High Magnetic Zones Research Articles

Magnetic response contrast of Oligo-Miocene basaltic magmatism and structural control during its ascent at the northern tip of the Red Sea Rift

Oligo-Miocene volcanic activity has occurred along the northern Egyptian margin of the Red Sea Rift system, leading to the creation of small-volume monogenetic volcanoes. This research delves into the morphology and setting of these rift-related volcanics by combining aeromagnetic data, remote sensing, and field verification. Basaltic exposures in the study area were mapped using False Color Composite (FCC) and band ratios in Landsat 8 OLI data. In a map view, these mappable volcanic rocks appear as discrete semi-circular basaltic bodies and as discontinuous dikes. The basaltic bodies are aligned along ENE and NW-oriented zones, with most dikes orientated NW-SE. Multiple isolated high magnetic zones with NE and NW orientations were identified. Most basaltic outcrops coincide with high magnetic zones except for Gabal Abu Treifiya along the Cairo-Suez Road. The integration and combination of three layers of surface and subsurface structure patterns reveal an array of variance. The dominant trend of surface and deeper structural lineaments is NW-SE and NE-SW respectively. In contrast, shallow structural lineaments have NW-SE, NE-SW, E-W, and N–S trends. Long linear grabens result from the pairing of NW-SE normal faults. The basaltic occurrences such as Qattamiya and Abu Treifiya are located at the intersection of the NW-SE and E-W normal faults along the NW-SE linear grabens. The field investigation at both locations reveals a polyphase magmatic feeding system manifesting in the surface today as coherent and volcaniclastic rock units. This suggests a diverse range of eruptive styles through a complex feeder system that involves more than one volcanic cone. Though the two areas have good basaltic exposures, yet they have different magnetic responses. The severe alteration of Abu Treifiya's volcanic rocks into nonmagnetic minerals assemblage of zeolite, smectite, calcite, and hydrolysis of basaltic glass diminishes the expected high magnetic anomaly. This study contributes to our knowledge of Rift-related volcanism and the factors influencing the distribution of these volcanic materials.

Solid mineral potential evaluation using integrated aeromagnetic and aeroradiometric datasets

The analytical hierarchy process (AHP) was employed to delineate the mineralisation potential across the notable schist belts in northwestern Nigeria. High-resolution aeromagnetic and aeroradiometric datasets were taken into consideration. This was achieved by using advanced signal enhancement techniques to study the structures, identify the hydrothermal alteration zones (that could serve as a pathway for mineralisation), and understand the geologic settings. Amongst the enhancement techniques are first vertical gradient, analytic signal, CET grid analysis and porphyry, Euler deconvolution, and K/Th ratio. The analytic signal reveals lithologic contact, structures and anomalous occurrences that aided the classification of the site into three magneto-lithologic zones: high (> 0.094 nT/m), intermediate (0.028 to 0.094 nT/m), and low magnetic zones (< 0.028 nT/m). The high magnetic zones (HMZ) were considered the main magnetic source outlines, which are inferred to be dominantly intrusive zones for hydrothermal activities. The 3-dimensional Euler deconvolution reveals highly magnetic and intrusive depth sources to be within the range of < 100 to 500 m. The Centre for Exploration Targeting (CET) grid technique revealed the structural distribution from which the lineament density map was produced. The orientations of the prevalent structural anomalies are E-W, NE-SW, WNW-ESE and NW–SE, with similar orientations observed from the first vertical gradient and the analytic signal. The highly dense structural zones coincide with the high magnetic zones and high-frequency amplitudes of the analytic signal and the vertical gradient map, respectively. Additionally, the CET porphyry detects the centres of the intrusive porphyries to be within zones of high lineament density. This reveals that the mineralisation potential of the area is structurally controlled. On the other hand, radioelement maps (eU, eTh, and K%) and ternary maps were used for lithological classification. The radiometric ternary map revealed the highly radioactive zones and the superior concentration of individual radioelements in their respective areas. The K/eTh ratio map delineates highly potassic alteration zones. The AHP model and weighted overlay tool were employed to integrate the analytic signal, lineament density, and K/Th ratio. Consequently, the mineralisation potential of the study site was revealed and classified into high, moderate, and low. This result was validated using known mine sites. There was a total agreement, with 87.5% of mines plotting within the high mineralisation potential class and 12.5% in the moderate class. Promising targets were identified for development.

Structural Characteristics of the Yushuwan Fault in the Northeastern Ordos Basin: Implications for Basin Tectonics and Hydrocarbon Exploration

The Yushuwan fault is located in the transition zone among the Yimeng uplift, Jinxi fold belt and Shanbei slope in the northeastern Ordos basin and marks the boundary between Palaeozoic and Mesozoic strata. According to the field geological survey and geophysical data, this study shows that the Yushuwan fault is a deep-cut northwest-southeast-oriented strike-slip basement fault. A total of 17 apatite and zircon fission track points show that the basin experienced several stages of uplift at 190 Ma, 150–170 Ma, 110–130 Ma, 60–80 Ma, and 20–40 Ma. The characteristics of the Yushuwan fault indicate that the northeastern part of the basin is not a simple slope but rather a step with a large drop. Therefore, in the area to the southwest of the Yushuwan fault, there should be high natural gas resource potential and good exploration prospects. The contour values of the magnetic anomaly on the southwestern side of the fault are high, and the contours are dense; in contrast, the contour values of the northwest magnetic anomaly are low, and the gradient is gentle. Therefore, the fault represents the eastern boundary of the Yimeng uplift, and restricts the eastward extent of the high magnetic anomaly zone in the Yimeng uplift area.

Investigation of iron ore potential in north-central Nigeria, using high-resolution aeromagnetic dataset and remote sensing approach

The instability of fuel prices in the international market has adversely affected the economy of Nigeria, which is a developing country that solely relies on crude oil. This has led the country to consider diversifying its economy into mineral resources. The consideration of iron ore as an important mineral for global infrastructure, steel production, and wealth creation has prompted this study to consider its potential within north-central Nigeria. Additionally, no geophysical reports considering the regional distribution of iron ore potential in both the Basement Complex and the sedimentary basins within north-central Nigeria have been previously carried out. This study employs an analytical hierarchy process (AHP) and weighted overlay to integrate the analyses of the high-resolution aeromagnetic data set (HRAMD) and Landsat 8 OLI/TIRS to assess the iron ore potential of the different geological terrain in north-central Nigeria. The ferrous mineral band ratio technique was adopted in processing the Landsat data, and several signal enhancement techniques were performed on the HRAMD, including the first vertical derivative (1VD), analytical signal, center for exploration targeting (CET) grid analysis, Euler deconvolution, and radially averaged power spectrum (RAPS). The shallowest magnetic sources and the high amplitude zones were delineated, and the dominant structural trend of the area was revealed to be WNW-ESE and NE-SW, and both orientations appeared to be favorable for mineralization. The apparent depth estimates of shallow and deep-seated magnetic structures obtained from Euler deconvolution ranged from 127.65 to 3540.64 m, whereas RAPS delineated a range of 285–3710 m. The analytical signal (AS) categorized the magnetism of the area into low, intermediate, and high magnetic zones. The higher magnetic zones were considered to be favorable for mineralization. The ferrous mineral ratio map reveals areas of higher reflectance, which depict zones of iron ore potential. The iron ore potential map was validated using known iron ore mines and deposits, which showed a perfect correlation, and new sites of probable iron ore mineralization within north-central Nigeria were revealed. This article will serve as a guide for stakeholders in the exploration of iron ore deposits.

Predictive Mapping of Prospectivity for Gold in the Central Portion of the Tapajós Mineral Province, Brazil

This work aims to model mineral prospectivity for intrusion–related gold deposits in the central portion of the Tapajós Mineral Province (TMP), southwestern Pará state. The scope includes experimentation and evaluation of knowledge and data-driven methods applied to multisource data to predict potential targets for gold mineralization. The radiometric data processing allowed to identify a hydrothermal alteration footprint of known gold deposits, providing information in regions with little or no field data available. The aeromagnetic data analysis prompted the identification of high magnetic zones, which are probably related to hydrothermal fluid transport. Linear features extracted from digital elevation data revealed an NNW–SSE general trend, which is consistent with the main structural control of deposits. The data were integrated through three modeling techniques—fuzzy logic (knowledge-driven), weights of evidence (WofE, data-driven), and a machine learning algorithm (SVM, data-driven)—resulting in three prospective models. In all models, the majority of indicated prospective regions coincide with the known deposits. The results obtained in the models were combined to generate an agreement map, which mapped the overlapping of their highest prospective scores, indicating new areas of prospective interest in the central portion of the TMP.

Programing Cell Assembly via Ink-Free, Label-Free Magneto-Archimedes Based Strategy.

Tissue engineering raised a high requirement to control cell distribution in defined materials and structures. In "ink"-based bioprintings, such as 3D printing and photolithography, cells were associated with inks for spatial orientation; the conditions suitable for one ink are hard to apply on other inks, which increases the obstacle in their universalization. The Magneto-Archimedes effect based (Mag-Arch) strategy can modulate cell locomotion directly without impelling inks. In a paramagnetic medium, cells were repelled from high magnetic strength zones due to their innate diamagnetism, which is independent of substrate properties. However, Mag-Arch has not been developed into a powerful bioprinting strategy as its precision, complexity, and throughput are limited by magnetic field distribution. By controlling the paramagnetic reagent concentration in the medium and the gaps between magnets, which decide the cell repelling scope of magnets, we created simultaneously more than a hundred micrometer scale identical assemblies into designed patterns (such as alphabets) with single/multiple cell types. Cell patterning models for cell migration and immune cell adhesion studies were conveniently created by Mag-Arch. As a proof of concept, we patterned a tumor/endothelial coculture model within a covered microfluidic channel to mimic epithelial-mesenchymal transition (EMT) under shear stress in a cancer pathological environment, which gave a potential solution to pattern multiple cell types in a confined space without any premodification. Overall, our Mag-Arch patterning presents an alternative strategy for the biofabrication and biohybrid assembly of cells with biomaterials featured in controlled distribution and organization, which can be broadly employed in tissue engineering, regenerative medicine, and cell biology research.

Reconstruction of the subsurface crustal and radiogenic heat models of the Bornu Basin, Nigeria, from multi-geophysical datasets: Implications for hydrocarbon prospecting

This study aims to reconstruct the subsurface crustal and radiogenic heat models from integrated airborne magnetic and gamma-ray spectrometric and satellite gravity methods for implications on hydrocarbon exploration in the Bornu Basin, NE Nigeria. The results revealed the basin’s lithologies and shallow-to-deep structures (faults and lineaments) and their trends, depicting contrasting magnetic intensities, gravity anomalies, and radionuclide concentrations. High magnetic anomalous zones in the southern and southeastern parts of the basin could be associated with the influence of the intrusive body (porphyritic granite) of high magnetization as observed in the 2D magnetic/gravity models. Moderate to low anomalous zones suggested thick sedimentation in the northeastern and central areas. The total horizontal derivatives (THDR) and Rose diagrams of the magnetic and Bouguer anomalies revealed major lineaments/faults in the NE-SW, NNE-SSW, and E-W directions. These structures likely serve as the migratory pathways/traps for the hydrocarbons. The structures delineated on the radioelements ratio, and composite maps also correlate with those identified on the THDR maps, especially the major SE-NW structure created by magmatic intrusion in the southeastern part of the basin. The estimated total radiogenic heat production (RHP) rates for the study area decreased in the order of 740.46 > 674.16 > 665.35 > 462.23 > 415.17, with the maximum obtained from the Yolde Formation (YF) and Pindiga Formation (PF). These are attributable to clay, limestone, shale, and ferruginized sandstone. The Chad Formation (CF), on the other hand, had thicker sediment (6.8 km) and a maximum RHP of 550 ρWkg-1, which is relatively low (<750 ρWkg-1) for hydrocarbon maturation and accumulation. Hence, CF is not viable for hydrocarbon prospecting as the RHP obtained might not be sufficient. The RHP > 797.87 ρWkg-1 obtained in the PF and the YF falls within the moderate RHP windows (750–1500 ρWkg-1) for sufficient hydrocarbon maturation and accumulation in the Bornu Basin.

Geophysical and morphological constraints on active tectonics in the Eastern Pontides (Turkey)

The Eastern Pontides in northern Anatolia (Turkey) offers an exceptional record of significant tectonic evolution from subduction to crustal thickening and Anatolian escape tectonics. Here, we provide ground magnetic measurements at 435 points integrated with aeromagnetic data, remote sensing, and surface geological features from the literature reveal all these tectonic deformation effects on the Eastern Pontides. The region, which is tectonically separated into three sub-zones, clearly revealed from both aero and ground magnetic data. Lineament analysis was carried out to reveal the deformation styles of the region and to compare different data sets. According to the lineament analyzes obtained from the aero&ground magnetic data and all the other data sets, similar results were obtained for the three sub-zones. Although slight differences were obtained results between the used methods and zones, the main lineament orientation is calculated in the E-W direction. Thus, the obtained results emphasize that tectonic deformation effects resulting from long-term ∼N-S convergence and crustal shortening (subduction, collision, obduction) may still dominate the region. In addition to the lineament results, high (low) magnetic zones in the E-W orientation are also evident. Considering the resolution of the data used, these magnetic changes can be associated with large scale geological structures rather than surface lithological changes. Despite the existence of an active dextral the North Anatolian Fault Zone, the zone is not evident from the magnetic results so the transcurrent tectonic regime probably still not dominant in the entire region.

Easy axis anisotropy creating high contrast magnetic zones on magnetic tunnel junctions based molecular spintronics devices (MTJMSD)

Magnetic tunnel junction-based molecular spintronics device (MTJMSD) may enable novel magnetic metamaterials by chemically bonding magnetic molecules and ferromagnets (FM) with a vast range of magnetic anisotropy. MTJMSD have experimentally shown intriguing microscopic phenomenon such as the development of highly contrasting magnetic phases on a ferromagnetic electrode at room temperature. This paper focuses on Monte Carlo Simulations (MCS) on MTJMSD to understand the potential mechanism and explore fundamental knowledge about the impact of magnetic anisotropy. The selection of MCS is based on our prior study showing the potential of MCS in explaining experimental results (Tyagi et al. in Nanotechnology 26:305602, 2015). In this paper, MCS is carried out on the 3D Heisenberg model of cross-junction-shaped MTJMSDs. Our research represents the experimentally studied cross-junction-shaped MTJMSD where paramagnetic molecules are covalently bonded between two FM electrodes along the exposed side edges of the magnetic tunnel junction (MTJ). We have studied atomistic MTJMSDs properties by simulating a wide range of easy-axis anisotropy for the case of experimentally observed predominant molecule-induced strong antiferromagnetic coupling. Our study focused on understanding the effect of anisotropy of the FM electrodes on the overall MTJMSDs at various temperatures. This study shows that the multiple domains of opposite spins start to appear on an FM electrode as the easy-axis anisotropy increases. Interestingly, MCS results resembled the experimentally observed highly contrasted magnetic zones on the ferromagnetic electrodes of MTJMSD. The magnetic phases with starkly different spins were observed around the molecular junction on the FM electrode with high anisotropy.

Influence of Early Low‐Temperature and Later High‐Temperature Diagenesis on Magnetic Mineral Assemblages in Marine Sediments From the Nankai Trough

AbstractDiagenesis can have a major impact on sedimentary mineralogy. Primary magnetic mineral assemblages can be modified significantly by dissolution or by formation of new magnetic minerals during early or late diagenesis. At International Ocean Discovery Program Site C0023, which was drilled in the protothrust zone of the Nankai Trough during Expedition 370, offshore of Shikoku Island, Japan, non‐steady state conditions have produced a complex sequence of magnetic overprints. Detailed rock magnetic measurements, which characterize magnetic mineral assemblages in terms of abundance, grain size, and composition, were conducted to assess magnetic mineral alteration and diagenetic overprinting. Four magnetic zones (MZs) are identified down‐core from ∼200 to 1,100 m below sea floor based on rock magnetic variations. MZ 1 is a high magnetic intensity zone that contains ferrimagnetic greigite, which formed at shallow depths and is preserved because of rapid sedimentation. MZs 2 and 4 are low magnetic intensity zones with fewer magnetic minerals, mainly coarse‐grained (titano‐)magnetite and hematite. This magnetic mineral assemblage is a remnant of a more complex assemblage that was altered diagenetically a few million years after deposition when the site entered the Nankai Trough. MZ 3 is a high magnetic intensity zone between MZs 2 and 4. It contains authigenic single‐domain magnetic particles that probably formed from fluids that circulated through faults in the accretionary prism. Varying sediment supply and organic matter input through time, burial temperature, and tectonic fluid circulation are the primary drivers of magnetic mineral assemblage variations.

Lithologic modeling using VES method for subsurface structure delineation with geological constraints, eastern margin of Chiang Mai Basin and southwestern margin of Mae On Depression, San Kamphaeng District, Chiang Mai Province, northern Thailand

We conducted a vertical electrical sounding (VES) survey over the Quaternary sediments covering a transitional region between the eastern part of the Chiang Mai rift basin and the Mae On Depression, northern Thailand and constructed 1D lithologic subsurface models for delineating geological structures of the underlying basement rocks. Thirteen VES stations were collected and divided into northern and southern sub-parallel alignments trending WSW-ENE across the high magnetic contrast zone. We employed a Schlumberger array with the maximum half current electrode separations around 465–500 m, using the ABEM Terrameter SAS4000 resistivity meter. The VES data were inverted using EarthImager™ 1D program. We classified four geoelectric units composed of (1) very-low-resistivity saturated sediments (17–39 Ωm), (2) low-resistivity unsaturated sediments (33–581 Ωm), (3) medium-resistivity sedimentary rocks (57–110 Ωm), and (4) high-resistivity mafic volcanic rocks (176–804 Ωm). Shallow zones of the geoelectric models were interpreted as sediment fill with layered strata with variable resistivity because of different water saturation and clay content. The sediment fill thickens westward to the Chiang Mai rift basin. Deeper zones of higher resistivity are interpreted as the underlying basement rocks. These basement rocks show contrasting resistivities with the western part modeled as clastic sedimentary rocks of sandstone and the eastern part as mafic volcanic rocks. Geological observation evidence of westward-dipping faults and chronostratigraphic results integrated with the 1D lithologic models and drilled well information support a structural model of a reverse fault trending SW-NE between the Permian volcanic and Carboniferous sedimentary basements. This structure is part of the Chiang Mai-Chiang Rai Suture Zone associated with the eastward subduction model during the Permo-Triassic Indosinian Orogeny in northern Thailand.

ASTER multispectral bands, ground magnetic data, ground spectroscopy and space-based EIGEN6C4 gravity data model for identifying potential zones for gold sulphide mineralization in Bhukia, Rajasthan, India

We have used visible near infrared (VNIR) and shortwave infrared (SWIR) spectral bands of Advanced spaceborne thermal emission and reflectance radiometer (ASTER) to identify prospective zones of mineralization associated with the gold deposit in Bhukia area, Rajasthan. Lineament density, the extent of influence of regional structures/lineament, updated lithological map and the reference magnetic anomaly map are used for prospectivity mapping based on Fuzzy-logic based gamma overlay analysis. We have used high pass filtered (5 × 5 windows) image of selected bands of ASTER to delineate geological lineaments; which are believed to have a significant role in localizing hydrothermal fluid movement and also have influenced the ore deposition. In this regard, two evidential layers are prepared for geological structures. One layer is prepared to delineate a zone of influence of geological structures and other is on lineament density. For this, we also have used ground magnetic data and EIGEN6C4 model based Bouguer gravity data to delineate regional structures; which are combined with the ASTER-derived lineaments to derive composite lineament density map. Principle component image of ASTER bands, suitable false color composite (FCC) images of selected bands are derived to update litho-contacts of reference geological map to use as an evidential layer. Weightages for the themes, on favorable structure, lineament density, and lithology are assigned based on the geological understanding of their relative influence on the formation of the deposit. Weightage given to each layer is rescaled to the range of 0–1 using suitable fuzzy function. In addition to the above-mentioned geogenic factors, magnetic anomaly map is also used as an evidential layer. In this regard, empirical relation between a magnetic anomaly and gold sulphides is used as the reference to give gradational higher weightages to high magnetic anomaly zones; which are occurring within the spatial extent of the host rocks. Highly prospective zone in the Fuzzy prospective map prepared using above evidential layers is segmented for “potential or priority areas” and consequently validated. In this regard, high Zr/Hf anomaly map, Fe2O3 oxide anomaly map, “basement-highs” derived from EIGEN6C4 gravity model and ASTER-derived gossan exposure map are used to identify few priority areas within the highly prospective zone of prospect map. Gossan exposure map used in above purpose has been derived based on the implementation of the matched filtering method on ASTER image using gossan broadband spectra as a reference. Highly prospective zone identified using few regional evidence layers are validated using the localized evidences; which have been used to identify few priority areas within the highly prospective zones. Present approach of conjugate use of regional and local evidence layers to identify the priority areas within the highly prospective zone can be regarded as an innovative method to implement for gold exploration in any part of the world.

English

Geoscience prospecting for gold mineralisation was conducted in Kwakuti town located within Latitudes 9.362500°N to 9.387500°N and Longitudes 6.920833°E to 6.945833°E in Northern Nigeria. The deployed geosciences techniques comprise surface geological mapping, processing and analysis of aeromagnetic total magnetic field intensity data using Oasis Montaj software and X-ray fluorescence analysis of soil samples. Migmatites and gneiss dominate the rock outcrops in the area. The migmatites occupy high elevations on the north-eastern and eastern portion of the area, where they display low magnetic field intensity values. The schist occupies moderate to low elevation areas and they display high magnetic field intensity values. The migmatites are dotted with quartz veins which constitute the gold mineralisation zone. Oval shaped high magnetic anomalous zones within the schist indicate basic intrusive into the schist. First derivative map of the magnetic field intensity data reveal NE-SW trending lineaments. They likely conducted hydrothermal fluids from the basic intrusive into the migmatites on the northeast, where gold mineralisation occurred by metasomatic ionic exchange. Spatial concentration of TiO2 and MnO2 are the highest within the oval shaped high magnetic anomalous zones in the southern portion of the study area. This strengthens the inference that basic intrusive underlies the area. Gold concentration distribution pattern in the area is skewed NE-SW, thereby suggesting that the NE-SW structures control the mineralisation. Mining activities will be more efficient if directed along the NE-SW structural trend. Key words: Aeromagnetic data, aero-radiometric data, geologic mapping, gold mineralisation, X-ray fluorescence.

Geoimaging of subsurface fabric in Awgbagba, Southwestern Nigeria using geomagnetic and geoelectrical techniques

The heterogeneous nature of the earth has given rise to variations experienced in the subsurface. Some parts are good for hydrological exploration while others are good for civil engineering activities. These variations experienced in the subsurface could not be detected except through geophysical survey and analysis. Integration of ground magnetic and Vertical Electrical Sounding (VES) techniques were carried out in Awgbagba to image the subsurface fabric with a view to mapping subsurface geological features, such as the weak and competent zones, to determine the overburden thickness, and suitability of Awgbagba for civil engineering and hydrogeological purposes. Ten ground magnetic traverses were established in W-E and N-S azimuths. Twenty VES points were randomly sounded for the study in order to cover the entire study area. The magnetic residual field anomaly values ranged from -1, 600 to 700 nT. The study area is grouped into low, average, and high magnetic zones. The depth to magnetic sources ranged from 8.1 to 48.9 m with a mean value of 17.4 m. Three planar feature orientations in the study area are in NE-SW, NW-SE, and SE-NW orientations. VES results showed 7 QH-curve type, 6 HA-curve type, 4 KH-curve type, 1 AA-curve type, 1 HK-curve type and 1 QQ-curve type respectively. The ratio of thin-to-thick overburden and fresh-to-fractured bedrock are 3:7 and 2:3 respectively. It is concluded that the study area cannot withstand high-rise building constructions. However, the mapped fracture zones in the study area would serve as promising zones for borehole development.

Rock record and magnetic response to large earthquakes within Wenchuan Earthquake Fault Scientific Drilling cores

AbstractFault‐related pseudotachylytes are often considered to be produced by large seismic events. To investigate the rock record and magnetic response to large earthquakes within cores from the Wenchuan Earthquake Fault Scientific Drilling borehole 2 (WFSD‐2), we carried out microstructural, geochemical, and rock‐magnetic analyses of representative cores. Based on microstructural observations and powder X‐ray diffraction analyses, we found 21 layers of melt‐origin pseudotachylytes from 579.62 to 599.31 m‐depth in the cores. The presence of early‐formed pseudotachylyte fragments in the new layer suggests that seismic faulting processes exploited the same fault strand more than once. Pseudotachylyte veins have higher values of magnetic susceptibility relative to wall rocks. Rock‐magnetic results indicate that the magnetic minerals within the pseudotachylyte veins are magnetite with varying amounts of paramagnetic minerals. Magnetic hysteresis loops show that a reduction of the grain size of ferromagnetic minerals is not a plausible explanation for the higher magnetic susceptibility values in pseudotachylyte veins. Rock‐magnetic analyses indicate that frictional heating (&gt;500°C) occurred in the pseudotachylyte veins during large earthquakes. The resulting high temperatures induced thermal decomposition of paramagnetic minerals, forming magnetite and contributing to the higher magnetic susceptibility values. Different generations of pseudotachylytes and numerous high magnetic susceptibility zones together demonstrate that ancient powerful earthquakes may have occurred repeatedly in the Longmen Shan thrust belt.

Deep seismic structure of the northeastern South China Sea: Origin of a high‐velocity layer in the lower crust

AbstractWe present a 2‐D seismic tomographic image of the crustal structure along the OBS2012 profile, which delineates the Moho morphology and magmatic features of the northeastern South China Sea margin. The image was created by forward modeling (RayInvr) and traveltime tomographic inversion (Tomo2D). Overall, the continental crust thins seaward from ~27 km to ~21 km within the continental shelf across the Zhu I Depression and Dongsha Rise, with slight local thickening beneath the Dongsha Rise accompanying the increase in the Moho depth. The Dongsha Rise is also characterized by ~4–7 km thick high‐velocity layer (HVL) (~7.0–7.6 km/s) in the lower crust and exhibits a relatively high velocity (~5.5–6.4 km/s) in the upper crust with a velocity gradient lower than those of the Zhu I Depression and Tainan Basin. Across the continental slope and continent‐ocean transition (COT), which contain the Tainan Basin, the crust sharply thins from 20 km to 10 km seaward and a ~2–3 km thick HVL is imaged in the lower crust. We observed that volcanoes are located only within the COT, but none exist in the continental shelf; the Dongsha Rise exhibits a high magnetic anomaly zone and different geochemical characteristics from the COT. Based on those observations, we conclude that the HVL underlying the COT is probably extension related resulting from the decompression melting in the Cenozoic, whereas the HVL beneath the Dongsha Rise is probably arc related and associated with the subduction of the paleo‐Pacific plate. These findings are inconsistent with those of some previous studies.

Reconstruction and appraisal of Akunu–Akoko area iron ore deposits using geological and magnetic approaches

Abstract Geological mapping and magnetic methods were applied for the exploration of iron ore deposits in the Akunu–Akoko area of Southwestern Nigeria for the purpose of evaluating their geological characteristics and resource potentials. A proton magnetometer measures the vertical, horizontal and total magnetic intensities in gammas. The subsurface geology was interpreted qualitatively and quantitatively. The downward continuations and second vertical derivatives, the small-sized mineralised bodies and shallow features in the study area were mapped. The faults are trending in the following directions: NE–SW, NW–SE, N–S and E–W groups, while the iron ore mineralisation is structurally controlled by two major groups of fault trends, namely, the NE–SW and NW–SE; the N–S and E–W groups are mere occurrences that do not contribute to the structural control of the iron ore mineralisation in Akunu. The upward continuation has a linear feature similar to the principal orientation of the regional faults, while Locations 2 and 3 have relatively high magnetic susceptibility zones; suspected to be iron ore deposits. The depths to the magnetic sources ranged from 25 m to about 250 m.

Coal fire mapping of East Basuria Colliery, Jharia coalfield using vertical derivative technique of magnetic data

The present study deals with the coal fire mapping of East Basuria Colliery, Jharia coalfield, India, using the magnetic method. It is based on the fact that rise in temperature would result significant changes in magnetic susceptibility and thermo-remanent magnetization (TRM) of the overlying rocks. Magnetism increases slowly with the rise of temperature until the Curie temperature. Generally, rock/ overburden loses magnetization and becomes paramagnetic due to heating to Curie temperature, which results with significant reduction in magnetic susceptibility. However, magnetism increases significantly after cooling below the Curie temperature. Several data processing methods such as diurnal correction, reduction to pole (RTP), first and second vertical derivatives have been used for analysis of magnetic data and their interpretation. It is observed that the total magnetic field intensity anomaly of the area varies approximately from 44850 to 47460 nT and the residual magnetic anomaly varies approximately from −1323 to 1253 nT. The range of the magnetic anomaly after RTP is approximately 1050–1450 nT. About 20 low magnetic anomaly zones have been identified associated with active coal fire regions and 11 high magnetic anomaly zones have been identified associated with non-coal fire regions using vertical derivative techniques.

Environmental conditions for the presence of magnetofossils in the Last Glacial Maximum inferred from magnetic parameters of sediments from the Ulleung Basin, East Sea

Magnetic characterization of sediments from the Ulleung basin in the East Sea (Japan Sea) was carried out to decipher changes in environmental conditions over the last ~39,000years. Magnetic susceptibility (χ), intensities of natural remanent magnetization (NRM), anhysteretic remanent magnetization (ARM), partial ARM (pARM), and saturation isothermal remanent magnetization (SIRM) were measured. For selected samples, IRM acquisition, hysteresis and first-order reversal curve (FORC) measurements were made. The ratios of rock-magnetic parameters, such as χARM/χ, SIRM/ARM and S-ratio, were determined to identify magnetic grain size and mineralogical variations along with electron microscope observations of magnetic mineral extracts. Down-core variations in rock-magnetic parameters for the 5.21-m-long sediment core reveal distinct intervals with different grain size, concentration and composition of magnetic minerals. The uppermost portion and the high magnetic intensity zone (HIZ) in marine isotope stage (MIS) 2 are characterized by higher concentrations of fine-grained magnetite, whereas the MIS 1 and 3 intervals contain lower concentrations of coarser-grained magnetic minerals with increasing influence of hematite and/or iron sulphide minerals. The HIZ interval is inferred to contain high occurrences of fine-grained bacterial magnetosomes. Based on geochemical proxies, East Sea conditions for the HIZ interval, coincident with the last glacial maximum (LGM), can be summarized as having low total organic carbon (TOC), enhanced eolian contributions from terrestrial organic particles and iron-bearing minerals under the influence of an intensified East Asian winter monsoon, along with iron-reducing, but not sulphidic, conditions that favoured growth and preservation of magnetotactic bacteria. The delicate balance between reductive dissolution of magnetofossils under sulphidic conditions and production and preservation of magnetotactic bacteria in overlying iron-reducing sediments for the HIZ interval is responsible for the resultant abundance variations of magnetosomes in the studied sediments.

Integrated Gravity-Magnetic Study for Delineation of Structural Guided Uranium Mineralization Zones at Kutni, Purulia, West Bengal

Abstract The hydrothermally altered vein type uranium mineralization has been reported at different places along the South Purulia Shear Zone (SPSZ) towards 50 km north of the Singhbhum Shear Zone. An integrated close grid gravity-magnetic study over nearly one km2 area between Kutni and Dandudih villages within this shear zone has been performed to delineate the detail structural features, depth continuation, and associated uranium mineralization zones. The observed Bouguer and calculated residual gravity anomaly map depicts an ESE-WNW (on the east) to E-W (on west) trending low gravity zone on the extreme northern side of the area. The total field magnetic anomaly map also depicts the presence of high anomaly patches which is in accordance with the field observation of the exposed iron oxides (e.g. magnetite and goethite) over the same part of the study area. The 2D inverse modeling across the northern low residual gravity anomaly zones reveals steeply dipping low density structures with dip due north, south or vertical at places. The NS widths of these zones vary from 50-300 m with maximum probable depth extent of ~220 m. Previous researcher has also reported high radioactivity over the exposed quartz-magnetite-apatite bands on the northern part of the area. This band is exactly coinciding with the presently identified low gravity and high magnetic anomaly zones. These anomaly zones on the northern part of the study area are concluded to be hydrothermally altered uranium mineralization target zones. Thus, the results of this study will provide important guiding parameters for the future exploration programme over this region.