Asian Continental Margin Research Articles

Detrital Zircon U-Pb Geochronology and Petrography Analysis of the Haenam Basin

The detrital zircons of the Cretaceous Haenam basin were examined to determine how they responded to the subduction of the oceanic plate to the eastern Asian continental margin during the Cretaceous. A sinistral strike-slip movement caused by the oblique subduction of the Paleo-Pacific plate created these two nonmarine subbasins in the northern marginal Okcheon Belt of the Korean Peninsula. In the late Cretaceous, due to the orthogonal subduction of the oceanic plate, sediments were deposited in terrestrial environments with associated volcanism. A total of 247 ages obtained from 300 zircon grains reveal that the maximum depositional ages of the Haenam basin is ca. 79.5 0.17 Ma, respectively. The detrital zircon age spectra indicate that their basin fills were mainly derived from the adjacent basement rocks comprising Paleoproterozoic metamorphic rocks and Jurassic granitoids with a minor supply from the Paleozoic metasedimentary rocks in the western Gyeonggi Massif and Okcheon Metamorphic Belt.

Mesozoic Magmatic Arc in East Asian Continental Margin Triggered by Paleo-Pacific Plate Subduction: Constraints from Gravity and Magnetic Anomalies

Mesozoic Magmatic Arc in East Asian Continental Margin Triggered by Paleo-Pacific Plate Subduction: Constraints from Gravity and Magnetic Anomalies

Kinematic reconstruction of the Raohe accretionary complex, Northeast China: Integration of onshore geologic evidence and global plate model

The Raohe accretionary complex (RHC) is located at the eastern of northeast China and adjacent to Russian Far East. As a part of the Circum-Pacific Orogenic Belt, it is the unique region of the accretionary orogenic belt, which is associated with the subduction process of the Panthalassic-Pacific Plate (PPP). We synthesize the detrital zircon ages of terrigenous clastic rocks of the RHC and tectonic units along the East Asian Continental Margin (EACM) to clarify its provenance. Then we place the docking position of the RHC adjacent to the South China Block, and determine that the final accretion of the RHC occur during the later Late Jurassic (~150 Ma) according to combination for ages of stitching plutons and terrigenous clastic rock. Integrating with the published global-scale plate kinematic frame, we restored the pre-docking motion path of the RHC using the Gplates software. The reconstructed scenario shows that it is a long distance of at least 1000 km between the proto-RHC and continent margin when the basaltic volcanism occurred subaqueously within the abyssal basin of the PPP. This model also provides a probability that the proto-RHC and the proto-Yuejingshan accretionary complexes have the same drift history, before their simultaneous emplacement into the continental margin.

Provenance and depositional history of the Mesozoic Sanjiang Basin (northeastern China): implications for the uplift history of the northeastern Asian continental margin

AbstractThe uplift history of northeastern Asian continental margin has been the subject of much debate. The Sanjiang Basin is an ideal area to investigate the uplift history of this margin. We present new data from sandstone petrology, whole-rock geochemistry, U–Pb geochronology and Hf isotopy of detrital zircons to trace the provenance of the basin and to unravel the uplift history of the northeastern Asian continental margin. We investigated the chemical and plagioclase indices of alteration and index of compositional variability (CIA, PIA and ICV, respectively) in samples from two formations. The geochemical proxies indicate that source rocks were subjected to an intensified weathering process. Based on the high SiO2/Al2O3 values and low K2O, we infer that the two basinal strata experienced silicification and insignificant potassium metasomatism. Sandstone petrography is indicative of low degrees of sedimentary sorting, suggesting a proximal deposition. The provenance fingerprints of light rare earth, high-field-strength and transition metal elements indicate that parts of the provenance included recycled sediments, and that first-cycle sediments were derived mainly from felsic rocks with a minor contribution from intermediate and mafic rocks. The combined detrital zircon U–Pb ages and Hf isotopic results constrain the contributions of different source terranes over time. The provenance results, in combination with seismic profile interpretations from the Sanjiang Basin, suggest that the northeastern Asian continental margin experienced at least three stages of uplift, which were driven by subduction initiation of the Paleo-Pacific Ocean during the Jurassic Period, a plate motion change during the Early Cretaceous Epoch, and a shallowing of the slab dip angle during the Late Cretaceous Epoch.

A Plate‐Mantle Convection System in the West Pacific Revealed by Tertiary Ultramafic‐Mafic Volcanic Rocks in Southeast China

AbstractCenozoic volcanic rocks in the East Asian continental margin usually form because of asthenosphere upwelling and resultant decompression melting, and their origins are important in deciphering mantle geodynamics attributed to intraplate volcanism. In this study, Oligocene basanite‐tephrite rocks were found in continental interior outcrops, and Miocene to Pliocene basanite, alkaline basalt, and olivine tholeiite were abundant in coastal areas in Zhejiang, Southeast China. Episodic magmatic activities appeared to migrate oceanward with an ∼100‐km spatial gap, which was temporally and spatially synchronized with repeated retreat and rollback of the Tertiary westward subducting Pacific slab. All rocks had OIB‐like trace‐element patterns, but the basanite‐tephrite rocks contained higher LILE, HFSE, and LREE than the alkaline basalt and olivine tholeiite. Geochemical modeling indicated the basanite‐tephrite and the alkaline and olivine tholeiite basalts had low‐degree (7%–15%) and slightly higher degree (20%–30%) partial melting of heterogeneous pyroxenite sources, respectively. In addition, their magma sources showed a progressive weakening of carbonatite metasomatism and decreasing melting depths. Those features were tightly associated with punctuated upwelling of asthenospheric materials, most likely metasomatized by the stagnant Pacific slab‐derived silica/carbonatite melts. A plate‐mantle convection system is proposed in which asthenosphere materials pulled by the subducting Pacific slab explain the intraplate volcanism in East Asia as asthenospheric upwelling, which formed by escaping from either slab tears or slab front edges in the mantle transition zone.

Pressure-temperature evolution of andalusite-kyanite-sillimanite-bearing pelitic schists from Nishidohira, southern Abukuma Mountains, Northeast Japan: Implications for Cretaceous rapid burial and exhumation in the Northeast Asian continental margin

We provide new petrological data of andalusite-kyanite-sillimanite-bearing pelitic schists from Nishidohira in the southern Abukuma Mountains, Northeast Japan. Investigation of the pressure-temperature (P–T) evolution of amphibolite-facies metamorphism in the region described the timing and tectonics of early Cretaceous metamorphic processes. The pelitic schists are composed of garnet, biotite, muscovite, quartz, plagioclase, K-feldspar, ilmenite, and graphite, in addition to Al2SiO5 polymorphs. The polymorphs occur predominantly as rounded or rhombic minerals, and are often composite aggregates. This suggests that the aggregates are pseudomorphs of andalusite (chiastolite). The andalusite, which rarely contains xenoblastic kyanite, is subidioblastic to xenoblastic, and partly replaced by kyanite, sillimanite, and muscovite. The matrix kyanite is coarse grained and porphyroblastic, surrounded by muscovite, which is in turn replaced by coarse-grained sillimanite. Fibrolite occurs as the latest phase around kyanite and sillimanite, or replacing muscovite. These textures suggest a crystallization sequence of kyanite > andalusite > sillimanite > kyanite > sillimanite > fibrolite for the Al2SiO5 polymorphs. Phase equilibria modeling of a prograde mineral assemblage (garnet + plagioclase + K-feldspar + biotite + kyanite + muscovite + ilmenite + quartz) yielded a P–T condition of 4.8–6.7 kbar and 530–570 °C. The maximum-pressure condition prior to the peak stage has been inferred as ~9 kbar (at 600 °C) based on Raman elastic geobarometry. Garnet-biotite-plagioclase-sillimanite/fibrolite-quartz assemblages yielded the peak condition of 4.5–6.9 kbar and 630–660 °C, which is within the stability field of the inferred peak assemblage (garnet + plagioclase + K-feldspar + biotite + sillimanite + muscovite + ilmenite + quartz). Our results indicate a clockwise P–T evolution with rapid burial (from the stability field of andalusite to ~9 kbar) and subsequent rapid erosion and exhumation at ~6 kbar. Electron microprobe Th-U-PbTotal dating of monazites intergrowing with the matrix biotite yielded a metamorphic age of 139 ± 24 Ma. The high-pressure amphibolite-facies metamorphism recorded in the rocks is likely associated with the thrusting of an allochthonous crustal block (including the protolith of the Hitachi metamorphic rocks) upon the Nishidohira metamorphic rocks, with serpentinite as lubricant. This process could be related to large-scale sinistral strike-slip movement and complex metamorphic processes in Northeast Asian continental margin during late Jurassic to early Cretaceous.

Permian to Cretaceous tectonic evolution of the Jiamusi and Songliao blocks in NE China: Transition from the closure of the Paleo-Asian Ocean to the subduction of the Paleo-Pacific Ocean

Zhangguangcailing Orogen constitutes a significant part of the Eastern Asian Continental margin. Its evolution was constrained by the oceanic consumption processes between the Jiamusi and Songliao blocks during late Paleozoic and Mesozoic time, resulted from the closure of the Paleo-Asian Ocean and subsequent Paleo-Pacific Ocean subduction. Considerable and continuing controversy has surrounded when, where, and how the ocean was subducted and the two blocks were eventually amalgamated, which plays a crucial role in understanding the tectonic transition from the closure of the Paleo-Asian ocean to the subduction of the Paleo-Pacific Plate, especially the initial formation of the Mesozoic foreland basin of the Songliao Basin. This review synthesizes the most recent data and geologic evidence that place critical constraints on the evolution of this oceanic domain.Available data and geological evidence indicate that there exists a stable micro-continent consisting of several microblocks of NE China and adjacent area within the Paleo-Asian Ocean regime during the Late Paleozoic. In the early Permian, voluminous magmatism occurred in an extensional setting induced by the subduction of the Paleo-Asian Ocean to the south and the Mongol-Okhotsk Ocean to the northwest. As the crustal extension continued, a continental rifting was initiated and further developed into an oceanic domain (the Heilongjiang Ocean or the Mudanjiang Ocean) separating the Jiamusi and Songliao blocks during Permian and Early Triassic time. Later, an active continental margin was situated along the Zhangguangcailing Orogen due to the subduction of the Heilongjiang oceanic plate in the Late Triassic, followed by a change in the regional stress regime controlled by Paleo-Pacific plate subduction. The high-pressure metamorphism occurred synchronously with the subduction/collision between the Jiamusi and Songliao blocks, forming the Heilongjiang Complex. The oceanic closure and collision events were sequentially followed by the A-type granitic magmatism, which commenced in the earliest Cretaceous.

Seismic Expression of Paleogene Talangakar Formation - Asri & Sunda Basins, Java Sea, Indonesia

The Asri and Sunda Basin are located offshore Southeast Sumatra under the SE Sumatera Production Sharing Contract (PSC). Currently, Pertamina Hulu Energi OSES operates in the Asri and Sunda Basins, known as Southeast Sumatra (SES) Block. As one of Indonesian prime oil and gas producing area in the past 50 years, these basins have produced cumulatively more than 1,500 MMBOE with an average of 90,000 BOPD. Both Asri and Sunda Basins are part of a series of Cenozoic half grabens developed on the Asian continental margin, that have occupied a retro arc setting since early Neogene times.Hydrocarbon exploration activities in the offshore southeastern Sumatra began with a signed agreement of Contract Sharing between IIAPCO (Independent Indonesian American Petroleum Company) and Pertamina on 6 September 1968.The contract area included the offshore areas east of Sumatra and south of Bangka - Belitung Islands, covering an area of 124,000 km2. After the final relinquishment in 2008, the current PSC boundary covers an area of 5,851 km2.This paper presents the results of integrated petroleum geosciences framework, its implications for Oligo-Miocene reservoirs and hydrocarbon prospectivity of the basins using near field exploration approaches. A four-fold division comprising rift initiation, rift climax, immediate post rift and late-post rift stages of basin evolution characterize the basin filling. Seismic expression of these kinematic units gives an idea about the linkage of their deposition with different stages of rift evolution. The present study has also identified a new potential play concept in Oligo -Miocene reservoirs.

Syn‐Subduction Strike‐Slip Faults Shape an Accretionary Orogen and its Provenance Signatures: Insights From Sikhote‐Alin in NE Asia During the Late Jurassic to Early Cretaceous

AbstractSedimentary rocks provide key information on source‐to‐sink systems and the regional expression of plate tectonics. The abruptly changing provenance signature of sedimentary rocks is commonly ascribed to changes in the source regions, such as continental collision events, the rise of mountain chains, reorganization of river systems and climatic change. However, moving the positions of the sinks (depocenters) geographically can also change the provenance record, but this is rarely considered. Typically, large‐scale strike‐slip faults can transport depocenters for long distances, thus altering the provenance information. In this study, we present a systematic investigation of the provenance changes in the Sikhote‐Alin orogenic belt in the Russian Far East to evaluate this process. The Central Sikhote‐Alin Fault is a margin‐parallel strike‐slip fault that developed along the NE Asian continental margin from the late Mesozoic. Our new detrital zircon U‐Pb data, together with a compilation of geochronological data, indicate two different types of provenance signatures available: the sediments to the west of the fault were mainly derived from the adjacent Central Asian Orogenic Belt, but to the east of the fault, they were mainly supplied from the Korean Peninsula, which is several hundred kilometers away. It is proposed that these different sedimentary rocks were juxtaposed by syn‐ and post‐subduction sinistral displacements along the fault, and were not related to local variations in the source regions. Thus, understanding the shifting positions of depocenters is important when decoding provenance change because lateral displacement is critical for reconstructing regional paleogeography along oblique convergent plate margins.

Cretaceous large-scale metal accumulation triggered by post-subductional large-scale extension, East Asia

Effective exploration for ore deposits usually seeks distinct conditions within fundamentally prospective geological settings that might lead to enhanced ore-forming processes. Large-scale metal accumulation to form ore deposits is usually associated with specific tectonic events. Many studies have found that significant mineralization is genetically related to plate subduction and accretion/collision tectonic regimes. Here we describe how an active continental margin extensional setting can also define an important tectonic environment for the development of a world-class regional metallogenic province subsequent to subduction and compressional stresses. The extensive and diverse group of mineral deposits formed from 145 to 70 Ma along the eastern Asian continental margin (EACM), from northern Vietnam, through eastern China, Korea, Japan, and to the Sikhote-Alin region of Far East Russia, represents a good example of a regional extension-related metallogenic province. The varied mineral deposits with distinct fluid and metal sources occur in a series of NE- or NNE-trending pull-apart basins and metamorphic core complexes, as well as along associated transtensional faults, reflecting the controls of an extensional regime. A new tectono-metallogenic model for the Cretaceous ore-forming events of eastern Asia is proposed to highlight the exploration potential in such tectonic settings throughout the world.

Nature and timing of anatectic event of the Hida Belt (Japan): Constraints from titanite geochemistry and U-Pb age of clinopyroxene-bearing leucogranite

The Hida Belt, central Japan, is a continental fragment, which was once a part of the crustal basement of the East Asian continental margin. It consists mainly of Permo-Triassic granite-gneiss complexes with both syn-to-late-metamorphic migmatite or granite bodies. Clinopyroxene-bearing leucogranite, locally called as ‘Inishi’-type migmatite, occurs characteristically in the migmatite zone associated with amphibolite and marble. The leucogranite is characterized by the presence of coarse-grained diopside–hedenbergite series clinopyroxene and titanite in plagioclase-dominated matrix. Clinopyroxene contains abundant calcite inclusions. Euhedral titanite with high Th/U ratios (~2.8–7.8) and REE contents (~4514–14,069 μg/g) contains polycrystalline ‘granitic’ inclusions. Those mineralogical features indicate the involvement of carbonate during anatexis. Considering a nominal pressure of 0.4–0.7 GPa of the Hida gneiss, Zr-in-titanite thermometry yields a temperature of 730–810 °C and 770–850 °C at aTiO2 = 0.5 and 1, respectively. The titanites show highly variable U/Pb (238U/206Pb = 15.0–24.0) and Pb (207Pb/206Pb = 0.172–0.419) isotope ratios, and the scattered trend follows a discordia line with a lower intercept at 225.4 ± 1.9 Ma. This titanite age would represent the timing of regional anatexis that have occurred in a later stage of the regional metamorphism of the Hida Belt. A high apparent thermal gradient at middle crustal levels might have been caused by regional extension.

Geochronological and Paleomagnetic Constraints on the Lower Cretaceous Dalazi Formation from the Yanji Basin, NE China, and its Tectonic Implication

The Lower Cretaceous Dalazi Formation in the Yanji Basin, eastern Jilin Province is of particular interest because it contains key fresh water fossil taxa, oil and gas resources, a potential terrestrial Albian–Cenomanian boundary, and regional unconformities. However, the lack of a precise chronology for the non-marine strata has precluded a better understanding of the regional stratigraphic correlation and terrestrial processes. Here, we report magnetostratigraphic and U–Pb geochronologic results of a sedimentary sequence from the Xing’antun section in the Yanji Basin. Thirty-two zircons from the tuff sample were analyzed by secondary ion mass spectrometry (SIMS); the U–Pb zircon dating method yielded a weighted mean of 206Pb/238U age of 105.7 ± 0.8 Ma (2σ, internal error). Paleomagnetic results show that the Dalazi Formation is of normal polarity, which was correlated to the early chron C34n constrained by the SIMS U–Pb zircon geochronologic data, further demonstrating that the terrestrial sedimentary sequence of the upper Dalazi Formation is of late Albian age. The established geochronologic framework allows the regional correlation of the Dalazi Formation in the Yanji Basin to the strata from other terrestrial sequences in northeastern China. The similar geodynamic and geologic background between the Yanji Basin and other terrestrial rift basins in northeastern China suggests that the unconformity between the Dalazi and Longjing formations may represent syn-rift and post-rift stages in the Yanji Basin, and thus the switch from extension to contraction during the mid-Cretaceous, precisely constrained to ~106–101 Ma based on our new chronology and previously published high-precision U–Pb dating of the lower Longjing Formation. It is most likely attributable to the docking of the west Pacific plate along the East Asian continental margin.

Extensive mineralization in the eastern segment of the Xingmeng orogenic belt, NE China: A regional view

• There are four types of EXOB deposits: porphyry , skarn , hydrothermal vein, and epithermal types. • There are five stage mineralizations in EXOB: 480–440, 300–220, 200–165, 155–130, and 125–100 Ma. • The five metallogenic periods correspond to different geological setting. The eastern segment of the Xingmeng orogenic belt (EXOB) hosts numerous Mo, polymetallic Ag, Pb–Zn , Sn, Cu, Au (Ag), and rare-earth element (REE) deposits and is one of the most important polymetallic-ore regions globally. The mineralization type includes i) porphyry Cu (Mo, Au), Mo (Cu, Pb–Zn, W), Sn (Cu–Ag) deposits, ii) skarn Cu–Au, polymetallic Pb–Zn and Fe–Sn deposit, and iii) veined polymetallic Ag, Pb–Zn, Sn, Cu deposits and W, Mo deposits, with an REE deposit related to magmatic hydrothermal fluid; and iv) epithermal Ag and Au (Ag, Te) deposits, respectively. The mineralization-related rocks in the EXOB are mainly high-K calc-alkaline granites together with minor alkaline granites that were derived from the melting of juvenile crust. The mineralizations in the EXOB are typically related to magmatic-hydrothermal events. In this study we attempt a systematic analysis of the temporospatial distribution of ore deposits in the EXOB, in order to constrain the geodynamic setting of mineralization. Based on available geochronological data, we suggest five mineralization stages for the EXOB at 480–440, 300–220, 200–165, 155–130, and 125–100 Ma, respectively. The porphyry Cu–Mo mineralization occurred during the early Paleozoic (480–440 Ma). The extensive Mo (W, Sn), and Ag (Pb–Zn) polymetallic mineralization was formed during the Mesozoic. The Ordovician–Triassic mineralization (480–220 Ma) is related to the Paleo-Asian Ocean tectonism and developed in an island arc setting. The Permian–Triassic mineralization (300–220) was formed in a post-collisional tectonic setting related to the development of Hegenshan suture. The early–middle Jurassic deposits (200–165 Ma) are related to the subduction of the Paleo-Pacific oceanic plate on the East Asian continental margin. The Late Jurassic–Early Cretaceous deposits (155–130 Ma) were formed in an extensional setting related to rollback of the Paleo-Pacific Plate. The Jurassic–Cretaceous mineralization (200–100 Ma) in the northern segment of the Da Hinggan Mountains is associated with the subduction and post-collisional tectonic setting of the Mongol–Okhotsk oceanic plate. The latest stage of epithermal deposits (125–100 Ma) were developed in a continental margin back-arc setting associated with subduction of the Paleo-Pacific Plate beneath the Eurasia.

Transition from a passive to active continental margin setting for the NE Asian continental margin during the Mesozoic: Insights from the sedimentary formations and paleogeography of the eastern Jiamusi Massif, NE China

Abstract The Mesozoic tectonic evolution of the NE Asian continental margin has received much attention in recent years. However, previous studies focused mainly on the petrogenesis of igneous rocks and their relationship with Mesozoic tectonics, and there have been few studies of the Mesozoic sedimentary formations of the NE Asian continental margin. We combined zircon U-Pb ages with Hf isotopic and biostratigraphic data to reconstruct the Mesozoic paleogeography of the NE Asian continental margin. The results indicate that Mesozoic strata of the eastern Jiamusi Massif, NE China, include the Upper Triassic Nanshuangyashan Formation (Norian), Lower Jurassic volcanic rocks, and Lower Cretaceous Longzhaogou Group. The Upper Triassic Nanshuangyashan Formation consists of a suite of alternating marine and terrestrial sedimentary rocks with abundant fossils that formed in a passive continental margin setting. The Lower Jurassic strata comprise a suite of calc-alkaline volcanic rocks that include basaltic andesites, andesites, and rhyolites that formed in an active continental margin setting related to initial subduction of the Paleo-Pacific Plate beneath Eurasia. The Lower Cretaceous Longzhaogou Group belong to alternating marine and terrestrial sedimentary formations that formed in an active continental margin setting related to subduction of the Paleo-Pacific Plate. Here, we integrate these data to reconstruct the Mesozoic tectonic history of the NE Asian continental margin, which comprises a Late Triassic passive continental margin, the initiation of subduction of the Paleo-Pacific Plate in the Early Jurassic, and westward subduction and rollback of the Paleo-Pacific Plate in the Early Cretaceous.

Thrust duplexing and transpression in the Yanshan Mountains: Implications for early Mesozoic orogenesis and decratonization of the North China Craton

AbstractThe configuration and kinematics of fold‐thrust belts are essential for clarifying the plate boundary conditions through which ancient orogenic belts evolved. Temporal and spatial variations in extension versus contraction within convergent plate margins are related to changes in plate kinematics and subduction geometry. The Yanshan Mountains, located along the northeastern margin of the North China Craton and bounded by the Central Asian Orogenic Belt to the north and the Pacific subduction zone to the east, underwent multiple phases of intra‐continental deformation during the Mesozoic. The issue of whether early Mesozoic deformation was extensional or contractional has remained controversial. New regional structural mapping and detailed sedimentological investigations, coupled with age constraints provided by zircon U‐Pb geochronology, provide sufficient temporal and spatial resolution to show that shortening and thrust‐top basin systems dominated during the early Mesozoic. Our results show that the Dengzhangzi Basin within the Lingyuan fold‐thrust belt developed during the construction of a major Late Triassic Chapeng antiformal‐stack duplex system, with episodic, east‐vergent forward‐breaking thrusts and subsequent ‘beheading’ by out‐of‐sequence, top‐to‐east thrusting in the latest Triassic or earliest Jurassic. The neighbouring Guojiadian Basin in the east was generated during top‐to‐SSE sinistral transpression along the Niuyingzi thrust system in the Middle to earliest Late Jurassic. These results support a new model of early craton decratonization involving (a) thrusting along the northern margin of the North China Craton and (b) transpressional deformation in the east driven by the Central Asian Ocean collision and flat‐slab subduction of the Palaeo‐Pacific Plate beneath the East Asian continental margin. Our findings suggest that large‐scale shortening and crustal/lithospheric thickening, triggered by flat‐slab subduction in the North China Craton, were fundamental components of craton decratonization. Subsequent slab roll‐back during the Cretaceous led to lithospheric thinning and crustal extension in the North China Craton.

Phylogenomic Analysis of Ultraconserved Elements Resolves the Evolutionary and Biogeographic History of Segmented Trapdoor Spiders.

The segmented trapdoor spiders (Liphistiidae) are the sole surviving family of the suborder Mesothelae, which forms the sister lineage to all other living spiders. Liphistiids have retained a number of plesiomorphic traits and their present-day distribution is limited to East and Southeast Asia. Studying this group has the potential to shed light on the deep evolutionary history of spiders, but the phylogeny and divergence times of the family have not been resolved with confidence. We performed phylogenomic and molecular dating analyses of 2765 ultraconserved element loci from 185 liphistiid taxa. Our analyses show that the crown group of Liphistiidae appeared in the mid-Cretaceous at 102 Ma (95% credibility interval 92-113 Ma), but it was not until the Neogene that much of the diversification within the family occurred in mainland Southeast and East Asia. This diversification was coincident with tectonic events such as the extension of the East Asian continental margin, as well as geological upheavals in Indochina induced by the collision between India and Asia. Our study highlights the important role of major tectonic events in shaping the evolutionary history, present-day diversity, and geographical distribution of mesothele and liphistiid spiders. [biogeography; concatenation; Liphistiidae; molecular dating; summary coalescent; UCEs.].

Paleocene–early Eocene post-subduction magmatism in Sikhote-Alin (Far East Russia): New constraints for the tectonic history of the Izanagi-Pacific ridge and the East Asian continental margin

New isotopic, geochemical and geochronological data justify the widespread occurrence of middle Paleocene to early Eocene (60–53 Ma) post-subduction felsic magmatism across the entire Sikhote-Alin territory (southeastern Russia), conform with previous observations in Northeast China, the southern Korean Peninsula, and the Inner Zone of Japan. This igneous activity in East Asia coincided with the reactivation (after tectonic quiescence between ~93–60 Ma) of left-lateral strike-slip displacements along the Tan-Lu and Central Sikhote-Alin faults and with the post-60 Ma cessation of subduction/accretion recorded in the Shimanto belt of SW Japan. The Sikhote-Alin post-subduction igneous A-type rocks present diverse mineralogical and geochemical features that suggest interactions of the subducting plate with anhydrous mantle upwelling through slab tears in the continental margin. The middle Paleocene–early Eocene magmatism is not related to subduction but is synchronous with strike-slip tectonics and the termination of accretionary prism development, suggesting a shift in tectonic regime from oceanic plate subduction at a convergent margin to parallel sliding and initiation of a transform continental margin. These new observations are inconsistent with the current tectonic model of 60–50 Ma Izanagi-Pacific ridge subduction beneath East Asian continental margin.

Slab‐fluid metasomatism in the Early Paleozoic forearc mantle deduced from the Motai serpentinites, South Kitakami Belt, northeast Japan

AbstractThe present study examines the petrology and geochemistry of the Early Paleozoic Motai serpentinites, the South Kitakami Belt, northeast Japan, to reveal the subduction processes and tectonics in the convergent margin of the Early Paleozoic proto‐East Asian continent. Protoliths of the serpentinites are estimated to be harzburgite to dunite based on the observed amounts of bastite (orthopyroxene pseudomorph). Relic chromian spinel Cr# [=Cr/(Cr + Al)] increases with decreasing amount of bastite. The compositional range of chromian spinel is similar to that found in the Mariana forearc serpentinites. This fact suggests that the protoliths of the serpentinites are depleted mantle peridotites developed beneath the forearc regions of a subduction zone. The Motai serpentinites are divided into two types, namely, Types 1 and 2 serpentinites; the former are characterized by fine‐grained antigorite and lack of olivine, and the latter have coarse‐grained antigorite and inclusion‐rich olivine. Ca‐amphibole occurs as isolated crystals or vein‐like aggregates in the Type 1 serpentinites and as needle‐shaped minerals in the Type 2 serpentinites. Ca‐amphibole of the Type 1 serpentinites is more enriched in LILEs and LREEs, suggesting the influence of hydrous fluids derived from slabs. By contrast, the mineral assemblage, mineral chemistry, and field distribution of the Type 2 serpentinites reflect the thermal effect of contact metamorphism by Cretaceous granite. The Ca‐amphibole of the Type 1 serpentinites is different from that of the Hayachine–Miyamori Ophiolite in terms of origin; the latter was formed by the infiltration of melts produced in an Early Paleozoic arc–backarc system. Chemical characteristics of the Ca‐amphibole in the ultramafic rocks in the South Kitakami Belt reflect the tectonics of an Early Paleozoic mantle wedge, and the formation of the Motai metamorphic rocks in the forearc region of the Hayachine–Miyamori subduction zone system, which occurred at the Early Paleozoic proto‐East Asian continental margin.

The δ13C–δ18O variations in marble in the Hida Belt, Japan

AbstractMarble has a great potential to understand a history of various geological events occurring during tectonic processes. In order to decode metamorphic–metasomatic records on C–O isotope compositions of marble at mid‐crustal conditions, we conducted a C–O–Sr isotope study on upper amphibolite‐facies marbles and a carbonate–silicate rock from the Hida Belt, which was once a part of the crustal basement of the East Asian continental margin. Carbon and oxygen isotope analyses of calcite from marbles (Kamioka area) and a carbonate–silicate rock (Wadagawa area) show a large variation of δ13C [VPDB] and δ18O [VSMOW] values (from −4.4 to +4.2 ‰ and +1.6 to +20.8 ‰, respectively). The low δ13C values of calcites from the carbonate–silicate rock (from −4.4 to −2.9 ‰) can be explained by decarbonation (CO2 releasing) reactions; carbon–oxygen isotope modeling suggests that a decrease of δ13C strongly depends on the amount of silicate reacting with carbonates. The occurrence of metamorphic clinopyroxene in marbles indicates that all samples have been affected by decarbonation reactions. All δ18O values of calcites are remarkably lower than the marine‐carbonate values. The large δ18O variation can be explained by the isotope exchange via interactions between marble, external fluids, and/or silicates. Remarkably low δ18O values of marbles that are lower than mantle value (~+5 ‰) suggest the interaction with meteoric water at a later stage. Sr isotope ratios (87Sr/86Sr = 0.707255–0.708220) might be close to their protolith values. One zircon associated with wollastonite in a marble thin‐section yields a U–Pb age of 222 ± 3 Ma, which represents the timing of the recrystallization of marble, triggered by H2O‐rich fluid infiltration at a relatively high‐temperature condition. Our isotope study implies that the upper amphibolite‐facies condition, like the Hida Belt, might be appropriate to cause decarbonation reactions which can modify original isotope compositions of marble if carbonates react with silicates.

АУТИГЕННЫЕ МИНЕРАЛЫ ВУЛКАНОГЕННО-ОСАДОЧНЫХ ПОРОД ПАЛЕОЗОЙСКО-КАЙНОЗОЙСКОГО ВОЗРАСТА ЮЖНОГО ПРИМОРЬЯ

Authigenic minerals in volcanogenic-sedimentary and sedimentary rocks of Southern Primorye from Permian to Miocene time have been studied. Corrensite, rectorite, highly ordered mixed-layer differences of the chlorite-smectite (corrensite-like) and mica-smectite (rectorite-like) types, mica, vermiculite-like differences (?), chlorite, defective chlorite, kaolinite, smectite, calcite, and zeolites were found. Such a set of minerals indicates that the sedimentary layer in the studied sedimentary basins could be three to five kilometer thick, and the temperature of their formation is more than 150°C. The formation of the Lower Cretaceous and Paleocene sedimentary strata has similar features, and probably proceeded first in a shallow sea basin setting of the continental margin (rift stage), sometimes under conditions close to evaporitic (presence of corrensite?), with a frequent change of the facial situation from shallow to deep sedimentation, episodic supplies of volcanic material, and gradual deepening of sedimentation basins. It can be assumed that in Early Cretaceous and Paleogene times, a series of discrete sedimentation basins along the northeastern Asia continental margin developed in a single mineralogical, tectonic, and sedimentological regime of crustal extension: minerals accumulated in the sediments, which in the process of epigenesis transformed in the following directions: a) smectite-rectorite-mica; and b) smectite (palygorskite, sepiolite?) - corrensite-chlorite. In the studied sedimentary complexes three mineralogical «layers» are distinguished: 1) chlorite-mica – mica-chlorite (Permian - Cretaceous); 2) transitional from chlorite and mica to smectite - developed are corrensite, rectorite and highly ordered mixed-layer corrensite-like and rectorite-like minerals (Cretaceous - Paleocene-Eocene), and 3) smectite (from Oligocene to the present).