Gyeongsang Basin Research Articles

Preliminary Study on the Jinju Formation in the Gyeongsang Basin to Evaluate Host Rock for High-level Radioactive Waste Geological Disposal: Focusing on Lithological and Mineralogical Characteristics

Preliminary Study on the Jinju Formation in the Gyeongsang Basin to Evaluate Host Rock for High-level Radioactive Waste Geological Disposal: Focusing on Lithological and Mineralogical Characteristics

Non-marine ostracods from the Cretaceous Banyaweol and Geoncheonri formations (Gyeongsang Basin, Korea) in the collections of the Geological Museum, KIGAM: Taxonomy, biostratigraphy, palaeobiogeography and palaeoenvironment

ABSTRACT The Geological Museum of the Korea Institute of Geoscience and Mineral Resources (KIGAM) houses a large collection of microfossils. From these collections, 14 species belonging to six genera of non-marine ostracods from the Banyaweol and Geoncheonri formations of the Gyeongsang Basin, South Korea are identified: Cypridea sp. 1, C. sp. 2, C. sp. 3, C. sp. 4, C. sp. 5, C? sp. 6, Lycopterocypris triangularis, L. cf. profunda, Candona declivis, C. sp. Cypria cf. elata, Djungarica sp. Ziziphocypris costata and Z. rugosa. The biostratigraphic correlation based on L. triangularis and C. declivis from the Cenomanian Banyaweol Formation suggests that these species appeared at a much earlier stage than the previous records from China. Ostracods of the Banyaweol Formation may represent a transitional fauna during the mid-Cretaceous faunal turnover in East Asia. However, L. cf. profunda and C. cf. elata suggest a Turonian – Santonian age (probably extend upwards into Campanian) for the Geoncheonri Formation. Many non-endemic species of both formations indicate that these faunas were linked with ostracod faunas of East Asia, providing evidence of active faunal exchanges and migration. The palaeoenvironments of both formations are interpreted as ephemeral waterbodies and unstable shallow-littoral zone of lakes based on faunal compositions.

Construction of Exploration Data for Greenhouse Gas Geologic Storage: Focusing on Geological Cross-section Data

In this study, the most basic data, underground geological structure data, that is, geological cross-section data, were established to select a candidate site for underground storage of greenhouse gases based on AI. As a target area, the Gyeongsang Basin, where a large amount of sedimentary rocks are distributed, was selected as the greenhouse gas can be stored most effectively in sedimentary rocks. To this end, the acquisition and edit step, the refinement step, and the labeling step were carried out in the order of raw data collection, source data and labeling data construction to construct the geological cross-section data. This data can be downloaded through the AI hub site (https://aihub.or.kr/aihubdata/data/view.do?curr Menu=115&topMenu=100&aihubDataSe=realm&dataSetSn=71390) operated by the Korea Institute for Intelligent Information Society Promotion.

Constraining mixing and mingling processes from zoned magmatic enclaves: An example from the Taejongdae granite in Busan, Korea

Zoned magmatic enclaves (MEs) within granitic intrusions provide valuable insights into the pre-intrusive processes involved in the formation of granitic magmas. This study focuses on the Taejongdae granite in Busan, Korea, which features uniquely zoned MEs, and aims to constrain the processes involved in their formation. Through a whole-rock geochemistry analysis, SHRIMP U-Pb zircon dating, and Lu-Hf isotope data, we investigated the geochemical characteristics and origin of the two magmas responsible for the zoned MEs (that is, dioritic and host granite magmas). Our findings have shown that the host granite aligns with the Bulguksa granites in the Gyeongsang Basin, which represent the I-Type magma resulting from the fractionation of magma generated through partial melting of the continental crust. In contrast, the dioritic magma exhibits a low K2O composition, high Sr content, and a positive-to-no Eu anomaly, indicating its formation via amphibole-dominated fractional crystallization under different pressure–temperature conditions. Both magmas share the same Cretaceous age and Hf ratios. The zoned MEs in the study area provide compelling evidence of magma mixing processes that occur between the outer dioritic zone and mafic core of the enclaves. However, the relationship between the outer dioritic zone and the host granite suggests limited interaction between the enclaves and host granite. This implies that mixing within the zoned enclaves occurred prior to enclosure in the host granite. By highlighting the significance of zoned magmatic enclaves, this case study advances our understanding of the pre-intrusive processes involved in the formation of granitic magmas, including magmatic mixing, mingling, and fractionation. These results offer important insights into the origin and geochemical characteristics of the two magmas and their interactions, thus contributing to a broader understanding of crustal evolution in subduction-related settings.

SHRIMP U–Pb ages of the detrital zircons from the Cretaceous Ullyeonsan Formation, SE Korea: A small piece for stratigraphic correlation in eastern Asia

To constrain the maximum depositional age of the Ullyeonsan Formation in the Yeongyang Subbasin, southeast Korea, detrital zircon U–Pb age determination was conducted using high-resolution secondary ion mass spectrometry (HR-SIMS). The U–Pb populations of detrital zircons from the Ullyeonsan Formation show a broad range of ages from the Archean to early Cretaceous and diverse patterns depending on the temporal and spatial distribution of sedimentation. The difference in location-dependent age distribution indicates that sediments originated from adjacent areas and source rocks were exposed during the sedimentation period of the Ullyeonsan Formation. The youngest detrital zircon cluster of the Ullyeonsan Formation yields concordia age of 110.4 ± 2.5 Ma (n = 5, MSWD = 1.2), indicating the maximum depositional age. Therefore, the Ullyeonsan Formation began to be deposited from the Albian period, and the Yeongyang Subbasin started to form around the same time as the deposition of the Hayang Group in other subbasins of the Gyeongsang Basin (e.g., Miryang and Uiseong).

Tectonic evolution of the Yangsan Fault, SE-Korea, by gravity field interpretation

SUMMARY To estimate the tectonic displacement history of the Yangsan Fault (YSF) in the Cretaceous Gyeongsang Basin (GB), SE-Korea, gravity field measurements were implemented and interpreted. A total of 8000 ground-measured gravity data were compiled and deployed to calculate the Bouguer and residual gravity field. The residual field shows that the mean value in the southern segment of the YSF is very similar to that of the northern segment, meanwhile that of the central segmentation is significantly lower than those of the other segmentations. This implies that the crustal structure and petrophysical properties of the central segment differ significantly from those of the other segments. Applying dip curvature analysis to enhance the image of the residual gravity field, we find that (1) the Gaum fault system (GF) in the western and middle part of the GB extends up to the YSF, where A-type granite outcrops are observed. (2) The middle- and eastern segments of the GF that formed before tectonic motion of the YSF are revealed in turn in areas east of the YSF, which were now covered by the younger sedimentary basin. (3) The middle segment of the GF which occupies between the YSF and the Yeonil tectonic line (YTL) has been displaced approximately 25 km southward from the main GF trace. (4) The easternmost segment is revealed buried in the eastern side of the YTL, which is displaced southward about 18 km from the main GF trace. Comparing all the available geoscientific data sets, we conclude that the right-lateral tectonic movement of the YSF, and the deeply buried middle segment of the GF remained stationary, while the most eastern segment of the GF is reversely moved to the north through left-lateral strike movement of the YTL.

Zircon U–Pb ages and Lu–Hf isotopes of the Jurassic Granites on the east coast of the Korean Peninsula and Southwest Japan: Petrogenesis and tectonic correlation between the Korean Peninsula and Japanese Islands

Jurassic granitoids in the Korean Peninsula and Japanese Islands are key for deciphering the tectonic evolution of East Asia and the tectonic correlation between them. Zircon U–Pb age dating of seven granitoids in Southwest Japan gives 247.3 ± 1.8 and 192–191 Ma for the Ebi Granites and 200.8 ± 2.1 and 184–183 Ma for the Hida Granites. On the east coast of the Korean Peninsula, 188–186 Ma, 183–177 Ma, 167.7 ± 1.5 Ma, and 173–166 Ma are obtained for the granitoids in the Gyeongsang Basin, Yeongnam Massif, Taebaeksan Basin, and Gyeonggi Massif, respectively. All granitoids show depletions in Nb, Ta, P, and Ti in whole-rock compositions, suggesting an arc environment. The studied granitoids can be divided into two types. The early-stage (∼201–183 Ma) granitoids from Southwest Japan and the Gyeongsang Basin are characterized by the absence of Precambrian inherited zircons, high εHf(t) values of +13.0 to −0.8, and higher crystallization temperatures of ∼750–830 °C. The later-stage (∼183–166 Ma) granitoids from the remaining areas have abundant Paleoproterozoic inherited zircons, low εHf(t) values of −13.9 to −25.0, and lower crystallization temperatures of ∼680–750 °C. The early-stage granitoids formed during a steep subduction regime under higher temperatures due to the heat supply from the asthenospheric mantle. In contrast, later-stage granitoids mostly have an adakitic nature and formed by the partial melting of garnet- and hornblende-bearing thickened Precambrian lower crust under lower temperature conditions during flat subduction. The Ebi and Hida Granites in Southwest Japan formed along the continental arc passing through the Gyeongsang Basin to the Dumangang Belt of the Korean Peninsula, indicating their paleopositions. A change in the subduction angle from steep to flat caused the migration of magmatism to the northwest or west throughout Northeast Asia, including Japan, Korea, and China.

Interactions of pāhoehoe and ‘a‘ā lavas and fluvial sediments on an alluvial plain (the Cretaceous Gyeongsang Basin, Republic of Korea)

Two lava units of basaltic composition named the Cheongryongsa and Hakbong basalts are intercalated in the Cretaceous nonmarine backarc basinfill of Korea. The Cheongryongsa Basalt is 5 to 6 m thick and laterally extends for about 12 km. It is underlain by massive sandy mudstone and overlain erosionally by a basaltic conglomerate, suggesting lava emplacement on a floodplain, followed by fluvial erosion and deposition. The basalt comprises flow-base breccia throughout the 12 km-long outcrop extent and is overlain by flow-top breccia at one locality. These breccias are composed mainly of irregular/fluidally shaped clasts in a massive mudstone matrix, and are interpreted as fluidal peperite produced by molten lava and wet sediment mingling. The Hakbong Basalt comprises three lava units, a few meters to several tens of meters thick, and intervening stratified sandstones. Three breccia types occur beneath or above the basalt: 1) irregular-clast breccia, 2) angular-clast breccia, and 3) fluidal-clast breccia. They are interpreted to be clinker, blocky peperite, and fluidal peperite, respectively, associated with ‘a‘ā lavas. The clinker could be distinguished from the peperite based on undeformed sedimentary structures in the interstitial sediment and the contrast in matrix grain size with the substrate sediment. Peperite formation was possible when the lava was emplaced upon very fine sandy to muddy sediment, which was probably under-consilidated and susceptible to liquefaction. The liquefied sediment needed to penetrate the carapace of clinker to come into contact with the coherent lava and generate peperite. Blocky peperite was produced by the interaction of semi-solidified lava with very fine sand, whereas fluidal peperite was produced by the mingling of molten lava and mud. These basalts show that various breccia types can be produced along subaerial lavas, involving autobrecciation, quench fragmentation, and peperitic fragmentation of lava. Many factors are interpreted to have been involved in forming these breccias, including the sediment type and consolidation state, the viscosity and emplacement setting of lava, and even a coincidental depositional event in the basin.

Late Cretaceous-early Paleogene magmatism in the Gyeongsang basin, southeast Korea and its implications for middle Paleogene climate change

• The youngest volcanic rocks in the Gyeongsang basin were formed at ∼ 56 Ma. • Paleogene magmatic activities indicate a transformation in tectonic regime. • Magmatic lull along east Asian and global cooling start from Cenozoic. The Gyeongsang basin is the largest volcanic-sedimentary basin in Korean Peninsula. It is characterized by widespread late Cretaceous-early Paleogene plutonic and volcanic rocks called Gyeongsang Supergroup, with its uppermost part of Yucheon group consisting mainly of andesitic to rhyolitic lavas and their derivative pyroclastic rocks. However, the precise geochronological framework of this supergroup remains elusive. In this study, we present new SIMS zircon U-Pb and whole-rock 40 Ar/ 39 Ar ages for the volcanic rocks of the Yucheon group and associated Bunam granites in the Juwangsan area, north of the Gyeongsang basin. Seven volcanic samples range in age from 71 to 56 Ma, while a granite sample has an age of 74 Ma. Combined with literature data, the Yucheon volcanic activities can be divided into two stages: 94–78 Ma and 71–56 Ma, comparable to 91–48 Ma of the granites in the basin. Actually, a gigantic igneous belt developed along the continental margin of NE Asia during the Mesozoic-Cenozoic, with a length over 10000 km and a width ∼ 1500 km. Exhibiting an eastward migration trend during 145–48 Ma and a magmatic tempo from flare-up to lull, this magmatic belt experiences a transformation in tectonic regime from active continental margin to back-arc due to subduction retreating of the Pacific subduction. The continental arc magmatic flare-up and associated contact metamorphism during Jurassic-early Paleogene may provide a huge amount of CO 2 for the global warming. With the magmatic lull after about 50 Ma, the cool Earth may be caused by less CO 2 inputs.

Fish swim traces from the Jindong Formation (Cretaceous) Korea: Implications for lake basin ichnofacies and paleoecology

The track rich Cretaceous Jindong Formation is famous for abundant tetrapod track assemblages, which occur at multiple, closely-spaced stratigraphic levels in the Gyeongsang Basin lake deposits of South Korea. However, until now no fish traces or fish body fossils had been reported. Thus, the discovery of three Undichna ichnospecies (U. simplicitas, U. bina, and U. britannica) provide the first direct evidence of fish in the basin. Paired in-phase traces vary in width and indicate a range of fish sizes among the tracemakers. Undichna assemblages represent a shallow lacustrine Undichna ichnocoenosis. Bird tracks (Ignotornis) registered on the same surface represent the superposition of a shorebird ichnocoenosis. Dinosaur tracks (Brontopodus and Caririchnium) associated with surfaces only ∼1 m below the bird and fish traces, represent additional large tetrapod ichnocoenoses, and demonstrate multiple, superimposed ichnocoenoses, interpenetrating localized sedimentary successions which are little separated in space or time. The Undichna and shorebird, ichnocoenoses are considered, in schema followed here, as representative of higher-level ‘archetypal’ Charachichnos and Grallator ichnofacies respectively. However, it is not clear whether a third ichnocoenosis and ichnofacies is represented by the Brontopodus and Caririchnium assemblages or the extent to which the overprinting of different ichnofaunas in localized areas always allows for the unambiguous application of ichnocoenois and ichnofacies schema.

Diminutive pterosaur tracks and trackways (Pteraichnus gracilis ichnosp. nov.) from the Lower Cretaceous Jinju Formation, Gyeongsang Basin, Korea

Small pterosaur tracks were discovered from the Cretaceous Jinju Formation of Jinju City, South Korea, now famous as an ichnological Konservat-Lagerstätte and source of large numbers of well-preserved tetrapod track assemblages. These small tracks are among ∼2500 reported from the vicinity of the Jinju Pterosaur Tracks Museum which represented the world's largest concentration of pterosaur tracks and multiple pterosaur track-bearing strata. The small tracks, which are the focus of this study are preserved as natural impressions (concave epirelief) on the surface of a dark grey shale slab. Manus tracks have average values of 27.2 mm length, 9.3 mm width and 3.0 length/width ratio. Pes tracks have average values of 27.8 mm length, 12.3 mm width and 2.2 length/width ratio. The sizes of the tracks are about as small as other small pterosaur tracks previously reported from Korea (P. koreanensis), Japan (P. nipponensis) and Spain (P. longipodus and P. parvus). Nevertheless, morphological characteristics of these tracks are clearly distinguished from those of ichnospecies mentioned above in divarication angles between manus digits, shape of pes imprints and length/width ratio of manus. Especially, the high elongation obtained from the narrow width of the manus and the length of elongated digit Ⅲ and the shape of isosceles triangular pes are the most prominent and diagnostic features. Thus, the small pterosaur tracks described herein, Pteraichnus gracilis ichnosp. nov., are considered a different type among established pterosaurian ichnotaxa.

Palynofloras from the Itsuki and Kuwajima Formations of the Tetori Group and their Correlation with Paleofloristic Provinces of Eastern Asia

We collected samples from the middle part of the Tetori Group and obtained diverse spores (28 species) and gymnosperm pollen (8 species) from the Barremian Itsuki and Kuwajima formations. This is the first report of Barremian palynofloras from the Tetori-type floristic province in Japan and we compared them to coeval ones from the Ryoseki-type floristic province. These palynofloras include various cyathealean spores and cheirolepidiaceous pollen, indicating that floristic differences between the Tetori- (Inner Zone) and Ryoseki-type (Outer Zone) provinces are not so distinct during the Barremian in Japan. They also show similarities with floras of the Sindong Group of the Gyeongsang Basin, southeast Korea and Shitouhezi Formation in northeast China, thus indicating that some Ryoseki-type elements may have extended their distribution northward in the Barremian.

Taxonomy, biostratigraphic and paleoecological aspects of non-marine ostracod fauna from the Jinju Formation (Albian) of the Gyeongsang Basin, South Korea

A total of seven species belonging to six genera of cypridoidean ostracods were recovered from three sections (Jeongchon, Hotan, and Tapri) of the Albian Jinju Formation from the Gyeongsang Basin, South Korea: Scabriculocypris yanbianensis, Cypridea khandae sp. nov., Cypridea samesi, Mongolocypris kohi, Lycopterocypris? cf. celsa, Candona sp., and Djungarica sp. Based on its morphology, C. khandae sp. nov. is considered to be the first record of the Cypridea alta-group in Korea. Although many species are endemic to the Jinju Formation, the occurrence of Scabriculocypris yanbianensis indicates that this fauna was linked with the contemporaneous fauna of the Tongfosi Formation of the Yanji Basin, northeast China. Based on previous studies and newly collected specimens, the taphonomic features of the sections indicate the presence of two distinct thanatocoenoses: a low-energy autochthonous thanatocoenosis (Jeongchon and Tapri sections) and a slightly higher energy thanatocoenosis (Hotan section). Finally, the absence of cytheroidean and darwinuloidean ostracods could be explained by the presence of unstable lake/temporal waterbodies and their low competitiveness.

Mineralogy, Geochemistry, and Age Constraints on the Axinite-Bearing Gukjeon Pb–Zn Skarn Deposit in the Miryang Area, South Korea

The axinite-bearing Gukjeon Pb–Zn deposit is hosted by the limestone, a member of the Jeonggaksan Formation, which, in turn, forms the part of the Jusasan subgroup of the Yucheon Group in the Gyeongsang Basin in the southeastern part of the Korean Peninsula. In this study, we attempted to interpret the spatial and temporal relationships among geologic events, including the mineralization of this deposit. We constructed a new 3D orebody model and suggested a relationship between skarn alteration and related mineralization. Mineralization timing was constrained using SHRIMP zircon age dating results combined with boron geochemistry on coeval intrusive rocks. Skarn alterations are restrictively found in several horizons of the limestone formation. The major skarn minerals are garnet (grossular), pyroxene (hedenbergite), amphibole (actinolite and ferro-actinolite), axinite (tizenite and ferro-axinite), and epidote (clinozoisite and epidote). The three stages of pre-skarn, syn-skarn, and post-skarn alteration are recognized within the deposit. The syn-skarn alteration is characterized by prograde metasomatic pyroxene and garnet, and the retrograde metasomatic amphibole, axinite, and epidote. Major skarn sulfide minerals are sphalerite, chalcopyrite, galena, and pyrite, which were predominantly precipitated during the retrograde stage and formed amphibole and axinite skarns. The skarn orebodies seem to be disc- or flat-shaped with a convex form at the central part of the orebodies. The vertical ascending and horizontal infiltration of boron-rich hydrothermal fluid probably controlled the geometry of the orebodies. Considering the whole-rock major, trace, and boron geochemical and geochronological results, the timing of Pb–Zn mineralization can be tightly constrained between the emplacement of boron-poor intrusion (fine-grained granodiorite, 82.8 Ma) and boron-rich intrusion (porphyritic andesite in Beomdori andesitic rocks, 83.8 Ma) in a back-arc basin setting. The boron for mineralization was sourced from late Cretaceous (Campanian), subduction-related magmatic rocks along the margin of the Pacific plate.

Regional variations of sulfur isotope compositions for metallic deposits in South Korea

AbstractSouthern Korean peninsula comprises five major geotectonic provinces, throughout which various metallic deposits are distributed. We reviewed sulfur isotope data (n = 1,574) of sulfide minerals collected from previous works for 177 metallic deposits in the provinces to interpret the sulfur isotope characteristics of each province, comprising different wall rocks and geologic settings. The averaged δ34S values of each metallic deposit associated with Precambrian metamorphic rocks and Jurassic granitoids in the Gyeonggi massif and Yeongnam massif range from −7.1 to +10.3‰ (av. +4.5‰) and from −3.6 to +7.8‰ (av. +3.5‰), respectively. The Taebaeksan basin produced the highest δ34S value among the five, −0.4 to +13.2‰ (av. +6.1‰). This was influenced by sulfate sulfur derived from marine carbonate host rock. The Okcheon metamorphic belt, comprising metasedimentary and metavolcanics rocks, shows an isotope range from +1.9 to +8.3‰ (av. +5.7‰). The sulfur isotope distribution of the Gyeongsang basin with a range from −1.2 to +11.7‰ (av. +5.2‰) can be divided into two zones: higher δ34S values from the inner zone related to the volcanic rocks and magnetite‐series Cretaceous granitoids, and lower δ34S values from the outer zone related to the organic‐rich sedimentary rocks. Sulfur isotope variations of metallic deposits in each geotectonic province were mainly influenced by igneous sulfur and inherent wall rock sulfur sources, 32S‐enriched sedimentary sulfur (e.g., Precambrian metasedimentary rocks and biogenic sulfur‐rich sedimentary rocks), and 34S‐enriched seawater sulfur (e.g., carbonates and acid to intermediate volcanic rocks). These wall rocks also contributed to the changes in δ34S values for granitoid rocks and metallic deposits by time, ore genetic type, and ore species in South Korea.

한반도 남부의 지질 유형별 지형 기복과 삭박 저항력

This study tried to reveal relative surface denudation resistance and ranking by geologic types in the Southern Korean Peninsula using an 1:250,000 digital geologic map and ASTER GDEM. Among rock types such as igneous, sedimentary and metamorphic rocks, metamorphic rock showed the greatest resistance to surface denudation. The most resistant rock to surface denudation by geologic periods (e.g., the Precambrian, Paleozoic, Mesozoic and Cenozoic) was found from the Precambrian. Among the major tectonic settings in the Southern Korean Peninsula such as the Gyeonggi massif, Okcheon belt, Yeongnam massif, Gyeongsang basin and Pohang basin, the Okcheon belt indicated the greatest resistance. The most and least resistant rocks from the representative nine rocks in the Southern Korean Peninsula were Paleozoic limestone, and Cretaceous sedimentary rock and Cenozoic sedimentary rock, respectively. This study suggests that Paleozoic limestone, Cretaceous volcanic rock, Paleozoic sedimentary rock and Precambrian gneiss can be regarded as hard rocks with high elevation, steep slope and complicated relief, while soft rocks with low elevation, gentle slope and simple relief are Jurassic granite, Cretaceous sedimentary rock and Cenozoic sedimentary rock.

The relationships among faults, geology and geophysical data in the southwestern Korean Peninsula, including the Haenam area, and their application for the interpretation of earthquakes

The southwestern Gyeonggi Massif features many faults. The main faults are NE-striking faults, including the Jeonju and Kwangju faults, and they are cut by N-S- and NNE-striking faults. Additionally, E-W-, WNW-, and NW-striking faults occur as minor faults. Historical earthquakes with magnitudes greater than 5.0 have occurred on or near these faults. Earthquakes with focal depths shallower than 12 km have generally occurred on the NE-, NNE-, and N-S-striking faults, while those with hypocenters deeper than 12 km have occurred on the NW-, WNW-, and NE-striking faults or do not show a relationship with any faults. The instrumental earthquakes with magnitudes greater than 5.0 in the Korean Peninsula mainly have hypocenters deeper than 12 km, and their epicenters form a NW-oriented trend. These data effectively reflect two seismogenic layers with a boundary at ca. 12 km, as suggested in a previous study. The recent earthquake in the Haenam area occurred in the lower seismogenic layer. The shallower earthquakes seem to be the result of the reactivation of pre-existing faults. In comparison, the deeper earthquakes tend to occur on recently formed or newly formed faults related to the present regional stress of the Korean Peninsula. The shear-wave velocity tomography of the southwestern Gyeonggi Massif at 1 km corresponds well to the geology. In contrast, the shear-wave velocity variations at 5–9 km generally correspond to the locations of brittle faults. The depth of the ~3.5 km/s isovelocity line is quite irregular and is deeper in areas with active fault movement. In the Gyeongsang basin, the relationship between the shear-wave velocity and faults is somewhat unclear because the thickness of low-density sedimentary rocks is up to 8 km. Shear-wave velocity tomography at 13 km can be used to identify areas with high shear stress. These data indicate that shear-wave velocity tomography may provide different information depending on the depth and that the brittle faults in this region may not extend to a depth of 13 km.

Zircon U-Pb geochronology and Sr–Nd–Pb–Hf isotope geochemistry for Permian–Early Triassic arc-related magmatism in Pohang, Jangsari, and Yeongdeok, southeastern Korean Peninsula

Granitoids in the southern Korean Peninsula range in age from Precambrian to Cenozoic, but were emplaced mostly during the Jurassic and Cretaceous periods. The late Permian–Early Triassic granitoids are minor in volume and restricted to the northeastern part of the Cretaceous Gyeongsang Basin (Yeongdeok and Jangsari). Here we report on Permian granites from the Cenozoic Pohang Basin of southeastern Korea. The samples were obtained during the drilling of injection wells in the Pohang enhanced geothermal system (4219 m depth for the drill core; 3658–4174 m for the drill cuttings). We also analyzed outcrop samples from Yeongdeok and Jangsari. We present zircon U-Pb ages and Hf isotope, whole-rock geochemical, and Sr–Nd–Pb isotope data for these Phanerozoic granitoids. The zircons yielded early–late Permian ages (ca. 283–264 Ma) for the Pohang granites, late Permian ages (ca. 261–256 Ma) for the Jangsari granitoids, and late Permian–Early Triassic ages (ca. 253–249 Ma) for the Yeongdeok granitoids. The granitoids are enriched in large-ion lithophile elements and depleted in high-field-strength elements, indicating formation in an arc-related tectonic setting. The Permian Pohang and Jangsari granitoids have a non-adakitic arc-like geochemistry, whereas the Yeongdeok intrusions are adakite-like and show high Sr and low Y-Yb contents. The granitoids have low (87Sr/86Sr)i (0.7038–0.7049) and positive εNd(t) values (+3.7 to +4.7 for the Pohang, +4.4 to +4.9 for the Jangsari, and + 2.4 to +4.1 for the Yeongdeok plutons), and are distinct from the basement of the Yeongnam Massif. The Pb isotope ratios of the studied samples plot in the field for Indian mid-ocean ridge basalts. Zircons from the plutons have positive εHf(t) values: +9.8 to +17.6 for the Pohang, +8.8 to +17.7 for the Jangsari, and + 8.5 to +18.3 for the Yeongdeok plutons. The Permian granitoids were likely the product of extensive fractional crystallization of H2O-rich parental magmas, which were possibly generated in a hydrated mantle wedge. The Yeongdeok adakite-like granitoids were derived from a normal H2O-rich arc magma by amphibole ± garnet fractionation. The juvenile Sr–Nd–Pb–Hf isotope compositions are comparable with those of coeval igneous rocks in the Central Asian Orogenic Belt, rather than intrusive counterparts in the North and South China cratons. We propose two possibilities that Permian–Early Triassic arc-related magmatism in the southeastern Korean Peninsula might have been associated with (1) northward subduction of the Paleo-Asian oceanic plate; or (2) westward subduction of the Paleo-Pacific plate at the eastern margin of the North China Craton.

Detrital zircon U-Pb ages of the uppermost Jinju Formation in the Natural Monument No. 534 ‘Tracksite of Pterosaurs, Birds, and Dinosaurs in Hotandong, Jinju’, Korea

LA-MC-ICP-MS zircon U-Pb dating was conducted to constrain the timing of fossil formation and the depositional age of the uppermost Jinju Formation located in Natural Monument No. 534 (Tracksite of Pterosaurs, Birds, and Dinosaurs in Hotandong, Jinju), and 87 Cretaceous, 1 Precambrian, and 2 Jurassic zircons were obtained from 90 valid analytical points. Most Cretaceous zircons were found to have a youngest graphical peak age of ca. 106.5 Ma, suggesting the depositional age of the uppermost Jinju Formation. Based on this study and previous works, the average sedimentation rate of the Jinju Formation was calculated to be approximately 0.17-0.31 mm/year in the Milyang Subbasin, and the Cretaceous zircons of the uppermost Jinju Formation seem to have originated mainly from the western or northwestern parts of the Gyeonggi Massif. Unlike the Nakdong and Hasandong formations of the Sindong Group, most zircons analyzed in the uppermost Jinju Formation were Cretaceous. This suggests that volcanic activity occurred in the area closer to the Gyeongsang Basin due to the roll-back of subducting paleo-Pacific Plates during the Jinju period.

묘곡층과 백악기 경상누층군 영양소분지 하부퇴적층의 층서와 퇴적상 분석

경북 봉화군 재산면 일대에 경상분지 영양소분지 퇴적층 산출지 북단에 소규모로 분포하는 묘곡층은 쥐라기의 대동층군과 백악기의 경상누층군을 연결하는 중요한 층서 단위임에도 불구하고 그동안 퇴적학적 연구는 매우 제한적이었다. 이번 연구에서는 이러한 묘곡층과, 영양소분지 하부 퇴적층(울련산층, 동화치층)에 대한 층서 및 퇴적학적 연구를 통해 퇴적환경과 퇴적층 발달 및 진화 과정을 제시하고자 하였다. 이번 연구에서는 야외에서 수집된 퇴적층 노두 기재 자료를 기반으로 11개의 퇴적상을 분류한 후 각각의 퇴적상을 수평 · 수직적 연장성과 산출 빈도를 고려하여 6개의 상조합으로 설정하였다. 연구 결과 묘곡층은 상조합 Ⅰ이 단일로 산출되며 호성 환경에서 퇴적된 것으로 해석된다. 울련산층은 상조합 Ⅱ와 상조합 Ⅲ이 우세하게 산출되고 있는데, 이는 간헐적으로 화산쇄설물이 공급되는 충적선상지 상부 퇴적환경에서 형성된 것으로 해석되고, 동화치층은 상조합Ⅳ와 상조합 Ⅴ가 우세하게 산출되는 퇴적층으로서 역시 간헐적으로 화산쇄설물이 공급되던 충적선상지 또는 선상지 평원과 역질 및 사질의 망상하천에서 범람원으로 전이하는 퇴적환경에서 퇴적된 것으로 해석된다. 퇴적환경 해석과 퇴적층 발달과정을 바탕으로 퇴적층 발달과정을 해석한 결과, 묘곡층은 경상분지에서 본격적으로 퇴적활동이 일어나기 전부터 이미 존재하였던 퇴적층으로 생각되는데, 부정합 관계인 묘곡층과 울련산층의 퇴적환경이 연속적이지 않는 점과 지각 운동에 의해 묘곡층 내에 수반된 여러 변형 구조들이 울련산층에는 없는 점은 묘곡층을 선경상누층군 지층으로 해석할 수 있는 근거이다.