Northeastern Eurasia is one of the least studied regions in the world, with limited geophysical data available due to its inaccessibility. The exact location of the plate boundary between Eurasia and North America is still under debate. The effective elastic thickness (EET) of the lithosphere serves as an indicator of lithospheric strength and provides valuable information on thermal conditions and tectonic activity. We have produced a high-resolution EET map for northeastern Eurasia using the fan wavelet coherence method applied to Bouguer gravity anomalies and topography/bathymetry data, with appropriate adjustments for density variations within sediments. The resulting EET variations provide valuable insights into the different tectonic regimes of this largely unexplored region. In particular, we identify the boundary between the Eurasian and North American plates in Siberia as a rheologically weak, diffusive zone extending from the Verkhoyansk and Sette-Daban Ranges to the eastern edge of the Chersky Range. In contrast to the Sette-Daban and Verkhoyansk Ranges, which were formed by plate collisions and have an EET of 30-50 km, other mountainous regions have much lower EET values, often less than 15 km, indicating recent tectonic activity that has weakened the lithosphere. This is fully consistent with the distribution of earthquakes and focal mechanisms. The majority of earthquakes are concentrated on the western and eastern boundaries of this zone (the eastern slope of the Verkhoyansk Range on one side and the eastern slope of the Chersky Range on the other). In the center of the diffuse zone only weak earthquakes of small depth occur.

How to cite: Kaban, M., Chen, B., Gvishiani, A., Soloviev, A., and Sidorov, R.: Variations of the effective elastic thickness of the lithosphere suggest a broad diffusive boundary between the North American and Eurasian plates in Siberia, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-18877, https://doi.org/10.5194/egusphere-egu25-18877, 2025.

    Magmatism in continental margin arc is generally episodic, and alternatively appearance with magmatic flare-up and lull. Accompanying the magmatic flare-up and lull, the arc could move toward the oceanic trench or continental. Although numerous researches are related to arc tempos, there is little understanding for the genetic mechanism of magmatism during lull. According to the distribution characteristics of magmatic rocks in the Neo-Tethyan continental margin arc, this study selected the Late Cretaceous granitic rocks in Tengchong block to further constrain their petrogenesis and dynamics process during lulls. The monzogranite ages in Husa area of Tengchong Block are 73Ma, formed within the Late Cretaceous magmatic lull (85-65Ma) at the continental margin arc related to Neo-Tethyan subduction. They are metaluminous to weakly peraluminous and calc-alkaline, with enriched Nd-Hf isotopic characteristics. These signatures reveal their sources are orthometamorphic gneisses in the Gaoligongshan Formation. In addition, the continental margin arc related to the flat subduction of Neo-Tethyan slab should be compressional state during Late Cretaceous (ca. 85-65Ma). Comparing Late Cretaceous magmatic rock assemblages, geochemical signatures, Zr saturated temperature and zoning in plagioclase with these in Early Eocene rocks during magmatic flare-up (ca. 55-50Ma), the granitic rocks during magmatic lull formed at a thicken crust and partial melting of gneisses induced by a long-time thermal accumulation. Therefore, the continental margin arc could form granitic magma by a long-time thermal accumulation within a thicken crust during magmatic lull with the low mantle-derived magma flux at a local region.

Supported by National Natural Science Foundation of China [Grant Nos. 42272052 and 41902046].

How to cite: Zhao, S.: Origin of Granitic Magma within continental margin arc magmatic lull: constraints by the Late Cretaceous granites in Tengchong Block, SE Tibet, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-225, https://doi.org/10.5194/egusphere-egu25-225, 2025.

The Himachal Pradesh Seismological NETwork (HiPSNET) was installed in 2019 across the northwestern Himachal Himalaya, comprising seven broadband seismograph systems. Teleseismic data with magnitude greater than 5.5 and in the distance range of 30-90° have been used to compute P-wave receiver functions (P-RFs). These P-RFs have been depth migrated to form a 2D common conversion point (CCP) stack profile across the strike of the Himalaya. The main features on the CCP profile are the positive impedance boundaries of the Moho and the mid-crust, and the negative impedance contrast boundary of the Main Himalayan Thrust (MHT). The Indian crust underthrust the Himachal Himalaya with a gentle dip of ~5-7° in the NE direction. The Moho depth increases from the foreland (SW) to the hinterland (NE). Beneath the Sub-Himalaya, the Moho is at a depth of ~45 km, and gradually deepens to ~60 km beneath the Lesser-Himalaya, and further to ~70 km beneath the Higher-to-Tethyan Himalaya. The MHT, associated with a low-velocity layer, has a flat-ramp geometry, and ranges in depth from ~10 km beneath the Sub-Himalaya to ~20 km beneath the Higher-Tethyan Himalaya. Precisely located small-to-moderate earthquakes, from previous studies, concentrate on or above the MHT frontal ramp structure. This possibly marks the locked-to-creep transition on the MHT, lying below the Higher-Himalaya. 

How to cite: Shamim, S., Sharma, S., Chaudhuri, D., Mitra, S., Priestley, K., and Wanchoo, S. K.: Himachal Pradesh Seismological NETwork (HiPSNET): Structure of the Crust and the geometry of the Main Himalayan Thrust, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-14787, https://doi.org/10.5194/egusphere-egu25-14787, 2025.

The Maguan area in Yunnan Province is situated at the southeastern edge of the Tibetan Plateau and the southwestern margin of the South China Block, representing a significant junction between these tectonic units. The structural characteristics of the deep lithosphere, particularly the lower crust, remain unclear. Deep-Seated xenoliths, which are rapidly transported to the surface by igneous rocks, reflect in-situ information about the deep lithosphere and serve as direct samples for studying lower crustal growth and evolution.This report focuses on the discovery of mafic granulite xenoliths within Cenozoic basalt in the Maguan area, located at the southwestern margin of the South China Block. We conducted a comprehensive analysis including mineralogical and geochemical characterization, zircon U-Pb geochronology, and in-situ Hf isotopic analysis.

Eequilibrium temperature and pressure estimates of the xenoliths suggest that they have a balance temperature exceeding 950 °C and a pressure of approximately 1.2 GPa, showing that these xenoliths originate from the lower crust. The SiO₂ content of the xenoliths ranges from 50.3 to 51.7 wt.%, with Mg# values of 67 to 68. The whole rock exhibits slight enrichment of light rare earth elements (LREE) and shows weak negative europium anomalies (δEu = 0.82 to 0.86). Additionally, the Sr-Nd isotopic composition is enriched and resembles EM II (⁸⁷Sr/⁸⁶Sr = 0.7081 to 0.7088, εNd = -3.8 to -4.7), while the Hf isotopic ratios of the magmatic zircon vary between -4.5 and +4.8.The concordia and near-concordia zircon U-Pb ages display several significant events, including 62 Ma, 41 Ma, 34 Ma, 26 Ma, 20 Ma, and 15 Ma, likely corresponding to collision and post-collision magmatic events related to the Tibetan Plateau and its southeastern margin, as well as the extensive left-lateral strike-slip movements in the Ailao Shan Belt. Inherited zircon concordia U-Pb ages include 874 Ma, 461 Ma, 338 Ma, 271 Ma, 131 Ma, and 102 Ma. Together with the zircon Hf isotopic data, the mafic xenoliths from Maguan record the lower crustal growth associated with the early Paleozoic orogeny in the Jiangnan Orogen, linked to the South China Sea (εHf = +1.4 to +4.5, T_DM1 = 1.0 to 0.9 Ga), late Paleozoic events (εHf = -5.7 to +6.0, T_DM2 = 1.9 to 1.0 Ga), and early to mid-Mesozoic growth and reworking related to the closure of the Paleo-Tethys Ocean (εHf = +0.4, T_DM1 = 0.8 Ga). Additionally, the late Mesozoic tectonic activities post-Indochina collision (εHf = -7.1 to +2.3, T_DM2 = 1.5 to 1.0 Ga) and Cenozoic reworking events (εHf = -4.5 to +4.8, T_DM1 = 0.9 to 0.5 Ga) are also recorded.

Considering the tectonic position of the Maguan area, this study suggests that the mafic granulite xenoliths represent ancient material from the Paleoproterozoic, reflecting the complex evolutionary history of the South China Block. Furthermore, they have been influenced by multiple magmatic and metamorphic events related to the Cenozoic collision of the Tibetan Plateau and the Indo-Asian region.

Keywords: Mafic granulite xenoliths; magmatic underplating; continental lower crust growth and reworking; southeastern Tibetan Plateau.

How to cite: Ma, X. and Zheng, J.: Underplating along the SE Margin of the Tibetan Plateau and Its Implications for the Reworking of the Lower Crust in SW South China: Evidence from Mafic Granulite Xenoliths in Maguan, Yunnan Province, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-5511, https://doi.org/10.5194/egusphere-egu25-5511, 2025.

The Xiazhuang uranium deposit is the largest granite-type uranium orefield in China, located in east of the Triassic to Jurassic-Cretaceous Guidong complex granite. Uranium orebodies mainly outcroped insides NNE-, NWW-, and NEE-trending quartz-breccia zones, especially intersection areas. These three groups of fault zones have approximate equidistant distribution characteristics and cut with each other, forming a kind of checker-board lattice pattern. The formation genesis of the granite, ore-forming metallogenesis and ore-controlling factors have been well documented, however the tectonic stress field evolution of Xiazhaung uranium orefield and the structural controlling effect to the mineralization have been controversial for long time.

In this study, the scratch-lineation method was mainly adopted to inversion of evolution of tectonic stress field, mostly depending on the detailed field measurements to collect the scratch-lineation data, using the Wulff's net to determine the principal stress direction and the properties of each point, and combining the scratch intersection relations and other geological evidence to reveal the scratch activity periods, and further dividing the tectonic stress evolution stage to discuss the features of the tectonic stress field and its constraints to the uranium mineralization, and finally guiding the ore-prospecting prediction.

45 group data of scratch-lineation with total 250 pieces of data have been collected in the field. Combining with the field observations of kinematics, tectonic and metallogenic relationship signs, it is suggested that the Meso-Cenozoic tectonic stress field process of the Xiazhuang uranium orefield can be divided into three periods of eight stages, including two stages before-, four stages during- and two stages after the metallogenic period, respectively.

The first stage was happened during the late Indosinian, about 230Ma to 200Ma, when the maximum principal stress in the ore-field was probably S-N direction during the late stage collision between the southern China with the northern China blocks, leading the intrusion of the  Guidong complex granite. During the second stage (~200-165Ma) with nearly EW-trending (about 80°±) compressional and NNE-SSW-trending extensional stress, the NWW-trending diabasic dikes intruded along the NWW-trending transtensional fault zone, and nearly NS-trending ductile zone inside the Xiazhaung orefiled and NW-trending（320-330°）sinistral strike-slipping mylonite zones in Late Silunan Donggualing granite in the eastern side of the Guidong complex granite and insides the Late Trassic Damofeng granite. Two groups of conjugate fracture system formed, including the NE-SW and NW-SE trending fault zones, as main tectonic structures of the ore-field

During the first stage of the metallogenic period about 160Ma to 135Ma with main compressional stress trend to NW-SE (290-300°±), led to the emplacement of parts of NWW-trending diabasic dykes and development of NNE-trending density cleavage belt as the compressional property, while the the first stage of regional large-scale mineralization happened for the NWW-trending fault zone behaving as extensional fracture.

The maximum principal stress might be approximate vertical during the fifth stage (the second stage of the metallogenic period, about 135Ma to 115Ma) with intermediate principal stress trending to NE60° and the minimum principal stress to NW330° in the horizontal plane.

How to cite: Chen, Z., Huo, H., Zhu, W., Li, H., Yan, J., Pan, J., Zhong, F., and Sun, Y.: Evolution of the tectonic stress field in the Xiazhuang uranium ore-field,southern China, and its coupling with the ore-formation, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-14350, https://doi.org/10.5194/egusphere-egu25-14350, 2025.

For hundreds of millions of years, Gondwana and Laurasia were separated by the Paleo- and Neotethyan oceans. Their eventual collision led to the amalgamation of various continental fragments, initiating multiple subduction cycles in the broader Anatolian region. This study presents, for the first time, five finger-like high-velocity anomalies beneath northern Anatolia (Türkiye), identified through high-resolution P-wave tomography at depths ranging from 80 to 250 km. These anomalies may represent shallow remnants of the Neotethyan slab, which may have remained buoyant due to underplating since the early Cenozoic. Their unique geometry and location suggest active mantle flow, possibly linked to either continental-continental subduction or recent lithospheric foundering.

How to cite: Confal, J., Taymaz, T., Eken, T., Bezeda, M. J., and Faccenda, M.: Remnant Tethyan Slab Fragments Beneath Northern Türkiye, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-21713, https://doi.org/10.5194/egusphere-egu25-21713, 2025.

The Iranian lithospheric plate (ILP), positioned at the boundary of Euro-Asia and Gondwana, is vital from a tectonic-geodynamic perspective. Its role in the evolution of the South Caspian Basin (SCB) and the Mesozoic Terrane Belt (MTB) (confined to the northern rim of the Arabian Plate) is undeniable. The boundary between Eurasia and Gondwana separates the western – Aegean-Anatolian Plate and the relatively young Neoproterozoic belt associated with the MTB and the eastern – ILP, which contains fragments of Arabian craton and terranes of Archean-Early Proterozoic age. The northwestern regional block of the ILP is influenced by the deep rotating mantle structure (DRMS) (Eppelbaum et al., 2021) and rotates counterclockwise. The central block of the ILP is in the marginal periclinal zone of the DRMS, and its movement is oriented mainly to the north. The ILP's eastern part is outside the DRMS's guidance and rotates NNE clockwise.

Immediately to the north of the MTB, already to the west of the discordant junction of the Mesozoic ophiolites and the Zagros terrane, an extensive collision-subduction belt is formed in the former development of the Neotethys Ocean. It occupies approximately half of the ILP in the meridional direction. The belt is composed of a complex of volcanic, metamorphic, and sedimentary rocks and hypsometrically forms a vast zone of the Iranian plateau (in the southern part of the ILP). This sublatitudinal zone of uplifts is closely adjacent to the SCB and the south boundary of the Eurasian Plate and is located near the Earth’s critical latitude of 35^o.

Combined tectonic-structural and geophysical analyses suggesting the Zagros suture imply that it might have been a piece of the MTB in the southern zone of the Neotethys Ocean. Since the post-Carboniferous, its uplift has been attested as an isolated tectonic feature of the Neotethys Ocean. The sharp divisions of the ILP on the western, central, and eastern fragments have a rational explanation – deep-seated geodynamics inspired by the impact of the DRMS, critical Earth’s latitude, and the Ural-African geoid anomaly. These factors overwhelmingly impact the mantle-lithospheric structure in the region.

This study generalized tectonic-structural, GPS, paleomagnetic, gravity, magnetic, and thermal data. The ILP is known to have high heat flow values along with a thickened Earth’s crust. Combined geophysical data analysis indicates that the ILP’s blocks press on the southwestern edge of the SCB, causing it to rotate clockwise.

Thus, we can conclude that the SCB is affected by three main geodynamic components: (1) the counterclockwise rotation of the DRMS, causing the complex movement of the ILP’s western and central lithospheric blocks; (2) the multipart direct pressure of the ILP on the SCB; and (3) the impact of the giant Ural-African step and critical Earth’s latitude. The current rapid reduction of the Caspian Sea level is associated, along with other factors, with the unevenness of the modern collision processes.

Eppelbaum, L.V., Ben-Avraham, Z., Katz, Y., Cloetingh, S. and Kaban, M., 2021. Giant quasi-ring mantle structure in the African-Arabian junction: Results derived from the geological-geophysical data integration. Geotectonics, 55, No.1, 67-93.

How to cite: Eppelbaum, L., Katz, Y., Gadirov (Kadirov), F., Guliyev, I., and Ben-Avraham, Z.: Why is the South Caspian Basin clockwise rotating?, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-1841, https://doi.org/10.5194/egusphere-egu25-1841, 2025.