Eastern Eurasian Margin Research Articles

Thermal Modeling of the Sanbagawa and Ryoke Belts

The Sanbagawa and Ryoke belts were formed in a convergent plate boundary along the eastern margin of Eurasia. Thermal modeling using the geological records of these belts as constraints allows quantitative estimates of both shear heating along the Wadati-Benioff zone and magma fluxes beneath the volcanic arc. In contrast to real-time observations of crustal movement and heat flow, rocks record changes in pressures and temperatures that occur over periods of several million years and can be used to examine conditions from the surface to the mantle. Thermal modeling combined with such geological records helps to bridge the gap in our knowledge between real-time observations of ongoing geological processes and the development of orogenies in convergent plate margins over geological time.

Cenozoic thermal-rheological evolution of the coastal cathaysia block of East Asia: geodynamic implications

ABSTRACT The thermal-rheological structure of the continental lithosphere is an important indicator of geodynamic processes. Tectonic compression and extension may change thermal-rheological structure of lithosphere. However, the thermal-rheological evolution over geological time may be difficult to determine because of overprints of subsequent geological events. Mantle xenolith temperature and pressure conditions can differentiate the lithospheric thermal structure over geological time and indicate the lithospheric rheological strength. Chronological and temperature-pressure condition data from 11 mantle xenoliths in Cenozoic basalts from the Wuyishan terrane (WYT) and coastal terrane (CT) on both sides of the Zhenghe-Dapu Fault (ZDF) in the Cathaysia Block indicate that these terranes have experienced distinct thermal-rheological evolution since the Eocene. Moreover, the thermal-rheological evolution of the CT and WTY were differently influenced by the West Pacific tectonic domain, respectively, because the ZDF has been relatively active since 23 Ma and thus has partly accommodated and thus reduced the tectonic impact of Eurasia-Pacific convergence. Therefore, we classified the thermal-rheological structural evolution of the Cathaysia Block into three stages (35.5–20 Ma, 20–6.5 Ma, and 6.5–0 Ma) that sequentially recorded the opening and closing dynamics of the northeastern margin of the South China Sea and uplift of Taiwan Island on the eastern margin of Eurasia.

Geological Characteristics of the Mesozoic Unconformities in Eastern Heilongjiang, NE China: Implications for the Mesozoic Continental Margin Evolution of Northeast Asia

Most of the significant petroleum- and coal-bearing sedimentary basins in Northeast Asia originated via rifting and thermal subsidence during the Late Jurassic-Early Cretaceous, followed by basin inversion in the Late Cretaceous. However, the tectonic background governing these basin prototype shifts has not been fully explored. The unconformities are excellent archives of plate boundary interactions and geodynamic switches in subduction zones. The Eastern Heilongjiang Province (EHLJ), Northeast China (NE China), comprises a series of Mesozoic-Cenozoic residual basins with well-preserved successions and provides significant insights into the tectonic characteristics and background of Northeast Asia. Mesozoic unconformities and large-scale contractional structures in the basins mark a series of important tectonic transitions in Northeast Asia. Based on the synthesis information of regional Mesozoic unconformities identified in the seismic reflection profiles and field outcrops of EHLJ, the tectonic characteristics and geodynamic background of the Mesozoic continental margin basins in Northeast Asia are analysed. The Middle-Upper Jurassic/basement unconformity (U1) can only be found in some areas of the Sanjiang and Hulin basins. It was a response to the continental collision of Siberia and the northern China–Mongolia tract along the Mongolia–Okhotsk suture during the Jurassic. The Paleo-Pacific Plate rapidly subducted in the NNW direction towards the eastern margin of Eurasia in the early Lower Cretaceous resulting in a mass of strike-slip faults and the widespread absence of deposits (Valanginian) (U2) in the EHLJ. Because of the subduction slab rollback of the Paleo-Pacific Plate during the late Lower Cretaceous, the local asthenospheric material upwelled, and fault and volcanic activities intensified in Northeast Asia. The Lower Cretaceous Dongshan Formation (Fm)/Muleng Fm unconformity (U3-1) reflects a specific scale of bimodal magmatism in the Songliao Basin and the EHLJ. The Pacific Plate subducted in a transformation from NNW to WNW during the early Upper Cretaceous (Cenomanian). The Houshigou Fm (Qixinhe Fm)/Lower Cretaceous angular unconformity (U3) reflects that on the basins experienced denudation after being extensively uplifted from the subduction events. With the subduction of the Kula Plate, a compression stress field during the later Upper Cretaceous Period controlled NE China. The basins underwent a widely compressive deformation, accompanied by large-scale thrusts, denudation and deplanation, resulting in Paleogene/Cretaceous unconformity (U4) was formed.

Stratigraphic reconstruction of the lower–middle Miocene Goto Group, Nagasaki Prefecture, Japan

AbstractThe Goto Islands are located at the westernmost tip of the Japan archipelago, and preserve a lower–middle Miocene sedimentary sequence deposited during rifting of the continental margin and opening of the Sea of Japan. The stratigraphy of the Goto Group and new K–Ar, fission‐track, and U–Pb age data were used to determine the initial conditions of rifting in southwest Japan. The thickness of the Goto Group is 2000–3000 m. The lower unit (ca. 22–17.6 Ma) consists of thick, greenish, volcaniclastic rocks with basaltic volcanic material, representing the initial stages of continental rifting. The middle unit (ca. −17.6 Ma) consists of alternating sandstones and shales deposited in lacustrine and meandering fluvial environments in a syn‐rift sedimentary basin during a period of volcanic activity. The upper unit (ca. 17.6–16.8 Ma) consists of thick sandstones of fluvial–deltaic facies that were deposited during rapid subsidence at the continental margin. This unit was deposited by a large fluvial system that flowed into the Sea of Japan. These sequences contain relatively cooler to warmer flora (Daijima‐type) and record the warm period of the Miocene Climatic Optimum. The Goto felsic volcanic rocks (16.8 ~ 15.4 Ma) unconformably overlie the Goto Group, and granitic magmatism (ca. 16–14.5 Ma) occurred after sedimentation of the Goto Group. The widespread lacustrine, meandering–braided fluvial, and vast deltaic systems of the Goto Group, and felsic volcanism, were formed due to rapid subsidence that produced a horst‐and‐graben basin during the early stages of rifting of a volcanic arc along the eastern margin of Eurasia. These events occurred from 22.0 to 16.8 Ma before and during the formation of the Sea of Japan.

Pressure–temperature–time–deformation (P–T–t–d) path for Devonian forearc deposits in the Imjingang Belt, South Korea: Implications for Permian–Triassic collisional orogenesis on the eastern margin of Eurasia

AbstractThe Taean Formation on the central western margin of the Korean Peninsula contains layered metapelite, quartzo‐feldspathic schist, amphibolite, calcsilicate, and impure marble. The layering was formed by tectonic transposition of the original bedding. The formation has lithologic and isotopic age signatures similar to those of metasedimentary rocks of the Imjingang Belt, which is the eastern extension of the Qinling–Dabie–Sulu Belt, and the depositional age of the formation can be constrained to the Late Devonian. The metamorphic pressure–temperature (P–T) path for the formation is characterized by a high ΔP/ΔT prograde part and subsequent near‐isothermal decompression with peak P–T conditions of 600–620°C and 9.0–10.5 kbar, based on observations of mineral assemblages and phase equilibria modelling. Strong compressional deformation (Dn) produced a N–S‐trending Sn foliation that transposed the earlier Sn–1 and isoclinal Fn folds and was coeval with an episode of medium‐ to high‐pressure amphibolite‐facies metamorphism. The NNE–SSW‐trending left‐lateral Ryeseonggang Fault played a key role in producing the N–S distribution of the metamorphosed forearc deposits along the western coast of the Korean Peninsula. The peak metamorphism and deformation occurred during the early Permian to Early Triassic (276–250 Ma), with retrogression taking place during the Late Triassic (c. 230 Ma), as inferred from new U–Pb zircon age data and previous titanite and muscovite ages for the formation. The clockwise P–T–t (time)–d (deformation) path for the Taean Formation indicates rapid burial during Dn followed by rapid exhumation during the Permian–Triassic collision between the Qinling microcontinent and the North China Craton.

Early Cretaceous partial melting recorded by pelitic gneiss from the Nagasaki Metamorphic Complex, western Kyushu, Japan: initiation of Cretaceous high-T metamorphism at eastern margin of Eurasia

ABSTRACT We present Early Cretaceous partial melting recorded by pelitic gneiss from the Nagasaki Metamorphic Complex, western Kyushu, Japan. The existence of the partially melted rock in the complex may imply initiation of long-lived high-T metamorphism in proto-southwest Japan at Cretaceous eastern margin of Eurasia. The gneisses are composed of three garnet-bearing layers and one felsic lens, and they show mylonitic foliation. Application of Zr-in-rutile geothermometer and quartz-in-garnet Raman geobarometer for inclusion rutile and quartz in garnet provided prograde condition of 670–692°C and 1.01–1.26 GPa and peak condition of 755–769°C and 1.17–1.38 GPa. These conditions correspond to those of lower crustal depths between volcanic arc and forearc where thermal advection by wedge mantle corner flow occurs. Presence of the felsic lens whose mineral assemblage can undergo partial melting under the peak condition and presence of a crystallized melt inclusion in garnet suggest that distinct partial melting occurred during the metamorphism. The concordant U–Pb ages of metamorphic zircon rims are ranging from 131.5 ± 4.3 to 120.1 ± 3.9 Ma. The mean square weighted deviation (MSWD) of the metamorphic age was as large as 6.4, implying a significant duration of high-T condition for growth of metamorphic zircon. The obtained ages are within the period of the magmatic hiatus that has previously proposed around southwest Japan and Korean Peninsula (ca. 160–120 Ma) that is due to stop of wedge mantle corner flow by flatting of subducting slab. Our results suggest that high-T metamorphism associated with partial melting at lower crustal depths along Early Cretaceous proto-southwest Japan has already been initiated at the late stage of the hiatus (ca. 130 Ma). This may be due to slab rollback and trench retreat at eastern margin of Eurasia.

Subduction of the Izanagi-Pacific Ridge–transform intersection at the northeastern end of the Eurasian plate

Recent reconstructions of global plate motions suggest that the Izanagi-Pacific Ridge was subducted along the eastern margin of Eurasia at ca. 50 Ma. In the Hidaka magmatic zone (HMZ), which was located at the northeastern end of the Eurasian plate, three magmatic pulses occurred (46–45, 40–36, and 19–18 Ma). We report whole-rock geochemical and Sr-Nd-Pb isotopic data for 36 Ma high-Sr/Y (adakitic) rocks from the HMZ and show that these rocks formed by partial melting of oceanic crust and were emplaced as near-trench intrusions during ridge subduction. We reevaluate the nature of plutonic rocks in the HMZ and show that both the 46–45 and 40–36 Ma granitoids have essentially identical geochemical features. The distribution of plutons and magmatic cessation between 45 and 40 Ma are best explained by subduction of a ridge-transform intersection with a large offset of the ridge axis. The boundary between the Eocene granitoids corresponds to the position of a paleo–transform fault, and adakitic magmatism was caused by partial melting triggered by slab tearing at an overlapping spreading center. The paleoridge-transform configuration coincides with the locations of later large faults and a peridotite body.

Astronomical time scale of the Paleogene lacustrine paleoclimate record from the Nanxiang Basin, central China

The chronostratigraphy of the Paleogene succession of the Nanxiang Basin, a petroliferous basin in central China with a predominantly lacustrine fill, remains controversial due to the lack of an accurate geological time scale. In this work, high-resolution gamma-ray (GR) logs from the B270 and BS1 boreholes from depocenters in the Biyang Depression of the Nanxiang Basin were used to analyze the cyclostratigraphy. Paleocene floodplain facies of red clastics and purple mudstone is conformably overlain by Eocene through Oligocene lacustrine facies of gray mudstone, oil shale, and clayey dolomite interbedded with siltstone-sandstone. The multitaper method of spectral analysis and Gaussian bandpass filtering were used to identify Milankovitch cycles. Based on the recognition of 405-kyr long-eccentricity cycles, the series was tuned to establish a 46-Myr floating astronomical time scale (ATS), which was then anchored to the Paleogene/Neogene boundary age of 23.03 Ma. The ATS enables the assignment of the ages to non-marine biozones, depositional facies and rifting episodes within the Nanxiang Basin. The subsidence and rates of sediment accumulation within the rift basin of the Nanxiang Basin has an inverse relationship to the convergence rates between Pacific plates and the eastern margin of Eurasia. The middle Eocene slowing of the convergence rate at ~50 Ma, coupled with the onset of the collision of the Indian and Eurasian plate, corresponds to the beginning of the climax stage of rifting, the development of deep lakes and the deposition of oil shales in the Nanxiang Basin. The organic-rich 3rd member of the Hetaoyuan Formation (Eh3) during this middle Eocene interval preserved a range of climatic cycles from annual varves to a ~1.2-Myr modulation of obliquity cycles, indicating the importance of climatic oscillations on the development of those hydrocarbon source rocks.

Nature and evolution of the Cretaceous basins in the eastern margin of Eurasia: A case study of the Gyeongsang Basin, SE Korea

The Gyeongsang Basin is the largest among the non-marine Cretaceous sedimentary basins in Korea and Japan, formed by sinistral strike-slip movement induced by the oblique subduction of the paleo-Pacific plate under the eastern Asian continental margin, and has an evolutionary history of about sixty million years. SHRIMP and LA-MC-ICPMS U-Pb analyses were performed on the detrital zircons separated from the constituent formations of the Gyeongsang Basin. We determined the maximum depositional ages of the four formations based on their ages of the youngest groups as follows; 118.0 ± 2.6 Ma (n = 4) for the Hasandong Formation, 108.7 ± 0.8 Ma (n = 11) for the Chilgok Formation, 106.0 ± 0.2 Ma (n = 63) for the Silla Conglomerate, 105.4 ± 0.4 Ma (n = 14) for the Haman Formation, and 99.9 ± 0.7 Ma (n = 5) for the Jindong Formation, belonging to the period from Aptian to Cenomanian. The pre-Cretaceous detrital zircons of the lower to middle Gyeongsang Supergroup indicate their derivation from the Yeongnam Massif and the Okcheon Metamorphic Belt surrounding the Gyeongsang Basin. At around 115 Ma, as reflected as enhanced proportion of such ages in the detrital zircons and occurrence of farther inland igneous activities, the tectonic environment of Gyeongsang Basin changed to intra-arc due to the more active reestablishment of subduction-related igneous activity. However, the upper Gyeongsang Supergroup has a new age component of the mid-Permian, along with changes in the direction of paleocurrent flowing from the northeast and east, indicating sediment supply from SW Japan with rocks of that age. The changes in the direction of the paleocurrent flow seem to indicate that the center of igneous activity has moved in the direction of the subduction zone to the east. As the intensity of igneous activity continued to increase, the Gyeongsang Basin was closed by the filling of the basin by volcanic activity. The Cretaceous basins of the Korean Peninsula and southwest Japan show similar evolutionary histories, suggesting a type model of the creation and the evolution of basin around the continental margin subduction zone; creation of sedimentary basin by pull-apart or rifting in the backarc region, evolution to intra-arc basin by the landward progress of subduction-related igneous activity, and finally the closure of the basin due to the advent of highly enhanced igneous activity.

Ascent of migmatites of a high-temperature metamorphic complex due to buoyancy beneath a volcanic arc: a mid-Cretaceous example from the eastern margin of Eurasia

ABSTRACTMixtures of melt and residue in a high-T metamorphic complex have a lower density and viscosity than the surrounding host crust, and the mixtures should ascend due to buoyancy. The mixtures are recognized as migmatites in the high-T metamorphic complex. To confirm ascent of migmatites, we conducted numerical simulations of ascent of a model migmatite (buoyant viscous fluid). The numerical simulations show that the model migmatite could rise to shallow levels of a model crust so long as it is continuously produced at the bottom of the model crust. Otherwise it ceases to rise at depth due to loss of buoyancy by cooling. The numerical simulations also show that the model migmatite experiences vertical thinning during the ascent. The ascent mechanism proposed in this paper requires the continuous production of partially melted rocks at the base of the crust, which is provided by a continuous input of energy into the crust from the mantle. Given that high-T metamorphic complexes are associated with igneous activity beneath a volcanic arc, the igneous activity reflects the energy input into the lower crust from the mantle. A high-grade part (migmatites) of a high-T metamorphic complex in the Omuta district of northern Kyushu, southwest Japan, experienced thinning during ascent. Large amount of igneous rocks, such as plutonic and volcanic rocks, are also distributed in northern Kyushu. Zircon U–Pb ages of igneous rocks from northern Kyushu reveal that igneous activity continued from 115 to 93 Ma, and that peak igneous activity at 110–100 Ma was synchronous with the ascent of migmatites of the high-T metamorphic complex in northern Kyushu. Therefore, the numerical simulations may provide an appropriate model of the ascent of migmatites of the high-T metamorphic complex beneath a volcanic arc, at the eastern margin of Eurasia in the mid-Cretaceous.

Subsidence transition during the post-rift stage of the Dongpu Sag, Bohai Bay Basin, NE China: A new geodynamic model

The Dongpu Sag, located in the Bohai Bay Basin, NE China, is a Cenozoic continental rift basin. The post-rift evolution of the Dongpu Sag is associated with the development of petroleum reservoirs and has implications for Neogene–Quaternary basin evolution along the eastern margin of Eurasia. To determine the nature and origin of post-rift subsidence in the Dongpu Sag, we apply backstripping, modified strain-rate inversion, and revised finite extension modelling techniques, using data from 14 real and synthetic wells that are intersected by three seismic lines. Our results reveal discrepancies by subsidence based on backstripping of well data (the observed subsidence) minus that predicted by modified strain-rate inversion and revised finite extension modelling (the predicted subsidence). During the Miocene, the observed subsidence was smaller than the predicted subsidence, leaving negative discrepancies referred to here as “insufficient subsidence” ranging from −343 to −96 m. In contrast, during the Pliocene–Quaternary the observed subsidence was greater than the predicted subsidence by +123 to +407 m, which left positive discrepancies referred to as “over-sufficient subsidence”. Therefore, we infer a transition from insufficient to over-sufficient subsidence during the post-rift stage. Normal faulting that started at ca. 5.3 Ma is estimated to have produced only ∼20% of the over-sufficient subsidence. Therefore, the remaining over-sufficient subsidence, as well as the preceding insufficient subsidence and the transition between the two, were likely controlled by lithosphere processes. We propose a new tectonic model in which variations in the conditions (e.g. rate, direction, and angle) associated with subduction of the Pacific plate resulted in a change of heat flow decreasing from a linear to a curvilinear pattern, leading to a transition from insufficient to over-sufficient subsidence.

Porphyry Copper Potential in Japan Based on Magmatic Oxidation State

AbstractThe formation of large porphyry Cu deposits requires shallow intrusion of felsic stocks that are injected into the focused sites of the mineralization. The parental magmas for these igneous rocks are oxidized, with a fO2 above FMQ + 2 (2 logarithmic units above fO2 of fayalite‐magnetite‐quartz buffer), which enables the magmas to transport large amounts of S and metals. The Japanese islands contain continental and island‐arc magmatic belts of various ages, from Mesozoic to recent, that were formed in response to westward subduction of oceanic plates. Mesozoic–Paleocene magmatism formed arcs on the eastern margin of Eurasia before the opening of the Sea of Japan. The back‐arc rifting that formed the Sea of Japan separated these igneous rocks from Eurasia, with their relics exposed as volcano–plutonic belts in Japan. Among these, the San‐in belt is the only representative of an upper crustal section that includes magnetite‐bearing granitic rocks. The opening of the Sea of Japan was accompanied by the asthenospheric mantle upwelling and eruption of oceanic magma. This was followed by extensive Miocene submarine volcanic activity that formed the Green Tuff belt. The current configuration of the Japanese islands was established in Pliocene. Young arcs, such as the Kuril, Izu‐Bonin, and Ryukyu arcs, are largely composed of mafic magmas, with accompanying asthenospheric upwelling in back‐arc extension. As the asthenospheric mantle and associated magmas have a reduced fO2, below FMQ (fayalite‐magnetite‐quartz buffer), continual upwelling of asthenospheric mantle creates limited areas of potential porphyry Cu mineralization in Japanese islands. Possible prospective areas include young stratovolcanoes in Honshu and Kyushu islands. Fertile rocks that may be associated with porphyry Cu mineralization can be identified by evaluating their magmatic oxidation conditions through mineralogical and mineral chemistry studies.

Genesis of the boron potential of the Taukha metallogenic zone, Sikhote Alin, and boron sources during formation of the Dal’negorsk boron deposit

It is shown that the formation of borosilicate skarn in the Taukha metallogenic zone completes a series of successive stages of the formation and transformation of the folded sequences of the Taukha accretionary wedge. The Early Cretaceous sedimentary stage, including accumulation of detrital tourmaline-rich sedimentary rocks, was implemented in the marginal sea of the Paleopacific Sino-Korean segment. In the Turonian–Campanian, the boron-bearing folded sequences of the accretionary wedge were involved in anatexis to generate siliceous S-type boron-bearing melts. The thus-formed magmatic chambers were emptied during catastrophic volcanic eruptions. At the final Middle Campanian volcanic stage, fluid-magmatic differentiation of the melt in the residual chambers generated fluid flow. The infiltration interaction of the fluids, which inherited enrichment in boron, with limestones of the olistostrome sequence resulted in the formation of a giant zone of grossular–wollastonite skarns and danburite lodes. The boron potential of the Taukha boron–lead–zinc metallogenic zone may be considered as a reproduction of the Precambrian boron metallogeny of the eastern Eurasian margin, which was implemented in the Late Mesozoic during recycling of the continental crust.

Paleomagnetic study of Plio-Pleistocene sediments in the concentrated deformation zone along the eastern margin of the Japan Sea

Paleomagnetic results are presented for a concentrated deformation zone (CDZ) in a convergent plate boundary region along the eastern Eurasian margin. We sampled late Pliocene–early Pleistocene tuffs and clastic sediments of the Sasaoka Formation (∼750 m thick) in northeastern Honshu, Japan, to test whether the late Pliocene–Quaternary crustal deformation within the CDZ along the eastern margin of the Japan Sea was accompanied by rotation about a vertical axis. Rock magnetic experiments suggest that the principal magnetic carrier is magnetite in the fine tuffs, and magnetic iron sulfide in the fine sandstones. Pre-folding characteristic remanent magnetization was confirmed using a positive bootstrap fold test. We obtained 21 acceptable site-mean characteristic directions that include our preliminary published results, and which cover an interval from ca. 2.7 to 1.7 Ma on the basis of magnetostratigraphic correlations. An updated age–depth model is given, and this allowed us to make numerical age estimates for key interbedded tuff beds (tephras). A positive fold test also suggests that the Gojome syncline began to develop after 1.7 Ma, which means the folding began long after the initiation of late Pliocene crustal shortening in northeastern Honshu. After 100% unfolding, the overall mean direction (D = 359.7°, I = 54.9°, α95 = 6.7°) is indistinguishable from the geocentric axial dipole field direction, indicating that the Gojome syncline, the most prominent structural element in the study area, developed without vertical-axis rotation. A comparison of our results with Plio-Pleistocene directions reported from other areas reveals no paleomagnetically detectable rotation in or adjacent to the CDZ, except for local rotation near strike-slip faults in central Honshu.

Paleo-Pacific subduction-accretion: Evidence from Geochemical and U-Pb zircon dating of the Nadanhada accretionary complex, NE China

The Nadanhada Terrane, located along the eastern margin of Eurasia, contains a typical accretionary complex related to paleo-Pacific plate subduction-accretion. The Yuejinshan Complex is the first stage accretion complex that consists of meta-clastic rocks and metamafic-ultramafic rocks, whereas the Raohe Complex forms the main parts of the terrane and consists of limestone, bedded chert, and mafic-ultramafic rocks embedded as olistolith blocks in a weakly sheared matrix of clastic meta-sedimentary rocks. Geochemical data indicate that the Yuejinshan metabasalts have normal mid-ocean ridge basalt (N-MORB) affinity, whereas the Raohe basaltic pillow lavas have an affinity to ocean island basalts (OIB). Sensitive high-resolution ion microprobe (SHRIMP) U-Pb zircon analyses of gabbro in the Raohe Complex yield a weighted mean 206Pb/238U zircon age of 216 ± 5 Ma, whereas two samples of granite intruded into the complex yield weighted mean 206Pb/238U zircon ages of 128 ± 2 and 129 ± 2 Ma. Laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) U-Pb zircon analyses of basaltic pillow lava in the Raohe Complex define a weighted mean age of 167 ± 1 Ma. Two sandstone samples in the Raohe Complex record younger concordant zircon weighted mean ages of 167 ± 17 and 137 ± 3 Ma. These new data support the view that accretion of the Raohe Complex was between 170 and 137 Ma, and that final emplacement of the Raohe Complex took place at 137–130 Ma. The accretion of the Yuejinshan Complex probably occurred between the 210 and 180 Ma, suggesting that paleo-Pacific plate subduction was initiated in the Late Triassic to Early Jurassic.

Permo-Triassic changes in bulk crustal shortening direction during deformation and metamorphism of the Taebaeksan Basin, South Korea using foliation intersection/inflection axes: Implications for tectonic movement at the eastern margin of Eurasia during the Songrim (Indosinian) orogeny

The Permo-Triassic Songrim (Indosinian) orogeny in South Korea was a major tectonic event involving complicated continental collisions at the eastern margin of Eurasia. Previous studies have examined the structural and metamorphic features of the Songrim orogeny in each of the Paleozoic terranes of the orogenic belt (i.e., the Taebaeksan Basin, the Okcheon Basin, and the Imjingang Belt), but correlations of these features among the terranes remain uncertain. The aim of this paper is to reveal deformation history including bulk crustal shortening directions in the Taebaeksan Basin, and to correlate the tectono-metamorphic evolution of the Taebaeksan Basin with other Phanerozoic mobile belts in eastern Asia based on a combined analysis of foliation intersection/inflection axes (FIA) trends and metamorphic P–T and T–t (time) paths. The orientations and relative timing of FIA preserved as inclusion trails within porphyroblasts of andalusite, chloritoid, garnet, and staurolite reveal two age groups of inclusion trails in the Pyeongan Supergroup at the northeastern margin of the Taebaeksan Basin. These microstructures indicate the development of early NNW–NNE-trending structures and fabrics, followed by later E–W-trending ones. These observations suggest a change in the orientation of bulk crustal shortening from E–W to N–S during the Songrim orogeny. Based on the similar microstructures and temperature–time paths of the three Paleozoic terranes, we interpret that the E–W bulk crustal shortening influenced the eastern part of the Korean Peninsula during the early stages of the Songrim orogeny, presumably related to amalgamation between the proto-Japan terrane and the eastern margin of Eurasia, whereas the N–S bulk crustal shortening was stronger in the western part of the peninsula during the later stages of the orogeny, related to collision between the South and North China blocks.

The great triangular seismic region in eastern Asia: Thoughts on its dynamic context

A huge triangle-shaped tectonic region in eastern Asia plays host to numerous major earthquakes. The three boundaries of this region, which contains plateaus, mountains, and intermountain basins, are roughly the Himalayan arc, the Tianshan-Baikal, and longitude line ∼105°E. Within this triangular region, tectonism is intense and major deformation occurs both between crustal blocks and within most of them. Outside of this region, rigid blocks move as a whole with relatively few major earthquakes and relatively weak Cenozoic deformation. On a large tectonic scale, the presence of this broad region of intraplate deformation results from dynamic interactions between the Indian, Philippine Sea-West Pacific, and Eurasian plates, as well as the influence of deep-level mantle flow. The Indian subcontinent, which continues to move northwards at ∼40 mm/a since its collision with Eurasia, has plunged beneath Tibet, resulting in various movements and deformations along the Himalayan arc that diffuse over a long distance into the hinterland of Asia. The northward crustal escape of Asia from the Himalayan collisional zone turns eastwards and southeastwards along 95°–100°E longitude and defines the eastern Himalayan syntaxis. At the western Himalayan syntaxis, the Pamirs continue to move into central Asia, leading to crustal deformation and earthquakes that are largely accommodated by old EW or NW trending faults in the bordering areas between China, Mongolia, and Russia, and are restricted by the stable landmass northwest of the Tianshan-Altai-Baikal region. The subduction of the Philippine and Pacific plates under the Eurasian continent has generated a very long and narrow seismic zone along trenches and island arcs in the marginal seas while imposing only slight horizontal compression on the Asian continent that does not impede the eastward motion of eastern Asia. In the third dimension, there may be southeastward deep mantle flow beneath most of Eurasia that reaches the marginal seas and may contribute to extension along the eastern margin of Eurasia.

Formation and deformation processes of late Paleogene sedimentary basins in southern central Hokkaido, Japan: Paleomagnetic and numerical modeling approach

AbstractFormation and deformation processes of the late Paleogene sedimentary basins related to a strike–slip fault system in southern central Hokkaido are described by a combination of paleomagnetic study and numerical analysis. After correction of the Miocene counter‐clockwise rotation associated with back‐arc opening of the Japan Sea, paleomagnetic declination data obtained from surface outcrops in the Umaoi and Yubari areas show significant easterly deflections. Although complicated differential rotation is anticipated as a result of recent thrust movements, clockwise rotation in the study areas is closely linked with development of the Paleogene Minami‐naganuma Basin as a pull‐apart depression along the north–south fault system. Numerical modeling suggests that 30 km of strike–slip is required to restore the distribution and volume of the Minami‐naganuma Basin. The relative slip rate on the long‐standing fault system is about 10 mm/yr, which corresponds to global‐scale plate motion. It has inevitably caused regional rearrangement of the eastern Eurasian margin. A rotation field simulated by simplified dextral motion using dislocation modeling basically accords with the paleomagnetic data around the pull‐apart basin.

Mid-crustal horizontal shear zone in the forearc region of the mid-Cretaceous SW Japan arc, inferred from strain analysis of rocks within the Ryoke metamorphic belt

The strain geometry of a Cretaceous low- P/high- T metamorphosed accretionary complex in SW Japan is one of the key geological constraints in understanding the Cretaceous tectonics of the forearc region at the eastern margin of Eurasia. We studied the strain geometry of upper greenschist to lower amphibolite facies metamorphic rocks from the mid-Cretaceous low- P/high- T Ryoke metamorphic belt, which during the mid-Cretaceous were located at mid-crustal depths close to the volcanic front. Strain analysis focused on deformed radiolarian fossils in metachert and sandstone/chert clasts within metapelite. The maximum stretching direction ( X-direction) of the strain ellipsoid of the schistosity-forming deformation at the peak metamorphism is oriented E-W, and the XY-plane of the strain ellipsoid developed parallel to schistosity. The metachert and metapelite exhibit plane strain to general flattening strain that probably largely resulted from the schistosity-forming deformation. Based on the results of the previous strain analyses and this study, it reveals that the schistosity-forming deformation occurred under bedding-normal shortening, resulting in general flattening strain throughout the Ryoke metamorphic belt. The X-direction of the strain ellipsoid of the schistosity-forming deformation is oriented parallel to the length of the Ryoke metamorphic belt throughout the SW Japan arc. The schistosity formed parallel to bedding planes that developed horizontally in non-metamorphosed accretionary complexes of SW Japan, indicating horizontal XY-plane of the strain ellipsoid. These observations indicate the occurrence of horizontal shear deformation within the middle crust (∼15 km depth) at the time of the low- P/high- T metamorphism. The deep-seated horizontal shear zone and the shallower incipient Median Tectonic Line (MTL) fault system could result in a crustal-scale detachment fault in the forearc region of the mid-Cretaceous SW Japan arc. After the formation of the detachment, the left-lateral pull-apart forearc basins, Onogawa-Izumi Group distributed on the north side of the MTL and the southern marginal mylonite zone might be formed by the sinistral movement along the detachment fault.

Geochemical modeling of sedimentary rocks in the central Hokkaido, Japan: Episodic deformation and subsequent confined basin-formation along the eastern Eurasian margin since the Cretaceous

Late Cretaceous to Paleogene tectonic episodes around the eastern Eurasian margin are described utilizing one-dimensional (1D) basin modeling technique on the basis of organic maturation; vitrinite reflectance and T max parameter values of the Rock-Eval pyrolysis. Fine-grained marine sediment of the Yezo Supergroup in central Hokkaido is one of the most extensive Cretaceous constituents along the plate margin. To evaluate organic maturation within the Yezo Supergroup, a surface section (Hakkin-zawa) and a deep borehole (MITI Yubari) were selected, from which quite different maturation trends were obtained. Notably low and constant maturation was suggested throughout the thick Hakkin-zawa section, and it was confirmed on the basis of various biomarker analyses. It requires anomalously rapid burial, probably related to thrust-stacking or large-scale slumping, followed by prompt tilting/exhumation event. In sharp contrast, Late Cretaceous strata in the MITI Yubari do not indicate coeval tectonic disturbances in a short period. A wider view of the ancient convergent margin suggests that deformation of the forearc propagated westward during the Cretaceous. High maturation levels in the uppermost thrust sheet around the MITI Yubari are optimized in 1D geochemical modeling on the assumption that a thick missing Paleogene unit has been eroded out as a result of thrust-forming contraction scheme emerged during the Neogene. A similar Cenozoic burial pattern is adopted for the once tilted and exhumed Hakkin-zawa section in order to match the present maturation levels. Although such an active subsidence of a ‘forearc’ is generally interpreted as an effect of the subduction erosion, heavy mineral composition of the shallow marine to fluvial Paleogene implies uplift and exhumation of ultramafic rocks to the east (trench-side). Thus the confined basin-formation in central Hokkaido is to be understood in a different tectonic framework that may reflect transcurrent motions on the plate margin.