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The Gulong Sag in the northern Songliao Basin, China, possesses abundant shale oil resources and represents a highly prospective area for shale oil exploration. However, the Qingshankou formation shale oil reservoir within this region exhibits characteristics such as thin longitudinal thickness, pronounced horizontal heterogeneity, limited frequency range, and significant anisotropy that pose difficulties in accurately predicting the “sweet spot” of shale oil within the target interval. The azimuthal anisotropy characteristics of the target layer in the Qingshankou formation are analyzed in this manuscript, utilizing wide-azimuth and small bin seismic data from the Y3 research area. Considering the limitations of existing methods for fitting elliptical velocities in azimuthal anisotropy correction, the influence of azimuthal anisotropy time difference on the non-in-phase superposition of seismic in-phase axis is eliminated by employing a non-rigid dynamic matching method, thereby enhancing the resolution and imaging accuracy of seismic data. The azimuth anisotropy correction effectively broadens the frequency range of the stack profile by 7 Hz, thereby enhancing the reliability of data for shale oil reservoir prediction in the study area.

Jumun Island is tectonostratigraphically situated on the marginal zone of the Gyeonggi Massif. The Massif is in contact with the southwestern margin of the Imjingang Belt and adjacent to Boreum Island, where ultramafic rock with magmatic Fe-Ti oxide deposits occurs. The northwest of Jumun Island, facing the Boreum ultramafic rock with Fe-Ti oxide ores, is composed of Precambrian Boreumdo schists containing a few magmatic intrusives, the exact ages of which are unknown. In Jumun, the ultramafic intrusion (Mg# = 75), which is confined to a narrow zone along the seaside, mainly consists of olivine (Fo = 81–82), amphibole (magnesio-horn-blende to tremolite), and phlogopite. The olivine is strongly serpentinized and encompassed by amphibole and phlogopite. The Ni-Cu sulfide mineralization found in the ultramafic rock is weak but has a typical assemblage of pyrrhotite-pentlandite-chalcopyrite with a small amount of magnetite. Notably, the Ni-Cu sulfides are closely associated with amphibole and phlogopite and are found in the fractures and interstitials of the olivine grains. The pyrrhotite (n = 2) and chalcopyrite (n = 1) are compositionally close to pure samples, whereas the pentlandite (n = 2) is characterized by enrichment with Co (up to 6.9 wt%). The sphalerite-bearing quartz vein cuts across the Precambrian gneissic rock and strikes N70 °W with an 80 °NE dip. This vein, which is traceable to a limited extent and approximately 40 cm wide, shows mineralogical zonation in the inward direction from pyrite to sphalerite-dominant. Consisting of sphalerite, pyrite, quartz, and chlorite with minor amounts of chalcopyrite, pyrrhotite, and pentlandite, it is composed of 9.56 wt% Zn with < 1.0 wt% As, Co, Cu, In, Mn, Ni, and Pb and below-detection limits (0.001 ppm) amounts of Bi, Ge, Mo, Se, Sb, Te, and W. Sphalerite, a principal ore mineral, is coarse-grained and reddish-brown and is composed of 57.3–58.8 wt% ZnS, 8.0–9.2 wt% FeS, and 32.0–32.4 wt% S with small amounts of Cu, Mn, As, and Cd. The recently discovered Ni-Cu sulfide mineralization and quartz vein with sphalerite, along with the linear array of magmatic Fe-Ti oxide deposits, provide conclusive evidence that the marginal zone of the Gyeonggi Massif may be a geologically favorable area for the formation of magmatic and magmatic-hydrothermal deposits. For exploration purposes, it is necessary to contextualize the source, tectonic setting, and magmatic evolution.

In order to constrain the granitic magma source at the northeastern continental margin of the Eurasian plate, noble gas isotopic ratios such as helium (³He/⁴He), argon (⁴⁰Ar/³⁶Ar) and neon (²⁰Ne/²²Ne, ²¹Ne/²²Ne) were determined for Mesozoic quartz and biotite minerals from granitic rocks in the Korean peninsula. ³He/⁴He ratios in fluid inclusions of quartz samples have a wide range from 0.005 to 0.522 R_A (av. 0.095 R_A) and 0.013 to 1.27 R_A (av. 0.37 R_A) (R_A =1.40 × 10⁻⁶, atmospheric value) for Jurassic (Daebo) and Cretaceous (Bulguksa) granites, respectively. The ³He/⁴He ratios clearly show a contribution of mantle-derived He to the granitic rock at the formation, then the helium has been deeply affected by accumulation of in situ produced radiogenic ⁴He and/or crustal helium. Although these ratios are lower than those of the subcontinental lithospheric mantle (SCLM) (6.1 ± 0.9 R_A), mantle helium has been traced in these Mesozoic I-type granites from South Korea. The observations imply that the helium of SCLM source predominates over all of the Jurassic granites in South Korea and the Cretaceous granites in the Ogcheon belt (OB), and suggests that the granitic magma was derived from the partial melting product of SCLM materials with appreciable amounts of radiogenic helium. Meanwhile, Cretaceous granites were originated from igneous mantle source materials beneath the Gyeongsang basin, south-eastern area of the Korean peninsula. A presence of mantle components (²⁰Ne/²²Ne ≈ 10.13) and/or nucleogenic Ne were identified in some quartz and most biotite samples of granitoids in Jurassic age. Argon isotopic ratios (av. ⁴⁰Ar/³⁶Ar = 2370) of fluid inclusions in quartz for Jurassic granites are considerably higher than those in Cretaceous granites (av. ⁴⁰Ar/³⁶Ar = 414), indicating a clear aging effect. He-Ar isotopic signatures together with the characteristics of Nd, Sr, and O isotopes can lead to the conclusion that the generation of Jurassic granitic magma was responsible for the subduction of the Izanagi oceanic plate. Meanwhile, the subduction ridge (e.g., the Kula-Pacific Ridge) model is likely to be a suitable scenario for formation of the Cretaceous granitic magma in the Korean peninsula.

Fault location and geometry are the most fundamental input data in seismic hazard analysis, the ultimate aim of which is to mitigate damage from future large earthquakes. In regions prone to large earthquakes or where cumulative deformation by multiple earthquake events are well expressed in the landscape, fault models are constructed primarily by (1) identifying active fault traces, mapped mostly by the surface ruptures associated with large earthquakes; (2) simplifying fault traces while capturing their geometrical characteristics; and (3) segmenting the simplified geometry, given that a single earthquake does not always rupture the entire length of a fault system. In slowly deforming regions, however, the construction of fault models is challenging, even though geologic records of large earthquakes exist, because of the lack of clear active fault traces. Indeed, surface-rupturing earthquakes may not be part of the historical periods owing to their long recurrence time of thousands of years or more. Nevertheless, seismic hazard analysis is required for densely populated and industrial areas in slowly deforming regions, such as South Korea. On the basis of criteria established previously for determining segmentation geometry in fault models, here we propose a methodology for identifying the segmentation geometry of strike-slip fault systems in slowly deforming regions. In terms of the criteria used to identify segment boundaries, we examine along-fault variations not only in fault geometry but also in fault-surrounding lithology and fault-related geomorphic features. We test the methodology for assessing the fault segmentation geometry in a case study of the Yangsan Fault, which is one of the most active seismogenic strike-slip faults on the Korean Peninsula. Results show that the ∼200 km length of the Yangsan Fault on land consists of 12 to 15 distinct fault segments. We discuss how models of fault segmentation geometry are able to improve seismic hazard analysis in regions that have not experienced surface-faulting earthquakes in historical period.

The 2017 Pohang earthquake (M_L 5.4) ranks as the second-largest instrumental earthquake in the Korean Peninsula and the country’s most destructive seismic event. The earthquake history of the Pohang area prior to the 2017 event is unknown due to the absence of instrumental seismic activity and the lack of mapped Quaternary faults near the 2017 epicenter. The aim of the present study is to identify evidence for previous earthquake ruptures along the surface projection of the seismogenic fault and interpret their paleoseismic implications. The study involved comprehensive paleoseismological investigation, including geomorphic analysis, field-work, drillhole surveys, trench excavation, and numerical age dating. Geomorphic analysis and drillhole surveys revealed two lineaments presumed to have originated from Quaternary faulting: NNE-SSW-striking Fault-1 and NE-SW to NNE-SSW-striking Fault-2. At the excavation site of Fault-1, which is regarded as the seismogenic fault of the 2017 Pohang earthquake, stratigraphic features and numerical ages show that the penultimate event occurred between 11 ± 1 and 2.6 ± 0.1 ka and that the most recent event took place after 0.17 ± 0.01 ka. Combined results from two outcrops of Fault-2 give occurrence ages for the penultimate and most recent events of ca. 200 ka and between 148 ± 7 ka and the analytical limit of ¹⁴C dating (> 43,500 BP), respectively. Our findings reveal that at least three seismic events causing surface ruptures have occurred in the Pohang area during the late Quaternary before the 2017 Pohang earthquake.

The origin of the late Cenozoic intraplate volcanoes in the NE Asia has sparked debate, with explanations ranging from deep mantle plume to lithospheric extension and decompression melting of mantle upwelling by distal subduction tectonics. The Jeju volcanic field (JVF), being the closest late Cenozoic intraplate volcano to the subduction zone, sheds light on whether the intraplate volcanism is primarily plume-related or linked to plate tectonics. This study determined the primary magma composition for JVF basalts, using the most primitive bulk-rock samples (MgO > 8.5 wt%), by incrementally adding olivine to melt until reaching equilibrium with olivine (Mg# = 90) in the residual mantle. The estimated temperature and pressure of mantle melting are 1,466–1,587 °C and 2.1–4.1 GPa for anhydrous primary magma and 1,347–1,512 °C and 2.0–3.6 GPa for hydrous primary magma within the acceptable range of water contents (H₂O = 2–4 wt%) reported from the Chinese intraplate basalts. The pressure estimates suggest that the minimal depth of the lithosphere-asthenosphere boundary is approximately ∼50–55 km. The mantle potential temperature for anhydrous primary magma is estimated to be 1,460–1,580 °C, higher than 1,300–1,400 °C of the ambient upper mantle, indicating a hot thermal regime below the JVF. Despite the absence of geophysical evidence for a mantle plume beneath the JVF, this study proposes that the hot mantle wedge is likely caused by the lateral influx or edge-driven convective upwelling of thermal plume near the leading edge of the stagnant Pacific Plate slab, contributing to the big mantle wedge. Intraplate volcanism in the JVF is proposed to be driven by lithospheric extension and decompression melting of the convective upwelling of hot sub-lithospheric mantle, influenced by distal subduction tectonics in the hot subduction zone. This model is supported by the present-day tectonics observed in the hot Ryukyu subduction zone, SW Japan.