How to cite: Schanner, M. A., Bohsung, L., and Korte, M.: Kalman filter based modeling of the Holocene geomagnetic field, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-2794, https://doi.org/10.5194/egusphere-egu25-2794, 2025.
One of the most prominent changes in Earth’s magnetic field over the past two centuries is the growth of the South Atlantic Anomaly (SAA)—a region of significantly weakened field intensity. Recent studies have suggested that weak field anomalies such as the SAA are recurrent features of the geomagnetic field, preferentially occurring around certain longitudes and generally drifting westward. These observations have sparked hypotheses linking the weak field anomalies to heat-flux heterogeneities at the core-mantle boundary and/or an eccentric planetary-scale gyre as observed in modern core surface flow reconstructions. To further investigate the underlying mechanisms, we generate core surface flow models that are compatible with the observed geomagnetic field changes. Several recent studies have made use statistics derived from geodynamo simulations to provide physically motivated priors on the core surface flow. Here, we adapt these methods to infer possible core flow solutions spanning the past 9000 years, constrained by archaeomagnetic and sedimentary palaeomagnetic data. Synthetic data are used to explore the extent to which archaeo-/palaeomagnetic observations can recover large-scale core flow variations. The integrated core-field and core-flow modelling approach is then applied to real-world data and the results are discussed within the context of recurrent weak field anomalies.
How to cite: Nilsson, A., Suttie, N., Gillet, N., and Aubert, J.: Core surface flow and geomagnetic field changes on millennial timescales, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-4289, https://doi.org/10.5194/egusphere-egu25-4289, 2025.
In this presentation, we review the methodological, geomagnetic, and chronological insights gained from a newly developed high-resolution archaeomagnetic intensity curve for the Levant spanning over the past 9 millennia. The new compilation includes a variety of archaeological materials collected from Israel, Syria, Iraq, and Jordan, primarily dated using radiocarbon and/or historical constraints. Paleointensity data were obtained using two different methods: Triaxe, and Thellier-IZZI-MagIC incorporating anisotropy and cooling rate corrections. To validate the two methods, we tested a set of 30 self-made pottery vessels crafted from six different clay types using traditional techniques. The results demonstrated that baked clay materials dominated by pseudo-single domain vortex state serve as excellent recorders of the field. Yet, to ensure robust results, data should be averaged from at least three fragments per archaeological context, with a minimum of three specimens per fragment. We confirm that in non-single domain recorders, thermoremanent magnetization exhibits a logarithmic dependence on the cooling rate, consistent with single domain theory. After applying stringent selection criteria, the current dataset comprises nearly 1700 samples (fragments) - mainly indicative pottery, fired structures, tiles, furnaces, and slag - from 310 different well-dated archaeological contexts. With the exception of a few short time intervals, the temporal resolution of the data is a century or better, enabling the construction of a continuous Bayesian curve covering the past nine millennia. This curve provides geomagnetic insights into the amplitude and rates of geomagnetic secular variations. Finally, we present a few case studies demonstrating the application of the curve for archaeomagnetic dating.
How to cite: Shaar, R., Gallet, Y., Hassul, E., Bar-Sovik, L., and Vaknin, Y.: Archaeomagnetic Intensity Curve for the Levant (9000 BP - Present) in Sub-Centennial Resolution: Methodological, Geomagnetic, and Chronological Applications, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-12593, https://doi.org/10.5194/egusphere-egu25-12593, 2025.
The study of geomagnetic field variations provides information on the Earth's inner dynamics, helps understanding the role of the Earth's magnetic field as the primary shield against cosmic radiations and is also used as a geochronological tool for dating archaeological artefacts. Geomagnetic field variations during the Early Medieval Age (EMA) in Central Europe are generally poorly constrained due to the scarcity of archaeological sites. While a rapid intensity increase in the 6th century, along with high intensity values for the 7th to 9th centuries, have been reported for Western Europe, new archaeointensity data from other regions is thus needed in order to reconstruct more closely the spatio-temporal geomagnetic field evolution.
This work focuses on the study of the secular variations during the EMA period for selected regions in Central Europe : Germany, Austria and Poland. We analyzed potsherds from Ternitz and Unterrohrbach, and baked clay from Frauenkogel in Austria. In Poland, we examined potsherds from Klenica and Chobienia from two different locations; for the latter also daub and baked clay of a drying pan have been investigated. Finally, we studied kiln rocks from Schnapsweg and baked clay of a rampart from Fergitz in Germany.
For setting up the archaeointensity measurements, we used thermal demagnetization of the NRM and thermal κ(T) cycling to determine the unblocking temperature spectra and alteration behavior. The MT4 protocol – a Coe variant of the Thellier method - was used, including pTRM, tail checks and additivity checks, as well as corrections for anisotropy and cooling rate effects. Modified selection criteria sets TTA and TTB were applied. For Ternitz and Fergitz sites, we also used the multi-specimen domain-state corrected (MSP-DSC) protocol. Rock magnetic experiments comprised hysteresis and backfield curve measurements.
Between 500 and 700 AD, results of Unterrohrbach and Ternitz yield palaeointensities around 50 µT. While the MT4 site mean for Ternitz is characterized by high scatter, MSP-DSC experiments revealed a reliable archaeointensity. For Unterrohrbach site, a similar value with a lower scatter is determined. Finally, results from Frauenkogel site, suggest a rapid and strong increase of the archaeointensity within 100 to 150 years to high values around 85 µT. Similar high values were obtained in France. After this maximum, a strong intensity decline is indicated by the results from the remaining sites.
How to cite: Ségué-Passama, G., Schnepp, E., Arneitz, P., Leonhardt, R., and Egli, R.: Strong secular variation in Central Europe during the Early Medieval Ages, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-15788, https://doi.org/10.5194/egusphere-egu25-15788, 2025.
The Earth’s magnetic field, generated in the liquid outer core, predominantly behaves as a dipole over time. The processes generating the magnetic field, however, are complex and therefore also generate higher order pole signals. These higher order pole signals can alter the Earth’s magnetic field on a short time scale, even when the dipole signal is strong. A substantial deviation from the current dipole field is the South Atlantic Anomaly (SAA), a large weak spot in the Earth’s magnetic field above South-America. In the SAA’s center, the magnetic field strength is ~22 μT, approximately half of the field strength at the same latitude in Australia.
To better understand the origin and evolution of the SAA, it is essential to develop high-quality geomagnetic models of the Earth’s magnetic field over the past millennia. A major challenge for the current geomagnetic models is the significant data absence from the Southern Hemisphere, where the SAA is located. This lack of data hinders accurate modeling of the SAA’s evolution over time.
Our research aims to increase the amount of data on the Southern Hemisphere, particularly at locations on the same latitude as the current SAA. These locations are chosen based on the observation that the SAA has been moving westward over the past few decades, leading to the hypothesis that this westward movement has been ongoing for a longer period. We are currently working on enhancing the amount of data by adding high-quality full-vector paleomagnetic data from volcanic deposits on Réunion Island, Bali and Fiji.
Here we present the results of our paleomagnetic study of lava flows from Taveuni, Fiji, revealing a remarkable weak magnetic field of approximately 12 μT in flows dated to around 600 years ago. These flows also have a 20-30 degrees deviation in declination and inclination from expected values. Incorporating this new data into the global geomagnetic dataset allows us to refine existing models, leading to the unexpected conclusion that this exceptionally low field intensity cannot be attributed to the South Atlantic Anomaly—located below Africa at the time—but rather points to the presence of another geomagnetic feature: a West Pacific Anomaly.
How to cite: van Grinsven, L., Out, F., van den Bosch, M., Meyer, R., and de Groot, L. V.: New Evidence for a West Pacific Anomaly: Paleomagnetic Data from Taveuni, Fiji, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-18828, https://doi.org/10.5194/egusphere-egu25-18828, 2025.
This study presents new paleomagnetic results from La Barrancosa Lake (37°19’ S, 60°06’ W), located in the Argentinian Pampean region. The region's sparse paleomagnetic studies and its location under the South Atlantic Anomaly (SAA) make it a key area to investigate past geomagnetic field behavior. A 1-meter-long sediment core (covering approximately the last 2500 years), the longest paleomagnetic record recovered from the lake to date, was collected and analyzed. This work aims to improve the understanding of paleosecular variations (PSV) and the geomagnetic field's non-dipole behavior in the Southern Hemisphere.
The magnetic susceptibility (k) profile was used to correlate this core with previous records from La Barrancosa. Standard paleomagnetic measurements were performed, including natural remanent magnetization (NRM) intensity and directions (declination D and inclination I). Stepwise alternating field (AF) demagnetization revealed a stable single-component NRM after removing a low-coercivity viscous component. Characteristic remanent magnetization (ChRM) directions were determined using principal component analysis. Additional rock magnetic experiments, such as anhysteretic remanent magnetization (ARM), isothermal remanent magnetization (IRM) until saturation (SIRM), thermomagnetic curves, hysteresis loops and First Order Reversal Curve (FORC) analysis provided insights into the concentration, coercivity and grain size of magnetic minerals. The measurements were carried out at the Istituto Nazionale di Geofisica e Vulcanologia (INGV), Rome, Italy.
Preliminary results demonstrate well-preserved paleomagnetic signals with inclinations ranging from -64° to -17°. MAD values lower than 5° in the samples indicate reliable ChRM directions. These data will be compared with global geomagnetic models to address potential discrepancies and explore the contributions of non-dipole features in the region, likely associated with the influence of the South Atlantic Anomaly (SAA).
The new paleomagnetic record from La Barrancosa Lake enhances the temporal resolution of paleomagnetic studies in the Pampean region and provides critical data to investigate geomagnetic field variations in the Southern Hemisphere. The results underscore the region’s potential for refining global and regional geomagnetic models and highlight the importance of further research to explore the implications of these deviations for understanding the evolution of the SAA.
How to cite: Achaga, R., Gogorza, C., Iruruzun, M. A., Orgeira, M. J., Spagnuolo, L., Sagnotti, L., and Winkler, A.: Paleomagnetic results from La Barrancosa Lake, Argentina, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-714, https://doi.org/10.5194/egusphere-egu25-714, 2025.
North Anatolia is defined by a magmatic arc which occurred as a result of the subduction of the Late Cretaceous Neotethys ocean under the Pontides. This arc shape is 2700 km long, and can be observed from the Lesser Caucasus on the southern edge of Eurasia to the Apuseni, Banat, Timok, Srednogorie line along the northern edge of the Pontides. The paleolatitude of the Pontide volcanic belt was the aim several studies that pointed to a position of 28°N-24°N, while paleomagnetic rotations were interpreted with either by an oroclinal bending model or the excursion of Anatolia to the west. In the Balkans, however, Middle Triassic and Jurassic rocks showed no rotation or remagnetization in several areas. This study depend of the paleomagnetic results from the Upper Cretaceous İğneada Group in the northernmost part of the Western Pontides, Turkiye and the Burgas groups rocks outcropped in the Srednogoria belt in Bulgaria. The results showed that the arc type rocks were remagnetized in localy areas due to hydrothermal alteration associated with Cu-Au mineralization. The paleolatitudes obtained from both volcanic and sedimentary rocks, on the other side, were compared with the results from the Pontide magmatic arc.
How to cite: Cengiz, M. and Karabulut, S.: Paleomagnetic Constraints on the Tectonic Evolution of the Upper Cretaceous magmatic arc rocks in Western Pontides, Türkiye and Srednogorie, Bulgaria, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-14640, https://doi.org/10.5194/egusphere-egu25-14640, 2025.
Pyrrhotite is conspicuous by its very strong magnetocrystalline anisotropy of magnetic susceptibility (AMS) that can be in turn used in the investigation of the preferred orientation of this mineral by crystal lattice in rocks. Unfortunately, the AMS of pyrrhotite-bearing rocks is often composite, carried not only by pyrrhotite, but also by magnetite and mafic silicates; contribution of pyrrhotite can even be overwhelmed by that of the other minerals. It is therefore desirable to separate the AMS component due to pyrrhotite from that due to the rest of the rock. This can be made using the anisotropy of the out-of-phase component of magnetic susceptibility (opAMS), which can be obtained through AMS measurement in alternating magnetic field. Namely, the out-of-phase susceptibility (opMS) of paramagnetic minerals as well as of pure magnetite is virtually zero, while it is clearly non-zero in pyrrhotite. However, the problem is with measuring precision. As shown earlier, the error in opMS determination increases with decreasing phase angle, reaching extremely high values for phase near zero. And the phase is affected not only by opMS but also by ipMS of the measured specimen. It is therefore desirable to study the properties of the opMS and opAMS of real rocks.
The opMS of the pyrrhotite-bearing rocks investigated increases significantly with the field intensity and the increase is faster in very low fields (<100 A/m) than in stronger low-fields. The Rayleigh Law range, in which magnetization is linearly related to the field, is relatively narrow, less than 40 A/m. The principal directions of the opAMS are virtually field independent in the entire low-field range used (10 to 700 A/m) being also very well parallel to the ipAMS directions. The degree of opAMS is also virtually field independent, but much higher than the degree of ipAMS. The shape parameter in opAMS is also field independent and resembles that in ipAMS. Theoretical quadratic relationship exists between the degree of anisotropy of initial ipMS and that of the tensor of Rayleigh coefficient characterizing the opAMS. Searching for empirical relationship between degrees of the opAMS and ipAMS measured in stronger low fields is the purpose of the present paper.
Physically purest is evidently measurement of opAMS in very weak field, conveniently within the Rayleigh Law range. On the other hand, measurement of the opAMS in the strongest low-field available (700 A/m) is more convenient from the point of view of measuring precision. This is fully convenient if one is interested above all in principal directions and ellipsoid shapes, which are evidently field independent and closely resemble those of ipAMS, and less precise as for the degree of opAMS, which is significantly higher than degree of ipAMS. This must be respected in geological interpretation of the data.
How to cite: Hrouda, F., Chadima, M., and Ježek, J.: Low-field variation of out-of-phase susceptibility of pyrrhotite-bearing rocks and its implications for rock fabric studies, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-10873, https://doi.org/10.5194/egusphere-egu25-10873, 2025.
We present a paleomagnetic study of 471–454 Ma (Dapingian to Sandbian) limestones from the Siljan (Sweden) impact structure, offering new insights into Baltica’s paleogeography and the Mid–Late Ordovician geomagnetic polarity timescale. Stepwise thermal demagnetization revealed a primary magnetization component that passes fold and reversal tests. Our data indicate Baltica’s initial stationary phase at ~55°S during the Dapingian–Early Darriwilian, followed by rapid northward drift (~35 cm/year) starting in the Middle Darriwilian and slowing to ~15 cm/year by the Sandbian (~33°S). Furthermore, we established a detailed polarity timescale and correlated it with Ordovician outcrops across Baltoscandia and the Siberian platform. Based on our magnetostratigraphic data, we defined the end of the Ordovician superchron at 465.7 Ma, further advancing its temporal resolution. Our findings align with prior studies, including normal polarity chrons in the Mid and Late Darriwilian stages, and limit the superchron's maximum duration to about 14 Myr.
How to cite: Ahanin, N., Gilder, S., Ebbestad, J. O., and Almqvist, B.: Reconstructing Baltica’s Ordovician paleogeography and pinning the end of the Ordovician Superchron: Paleomagnetic data from Siljan, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-20844, https://doi.org/10.5194/egusphere-egu25-20844, 2025.
Posters on site: Thu, 1 May, 10:45–12:30 | Hall X2
Data-based geomagnetic models are key for mapping the global field, predicting the movement of magnetic poles, understanding the complex processes happening in the outer core, and describing the global expression of magnetic field reversals. There exists a wide range of models, which differ in a priori assumptions and methods for the interpolation of data in space and time. A frequently used modeling procedure is based on regularized least squares (RLS) spherical harmonic analysis, which has been used since the 1980s. This technique minimizes the error between modeled observations and data while constraining the model to realistic values, although some of these constraints have (partially) lost their physical foundation.
The first version of this algorithm has been written in Fortran and led many different research groups to produce versions of the algorithm in other programming languages, either published open-access or only accessible within the institute. To open up the research field and allow for reproducibility of results between existing versions, we provide a user-friendly open-source Python version of the RLS algorithm accompanied by six spatial and two temporal damping methods from literature to enforce a spatially and temporally realistic magnetic field. We also provide a comprehensive discussion of key background concepts - concerning Maxwells equations, spherical harmonics, cubic B-Splines, and regularization – for a deeper understanding of the theoretical foundation of RLS geomagnetic models.
While Python is known for its readability, it is often criticized for its high overhead costs. We addressed this issue by leveraging the banded structure of the normal equations and incorporating C-code (via Cython) for matrix operations, significantly improving speed. As a result, the algorithm can run on a standard laptop with performance comparable to its Fortran predecessor. We show how to employ the new lightweight and quick algorithm with ample examples from our four included tutorials. With this well-documented open-source Python version, we aim to encourage both existing and new users to create their own geomagnetic models and further advance the method.
How to cite: Out, F., Schanner, M., van Grinsven, L., Korte, M., and de Groot, L.: pymaginverse: a python package for global geomagnetic field modeling, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-18949, https://doi.org/10.5194/egusphere-egu25-18949, 2025.
The South Atlantic Anomaly (SAA) represents the region of Earth’s weakest magnetic field intensity, where the inner radiation belt approaches closer to the planet’s surface. This anomaly is a critical region for understanding radiation belt dynamics and their responses to solar activity-induced magnetospheric changes.
This study is based on our recent numerical simulations of the inner proton radiation belt [Girgis et al., JSWSC (2021), SW (2024)], extending the model to include electron dynamics in the inner magnetosphere. The simulations adopted the IGRF and Tsyganenko models to provide a time-dependent magnetic field driven by solar input conditions detected by ACE mission, including the associated inductive electric field. A key feature of this research is the incorporation of wave-particle interactions, identified through Pc4-Pc5 wave detections using the MAGDAS ground magnetometer network. The primary objective is to simulate the enhancement of electron flux in the northern SAA region due to wave-particle interactions.
Understanding particle dynamics within the SAA is essential for predicting the radiation environment in low Earth orbit (LEO) missions, forecasting ionospheric responses to severe space weather, and assessing potential long-term impacts on Earth's climate system.
How to cite: Girgis, K., Hada, T., Yoshikawa, A., and Matsukiyo, S.: Multi-Scale Observation-based Simulation Model for Investigating the Wave-Particle Interactions in the South Atlantic Magnetic Anomaly: Preliminary Results, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-554, https://doi.org/10.5194/egusphere-egu25-554, 2025.
The tectonic evolution of the Aegean Region can be divided into two main geological phases in the Cenozoic era. The first phase began at the end of the Mesozoic and is characterized by a compressional regime resulting from the closure of the Tethys Ocean and the formation of the Alpine system. This period was dominated by subduction tectonics, which shaped the geological evolution of the region. During this time, the formation of rift valleys, which are the most prominent structural elements of the Aegean today, was triggered. These rift valleys, typically bounded by faults on both sides, developed asymmetrically. They are the most dominant geological and morphological feature of Western Anatolia. The rift valleys, which are mostly bounded by normal faults, are seismically active. These rift valleys can be listed from north to south as follows: Edremit Gulf, Bakırçay-Simav Rift, Gediz-Küçük Menderes Rift, Büyük Menderes, and Gökova Rift. The second phase is characterized by a regional North-South extensional period. During this time, the fault systems in the region became more pronounced under the influence of extension. This extensional regime is related to changes in the stress environment within the lithosphere.
The Menderes Massif, with its unique geological structure and evolution, is another important feature of the region. It is particularly notable for being cut by numerous late-stage rifts, resulting in a dynamic structural evolution. The majority of the massif contains high to medium enthalpy geothermal reservoirs, with temperatures ranging from 120°C to 240°C. These reservoirs generally lie within metamorphic rocks and are located in lithologically diverse units. This study will focus on magnetism studies of geothermal wells in Western Anatolia, with samples taken from different depths and temperatures. The aim is to investigate the magnetic characteristics of the geothermal wells under pressure and temperature conditions. The methods applied will include the following: Magnetic susceptibility study (frequency-dependent susceptibility), Thermal magnetic susceptibility study, Hysteresis measurement, Isothermal remanent magnetization (IRM), Saturation isothermal remanent magnetization (SIRM). The result really Show the transformation of magnetic minerals in geothermal wells which have undergone different pressure and temperature conditions. Additionally, paleomagnetic measurements will be carried out to determine the movement and evolution of rocks over geological time.
How to cite: Cabuk, B. S., Cengiz, M., and Karabulut, S.: The Role of Magnetic Properties of Rocks in Determining the Geothermal Potential in the Western Anatolia Region, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-889, https://doi.org/10.5194/egusphere-egu25-889, 2025.
The loess-paleosol series (LPS) of Southern Tajikistan are one of the most complete and long-lasting geological archives of the Quaternary, beginning to form 2.7 million years ago (Zan et al., 2022). The LPS contain a record of past climate and can be used to reconstruct paleogeographic settings in the region. In addition to their significance as a paleoclimatic archive, the loess-paleosol series of Southern Tajikistan are archaeological sites due to the presence of Early and Middle Paleolithic tools in buried Pleistocene paleosoils. The geological and archaeological significance of the LPS of Southern Tajikistan determines the importance of developing a reliable chronostratigraphic scale for the sections. Currently, the basis for such a scale is the correlation analysis of the magnetic susceptibility of sediments and the oxygen isotope scale of the World Ocean (Forster, Heller, 1994; Shackleton et al., 1995; Dodonov et al., 2006). To verify the correlation comparison, absolute dating methods and the paleomagnetic method are used. The optically stimulated luminescence (OSL) method can be used for the upper part of the sections along the second pedocomplex (PC), for the lower units only paleomagnetic method can be used.
The position of the main magnetostratigraphic reference point in the Tajikistan LPS – the Matuyama-Brunhes boundary – is accepted in the loess layer between PC 9 and 10 (Penkov, Gamov, 1980), formed during the cold climatic stage MIS 20. While the generally accepted position of the Matuyama-Brunhes boundary in bottom ocean sediments corresponds to the warm climatic stage MIS 19 (Channell et al., 2010). Such a difference in the position of the Matuyama-Brunhes boundary in continental and oceanic sediments calls into question the correctness of the correlation and the synchronicity of the manifestation of climatic events on land and in the ocean as a whole. A similar age difference was established for the Jaramillo subchron. Therefore, the position of the main magnetostratigraphic benchmarks in the Tajikistan LPS requires a detailed study at the current level of quality and reliability to verify the age correlation of sediments.
We conducted such detailed magnetostratigraphic studies at the current level of quality and reliability and discovered the following:
1) Matuyama-Brunhes boundary is recorded higher in both reference sections under our study than previously thought and corresponds to the soil horizon of PC 9 (MIS 19).
2) The upper boundary of the Jaramillo normal polarity sunchron is also located higher – in the lower part of PC 14. The lower boundary, as was established earlier, is located at the base of PC 16.
The new position of the main magnetostratigraphic benchmarks resolves the previously existing climatic unconformity. The Matuyama-Brunhes boundary and the Jaramillo subchron boundaries are located in the same climatic stages as in the oceanic bottom sediments, by which the MIS are distinguished. However, it is easy to notice that their position is slightly lower than the expected position (by 1-1.5 m), which is associated with the mechanism of fixing the paleomagnetic signal by loess-paleosol series and its delay (lock-in depths).
How to cite: Kulakova, E.: New data on the position of magnetostratigraphic benchmarks in the loess-paleosol series of Southern Tajikistan, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-1063, https://doi.org/10.5194/egusphere-egu25-1063, 2025.
The reorientation of Earth through rotation of its solid shell relative to its spin axis is known as True polar wander (TPW). It is well-documented at present, but the occurrence of TPW in the geologic past remains controversial. This is especially so for Late Jurassic TPW, where the veracity and dynamics of a particularly large shift remain debated. Here, we report three palaeomagnetic poles at 153, 147, and 141 million years (Myr) ago from the North China craton that document an ~12° southward shift in palaeolatitude from 155–147 Myr ago (~1.5° Myr-1), immediately followed by an ~10° northward displacement between 147–141 Myr ago (~1.6° Myr-1). Our data support a large round-trip TPW oscillation in the past 200 Myr. By comparison of Jurassic paleomagnetic poles of the NCC and SIB, we suggest that the Late Jurassic true polar wander event may have biased paleomagnetic results and thereby affected the interpretation of the final closure of the Mongol-Okhotsk Ocean. Combining paleomagnetic data with regional geological evidence, we propose that the Mongol-Okhotsk Ocean was closed in its eastern segment in the Late Jurassic, marking the formation of the central Asian continent. We suggest that the shifting back-and-forth of the continents may contribute to the biota evolution in East Asia and the global Jurassic–Cretaceous extinction and endemism.
How to cite: Zhao, P., Hou, Y., Qin, H., Mitchell, R., Li, Q., Hao, W., Zhang, M., Ward, P., Yuan, J., Deng, C., and Zhu, R.: Late Jurassic true polar event revealed by paleomagnetic study in the North China Craton and its implication for regional tectonics and biota evolution in East Asia, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-4914, https://doi.org/10.5194/egusphere-egu25-4914, 2025.
The West Pacific-East Asia transition zone is characterized by a remarkable continental mosaic system and a chain of marginal basins. However, the complexity of the continental amalgamation process has led to controversy regarding the origin and migration of many micro-continents. In particular, the origin of the East China Sea (ECS) remains a subject of debate. The question of whether the ECS is "part of the South China Block (SCB)" or an "exotic microcontinent" has yet to be definitively resolved (Niu et al., 2015; Fu et al., 2022). Furthermore, there is divergence in perspectives concerning the evolution of the ECS, exemplified by models such as "back-arc spreading" and "strike-slip pull-apart", which in turn fuel disputes regarding the nature of the ECS basin.
In this study, we conducted paleomagnetic sampling of Cretaceous-Eocene cores from nine boreholes in the ECS basin. A systematic paleomagnetic study was undertaken, employing rock magnetic experiments, scanning electron microscope (SEM) analysis, and stepwise thermal demagnetization. Utilizing the inclination data of characteristic remanent magnetization (ChRM) obtained from thermal demagnetization experiments, we have, for the first time, derived paleomagnetic records for Early Cretaceous to Eocene cores from the ECS boreholes. The results indicate that the paleolatitudes of the ECS were 18.7° ± 4.5° (134 Ma), 21.4° ± 6.4° (107.2 ± 0.6 Ma), 18.1° ± 4.5° (66-61 Ma), 20.3° ± 4.3° (61-56 Ma), and 26.4° ± 8.2° (49-34 Ma). The investigation and comparison of the paleomagnetic data reveal that the paleolatitudes of the ECS are similar to those of the SCB from the Early Cretaceous to the Eocene. This suggests that the ECS and SCB have been part of the same tectonic block since the Early Cretaceous.
Further analysis of the spatial relationship between the ECS and SCB confirms that their relative motions can be delineated into three distinct phases: (1) During the Cretaceous period, the ECS and the SCB moved in the same direction, albeit with a disparity in their velocities; (2) During the Late Cretaceous to Early Paleocene period, the ECS migrated northward while the SCB shifted southward; (3) During the Middle Paleocene to Eocene period, the ECS and the SCB moved in concert, with negligible differences in velocity, thereby establishing a stable connection. It is concluded that the kinematic transitions of the ECS and the SCB from the Early Cretaceous to the Eocene were directly governed by changes in the subduction direction of the Izanagi/Pacific Plate.
How to cite: Xu, M., Yang, F., and Hu, P.: Is the East China Sea an exotic microcontinent from the Paleo-Pacific? ——Paleomagnetic Insights from the East China Sea Basin, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-6135, https://doi.org/10.5194/egusphere-egu25-6135, 2025.
The Deccan Continental Flood Basalts (DCFB) are associated with three major dyke swarms: the Narmada-Satpura-Tapi (N-S-T), the Western Coastal, and the Nasik-Pune swarms. The DCFB around Pachmarhi is characterized by a lower Magnesium number (Mg#) and higher TiO₂ content, suggesting a more evolved composition compared to other Deccan basalts. Located in the eastern segment of the N-S-T swarms, the Pachmarhi dyke swarms comprise approximately 244 mapped doleritic and basaltic dykes, with lengths ranging from 140 m to 22 km, averaging ~5.15 km. These dykes are emplaced along pre-existing fractures and predominantly exhibit an E-W orientation. Petrographic and rock magnetic analyses indicate that the primary remanence carriers are high- and low-titanium magnetite particles, primarily in the pseudo-single domain state, with a minor contribution from multi-domain grains.
Paleomagnetic studies have been conducted on 12 dykes, revealing that five exhibit normal polarity while seven display reverse polarity. The normal-polarity dykes are characterized by a mean ChRM direction of Dm = 332°, Im = -39.8° (k = 90.24, α95 = 8.16°, N = 32), whereas the reverse-polarity dykes exhibit Dm = 156.1°, Im = 38.1° (k = 55.4, α95 = 8.02°, N = 63). The combined mean ChRM direction has been determined as Dm ≈ 334° and Im ≈ -38.95° (k = 72.82, α95 = 8.09°, N = 95). The calculated paleopole for the Pachmarhi dykes (37.97° N, 88.38° W) closely corresponds to that of the Nandurbar-Dhule (N-D) dykes (38.3° N, 79.9° W), which represent the western segment of the N-S-T dykes. The averaged paleopole position (38.14° N, 83.84° W) aligns well with the Deccan Superpole (36.96° N, 78.7° W). This similarity suggests that the emplacement of these dykes occurred synchronously during the late stages of Deccan volcanism. The Pachmarhi dykes with normal polarity have been tentatively linked to magnetic chron 29N, while those with reverse polarity correspond to chron 29R. It is inferred that these dykes may have fed late-stage Deccan flow units, such as the Ambenali and Mahabaleshwar formations of the Wai Subgroup. The paleolatitudes of the Pachmarhi (22.4° S) and N-D (25.4° S) dykes indicate minimal latitudinal variation, supporting the hypothesis of near-synchronous emplacement across the Narmada-Son-Lineament (NSL) region.
How to cite: Shukla, G., Lakshmi, B., and Mallik, J.: Paleomagnetic evidence of synchronous emplacement of Deccan dykes along the Narmada-Son Lineament witnessing the magnetic reversal, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-6567, https://doi.org/10.5194/egusphere-egu25-6567, 2025.
Global plate reconstructions with continuously closed topological boundaries, also known as full-plate models, offer a crucial framework for understanding Earth’s history and its dynamic inner engine. These models serve as vital boundary conditions for investigating mantle thermochemical evolution, surface dynamic processes, and global climatic changes. Here, we present a new full-plate reconstruction spanning the Phanerozoic, linking with our previously published 2000–540 Ma model to create the world’s first full-plate model extending back to 2 Ga. Our model is constructed using a set of newly developed apparent polar wander paths (APWPs) based on innovative weighted-mean statistics and a carefully assessed list of paleomagnetic poles. The model is then optimized with geodynamic considerations and tested against geological constraints. Key differences between our model and existing ones include: (i) Our model is built on a paleomagnetic reference frame, ensuring that all plates with paleomagnetic constraints are positioned at their "correct" paleolatitudes; (ii) Our model constrains paleolongitude using the orthoversion hypothesis, which suggests that supercontinents form approximately 90 degrees longitudinally away from their predecessors; (iii) We provide two alternative models with different paleolongitudinal options; (iv) We offer distinct reconstructions for the early Cambrian motion of Baltica, the evolution of the Iapetus and Rheic Oceans, and the assembly of the Chinese blocks. While our model inevitably contains significant uncertainties due to the limitations of available data and should be used cautiously, it establishes a foundation for further testing and refinement by the scientific community as new critical data and enhanced databases become available.
How to cite: Liu, Y., Li, Z.-X., and Pisarevsky, S.: A new global full-plate model for the Phanerozoic, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-19872, https://doi.org/10.5194/egusphere-egu25-19872, 2025.
Low-field magnetic susceptibility of diamagnetic and paramagnetic minerals as well as that of pure magnetite and all single-domain ferromagnetic (s.l.) minerals is field-independent. In contrast, magnetic susceptibility of multi-domain pyrrhotite, hematite and titanomagnetite may significantly depend on the field intensity. Hence, the AMS data acquired in various fields have a great potential to separate the magnetic fabric carried by the latter group of minerals from the whole-rock fabric. The determination of the field variation of AMS consist of separate measurements of each sample in several fields within the Rayleigh Law range and subsequent processing in which the field-independent and field-dependent susceptibility tensors are calculated.
Using a 3D rotator developed for the MFK1/2/KLY5 series of AGICO Kappabridges, the measurement is fully automated in such a way that, once the sample is mounted into the rotator, it requires no additional positioning to measure the full AMS tensor. The important advantage of the 3D rotator is that it enables to measure AMS in a sequence of pre-set field intensities without any operator manipulation. Whole procedure is computer-controlled and, once a sequence of measurements is finished, the acquired data are immediately processed and visualized. Examples of natural rocks demonstrating various types of field dependence of AMS are given.
How to cite: Chadima, M. and Hrouda, F.: A simple toolbox for separation of field-independent and field-dependent AMS tensors using a sequence of fully automated measurements, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-11075, https://doi.org/10.5194/egusphere-egu25-11075, 2025.
