Main Magnetic Carrier Research Articles

Paleomagnetism and rock magnetic properties of Late Pleistocene volcanism from El Pinacate Volcanic Field, northwest Mexico

The Pinacate Volcanic Field is a Neogene volcanic sequence at the northwestern part of Mexico. This paleomagnetic study (directional and paleointensity) will continue to improve the global paleomagnetic databases. It is the first full vector paleomagnetic analysis within the more than 400 volcanic structures of the Pinacate Volcanic Field, e.g. scoria, cinder cones, large ‘pahoehoe’ and ‘aa’ lava flows. A total of 140 oriented standard paleomagnetic cores from 16 sites of lava flows were collected. Rock magnetic experiments such as thermomagnetic analyses show that Ti-poor titanomagnetite is the main magnetic carrier of the remanence. Microscopy of oxide minerals supports this observation. Parameters from hysteresis and FORC diagrams suggest pseudo-single domain grain sizes or a mixture of single and multidomain particles. The directional analysis of 11/12 volcanic structures provides a mean paleomagnetic direction and corresponding mean paleomagnetic pole (calculated from the site VGPs): Dec = 0.1°, Inc = 54.5°, n = 11, α95 = 6.7°, k = 48, Plat = 86.5°, Plong = 247.8°, A95 = 7.5°, respectively. The mean paleointensity, obtained by using the Thellier-Coe protocol, and estimated from seven volcanic structures is 42.2 ± 3.6 μT, and VDM is 8.1 × 1022Am2. The obtained directions are compatible with the Late Pleistocene paleodirection as expected from the geocentric axial dipole (GAD) field. The virtual geomagnetic pole scatter (Sb) was calculated at 13.5° which is consistent with the paleosecular variation value expected from El Pinacate latitude.

An experimental approach to the preservation potential of magnetic signatures in anthropogenic fires.

Archaeomagnetic and rock-magnetic methods are of great value in the identification of archaeological fire, especially in Palaeolithic sites where evidence is usually scarce, ambiguous or poorly preserved. Although taphonomic processes can significantly modify Palaeolithic combustion structures, the extent to which such processes affect the magnetic record remains unknown. Here we report the results of an archaeomagnetic study involving five, two-to-five-year-old experimental combustion structures in open-air and cave settings. Some of these combustion structures involved post-combustion human actions such as trampling and relighting. Our results show pseudo-single domain (PSD) magnetite as the main magnetic carrier. Wood ash layers of combustion structures are the most magnetic facies followed by thermally altered sediments constituting the combustion substrates. A decreasing magnetic concentration pattern in depth was observed as a function of temperature. Positive correlation was found between good-quality directional data and macroscopically well-preserved combustion structures. Partial thermoremanent magnetization (pTRM) was the main magnetization mechanism identified in the combustion substrate facies. These data coupled with partial thermomagnetic curve experiments show the potential of these methods to estimate maximum temperatures of the last combustion event. Relightings show very good directional results, but they cannot be identified because the time between them is not enough to statistically distinguish directional variations of the local Earth´s magnetic field. The substrate sediment of an intensively trampled combustion structure yielded reliable archaeomagnetic directions. The results are discussed in terms of magnetization preservation potential and the effects of taphonomic processes on the archaeomagnetic record.

Rock magnetic and palaeomagnetic studies on the alkaline complexes of western Rajasthan, India

Forty-five oriented block samples were collected from 12 sites of Mundwara and Sarnu-Dandali alkaline complexes of western Rajasthan for palaeomagnetic and rock magnetic investigations to constrain the timing of the emplacement of alkaline suites in relation to the time-frame of main Deccan volcanism. The rock magnetic studies indicated (titano) magnetite as the main magnetic carrier of stable remanence. The Lowrie–Fuller (L–F) test revealed SD and PSD type magnetic carrier in samples. AF and thermal demagnetizations were piloted on the samples to isolate Characteristic Remanent Magnetisation (ChRM) directions. The samples exhibited stable remanence between 5 and 35 mT during AFD. Thermal demagnetizations unblocked remanence between 350° and 500°C. The mean ChRM directions with Declm = 342, Inclm = –35 (α95 = 4.39, K = 121, N = 9) corresponds to paleopole position at 42°N and 274°E with a palaeo-latitude of 24.5°S is coincident with the Deccan Super pole position. The comparison of ChRM in alkaline complexes with those documented in DVP implies coeval emplacement of alkaline suites with Deccan eruption. Further, the ChRM marked largely by normal polarity suggests that alkaline intrusions were accomplished within the magnetic Chron C30N, during the onset of Deccan volcanism. The results also indicate that Deccan volcanism extended far beyond the present day boundaries of the traps, especially in the north.

Paleomagnetism and age constraints of historical lava flows from the El Jorullo volcano, Michoacán, Mexico

The present study employs petrology, rock magnetics, and paleointensity on four lava flows (seven sites) of the El Jorullo volcano; a historical (1759–1766 AD) cinder cone in the Michoacán-Guanajuato Volcanic Field. El Jorullo lava flows cover an extensive area of ∼10.8 Km2, and a volume ∼0.35 Km3. Rock-magnetic experiments and microscopic observations reveal pseudo-single domain size Ti-poor titanomagnetite as the main carrier of magnetization in all sampled flows. Notably the calculated mean direction from all the sites show a slight westwards bias when compared against the secular variation global models. The mean paleo-direction for the El Jorullo historical eruption is: Dec = 2.5°, Inc = 37.5°, kappa = 214, α95 = 1.8, N = 31; the Virtual Geomagnetic Pole has Plat = 87° N, Plong = 307° E, Kappa = 260, A95 = 1.6; paleointensity data: 45.4 ± 3.6 μT; and Virtual Dipole Moment: 10.3 ± 1 × 1022 Am2. The paleodirections and the paleointensity values from these lava flows are well-constrained and reliable. These results allow to improve the accuracy and reliability of the global and regional secular variation models available for the last centuries.

Cooling rate effect on thermoremanent magnetization in archaeological baked clays: an experimental study on modern bricks

The influence of cooling rate on the intensity of thermoremanent magnetization (TRM) and the necessity to correct archaeo/palaeointensities for this effect have long been recognized. However the reliability of the correction is still questioned. We studied 35 bricks baked in two modern kilns (SK and BK) in known experimental conditions and with measurements of the direction and intensity of the geomagnetic field at the site. The smallest kiln (SK, 0.2 m 3) cooled in around 12 hours and the biggest kiln (BK, 8 m 3) in around 40 hours. Thermomagnetic, hysteresis and backfield curves indicated that the main magnetic carriers were Ti-poor titanomagnetites and Tipoortitanohematites. The fraction of the TRM carried by Ti-poor titanohematites is the maindifference between the two sets of bricks. This fraction is around 5-10% in bricks from BK kilnand up to 40% in those from SK kiln. Intensities of the Earth's magnetic field were determinedusing the original Thellier-Thellier protocol with correction of TRM anisotropy. The averageintensities overestimate the expected field intensity by 5% (SK) and 6% (BK). This resultemphasizes the necessity of the cooling rate correction. In order to have a detailed evaluation ofthe cooling rate effect, we used several slow cooling rates: 0.8, 0.4, 0.2 and 0.1{\textdegree}C/min. Thecorrection factors obtained with the 0.8{\textdegree}C/min cooling ranged between -2% and 21% and wereproportional to the TRM fraction carried by Ti-poor titanohematite. The higher proportion ofthese grains in bricks from SK kiln led to an overestimate of the correction factor and anunderestimate of the intensity by 7%. However, the expected intensity is recovered whentemperature steps higher than 580{\textdegree}C (i.e. in the range of Ti-poor titanohematites unblockingtemperatures) were excluded from the calculation of archaeointensity and cooling rate correction.In the case of the BK kiln bricks, for which Ti-poor titanohematites does not contributesignificantly to the TRM, all tested cooling rates give average intensities close to the expectedvalue. Incorrectly estimating the duration of the archaeological cooling has therefore a lowimpact on the accuracy of the archaeointensity data on these kinds of material.

Palaeomagnetism of Bahariya iron ores and their genetic implications, North Western Desert, Egypt

ABSTRACTIntegrated rock magnetic, palaeomagnetic and magnetic fabric studies have been carried out, in order to rely a better understanding of the age and origin of the Bahariya iron ores. A total of 80 oriented block samples were collected from 10 sites covering the three iron mining areas at Bahariya Oasis (El Gedida, Ghorabi and El Harra) within an area located between latitudes (28° 24` to 28° 29`N) and longitudes (29° 02` to 29° 11`E). Rock magnetic measurements reveal that, the main magnetic carrier is the haematite. Goethite and pyrrhotite, that are also present as subordinate constituents in the studied samples. Careful thermal demagnetization successfully enabled the isolation of the characteristic remanent magnetization of primary origin, which yields a magnetic position at Lat. 75.3º N and Long. 137.9º E revealing that, both the ore and host rocks are of the same age. Magnetic fabric studies on these ores indicated that, they are still holding their primary Fabrics of sedimentary origin.

Comprehensive magnetic surveys of kilns for bell and tile fabrication in Castile (Spain)

In absence of organic remains suitable for radiocarbon dating, archaeomagnetic dating can contribute significantly to the better understanding and rescue of our past and cultural heritage. Nowadays, in the archaeological research is fundamental to obtain as much information as possible, including precise dates of the archaeological sites; indeed, the rapid expansion of urban areas implies the destruction of archaeological structures, causing the loss of knowledge about our past. In this effort, we report an archaeomagnetic and rock-magnetic study on burnt archaeological samples from two kilns discovered at the monastery of San Pelayo del Cerrato (Cevico Navero, Palencia, Spain). This was a rescue excavation where arose the possibility of studying archaeomagnetically two ovens for the manufacture of tiles and bells before their destruction due to the restoration works of the monastery. Given their suitable conditions and the lack of independent absolute dating, the aim of the study is to archaeomagnetically date their last use. The main magnetic carrier is pseudo-single domain (PSD) Ti-poor titanomagnetite, indicating that the magnetic signal is stable. The comparison of mean full-vector values (mean direction and absolute archaeointensity determinations) with the SCHA.DIF.14k model determined their last use between 1301–1391 CE (bells´ kiln) and 1295–1404 CE (tiles´ kiln) at 95% confidence level, respectively. These dates agree well with the archaeological evidence indicating that the last use of both kilns occurred almost simultaneously or closely confined in time (14th century AD). The dating resolution obtained is comparable to the radiocarbon with the benefit that archaeomagnetism dates the last use (abandonment) of the structures and no material associated to it which might be slightly younger.

Emplacement temperature resolution and age determination of Cerro Colorado tuff ring by paleomagnetic analysis, El Pinacate Volcanic Field, Sonora, Mexico

The lithic in the Cerro Colorado tephra deposits is dominated by clasts from shallow country rocks for a diatreme not well mixed, or, by a combination of material that reflects the relative proportions of country rock in a well-mixed diatreme. The clasts in a tephra ring bed might be ideal to use the paleomagnetic study to determine the emplacement temperature. We report emplacement temperature ranges of pyroclastic lithic fragments from the Cerro Colorado Tuff Ring in the El Pinacate Volcanic Field, Sonora, northwestern Mexico. Paleomagnetic analyses, particularly the thermoremanent magnetization procedure was carried out in 38 small cores, drilled from the 10 to 30 cm in size lithic fragments. It estimates the emplacement temperature ranges of the juvenile basaltic cauliflower bombs and not-juvenile diverse blocks produced in the phreatomagmatic eruption. Titanomagnetite and titanohematite are the main magnetic carriers, along with titanomaghemite. The 22 emplacement temperatures determined in this study are grouped in three temperature ranges, low (310–370 °C), medium (400–460 °C) and high (>500 °C). The low and medium temperature samples show two or more components, while the high temperature samples have, in general, only one component. This wide range of emplacement temperature could be related to the composite thermal history of the pyroclastic material incorporated in the water saturated pyroclastic flow deposit. The comparison of the mean direction of the secondary magnetization component with the secular variation curve of the global model SHA.DIF.14, suggests an age of 3915 ± 59 yr. BP.

Insights on the geomagnetic secular variation in the Eastern Mediterranean: First directional data from Cyprus

The first directional archaeomagnetic results from Cyprus are presented, based on the study of seven baked clay structures excavated at the copper smelting site of Agia Varvara-Almyras, situated some 20 km south of Nicosia. This ancient copper working site represents a unique example of complete primary production of the copper metal in Cyprus. In total five furnaces, one ore roasting pit and an oven were sampled in situ for an archaeomagnetic investigation. All the studied structures are well dated, based on pottery finds and radiocarbon dates, with ages ranging from 600 BCE to 50 BCE. Several rock magnetic experiments were performed to determine the main magnetic carrier and to investigate the thermal stability of the studied material. The direction of the Characteristic Remanent Magnetization (ChRM) for each structure has been determined by demagnetization of 7–15 specimens in alternating magnetic fields up to 100–120 mT. Mean directions were calculated assuming a Fisherian distribution and are well defined for five out of the seven structures studied. The new directions are compared with the directional data and the secular variation curves available from the surrounding area, and with the predictions of global geomagnetic field models. They are an important contribution to improving our knowledge of the geomagnetic field variations in the Eastern Mediterranean and offer new insights about fast geomagnetic directional changes during the first millennium BCE that were not previously identified by the available geomagnetic field models’ predictions in this area.

Palaeo- and rock magnetic investigations of Late Quaternary sediments from the Upper Congo deep-sea fan: on the difficulty in obtaining palaeomagnetic secular variation records from low latitudes

We report here on results of palaeo- and rock magnetic investigations of two sediment cores from the Upper Congo deep-sea fan. The sediments have a high organic content and contain a heterogeneous Fe-mineral assemblage with biogenic magnetite and detrital (Ti-)magnetite as the main magnetic carrier minerals. Pyrite, hematite, and Fe-oxyhydroxides were identified by comparing high-temperature magnetic susceptibility curves with those from Fe-minerals of known composition. According to AMS 14C dates, the 6.8 m-long profile spans the last 37 kyr. Sediments older than 20 ka are affected by reductive diagenesis that has led to a loss of the fine-grained magnetic mineral fraction. Sediments younger than 20 ka have stable magnetizations. Characteristic remanent magnetization records of inclination and declination were obtained for each core. There is a little agreement between these records, modelled curves, and other sediment records from Equatorial Africa, so no composite record could be established. The cores are not ideal relative palaeointensity recorders and estimates using different normalizers did not yield consistent signals from both cores. Normalization methods used for relative palaeointensity estimation were not developed for sediments that contain large amounts of ultra-fine-grained biogenic magnetite; therefore, the relative palaeointensity estimates should be considered with caution. However, in view of the incoherent picture given by the scarce available palaeointensity information from the region off South-West Africa, the GeoB6517-2 record may provide a tentative relative palaeointensity record for comparison, at least for the past 10 kyr.

Archaeomagnetic evidence of pre-Hispanic origin of Mezcal

For centuries, and until a few years ago, it was considered that the distillation process had been brought to the new world by the Spaniards, who in turn learned it from the Arabs. For this reason, it was believed that the only alcoholic beverage of the Mesoamerican societies was pulque - a ferment of maguey. However, recent archaeological investigations revealed that the alcohol distillation was known in Mesoamerica long before the arrival of Europeans, for at least 25 centuries. The direct evidence comes from the ceremonial and administrative center of Xochitécatl-Cacaxtla (state of Tlaxcala) with several ovens where cooked maguey remains were discowered. The corresponding archaeological context was radiometrically dated from 600 to 400 BCE. Here, we report a detailed archaeomagnetic study on burned archaeological artifacts found in these cooking ovens. 35 specimens belonging to two pottery fragments, one burned rock and two burned soil samples were pre-selected for archaeointensity experiments. Pottery samples exhibited essentially reversible behavior during thermomagnetic experiments pointing to Ti-poor titanomagnetite (almost magnetite phase) as main magnetic carrier while two ferromagnetic phase seems to co-exist in burned soils. In contrast, burned rock samples exhibited some instabilities during the heating at high temperatures and indirect evidence of the presence of antiferromagnetic hematite grains. In total, 29 specimens allowed the estimation of absolute geomagnetic intensity recorded during the last use of the furnace. Archaeomagnetic dating yielded two possible time intervals between 878 to 693 BCE and 557 to 487 BCE. These new data reinforce the initial hypothesis and corroborate the temporality of these pre-Hispanic kilns.

Mineralogical, Rock-Magnetic and Palaeomagnetic Properties of Metadolerites from Central Western Svalbard

A combination of mineralogical, rock-magnetic and palaeomagnetic methods were employed in an attempt to shed a new light on the tectonism and paleogeography of Central Western Svalbard. The focus is on six metadolerite sites from the metamorphic Proterozoic–Lower Palaeozoic complex of south-western Oscar II Land (Western Spitsbergen). The primary mineral compositions of the metadolerites were strongly remineralized during Caledonian (sensu lato) greenschist-facies metamorphism although some younger tectonothermal modification is also apparent from the rock-magnetic studies. Rock-magnetic experiments supported by thin-section mineral identification and separation of Fe-containing fractions indicate that the main ferromagnetic carriers of the Natural Remanent Magnetization are represented by low-coercivity pyrrhotite and magnetite/maghemite. The investigated metadolerites are characterized by complex pattern of magnetization. The low-temperature palaeomagnetic components which demagnetized up to 250 °C, are characterized by high inclinations (~70–80°) potentially representing Mesozoic–Cenozoic remagnetization. The most stable middle-high temperature directions which demagnetized from 250 °C, were obtained from only two of six sites. Two Virtual Geomagnetic Poles calculated from two of the middle-high temperature site means do not correlate with the Laurussia reference path for syn- to post-Caledonian time. Two possible explanations of observed inconsistency are discussed. These are a modification of the Oscar II Land Caledonian basement geometry by listric faulting and/or tectonic rotations related to Daudmannsdalen–Protectorbreen high-strain (shear) zone. The results presented here suggest that post-Caledonian tectonic modification of the palaeomagnetic directions may be more a widespread feature of Western Svalbard.

Behavior of Natural Radionuclides, Radiological Hazard Parameters, and Magnetic Minerals in Rock Samples of Kolli Hills, Eastern Ghats, India

The natural radionuclide (238U, 232Th, and 40K) content of the 15 rock samples collected from Kolli hills along the Eastern Ghats, India, has been analyzed using a 4″ × 4″ NaI (Tl) scintillation detector-based gamma-ray spectrometer. The activity concentrations of 238U, 232Th, and 40K were between 12.97–49.89, 6.4–27.05, and 51.85–95.84 Bq kg−1, respectively. To understand the entire radiological characteristics of the collected samples, the various radiological hazard parameters have been calculated and were compared with the global recommended mean values. From the magnetic studies, values of χ ranged from 33.1 to 510.7 × 10−8 m3 kg−1. Moreover, in rock samples of Kolli hills, ferrimagnetic minerals are found to be the main magnetic carrier. Statistical analyses were performed to study the relation between the natural radionuclides, radiological hazard parameters, and magnetic minerals. The results of Pearson’s correlation analysis shows that 238U and 232Th strongly correlate with the radiological hazard parameters, and a poor correlation was noted between magnetic and radiological parameters. However, a positive and a near-positive correlation was also observed between SIRM/χ (0.501–0.578) and κFD% (0.471–0.481) with U and Th activity concentrations, respectively. This indicates that the carrier and grain size dependence parameters also play a significant role in increasing the activity concentrations of U and Th and its associated radiological parameters. From the cluster analysis, it is found that the content of ferrimagnetic minerals is higher in sample nos. 4, 5, 11, and 12, which greatly improves the properties of concrete when used for building construction purposes.

Constrained Magnetostratigraphic Dating of a Continental Middle Miocene Section in the Arid Central Asia

The Neogene succession of the Aktau Mountains in the Ili Basin, southeast Kazakhstan, is a terrestrial archive well suited for researching the role of Central Asia in Miocene climate evolution. We present an integrated approach for dating the well-exposed Bastau Formation, based on magnetostratigraphy and constraints from cyclostratigraphy and biostratigraphy. Stepwise demagnetization yielded characteristic remanence directions that are consistent with those expected for the Miocene in Central Asia. The reddish-colored alluvial floodplain deposits and grey lacustrine deposits show partly complex magnetic behavior with magnetite and hematite as the main magnetic carriers, with variable demagnetization behavior and non-dipolar normal and reverse polarity directions. The observed magnetic properties are best explained by depositional variability and magneto-mineralogical alteration effects of both dissolution and neo-formation of magnetite, including significant secondary magnetization. The mean of reverse polarity directions is flatter than the expected Middle Miocene Earth magnetic field, which is an indicator for the existence of inclination shallowing that supports a primary origin. Detailed rock magnetic analyses are used to analyze the nature of the characteristic remanent magnetization and to discriminate primary and secondary remanence directions in order to obtain a reliable magnetostratigraphic result. The proposed age of 15.3 Ma to 13.9 Ma for the Bastau Formation corresponds to the known biostratigraphic setting, matches with typical sedimentation rates of foreland basins in Central Asia, and coincides with spectral analysis of geochemical proxies of that section. The resulting age model serves as a robust framework for paleoclimate reconstruction of Neogene climate dynamics in Central Asia.

Evaluation of Potential Hazards Associated With Qattara Depression as a National Hydropower Project in Egypt

THE OBJECTIVE of this paper is to evaluate, assess and study the potential hazard associated with the proposed national hydropower project at Qattara Depression. To achieve this objective, the two elements of sustainable development of this vast area, water and power, were developed. The Old Delta lies in the Western Desert of Egypt. It begins from the west of Assiut aligning the western outskirts of El Minya and Al Faiyum and terminated by Qattara Depression in the northern portion. It is a natural depression that covers about 2% of Egypt area. The depression depth is about 134 m below the sea level.A Clean electric generation could be achieved from the net head of filling Qattara Depression. Qattara Depression has the super potentiality of generating clean hydropower during the filling period and also has a high potentiality after filling period. This hydropower can provide Egypt with future needs and could be exported to Eastern Nile countries. The hydropower generation from Qattara Depression does not need filling time, high cost building dam. Previously eleven sites comprising 76 core samples of Lower (sandstone) and Middle (limestone) Miocene age were collected from the northeastern tip of the Qattara Depression in the north of the Western Desert of Egypt. The majority of samples showed weak to very weak remnant magnetization with goethite, hematite and titan magnetite as the main magnetic carriers. However, with a careful, detailed thermal demagnetization, they yielded stable, probably primary, magnetization. The resulting overall mean direction corresponded to a palaeomagnetic which seems to agree with other known African Miocene poles. This result implies that the Qattara area has been stable at least since Early Miocene. The presence of goethite as the main magnetic carrier supports the assumption that weathering has been playing an important role in the development and shaping of the depression. The possible site hazards associated with this project could be evaluated from different points of view, in order to give an overview to this promising project. The Qattara Depression, as a national project in Egypt, can achieve some accomplishments such as ; defending the Nile River Delta and the Mediterranean countries from the sinking or inundation of the Sea Level Rise (SLR) and reduce the cost of protecting coastal shores , generate hydroelectric power between 1000 to 2000 MW (depending on the filling water rate ) from Qattara Depression (under the sea level by depth of -60 (MSL) in average, desalinating water in large quantities; providing new areas, particularly for the refugee in case of the sinking or inundating of some parts of the Delta, due to the (SLR); Supporting housing, fish farming, tourism, and agriculture.

A high-resolution record of the Matuyama-Brunhes transition from the Mediterranean region: The Valle di Manche section (Calabria, Southern Italy)

High-resolution palaeomagnetic and rock magnetic investigations on the Valle di Manche section (Crotone Basin, Calabria, Southern Italy) provide a detailed record of the Matuyama-Brunhes (M-B) reversal that, to our best knowledge, is the only available record of the last geomagnetic reversal for the Mediterranean on-land marine stratigraphy. The M-B transition can be pinpointed precisely, as it develops within a 3-cm-thick interval located just above a prominent tephra layer (the “Pitagora ash”) where the sedimentation rates are about 27 cm/kyr. Demagnetization analyses indicate a stable palaeomagnetic behaviour throughout the section for both normal and reversed polarity directions, with demagnetization vectors aligned toward the origin of Zijderveld diagrams after the removal of a small viscous low-coercivity remanence component. In the lower part of the studied interval, some samples acquired a spurious gyromagnetic remanent magnetization (GRM) during AF demagnetization in high fields. Rock magnetic analyses confirm that magnetite is the main magnetic carrier for all measured specimens, which also have an abundant paramagnetic fraction. Only the lower part of the record, well below the M-B boundary, is characterized by a downward-increasing presence of iron sulphides (greigite). According to our chronology, which is based on a robust, cross-validated age model, the final reverse-to-normal directional change of the M-B transition occurred at ca. 786.9 ± 5 ka (error includes uncertainty in orbital tuning) and was very rapid, of the order of 100 years or less.

Cenozoic to Cretaceous paleomagnetic dataset from Egypt: New data, review and global analysis

Different phases of igneous activity took place in Egypt during the Mesozoic and the Cenozoic and oriented samples were collected from three Cenozoic localities (Baharya oasis in the Western Desert, Abu Had in the Eastern Desert and Quseir along the Red Sea coast), and four Cretaceous localities (Toshki & Abu Simbel south of Aswan, and Shalaten & Abu Shihat along the Red Sea coast). Rock magnetic properties of the samples indicate magnetite and titanomagnetite as the main carrier of the remanent magnetization. Following stepwise demagnetization, characteristic remanent directions were identified only for 62% of the samples, a fairly low rate for that type of samples, and 8 new paleomagnetic poles were calculated. All our Cenozoic poles fall clearly off Master Polar Wander Paths proposed for South Africa. Therefore, all paleomagnetic results, previously published for Egypt, were compiled from Cretaceous to Quaternary. The published poles largely overlap, blurring the Egyptian Apparent Polar Wander Path. A new analysis at the site level was then carried out. Only poles having a kappa larger than 50 were selected, and new pole positions were calculated by area and by epoch, when at least 3 sites were available. Even though the selection drastically reduced the number of considered poles, it allows definition of a reliable Cenozoic apparent polar wander trend for Egypt that differs from the South African Master Polar Wander Path by about 10–15 °. If the Cretaceous igneous poles are in good agreement with the rest of the African data, the sedimentary poles plot close to the Cenozoic portion of the South African Master Polar Wander Path, a discrepancy that could be related either to inclination flattening and/or error on age and/or remagnetization in the Cenozoic.

Specific features of the mineral composition and petromagnetic properties of rocks from the Minami-Khiosi submarine volcano (Mariana island arc)

Complex studies of the mineral composition and petromagnetic properties of the rocks which compose an edifice of the Minami–Khiosi submarine volcano located in the Mariana island arc are carried out for the first time. The Minami–Khiosi Volcano is a part of the Khiosi volcanic complex within the alkaline province of the Idzu–Bonin and Mariana island arcs. All of the rocks analyzed are enriched in K2O (1.34–3.30%), Ba (370–806 ppm), and Sr (204–748 ppm). The basalt has a porhyric texture and contains mosTy olivine phenocrysts as individual crystals and growths with a size up to 2 cm; the groundmass is finecrystalline. The samples studied contain at least three Fe-bearing oxide minerals. These are predominant magnetite and less abundant ilmenite and Fe hydroxides. It is established that the samples studied are magnetically isotropic and have high values of natural remanent magnetization and Konigsberger ratio. Similarly to the other island-arc Late Cenozoic submarine volcanoes in the western part of the Pacific Ocean, the samples studied are strongly differentiated by the value of natural remanent magnetization and magnetic susceptibility. The low-coercivity magnetic minerals (titanomagnetite and magnetite) of the pseudo-single-domain structure, as well as high-coercivity minerals (hematite) are the main carriers of magnetization. The high values of natural remanent magnetization are explained by the pseudo-single-domain structure of the titanomagnetite grains, whereas the high values of magnetic susceptibility result from the high concentration of ferromagnetic grains.

The nature of magnetic anomalies in subduction zones

The analysis of the magnetic survey data suggests the presence of a frontal zone of intense magnetic anomalies in a number of the Pacific island-arc systems. These zones with amplitudes of 100–300 nT are observed within the Kuril–Kamchatka and Aleutian island arc systems, in Southern and Central America, and Alaska. As demonstrated by the solution of the inverse problem and petromagnetic investigation of the rocks, these zones are presumably related to the serpentinite bodies which form as a result of the hydration of the upper mantle peridotites by the oceanic water penetrating through a system of cracks and fractures into the subducting slab at its bend. The rock magnetic studies show that magnetite is the main carrier of magnetization in these serpentinite bodies. Hydration of the subducting slab also causes hydration of the mantle rocks of the overriding plate with the formation of the magnetized serpentinite wedge. The decompaction of ultrabasic rocks under hydration is marked by a decrease in the gravity field and velocities of elastic waves. As the subducting plate loses water, it becomes embrittled and becomes the localization region for the epicenters of the strongest earthquakes. Magnetic survey can be used for revealing the potential sources of catastrophic earthquakes.

Magnetostratigraphy of two deep boreholes in southwestern Bohai Bay: Tectonic implications and constraints on the ages of volcanic layers

Abstract The sediments of Bohai Bay Basin have recorded the processes of basin infilling and structural evolution, reflecting the destruction of the North China Craton during the late Mesozoic and the early Cenozoic. However, the absence of a reliable chronostratigraphic framework for the sedimentary sequences in the basin has precluded a comprehensive understanding of these processes. This is especially the case for southwestern Bohai Bay, the evolution of which has been affected by two Quaternary volcanoes, Dashan and Xiaoshan. In this study, we combine paleomagnetic and sedimentary analyses of the sediments from two boreholes (CK3 and G4) from southwestern Bohai Bay Basin in order to provide new insights into the sedimentary history and regional tectonic processes since the Pliocene. The main findings are as follows. (1) Magnetite and hematite are the main carriers of the characteristic remanent magnetization in these sedimentary sequences, with hematite the main component. (2) The two drill cores record the Brunhes, Olduvai and Gauss normal chrons, and the Matuyama and Gilbert reverse chrons. (3) Subsidence-related differences in the depths of the Matuyama/Brunhes (M/B) and Gauss/Matuyama (G/M) boundaries in boreholes across Bohai Bay, distributed within different structural units, reflect the fact that northern Bohai Bay is the regional subsidence center and that the development of the WNW-orientated structures of the basin has intensified during the Quaternary. (4) The trend of changes in the elevation of three marine layers is similar to that of the M/B and G/M boundaries across Bohai Bay. Based on magnetostratigraphic constraints of the boreholes, the ages of the second and third marine layers are assigned to Marine Oxygen Isotope (MIS) 3 and MIS 5, respectively. The WNW-orientated structures and ‘super interglacials’ resulted in the formation of transgressions in the Huanghua depression. (5) In borehole CK3, the ages of four volcanic layers at the depth intervals of 13.14–16.18 m, 33.37–48.02 m, 145.8–154.05 m and 222.16–233.80 m, are 10–18 ka BP, 80–90 ka BP, 2.2 Ma and 3.1 Ma, respectively. In borehole G4, the age of the volcanic layer at the depth of 46.4–55.8 m is 0.33–0.55 Ma. Comparative analysis of subsidence-related differences between the NEE and NE-orientated uplifts and depressions in western and southern Bohai Bay indicates that structural evolution in the vertical plane during the Quaternary was mainly controlled by the subduction of the Pacific Plate.