Paleomagnetism and magnetic fabrics: Recent advances and geological applications

The recent methodological and instrumental advances in paleomagnetism and magnetic fabric research further increased their already high potential in solving geological, geophysical, and tectonic problems. Integrated paleomagnetic and magnetic fabric studies, together with structural geology and petrology, are very efficient tools in increasing our knowledge about sedimentological, tectonic or volcanic processes, both on regional and global scales. This session is intended to give an opportunity to present innovative theoretical or methodological works and their direct applications in various geological settings. Especially welcome are contributions combining paleomagnetic and magnetic fabric data, integrating various magnetic fabric techniques, combining magnetic fabric with other means of fabric analysis, or showing novel approaches in data evaluation and modelling. We also highly solicit contributions showing all aspects of paleomagnetic reconstructions, acquisition of characteristic remanence and remagnetisations applied to solving geotectonic problems.

Convener: Martin Chadima | Co-conveners: Andrea Regina BiedermannECSECS, Juan José Villalaín
vPICO presentations
| Thu, 29 Apr, 09:00–10:30 (CEST)

vPICO presentations: Thu, 29 Apr

Chairpersons: Martin Chadima, Andrea Regina Biedermann, Juan José Villalaín
Magnetic fabric in deformed sediments
Sara Satolli, Claudio Robustelli Test, Elena Zanella, Dorota Staneczek, Fernando Calamita, and Evdokia Tema


The aim of this study is to investigate how structural deformation in shear zones is documented by the anisotropy of magnetic susceptibility (AMS). The study area is located in the Pliocene outer thrust of the Northern Apennines, which involved Cretaceous to Neogene calcareous and marly rocks. Here, brittle-ductile tectonites show different characteristics along two differently oriented thrust ramps: the NNE-SSW-trending oblique thrust ramp is characterized by the presence of S tectonites, while the NW-SE-trending frontal ramp is characterized by the presence of SC tectonites.

Samples for AMS fabric investigation were collected on shear zones from three sectors of the belt, at different distance from the main thrust to detect possible magnetic fabric variations. The three study area are characterized by different combinations of simple and pure shear, thus different degree of non-coaxiality, which has been quantified through the vorticity number Wk.

Specimens were measured with an AGICO KLY-3 Kappabridge at the CIMaN-ALP Laboratory (Italy) on 15 different directions mode. Only measurements with all three F-statistics of the anisotropy tests higher than 5 were accepted as reliable. Moreover, outliers characterized by ± 2σ difference with respect to the mean value of AMS scalar parameters were excluded from further analysis. In order to distinguish groups of specimens affected by different sedimentary or tectonic processes, we identified clusters of AMS scalar parameters; when clusters were not defined by these parameters, we applied a combination of contouring and cluster analysis on each principal axis to identify different subfabrics.

The magnetic fabric revealed straightforward correlations with structural data and specific changes of AMS axis orientation depending upon the increasing of deformation (lower vorticity number) and proximity to the main thrust. Similar evolution was detected in different deformation regimes. Overall, the magnetic fabric is more sensitive to the simple shear deformation, as the magnetic lineation tends to parallelize mostly with the computed slip vector; however in pure-shear dominated regimes, the magnetic lineation becomes parallel to the transport direction when the deformation is really intense (sites at less than 15-30 cm from the thrust plane).

The applied combination of density diagrams and cluster analysis on AMS data successfully allowed discriminating subfabrics related to different events, and shows a great potential to unravel mixed sedimentary and/or tectonic features in magnetic fabrics.

How to cite: Satolli, S., Robustelli Test, C., Zanella, E., Staneczek, D., Calamita, F., and Tema, E.: Magnetic fabric in carbonatic rocks from thrust shear zones, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1686, https://doi.org/10.5194/egusphere-egu21-1686, 2021.

Claudio Robustelli Test, Elena Zanella, Andrea Festa, and Francesca Remitti

Deciphering the stress and strain distribution across plate boundary shear zones is critical to understanding the physical processes involved in the nucleation of megathrust faults and its behaviour. Plate boundaries at shallow depth represent complex and highly deformed zones showing structures from both distributed and localized deformation.

As magnetic minerals are sensitive to stress regime, the investigation of the magnetic fabric has proven to be an effective tool in studying faulting processes at intraplate shear zones.

Anisotropy of magnetic susceptibility (AMS) provides insights into the preferred orientation of mineral grains and the qualitative relationships between petrofabrics and deformation intensity.

We present an approach of combined Contoured Diagram and Cluster Analysis to isolate the contribution of coexisting petrofabrics to the total AMS and evaluating the significance of magnetic fabric clusters.

Our results reveal distinct subfabrics with reasonably straightforward correlations with structural data. Specific AMS pattern may be associated to the intensity of the reworking related to tectonic shearing and the structural position within the shear zone (i.e., the proximity to the main thrust faults).

Close to the main thrust the magnetic fabric is dominantly oblate with magnetic foliation consistent to the S-C fabric and/or mélange foliation and the magnetic lineation parallel to the shear sense.

Away from the thrust faults the degree of anisotropy as well as the ellipsoids oblateness gradually diminishes. Thus, the presence of subfabrics related to previous tectonic events or less intense deformation (i.e. intersection lineation fabric) became dominant. The discrimination of subfabrics also allowed to unravel the presence of minor thrust plane and qualitatively evaluate the heterogeneous registration of strain (i.e. distributed versus localized deformation).

An abrupt change in magnetic ellipsoid shape and parameters is also observed below the basal décollements showing purely sedimentary magnetic fabric or previous deformation history with minor to absent evidences of shearing in the hanging wall.

Then, the integration with anisotropy of magnetic remanence experiments in different coercivity windows (ApARM) allow to separate the contribution of different ferromagnetic subpopulation of grains, constraining the significance of the different magnetic pattern/clusters detected through the AMS analysis.

In conclusion, our results show the potential of a combination of density diagrams and cluster analysis validated by ApARM experiments in distinguishing the superposition of deformation events, unravelling strain partitioning/concentration and thus to better understand the geodynamic evolution of subduction-accretion complexes.

How to cite: Robustelli Test, C., Zanella, E., Festa, A., and Remitti, F.: Unravelling heterogeneities in magnetic fabric record of the strain: a combined AMS and ApARM data analysis applied to intraplate shear zones., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1481, https://doi.org/10.5194/egusphere-egu21-1481, 2021.

Francho Gracia Puzo, Charles Aubourg, and Antonio Casas Sainz

With the objective of mapping strain on the footwall of a thrust in an orogenic context (Leyre thrust, South Pyrenean Range), more than 1500 unoriented shale fragments (0.7-6.2 g) have been collected. Scalar data (degree of anisotropy P and shape parameter T), together with ellipse of confidence of individual axes provide a proxy of strain acquired by shales in the footwall of the main thrust (Saur et al. 2020).

Normally, sampling is done by two methods: collecting oriented decimetric hand specimens; or drilling 2.5 cm diameter cylinders. This presents the advantage to deal with oriented samples. However, those techniques are time consuming and it is difficult to collect numerous samples in loose materials such as shales. On the contrary, collecting rock fragments presents the net advantage to provide a much better statistical characterization of the site.

All samples belong to the Eocene shaly formations from the Jaca Basin. Rock fragments are mostly fractured according to the bedding and/or cleavage surfaces. We demonstrate that the anisotropy parameters P and T maintain their values, regardless the shape and size of fragments. Rock magnetism indicates that AMS is primarily governed by illite, with little contribution of magnetite. AMS provides therefore a proxy of illite organisation within the matrix.

In the footwall of the Sierra de Leyre we have defined up to 7 parallel sampling sections, whose traces are perpendicular to the direction of the main thrust. On average, each section is made up of about 10 sampling sites and about 15 fragments are collected per site, covering a few square meters.

We are restricted by the dimensions of AGICO holders (8cm3 for cubes, or 10 cm3 for cylinders). It is possible to use an empty 10 cm3 cylinder, which can be filled with smaller fragments of rock. The automatic rotator allows a fast and precise description of the AMS tensor. We removed from analysis low susceptibility, carbonate-rich samples, that show a higher variety of magnetic minerals. All sites present homogenous results at the site scale, but with significant differences with respect to strain. P and T parameters are very sensitive to strain as illite is the dominant carrier. In addition, the ellipse of confidence of the minimum AMS axis (K3) provides a sensitive proxy to characterize the competition between bedding and cleavage.

The comparison between the different sections allows to map the areas of damage linked to the propagation of faults associated with the folds. 5 stages of development of the magnetic fabric allows the detection of damage gradients. The mapping has allowed the identification of hidden faults.    

This new approach is very promising, and allows much more detailed samplings in difficult areas, providing more robust statistical description of scalar AMS data. This methodology could be useful for the study of outcrops that are difficult to access, and more interestingly, from borehole cuttings.

How to cite: Gracia Puzo, F., Aubourg, C., and Casas Sainz, A.: AMS of strained shales fragments: a fast way to quantify the matrix damage, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6377, https://doi.org/10.5194/egusphere-egu21-6377, 2021.

Andrea Regina Biedermann, Yi Zhou, and Michele Pugnetti

The magnetic pore fabric method was proposed in the 1990s as a fast and efficient way to characterize the preferred shape and alignment of pores in sedimentary rocks. Since then, magnetic pore fabrics have been interpreted based on empirical relationships, stating for example that the orientation of magnetic pore fabric relates to pore alignment, and that degree and shape of the magnetic anisotropy correlate with pore aspect ratio and pore shape. Unfortunately, these empirical relationships vary largely between studies, making it challenging to interpret the data quantitatively. Here, we are presenting numerical models and experiments that help explain the variability in published relationships. Magnetic pore fabrics are determined by first impregnating the samples with a suitable ferrofluid, followed by magnetic anisotropy measurements in various fields and at multiple frequencies. These data are compared to known pore properties in synthetic samples, or X-ray-computed-tomography-based models of the pore space in natural samples. Our data show how the magnetic properties of various ferrofluids vary with measurement conditions, in particular measurement frequency. Ultimately, the effective susceptibility of the ferrofluid at measurement conditions largely affects measured magnetic anisotropy, and thus the observed empirical relationships. These findings allow to re-interpret existing empirical relationships and their variability, and also provide recommendations for future magnetic pore fabric studies. 

How to cite: Biedermann, A. R., Zhou, Y., and Pugnetti, M.: Explaining the large variability in empirical relationships between magnetic pore fabrics and pore space properties, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1278, https://doi.org/10.5194/egusphere-egu21-1278, 2021.

Magnetic fabric in dikes
Sofya Fursova and Roman Veselovskiy

Nowadays, anisotropy of magnetic susceptibility (AMS) is a very widespread method to investigate the magnetic fabric of the rocks. AMS studies provide the information on the magma flow during formation of intrusives (dikes and sills) based on the orientation of the AMS ellipsoid. In this work we present the results of the AMS measurements of Devonian, Paleoproterozoic and Neoarchean dikes, which are located on the northeastern part of the Kola Peninsula. We use these data to reconstruct the direction of magma flow with the final aim to reconstruct the spatial distribution of the magma sources.

Laboratory studies of AMS were carried out on 1282 samples representing 102 dikes. At this stage, all studied dikes were typified based on their geological age and type of AMS (normal, reversed, intermediate). It is shown that not all the studied intrusive bodies retained the primary magnetic fabric and only a few of them can be used to reconstruct the position of magmatic centers and are promising for obtaining reliable paleomagnetic data. We also present the first results of interpretation of the direction of the magma flow during dike swarms forming.

The study is supported by the grant of RSF #16-17-10260.

How to cite: Fursova, S. and Veselovskiy, R.: Magnetic fabric of Devonian and Precambrian dikes from the northeastern Kola Peninsula, Russia, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8720, https://doi.org/10.5194/egusphere-egu21-8720, 2021.

Emilio Herrero-Bervera and Manuel Calvo Rathert

The Miocene Tejeda Complex on Gran Canaria (Canary Islands) is characterized by more than 500 trachytic and phonolitic conesheets, dikes, hypabyssal syenite stocks and subordinate radial dikes from a 20-km diameter intrusive complex in the volcaniclastic fill of the Miocene Tejeda caldera (20 x 35 km) on Gran Canaria, Canary Islands. The dikes intruded concentrically around a central axis or radial symmetry and dip uniformly an average of~41 degrees toward the center.We have conducted a pilot study of magnetic properties as well as Anisotropy of Magnetic Susceptibility (AMS) on a variety of dikes (trachytic and phonolitic and basaltic in composition) to investigate the possibility of obtaining petrofabrics results that would allow us to test the origin of the formation of the Tejeda conesheets that most likely resulted from deformation processes due to resurgent doming initiated by recurrent replenishment of a flat  laccolith-like magma chamber. The current ideas indicate that the formation of the cone-shaped fractures were originated by a magma supply exceeding the volume that could be compensated for by up-doming of the overlying caldera fill. Thus far, our AMS results indicate that all the ten intrusives studied despite their different lithologies are susceptible of carrying a measurable magnetic signal. Low-field magnetic susceptibility vs temperature (k-T 28-700oC) experiments have identified mainly one primary magnetic mineral phase namely stoichiometric magnetite, Curie temperature of 585oC,  SIRM, hysteresis loops and back-field were performed and yielded a series of secondary magnetic mineral present as well and corroborated by FORC’s results. The petrofabric in the intrusive bodies results show coherent flow azimuths regardless of their time of emplacement. Three main types of magnetic fabrics, (i.e. A to C) were found. Fabric type A (plane Kmax-Kint parallel to the dike plane) represents magma flow direction within the intrusives and is the dominant fabric (~60% of all the intrusives) studied thus far. The Kmax axis inclinations show that about 70% of the intrusives were fed by inclined vertical magma fluxes (inclinations greater than 30o), and the rest of them (~30%) by horizontal to sub-horizontal magma fluxes. Vertical magma flow means inclined magma injection inside fractures, and become more probable as the source is therefore located very close to the origin of the caldera

How to cite: Herrero-Bervera, E. and Calvo Rathert, M.: On the rock magnetic properties and petrofabric (AMS) analyses of trachytic, phonolitic, basaltic dikes and conesheets emplaced on the Miocene Tejeda caldera complex, on Gran Canary Island, Spain., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9019, https://doi.org/10.5194/egusphere-egu21-9019, 2021.

Anisotropy of out-of-phase magnetic susceptibility
František Hrouda, Jan Franěk, Martin Chadima, Josef Ježek, Štěpánka Mrázová, and Michal Poňavič

Magnetostatic susceptibility of single crystals of graphite is negative (the mineral is diamagnetic) and strongly anisotropic. The in-phase component of dynamic susceptibility (measured in alternating magnetic field) is also negative, but an order-of-magnitude stronger than the magnetostatic susceptibility. The out-of-phase component, which is no doubt due to electrical eddy currents, is positive and strong. Consequently, if the graphite crystals in graphite ore are oriented preferentially by crystal lattice (LPO), one would expect strong anisotropy of magnetic susceptibility (AMS) of graphite ore in both in-phase (ipAMS) and out-of-phase (opAMS) components. The ipAMS is controlled not only by the LPO of graphite, but also by the preferred orientation of paramagnetic and ferromagnetic minerals of the barren rock, while the opAMS indicates only the LPO of graphite. In graphite ores occurring in the Moldanubian Unit of Southern Bohemia, the in-phase susceptibility ranges from negative values in the order of 10-5 [SI units] to positive values in the order of 10-4. This probably indicates simultaneous control by graphite and paramagnetic and/or ferromagnetic minerals. On the other hand, the out-of-phase susceptibility is much higher, in the order of 10-4, and no doubt indicates its graphite control. The degree of ipAMS is moderate, that of opAMS is truly high. The ipAMS foliation is roughly parallel to the metamorphic foliation in ores and wall rocks and the ipAMS lineation is parallel to the mesoscopic lineation. The opAMS is inverse to the ipAMS with the opAMS lineation being perpendicular to the metamorphic foliation. All this indicates a conspicuous LPO of graphite in the ore that was probably created during Variscan regional metamorphism and associated ductile deformation. The opAMS has therefore shown an effective tool for the investigation of the LPO of graphite in graphite ore or graphite-bearing rocks provided that the opAMS is strong enough to be determined with sufficient precision and graphite is the only conductive mineral in the samples investigated.

How to cite: Hrouda, F., Franěk, J., Chadima, M., Ježek, J., Mrázová, Š., and Poňavič, M.: Anisotropy of out-of-phase magnetic susceptibility due to eddy currents in graphite ore and its structural implications, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2385, https://doi.org/10.5194/egusphere-egu21-2385, 2021.

Josef Ježek and František Hrouda

In order to better understand the anisotropy of out-of-phase magnetic susceptibility (opAMS) due to eddy currents of rocks and minerals, we investigated the ipAMS and opAMS of copper cylinders, because copper is diamagnetic, internally isotropic magnetically, and its opMS is no doubt due to eddy currents. Results of laboratory measurements were compared to theoretical models.  The degree of AMS (P=k1/k3, k1>k2>k3) of internally isotropic substance is controlled by the internal susceptibility and demagnetizing factors solely controlled by the substance shape. If the internal susceptibility is very low and this is the case of magnetostatic susceptibility of most diamagnetic substances, there is virtually no shape anisotropy of such material. The magnetostatic susceptibility of copper is about -8 x 10-6 [SI units], which gives rise to undetectably low degree of magnetostatic AMS. The ipMS is also negative, but several orders lower depending on grain size, grain shape, and operating frequency. The opMS is on the other hand positive, relatively high and also dependent on operating frequency. Calculated from signed (negative) k1, k2, k3 values, the degree of ipAMS, P<1, and decreases with increasing elongation of copper cylinders. It also depends on grain size and operating frequency. The degree of opAMS, P>1 and increases with increasing cylinder elongation. It also depends on grain size and operating frequency. Parallel to the axes of elongated cylinders are the ipk1 and opk3 directions, while in flattened cylinders are parallel to the cylinder axes the ipk3 and opk1 directions. The above results are useful in the structural interpretation of the opAMS of rocks and ores whose opAMS is due eddy currents (in electrically conductive materials).

How to cite: Ježek, J. and Hrouda, F.: Unexpectedly strong shape anisotropy of out-of-phase magnetic susceptibility due to eddy currents of internally isotropic copper cylinders, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3076, https://doi.org/10.5194/egusphere-egu21-3076, 2021.

Cláudia Cruz, Helena Sant'Ovaia, William McCarthy, and Fernando Noronha

The Anisotropy of Magnetic Susceptibility (AMS) represents the contribution of all minerals in rock samples (paramagnetic, diamagnetic, and/or ferromagnetic minerals). An intermediate AMS tensor may be recorded in rocks where a composite fabric is present, due to the presence of both paramagnetic and ferromagnetic minerals, being possible to be resolved into two distinct subfabrics using techniques as out-of-phase AMS (opAMS). The magnetic susceptibility measured in alternating field can be resolved into in-phase and out-of-phase components. In-phase AMS (ipAMS) measures the bulk response of all minerals in a sample however, opAMS is only sensitive to selected ferromagnetic minerals such as hematite, titanomagnetite, and ultrafine magnetite. The opAMS can be harnessed as a tool for direct determination of magnetic subfabrics defined by ferromagnetic minerals. This work focuses on three Portuguese plutons: Lamas de Olo, Lavadores-Madalena, and Santa Eulália. The preliminary results show that magnetic susceptibility is lower in opAMS, the degree of magnetic anisotropy is much higher in opAMS and the ellipsoid shape parameter has no significant differences in opAMS or ipAMS. The ipAMS and opAMS tensors are in general coaxial, pointing out that standard AMS fabric is parallel to the subfabric of minerals like hematite, titanomagnetite, and ultrafine magnetite. Two sites from Lamas de Olo Pluton with low in-phase magnetic susceptibility (ipKm) values were also measured, showing two different scenarios: (i) the coaxially is present in one site, pointing out the presence of minerals like hematite (after magnetite) but with the same orientation as the matrix; (ii) different orientation of K1 and K3 in ipAMS and opAMS suggesting the presence of a ferromagnetic oxide like hematite (after magnetite) but with a different orientation from the paramagnetic minerals. Nevertheless, it should be noted that in samples with low Km values, the presence of ferromagnetic minerals is scarce (or absent) and the opAMS has minor accuracy (the associated error is greater). The opAMS findings attain similar results to the anisotropy of anhysteretic remanent magnetization (AARM) studies, once both are related to the presence of ferromagnetic minerals, and their magnetic properties. However, the opAMS does not require the permanent magnetization of samples and is measured simultaneously with the ipAMS. With further works, a larger number of samples will be measured to accomplish more information, and AARM measurements will be performed on the same samples to compare the ipAMS, opAMS, and AARM tensors.

Acknowledgements: This work was funded by the Fundação para a Ciência e a Tecnologia (FCT) under UIDB/04683/2020 project.

How to cite: Cruz, C., Sant'Ovaia, H., McCarthy, W., and Noronha, F.: Anisotropy of out-of-phase magnetic susceptibility: an approach for magnetic subfabrics determination, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2835, https://doi.org/10.5194/egusphere-egu21-2835, 2021.

Rock magnetism and experimental paleomagnetism
Helena Sant Ovaia, Cláudia Cruz, Ana Gonçalves, and Fernando Noronha

The magnetic susceptibility (Km) of granites is an important characteristic and it is mainly controlled by the presence of certain oxide minerals like magnetite and/or ilmenite, as well as ferromagnesian phyllosilicates such as biotite. The abundance of magnetite or ilmenite can be explained by different redox conditions in the magma chamber and distinct magma sources. The presence of magnetite or ilmenite as accessory minerals represents oxidized- or magnetite-type granites and reduced- or ilmenite-type granites, respectively.

This work focuses on the Km of 20 Variscan granitic massifs from northern and central Portugal and considers the results obtained in about 750 sampling sites, in order to deduce the redox conditions in the magma system. These granites are essentially two mica mesocrustal and biotite-rich basicrustal/infracrustal in origin and their emplacement was related to Variscan orogeny. In the northern and central Portugal, three main ductile deformation Variscan phases were recognized and described: D1, D2 and D3. The studied granites were subdivided in three main groups according to U-Pb dating, field observations and emplacement relative to the D3 phase. Therefore, the studied granites are subdivided as following: (1) syn-D3 two-mica (mosc=biot) granites, ca. 311 Ma; (2) late-D3 monzogranites, biotite-rich and two-mica granites (biot>mosc), ca. 300 Ma; (3) post-D3 monzogranites and biotite-rich granites, < 299 Ma.

The evaluation of the Km variation of the different granite groups shows that, as granites become progressively younger, the Km parameter tends to increase as a result of the increasing in the mantellic contribution to the genesis of the magmas. Syn-D3 granites display Km between 17.8 μSI and 186 μSI. This variation is due to the high textural and compositional diversity, including two-mica granites with different relative proportions of muscovite and biotite. Late-D3 granites are represented by two-mica and biotite-rich granites with calcium plagioclase, with several degrees of post-magmatic alteration, implying iron leaching processes. These processes also promote the crystallization of secondary muscovite, which implied a decrease in the Km values, and also a wide dispersion of Km values ranging between 7.3 μSI and 276 μSI. Post-D3 granites are mostly represented by biotite-rich granites with calcium plagioclase close to I-type granites. This granite group is divided into two subgroups: (i) post-D3 ilmenite-type granites with Km values of ca. 113 μSI, typical of biotite-rich granites; and (ii) post-D3 magnetite-type granites with Km values between 2078 μSI and 11676 μSI representing magnetite-type granites. In N and Central Portugal, these magnetite-type granites can occur in homogeneous plutons or in composite plutons constituted by ferromagnetic and paramagnetic facies.

As a conclusion, mostly of the granites from northern and central Portugal exhibit average Km values below 1000 μSI and are characterized by a paramagnetic behavior corresponding to reduced- or ilmenite-type granites. Among the all studied granites, only one pluton showed to be a truly oxidized- or magnetite-type granite, with Km of the order of 11676 μSI.

Acknowledgements: This work was funded by the Fundação para a Ciência e a Tecnologia (FCT) under UIDB/04683/2020 project.

How to cite: Sant Ovaia, H., Cruz, C., Gonçalves, A., and Noronha, F.: Reduced- or ilmenite-type granites versus oxidized- or magnetite-type granites: occurrence in Iberian Variscan Belt, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-11885, https://doi.org/10.5194/egusphere-egu21-11885, 2021.

William McCarthy, Vincent Twomey, Craig Magee, Michael Petronis, and Tobias Mattsson

Injection and inflation of magma in the shallow crust is commonly accommodated by uplift of the surrounding host rock, producing intrusion-induced forced folding that mimics the geometry of the underlying intrusion. Whilst such forced folds have previously been described from field exposures, seismic reflection images, and modelled in scaled laboratory experiments, the dynamic interaction between progressive emplacement of hot magma, roof uplift, and any associated fracture/fault development remains poorly understood. Analysis of ancient examples where magmatism has long-since ceased typically only provides information on final geometrical relationships, while studies of active intrusions and forced folding only capture brief phases of the dynamic evolution of these structures. If we could unravel the spatial and temporal evolution of ancient forced folds, we could therefore acquire critical insights into magma emplacement processes and interpretation of ground deformation data at active volcanoes.


We put forth a new hypothesis suggesting that thermoremanent magnetization records progressive deflection of the host rock during laccolith construction where these measurements can be used to measure the rate and dynamics of the magma emplacement of. Our test site is located within the basaltic lava pile of the ~800 m wide structural aureole surrounding the rhyolitic Sandfell Laccolith in SE Iceland, which intruded <1 Km below the palaeosurface at ~11.7 Ma. Our results show heat from the laccolith resets the remanence from samples within 50 m of the contact. Several variations in thermoremanent vectors observed further outward along the structural aureole reflect stepwise folding from incremental injection of magma suggesting as and the laccolith develops, different sections of the host rock are incrementally tilted and possibly reheated. This procedure could be tested in other ancient structure aureoles to investigate whether single or multiple thermal wm37@st-andrews.ac.ukoverprints coupled with structural observations could be used a proxy for ground deformation patterns in volcanic hazard assessment.

How to cite: McCarthy, W., Twomey, V., Magee, C., Petronis, M., and Mattsson, T.: Unravelling the dynamics of magma emplacement through palaeomagnetic backstripping of intrusion-induced host rock deformation: Analysis from the Sandfell Laccolith, SE Iceland, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15561, https://doi.org/10.5194/egusphere-egu21-15561, 2021.

Grachev Roman and Valery Maksimochkin

It is well known that low-temperature oxidation of rocks in the magnetic field could lead to the formation of secondary magnetization of chemical nature (CRM). This case is a common reason for errors in determining the strength and direction of the ancient magnetic field related to primarily thermoremanent magnetization (TRM). The absence of explicit criteria for separating these types of magnetization in rocks is a fundamental problem of paleomagnetic studies.

To try to recognize CRM and TRM components of magnetization in the basalt we have carried out a laboratory simulation of the low-temperature oxidation of the Red Sea basalt P72/4 by annealing samples in the magnetic field. The main magnetic mineral of carried basalt is homogeneous titanomagnetite with a median Curie temperature Tc=260°C and NRM=55 A/m. The Ti content in titanomagnetite grains varies in the range of 6.8-7.5 wt. %, lattice constant a=8.455A. Titanomagnetite magnetic grains are mostly in the PSD state and have a dendritic structure. Laboratory TRM was created when demagnetized samples were cooled from 400 °C to room T in an argon atmosphere and a magnetic field B=50 µT. The TRM value is about 85-90% of the NRM (TRMmean=49.2 A/m). 

Annealing of samples in the air was carried out at Tan=260 °C in a magnetic field with induction B=50 µT parallel and perpendicular to the previously created TRM. As a result of annealing during 12.5-1300 hours, Tc increased by 40-170 °C, Is increased from 2400 A/m to 3050 A/m, Hc decreased from 17.3 mT to 14.3 mT, Hcr – from 22.1 c to 19.2 mT. The oxidation state (Z) after annealing for 12.5, 100, 400, and 1300 hours were found to be 0.16, 0.31, 0.58, and 0.67, respectively.

Regardless of the field B direction, magnetization co-directed with the initial TRM and blocking temperatures above 400 °C occurs. The effect increases with the annealing time: particularly t=12.5 h, the proportion of magnetization with unblocking temperatures T>400 °C is about 15-20% of TRM (260-400 °C), while for t=400 hours it is already 35-40% of TRM (260-400 °C). Then again the magnetization obtained as a result of annealing in B⊥TRM, two components are detected by thermal cleaning: one co-directed with B (CRM component), the other in the direction of the original TRM. The thermal stability of the TRM component is significantly higher than that of the CRM: most (75-80%) of the blocking temperatures of the CRM are confined to a narrow range near Tan (260-340 °C), while the TRM is destroyed after heating to 340 °C only by 35-40%. Also, CRM and TRM components differ in their resistance to the influence of an alternating magnetic field. In the case of B∥TRM, only one component is diagnosed with magnetic and thermal cleaning. In this case, it is not possible to detect the presence of two superimposed magnetization components of different genesis.

This work was supported by the Russian Foundation for Basic Research, project 20-05-00573.

How to cite: Roman, G. and Maksimochkin, V.: Paleomagnetic properties of laboratory TRM of natural titanomagnetite during it`s low-temperature oxidation, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10918, https://doi.org/10.5194/egusphere-egu21-10918, 2021.

Paleomagnetism applied to tectonics
Dario Bilardello, William Callebert, and Joshua Davis

South American Jurassic/Cretaceous rocks has been troubled by elongated virtual geomagnetic pole (VGP) distributions, while many discordant poles from the Carboniferous to the Triassic have also been recognized, rendering the South American apparent polar wonder path (APWP) problematic. 

We have conducted a paleomagnetic study of the sedimentary Permo-Carboniferous Itararé Group rocks within the state of São Paulo, Brazil, including three intruding mafic sills that are attributed to Early Cretaceus magmatic activity. The site-mean VGP distributions obtained from the sedimentary rocks define elongations that include the VGPs of the mafic intrusions, and are interpreted as remagnetization paths toward the directions characteristic of the sills. These interpretations are supported by extensive rock-magnetic data that provide a viable mechanism for the secondary magnetizations. Careful analysis of the paleomagnetic data of the sedimentary rocks enables isolation of a primary VGP distribution that is consistent with the reference Carboniferous pole position.

Analysis of other Carboniferous to Triassic South American paleomagnetic VGPs reveals that the majority of these data are also elongated: regardless of the age of the rocks, the elongations dominantly intersect at the location of the Late Cretaceous reference pole, and a second location similar to the intersection of the VGP elongations of some Jurassic/Cretaceous rock formations, and also coincides with the cusp of the debated loop in the Carboniferous-Triassic APWP. Based on multiple lines of evidence, we interpret the elongations and their intersections to reflect remagnetizations that occurred as a result of the widespread magmatism associated with the opening of the South Atlantic. We suggest that the extent of the remagnetizations is formation-specific, and that other rock-formations should be carefully re-evaluated.

How to cite: Bilardello, D., Callebert, W., and Davis, J.: Great circle analysis of South American VGP-distributions suggests widespread Cretaceous remagnetizations: case study of the Itararé Group Rocks from Brazil, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6270, https://doi.org/10.5194/egusphere-egu21-6270, 2021.

M. Irene B. Raposo, Marcia Ernesto, Daniele Brandt, and Daniel Ribeiro Franco

The sedimentation in the Paraná Basin started during the Ordovician and ended with the Early Cretaceous magmatic event. Thick lava pilescovered the entire basin, and voluminous dikes and sills occur in the sedimentary sequences, mainly in the northeastern border of the basin. Despite the thermal effect, some Paleozoic sedimentary formations preserved their primary magnetization. The paleomagnetic results from the glacial Aquidauana Formation, an equivalent to the Itararé group in the north-western portion of the basin, indicated that the magnetization is compatible with the Middle-Late Permian age assigned in literature. A detailed investigation of the magnetic mineralogy and the magnetization of other Paleozoic sedimentary rocks led to the conclusion that the intrusive rocks were more effective than the lavas in disturbing the primary magnetization, especially in the low-clay content rocks. The secondary magnetizations identified in the different areas of the basin are not always compatible with the Early Cretaceous magnetization imprinted by the Paraná magmatism. This component prevails in the northeastern area, while a Permo-Triassic magnetization was identified in other areas. The results obtained so far are coherent with the geomagnetic reversal scale for the considered time interval, and the paleomagnetic poles agree with the APWP for South America.

How to cite: B. Raposo, M. I., Ernesto, M., Brandt, D., and Franco, D. R.: The Paraná magmatism (Brazil) as the villain regarding the remagnetization of the Paleozoic sediments in the basin – half-truth!, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16483, https://doi.org/10.5194/egusphere-egu21-16483, 2021.

Maiia Anosova, Anton Latyshev, and Alexey Khotylev

     The research object is magmatic bodies from the southern, central and northern parts of the Bashkirian megazone (the Southern Urals, meridian length of the Bashkirian megazone - 300 km). Most of the study intrusions have the Riphean age. In the Riphean the Bashkirian megazone was part of the East European craton. And in the Late Paleozoic rocks of the Bashkirian megazone were involved in the collision process. The formation of most studies bodies is associated with the Mashak magmatic event (the Riphean), which marks the collapse of the super-continent Nuna.

     The Middle Paleozoic component was isolated in 28 bodies. Probably it is the secondary component, that is widespread on the Southern Urals and has been repeatedly identified by other researchers. Directions comparison from different districts showed that there was a rotation of the southern, northern and central blocks of Bashkirian megazone relative to each other during the Late Paleozoic collision. At the same time, paleomagnetic directions from the northern regions (which are about 40-50 km apart from each other) statistically coincide or differ not so much. Which means that they were stable or relatively stable.

     Besides, the Riphean component was isolated and the paleomagnetic pole for the boundary of the Lower and Middle Riphean of the East European Craton (1349+/-11 Ma) is calculated from 8 thin sheet intrusions. Plat=8.4; Plong=162.4; A95=4.1.

How to cite: Anosova, M., Latyshev, A., and Khotylev, A.: Relative movements of the Bashkirian megazone blocks(the South Urals) according to paleomagnetic data., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14264, https://doi.org/10.5194/egusphere-egu21-14264, 2021.

Ivan Lebedev, Elizaveta Bobrovnikova, Artem Moiseev, and Bagdasarian Tatiana

The Cretaceous Okhotsk-Chukotka volcanic belt (OChVB) is one of the largest provinces of continental marginal magmatism with length more than 3000 km along the Pacific edge of Asia. In the field studies of 2019 and 2020 we sampled 21 sections in the northern part of the OChVB and 3 sections from basement of OChVB. These sections are represented by basalts and andesites; their tuffs, ignibrites and other volcanic rocks are much less common. The age of these volcanics is estimated based on U-Pb and Ar-Ar published data and our new Ar-Ar dates.

Based on the obtained data, a new paleomagnetic pole for the Chukotka part of the OChVB was calculated. The latitude of this paleomagnetic pole differs from the expected one when compared with that calculated for Chukotka from published data from Besse and Courtillot, 2003; Torsvik et al., 2012. These results are inconsistent with most of the existing geological data. Only a few works admit younger displacements in the southern part of the Verkhoyansk fold belt or in modern diffuse boundary of the Eurasian and North American plates. Moreover, we compare our OChVB pole with results from basaltic complexes from the basement, which has been likely remagnetized when OChVB was formed.

Acknowledgements: study of cretaceous volcanics is supported by RSF grant № 19-47-04110 and jurassic by RSF grant №18-77-10073.

How to cite: Lebedev, I., Bobrovnikova, E., Moiseev, A., and Tatiana, B.: Cretaceous paleomagnetic directions in different volcanics in Chukotka (NE Russia), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9596, https://doi.org/10.5194/egusphere-egu21-9596, 2021.

Emö Márton, Vesna Cvetkov, Martin Đaković, Vesna Lesić, and Slobodan Radusinović

In the Southern part of Montenegro three main tectonic units are distinguished. The Dalmatian (South-Adriatic) zone is in the lowermost tectonic position and comprises shallow water Cretaceous carbonates, bauxites, Middle Eocene nummulitic limestones, transitional marls, and Upper Eocene flysch. It is thrusted over by the Budva zone characterized by deep water sediments of Triassic through Paleogene ages with tuffitic layers in the Ladinian. The uppermost unit is the High Karst zone developed in a carbonate platform environment from the Middle Triassic till the end of the Cretaceous. In this unit flysch sedimentation started after the K/T boundary.
From the above units we drilled and oriented in situ a total of 248 samples representing nine localities from the Dalmatian, six from the Budva and five from the High Karst zone, respectively. The harder rocks were drilled with portable gasoline powered, the softer ones with an electric drill. The laboratory measurements, demagnetizations, statistical evaluations of the measurements were carried out using standard methods in the Paleomagnetic Laboratory of the Mining and Geological Survey of Hungary. 
From the Dalmatian zone, the Upper Cretacous limestones have diamagnetic susceptibilities, very weak NRM, which either failed to provide statistically acceptable results (two localities) or obtained their remanence quite recently. The five flysch and transitional marl localities have well-defined AMS fabrics as well as paleomagnetic directions. The AMS fabrics must have been imprinted in a NE-SW oriented compressional strain field, which resulted in moderately strong fabrics with NW-SE oriented AMS lineations, perfectly following the Dinaridic trend recognized in Croatia. One locality, the only one with moderate tilt and rather poor statistical characteristics suggest post–Eocene CCW, while those with steep dips have well-defined paleomagnetic directions, which are aligned with the intermediate AMS direction. The positive tilt test, however, provides an overall mean direction with absurdly shallow inclinations, which suggest that we are not dealing with “real” paleomagnetic directions, but maybe with magnetic anisotropy governed NRMs.
The rocks in the Budva zone show a large variation in lithological sense as well as in their magnetic properties. Except a Pietra Verde localitiy, the magnetic susceptibilities are weak, we can not define AMS fabrics, yet most of the localities yielded fairly good paleomagnetic results, which suggest pre-folding age of the acquisition followed by a large CW rotation of unit.
From the High Karst, only a single Lower Jurassic pelagic limestone provided paleomagnetic results which exhibit CW rotation. The others had weak magnetizations, diamagnetic susceptibilities and no or scattered magnetic signals.
This work was financially supported by the National Development and Innovation Office of Hungary project K 128625, by the Geological Survey of Montenegro and Ministry of Science and Education of Serbia project No. 176016.

How to cite: Márton, E., Cvetkov, V., Đaković, M., Lesić, V., and Radusinović, S.: First paleomagnetic and magnetic anisotropy results from Montenegro – the coastal area, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12596, https://doi.org/10.5194/egusphere-egu21-12596, 2021.

Juan José Villalaín, Pablo Calvín, Puy Ayarza, Ruth Soto, and Manuel Calvo

The Iberian microplate and its evolution during the Mesozoic have been in-vogue topics in the field of Geodynamics, because of its location between two of the major tectonic plates, its interaction with both of them, and its significance in relation with the evolution of the western Tethys domain. Geodynamic models of Mesozoic Iberian evolution are based upon the knowledge of the kinematics of the microplate obtained from the ocean floor magnetic anomalies and particularly its apparent polar wander path (APWP) defined by existing paleomagnetic data. In this sense, the most important feature is the anticlockwise 30º-40º rotation that Iberia underwent during the Cretaceous. Nevertheless, there are still uncertainties about the chronology of this movement due to the poor definition of oceanic magnetic anomalies and the scarcity of high-quality paleomagnetic data in the continent. According to recent works, existing paleomagnetic poles are contradictory and inconsistent with the global apparent wander path (GAPWP) and ocean floor anomalies. This is due to the widespread presence of remagnetizations in the Mesozoic basins within Iberia.

To address this question we are starting to develop a new project that aims to obtain new paleomagnetic data from unexplored geological units meeting the necessary condition to obtain new Cretaceous paleomagnetic poles representative of the Iberian plate. In this talk we show new paleomagnetic data from detritic deposits with siliceous cement located at the Duero basin (North Iberian Meseta) ascribed to the upper Cretaceous-Paleocene (Areniscas de Salamanca Formation). A stable paleomagnetic component carried by hematite, showing normal and reversed polarities has been isolated. A stable paleomagnetic component carried by hematite, showing normal and reversed polarities has been isolated. We discuss the primary character of this magnetization in terms of inferring the age of this unit in the frame of the kinematic evolution of the plate.  

How to cite: Villalaín, J. J., Calvín, P., Ayarza, P., Soto, R., and Calvo, M.: New paleomagnetic results obtained in Upper Cretaceous-Paleocene detritic deposits of the North Iberian Meseta, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15885, https://doi.org/10.5194/egusphere-egu21-15885, 2021.

Methods and data processing
Ivan Efremov and Roman Veselovskiy

There are many programs for the analysis and visualization of paleomagnetic data, but each of them is good only in a certain use case and does not allow to perform a full cycle of paleomagnetic operations. Therefore, one has to resort to using a number of programs to complete the full path of processing paleomagnetic data. You often have to convert data from one format to another, manually vectorize charts, and generally spend more time and effort than could theoretically be spent. Thus, there is a long overdue need for a universal program capable of fast, convenient and high-quality performance of a full cycle of paleomagnetic operations. A set of programs written by Randy Enkin (Enkin, 1996) for DOS was taken as a time-tested example of such a program. The choice fell on them, since these programs (although they are very outdated) allow performing a full cycle of paleomagnetic operations and do it as conveniently and efficiently as possible for that time.

Our goal is to create a program devoid of all of the above disadvantages and capable of developing indefinitely as modular opensource software by the efforts of all people interested in this.

The result of our work is PMTools – a cross-platform software for statistical analysis and visualization of paleomagnetic data. PMTools supports all widely used paleomagnetic data formats and allows you to work with them simultaneously. All charts created in PMTools are vector, adapted for direct using in publications and presentations, and can be exported in both vector and raster formats. At the same time, PMTools implements a full cycle of routine paleomagnetic operations: from finding the best-fit directions to calculating the mean paleomagnetic poles. Moreover, all operations can be performed both with a mouse through a graphical user interface and with hotkeys, which significantly speeds up the data analysis process. 

In the near future, PMTools will become a modular open source application, so that each user will be able to add its own modules, thereby expanding the program's functionality.


Enkin, R.J., 1996. A Computer Program Package for Analysis and Presentation of Paleomagnetic Data, Pacific Geoscience Center, Geological Survey of Canada, http://www.pgc.nrcan.gc.ca/tectonic/enkin.htm.

How to cite: Efremov, I. and Veselovskiy, R.: PMTools – a new program for paleomagnetic data analysis and visualization, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3474, https://doi.org/10.5194/egusphere-egu21-3474, 2021.

Leonid Surovitskii, Andrei Kosterov, Mary Kovacheva, Maria Kostadinova-Avramova, Natalya Salnaya, and Aleksey Smirnov

The three-axis isothermal remanent magnetization (IRM) test (the Lowrie test; Lowrie, 1990, Geophys. Res. Lett., 17, 159-162) is a useful tool to identify ferromagnetic minerals by their coercivity and unblocking temperature spectra. In this study, we explore a variant of the Lowrie test in which measurements are conducted directly at elevated temperatures, and compare its performance with the results of the conventional stepwise procedure. IRM acquisition fields applied along three orthogonal axes were 1 T, 200 mT and 40 mT, respectively. The field value for the soft component was chosen so as to include ca. 90% of its coercivity spectrum. For the hard component the maximum available field was used. The test is applied to characterize the magnetic mineralogy of archaeological baked clays and bricks from Bulgaria and Russia. Bulgarian samples are baked clays from various Neolithic (5700-5300 BCE) archaeological sites and several bricks of the Roman epoch (III-IV c. AD). Samples from Russia are bricks originating from several regions with ages from XIII to early XIX c. AD.

The low- and intermediate-coercivity components of IRM in the studied samples are typically demagnetized by 520-550°C, compatible with substituted or cation-deficient magnetite or, possibly, maghemite. This is supported by the absence of the Verwey transition in studied samples (Kosterov et al., 2021, Geophys. J. Int., 224(2), 1256-1271). The high-coercivity component appears to be carried by two mineral phases with very distinct unblocking temperatures, 120-200°C and 500 to 640°C. The first phase is similar to the high coercivity, low unblocking temperature (HCSLT) phase described by McIntosh et al., 2007 (Geophys. Res. Lett., 34, L21302, doi: 10.1029/22007GL031168), and the second one appears to be hematite with variable degree of substitution.

Performance of the high-temperature variant of the Lowrie test compares favorably with the classical procedure, while the former is also significantly faster and yields a superior temperature resolution.

This study is supported by Russian Foundation of the Basic Research, grant 19-55-18006, and by Bulgarian National Science Fund, grant KP-06-Russia-10.

How to cite: Surovitskii, L., Kosterov, A., Kovacheva, M., Kostadinova-Avramova, M., Salnaya, N., and Smirnov, A.: High-temperature three-axis IRM Lowrie test, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1782, https://doi.org/10.5194/egusphere-egu21-1782, 2021.

Martin Chadima and Josef Jezek

The growing interest in ferromagnetic fabric, i.e. magnetic fabric carried solely by ferromagnetic (sensu lato) grains, creates the need for a simple way to obtain anisotropy of magnetic remanence (AMR) tensors from a set of laboratory imparted, spatially distributed vectors of remanent magnetization (RM). Here, we present a simple, user-friendly toolbox embedded into Anisoft software which facilitates AMR tensor fitting from an array of RM vectors acquired according the prescribed data acquisition protocol (6, 9, 12, 15, 18 magnetizing directions). As these protocols are usually quite laborious involving a series of demagnetizations and directional magnetizations, this toolbox provides a graphical visualization of the measured RM vectors in terms of their directional comparison with the respective magnetizing directions and their intensity in comparison with the intensity of demagnetized state (background magnetization). This visualization provides a convenient way to check whether there are no evident missteps and the measured vectors correspond to the acquisition protocol. Prior to tensor fitting, the correction for the background magnetization is optionally done by a direct subtraction of the measured background or by mutual subtraction of antipodally magnetized RM vectors, if available. The AMR tensors are fitted by the least-square algorithm using two slightly different approaches involving either (1) all three orthogonal components of magnetized RM vectors (Vector Method) or (2) their projections to the respective magnetizing directions (Projection Method). The differences between these two approaches are discussed. The calculated AMR tensors, including their confidence limits, are stored in the same file format as AMS data and can be thus instantly visualized and further processed using Anisoft software.

How to cite: Chadima, M. and Jezek, J.: A toolbox for convenient calculation of anisotropy of magnetic remanence, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1155, https://doi.org/10.5194/egusphere-egu21-1155, 2021.