone of Southwest Japan

REE-bearing minerals of the Late Cretaceous ilmenite-series granites of the Inner Zone of Southwest Japan

Genesis and Mixing/Mingling of Mafic and Felsic Magmas of Back‐Arc Granite: Miocene Tsushima Pluton, Southwest Japan

AbstractThe Middle Miocene Tsushima granite pluton is composed of leucocratic granites, gray granites and numerous mafic microgranular enclaves (MME). The granites have a metaluminous to slightly peraluminous composition and belong to the calc‐alkaline series, as do many other coeval granites of southwestern Japan, all of which formed in relation to the opening of the Sea of Japan. The Tsushima granites are unique in that they occur in the back‐arc area of the innermost Inner Zone of Southwest Japan, contain numerous miarolitic cavities, and show shallow crystallization (2–6 km deep), based on hornblende geobarometry. The leucocratic granite has higher initial 87Sr/86Sr ratios (0.7065–0.7085) and lower εNd(t) (−7.70 to −4.35) than the MME of basaltic–dacitic composition (0.7044–0.7061 and −0.53 to −5.24), whereas most gray granites have intermediate chemical and Sr–Nd isotopic compositions (0.7061–0.7072 and −3.75 to −6.17). Field, petrological, and geochemical data demonstrate that the Tsushima granites formed by the mingling and mixing of mafic and felsic magmas. The Sr–Nd–Pb isotope data strongly suggest that the mafic magma was derived from two mantle components with depleted mantle material and enriched mantle I (EMI) compositions, whereas the felsic magma formed by mixing of upper mantle magma of EMI composition with metabasic rocks in the overlying lower crust. Element data points deviating from the simple mixing line of the two magmas may indicate fractional crystallization of the felsic magma or chemical modification by hydrothermal fluid. The miarolitic cavities and enrichment of alkali elements in the MME suggest rapid cooling of the mingled magma accompanied by elemental transport by hydrothermal fluid. The inferred genesis of this magma–fluid system is as follows: (i) the mafic and felsic magmas were generated in the mantle and lower crust, respectively, by a large heat supply and pressure decrease under back‐arc conditions induced by mantle upwelling and crustal thinning; (ii) they mingled and crystallized rapidly at shallow depths in the upper crust without interaction during the ascent of the magmas from the middle to the upper crust, which (iii) led to fluid generation in the shallow crust. The upper mantle in southwest Japan thus has an EMI‐like composition, which plays an important role in the genesis of igneous rocks there.

西南日本内帯花崗岩類に伴うferriallanite-(Ce)様鉱物及びYに富むuedaite-(Ce)

西南日本内帯花崗岩類に伴うferriallanite-(Ce)様鉱物及びYに富むuedaite-(Ce)

Carboniferous and Permian foraminifers from limestone fragments contained in the Upper Cretaceous Hiromine Formation in north of Himeji, Hyogo, Japan

Limestone fragments with Late Paleozoic foraminifers were distinguished at Okusukain and Aisaka, north of Himeji, Nishi-Harima district, Hyogo, Japan. They are contained within matrix-supported conglomerate of caldera-originated lacustrine deposits in the Upper Cretaceous Hiromine Formation, which is mostly composed of volcaniclastic rocks. Age distribution of the limestone fragments is variable in comparison with their very rare occurrence. Late Visean, Serpukhovian or early Bashkirian, Moscovian, Wordian and Lopingian ages were attached to the fragments based on their age diagnostic foraminifers, in addition to questionable post-Triassic limestone cobble. All of these limestone fragments are thought to have been derived from the Akiyoshi and Maizuru terranes, and have special paleogeographic and tectonic implications in relation to the nappe tectonics of the Inner Zone of Southwest Japan.

Paleogeographic reconstructions of the East Asia continental margin during the middle to late Mesozoic

AbstractThe currently available paleogeographic maps of the East Asia continental margin during the Mesozoic have been recast in the light of recent research results on sediments distributed in Korea and Japan. Both the Korean peninsula and the Inner zone of Southwest Japan exchanged sediment supply during the Middle to Late Mesozoic, suggestive of a close paleogeographic relationship between the two countries at the active continental margin setting. During the latest Middle to earliest Late Jurassic the Mino–Tamba trench was developed along the southeastern Korean peninsula, from which trench‐fill sediments were sourced and to which an accretionary complex was accreted. Lower Cretaceous quartz‐arenite clasts of the Tetori Group in the Hida Marginal Belt of Southwest Japan were derived from pre‐Mesozoic quartz‐arenite strata distributed in the southern central and east central Korean peninsula, suggesting that the Tetori Basin was located close to the central eastern part of the Korean peninsula at the time of deposition of quartz‐arenite clasts, contrary to conventional thought of far distance between the two areas based on paleomagnetic data. During the early Late Cretaceous radiolaria‐bearing chert pebbles and sands in the northern part of the non‐marine Gyeongsang Basin distributed in the southeastern Korean peninsula were derived from the uplifted Mino–Tamba accretionary complex distributed in southwest Japan, suggesting that the Mino–Tamba terrane was land‐connected with the eastern Korean peninsula. These new findings suggest that in contrast to conventional thought, the collage of tectonic blocks in Southwest Japan has assembled in post‐early Late Cretaceous time.