Anomalous Geomagnetic Field Research Articles

Paleomagnetism and Cyclostratigraphy of the Permian-Triassic Boundary Interval of the Staroe Slukino Section, Vladimir Region

The paper presents the results of paleomagnetic, rock magnetic, and cyclostratigraphic studies of continental red beds that compose the Permian-Triassic boundary interval of the Staroe Slukino section in the Vladimir Region, Russia. Based on the directions of the characteristic components of the natural remanent magnetization of the studied rocks in the sampled stratum, the intervals of normal and reverse polarity related to the regional magnetozones r2RnP, r3RnP, and N3P-T were identified. Within the r3RnP zone, there is an interval of anomalous paleomagnetic directions, which has similar characteristics to those identified earlier in the coeval intervals of the Nedubrovo, Zhukov Ravine, and Okskiy Siyezd sections. Revising the biostratigraphy allows us to assume that the two zones of anomalous paleomagnetic directions in the composite magnetic polarity scale of the upper Permian of the Russian Platform are a reflection of the same epoch of an anomalous geomagnetic field configuration. It is estimated by the cyclostratigraphic method that the studied 16 m interval of the Staroe Slukino section took 900±20 thousand years to accumulate, which limits the duration of the geomagnetic field anomalous state near the Permian-Triassic boundary to ~110 000 years. A new Permian-Triassic (~252 Ma) paleomagnetic pole of the East European Platform is calculated: plat = 36.3°; plong = = 155.0°; dp/dm = 2.8°/4.8°.

GRAVITAIONAL FIELDS AND EARTHQUAKES IN CENTRAL KAZAKHSTAN

The article gave a characteristic of used materials; outline of issues of hardware support and methods for mapping the anomalous geomagnetic, gravitational, geothermal fields of various and the interpretation results from the data obtained, the research of linking the observed magnetic fields to separate isolated areas, registered the normal of magnetic field of the Earth. The main regularities of the distribution of the anomalous geomagnetic field (ΔТа) within the Central Kazakhstan, issues of nature of anomalous of the geomagnetic, gravitational, thermal fields in some regions, some large geological structures, of anomalous fields and anomalies. The results of investigations on revealing correlations between anomalous geomagnetic, gravitational, thermal fields and seismicity in the territory of Central Kazakhstan are analyzed.

Correlation between lithospheric magnetic anomalies and tectonic structures in the Norwegian-Greenland region of the Arctic

The studies of the deep structure and tectonics of the Arctic are important for solving the fundamental problems of modern geodynamics and developing its natural resources. This region is also of interest from the geopolitical point of view, in particular, considering the boundaries of the marginal seas. Our study aims to investigate the lithospheric (anomalous) geomagnetic field in the Norwegian-Greenland region of the Arctic and to correlate the identified anomalies with tectonic structures located in the region under study. The database includes the CHAMP satellite measurements of the modulus of the total geomagnetic field vector (the satellite operated at the altitude of ~280 km). This article describes the satellite data processing method applied to distinguish between the lithospheric part and other components of the geomagnetic field. Map showing the total vector modulus of the lithospheric field has been constructed for the studied area. The article discusses the possible nature of the lithospheric magnetic anomalies and their relation to the processes that occur under the territory of Greenland. According to our interpretation of the maps, the geomagnetic field anomalies are related to the modern large-scale geological and tectonic structures located in the studied area. The obtained results can facilitate further comprehensive geological and geophysical studies and contribute to modeling of the evolution of the lithosphere.

Systematics of Early Cambrian Paleomagnetic Directions from the Northern and Eastern Regions of the Siberian Platform and the Problem of an Anomalous Geomagnetic Field in the Time Vicinity of the Proterozoic–Phanerozoic Boundary

Representative paleomagnetic collections of Lower Cambrian rocks from the northern and eastern regions of the Siberian platform are studied. New evidence demonstrating the anomalous character of the paleomagnetic record in these rocks is obtained. These data confidently support the hypothesis (Pavlov et al., 2004) that in the substantial part of the Lower Cambrian section of the Siberian platform there are two stable high-temperature magnetization components having significantly different directions, each of which is eligible for being a primary component that was formed, at the latest, in the Early Cambrian. The analysis of the world’s paleomagnetic data for this interval of the geological history shows that the peculiarities observed in Siberia in the paleomagnetic record for the Precambrian–Phanerozoic boundary are global, inconsistent with the traditional notion of a paleomagnetic record as reflecting the predominant axial dipole component of the geomagnetic field, and necessitates the assumption that the geomagnetic field at the Proterozoic–Phanerozoic boundary (Ediacaran–Lower Cambrian) substantially differed from the field of most of the other geological epochs. In order to explain the observed paleomagnetic record, we propose a hypothesis suggesting that the geomagnetic field at the Precambrian–Cambrian boundary had an anomalous character. This field was characterized by the presence of two alternating quasi-stable generation regimes. According to our hypothesis, the magnetic field at the Precambrian–Cambrian boundary can be described by the alternation of long periods dominated by an axial, mainly monopolar dipole field and relatively short epochs, lasting a few hundred kA, with the prevalence of the near-equatorial or midlatitude dipole. The proposed hypothesis agrees with the data obtained from studies of the transitional fields of Paleozoic reversals (Khramov and Iosifidi, 2012) and with the results of geodynamo numerical simulations (Aubert and Wicht, 2004; Glatzmayer and Olson, 2005; Gissinger et al., 2012).

Anomalous Geomagnetic Field Modeling for Crustal Structure of a Graben in West Siberia

An intensive magnetic anomaly within the limits of West Siberia Danilov graben-rift indicates magnetic rocks while numerous wells encountered only weakly magnetized Triassic basalts and liparites in the basement covered by thick loose Jurassic and younger sediments. The wells penetrated only the first tens meters of the basement and could not tell us about the liparites structure at depth where supposedly they may form a big single body and magnetic rocks may be situated deeper. Geological ideas on a graben-rift structure may be proved (or rejected) by a computer modeling of its magnetic properties. For the anomalous geomagnetic field interpretation, a method of volume integral equations taking into account demagnetization effect was employed. To fit a model a trial-and-error procedure was utilized. The results show that 1) at the depth some rocks are magnetized in opposite direction to the present field; 2) highly magnetized rocks (magnetic susceptibility 0.06 - 0.1SI) coming up continuously from the bottom of the model and situated under the graben; 3) the studied structure is not a graben but the rift because the continental light crust is absent.

MW9 Tohoku earthquake in 2011 in Japan: precursors uncovered by natural time analysis

This paper reviews the precursory phenomena of the 2011 MW9 Tohoku earthquake in Japan that emerge solely when we analyze the seismicity data in a new time domain termed natural time. If we do not consider this analysis, important precursory changes cannot be identified and hence are missed. Natural time analysis has the privilege that enables the introduction of an order parameter of seismicity. In this frame, we find that the fluctuations of this parameter exhibit an unprecedented characteristic change, i.e., an evident minimum, approximately two months before Tohoku earthquake, which strikingly is almost simultaneous with unique anomalous geomagnetic field variations recorded mainly on the z component. This is consistent with our finding that such a characteristic change in seismicity appears when a seismic electric signal (SES) activity of the VAN method (from the initials of Varotsos, Alexopoulos, Nomicos) initiates, and provides a direct confirmation of the physical interconnection between SES and seismicity.

A re-assessesment of the shallow paleomagnetic inclinations of the Western Cyclades, Greece

Consistently shallow paleomagnetic inclinations measured in Early to Middle Miocene lacustrine and dacitic volcanic rocks of the Kymi-Aliveri basin have been cited as evidence for an anomalous geomagnetic field geometry or northward drift of the Aegean Sea region. We present new paleomagnetic data from the lacustrine beds that are instead not anomalously shallow and consistent with deposition near their present-day latitude as predicted by global apparent polar wander paths. Anomalously shallow inclinations and easterly declinations reported from the Oxylithos volcanics are an artifact of an inappropriate tilt correction. The excessively shallow paleomagnetic inclinations reported from the deformed Middle Miocene plutons on Mykonos and Naxos are consistent with reorientation of an original thermoremanent magnetization acquired during cooling below 580°C by subsequent ductile strain at temperatures of 400-500°C. Magnetization overprints observed in these rocks may reflect the acquisition of a stable chemical remanent magnetization lying parallel to the transposed high-temperature magnetization as the result of low-temperature (<350°C) maghemitization. We therefore find no convincing evidence for an anomalous Middle Miocene field geometry, northward drift of the Aegean, or back-tilting of the low-angle normal faults that constitute the North Cycladic Detachment System.

Иммунологическая эффективность использования ликопида у животных с экспериментальным кератоконъюнктивитом в условиях воздействия магнитного поля различной напряженности

The efficacy of using lycopid immunomodulator in the therapy of experimental keratoconjunctivitis in guinea pigs undergoing the background and elevated magnetic field values is investigated. The development of keratoconjunctivitis in laboratory animals under conditions of an abnormal magnetic field is accompanied by the formation of functional deficiency of phagocytic cells with oppression of the reserve bactericidal potential of neutrophils, as well as oxidative-antioxidant imbalance. It was shown that the use of lycopid in animals with staphylococcal keratoconjunctivitis made it possible to correct the indices of the phagocytic link of immunity and enzyme systems that had been unbalanced in the course of pathology development, ensuring the maintenance of homeostasis. This process was noted both in terms of background values of the geomagnetic field, and in simulating the effect of the anomalous geomagnetic field of the Kursk magnetic anomaly region.

Spectral analysis of the multiple-altitude anomalous geomagnetic field

The spectra of the anomalous geomagnetic field measured at ground and balloon (30 km) altitudes were analyzed. The ground-based data were adapted from a map of the anomalous magnetic field of the Earth. A balloon surveys was carried out by the authors. It has been shown that the ground and balloon spectra of the anomalous magnetic field of the Earth substantially differ. Suppositions explaining the differences in the obtained spectra have been suggested.

Anomalous geomagnetic variations associated with the volcanic activity of the Mayon volcano, Philippines during 2009–2010

Local anomalous geomagnetic variations preceding and accompanying the volcanic eruptions had been reported by several researchers. This paper uses continuous high-resolution geomagnetic data to examine the occurrence of any anomalous geomagnetic field variations that possibly linked with the volcanic eruption of the Mayon volcano, Philippines during 2009–2010. The nearest geomagnetic observing point from the Mayon volcano is the Legazpi (LGZ) station, Philippines; which is located about 13km South of the Mayon volcano. The amplitude range of daily variations and the amplitude of Ultra Low Frequency emissions in the Pc3 range (Pc3; 10–45s) were examined at the LGZ station and also were compared with those from the Davao (DAV) station, Philippines as a remote reference station. Both the LGZ and DAV stations belong to the MAGDAS Network. The result of data analysis reveals significant anomalous changes in the amplitude range of daily variations and the Pc3 amplitude at the LGZ station before and during the volcanic eruption of the Mayon volcano. From the obtained results, it appears that the observed anomalous variations are dependent on the change in the underground conductivity connected with variation in the physical properties of the Earth’s crust due to the activity of the Mayon volcano. Therefore, these anomalous geomagnetic variations are considered to be of a local volcanic origin.

On the anomalous changes of seismicity and geomagnetic field prior to the 2011 [formula omitted] 9.0 Tohoku earthquake

In Xu et al. (2013), it has just been reported that approximately 2months prior to the Mw 9.0 Tohoku earthquake that occurred in Japan on 11 March 2011 anomalous variations of the geomagnetic field have been observed in the vertical component at a measuring station about 135km from the epicenter for about 10days (4–14 January 2011). Here, we show that this observation is in striking agreement with independent recent results obtained from natural time analysis of seismicity in Japan. In particular, this analysis has revealed that an unprecedented minimum of the order parameter fluctuations of seismicity was observed around 5 January 2011, thus pointing to the initiation at that date of a strong precursory Seismic Electric Signals activity accompanied by the anomalous geomagnetic field variations. Starting from this date, natural time analysis of the subsequent seismicity indicates that a strong main shock was expected in a few days to one week after 08:40 LT on 10 March 2011.

The 6 April 2009 earthquake at L'Aquila: a preliminary analysis of magnetic field measurements

Abstract. Several investigations reported the possible identification of anomalous geomagnetic field signals prior to earthquake occurrence. In the ULF frequency range, candidates for precursory signatures have been proposed in the increase in the noise background and polarization parameter (i.e. the ratio between the amplitude/power of the vertical component and that one of the horizontal component), in the changing characteristics of the slope of the power spectrum and fractal dimension, in the possible occurrence of short duration pulses. We conducted, with conventional techniques of data processing, a preliminary analysis of the magnetic field observations performed at L'Aquila during three months preceding the 6 April 2009 earthquake, focusing attention on the possible occurrence of features similar to those identified in previous events. Within the limits of this analysis, we do not find compelling evidence for any of the features which have been proposed as earthquake precursors: indeed, most of aspects of our observations (which, in some cases, appear consistent with previous findings) might be interpreted in terms of the general magnetospheric conditions and/or of different sources.

Magnetic and petrochemical properties of rocks from the Red Sea rift zone

The magnetic and petrochemical properties of basalts dredged from the rift zone at the southern part of the Red Sea at approximately 18° N are studied. The conditions of their formation and evolution are elucidated. The formation of basalts is found to occur in two stages differing in their magma supply rates and redox conditions. Special features of magnetic behavior and the anomalous geomagnetic field in the region were used to determine the sites where the rocks had experienced strong thermodynamic effects (tectonic strain, deformation, heating, etc.). The conditions in which natural remanent magnetization of the basalts was formed are estimated.

High-resolution magnetic stratigraphy at Bosso Stirpeto (Marche, Italy): Anomalous geomagnetic field behaviour during early Pliensbachian (early Jurassic) times?

We report on a high-resolution magnetostratigraphic analysis of a continuous 97.5 m thick upper Sinemurian–Pliensbachian pelagic limestone section (“Corniola” formation) exposed at Bosso Stirpeto (Marche, Italy), where detailed ammonite and calcareous nannofossil biozonation is available. The early Pliensbachian (Carixian) is notably expanded (74.35 m excluding few slumps), implying an average sedimentation rate of 28.6 m/Myr. Both the Carixian boundaries and three additional intra-Carixian ammonite zone boundaries are tied up with polarity magnetozones. We find that a normal polarity characterizes the latest Sinemurian and Sinemurian/Carixian boundary, while a reverse polarity (punctuated by three short normal polarity magnetozones) dominates the Carixian. In the lower Carixian sediments, two 3.39–4.65 m thick (excluding the slumps) intervals are characterized by persistent transitional (between 45°N and 45°S) virtual geomagnetic pole (VGP) latitudes, but this seems not to be the result of mineral magnetic artefacts, antipodal polarity averaging, or sedimentary layer disturbance. Thus we suggest that in two ∼ 120 and 160 kyr-long time intervals during the early Carixian, the VGPs were predominantly confined at tropical–equatorial latitudes of the Earth. Such geomagnetic feature has never been documented before elsewhere in other time intervals, and requires further paleomagnetic investigation of other expanded Carixian section to be confirmed. The magnetic polarity profile (including the transitional direction intervals) of Bosso Stirpeto may compare with that from similarly expanded Carixian sections (Breggia, Switzerland), and cores (Montcornet, Paris basin). However, the matching of magnetic polarity zones requires a significant aging of some parts of the magnetostratigraphic profile at both Breggia and Montcornet.

Magnetic properties of basalts and geodynamics of the rift zone in the southern Red Sea

Magnetic properties of young rift basalts in the southern Red Sea are studied in detail. It is found that basalt samples dredged in the rift zone are characterized by a large spread in magnetic parameters. The magnetic properties of the basalts are shown to indicate a complex evolution of the Red Sea rift zone. Titanomagnetite grains of the basalts are mostly affected by single-phase oxidation processes and have preserved paleomagnetic information. However, basalts discovered near the rift axis yield evidence of multiphase oxidation of titanomagnetite grains, which is untypical of young oceanic basalts. These basalts have high Curie points and large values of the natural remanent magnetization and Koenigsberger parameter. The corresponding samples were taken in nontransform zones where rocks have experienced the action of significant tectonic forces and, moreover, anomalous geomagnetic field patterns correlate with the position of these zones. Using the magnetic properties of the basalts, the northern segment of the rift axis is relocated.

Global Correlation of the 223 ka Pringle Falls Event

New 40Ar/39Ar ages link records of anomalous geomagnetic field behavior from western North America and New Zealand, and suggest that a reversal event occurred at 223 ± 4 ka (± 1σ). Virtual geomagnetic poles from the Pringle Falls event recorded in both hemispheres lie on a common transition path, suggesting that the geomagnetic field during this interval was dominated by a dipolar component. The Pringle Falls event therefore may be a reverse polarity subchron within the Brunhes Normal Chron.

A detailed palaeomagnetic record for the last interglacial period

A palaeomagnetic investigation of a late Quaternary loess-palaeosol sequence in central China has revealed two episodes of anomalous geomagnetic field behaviour in the lower and upper parts of soil complex S1, which spans the last interglacial interval. Comparison of the magnetic susceptibility record with the deep-sea oxygen isotope stratigraphy provides a chronology which has been used to estimate the age and duration of these episodes. The age of the younger episode has been estimated at 89-75 ka, comparable with the age of a geomagnetic event recognised from sediments in the Norwegian-Greenland Sea and Arctic Ocean. The older episode is associated with a broad low in relative palaeointensity which has been assigned to the time interval 130-110 ka. This is consistent with existing age estimates of the so-called Blake Event. In contrast to a published record of the Blake Event from the loess at Xining, which shows three short episodes of almost full reversed polarity, in the current record the Blake Event is represented by a loop in the VGP path which passes through shallow southern latitudes. Although both episodes occur within the same part of soil complex S1, the difference between the two records implies a greater degree of smoothing of the geomagnetic signal in the Huanxian record. This is probably a reflection of the lower accumulation rate at Huanxian and may also indicate a greater time constant for the remanence aquisition at this site.

Paleomagnetic and cooling constraints on the duration of the Pahute Mesa‐Trail Ridge Eruptive event and associated magmatic evolution, Black Mountain Volcanic Center, southwestern Nevada

Two successive ash‐flow cooling units of the Black Mountain volcanic center, the Pahute Mesa and Trail Ridge members of the late Miocene Thirsty Canyon Tuff, possess nearly indistinguishable reverse polarity remanent magnetization directions of about 178° declination and −10° inclination. These are interpreted to reflect thermoremanent magnetization either during a single geomagnetic field excursion or polarity transition, or possibly from a field oriented near the limit of secular variation. Regardless of the type of geomagnetic field behavior, the Pahute Mesa Member was apparently overlain by the Trail Ridge Member before appreciable shift of the anomalous geomagnetic field. This strongly suggests that the time between eruption of the two ash‐flow sheets was very short, perhaps less than 100 years, a conclusion that is independently supported by the local absence of a complete cooling break between the two units. The interval between eruption of the Pahute Mesa and Trail Ridge members was too brief for reestablishment of chemical stratification in the Black Mountain magma system. Nevertheless, a small volume of highly evolved comendite magma developed and was erupted as lava within the Black Mountain caldera and as a component of the Trail Ridge Member. The short repose interval and rapid development of the comendite, together with evidence for mixing of magmas of silicic to near‐mafic composition (Vogel et al., this issue), and the presence of air‐fall tuff at the base of the Trail Ridge, lead us to conclude that eruption of the Trail Ridge Member was probably triggered by injection of hot, relatively unevolved magma into the Black Mountain magma body.

A magnetotelluric traverse across the Mauritanides orogenic belt (West Africa)

Summary. Magnetotelluric measurements, with reference magnetometers for study of the anomalous geomagnetic variation field, were made in the period range 10-10000 s at 10 sites crossing the main geological zones of eastern Senegal (West Africa). The results indicate that the electromagnetic fields are controlled by the dominant structural trends of the Mauritanides orogenic belt. Apparent resistivities reflect the major structural units identified from surface geology. Data in the principal directions for most of the stations exhibit anisotropic behaviour, and at certain sites the magnetotelluric responses are affected by surface distortion effects. The study of the geomagnetic variation field indicates the presence of a concentration of telluric current flow beneath the craton. A lack of obviously conductive geological structures at the surface suggests that this flow of telluric current may be attributed to increased electrical conductivity in the crust. The quantitative interpretation of the magnetotelluric results is given in terms of 1-D and 2-D resistivity models. It is shown that the data require the presence of a highly anomalous crustal structure (40 Rm) dipping to the east from a small depth beneath the outcropping margin of the West African craton. In the Mauritanides, the crust is resistive (3000 Rm) t o a depth of 30 km, and overlies a conducting layer of 10 km thick. In the depth range 250-300 km, resistivities decrease to less than 50 Rm. This may coincide with the top of the asthenosphere. The geoelectric model is discussed in relation to the general geological background of the region considered.

Geoelectrical structure of the northern part of the Senegal Basin from joint interpretation of magnetotelluric and geomagnetic data

Magnetotelluric (MT) and differential geomagnetic sounding (DGS) surveys were made in the Senegal basin, West Africa, to investigate the conductivity structure of the crust and upper mantle. Magnetic and electric variations were recorded at 11 sites along a east‐west profile 220 km in length. The period range of the measurements was 10–10,000 s. For the western stations, the MT response displays low skew with moderate anisotropy. The DGS data indicate the presence of an anomalous geomagnetic variation field in the deep basin and on the eastern margin of our study area. A telluric current concentration in the thick, conductive sediments of the deep basin might cause this anomaly. Both MT and DGS data are then compared with two‐dimensional model results to explain the observed features along the profile. Results reveal a strong anomalous crustal structure in the central part of the profile involving a conductive zone (2 ohm m) at depths in the range 4–23 km. Within the deep basin the results of the two‐dimensional modeling are not conclusive; acceptable fits between observed and calculated data have been obtained with alternate models. A continuous conductive zone (0.7–2 ohm m) may exist beneath the uppermost layers at a depth of approximatively 1 km, but the observed data can be explained equally well by poorly conducting material intruded into the highly conductive sedimentary cover. This ambiguity is inherent in the two‐dimensional models themselves. Both models show that the sediments increase westward to reach a thickness greater than 4000 m when approaching the coast. It is suggested that the flow of current observed in this study is largely controlled by strong lateral variation in the resistivity of the basin and/or by variation of the sedimentary thickness. The models also indicate the existence of two decreased resistivity layers in the eastern part of the profile, one at the base of the crust and the second at 80 km with a resistivity of 20 ohm m. No such layers were detected at these depths below the deep basin. At some depth greater than 150 km there is a general trend toward lower resistivities from 2000 to about 2 ohm m across the entire profile. This may indicate the presence of a small degree of partial melt.