Thermal Demagnetization Research Articles

Exothermic events in a fossil seismogenic fault acquiring thermoviscous remanent magnetization in an exhumed accretionary complex

Detecting exothermic events within fault rocks is key to understanding fault behavior. While paleo-temperature studies of fault rocks have been conducted via vitrinite reflectance or magnetic analysis, secondary remanent magnetization caused by exothermic events within fault rocks has not been examined directly. Direct detection of the secondary remanence has the advantage of revealing both paleo-temperature constraints and the paleomagnetic direction, which can help assess the relationship between secondary remanence and deformation. We conducted thermal and alternating-field demagnetization analyses and other rock–magnetic experiments on a fossil seismogenic fault zone and the surrounding host rocks in an exhumed accretionary complex. The fault zone comprises a few cataclastic zones, including thin-slip zones. The blocking temperature in the secondary magnetization site was detected at 300–360 °C only in cataclasite with magnetite or pyrrhotite as carriers. The paleomagnetic direction of the secondary magnetization is characteristic in the cataclasite, and the blocking temperature was higher than the paleo-maximum temperature of the host rocks. Therefore, the blocking temperature records a local exothermic event in the cataclasite. Thermoviscous remanent magnetization (TVRM) were suggested as candidate mechanisms for the blocking temperature.

Contractional deformation between extensional dome exhumation in Central Pamir at 17–15 Ma constrained by metamorphic and paleomagnetic data from the Bartang volcanic rocks, Tajikistan

The debated mechanism and timing of formation of the Pamir orogenic salient provides an ideal case to combine paleomagnetic and metamorphic analyses. Opposing views argue for either oroclinal bending of the Pamir since the India-Asia collision or for an originally arcuate shape, which can be tested using paleomagnetism to estimate vertical-axis rotations. Furthermore, Pamir deformation can be dated and characterized by the analysis of a well-expressed regional metamorphism. However, paleomagnetism is seldom applied to metamorphic rocks such that the significance of their rock magnetic signal with respect to deformation remains poorly understood. We studied a > 2 km thick sequence of slightly metamorphosed Cretaceous-Paleogene volcanic and volcaniclastic rocks from the western Central Pamir Mountains using metamorphic petrology, geochronology, and paleomagnetic analyses. These rocks present a medium-grade metamorphism and have undergone fine grained recrystallisation of biotite due to NW-SE compression. 40Ar/39Ar and Rb/Sr ages (∼17 to 15 Ma) on whole rock and biotite, confirm that the greenschist facies metamorphism is related to the coeval exhumation of Pamir gneiss domes during the Middle Miocene. The rocks generally show homogenous magnetic properties. High field magnetic properties and SEM/EDS data show that the main magnetic remanent carrier is titano-hematite. Thermal demagnetizations yield complex characteristic remanent magnetization with site-mean directions obtained for only 14 of the 44 studied sites but scattered mainly within the foliation plane defined by the anisotropy of magnetic susceptibility (AMS). These rocks have also unusual magnetic characteristics where strong-field isothermal remanent magnetizations induce an AMS fabric, likely related to the specific composition and structure of the metamorphic titano-hematite but independent of the original anisotropy fabrics. Combined paleomagnetic and geochronological data from the Bartang volcaniclastics rocks enable us to identify a ∼ 17–15 Ma compressive deformation event contemporaneous with the emplacement of the gneiss domes and suggest no oroclinal bending since that event.

Paleomagnetism and age of the Leucite Hills Volcanic complex, Wyoming: Implications for eruptive history, landscape evolution, and the geomagnetic instability timescale (GITS)

The Leucite Hills Volcanic Field, southwest Wyoming comprises two dozen volcanic features including necks, flows, dikes, and plugs. It has been the focus of many petrologic studies as its volcanic and shallow intrusive rocks are one of the only surficial manifestations of ultrapotassic lamproite. We build on paleomagnetic findings of Sheriff and Shive (1980) by providing further paleomagnetic data from the Boars Tusk dike and Black Rock flows. We also characterize the magnetic mineral assemblage of these lamproites. Principal component analysis of alternating field (AF) and thermal demagnetization data indicate that the dike and breccias of Boars Tusk record a reversed magnetic polarity and the Black Rock lava records a normal polarity, both consistent with previous findings. This recording is typically carried by minerals with coercivities >15 mT and susceptibility measurements indicate magnetite, maghemite, and titanomagnetite as likely magnetic carriers. AF and thermal demagnetization experiments evince secondary magnetizations held by lower coercivity grains, likely caused by lightning strikes. 40Ar/39Ar incremental heating experiments from Boars Tusk and Black Rock give plateau ages of ∼2500 ka and ∼900 ka, respectively. Recent advances in the chronology of geomagnetic field reversals and excursions during the Quaternary permit integration of the Boars Tusk dike into the lower Matuyama chron, whereas the Black Rock lavas most probably record the Kamikatsura excursion. Notably, Black Rock records high inclinations that suggest the short-lived excursion achieved a full geomagnetic reversal, something not observed at other localities recording the Kamikatsura excursion. The Leucite Hills offer further opportunities to refine the Quaternary geomagnetic instability time scale (GITS), and to improve understanding of the eruptive and geomorphic evolution of this unusual volcanism.

New paleomagnetic data from the central Tethyan Himalaya refine the size of Greater India during the Campanian

Knowledge of the size of Greater India is critical to deciphering the geodynamic processes of the India-Asia collision, as the size of this landmass primarily determines the amount of continental subduction and crustal shortening. Recently, paleomagnetic data revealed the rapid drift of a Himalayan microcontinent (i.e., the Tibetan Himalaya) during ca. 75–61 Ma, which resulted in a triple-stage India-Asia collision scenario. This scenario implies a relatively small Greater India at 75 Ma (Late Cretaceous), but this hypothesis is only based on one high quality Campanian paleomagnetic pole from the eastern part of the Tethyan Himalaya. Here we report comprehensive petrographic, rock magnetic and paleomagnetic studies on Upper Cretaceous oceanic affinity red beds (CORBs) of the Chuangde Formation from the central part of the Tethyan Himalaya at 83.6°E longitude. These CORBs are characterized by multi-component magnetizations carried dominantly by detrital hematite, which retains a primary remanent magnetization. Accordingly, the high temperature magnetization components (585−690°C) were isolated by high-resolution thermal demagnetization. The new paleomagnetic data provide, after applying an inclination shallowing correction, a Campanian paleopole of 43.3°N/258.3°E, which places the central part of the Tethyan Himalaya at a paleolatitude of 16.7° ± 1.8°S at ca. 75 Ma. Comparison of this paleolatitude with the expected paleolatitude of India, using its apparent polar wander path, shows that the N-S extent of central Greater India was ∼910 km during the middle Campanian. Our paleomagnetic data are in agreement with balanced cross section reconstructions and seismologic interpretations of the India-Asia collision zone and provide additional support for the North India Sea hypothesis. The improved estimate of the size of Greater India has several implications for Indian continental crust behavior and results in an updated reconstruction of the India-Asia collision system in the Campanian.

Lumped model for the calculation of harmonic eddy current losses in permanent magnets for homogeneous flux distributions considering eddy current reaction flux

This is the first of a series of papers on novel methods for the calculation of eddy current losses in permanent magnets (PMs) and the shortcomings of previously conducted analyses. Eddy current losses in PMs and their mitigation are significant factors when designing inverter-fed electric drives. Especially with the need for drives with high power density, a trend toward increased rotational speeds and, therefore, higher fundamental frequencies and inverter switching frequencies are observable. Higher harmonic frequency components of concentrated windings and fractional slot windings are widely taken into account when designing rare sintered earth magnets for permanent magnet synchronous machines (PMSMs). However, inverter-related losses are rarely simulated in Finite Element Method (FEM) co-simulations and can contribute as a major factor to the power losses in PMSMs. In the worst case, this neglection can lead to the thermal demagnetization of the permanent magnets. Segmentation provides an effective measure to limit eddy current losses. However, not every kind of segmentation proves effective in a drive application. Depending on the frequency behavior of the magnetic flux in a permanent magnet, segmentation can increase eddy current losses within the magnet compared to a non-segmented magnet. This behavior is due to the reaction to the flux caused by the eddy currents within the magnet. The reaction of the eddy currents is often neglected [1, 2], and, in other cases, the vicinity of ferromagnetic material is neglected in analytical calculations [3] but proves crucial for the exact calculation of higher frequency losses [4, 5]. In this paper, an analytical solution to calculate harmonic eddy current losses in permanent magnets, including the reaction flux with homogeneous excitation, is given.

Simultaneous enhancement of magnetic properties and electric resistance of hot-deformed Nd-Fe-B magnets by doping insulating nano-diamonds

The combination of high coercivity and resistivity is a strong requirement for permanent magnets to resist thermal demagnetization under complex operating conditions. However, this still remains a challenge for conventional strategies, which always sacrifice the maximum energy product as the coercivity or resistivity increases. In this work, we report a unique strategy of introducing only 0.1 wt% of insulating nano-diamonds (NDs) into the hot-deformed Nd-Fe-B magnets to significantly and simultaneously improve both magnetic properties (Hci, Br, (BH)max and Hk/Hcj) and resistivity. In particular, the coercivity increased by 60% from 746 to 1193 kA/m. Further microstructure observations indicate that this exceptional effect stems from NDs can effectively inhibit the formation of coarse grains and optimize the distribution of Nd-rich phases, ensuring a strong pinning effect to enhance hard magnetic performance. Meanwhile, the presence of additional grain boundaries and NDs can also lead to higher resistivity by preventing the movement of free electrons. Our work provides a distinctive strategy for interface modification with excellent comprehensive performance in permanent magnets.

Modulation of the Transient Magnetization of an EuO/Co Bilayer Tuned by Optical Excitation

AbstractThe magnetic proximity effect provides a promising way to increase the low Curie temperature (TC) of europium monoxide (EuO) toward or even above room temperature, while keeping its stoichiometry and insulating properties. This work studies EuO/Co bilayers using static and time‐resolved magneto‐optical Kerr effect measurements, and explores the influence of magnetic proximity on TC and on the spin dynamics in EuO. Excitation above the EuO bandgap results in an ultrafast enhancement of the EuO magnetization followed by a demagnetization within nanoseconds. This behaviour is also visible upon selectively photoexciting Co in the EuO/Co bilayer placed in an out‐of‐plane magnetic field, which is attributed to propagation of a superdiffusive spin current from Co into EuO. As the spin dynamics of Co shows a transient thermal demagnetization, the bilayer provides a system where the transient magneto‐optical signal can be tuned in amplitude and sign by varying external parameters such as the sample temperature or pump fluence. Moreover, in a strong excitation regime it is possible to measure the magnetic hysteresis of the underlying EuO, which is present up to room temperature – giving experimental evidence for the presence of a tuneable magnetic proximity coupling between Co and EuO.

Pressure-driven processes explain the decreasing magnetization of the subducting oceanic crust in the Japan Trench

Despite the decreases in temperature and permeability of oceanic plates with increasing age, hydrothermal circulation (HC) can be rejuvenated in the 130-Ma old Pacific plate in the vicinity of the Japan Trench, substantially affecting the thermal structure and remaining amount of magnetization (RAM). To decipher the roles of HC in the thermal structure and the RAM, the vigor and extent of HC in the vicinity of the Japan Trench should be quantitatively evaluated. Here we numerically show that HC is rejuvenated in the outer-rise zone but ceases after subduction owing to permeability evolution. The calculated thermal structure explains the measured heat flow evolution but negates the HC-driven thermal demagnetization, which was thought to decrease the RAM after subduction. Instead, we propose that the pressure-driven processes decrease the RAM after subduction through the demagnetizations of titanomaghemite and magnetite and the mineral phase transitions from maghemite to hematite.

Possible evidence for geomagnetic intensity anomaly around 5500 BP from archaeomagnetic analyses of San Jacinto pottery, Caribbean Colombia

San Jacinto archaeological site in Northern Colombia hosts America's oldest pottery, radiometrically dated between 5940 ± 60 BP and 5190 ± 40 BP. Eleven ceramic fragments were subjected to detailed archaeomagnetic and archaeointensity investigation. Alternating field and thermal demagnetization, as well as magnetic mineralogy experiments, were carried out before double heating paleointensity determination to select the most suitable samples. Magnetic susceptibility vs. temperature curves revealed that magnetite and hematite grains are the main carriers of the thermoremanent magnetization. However, the contribution of hematite in total remanence seems to be negligible. Thirty-six samples corresponding to six pottery fragments were selected for Thellier-type paleointensity determinations. Mean intensity values range from 50.7 ± 4.7 to 65.3 ± 11.9 μT while virtual Axial Dipole Moments vary from 12.6 ± 1.2 to 16.2 ± 2.9 × 1022 Am2, indicating high geomagnetic field strength compared to values predicted by global geomagnetic models. This represents the first evidence of relatively high archaeointesities before 1000 BC. Similar or even higher geomagnetic strengths are observed in the Levant around 1100–800 BC and SW Pacific Region at 1450 AD. The actual cause of such anomalously high intensity, often attributed to the processes in the vicinity of the Core-Mantle Boundary, is still not fully understood.

Magnetic characterization of the Daule chondrite (Ecuador's first meteorite fall): The case of elusive tetrataenite?

We investigated the magnetic properties of Ecuador's first reported meteorite fall (March 23, 2008), the Daule ordinary chondrite (L5, S4, W0) using thermomagnetic analyses at high and cryogenic temperatures, analyses of magnetic hysteresis and first-order reversal curves, and thermal and alternating field (AF) demagnetization of natural remanent magnetization (NRM). The mineralogical and chemical composition of Daule was examined using scanning electron microscopy with energy-dispersive x-ray spectroscopy. Most methods indicate that the magnetic properties of Daule are dominated by multidomain FeNi alloys (kamacite) with Ni content varying between ∼4% and ∼17%. However, backfield demagnetization (BFD) analyses revealed the presence of high-coercivity tetrataenite that survived shock metamorphism. The differential survival of tetrataenite at the millimeter scale indicates heterogeneity of the impact-related temperature and pressure fields within the Daule meteorite. BFD curves may serve as an efficient tool for identifying minor amounts of tetrataenite that otherwise cannot be discerned from the signal from magnetically-soft FeNi mineral phases by methods based on induced magnetization. Thermal demagnetization experiments unveiled the presence of a well-defined characteristic component of NRM, which remains resistant to AF demagnetization. We interpret this component as a pre-impact thermochemical remanence carried by tetrataenite and acquired during the thermal metamorphism of the parent body. At cryogenic temperatures, the magnetic properties of Daule are dominated by low-Mg magnesiochromite with the Curie temperature at 60–70 K.

Thermal Resolution of Unblocking Temperatures (TROUT): A Method for “Unmixing” Multi‐Component Magnetizations

AbstractSome rocks contain multiple remanence “components,” each of which preserves a record of a different magnetic field. The temperature ranges over which these remanence components unblock can overlap, making it difficult to determine their directions. We present a data analysis tool called Thermal Resolution Of Unblocking Temperatures (TROUT) that treats the process of thermal demagnetization as a function of temperature (or alternating field demagnetization as a function of coercivity). TROUT models the unblocking temperature/coercivity distributions of components in a demagnetization experiment, allowing these distributions to overlap. TROUT can be used to find the temperatures/coercivities over which paleomagnetic directions change and when two directional components overlap resulting in curved demagnetization trajectories. When applied to specimens given multi‐component Thermoremanent Magnetizations (TRMs) in the laboratory, the TROUT method estimates the temperature at which the partial TRMs were acquired to within one temperature step, even for specimens with significant overlap. TROUT has numerous applications: knowing the temperature at which the direction changes is useful for experiments in which the thermal history of a specimen is of interest (e.g., emplacement temperature of pyroclastic deposits, re‐heating of archaeological artifacts, reconstruction of cooling rates of igneous bodies). The ability to determine whether a single component or multiple components are demagnetizing at a given temperature is useful for choosing appropriate ranges of temperatures to use in paleodirection/intensity experiments. Finally, the width of the range of temperature overlap may be useful for inferring the composition, grain size and domain state of magnetic mineral assemblages.

Thermal failure analysis and control algorithm improvement for the active controlled mount with consideration of thermal–magnetic coupling

This paper proposes an improved narrowband filtered-x least mean square(FxLMS) algorithm to overcome the thermal failure problems of the active controlled mount(ACM). Firstly, the temperature-rising model and the thermal demagnetization model of the ACM are respectively developed. Combining these two models with the powertrain mounting system model, an analytical method is established for the thermal–magnetic coupling analysis of the ACM. Then a numerical simulation is carried out to obtain the permanent magnet(PM) temperature and the coil current. The ACM failure problem is discussed according to the working point trajectory. Finally, an improved algorithm is proposed. This algorithm is able to overcome the thermal failure problems by sacrificing part of the vibration isolation performance. The effectiveness of this algorithm is validated by numerical simulations and a comparison with conventional algorithms.

Thrust Ripple Reduction of Dual-Side Toroidal-Winding Linear Permanent Magnet Vernier Machine by Unequal End-Teeth Width

The linear permanent magnet vernier machines have attracted more and more attention last several years due to its high thrust density at low speed, which is very suitable for the direct-drive applications. A dual-side toroidal-winding linear permanent magnet vernier machine (DSTWLPMVM) with the unequal end-teeth width is proposed in this paper to reduce the detent force and thrust ripple. The low detent force in DSTWLPMVM is achieved by combination of two single-side detent forces with the opposite phase angle, which is obtained by optimizing the unequal end-teeth width of the primary two-sides. In the proposed machine, the symmetry three-phase flux linkage of the toroidal winding also can be obtained by optimizing the unequal end-teeth width. Therefore, the low thrust ripple of DSTWLPMVM is achieved by optimizing the unequal end-teeth width for obtaining the low detent force and symmetry three-phase flux linkage. The vertical magnetic force in the proposed LPMVM is also considered by finite element analysis (FEA) in this paper. Via optimizing the unequal end-teeth width, the detent force is reduced by 90% while the peak-to-peak thrust ripple is reduced by 69% than that of DSTWLPMVM with the equal end-teeth width, respectively. In this paper, the thermal and PM demagnetization behavior of the proposed machine are also analyzed by FEA.

Middle Jurassic Paleolatitude of the Tethyan Himalaya: New Insights Into the Evolution of the Neo‐Tethys Ocean

AbstractThe paleogeography of the Tethyan Himalaya (TH) during the Mesozoic is vital to constrain the evolutionary history of the Neo‐Tethys Ocean, but reliable paleomagnetic data from Jurassic rocks in this area are scarce. Here, we report the first high‐quality paleomagnetic results from the Middle Jurassic (∼175–173 Ma) Lanongla Formation limestones in the Nyalam area of the TH. For most specimens, stepwise thermal or hybrid demagnetization reveals two well‐defined magnetization components. A low‐temperature component, which is isolated between natural remanent magnetization and 200–300°C, is consistent with the present geomagnetic field direction. A high‐temperature component, which is isolated between 300–350°C/10–25 mT and 500–580°C/60–120 mT, passes fold tests at the 95% and 99% confidence level, indicating a prefolding primary magnetization. The tilt‐corrected site‐mean direction for 28 paleomagnetic sites is Ds = 331.0° and Is = −49.9° with α95 = 2.7°, which provides a Fisherian site‐mean paleopole at 23.7°N, 292.9°E with A95 = 2.8° and a paleolatitude of 31.7 ± 2.8°S for the Nyalam study area (28.6°N, 86.1°E). A comparison between the reliable Middle Jurassic‐Early Cretaceous poles obtained from the TH and those observed from the Lhasa terrane reveals that the Neo‐Tethys Ocean for the reference point (29.1°N, 86.1°E) had a latitudinal width of 3,500 ± 1,000 km at ∼174 Ma, reached its greatest width of 7,000 ± 1,000 km at ∼137 Ma, and had an average latitudinal spreading rate of ∼10.4 cm/year during ∼174–137 Ma.

Comparative evaluation of different permanent magnet materials for spoke interior permanent magnet motor suitable for automotive application

Spoke Interior Permanent Magnet Synchronous Motors (IPMSM) are regarded as a potential substitute to conventional IPMSM in electric four wheeler application due to its simple construction, wide speed torque characteristics, improved power factor, efficiency and high torque density. The traditional IPMSM employs Neodymium Ferrite Boron (NdFeB) magnets, which have high torque density. But, the problems associated with NdFeB magnets include demagnetization owing to high operating temperature, high investment cost, brittleness which leads to high stress distribution in the rotor with torque ripple, high cost of maintenance and existence of cogging torque. Therefore, this work attempts to design and analyze Spoke type IPMSM different permanent magnet materials such as Samarium Cobalt (SmCo) and Y30 type Ferrite Magnets (rare earth free magnet), to evaluate the torque density, efficiency, requirement of cooling in order to reduce thermal demagnetization, Von-Mises stress and cogging torque. The volume of the motor is varied for various permanent magnet materials. The results of this work provides insight into the various performance measures with respect to choice of different magnetic materials suited to electric four wheeler (automotive) application.

Palaeosecular variation in Northern Patagonia recorded by 0–5 Ma Caviahue–Copahue lava flows

SUMMARY Palaeosecular variation (PSV) determinations and studies of the geometry of the Earth’s main magnetic field provide important information about the field evolution, and to constrain numerical geodynamo models. Palaeomagnetic directional data from lava flows over the last few million years is of particular interest because the regional and global tectonic effects are minimal. However, the distribution of this type of palaeomagnetic data is uneven where the Southern Hemisphere is the destitute side. Therefore, the better knowledge of the geomagnetic field behaviour depends on the increased availability of high-quality data, especially in the Southern Hemisphere. A PSV and the time-averaged field (TAF) study was then performed in 0–5 Ma lava flows from the Caviahue–Copahue Volcanic Complex located in Northern Patagonia, Argentina (37°0′S, 71°10′W). The magnetic mineralogy of lava flows was investigated through thermomagnetic susceptibility curves, isothermal remanent magnetization (IRM) acquisition curves, hysteresis loops and first-order reversal curves (FORCs). Samples are essentially comprised of titanomagnetite with different Ti contents and magnetic domain structures typical of vortex state particles. A total of 50 volcanic sites were sampled, which provided 42 reliable palaeomagnetic site-mean directions after alternating field and thermal demagnetization. From these 42 sites, 36 are of normal and 6 are of reversed polarity. The mean direction from normal (reversed) sites is D = 356.2°, I = −50.1°, α95 = 4.0° and N = 36 (D = 176.5°, I = 59.5°, α95 = 14.1° and N = 6). Using only site-level data with the precision parameter k ≥ 100, we obtain 26 palaeomagnetic sites for PSV and TAF investigations in the study region. The filtered data set has a mean direction (D = 354.4°, I = −53.2° and α95 = 5.1°) close to the expected direction for a geocentric axial dipole (GAD) field (IGAD = −57.3°). The palaeopole (Plat = 84.4°, Plon = 229.1° and A95 = 5.7°) coincides with the Earth’s spin axis within the 95 per cent confidence interval. Virtual geomagnetic pole scatter ($S_{B} = {15.8^{18.9}_{11.8}}^{\circ }$) and the inclination anomaly ($\Delta I = 4.1_{-1.0}^{{9.2}^{\circ }}$) are both consistent at the 95 per cent confidence level with recent PSV and TAF models, respectively. Our results support the presence of small non-dipole field contributions (<3 per cent) superimposed on the GAD term, as reported by South American studies at mid southern latitudes.

New magnetostratigraphy constrains in the southern Paraná magmatic province (Herveiras and Gramado Xavier areas), Rio Grande do Sul state, South Brazil: Implications for the timing between volcanic sources

This paper undertakes a new paleomagnetic study of 51 lava flows that crop out in Southern Paraná Magmatic Province. The volcanic rocks are basalt of low-Ti content type (18 flows) and Palmas silicic type (33 flows) corresponding to the Caxias do Sul, Barros Cassal, and Santa Maria sub-types covering all lithostratigraphic units of the region. This study is part of a broad magnetic study in the Brazilian part of the Paraná-Etendeka LIP to understand the temporal relationship between the volcanic units of different compositions allowing the investigation of the contemporary nature of the multiple sources. The characteristic remanent magnetization (ChRM) for the lavas was obtained by alternating field (a.f.) and thermal demagnetization methods, and is interpreted as a thermo-remanent magnetization carried by stable pseudo-single domain magnetite grains, being acquired during the cooling of the lava flows. The high-quality of the ChRM results from both areas allows us to interpret that the emplacement of all volcanic rocks was geologically very fast, as verified by the symmetrical direction of both polarities (normal and reverse i.e. positive reversal test) of the geomagnetic field associated with the stratigraphy of all lava flows. The passage from pahoehoe to rubbly basalts was marked by a change of polarity of the geomagnetic field. Our results suggest that all volcanic rocks were emplaced by at least two volcanic events as shown by the polarity changes, and the sources of basaltic and silicic rocks of different compositions were active at the same time.

New Paleomagnetic Constraints on the Amalgamation of the Tuva and Mongolia Blocks From Late Carboniferous Andesites in the Mongolia Block

AbstractThe Tuva‐Mongolia collage system comprises the Tuva Block (TVB) and the Mongolia Block (MOB), which are located in the eastern Central Asian Orogenic Belt (CAOB). The timing of amalgamation of the TVB and MOB is unclear. To address this, we conducted a combined geochronological and paleomagnetic study of the andesitic Member 1 of the Baoligaomiao Formation, in the southeastern MOB. The andesites have a U‐Pb zircon age of 314.2 ± 1.7 Ma. A total of 65 paleomagnetic samples from 10 sites were subjected to stepwise thermal demagnetization and had stable magnetic signals. After removal of the low‐temperature viscous components acquired in the recent geomagnetic field, stable high‐temperature components (HTCs) were isolated from most samples. The HTCs passed the fold, conglomerate, and reversal tests, demonstrating their primary origins. Moreover, these andesites have averaged out paleosecular variations. The paleomagnetic pole for the MOB is located at 30.5°N/14.6°E (A95 = 7.4°) at ca. 314 Ma. Combining our new results with published late Paleozoic paleomagnetic poles for the MOB and TVB suggests that the two blocks were distant from each other during the Middle Devonian and merged into a single TVB‐MOB by ca. 315 Ma. The Devonian‐Carboniferous paleomagnetic, paleobiogeographic, and geological data indicate that the multiple microcontinents in the CAOB were separated by numerous ocean basins during the Middle Devonian. At ca. 315 Ma, the TVB‐MOB, Kazakhstan, and Siberia were amalgamated to form a ribbon‐like continent, which was associated with the evolution of the Mongol‐Okhotsk Ocean.

The Holy Cross Mountains (Poland) terranes palaeoposition and depositional environment in Silurian—new insights from rock magnetic studies

SUMMARY The Holy Cross Mountains (HCM) in Poland, is an isolated natural outcrop of Palaeozoic rocks located within the Trans-European Suture Zone, a tectonic collage of continental terranes adjacent to the Tornquist margin of the Baltica. This uniqueness made the HCM a target for palaeogeographic research. Based on the facies differences, the HCM had been divided into two major units, the southern (the Kielce Unit) and northern (the Łysogóry Unit) part (SHCM and NHCM, respectively). Their position in relation to each other and the Baltica continent during Silurian times is still a matter of discussion, whether both parts of the HCM were separated terranes located along the Baltica margin or they shared in common palaeogeographic history. Here, we present the results of comprehensive rock magnetic measurements applied as a tool to interpret palaeoenvironmental conditions during deposition and burial and therefore allow discussion about the terranes’ relative position. To recognize the magnetic mineral composition and texture of studied Silurian graptolitic shales several rock magnetic measurements were conducted including low-temperature Saturated Isothermal Remanent Magnetization, thermal demagnetization of three-component IRM and hysteresis measurements, as well as anisotropy of magnetic susceptibility (AMS). The sampled rocks come from both units of the HCM. In all analysed samples we found single domain (SD) stoichiometric magnetite of mostly diagenetic (i.e. post-depositional) origin and goethite resulting likely from weathering. In turn, detrital magnetite, even if observed in previously investigated Silurian rocks from the Baltica margin, was not identified in this study, what we attribute to dissolution during diagenesis in the deep-water environment. Solely in the NHCM, SD hematite and maghemite grains were observed, which we interpret as detrital in origin. These grains have been preserved in the suboxic environment of the NHCM sub-basin bottom waters due to their resistance to dissolution in marine waters. Considering the deposition conditions (oxygenation of the near-bottom zone) rather similar for both HCM parts, we associate the presence of aeolian hematite grains solely in the NHCM rocks with a more proximal position of the NHCM than the SHCM in relation to the Baltica continent during late Llandovery (Silurian). This conclusion agrees with some existing palaeogeographic models. In addition to petromagnetic studies focused on the analysis of ferromagnets, AMS measurements were also carried out. The results indicate that the magnetic susceptibility is mainly governed by paramagnetic minerals, mostly phyllosilicates with small ferromagnetic contributions. Oblate AMS ellipsoid and distinct bedding parallel foliation indicate prevailing sedimentary-compactional alignment. Observed magnetic lineation of tectonic origin resulting from weak strain is related presumably to Variscian deformations.

New palaeomagnetic results of Permian tectonic blocks in the western segment of the Yarlung Zangbo Suture Zone and their tectonic implications

The composition and structure of the Yarlung Zangbo Suture Zone (YZSZ) provide important constraints on long‐standing disputes about the evolution of the Neo‐Tethys Ocean and continental collision. The Permian tectonic blocks (mainly composed of limestones and volcanic rocks) are widely distributed in the YZSZ. There are still different understandings about the origin of these blocks and their relationship with the evolution of the Neo‐Tethys Ocean. Here, we report the first reliable palaeomagnetic results of the Permian tectonic blocks in the Gyanyima and Mayum areas, the western segment of the YZSZ. Fossil records and our new zircon U–Pb results provide 254–252 Ma and 249 ± 2 Ma age constraints for the strata in the Gyanyima and Mayum sections, respectively. A total of 109 specimens were subjected to stepwise thermal demagnetizations. Among them, 83 specimens yielded interpretable results. After removing a recent geomagnetic field direction at low to intermediate temperature ranges, characteristic remanent magnetization (ChRM) directions can be isolated from the upper Permian–lower Triassic Gyanyima Formation (Fm). Unblocking temperature spectra of the ChRM indicate magnetite and haematite as the main carriers in the basalt and limestone/andesitic‐basalt samples, respectively. The ChRM passed the fold tests and a reversal test at 95% confidence level, indicating that it is most likely a primary remanence. The tilt‐corrected mean direction for 14 sites is D = 18.0°, I = −14.4°, k = 20.5, and α95 = 9.0°, which corresponds to a palaeolatitude of 7.3°S and a palaeopole at 48.3°N, 233.3°E with A95 = 8.1°. Combining the rock assemblage, palaeomagnetic, palaeontological, and geochronologic data, we conclude that these blocks were most likely seamounts formed in the low‐latitude area of the Neo‐Tethys Ocean. The palaeolatitude and origin of these blocks indicate that the evolution of the Neo‐Tethys Ocean was a complex process interspersed with the development of a series of seamounts and microblocks.