Metamorphic and cooling history of the Shimanto accretionary complex,Kyushu, Southwest Japan: Implications for the timing of out‐of‐sequence thrusting

Illite crystallinity, K–Ar dating of illite, and fission‐track dating of zircon are analyzed in the hanging wall (Sampodake unit) and footwall (Mikado unit) of a seismogenic out‐of‐sequence thrust (Nobeoka thrust) within the Shimanto accretionary complex of central Kyushu, southwest Japan. The obtained metamorphic temperatures, and timing of metamorphism and cooling, reveal the tectono‐metamorphic evolution of the complex, and related development of the Nobeoka thrust. Illite crystallinity data indicate that the Late Cretaceous Sampodake unit was metamorphosed at temperatures of around 300 to 310°C, while the Middle Eocene Mikado unit was metamorphosed at 260 to 300°C. Illite K–Ar ages and zircon fission‐track ages constrain the timing of metamorphism of the Sampodake unit to the early Middle Eocene (46 to 50 Ma, mean = 48 Ma). Metamorphism of the Mikado unit occurred no earlier than 40 Ma, which is the youngest depositional age of the unit. The Nobeoka thrust is inferred to have been active during about 40 to 48 Ma, as the Sampodake unit started its post metamorphic cooling after 48 Ma and was thrust over the Mikado unit at about 40 Ma along the Nobeoka thrust. These results indicate that the Nobeoka thrust was active for more than 10 million years.     

Detrital zircon geochronology of the Cretaceous Sindong Group,Southeast Korea: Implications for depositional age and Early Cretaceous igneous activity

The Sindong Group forms the lowermost basin‐fill of the Gyeongsang Basin, the largest Cretaceous nonmarine basin located in southeastern Korea, and comprises the Nakdong, Hasandong, and Jinju Formations with decreasing age. The depositional age of the Sindong Group has not yet been determined well and the reported age ranges from the Valanginian to Albian. Detrital zircons from the Sindong Group have been subjected to U–Pb dating using laser ablation inductively coupled plasma mass spectrometry. The Sindong Group contains noticeable amounts of detrital magmatic zircons of Cretaceous age (138–106 Ma), indicative of continuous magmatic activity prior to and during deposition of the Sindong Group. The youngest detrital zircon age of three formations becomes progressively younger stratigraphically: 118 Ma for the Nakdong Formation, 109 Ma for the Hasandong Formation, and 106 Ma for the Jinju Formation. Accordingly, the depositional age of the Sindong Group ranges from the late Aptian to late Albian, which is much younger than previously thought. Lower Cretaceous magmatic activity, which supplied detrital zircons to the Sindong Group, changed its location spatially through time; it occurred in the middle and northern source areas during the early stage, and then switched to the middle to southern source areas during the middle to late stages. This study reports first the Lower Cretaceous magmatic activity from the East Asian continental margin, which results in a narrower magmatic gap (ca 20 m.y.) than previously known.     

Chert–clastic sequence in the Cretaceous Shimanto Accretionary Complex on the central Kii Peninsula,SW Japan

Ocean plate stratigraphy (OPS) within an ancient accretionary complex provides important information for understanding the history of an oceanic plate from its origin at a mid‐ocean ridge to its subduction at a trench. Here, we report a recently discovered chert–clastic sequence (CCS) that comprises a continuous succession from pelagic sediments to terrigenous clastics and which constitutes part of the OPS in the Akataki Complex within the Cretaceous Shimanto Accretionary Complex on the central Kii Peninsula, SW Japan. As well as describing this sequence, we present U–Pb ages of detrital zircons from terrigenous clastic rocks in the CCS, results for which show that the youngest single grain and youngest cluster ages belong to the Santonian–Campanian and are younger than the radiolarian age from the underlying pelagic sedimentary rock (late Albian–Cenomanian). Thus, the CCS records the movement history of the oceanic plate from pelagic sedimentation (until the late Albian–Cenomanian) to a terrigenous sediment supply (Santonian–Campanian).     

Parentage of low‐grade metasediments in the Sanbagawa belt,eastern Shikoku,Southwest Japan,and its geotectonic implications

This study examines the geology of low‐grade (chlorite zone) metamorphic rocks in the Sanbagawa belt and of a Jurassic accretionary complex in the Northern Chichibu belt, eastern Shikoku, Japan. The bulk chemistries of metasandstones and metapelites in the Sanbagawa belt of eastern Shikoku are examined in order to determine their parentage. The Sanbagawa belt can be divided into northern and southern parts based on lithology and geologic structure. Geochemical data indicate that metasediments in the northern and southern parts are the metamorphic equivalents of the KS‐II (Coniacian–Campanian) and KS‐I (late Albian–early Coniacian) units of the Shimanto belt, respectively. The depositional ages of the parent sediments of low‐grade metamorphic rocks found in the Sanbagawa belt and the Jurassic Northern Chichibu belt, indicate a north‐younging polarity. In contrast, sedimentological evidence indicates younging to the south. These observations suggest that a tectonic event has resulted in a change from a northerly to southerly dip direction for schistosity and bedding in the Sanbagawa and Northern Chichibu belts of eastern Shikoku. The younging polarity observed in the Sanbagawa and Northern Chichibu belts, together with previously reported data on vitrinite reflectance and geological structure, indicate that the Northern Chichibu belt was part of the overburden formerly lying on top of the Sanbagawa low‐grade metamorphic rocks.     

Timescale of material circulation in subduction zone: U–Pb zircon and K–Ar phengite double‐dating of the Sanbagawa metamorphic complex in the Ikeda district,central Shikoku,southwest Japan

We have estimated the timescale of material circulation in the Sanbagawa subduction zone based on U–Pb zircon and K–Ar phengite dating in the Ikeda district, central Shikoku. The Minawa and Koboke units are major constituents of the high‐P Sanbagawa metamorphic complex in Shikoku, southwest Japan. For the Minawa unit, ages of 92–81 Ma for the trench‐fill sediments, are indicated, whereas the age of ductile deformation and metamorphism of garnet and chlorite zones are 74–72 Ma and 65 Ma, respectively. Our results and occurrence of c. 150 Ma Besshi‐type deposits formed at mid‐ocean ridge suggest that the 60‐Myr‐old Izanagi Plate was subducted beneath the Eurasian Plate at c. 90 Ma, and this observation is consistent with recent plate reconstructions. For the Koboke unit, the depositional ages of the trench‐fill sediments and the dates for the termination of ductile deformation and metamorphism are estimated at c. 76–74 and 64–62 Ma, respectively. In the Ikeda district, the depositional ages generally become younger towards lower structural levels in the Sanbagawa metamorphic complex. Our results of U–Pb and K–Ar dating show that the circulation of material from the deposition of the Minawa and Koboke units at the trench through an active high‐P metamorphic domain to the final exhumation from the domain occurred continuously throughout c. 30 Myr (from c. 90 to 60 Ma).     

Lithological,structural, and chronological relationships between the Sanbagawa Metamorphic Complex and the Cretaceous Shimanto Accretionary Complex on the central Kii Peninsula,SW Japan

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.     

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

Zircons 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.     

Fission track and U–Pb zircon ages of psammitic rocks from the Harushinai unit,Kamuikotan metamorphic rocks,central Hokkaido,Japan: constraints on metamorphic histories

Accurate pressure–temperature–time (P–T–t) paths of rocks from sedimentation through maximum burial to exhumation are needed to determine the processes and mechanisms that form high‐pressure and low‐temperature type metamorphic rocks. Here, we present a new method combining laser ablation–inductively coupled plasma–mass spectrometry (LA–ICP–MS) U–Pb with fission track (FT) dates for detrital zircons from two psammitic rock samples collected from the Harushinai unit of the Kamuikotan metamorphic rocks. The concordant zircon U–Pb ages for these samples vary markedly, from 1980 to 95 Ma, with the youngest age clusters in both samples yielding Albian‐Cenomanian weighted mean ages of 100.8 ± 1.1 and 99.3 ± 1.0 Ma (2σ uncertainties). The zircon U–Pb ages were not reset by high‐P/T type metamorphism, because there is no indication of overgrowth within the zircons with igneous oscillatory zoning. Therefore, these weighted mean ages are indicative of the maximum age of deposition of protolithic material. By comparison, the zircon FT data yield a pooled age of ca. 90 Ma, which is almost the same as the weighted mean age of the youngest U–Pb age cluster. This indicates that the zircon FT ages were reset at ca. 90 Ma while still at their source, but have not been reset since. This conclusion is supported by recorded temperature conditions of less than about 300 °C (the closure temperature of zircon FTs), as estimated from microstructures in the deformed detrital quartz grains in psammitic rocks, and no shortening of fission track lengths in the zircon. Combining these new data with previously reported white mica K–Ar ages indicates that the Harushinai unit was deposited after ca. 100 Ma, and underwent burial to its maximum depth before being subjected to a localized thermal overprint during exhumation at ca. 58 Ma.     

U–Pb zircon age from the radiolarian‐bearing Hitoegane Formation in the Hida Gaien Belt,Japan

The dating of radiolarian biostratigraphic zones from the Silurian to Devonian is only partially understood. Dating the zircons in radiolarian‐bearing tuffaceous rocks has enabled us to ascribe practical ages to the radiolarian zones. To extend knowledge in this area, radiometric dating of magmatic zircons within the radiolarian‐bearing Hitoegane Formation, Japan, was undertaken. The Hitoegane Formation is mainly composed of alternating beds of tuffaceous sandstones, tuffaceous mudstones and felsic tuff. The felsic tuff and tuffaceous mudstone yield well‐preserved radiolarian fossils. Zircon grains showing a U–Pb laser ablation–inductively coupled plasma–mass spectrometry age of 426.6 ± 3.7 Ma were collected from four horizons of the Hitoegane Formation, which is the boundary between the Pseudospongoprunum tauversi to Futobari solidus–Zadrappolus tenuis radiolarian assemblage zones. This fact strongly suggests that the boundary of these assemblage zones is around the Ludlowian to Pridolian. The last occurrence of F. solidus is considered to be Pragian based on the reinterpretation of a U–Pb sensitive high mass‐resolution ion microprobe (SHRIMP) zircon age of 408.9 ± 7.6 Ma for a felsic tuff of the Kurosegawa belt, Southwest Japan. Thus the F. solidus–Z. tenuis assemblage can be assigned to the Ludlowian or Pridolian to Pragian. The present data also contribute to establishing overall stratigraphy of the Paleozoic rocks of the Fukuji–Hitoegane area. According to the Ordovician to Carboniferous stratigraphy in this area, Ordovician to Silurian volcanism was gradually reduced to change the sedimentary environment into a tropical lagoon in the early Devonian. And the quiet Carboniferous environment was subsequently interrupted, throwing it once more into the volcanic conditions in the Middle Permian.     

SHRIMP U–Pb ages of detrital zircons from the Early Cretaceous Nakdong Formation,South East Korea: Timing of initiation of the Gyeongsang Basin and its provenance

To constrain the depositional age of the lowermost Nakdong Formation in the Early Cretaceous Gyeongsang Basin, SHRIMP U–Pb age determination was carried out on zircon separates. The U–Pb compositions of detrital zircons from the Nakdong Formation yield a wide range of ages from the Archean to the Cretaceous but show a marked contrast in age distribution according to the geographical locations within the basin. The provenance of the southern Nakdong Formation is dominantly the surrounding Yeongnam Massif, which is composed of Paleoproterozoic metamorphic rocks and Triassic to Jurassic plutonic rocks, whereas the central to northern Nakdong Formation records significant sediment derivation from the Okcheon Metamorphic Belt, which is distributed to the northwest, in addition to the contribution from the Yeongnam Massif. It is suggested that the maximum depositional age of the Nakdong Formation is ca 127 Ma, based on its youngest detrital zircon age population. The onset of its deposition at 127 Ma coincided with the tectonic inversion in East Asia from a compressional to an extensional geodynamic setting, probably due to the contemporaneous change in the drift direction of the Izanagi Plate and its subsequent oblique subduction.     

The subduction of the Paleo‐Pacific Plate to the Jiamusi Block: Evidence from the Early Mesozoic sedimentary rocks of the eastern Jiamusi Block

In order to provide references of the subduction process of the Paleo‐Pacific Plate beneath the Jiamusi Block, this paper studied the clastic rocks of the Nanshuangyashan Formation using modal analysis of sandstones, mudstone elements geochemistry, and detrital zircon U–Pb dating. These results suggest the maximum depositional age of the Nanshuangyashan Formation was between the Norian and Rhaetian (206.8 ±4.6 Ma, mean standard weighted deviation (MSWD) = 0.17). Whole‐rock geochemistry of mudstone indicates that source rocks of the Nanshuangyashan Formation were primarily felsic igneous rocks and quartzose sedimentary rocks, which were mainly derived from the stable continental block and a magmatic arc. Detrital zircon analysis showed the Nanshuangyashan Formation samples recorded four main age groups: 229–204 Ma, 284–254 Ma, 524–489 Ma and 930–885 Ma, and the provenances were attributed to the Jiamusi Block and a Late Triassic magmatic arc near the study area. Furthermore, the eastern Jiamusi Block was a backarc basin, affected by the subduction of the Paleo‐Pacific Plate in the Late Triassic, but the magmatic arc related to the subduction near the study area finally died out due to tectonic changes and stratigraphic erosion.     

Zircon U–Pb geochronology of “Sashu mylonite”, eastern extension of Higo Plutono‐metamorphic Complex,Southwest Japan: Implication for regional tectonic evolution

Geochronological and geochemical studies reveal the possible origin of the restricted body of mylonite rocks occurring at the eastern edge of Kyushu Island, Japan, just in contact with the Sashu Fault, a part of the Paleo‐Median Tectonic Line (Paleo‐MTL). The LA‐ICP‐MS zircon U–Pb dating of the quartz diorite mylonite in this mylonitic body indicates a crystallization age of 114.0 ±1.7 Ma. Moreover, the two tonalite samples appear as thin layers within the Permian fine‐grained mafic mylonite; a part of the same body yields the age of 113.7 ±2.3 Ma and 116.9 ±1.3 Ma, with extremely low Th/U ratio. These quartz diorite mylonite and tonalite are consistent with the late Early Cretaceous magmatism and coeval metamorphism similar to those in the Higo Plutono‐metamorphic Complex in western Kyushu, Japan. This newly characterized complex occurs just south of the Cretaceous Sambagawa metamorphic rocks. The newly characterized mylonitic rocks are lying structurally above the Sambagawa Metamorphic Complex and are distributed along the Paleo‐MTL. The extension of the Higo Plutonometamorphic Complex, as well as the structural relationship between this complex and the Sambagawa Metamorphic Complex, is still controversial but holds a key to reconstruct the tectonic evolution of Southwest Japan during the Late Mesozoic to Early Cenozoic period. Hence, this article provides new insight into the reconstruction of the evolution history of East Asia as an active convergent margin.     

Depositional age of the Himenoura Group on the Amakusa‐Kamishima area,Kyushu, southwest Japan: Using zircon U–Pb dating of the acidic tuffs

The Upper Cretaceous Himenoura Group in the Amakusa‐Kamishima Island area, southwest Japan is subdivided into the Hinoshima and Amura Formations. In order to determine the numerical depositional age of the formations, zircon U–Pb ages were investigated using laser ablation inductively coupled plasma mass spectrometry (LA–ICP–MS) for acidic tuff samples from the lower part of the Hinoshima Formation and the upper part of the Amura Formation. Although the two samples contain some accidental zircons, the samples have a definite youngest age cluster and their weighted mean ages are 85.4 ± 1.3 and 81.5 ± 1.1 Ma, respectively (errors are 95 % confidence interval). These age data indicate that the Himenoura Group in the Amakusa‐Kamishima Island area was deposited mainly in the early Santonian to early Campanian which is consistent with biostratigraphic ages. Additionally, zircon age distributions of the two tuff samples from the upper part of the Hinoshima Formation do not show a distinct youngest peak of eruption age but characteristics of detrital zircons suggestive of maximum depositional age of the host sediments. These results demonstrate that the mean age of the youngest zircon age cluster of a tuff sample does not always indicate depositional age of the tuff, and statistical evaluation of age data is effective to determine depositional age of a tuff bed using zircon U–Pb ages.     

A middle Miocene post‐rift stress regime revealed by dikes and mesoscale faults in the Kakunodate area,NE Japan

Significant advances were made in the last century in the investigations of the Neogene stress history of the NE Japan arc. However, previous studies have failed to fully resolve middle Miocene post‐rift stress conditions owing to their assumption of Andersonian faulting and an inability to determine maximum and intermediate stress axes from dike orientations. We applied the latest methods of paleostress analysis in this study to igneous dikes and mesoscale faults in the Kakunodate area of the NE Japan arc to elucidate post‐rift stress conditions. Stratigraphic constraints and U–Pb dating indicate that the doleritic and dacitic dikes were formed at 16–12 Ma and 15–12 Ma, respectively. Dolerite and dacite dikes yielded NW–SE extensional stresses with intermediate and low stress ratios, respectively. Mesoscale faults in the middle Miocene formations of the studied area indicated similar stresses. We suggest the sluggish deformations resulting in the dike intrusion and faulting in the normal‐faulting stress regime after the termination of intra‐arc rifting at ca. 15 Ma.     

Denudation history of the Kiso Range,central Japan,and its tectonic implications: Constraints from low‐temperature thermochronology

Fission‐track (FT) and (U–Th–Sm)/He (He) analyses are used to constrain the denudation pattern and history of the Kiso Range, a Japanese fault‐block mountain range which has been uplifted since ca 0.8 Ma. We obtained nine zircon FT ages ranging 59.3–42.1 Ma, 18 apatite FT ages ranging 81.9–2.3 Ma, and 13 apatite He ages ranging 36.7–2.2 Ma. The apatite FT and He ages are divided into an older group comparable to the zircon FT age range and a younger group of <18 Ma. The younger ages are interpreted as a reflection of uplift of the Kiso Range because they were obtained only to the east of the Seinaiji‐touge Fault, and the event age estimated from apatite FT data is consistent with the timing of the onset of the Kiso Range uplift. On the basis of the distribution of the younger ages, we propose westward tilting uplift of the Kiso Range between the boundary fault of the Inadani Fault Zone and Seinaiji‐touge Fault, which implies a model of bedrock uplift that is intermediate between two existing models: a pop‐up model in which the Kiso Range is squeezed upward between the two faults and a tilted uplift model which assumes that the Kiso Range is uplifted and tilted to the west by the Inadani Fault Zone. The original land surface before the onset of uplift/denudation of the Kiso Range is estimated to have been uplifted to an elevation of 2700–4900 m. We estimated denudation rates at 1.3–4.0 mm/y and maximum bedrock uplift rates at 3.4–6.1 mm/y since ca 0.8 Ma. The Seinaiji‐touge fault is interpreted as a back thrust of the west‐dipping Inadani Fault Zone. The older group of apatite FT and He ages is interpreted to reflect long‐term peneplanation with a probable denudation rate of <0.1 mm/y.     

U–Pb zircon ages of the Nakanogawa Group in the Hidaka Belt,northern Japan: Implications for its provenance and the protolith of the Hidaka metamorphic rocks

Zircon U–Pb ages of two acidic tuff and two turbidite sandstone samples from the Nakanogawa Group, Hidaka Belt, were measured to estimate its depositional age and the development of the Hokkaido Central Belt, northeast Japan. In the northern unit, homogeneous zircons from pelagic acidic tuff from a basal horizon dated to 58–57 Ma, zircons from sandstone from the upper part of the unit dated to 56–54 Ma, and zircons from acidic tuff from the uppermost part dated to 60–56 Ma and 69–63 Ma. Both of the tuff U–Pb ages are significantly older than the youngest radiolarian fossil age (66–48 Ma). Therefore, the maximum depositional age of the turbidite facies in the northern unit is 58 Ma and the younger age limit, estimated from the fossil age, is 48 Ma. In the southern unit, homogeneous zircons from turbidite sandstone dated to 58–57 Ma. Thus the depositional age of this turbidite facies was interpreted to be 66–56 Ma from the fossil age, probably close to 57 Ma. Most of the zircon U–Pb ages from the Nakanogawa Group are younger than 80 Ma, with a major peak at 60 Ma. This result implies that around Hokkaido volcanic activity occurred mainly after 80 Ma. Older zircon ages (120–80 Ma, 180–140 Ma, 340–220 Ma, 1.9 Ga, 2.2 Ga, and 2.7 Ga) give information about the provenance of other rocks in the Hidaka Belt. It is inferred that the Nakanogawa Group comprises protoliths of the upper sequence of the Hidaka Metamorphic Zone, which therefore has the same depositional age as the Nakanogawa Group (66–48 Ma). The depositional ages of the lower sequence of the Hidaka Metamorphic Zone and the Nakanogawa Group are probably the same.