Displays

Shear zones, or their counterparts in near-surface conditions, the brittle fault zones, constitute crustal-scale, narrow, planar domains where deformation is strongly localized. The variation with depth of deformation conditions (P-T), rheology and strain rates entails a wide range of fault rock types, characterized by different petrofabrics and classically grouped into mylonitic (fault rocks undergoing crystalline plasticity) and cataclasitic (fault rocks undergoing frictional deformation) series. Magnetic fabric methods (most frequently anisotropy of magnetic susceptibility, AMS) have been established as a useful tool to determine fault rock petrofabrics in shear/fault zones, being interpreted as kinematic indicators with a considerable degree of success. However, mylonites and cataclasites show remarkable differences in magnetic carriers, shape and orientation of the fabric ellipsoid. Here, we present a study of ten brittle fault zones (one of them at the plastic-brittle transition) located in various locations in the Iberian Plate, with an aim  to interpret patterns of AMS in cataclasites.

Reviewing AMS studies dealing with SC mylonites, three fundamental features can be drawn: i) the presence of composite magnetic fabrics with shape and lattice-preferred orientations, ii) the fabric is carried predominately by ferromagnetic minerals and iii) surprisingly in composite fabrics, the absolute predominance of magnetic lineations parallel to (shear) transport direction (88% of the reviewed sites), independently of fabrics being defined by paramagnetic or ferromagnetic carriers. Based on our study, magnetic fabrics in cataclasites: i) are mainly carried by paramagnetic minerals and ii) show a strong variability in magnetic lineation orientations, which in relation with SC deformational structures, are either parallel to transport direction (44% of sites) or parallel to the intersection lineation between shear (C) and foliation (S) planes (41%). Furthermore, changes between the two end-members can be frequently observed in the same fault zone. Sub-fabric determinations (LT-AMS; AIRM and AARM) also indicate that the type of magnetic lineation cannot be consistently related with a specific mineralogy (i.e. paramagnetic vs ferromagnetic minerals).

The wide range of deformation conditions and fault rocks covered in our study allowed us to analyse the factors that control these different magnetic lineation orientations, especially in brittle contexts. Plastic deformation results into a mineral stretching parallel to transport direction which can be directly correlated with the development of transport-parallel magnetic lineation. In brittle fault zones, the degree of shear deformation can be directly correlated with the type of magnetic lineation. The fault cores, where strain and slip are localized, show a predominance of transport-parallel magnetic lineations, most probably related with the development of lineated petrofabrics. Furthermore, the minor development of shear-related petrofabrics enhance the frequency of intersection-parallel magnetic lineations, also contributing the presence of inherited, host rock petrofabrics in the fault rocks.

How to cite: Marcén, M., Casas-Sainz, A., Román-Berdiel, T., Oliva-Urcia, B., Soto, R., García-Lasanta, C., Calvin, P., Gil-Imaz, A., and Aldega, L.: Magnetic fabric in brittle faults and ductile shear-zones: Examples from cataclasites from the Iberian Peninsula, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-207, https://doi.org/10.5194/egusphere-egu2020-207, 2020.

With the objective of mapping strain around a thrust front in an orogenic context (Pyrenean Range), 757 shale fragments (0.7-6.2 g) have been collected in 49 sites. 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 like shales. On the contrary, collecting rock fragments present the net advantage to have a much better statistical description of the site. We are restricted by the dimensions of AGICO holders (8cm³ for cubes, or 10 cm³ for cylinder). It is possible to use an empty 10 cm³ cylinder, which could be filled with smaller fragments of rock. The homogeneity of magnetic field of MFK2 Kappabridge (AGICO) allows to measure sample with no distortion due to irregular shape. In addition, the automatic rotator allow a fast and precise description of the AMS tensor.

All samples belong to the Hecho Group (Eocene from Jaca Basin), consisting of cleaved or stratified marls. Rock fragments are mostly fractured according to the bedding and/or cleavage surfaces. Then we set the rock “horizontally” with the main surface parallel to the bottom of the box, to keep a geometrical reference. We assume that the anisotropy parameters P and T will 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.

It is noticeable the speed with which data can be acquired in a well-known regional geological setting (757 samples, 49 sites) during 5 field work days and 17 laboratory days. About 15 fragments per site, covering few square meters, display homogenous pattern of P, T, and ellipse of confidence. The data visualization is done thanks to Anisoft 5.1 Software (Chadima, M.). We removed from analysis low susceptibility samples which are carbonate-rich and with more varieties 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 minimum AMS axis (K3) provide a sensitive proxy to characterize the competition between bedding and cleavage.

This new approach is very promising, and allows much more detailed sampling in difficult area, with much more robust statistical description of scalar AMS data. Aubourg et al. (EGU, TS7.3 session) will use these data to show the pattern of strain in a ramp-related fold.

How to cite: Gracia-Puzo, F., Aubourg, C., Casas-Sainz, A., and Boiron, T.: AMS of strained shales fragments: a fast way to quantify the matrix damage., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11399, https://doi.org/10.5194/egusphere-egu2020-11399, 2020.

Literature data on single crystals of graphite shows that this mineral is diamagnetic and strongly anisotropic. In addition, it possesses high electrical conductivity and extremely strong conductivity anisotropy. The AC magnetic susceptibility of graphite exhibits weak and negative in-phase component and relatively strong out-of-phase component, which is no doubt due to electrical eddy currents. Consequently, if the graphite crystals are oriented preferentially by crystal lattice (LPO) in graphite ore, one would expect strong anisotropy of magnetic susceptibility (AMS) of the ore. Unfortunately, the standard AMS, which is in fact the anisotropy of the in-phase component of susceptibility (ipAMS), reflects not only the LPO of graphite, but also the preferred orientation of paramagnetic and ferromagnetic admixtures. On the other hand, the anisotropy of out-of-phase susceptibility (opAMS) indicates LPO of graphite, free of the effects of non-conductive paramagnetic and ferromagnetic minerals.

The above theoretical expectations were tested on natural South Bohemian graphite ores occurring in the wide vicinity of the town of Český Krumlov in the Moldanubian Unit and being mined for pencil industry in the past. The ores are metamorphic in origin and one can therefore expect strong LPO of graphite in them. The graphite ores were sampled in two localities near the town of Český Krumlov, one being a road cut outcrop and the other one being a graphite mine. In both cases, the in-phase susceptibility is very low, in the order of 10^-6 [in SI units], being positive in some specimens and negative in the others. This indicates simultaneous and more or less balanced 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 opAMS is truly high, P = 2 to 3, and the opAMS foliation is closely related to the metamorphic foliation in ores and wall rocks. All this indicates a conspicuous LPO of graphite in the ore that was probably created during Variscan regional metamorphism and associated ductile deformation.

How to cite: Hrouda, F., Chadima, M., Ježek, J., Mrázová, Š., and Poňavič, M.: Lattice preferred orientation of graphite in a graphite ore as investigated by the anisotropy of out-of-phase magnetic susceptibility, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2868, https://doi.org/10.5194/egusphere-egu2020-2868, 2020.

Field impressed AMS fabric, although it has been recognized for a very long time, has been the subject of very few publications in the paleomagnetic literature. This effect has been mainly described in samples with magnetite as a main magnetic carrier. This fabric is usually of low magnitude and observed mainly in nearly isotropic rock after application of static AF demagnetization or after acquisition of an isothermal remanent magnetization (IRM). Forty four paleomagnetic sites have been sampled in a >2 km thick sequence of Cretaceous volcano-clastic rocks from the western Central Pamir mountain (Tadjikistan). These rocks present a medium grade level of metamorphism characterized by fine grained recrystallisation of biotite. The magnetic properties are very homogeneous across the sequence. Bulk magnetic susceptibilities vary between 150-250 μ SI. The AMS magnetic fabrics correspond to triaxial tensors with a well defined foliation plane and a steeply dipping magnetic lineation. The degree of anisotropy varies between 1.03 and 1.2. This fabric was likely acquired during the deformation associated with the emplacement of Middle Miocene gneiss domes. SEM/EDS data indicate that the main iron oxide mineral is hematite with up to 15% of ilmenite in solid solutions. This is in agreement with unblocking temperatures of SIRM around 630 °C, lower than the one of pure hematite. One of the most surprising magnetic characteristics of these rocks is the effect of strong-field remanent magnetizations upon the AMS. During the acquisition of an Isothermal Remanent Magnetization (IRM), the initial AMS is progressively obliterated by a new AMS fabric. The field-impressed AMS is characterized by a decrease of the magnetic susceptibility along the direction of the IRM and an increase in magnetic susceptibility in the orthogonal plane. The field-impressed AMS is thus mainly oblate with a degree of anisotropy usually between 1.2 and 1.4. As far as we know, such a strong effect has never been reported. In sandstone with detrital hematite as the main carrier, the degree of the induced AMS fabric is less than 1.02 suggesting that the ilmenite content in the metamorphic hematite is the main cause of the large observed field induced fabric in these rocks.

How to cite: Roperch, P., Aminov, J., Dupont-Nivet, G., Guillot, S., and Lagroix, F.: Large field impressed anisotropy of magnetic susceptibility (AMS) in metamorphic volcanoclastic rocks from the western Central Pamir with ilmeno-hematite as the main magnetic carrier., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2702, https://doi.org/10.5194/egusphere-egu2020-2702, 2020.

The dewatering and subsequent drainage of fluids from porous sediments in forearc regions controls heat flux and the frictional behavior of the plate boundary decollement and all other forearc faults. Here we present new rock magnetic datasets that help to depict the strain history and locus of fluid and gas migration across a shallow subduction thrust near the deformation front of the Hikurangi subduction margin (New Zealand). Site U1518 of International Ocean Discovery Program (IODP) Expedition 375 penetrated hanging-wall, the roughly 60 m thick fault-zone, and footwall sequences of the Pāpaku fault up to a maximum depth of 504 mbsf.

Rock magnetic investigations include the measurement of Anisotropy of Magnetic Susceptibility (AMS), static three-axis alternating field demagnetization (AFD), magnetic hysteresis, anhysteretic remanence acquisition (ARM) and S-ratio measurement. The datasets are presented for an interval between 275 and 375 mbsf, and encompass both fault-zone and directly adjacent sequences.

Throughout most of the sedimentary sequence, samples yield intensities of the natural remanent magnetization (NRM) between 10^-5 and 10^-6 Am²/kg. Magnetic coercivities range from 40 to 60 mT. During static AFD samples acquired a gyroremanent magnetization. These observations indicate the presence of authigenic greigite (Fe₃S₄). In two intervals, between 304 and 312, and 334 - 351 mbsf, samples yield distinctively lower remanence intensities (~ 10^-7 Am²/kg) and lower coercivities around 20 mT. The upper interval coincides with the onset of brittle deformation at the top of the fault-zone. In the same interval AMS results change abruptly. We propose that the rock magnetic signature is due to the reduction of ferrimagnetic greigite to paramagnetic pyrite (FeS₂). This is most likely caused by the drainage of methane-, and sulfide rich fluids/gas along the upper fault-zone and supports interpretations that the fault zone acts as effective conduit. A continued transport of fluids/gases could have promoted a self-sustaining weakening and strain decoupling with episodic high pore-fluid pressure within localized parts of the fault-zone.

How to cite: Greve, A., Kars, M., Stipp, M., and Dekkers, M.: Characterizing sediment dewatering and constraining spatially limited fluid flux in accretionary systems. A rock magnetic approach., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-17918, https://doi.org/10.5194/egusphere-egu2020-17918, 2020.

The Eastern Galicia Magnetic Anomaly (EGMA) is one of the most conspicuous and, definitively, the best studied of all the magnetic anomalies in the Central Iberian Arc (CIA). This is probably due to its location, on the thoroughly researched Lugo-Sanabria gneiss dome and to the unique fact that its source rocks crop out in the Xistral Tectonic Window. Multiple studies and models of this anomaly have been carried out in the last 25 years and still, new results keep on shedding more light on its understanding. Rock magnetic analyses, natural remanent magnetization, anisotropy of the magnetic susceptibility and stable isotopes geochemistry carried out on the rocks that produce this anomaly have provided new insights on the processes that led to magnetization and on its age. Results suggest that magnetization of source rocks is a consequence of the increase in oxygen fugacity underwent by metamorphic and magmatic rocks affected by late-Variscan extensional tectonics. Extensional detachments were the pathways that allowed the entrance of fluids that led to syn-tectonic crystallization of magnetite and hematite in S-Type granites. Accordingly, magnetization is not really linked to primary lithologies but mostly to extensional structures. This process took place in the late Carboniferous to earliest Permian, during the Kiaman reverse superchron. Natural remanent magnetization exhibited by hematite-bearing samples confirms the age of the magnetization and adds complexity to the interpretation of the EGMA, where remanence has been often largely ignored or underestimated. Understanding the origin of the EGMA contributes to the interpretation of other anomalies existing in the CIA, also located on thermal domes. Furthermore, it provides new hints to interpret magnetic anomalies located in extensional tectonic contexts worldwide

How to cite: Ayarza, P., Villalaín, J. J., Martínez Catalán, J. R., Alvarez Lobato, F., Durán Oreja, M., and Recio, C.: New constraints on the role of late Variscan extension in the origin of the Eastern Galicia Magnetic Anomaly (NW Spain), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2064, https://doi.org/10.5194/egusphere-egu2020-2064, 2020.

The Atlas system is an ENE-WSW intracontinental chain that extends from Morocco to Tunisia. It is the result of the Cenozoic inversion of a set of intraplate extensional basins that started its development during the Triassic and continued during the Jurassic. The Central High Atlas (CHA) is located at the Moroccan part of the Atlas System, characterised by NE-SW to ENE-WSW tight anticlines that limit wide synclines with the same orientation.

In this work, we present a high resolution structural and paleomagnetic study in a representative area with a tectonic evolution characteristic of the CHA. The study area is formed mainly by the Anemzi syncline, a structure of about 28 km long and 12 km wide. This structure is filed in by lower to mid Jurassic marine carbonates, which gradually change upwards to continental red beds. Towards the south, the Anemzi syncline limits with a vertical set of Jurassic intrusive bodies together with Triassic shales and basalts. On the other hand, towards the north crops out Lower Jurassic carbonates in the north limb of the syncline, which overthrust Middle Jurassic rocks.

Alongside with other areas of the CHA, in the study area can be identified a widespread remagnetization that has been dated ca 100 Ma. This remagnetization happened after the extensional period, and before the Cenozoic deformation started. The fact that it is an inter-folding record, allows using an already proved method in the CHA to[o1]  restore the structures of the area, and so, erase the Cenozoic deformation to better understand all the structural evolution of the area.

Samples from 90 palaeomagnetic sites were collected from sedimentary rocks, together with 170 bedding sites. The paleomagnetic results can be divided depending on the lithology. (1)  Jurassic[o2]  limestones show, in addition to a viscous component, the remagnetization typical from the CHA: a component with maximum unblocking temperatures between 450⁰C and 550⁰C carried by magnetite. Also in this lithology, in few samples a component carried by pyrrhotite can be observed. (2) Red beds show also a Cretaceous overprint, but carried by hematite.

By applying Small Circles methods to the Cretaceous remagnetization, we have obtained the paleobedding at the remagnetization acquisition time (ca. 100 Ma). These results allow us to restore two geological cross-sections at the remagnetization time and compare their with the present-day geometry. Besides, two maps of dip domains have been done, one showing the present day structure and the other one using the dips at the remagnetization. This methodology is a remarkable tool to assess the evolution of singular structures and to separate the deformation related with the basinal period with those related with the subsequent inversion. This restoration is part of a bigger project, whose objective is to build two 3D model of the Anemzi syncline with both the present-day and the restored structure at ca. 100 Ma using the palaeomagnetic data.

How to cite: Jimenez-Sanz, A., Calvín, P., Villalaín, J. J., and Casas-Sainz, A. M.: 100 Ma palinspastic restoration of the Anemzi syncline from paleomagnetic results., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-16795, https://doi.org/10.5194/egusphere-egu2020-16795, 2020.

A recent trend in paleomagnetism is the study of samples of ever decreasing sizes, going down to (sub)millimeter scales and even microscopic scales (“nanopaleomagnetism”). These include studies of single-silicate-crystals, microscopic magnetic imaging of the cloudy zones in Iron meteorites, and recently even the determination of individual magnetic remanence carriers. As single-crystal and nanopalaeomagnetic methods are getting more adopted, it is getting increasingly important to assess the statistical reliability with which such small samples can record remanences from a physical perspective. We previously proposed a benchmark to assess small-scale samples of randomly oriented non-interacting single-domain (SD) particles and found that in most cases, the number of magnetic particles a sample must contain lies in the order of tens to hundreds of millions – or equivalently NRM strengths of the order of 10^-12 Am². In this talk, we present how this benchmark can be used as a simple yet indispensable tool to assess whether or not (sub)millimeter-size and nanopalaeomagnetic samples are able to statistically reliably record palaeomagnetic fields. Moreover, this talk will provide an outlook into future limitations but also opportunities of the statistical physics nature of microscopic magnetic particle systems. It will explore if multi-domain particles should ever be considered statistically reliable recorders, how interactions in SD particle clusters might affect statistical reliability, and will review the various challenges that Iron meteorites pose as a remanence recorder.

How to cite: Berndt, T., Muxworthy, A., Fabian, K., and Chang, L.: The Bigger The Better? - Updates on the Statistical Limits of Nano-Palaeomagnetic Works and (sub)millimeter Samples, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4429, https://doi.org/10.5194/egusphere-egu2020-4429, 2020.

The Neoproterozoic-Paleozoic transition (~541 Ma) was a turning point in Earth’s history resulting in great biological changes between the microbial Precambrian life and the Ediacaran biotic revolution with the occupation of the sedimentary substrate, the dawn of biomineralization and the appearance of the earliest multicellular organisms. In parallel, this period is marked by a large plate reorganization leading to the assembly of Gondwana and by major climatic changes (extreme glacial events). Due in part to a poor paleomagnetic database for the different cratons in the Ediacarian-Cambrian times, the global paleogeography at that time still remains controversial. In this study we present a new paleomagnetic pole (Q= 6) for the Monteiro dike swarms in the Borborema Province (NE Brazil). They are fine-grained hornblende dolerite dated by U-Pb on zircon at ~538 Ma. Rock magnetic data indicate that magnetite and pyrrhotite are the main remanence carriers. Positive baked-contact tests support the primary remanence obtained for these dikes (19 sites). A positive reversal test (classified C) was also obtained from the 14 sites with normal polarity and the 5 sites with reversed polarity, indicating that the secular variations was eliminated with our sampling. Our new key pole is not consistent with the classical Apparent Polar Wander Path of the West Gondwana which consists of a long track from a southern polar position at ~590 Ma to an equatorial position at ~520 Ma. The Monteiro paleomagnetic pole suggest instead rapid and small oscillations of the APW, or wobbles, after 560 Ma. These rapid oscillations may be related to inertial readjustments in response to true polar wander (TPW) of the spin axis. TPW events have been suggested from 615 to 590 and then from 575 to 565 Ma in previous works. These TPWs are supposedly caused by changes in the inertia tensor of the Earth due to internal mass redistribution, related to rapid changes in subduction velocity. Possible links between these events and life evolution will also be discussed.

How to cite: Antonio, P. Y. J., da Trindade, R. I. F., Hollanda, M. H. B. M., and Giacomini, B.: Wobbles in the Early Cambrian Earth's spin axis? New high-quality paleomagnetic data from NE Brazil, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-577, https://doi.org/10.5194/egusphere-egu2020-577, 2020.

The African-European convergent tectonic setting has resulted in a complex deformation history with several large-scale tectonic features in western Anatolia, where is dominated by a crustal-scale extension since the late Eocene. The Menderes metamorphic core complex, the İzmir-Balıkesir Transfer Zone, and the North Anatolian Fault Zone are some of these main tectonic features. To understand their spatio-temporal relationships we employ paleomagnetic, geochronologic and kinematic studies in the northernmost part of the western Anatolia, where these structures interacting with each other. 

Our results show that western Anatolia has experienced at least two separate rotational phases since the Miocene. The first rotational phase is clockwise and related volcanism is dated as 21–16 Ma. The second rotational phase is counterclockwise and related volcanic rocks are dated as 14–12 Ma. According to collected kinematic data, pervasive transcurrent tectonism was dominated during the first phase, while the second one was dominated by extensional (and/or transtensional) tectonism. Here, the mode of extension switched from distributed diffuse deformation to discrete local deformation, possibly due to tearing and retreating of the northward subducting African oceanic slab below the western  Anatolian crust. This interrelated process also led to the localization of the İzmir-Balıkesir Transfer Zone with the decoupling of strike-slip faults, and to the episodic exhumation of the Menderes metamorphic core complex. This study is supported by a Tübitak Project, Grant Number of 117R011.

How to cite: Uzel, B., Kaymakci, N., Cakir, E., Tosun, L., Ozkaptan, M., Sumer, O., Kuiper, K., Langereis, C., and Koralay, E.: Rotational Evolution of Western Anatolia since the Miocene and Its Implications on the Subduction Dynamics of Eurasia-Africa Collision, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2001, https://doi.org/10.5194/egusphere-egu2020-2001, 2020.

The paleogeography and chronology of the Paleoproterozoic supercontinent Nuna are highly debated. To further test the paleogeography of Australian cratons in the leadup to Nuna formation, we present new paleomagnetic results from two Paleoproterozoic rock formations in North Australia. First, we obtained paleomagnetic directions from the 1825±4 Ma, bimodal Plum Tree Creek Volcanics sequence located within the Pine Creek Inlier of the North Australian Craton. Second, we studied the 1855±2 Ma layered mafic-ultramafic ‘Toby’ intrusion from the Kimberley Craton (KC). Samples from both study areas reveal high quality, stable, magnetite related characteristic remanent magnetization directions. Combining within-site clustered mean directions, we obtained two paleopoles, which plot proximal to each other in the present day central Pacific Ocean, off the east coast of Australia. These results agree with previous interpretation that the Kimberly Craton was amalgamated with the rest of the North Australian Craton (NAC) prior to ca. 1.85 Ga. Comparing these new results with slightly younger poles from the NAC and slightly older, rotated poles form the West Australian Craton (WAC) reveal a high degree of clustering suggesting very minimal absolute plate motion between ca. 1.9-1.85 and 1.6 Ga before the final amalgamation of Nuna. All available paleomagnetic poles agree with an assembly, or close juxtaposition, of the two major Australian cratons (NAC and WAC) before 1.8 Ga. Furthermore, the individual virtual geomagnetic poles from the potentially slow cooled Toby intrusion show a non-fisherian distribution along a great circle. This spread might be related to previously interpreted major true polar wander events based on Laurentian data, which would be global if such an interpretation is correct. The assembly of proto-Australia prior to ca. 1.85 Ga roughly 250 to 300 Myr before the final stage of supercontinent Nuna’s amalgamation ca. 1.6 Ga suggests that assembling of major building blocks, such as Australia and Laurentia for the supercontinent Nuna and Gondwana for the supercontinent Pangea, is an important step in the formation of supercontinents.

How to cite: Kirscher, U., Mitchell, R., Liu, Y., Nordsvan, A., Cox, G., Pisarevsky, S., and Li, Z.-X.: Unification of the Australian cratons before the formation of Nuna, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20258, https://doi.org/10.5194/egusphere-egu2020-20258, 2020.

Accurate dating of marine sediments from the Arctic Ocean remains a subject of great debate over the last decades. Due to the lack of adequate materials for biostratigraphy and stable isotope analyses, paleomagnetic reconstructions came into play here but though yielded ambiguous interpretations. Moreover, sedimentation rates in the Quaternary, determined for isolated morphological features in the Arctic Ocean, are often applied to the entire Arctic Ocean realm resulting in an inappropriate oversimplification of probably diverging regional depositional regimes.

Paleomagnetic studies on four long sediment cores, collected from the Mendeleev Ridge and the Lomonosov Ridge, complemented by the results from one core from the Podvodnikov Basin, have provided an opportunity to compare the sedimentation history of these profound structures in the Arctic Ocean. Cores PS72/396-5 and PS72/410-3 (Mendeleev Ridge), PS87/023-1, PS87/030-1 (Lomonosov Ridge) and PS87/074-3 (Podvodnikov Basin) were retrieved during expeditions of RV Polarstern in 2008, and 2014. Paleomagnetic, rock magnetic and physical properties measurements were carried out at the Center for Geo-Environmental Research and Modeling (GEOMODEL) of the Research Park in St. Petersburg State University, at the University of Bremen, and the Alfred Wegener Institute.

According to the results on the Mendeleev Ridge’s cores, complemented with 230Th excess study on core PS72/396-5, the Brunhes Matuyama boundary (0.78 Ma) is observed at the first meters below the seafloor. That, together with the Matuyama Gauss transition (2.58 Ma) recorded in both cores, implies the mean sedimentation rate in this area to be in the order of mm/kyr.

In contrast to the Mendeleev Ridge, the cores from the Lomonosov Ridge and the Podvodnikov Basin have shown a more complex paleomagnetic record with a relevant shift to negative inclinations significantly deeper downcore. This could signify a relevant difference in the sedimentation regimes between both ridges during the Quaternary.    

How to cite: Elkina, D., Frederichs, T., Geibert, W., Matthiessen, J., Niessen, F., Piskarev, A., and Stein, R.: New paleomagnetic data on marine sediments from the Central Arctic Ocean, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21920, https://doi.org/10.5194/egusphere-egu2020-21920, 2020.

In the paleogeographic reconstruction of the Rodinia supercontinent, the Tarim Craton is placed either on the periphery of the supercontinent to the northwestern Australia or in the heart of the supercontinent between Australia and Laurentia. The mystery of the Tarim Craton is caused by the lack of paleomagnetic data, especially during the Rodinia assembly. We present here new primary paleomagnetic data from ca. 900 Ma volcanic strata in the Aksu region of the northeastern Tarim Craton. Rock magnetic investigations reveal magnetite and hematite as the main magnetic carriers. Characteristic remanent magnetizations isolated from 15 sites show both normal and reverse polarities. A site-mean direction is calculated at D_g/I_g = 155.2°/47.5° (k_g = 11.6, α_95g = 11.7°) in geographic coordinate and D_s/I_s = 205.2°/64.0° (k_s = 24.4, α_95s = 7.9°) after tilt-correction. The site-mean direction passes fold tests and a ~900 Ma paleomagnetic pole is calculated at λ/φ = -0.5°N/62.3°E (A₉₅ = 11.8°) corresponding to a paleolatitude of 45.7° N. The data reveal a ~20° latitude difference between the northern Tarim (N-Tarim) and southern Tarim (S-Tarim) terranes. Together with the late Meso- to early Neo-proterozoic arc magmatism identified both in the central Tarim Basin and along the north margin of the Tarim Craton, a post-900 Ma cratonization of the Tarim Craton resulting from a dual subduction system is proposed. Finally, a new paleogeographic reconstruction of the Rodinia supercontinent is made with the Tarim Craton being placed to the northwestern Australia and cratonization of the Tarim Craton may occur during the Rodinia assembly.

How to cite: Zhao, P. and He, J.: Neoproterozoic (post-900Ma) cratonization of the Tarim Craton and its role in assembly of the Rodinia supercontinent, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11742, https://doi.org/10.5194/egusphere-egu2020-11742, 2020.

Accumulation of the global paleomagnetic data, from both continental and oceanic plates, may suggest a true polar wander (TPW) event in Jurassic, with a rotation axis located in the present northwestern Africa, but no consensus has been reached regarding to the initiation, duration and velocity of the TPW. As one of the eastern Asian blocks, the north China block (NCB) is then located far from the rotation axis of the TPW and the plate convergence between Siberia and the Amur-NCB, known as the subduction in the Mesozoic Okhotsk-Baikal ocean, did exist. Paleogeographic changes observed of the eastern Asian blocks in Jurassic thus should contain the TPW component and plate moving component. To better estimate the influence of the TPW in the Eastern Asia blocks, we carried out a new paleomagnetic and precision U-Pb geochronological study on the middle Jurassic lavas in the NCB. Being profoundly different to the recent paleogeographic model (Yi et al., 2019, https://doi .org/10.1130/G46641.1) that suggest that the NCB experienced a large latitudinal displacement (monster-shift) responding to the TPW event between ~174 and ~157 Ma, we suggest that the NCB, as well as other blocks already connected with it, do not record any monster-shift between ~170 and ~160 Ma. The strata, ranging from 160 to 145 Ma, however, yield considerable paleomagnetic variations and need further investigation.

How to cite: Zhang, S., Gao, Y., and Ren, Q.: Enigma of the Jurassic monster shift of the North China block, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12816, https://doi.org/10.5194/egusphere-egu2020-12816, 2020.

The magnetic fabric of strongly magnetic materials originates from (1) self-demagnetization in bodies of non-isometric geometry, and (2) magnetostatic interactions between bodies with non-random distribution. These contributions, termed shape anisotropy and distribution anisotropy, carry information about a rock’s formation or deformation history. Both may be important when magnetite grains control the anisotropy of a rock, or when the pore space of a rock is impregnated with strongly magnetic fluid. The relative importance of each contribution to the overall anisotropy is debated, partly because it is influenced by many factors, including the body shape, orientation, or spacing. Another challenge is that existing models of distribution anisotropy consider infinite regular arrangements of equal bodies and take into account nearest neighbour interactions only. These simplifications make it difficult to predict distribution anisotropy in real rocks, where particles or pores are distributed irregularly, and display a range of sizes, shapes and orientations. A new model is presented here, which calculates both shape and distribution anisotropy for irregular assemblages of diverse bodies – differing in their size, shape, and orientation. The model assumes ellipsoidal bodies of equal intrinsic magnetic susceptibility in a non-magnetic matrix. Input parameters include the coordinates of each body centre, dimensions and orientation vectors of the three principal axes. These can be derived from imaging and X-ray computed tomography, or be pre-defined parameters of man-made samples. Calculations were verified against magnetic pore fabric measurements performed on synthetic samples with known pore parameters. The model is expected to advance our understanding of the interplay between shape and distribution anisotropies in natural samples. Hence, it will facilitate structural interpretations in samples whose magnetic fabrics are predominantly controlled by magnetite, as well as the interpretation of magnetic pore fabrics in future studies.

How to cite: Biedermann, A. R.: Shape and distribution anisotropy of irregular arrangements of diverse bodies – a 3D computational model, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1619, https://doi.org/10.5194/egusphere-egu2020-1619, 2020.

The Lamas de Olo Pluton (LOP) is a small outcrop located in the Northern part of Central Iberian Zone from the Iberian Variscan belt. The LOP is a post-tectonic (ca. 297.19 ± 0.73 Ma) pluton composed of different granites: Lamas de Olo (LO; medium to coarse-grained porphyritic granite, ilmenite, and magnetite-type), Alto dos Cabeços (AC; medium to fine-grained porphyritic, ilmenite-type granite), and Barragem (BA; leucocratic fine- to medium-grained, slightly porphyritic, ilmenite-type granite). The magnetic fabric was characterized by measurements of anisotropy of magnetic susceptibility (AMS), and anisotropy of anhysteretic remanent magnetization (AARM). Both techniques are based on the magnetic properties of rock minerals, but while AMS consider the contribution of all rock minerals (paramagnetic, diamagnetic and ferromagnetic s.l.), in the AARM, the fabric is exclusively given by the ferromagnetic s.l. minerals. A correlation between AMS and AMR tensor was established, in order to compare both fabrics. The magnetic lineation is K_max or AARM_max and the magnetic foliation is perpendicular to K_min or AARM_min. Considering the global magnetic fabric for all samples from all the granite set, the magnetic foliations (AMS: N166°, 82°NE; AARM: N167°, 83°NE) and the magnetic lineations (AMS: 23°- N166°; AARM: 68°- N163°) are coaxial in both tensors. On the other hand, the analysis of each site sampling shows some differences in the ilmenite-type granites. Magnetic lineations and foliations given by both tensors (AMS and AARM) are coaxial in the magnetite-type granites, meaning that the magnetite and paramagnetic (or diamagnetic) minerals have the same orientation. The coaxial AMS and AARM magnetic foliations are due to magnetite grains imitating the fabrics of paramagnetic phases, through preferred collage, or crystallization of magnetite along grain boundaries, or exsolutions of magnetite along biotite cleavage planes. However, in the ilmenite-type granites, the AMS and AARM foliations are parallel, but the AMS and AARM lineations are not coaxial. Previous magnetic mineralogy studies (e.g. thermomagnetic experiments and isothermal remanent magnetization) pointed out the presence of magnetite/Ti-poor magnetite in all LOP granites, even in the ilmenite-type, but in different proportions. The petrographic observations also showed that, in the ilmenite-type granites, the magnetite is often oxidized to hematite (martite). The presence of martite may justify non-coaxility linear fabrics. Regarding the LOP emplacement, WSW-ENE opening structures provided the space for magma ascending, with an NNW-SSE magmatic flow controlled by regional structures, as shown by the magnetic foliations and lineations ca. N170º trending. The absence of outcrop deformation and the lack of solid-state microstructures precludes the substantial deformation after full crystallization of LOP.

How to cite: Cruz, C., Sant'Ovaia, H., Raposo, M. I. B., and Noronha, F.: Magnetic fabric of Lamas de Olo Pluton: AMS and AARM fabrics comparison, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21596, https://doi.org/10.5194/egusphere-egu2020-21596, 2020.

This work focuses on the magnetic fabric of 20 variscan granitic massifs from northern and central Portugal and considers the Anisotropy of Magnetic Susceptibility (AMS) results obtained in about 750 sampling sites. In the northern and central Portugal, three main ductile deformation phases were recognized and described: D₁, D₂ and D₃, being the variscan magmatism events mainly related to D₃ phase. D₃ produced wide amplitude folds with NW-SE subhorizontal axial plane and subvertical dextral and sinistral ductile shear zones, forming obtuse angles with the maximum compression direction, σ1, NE-SW oriented. The post-D₃ brittle phase was responsible for the development of conjugate faults (NNW-SSE, NNE-SSW and ENE-WSW), related to a N-S maximum compression. The studied granites were subdivided according to U-Pb dating, field observations and considering the chronology of their emplacement relative to the D₃ phase of Variscan orogeny. Therefore, the studied granites are subdivided into: (1) syn-D₃ two-mica granites, ca. 311 Ma; (2) late-D₃ monzogranites, biotite-rich and two-mica granites, ca. 300 Ma; (3) post-D₃ monzogranites and biotite-rich granites, ca. 299 – 297 Ma. Magnetic fabric gives two types of directional data, magnetic foliations and magnetic lineations, which provide important information regarding the orientation of the magmatic flow, feeder zone location, relationship between the magma emplacement and tectonics and, also, the stress field. The data obtained for the magnetic fabric, based on AMS technique, allowed concluding: (i) syn-D₃ granites show magnetic foliations and lineations consistent with the syn-D₃ variscan structures ca. N110°-120°E, related to a NE-SW maximum stress field . The foliations are, mainly, subvertical (> 60º), which may indicate a high thickness of the granitic body and deep rooting; on the other hand, the magnetic lineations exhibit variables plunges. (ii) Late-D₃ granites are characterized by foliations and lineations, dominantly NNW-SSE to NNE-SSW oriented. The foliations are subvertical dips (> 60º) and the lineations have, generally, soft plunges. (iii) Post-D₃ granites have, in general, magnetic foliations and lineations associated with important regional post-D₃ brittle structures, which display NNE-SSW and ENE-WSW trending. The subhorizontal fabric may suggest a small thickness of the granitic bodies. In all granite sets under study there is a dominance of weakly dipping lineations (slope <60º), indicating that the feeding zones are deep, which supports the idea of an emplacement at high structural levels.

Acknowledgments: The authors thank Department of Geosciences, Environment and Spatial Planning at Faculty of Sciences of the University of Porto and the Earth Sciences Institute (Porto Pole, Project COMPETE 2020 (UID/GEO/04683/2013), reference POCI-01-0145-FEDER-007690).

How to cite: Sant Ovaia, H., Gonçalves, A., Cruz, C., and Noronha, F.: Magnetic fabrics in Portuguese Variscan granites: structural markers of the Variscan orogeny, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20879, https://doi.org/10.5194/egusphere-egu2020-20879, 2020.

The paleo-position of the North China Craton (NCC) within the Supercontinent Nuna/Columbia is controversial. Hindered by ubiquitous alteration of the very ancient rocks, paleomagnetic studies have not been able yet to conclusively solve this puzzle. Comprehensive analysis on the relatively limited Precambrian records is essential to understand the geological history of these cratons. Within the NCC, the tectonic setting of a ~1.78 Ga large igneous province (LIP) is long debated. It is considered to be related to a paleoplume, post-collision extension, or an Andean continental margin. Knowing its mode of formation constrains the geological evolution of the NCC and its paleo-position within the Supercontinent Nuna/Columbia. Here we conduct a study into the anisotropy of magnetic susceptibility (AMS) in the dykes and lavas of the ~1.78 Ga LIP, together with systematic rock magnetic experiments, to constrain the geological background of the igneous event(s), to understand the tectonic evolution of the NCC, as well as its paleo-position within the assembly of the Nuna/Columbia supercontinent.

Thirty-three dykes in the northern and middle parts and thirty lavas in the southern part of the NCC were collected. Detailed rock magnetic analyses indicate PSD magnetite to be the dominant magnetic mineral in the samples, occasionally with pyrrhotite in the dykes and hematite in the lavas. The often observed relatively weak anisotropy degree suggests that the AMS ellipsoids probably portray magma flow-related fabrics. The inferred directions from the AMS fabrics of the lavas reveal a radial flow pattern with an eruption center located on the south margin of the NCC. The studied dykes show a predominance of horizontally to subhorizontally northward magma flow, with only few vertical intrusions. These observations imply that the ~1.78 Ga LIP may have formed by magma source(s) at the south margin of the NCC. Some localized magma sub-chambers may have formed during the propagation of the magma and could have been responsible for the less common vertically intruded dykes and the EW-trending dykes. Therefore, we favor a plume-related tectonic setting for the ~1.78 Ga LIP with the eruption center along the margin of the NCC. It can serve as an essential criterion to search for possible neighbour(s) of the NCC within Nuna/Columbia, which should preserve the relics of the ~1.78 Ga LIP. Our study, in combination with extant geological and paleomagnetic results suggests a close linkage of the NCC with the São Francisco-Congo, Rio de la Plata and Siberia cratons in the Nuna/Columbia supercontinent.

How to cite: Xu, H., Yang, T., Dekkers, M., Peng, P., Ge, K., and Deng, C.: Magnetic fabric and rock magnetic studies of the ~1.78 Ga large igneous province in the North China Craton and its implication for the configuration of the supercontinent Nuna/Columbia, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9703, https://doi.org/10.5194/egusphere-egu2020-9703, 2020.

The Capinha area is located in the Central Iberian zone and is characterized by several Variscan granites intruded in the Neoproterozoic–Cambrian metasedimentary rocks. The main goal of the study is to identify the deformation patterns and provide crucial information to investigate the evolution of the magnetic fabrics in a post-Variscan granite emplaced during the crustal thinning, at the end of the Variscan orogeny. In order to achieve these purposes, fieldwork, petrography, microstructures and anisotropy of magnetic susceptibility (AMS) analysis were undertaken. The AMS was measured in 160 oriented cores, collected from 20 sampling sites homogeneously distributed, allowing the quantification of scalar (magnetic susceptibility, K; paramagnetic anisotropy, P_para; magnetic ellipsoid shape, T) and directional data (magnetic lineation, //K₁; magnetic foliation, perpendicular to K₃). The Capinha granite (CG), exposed over an area of about 7 km², is a small circular circumscribed outcrop in the NE-SW contact between the regional Belmonte–Caria granite (301.1±2.2 Ma) and the metasedimentary sequences. The CG is cut by two main fracturing systems: N30º-40ºE and N110º-120ºE, both subvertical. The contact is sharp, intrusive and discordant with the general trending of the D₁ and D₃ Variscan structures registered in the metasedimentary rocks. The CG is homogeneous in the whole area and consists of a fine- to medium-grained, muscovite-biotite leucogranite. The CG exhibit a paramagnetic behaviour with a K mean of 73 µSI, belonging to the ilmenite-type granites. At several scales, the CG does not show any magmatic flow or ductile deformation patterns displaying P_para of about 1.6%, which corresponds to dominant magmatic to submagmatic microstructures. The P_para highest values are concentrated in the NE border associated to prolate ellipsoids (linear fabric). Based on the interpretation of the magnetic fabric, is possible to observe that the orientation of the magnetic foliation is variable ranging from NNW-SSE to NNE-SSW. Generally, the magnetic foliations are sub-horizontal, being the vertical dips observed in the NE border, near the intersection of the N100º-120ºE and the N30º-40ºE fractures. The arrangement of the magnetic foliations follow concentric trajectories, with the symmetry axe parallel to the major axis of the outcrop (roughly NNE-SSW). The magnetic lineations are mainly sub-horizontal NNE-SSW parallel to the granite major axis; although, in the SW border the lineations tend to be parallelized to the contact. The magnetic lineation arrangement develops linear trajectories converging to the NE zone, where the dip is strong. The common gently magnetic fabric suggests the roof of the CG intrusion. During the late stages of the Variscan orogeny (D₃, 321-300 Ma), ductile extensional detachments promoted the thinning of a previously thickened crust, providing the opening of pre-existing structures and the production of new ones. These structures act as conduits for a passive magma ascending and emplacement at shallow levels. Therefore, it is suggested that the CG magma ascent and emplaced in the intersection of pre-existing fractures, located in the NE zone, and flowed to the SW, developing a small asymmetric laccolith, poorly eroded, with a tongue-shaped body.

How to cite: Gonçalves, A., Sant'Ovaia, H., and Noronha, F.: Deformation and magnetic fabric of the Capinha granite (Fundão, Central Portugal): ascent and emplacement mechanisms during the late-Variscan crustal thinning, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4940, https://doi.org/10.5194/egusphere-egu2020-4940, 2020.

Volcano eruption forecasting typically links ground deformation patterns to sub-surface magma movement. Injection and inflation of magmatic intrusions in the shallow crust is commonly accommodated by roof uplift, producing intrusion-induced forced folds that mimic the geometry of underlying igneous bodies. 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. For instance, analysis of ancient examples where magmatism has long-since ceased 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 and aim to test a new hypothesis suggesting that thermoremanent magnetization (TRM) records progressive deflection of the host rock during incremental laccolith construction and that these measurements can be used to measure the rate of laccolith construction. Here, we integrate palaeomagnetic techniques with semi-automated, UAV-based photogrammetric structural mapping to test: (1) whether we can identify variations in Natural Remanent Magnetisation (NRM), TRM, and magnetic mineralogy across an intrusions structural aureole; and (2) whether measured magnetic variations can be related to deflection caused by incremental sheet emplacement. Our test site is located within the basaltic lava pile of the ~800 m wide structural aureole of the rhyolitic Sandfell Laccolith in SE Iceland, which intruded <1 Km below the palaeosurface at ~11.7 Ma. We discuss whether palaeomagnetic backstripping can be an effective resource to constrain the rate and magnitude of intrusion-induced forced fold evolution, and thus an effective tool in volcanic hazard assessment.

How to cite: McCarthy, W., Twomey, V., Magee, C., and Petronis, M.: Unravelling magma emplacement through palaeomagnetic backstripping of an intrusion-induced forced fold: A case study from the Sandfell Laccolith, SE Iceland, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20895, https://doi.org/10.5194/egusphere-egu2020-20895, 2020.

Large, hot orogens are characterized by an orogenic plateau supported by a zone of weak ductile flow. During the collision phase, the magnitude of the belt and the temperature increase as radioactive crustal material is accreted, buried and heated. After convergence ends, no material is added to the orogenic system and the orogen undergo gravitational (or extensional) collapse that results from the lateral flow of the hot orogenic infrastructure. In the Araçuaí-West Congo orogen (AWO), the high temperatures, slow cooling, and excessive amount of melt in the hinterland, in the northern part of the belt, imply that a high temperature was maintained for a long time. Geochronologic results suggest that this internal domain was hot for a long time, cooling at < 3°/Myr since 600 Ma until 500 Ma, and cooling through the Ar/Ar retention temperature for biotite occurred around 470 Ma. In the south the collapse of the orogen is marked by the widespread intrusion of bimodal, composite plutons at ~500 Ma. Here we use the magnetic fabric (i.e. low-field anisotropy of magnetic susceptibility) of intrusions in the north and south sectors to track the kinematics and rheological changes across the belt. In the northern part of the AWO we studied the Padre Paraíso Charnockite and the southern part of the AWO we studied the Conceição de Muqui and Santa Angélica plutons. The Padre Paraíso charnockite has a coherent magnetic fabric, with magnetic foliations trending N-S, following the general structure of the belt in that sector. In turn, Conceição de Muqui and Santa Angélica plutons show a concentric distribution of foliations and lineations, in starking contrast with the general NE-SW trend of the belt in the south. This contrasting structural pattern for coeaval plutons along the AWO belt reveal the strain partitioning at the scale of the orogenic belt during the cooling of the AWO. At 500 Ma the hot northern sector remains warm enough to allow a coherent deformation of intrusions and host rocks. At the same time, more material was being added to the margins of the hot orogen, which already cold, with the diapire-like plutons structure being dominantly controlled by the forces of magma ascent and emplacement.

How to cite: Temporim, F., Trindade, R., Tohver, E., Egydio-Silva, M., and Valim, T.: Strain partitioning in a collapsing hot orogeny, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2986, https://doi.org/10.5194/egusphere-egu2020-2986, 2020.

As shown in the literature several times, the calculation of the anisotropy of magnetic susceptibility (AMS) of hematite single crystals using standard linear AMS theory reveals that the calculated minimum principal susceptibility is parallel to the crystallographic c-axis, but is negative, which is however not due to diamagnetism as evidenced by direct measurements of susceptibility along the principal directions.

Susceptibility of a few hematite single crystals from Minas Gerais, Brazil, was measured in 320 directions using a special 3D rotator and the measurements were processed through AMS calculation by means of standard linear theory and through constructing contour diagram in equal-area projection. In addition, the deviations of the measured directional susceptibilities from the directional susceptibilities calculated from the fitted AMS tensor were calculated. The crystals show extremely high anisotropy, the susceptibility measured along the basal plane is several hundred to several thousand times higher than that along the c-axis and the AMS ellipsoids are very oblate, nearly rotational. The contour diagrams show relatively simple patterns of directional susceptibilities, similar to those of the second-rank tensor. However, the calculated AMS ellipsoids are slightly more eccentric than is the surface connecting the directional susceptibility values. The present study is assessing whether, realizing that the susceptibility along the c-axis is about three orders lower than that along the basal plane and taking into account the directional distribution of the fitting errors, one can ascribe the existence of the negative minimum susceptibilities calculated through standard linear theory to imperfect techniques of second-rank tensor fitting rather than to complicated magnetization mechanisms.  

How to cite: Jezek, J., Chadima, M., and Hrouda, F.: On the origin of apparently negative minimum susceptibilities of hematite single crystals calculated from low-field anisotropy of magnetic susceptibility, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2993, https://doi.org/10.5194/egusphere-egu2020-2993, 2020.

A growing interest in isolating ferromagnetic fabric, i.e. magnetic fabric carried solely by ferromagnetic (sensu lato) grains, creates a need for optimization of laboratory protocols used to acquire the array of magnetic remanence vectors necessary to calculate the anisotropy of magnetic remanence (AMR) tensors. Before a laborious and tedious process of measuring large sample collections, several aspects shall be experimentally assessed, namely: (1) what type of magnetic remanence should be applied (e.g. isothermal, anhysteretic), (2) how magnetizing fields (AC and DC) control the acquired magnetization and its anisotropy, (3) how the viscous decay influences the measured remanence, and (4) how many and which magnetizing directions are necessary to obtain reliable and statistically sound AMR tensors. A careful examination of these factors considerably influences the quality of acquired AMR data. To facilitate the AMR tensor calculation, we present AREM – a simple and user-friendly toolbox embedded into Anisoft software. Prior to the tensor calculation, AREM provides a graphical visualization of a set of measured remanence vectors as spherical projections of unit vectors compared to the respective magnetizing directions and their intensity compared to the intensity of demagnetized state (background magnetization). The correction for the background magnetization is optionally done by a direct subtraction of measured background or by a mutual subtraction of antipodally magnetized vectors, if available. The AMR tensor is fitted by a least-square algorithm using a set of pre-selected full-vectors (full-vector method) or their projections to their magnetized directions (projection method). The calculated AMR tensors, including their confidence limits, can be immediately reviewed and processed using all features of the Anisoft software.

How to cite: Chadima, M.: AREM: A user friendly toolbox for calculating anisotropy of magnetic remanence, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-17562, https://doi.org/10.5194/egusphere-egu2020-17562, 2020.

The aim of this work is to apply the anisotropy of magnetic susceptibility (AMS) to determine the primary and tectonic fabrics of lava flows and volcanoclastic materials in one of the Pyrenean Stephano-Permian basins.

The Pyrenean Range is a double vergence orogen located at the northern end of the Iberian Peninsula. During Carboniferous-Early Permian times the extensional or transtensional regime dominant during the progressive dismantling of the Variscan belt resulted in the development of E-W elongated intra-mountainous basins. This process was coeval with an exceptional episode of magmatic activity, both intrusive and extrusive. The Cadí basin represents a good example of these structures were Stephano-Permian rocks are aligned along an E-W continuous outcrop and reach thickness of several hundreds of meters. The stratigraphy of the study area is characterized by fluviolacustrine sediments changing laterally to volcanoclastic and pyroclastic rocks with interbedded andesitic lava flows.  

A total of 75 sites (733 standard specimens) were studied and analysed throughout the volcanoclastic, volcanic and intrusive materials of the Stephano-Permian outcrops in the Cadí basin. Samples were drilled in the field along 5 sections with N-S or NW-SE direction in the Grey and Transition Unit. Afterwards, standard specimens were measured in a Kappabridge KLY-3 (AGICO) at the Zaragoza University to characterise the magnetic fabric. The susceptibility bridge combined with a CS-3 furnace (AGICO) was used for the temperature-dependent magnetic susceptibility curves (from 20 to 700 °C) to recognize the magnetic mineralogy. In addition, textural and mineralogical recognition in thin-sections of the samples was carried out in order to recognize the relationship between magnetic and petrographic fabrics.

The results shows that the bulk magnetic susceptibility of the specimens ranges between 118 and 9060·10^-6 SI but most of the values are bracketed between 160 to 450·10^-6 SI. Taking into account magnetic parameters (Km, Pj and T) there is no correlation between magnetic fabrics and magnetic mineralogy and there is a dominance of triaxial and prolate ellipsoids. Thermomagnetic curves indicate the dominance of paramagnetic behaviour in all the samples and except in one case there is a ferromagnetic contribution due to the generalised presence of magnetite.

Magnetic ellipsoids can be divided into four main types depending on the orientation of the main axes and associated with the lithologic types: 1) K_max vertical and K_int and K_min horizontal for small intrusive bodies (no restoring); 2) K_max horizontal or subhorizontal and K_int and K_min included in a subvertical plane (before and after restitution) for volcanic breccias; 3) K_min vertical (after restoring) and three directional maxima for lava flows and 4) non-defined fabric for the explosive materials (probably due to their complex depositional mechanisms). In general, a dominant E-W magnetic lineation is observed in many sites, resulting either from dominant flow direction, or to secondary processes. This is the case for some of the magnetic ellipsoids, that seems to be affected by deformation, K_min is not normal to bedding and therefore, they do not become vertical after bedding restitution.

How to cite: Simon-Muzas, A., Casas-Sainz, A. M., Soto, R., Gisbert, J., Román-Berdiel, T., Oliva-Urcia, B., Pueyo, E. L., and Beamud, E.: Anisotropy of magnetic susceptibility (AMS) of lava and volcanoclastic flows of the Stephano-Permian Cadi basin (central-eastern Pyrenees), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10284, https://doi.org/10.5194/egusphere-egu2020-10284, 2020.

The Basque-Cantabrian Basin is a hyperextended extensional basin that formed as the result of the opening of the Bay of Biscay at latest Jurassic-middle Cretaceous times. It is formed by upper mantle and crustal rocks affected by both high- and low-angle faults that die against an Upper Triassic salt layer that decouples the deformation and generate salt diapirs. From late Santonian (Late Cretaceous), the Basque-Cantabrian Basin was involved in the Pyrenean orogeny which reactivated the previous faults and salt décollement.

In this scenario, our study focuses at the northern margin of this basin where the salt overburden (Jurassic to Eocene in age) is displaced several km northwards and appears compartmentalized by several salt walls (Bakio, Bermeo, Guernica and Mungia diapirs). These walls are linked by narrow stripes of variable orientations in which the overburden appears strongly deformed by tight detachment folds and minor thin-skinned thrusts. The piercing salt is composed of Upper Triassic evaporites, red clays and volcanic ophites, and is flanked by Aptian-Albian syn-diapiric carbonate to terrigenous halokinetic sequences limited by angular unconformities that become conformable as distance to the diapir edges increases. Using a paleomagnetic study, we seek to better understand the kinematics of suprasalt deformation trying to detect and quantify vertical axis rotations recorded during both the extensional and later contractional reactivation of the basin margin. For that, 52 paleomagnetic sites have been analyzed in the overburden sequence. Of these, 50 sites were sampled in Aptian, Albian and Cenomanian marls, marly limestones and fine grained sandstones, and 2 sites were sampled in Eocene sandstones and marly limestones. Characteristic components are usually defined between 200-450 ºC pointing to titanomagnetite as the main remanence carrier. They show predominant anticlockwise rotations with some anomalous clockwise and larger anticlockwise rotations near the salt diapirs. All these components yield normal polarity, as expected by the age of the rocks, which (except the Eocene sites) coincide with the Cretaceous superchron C34n. However, some of the sites are clearly remagnetized as they yield negative fold tests, whereas some other sites show a prefolding magnetization. These results are also supported by several hysteresis analyses and back field experiments that confirm a clear remagnetization signal in the Day diagram in part of the studied rocks. However, the spatial location of remagnetized rocks does not show a distinct structural pattern. With the current data, the origin and age of this remagnetization is difficult to assess and further analysis will be necessary. It could be either an earlier Albian-Maastrichtian remagnetization or a remagnetization linked to the Pyrenean compression. Although these uncertainties, the obtained results allow establishing a preliminary kinematic model for the suprasalt deformation together with the underlying decoupled autochtonous materials.

How to cite: Beamud, E., Soto, R., Peigney, C., Roca, E., and Pueyo, E. L.: Paleomagnetism on salt-detached syndiapiric overburden rocks from the Northern margin of the Basque-Cantabrian extensional Basin (N Spain), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4598, https://doi.org/10.5194/egusphere-egu2020-4598, 2020.

Magnetic fabric has become a first-order tool for the study of the evolution of inverted sedimentary basins, as has been demonstrated in the last decade (García-Lasanta et al. 2018 and references therein). Its application is based on its broad and reliable applicability to characterize the structural context of a region where structural markers are often punctually located or scarce. Determining the contribution of basinal (extensional) and compressional (inversion) deformation to the total magnetic fabric is a major issue in understanding the internal deformation underwent by the basin fill.

The main goal of this work is to integrate the available data of anisotropy of magnetic susceptibility (AMS) performed during the last ten years in the Mesozoic series of the Central High Atlas. It has a total of 645 sites (7477 standard specimens), 484 of them (5657 standard specimens) are measured in the framework of the actual CGL2016-77560-C2-P research project (Spanish Ministry of Science and Innovation), and it has been integrated with 161 sites (1820 standard specimens) obtained in the precedent research projects (CGL2012-38481, CGL2009-08969 and CGL2009-10840). Samples were measured in a KLY3-S Kappabridge (AGICO) susceptometer at the Zaragoza University. Magnetic subfabric analysis were also done (AMS-LT and AARM) for representative selected sites, that allow us to identify anomalous fabrics. Magnetic carriers were determined by carrying out temperature-dependent susceptibility curves (from 40 to 700ºC) combining the susceptibility bridge with a CS-3 furnace, an also by means of the acquisition curves of isothermal remanent magnetization (IRM), backfield curves and hysteresis loops using a variable field translation balance MMVFTB at the Paleomagnetic Laboratory of the Burgos University. Rock magnetic experiments indicate the presence of paramagnetic behavior in most samples, the presence of magnetite as main ferromagnetic contribution, and of hematite in the red beds.

The application of the ASM has made it possible to obtain data of well-defined foliations and magnetic lines from the analysis of a large number of samples, and therefore representative of the Mesozoic rocks that emerge in the High Central Atlas. Viewing the data as a whole, magnetic ellipsoids can be divided into three main types depending on the orientation of the main axes, and can be related with the kinematic evolution of the Central High Atlas: 1) k_min normal to bedding and sub-horizontal k_max with a NW-SE main maximum, which is mainly associated with gentle synclines and can be related to Mesozoic extensional tectonic; 2) k_int normal to bedding and sub-horizontal k_max with a NE-SW main maximum, which can be interpreted as modified by compressional tectonics; 3) k_max normal to bedding, which are located near thrust planes or near the core of narrow and tight anticlines and can be interpreted as related with transport direction or salt tectonics and re-tightening of structures. The predominance of one or another type of fabric varies spatially; so that in the Western and Eastern sectors type 1 fabric dominates (more tan 60% of the samples), whereas in the central sector this percentage  decreases to 48% of the samples.

How to cite: Román-Berdiel, T., Oliva-Urcia, B., Casas-Sainz, A. M., Calvín, P., Moussaid, B., Izquierdo, E., Ruiz, V. C., Pocoví, A., Gil-Imaz, A., Torres, S., Villalaín, J. J., El Ouardi, H., Mochales, T., Santolaria, P., Marcén, M., Bógalo, M. F., Sánchez-Moreno, E. M., Herrejón, Á., Jiménez-Sanz, Á., and Falcón, I.: Contribution of magnetic fabric to the knowledge of Mesozoic and Cenozoic kinematic evolution in the Central High Atlas, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13734, https://doi.org/10.5194/egusphere-egu2020-13734, 2020.

The Jurassic carbonates of the Central High Atlas (CHA) are affected by a widespread and homogeneous chemical remagnetization. This is an interfolding remagnetization (dated in ca. 100 Ma by comparison with the GAPWP of the African plate) that separates two deformational events; the first one is related to the basinal period in the Atlas (Triassic and Jurassic times) that is responsible of the thick Jurassic sequence that crops out in the CHA. The second one is associated with the tectonic inversion during the Cenozoic caused by the African and European plates convergence, which resulted in the uplift of the cordillera.

Using the Small Circle tools, it is possible (i) to obtain the remagnetization direction and then (ii) use it as reference to obtain the paleodips of each site (i.e. the paleodips at the remagnetization time). This methodology applied to interfolding remagnetizations allows restoring the present-day geometry to the one at ca. 100 Ma and therefore separating the structure associated to the extensional and compressional periods.

This work is framed on an ambitious research project (CGL2016-77560-C2-P, Spanish Ministry of Science and Innovation) in which 700 AMS/paleomagnetic sites and additional 1000 bedding data are integrated to unravel the Mesozoic and subsequent Cenozoic evolution of the CHA. Based on the aforementioned datasets, field work, geophysical (gravimetric and magnetic surveys) constraints and the construction and restoration (at the remagnetization time) of cross-sections, the ultimate goal of the project is to reconstructed and restored the 3-D geometry of the CHA fold-and-thrust belt.  

Here in particular, we present the paleomagnetic data supporting the calculated paleodips. We also analyze how the deformation associated with each of the two deformational events is distributed along the study area. The comparison of dip-domains maps showing the present day attitude and also the pre-inversional one allows analyzing how extensional deformation is more or less associated with particular structures and to understand the importance of the different processes that acted during this period (i.e. deformation associated with extensional faults, salt walls, igneous intrusions, etc.).

How to cite: Calvín, P., Villalaín, J. J., Casas-Sainz, A. M., Román-Berdiel, T., Santolaria, P., Mochales, T., Falcón, I., Moussaid, B., Oliva-Urcia, B., Torres-López, S., Izquierdo, E., Bógalo, M. F., Gil-Imaz, A., Ruíz, V. C., Sánchez-Moreno, E. M., Marcén, M., Herrejón, Á., Jimenez-Sanz, Á., El Ouardi, H., and Pocoví, A.: Palinspastic restorations using interfolding remagnetizations. The case of the Cretaceous widespread remagnetization of the Central High Atlas (Morocco), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-17600, https://doi.org/10.5194/egusphere-egu2020-17600, 2020.

Chemical widespread remagnetizations are especially frequents during superchrons. An interesting issue is whether this relationship is due to especial requirements of the mechanism of acquisition of the remagnetizations and their timing. One example of this type of remagnetizations during a superchron is the one recorded by the Jurassic carbonates from the Central High Atlas (CHA) in Morocco. This normal polarity overprint has been dated ca 100 Ma, comparing the remagnetization direction with the African APWP, i.e. during the Cretaceous Normal Superchron (CNS) and also during the extensional stage of these inverted basins.

After several paleomagnetic studies performed in this area in the framework of a big research project, paleomagnetic and rock magnetic data from a set of more than 600 paleomagnetic sites distributed over an area of 10000 km2 are available. The CHA cretaceous remagnetization has been observed in all these sites with the same magnetic properties: a viscous paleomagnetic component with maximum unblocking temperatures of 200-250ºC and the remagnetization normal polarity component up to 450–500ºC. Both are carried by authigenic uniaxial SSD magnetite. The paleomagnetic direction calculated by small circle intersection method (SCI) is also similar in the different locations of this wide area.

The mechanism proposed for this type of widespread remagnetization is the generation of magnetite grains due to the heating related with burial. The homogeneous direction of the remagnetization seems to suggest an acquisition for a short event at 100 Ma. However, the extensional stage of these basins lasts tens of millions years keeping the necessary burial conditions for growth of magnetite grains covering several polarity chrons including the CNS.

In this work we address the question of timing under with these processes happened, i.e. short vs. long remagnetization periods. We propose the hypothesis that the ca. 100 Ma paleomagnetic direction shows by the remagnetization is just the average of magnetic moments of the entire SSD magnetite population that grow from the Middle Jurassic up to the Cenozoic. Grains block the magnetic moments when they grow above their critical volume, keeping the magnetic polarity generating over time a distribution of grains in normal and reverse polarity groups. To test this hypothesis we develop 1) simulations for the calculation of magnetization directions assuming a homogeneous and constant growth of magnetite crystals and 2) rock magnetic experiments to demonstrate the presence of SSD magnetite grains with opposed magnetic moments. These experiments intend to assess the effectiveness of the SSD grains carrying the remagnetization by comparing the NRM and the ARM signal through the pseudo-Thellier approach.

How to cite: Villalaín, J. J., Calvín, P., Bógalo, M. F., Falcón, I., and Casas-Sainz, A. M.: Links between remagnetizations and superchrons. New experiments and new results., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19781, https://doi.org/10.5194/egusphere-egu2020-19781, 2020.

In this study, we present preliminary results on paleomagnetic data collected in the Tunisian Tellian domain in both magmatic and sedimentary rocks of middle to lat Miocene ages from the Nefza-Mogods province, North-West of Tunisia. About 320 cores distributed over twenty one sites were collected both in magmatic rocks (16 sites) and in sedimentary rocks (5 sites), in order to obtain geometric constraints to establish a kinematic model along the North-East African margin. The sampled rocks are distributed between basanites, rhyodacites and microgranites. Some samples were taken from host sedimentary rocks host rocks in lacustrine limestones and jaspilites. Demagnetization process and Rock-Magnetism studies revealed a diversified magnetic mineralogy. In basalts, magnetite with an unblocking temperature of 580 °C is identified. In rhyodacites, the mineralogy is mixed with three types of minerals: a mineral with an unblocking temperature around 350°-400°C attributed either to a sulfide or to titanomagnetite, magnetite with unblocking temperature at 580°C, and a high temperature mineral with unblocking temperature between 600°C and 680°C attributed to hematite or titanohematite. The limestones, having a low magnetization intensity, are characterized by the presence of magnetite and the jaspilites by hematite. Basalts, which have been mainly demagnetized by AF process , show a characteristic component demagnetized between 20mT and 100mT. For rhyodacites, some sites have a characteristic component demagnetized between 400°C and 580°C and others up to 670°C. Although their low magnetization intensity, the lacustrine limestones show a magnetic component between 20mT and 140 mT. The first result indicate that the mean directions associated to the younger magmatic (basalts and rhyodacite) rocks (8 Ma, Tortonian) and their sedimentary host deposits are very close to the expected magnetic field after tilting in paleogeographic coordinates. By contrast, the older microgranites and rhyodacites(-12 Ma) display a vertical axis clockwise rotation of about 30°. This result suggests a significant tectonic phase between 12 Ma and 8 Ma, linked to the implementation of the Tell nappes.

How to cite: Robion, P., Arfaoui, M., Ahmadi, R., Derder, M. E. M., Amena, M., Maouche, S., and Rekihiss, F.: New constraints on the tectonic activity of the “Tellian Domain” from Northern Tunisia: preliminary paleomagnetic results., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10371, https://doi.org/10.5194/egusphere-egu2020-10371, 2020.

The Oman ophiolite is a natural laboratory for the study of processes operating above a nascent subduction zone. It formed in the Late Cretaceous by supra-subduction zone spreading and shortly afterwards was emplaced onto the Arabian continental margin. Twelve massifs in the ophiolite expose complete sections of the Neotethyan oceanic lithosphere, including upper mantle peridotites, lower crustal gabbros, and upper crustal sheeted dykes and lava flows.

Previous palaeomagnetic studies have suggested that the southern massifs of the ophiolite were affected by a large-scale remagnetization event during emplacement, that completely replaced original remanences acquired during crustal accretion. In contrast, primary magnetizations are preserved throughout the northern massifs. This study aimed to: (i) apply palaeomagnetic, magnetic fabric and rock magnetic techniques to analyse crustal sections through the southern massifs of the Oman ophiolite to investigate further the extent and nature of this remagnetization event; and (ii) use any primary magnetizations that survived this event to document intraoceanic rotation of the ophiolite prior to emplacement.

Our new data confirms that remagnetization appears to have been pervasive throughout the southern massifs, resulting in presence of shallowly-inclined NNW directions of magnetization at all localities. An important exception is the crustal section exposed in Wadi Abyad (Rustaq massif) where directions of magnetization change systematically through the gabbro-sheeted dyke transition. Demagnetization characteristics are shown to be consistent with acquisition of a chemical remanent overprint that decreased in intensity from the base of the ophiolite upwards. The top of the exposed Wadi Abyad section (in the sheeted dyke complex) appears to preserve original SE-directed remanences that are interpreted as primary seafloor magnetizations. Similar SE primary remanences were also isolated at a control locality in the Salahi massif, outside of the region of remagnetization. Net tectonic rotation analysis at these non-remagnetised sites shows an initial NNE-SSW strike for the supra-subduction zone ridge during spreading, comparable with recently published models for the regional evolution of the ophiolite.

How to cite: Koornneef, L., Morris, A., Harris, M., and MacLeod, C.: Unravelling the Remagnetization of the Oman Ophiolite, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2063, https://doi.org/10.5194/egusphere-egu2020-2063, 2020.

Nihewan Basin is one of a series of well-developed East Asian Cenozoic basins, located in Hebei Province, North China. It has abundant gullies developed along both banks of the Sanggan River during and after the demise of Nihewan paleo-lake, creating a number of outcrops of the Nihewan Beds of fluvio-lacustrine origin, which are underlain by the Pliocene eolian Red Clay and overlain by the late Pleistocene loess. The fluvio-lacustrine sequence is rich sources of mammalian faunas and Paleolithic sites, thus providing unique insights into our understanding of land mammal biochronology and early human settlements in East Asia. Among the Nihewan Fauna (sensu lato), the Danangou (DNG) and Dongyaozitou (DZ) faunas are two of the important Pleistocene and Pliocene mammalian faunas in the Nihewan Basin. Except for a biostratigraphy, precise age control on the DNG and DZ faunas remains unavailable. Here we report a high-resolution magnetostratigraphic results that stringently constrain their ages. Rock magnetism and thermal demagnetization results show that magnetite and hematite dominate the remanence carriers in the DNG and DZ fluvio-lacustrine sequences. High-resolution magnetic polarity stratigraphy indicates that the DNG sequence recorded the Brunhes normal chron, the Matuyama reverse chron and the late Gauss normal chron, yielding the fossil-rich layers of DNG fauna with an age of ca. 1.95 Ma to 1.78 Ma during the Olduvai normal subchron. The DZ sequence was located at the late Gauss normal chron, leading an age of ca. 3.04−2.58 Ma before the termination of the Kaena reverse subchron. This result, together with previously published magnetochronology data obtained in the eastern basin, constructs a precise age constraints on the chronological framework of the Nihewan faunas and Paleolithic sites, especially during the Plio-Pleistocene transition.

How to cite: Liu, P., Qin, H., Li, S., and Yuan, B.: Time constraint on Danangou and Dongyaozitou mammalian faunas in the Nihewan Basin, North China, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6878, https://doi.org/10.5194/egusphere-egu2020-6878, 2020.

In spite of the fact, that during the last two decades some number of new paleomagnetic poles, more or less meeting the modern standards of quality [Van der Voo, 1993], have been obtained for Mesoproterozoic of Siberia [Evans et al., 2016]. The problem of the Precambrian segment of the apparent polar wander path (APWP) for Siberia, rests still to be far from its solution.

The latter, obviously, hampers the elaboration of Precambrian paleogeographic reconstructions, solution of numerous other important tasks of the Earth Sciences.

The Late Precambrian key section of the Udzha Uplift seemed to be one of the most promising object to elaborate the Mesoproterozoic segment of APWP of the Siberian platform. Until recently, the rocks composing this section have been considered to be of the Mesoproterozoic and Vendian age.

As a result of isotope studies in recent years, the age of formations of the Udzha Uplift has been significantly increased (1386±30 Ma, apatite, U-Pb, [Malyshev et al., 2018]). In particular, age of the Udzha Fm, which forms the uppermost part of the Udzha riphean sequence is considered currently to be Mesoproterosoic. On the base of our new paleomagnetic data this formation has been formed about the same time as the Khaypakh Fm from the Olenek Uplift (NE Siberia), whose Mesoproterozoic age has been established earlier from independent isotopic data [Zaitseva et al., 2017].

During last several years we have carried out the paleomagnetic studies of Late Precambrian rocks of the Udzha Uplift including the Mesoproterozoic Udzha and Unguokhtakh formations as well as intrusions representing two Mesoproterozoic magmatic events.

In this abstract we present new paleomagnetic poles for the Mesoproterosoic rocks (1500 Ma, ca.1400 Ma, 1385 Ma) of the Siberian platform.

These paleomagnetic poles significantly complement the Mesoproterozoic segment of APWP of the Siberian Platform.

The studies were supported by the Russian Science Foundation project № 19-77-10048.

How to cite: Pasenko, A., Savelev, A., and Malyshev, S.: Paleomagnetic data of the Siberian Mesoproterozoic rocks (Udzha Uplift, Northern Siberia), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9315, https://doi.org/10.5194/egusphere-egu2020-9315, 2020.

         The studied objects are located in the core of the Bashkirian megazone and related to the Riphean stage of rift magmatism of the East European craton. Paleomagnetic studies of the Bashkirian megazone intrusive bodies can be a source of new information on the East European platform position in the Riphean, as well as on the process of remagnetization during the Late Paleozoic folding on the Southern Urals. At this moment, 42 thin basic intrusions and the Main Bakal dyke were investigated.

         According to the results of our previous paleomagnetic studies two remanence components were isolated in Bashkirian megazone intrusions. First, the primary remanence component of Middle Riphean age was isolated in 8 thin bodies. Pole for the boundary of the Early and Middle Riphean of the East European Craton was calculated from high-temperature component of remanence of 8 sheet intrusions. This pole is close to the known paleomagnetic poles of East European craton for close ages and agrees with U-Pb age of one of the studied bodies (1349 ± 11 Ma). Also, arguments in favor of the primary origin of the remanence and the absence of significant tectonic dislocations in the sampling area are discussed. In other 4 intrusive bodies, paleomagnetic directions that are close but slightly different from the Middle Riphean directions were found. Second, the Late Paleozoic directions were found in the studied objects. These directions are widespread in the Bashkirian megazone rocks and have been reported by other researchers. Presumably it is the result of the Late Paleozoic syn-collisional remagnetization.

            According to the new results another component of remanence was detected in the intrusive bodies of the Bashkirian megazone. In 2 sheet bodies and the Main Bakal dyke a component close to the Late Riphean identified earlier in sedimentary rocks of the same region was found (Pavlov, Gallet, 2009; Danukalov et al., 2019). Furthermore, in total 20 thin intrusive bodies and the Main Bakal dyke have paleomagnetic directions close to the Late Paleozoic directions. The comparison of mean paleomagnetic directions for the different studied regions demonstrates the absence of any traces of essential rotation of blocks within the Bashkirian megazone in the Later Paleozoic.

            At this moment the origin of the remanence of 8 thin bodies is unclear, the nature of the other components of remanence requires additional research. It is planned to sample more intrusive bodies and to perform the isotopic dating of the key objects.

References:

• 1) Pavlov V.E., Gallet Y. Katav limestones: A unique example of remagnetization or an ideal recorder of the Neoproterozoic geomagnetic field. Izvestiya, Physics of the Solid Earth, 2009, vol. 45, no. 1, pp. 31-40
• 2) Danukalov K. N., Salmanova R. Y., Golovanova I. V., Parfiriev N. P. New paleomagnetic data on sedimentary rocks of the Inzer and Zilmerdak formations in the Southern Urals// Materials of the XXV anniversary All-Russian School-Seminar on problems of paleomagnetism and magnetism of rocks. – IPE RAS Moscow, 2019. – P. 108-113

How to cite: Anosova, M., Latyshev, A., and Khotylev, A.: Prospects of paleomagnetic studies of the Riphean intrusive bodies of the Bashkirian megazone (Southern Urals), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11392, https://doi.org/10.5194/egusphere-egu2020-11392, 2020.

Here, we present the results of a study of the anisotropy of magnetic susceptibility (AMS) completed in the Early Cretaceous magmatic complexes from the Franz Josef Land (FJL). AMS was measured in the framework of paleomagnetic research as a leading indicator of the rock magnetic fabric to help in understanding the lava flow directions and forming mechanisms. The three types of magmatic bodies were available in these studies: dolerite sills, dykes and basaltic lava flows from several islands (Alexandra, Hall, Ziegler, Jackson and Heiss Islands) among FJL. During the experiments the different parameters of AMS ellipsoids were obtained which have a good correlation with the igneous body shapes and also could illustrate lava flows direction parameters. The degree of anisotropy P is 1.01-1.06 for most sites that is typical for the primary igneous magnetic fabric. The form factor T characterizing the shape of the AMS ellipsoids demonstrates both planar and linear magnetic fabric in studied magmatic bodies. What is remarkable the part of the dykes is characterized strictly oblate magnetic fabric and another dykes have the prolate AMS ellipsoids. The linear magnetic structure is also more typical for lava flows with the maximum axes K1 lying in the flow plane that is obviously could point to the flow direction. The part of the igneous bodies are characterized by the inverse type of magnetic fabric, when the principal axis K1 of the ellipsoid is oriented perpendicularly to the plane of the flow or the sill, that was likely caused by the effect of secondary processes. The previous studies (Abashev et al., 2019) demonstrated that the primary orientation of the AMS ellipsoid could be recovered after temperature demagnetization. Noticeable changes were revealed at heating up to ~450 deg C, which generally corresponds to deblocking temperatures of titanomagnetites identified in the rocks by rock-magnetic methods. The degree of anisotropy was decreased after heating in 2-3 times. The heating also resulted to the redistribution of magnetic axes and in several cases the axes becomes more grouped. Analysis of the AMS results from the basaltic lava flows of the Aleksandra Island defined the magma flow direction to be NW-SE. Similar behavior of the AMS ellipsoids and lava flow orientation is typical for Ziegler Island. Generally our results show that complex analysis of AMS data in basaltic rocks is promising for identifying magma flow direction and it can give more detailed information about forming mechanisms of the different magmatic bodies.

This work was supported by the RSF (project no. 19-17-00091) and the RFBR (project nos. 18-35-00273, 18-05-70035).

How to cite: Chernova, A., Abashev, V., Metelkin, D., Vernikovsky, V., and Mikhaltsov, N.: Magnetic fabric and flow directions in magmatic rocks of the Franz Josef Land, Arctic Ocean, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12382, https://doi.org/10.5194/egusphere-egu2020-12382, 2020.

Products of the Permian-Triassic magmatic activity in the Kotuy river valley consist of two contrasting in composition groups: 1) tholeiitic basalts, similar to the main volume of the Siberian Traps; 2) alkaline-ultramafic rocks which are extremely rare in other regions of the Siberian platform. Alkaline lavas and tuffs in the Kotuy river valley are exposed only in limited area (Arydzhangsky and Khardakhsky formations), however, multiphase circular plutons (Kugda, Odikhincha) and swarms of radial and parallel dikes marks the essentially wider territory of the manifestation of alkaline magmatic activity.

Here we present the preliminary results of the investigation of AMS in the dike complex of alkaline lamprophyres from the Kotuy river valley. The majority of dikes demonstrate I-type of the magnetic fabric, when the medium axes K2 of AMS ellipsoid is orthogonal to the contact of intrusion. In dikes where the minimal axis K3 is subvertical and maximal axis K1 is flat, we interpret this magnetic fabric as a result of cooling of the static magma column after the emplacement in the setting of horizontal extension (Park et al., 1988; Raposo and Ernesto, 1995). Also, N-type and R-type of magnetic fabric were identified as well. In some intrusions, the orientation of the axes of AMS ellipsoid changes from the contact zones to the inner part if intrusion. In this case, we used data from the contact zones for the magma flow reconstruction.

Analysis of the maximal axis K1 orientation in different dikes showed that in majority of bodies it shallowly plunges to the west. This corresponds to the lateral magma flow from west to east during the emplacement. Consequently, formation of the studied dikes is not directly related to Kugda pluton, which is located 8 km eastward. The emplacement of dikes occurred from the magmatic center located westward from the Kotuy river valley and is not associated with any known large massifs. Petrographic similarity of the studied dikes to the lavas of Arydzhangsky formation allows us to suggest that they are coeval. This implies the wider area of manifestation of the Arydzhangsky magmatic stage.

This work was supported by RFBR (projects 18-35-20058, 18-05-70094, 17-05-01121 and 20-05-00573).

How to cite: Latyshev, A., Chmerev, V., and Zaitsev, V.: Anisotropy of magnetic susceptibility in alkaline-ultramafic dikes of the Kotuy river valley: Reconstruction of magma transport during the Siberian Traps emplacement, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-15594, https://doi.org/10.5194/egusphere-egu2020-15594, 2020.

We have studied the AMS of metamorphic rocks (gneiss, granitoids, dykes) and soft sediments (mainly marine sediments or reworked diamicton) from the Kandalaksha and Onega Bay’s Islands of the White sea. The objects of research are located within the White sea mobile belt, represented by large tectonic nappes.

The magnetic susceptibility in soft sediment samples ranges from 78.6 E-6 to 1525E-6 (Km), and the degree (P) from 1.8% to 4.1%. Ellipsoids have a predominantly flattened type, such a distribution of AMS is typical for sedimentary rocks. At the same time, in a number of samples (from the Islands of Joker, Ipanchinikha and Olenevsky), the maximum axis is directed in a North-Westerly direction, which may indicate the flow direction. This is especially evident in flattened-triaxial ellipsoids (T=0.2-0.3). Values that have a T greater than 0.5 have a predominantly northerly direction and the orientation of minerals of the magnetic fraction and the direction of paleoflow is less pronounced.

The study of the anisotropy of the magnetic susceptibility of the Archean complexes composing the Islands of the Kandalaksha Bay of the White sea showed a high magnetic susceptibility-5E-6-1E-3 (Km), which confirms the change in the petrographic composition of gneiss. The degree of anisotropy (P) is 9% on average. It was found that the distribution of the main axes of the magnetic susceptibility ellipsoid coincides with shale and banding in the root outlets, while the maximum axis of the ellipsoid coincides with the West-North-West stretch of the regional fault. In the Onega Bay we sampled paleoproerozoic dykes, and there are AMS is coincided as contacts of studied dykes.

We done alternating field and thermal demagnetization of pilot collection which contains samples from all studied complexes. And it gives us not good results because of bad paleomagnetic record. Most of samples contain only low coercive or low temperature components and it mainly has modern direction.

How to cite: Kosevich, N., Lebedev, I., and Bagdasaryan, T.: New petro-paleomagnetic data of Kandalaksha and Onega Bay Islands in the White Sea, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21720, https://doi.org/10.5194/egusphere-egu2020-21720, 2020.

The Vardar zone divides units of African affinity from units of the European margin. It is characterized by extensional opening of an oceanic domain during the Triassic and Jurassic followed by divergent simultaneous obduction of the oceanic litoshphere over the continental units in the Upper Jurassic. However, a stripe of the oceanic domain persisted till the Cretaceous and Paleogene convergence. The remnants of the last closing part of the Vardar ocean are found in the Sava zone.

In this paper recently published and new paleomagnetic, AMS results in combination with structural observations will be presented from Upper Cretaceous sediments and Oligocene –Lower Miocene igneous rocks representing the areas bordering the Sava zone from the western and eastern sides, respectively and from the upper Cretaceous flysch deposited in the Sava zone.

In the areas W and E of the Sava zone, respectively, the primary remanences of the igneous rocks point to post-Oligocene CW rotation of about 30°. The sediments carry secondary magnetizations, imprinted during magmatic activity. Compared to the areas flanking it, the sediments of the Sava zone were intensively folded during the Upper Cretaceous and Paleogene and the paleomagnetic signals, which exhibit smeared distribution close to the present N, are of post-folding age. The AMS foliation and bedding planes are sub-parallel, thus the deformation must have been weak. Fold axes and AMS lineations are roughly N-S oriented, pointing to the deformational origin of the AMS lineations. These observations form the Sava zone will be discussed in the context of the post-Oligocene CW rotation of the flanking areas and the general NE-SW orientation of the compressional stress field outside of the zone.

Acknowledgement. This work was financially supported by the National Development and Innovation Office of Hungary, project K 128625 and by the Ministry of Education and Science of the Republic of Serbia, project 176015.

How to cite: Márton, E., Toljić, M., Lesić, V., and Cvetkov, V.: The final closure of the Vardar Ocean: paleomagnetic, AMS and structural results, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-17266, https://doi.org/10.5194/egusphere-egu2020-17266, 2020.

The Marmara region is located on the Alpine Himalayan orogenic belt which experienced a active tectonic deformation. The region consists of tectonic units such as the Istanbul Zone, the Strandja Zone and the Sakarya Continent. It is reported in the previous geological studies that the Istanbul Zone began to move southwards appart from the Moesia Platform with the effect of West Blacksea Fault in the west and West Crimea Fault in the east after the the opening of the Black Sea in the Cretaceous. It is known that the Intra Pontide suture is formed after the closure of the Intra-Pontide ocean during the Early Eocene due to the collision between İstanbulzone and the Sakarya continent which moved northwards. As a result of the continental collision, the region has completed its evolution under the influence of basin formation and the emplacement of North Anatolian Fault Zone from Miocene to the present.

In this study, Upper Cretaceous-Oligocene sedimentary and volcanic rocks were sampled at 103 sites to investigate the tectonic deformation of the area. As a result of rock magnetism studies, it was shown that magnetic minerals in sedimentary and volcanic rocks are defined by titanium-rich titanomagnetite showing low coercivity, while in limestone samples, magnetization is defined by hematite showing high coercivity. As a result of anisotropy of magnetic susceptibility (AMS) measurements, it was observed that most of the samples show magnetic foliation and a deformation ellipsoid which is oblate. Paleomagnetic results show counterclockwise rotation of 19.9°±10.9° for the Sakarya continent, 27.4°±11.6°for the Pontides and 15.6°±11.8°for the Strandja Zone from Eocene to present. The results indicate that the region has completed the collision in Eocene and rotated counterclockwise as a large block. Deformation due to basin development or fault bounded block rotations which developed after Miocene could not been detected in this study. Miocene paleomagnetic data from previous studies in the study area are compatible with counterclockwise rotations in Upper Cretaceous-Oligocene which shows that different blocks emplaced in the study area moved together as a single plate during Eocene-Miocene time.

How to cite: Cabuk, B. S. and Cengiz, M.: A Paleomagnetic Study of the Tectonic Deformation in Circum-Marmara Region, NW Anatolia, during the Late Cretaceous and Cenozoic Period, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-17635, https://doi.org/10.5194/egusphere-egu2020-17635, 2020.

The Okhotsk-Chukotka volcanic belt (OChVB),  located in the north-eastern part of Russia, is a unique volcanic structure, which has been formed over a wide time interval from Aptian (K1) to Cenomanian (K2) [Tihomirov, 2018]. Age of its formation nearly coincides with the occurrence of the Cretaceous geomagnetic superchron of normal polarity. Thus, the volcanic formations of the OChVB represent a promising object to study the characteristics of the geomagnetic field during the Cretaceous superchron (direction, paleointensity, secular variations) needed to test various models explaining superchrons’s existence .

During the reconnaissance field work of the summer 2019 we have sampled volcanic rocks in 9 sections each includes from 8 to 30 sites corresponding either to lava flow or to tuff layers.

Up to date we have carried out AF demagnetization, petromagnetic and AMS studies. Demagnetisations studies demonstrate that the rocks contain paleomagnetic record of the ancient (primary?) magnetization of good to excellent quality. Petromagnetic experiments indicate that the main magnetic mineral in majority of studied volcanics is titanomagnetite with pseudo-single domain grain size. We use the magnetic fabric derived from AMS studies to test either the modern attitude (slight dipping up to 10-15˚) of studied rocks is due to primary paleorelief or the rocks have experienced some tectonic deformations.

How to cite: Lebedev, I., Usanova, O., Fadeeva, T., Lhuillier, F., Eid, B., Murray-Bergquist, L., Pasenko, A., and Gavrushkin, D.: New paleomagnetic data for Ochotsk-Chukotka volcanic belt, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-803, https://doi.org/10.5194/egusphere-egu2020-803, 2020.

This paleomagnetic study is located at the north-west extremity of the Tarim Basin and has aimed to constrain the style of Neotectonic deformation where indentation of the Pamir Orogen into the southward-verging Tian Shan frontal zone has produced a complex zone of thrusting, folding and strike-slip. Sampling focused on two Pliocene to Pleistocene sedimentary formations folded across the Mingyaole Anticline, the major structural feature between the two frontal zones, has yielded well-grouped characteristic remanent magnetizations at 18 of 24 sites and a positive fold test. Together with fabric evidence, the results indicate a probable post-depositional detrital origin for the remanence. The results show that only small inter-locational vertical-axis rotations have occurred within the Kashi-Atushi fold and thrust system since the Miocene and imply that the Kashi depression has behaved as a quasi-rigid block. A common 15-30º counterclockwise (CCW) rotation relative to Eurasia since the Miocene of the Kashi Depression and the bordering Tian Shan range proves to be unrelated to the right lateral motion along the Talas-Ferghana intracontinental transform fault to the north west. This contrast is provisionally interpreted as taking place along a transfer fault between different segments of the thrust belt. Ongoing CCW rotation of the Tarim Basin is interpreted as a regional response to impingement by northward movement of the larger Tibetan Block to the south east.

How to cite: Qiao, Q.: Neotectonic deformation in the Southwestern Tian Shan, Western China: evidence from paleomagnetic study of Quaternary sediments from the Mingyaole Anticline, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12530, https://doi.org/10.5194/egusphere-egu2020-12530, 2020.

The mid-Cretaceous paleo-Pacific ocean witnessed increased mantle plume activity, high oceanic crust production rate, enhanced subduction-related magmatism, and widespread short-lived intense deformation. The Antarctic Peninsula located at the Pacific margin of Gondwana and strongly influenced by the exceptionally pan-Pacific tectonic events during the mid-Cretaceous. Therefore, plate reconstruction of the Antarctic Peninsula and its implication to the global geodynamics, paleo-ocean circulation and paleoclimate have become one major subject for pan-Pacific geoscience studies. However, this is difficult because of the small number of reliable paleomagnetic data of the Antarctic Peninsula at the early stage of mid-Cretaceous. In this study, we obtained a key ca. 120-105 Ma paleopole from the Byers Peninsula, Livingstone Island, South Shetland Island, during the global ocean crust peak production period. Plate reconstruction reveals that the Western-Central Domain of the Antarctic Peninsula experienced clockwise rotation between ca. 155 Ma and 120-105 Ma, and large-scale anticlockwise rotation from ca. 120-105 Ma to 90 Ma. This anticlockwise rotation was ascribed to induce the final formation of the Antarctic Peninsula. This assembly age coincides with the global-scale plate reorganization at 105-100 Ma, which might associate with the eruption of mantle superplume in the southern Atlantic region.

Acknowledgments

This research was funded by the National Key R&D Program of China (2018YFC1406904), the Natural Science Foundation of China (NSFC) (41930218, 41706222, 41372082), the open foundation project (KLPTR-03) of Key laboratory of paleomagnetism and Tectonic Reconstruction, Ministry of Land and Resources.

How to cite: Gao, L., Pei, J., Yang, Z., Liu, X., Zhang, S.-H., and Zhao, Y.: Paleomagnetic constrains on the assembly processes of the Antarctic Peninsula, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19033, 2020.

This study involves kinematic development and magnetostratigraphy of infill of the Datça Graben located at the southwesternmost corner of Anatolia (Turkey). The study comprises kinematic analysis based on fault slip data collected from the margins of the Datça Graben and the magnetostratigraphic analysis of infill of the Datça Graben. For the kinematic analysis, 977 fault-slip data were collected from 44 sites. The data are analyzed using a software which is based on Angelier’s reduced stress tensor algorithm. For the magnetostratigraphic analysis, 344 samples are used and the paleomagnetic measurements of those samples are performed in the Fort Hofddjik Paleomagnetism Laboratory, University of Utrecht.

The results of the kinematic analysis have shown that the Datça Basin has developed under the effects of N-S-directed tensional stress regime manifested by WNW-ESE- striking normal faults. As a result of paleomagnetic measurements, the infill sediments of the Datça Graben can be represented by a reversed-normal-reversed polarity pattern, which can be correlated to C2r.1r-C2r.1n-C2r.2r subchrons within the C2r chron of the Early Matuyama in Geomagnetic Polarity Time Scale. This means that the graben filling sediments deposited between 2.3 Ma to 1.9 Ma, in the Late Pliocene.

This age interval suggests that the Datça Graben has completed its development from half-graben to full-graben geometry by the effects of syn-sedimentary WNW-ESE-striking faults in the Late Pliocene.

This thesis is supported by TUBITAK (Grant No: 117R012).

How to cite: İnce, M. D., Kaymakcı, N., Sümer, Ö., Uzel, B., Şiş, S., Tosun, L., Langereis, C., and Stoica, M.: Magnetostratigraphy and Kinematic Characteristics of Datca Graben (Mugla, SW Turkey), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2, https://doi.org/10.5194/egusphere-egu2020-2, 2020.

In this work we present the overview of comprehensive research on paleomagnetism and rock magnetism of more than 120 Paleoproterozoic and Neoarchean dykes from the Kola Peninsula, NE Fennoscandia. We discuss our results in a context of Precambrian paleotectonic reconstructions of Fennoscandia and Murmansk craton in particular for the time intervals corresponding to the dyke magmatism. We also show our progress in dating of the remanent magnetization components using some approaches of isotope geochronology, such as U-Pb dating of baddeleyite and Ar/Ar dating of mica, feldspar and amphibole. In total, we have been able to calculate virtual geomagnetic poles for the selected dykes and to make some conclusions about paleotectonics of the Murmansk craton at the corresponding time. New paleointensity data for some 2.50 and 2.68 Ga dykes will be also presented and commented. The study is supported by the grant of RSF #16-17-10260.

How to cite: Veselovskiy, R., Stepanova, A., Samsonov, A., and Lebedev, I.: Paleomagnetism of the Kola Peninsula's dykes, NE Fennoscandia: the review, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20186, https://doi.org/10.5194/egusphere-egu2020-20186, 2020.