Positive Magnetic Anomalies Research Articles

Subsurface interconnection beneath the Mio-Plio-Quaternary volcanoes of the Ain Leuh causse (Middle Atlas, Morocco): Structural framework and emplacement mechanisms

In the Ain Leuh causse, located southwest of the Middle Atlas Volcanic Province (MAVP), a combined study using aeromagnetic data, fieldwork, and remote sensing is carried out to improve our understanding of the subsurface structure and its correlation with surface structural and volcanic features. The tectonically controlled emplacement mechanisms of the MAVP, which continue to hold undisclosed aspects, were investigated as well. Numerous techniques and mathematical filters for edge detection and 2D modelling were used. The main observations show that the positive magnetic anomalies in the study area are mainly related to the magmatic activity, which was organized into two main episodes: i) Triassic, with a tholeiitic affinity, well recorded along the NE-SW inherited Variscan faults; and ii) Mio-Plio-Quaternary, with an alkaline affinity, forming scoria cones, maars, and lava flows. They are also expressed as hidden domes, laccoliths, and interconnected magma bodies revealed by 2D modelling of the aeromagnetic data. Uncommon coarse-grained undersaturated alkaline rocks crop out in Talzast dome (Talzastite, Fasinites, and Ankaratrite). They are unique in the MAVP and restricted to this part of the Middle Atlas. 2D aeromagnetic modelling has revealed the presence of shallow magmatic rocks bodies beneath Talzast dome. The extracted structural lineaments using aeromagnetic data and remote sensing reveal that the structural framework of the study area is controlled by NE-trending faults reaching depths of up to 3000 m, which is the depth of the magnetic signature of the faults. Additionally, NNE, NNW and N-trending short-length lineaments are defined and rooted at depths up to 3000 m. The predominant NE-SW structural trend may be linked to the reactivation of Hercynian faults during Alpine tectonics. These faults likely controlled the emplacement of the Mio-Plio-Quaternary volcanism in the area. The magma reservoirs that would have fed the various volcanic edifices are located at a shallow depth of less than 500–2000 m.

Crustal architecture of the Dharwar craton and Southern Granulite Terrane, Southern India, from the analysis of gravity-magnetic data

The potential field (gravity & magnetic) data, along with results of available seismic and magnetotelluric studies over the Southern India Shield comprising the Dharwar craton (DC), Southern Granulite Terrane (SGT) and southern part of the Eastern Granulite Mobile Belt (EGMB), were synthesized. The study attempts to elucidate the geometry and continuity of greenstone belts/shear zones, isostatic condition and deeper crustal architecture. We observed a characteristic elevation-gravity relationship for each of the tectonic domains. The greenstone belts of the DC are characterized by N-S to NW-SE trending positive residual gravity anomalies and continue beneath the Cuddapah sediments. The basement of the Cuddapah basin is characterized by NW-SE trending greenstone belts with low-density intrusive rocks. The gravity anomaly suggests occurrence of a high-density rock unit occurs at mid-crustal depth beneath the ‘Closepet granite’ and ‘Chitradurga schist belt’. The potential field signatures of the DC and SGT are distinctly different. The Cauvery Shear Zone is characterized by positive residual gravity and magnetic anomalies with ∼E-W to ENE-WSW trend that continues further east under sediment covers of the east coast. The residual gravity signatures and horizontal gravity gradient signatures suggest that the orogenic systems encompassing the Cauvery Shear Zone and the eastern part of the EGMB may be a part of a common structural belt or tectonic regime associated with the assembly of the Gondwana supercontinent. The potential field modelling reveals the presence of a thick, high-density material resting over the base of the crust in the entire stretch of the east-coast of India.

Magnetic field map of the Wilhelm Archipelago shelf zone, West Antarctica

The Antarctic Digital Magnetic Anomaly Project is an international research effort to construct a magnetic map of the continent based on ground, satellite, marine, and aeromagnetic surveys. This paper reports the magnetic mapping of the shelf zone in the SE part of the Wilhelm Archipelago, West Antarctica, based on magnetic surveys conducted with Zodiac boats. A spectacular feature of this area is the strong magnetic anomaly of the Antarctic Peninsula (AP) batholith, which was the product of subduction-related Mesozoic–Cenozoic arc magmatism on the former margin of Western Gondwana. We constructed and analyzed a detailed magnetic map of magnetic field anomalies using field observations of rock exposures on the islands and magnetic properties of rocks from laboratory data. The oldest volcanic rocks of Jurassic to Lower Cretaceous age relate to N-NE trending bands of negative magnetic field. The largest feature in the study area is an Upper Cretaceous/Paleogene granodiorite complex that produces a positive magnetic anomaly. Many smaller anomalies are also present over gabbroid bodies of Cretaceous age. Two-dimensional magnetic modeling shows that heterogeneities in the upper crust may have magnetic susceptibilities in the range of 0.005–0.13 SI. Magnetic field anomalies also delineate an orthogonal system of tectonic faults, including the main NE fault along the Penola Strait (sub-parallel to the AP coastline) and four intersecting faults. These fault systems may be associated with different stages of continental margin evolution along the Antarctic Peninsula.

Reconnaissance Basement Geology and Tectonics of North Zealandia

AbstractNew rock dredge samples supply key information to establish the tectonic and geological framework of the northern two‐thirds of the 95% submerged Zealandia continent. The R/V Investigator voyage IN2016T01 to the Fairway Ridge, Coral Sea, obtained poorly sorted poly‐lithologic pebbly to cobbly sandstones, well sorted fine grained sandstones, mudstones, bioclastic limestones, and basaltic lavas. Post‐cruise analytical work comprised petrography, whole rock geochemical and Sr and Nd isotopic analyses, and U‐Pb zircon, Rb‐Sr, and Ar‐Ar geochronology. A Fairway Ridge cobbly sandstone has a ∼95 Ma (early Late Cretaceous) depositional age; two biotite granite cobbles are 111 ± 1 and 128 ± 1 Ma in age, and some volcanic pebbles are also likely Early Cretaceous. Fairway Ridge basalts have intraplate alkaline chemistry and are of Late Eocene age (∼40–36 Ma). By analogy with South Zealandia, we interpret strong positive continental magnetic anomalies of North Zealandia to mainly result from Late Cretaceous to Cenozoic intraplate basalts, many of them rift‐related lavas. A new basement geological map of North Zealandia shows the position of the Mesozoic Gondwana magmatic arc axis (Median Batholith) and other major geological units. This study completes onland and offshore reconnaissance geological mapping of the entire 5 Mkm2 Zealandia continent.

The geomagnetic field of the Ukrainian Carpathians and a 3D magnetic model of the Transcarpathian Depression

The article presents the results of a qualitative and quantitative analysis of the geomagnetic field of the Ukrainian Carpathians and 3D magnetic modeling along the PANCAKE geotransect and Transcarpathian Depression, as well as a comparison of the obtained results with fault-block tectonics, deep structure, and distribution of a number of types of endogenous ore deposits and hydrocarbon accumulations.
 It is shown that the overthrust northeastern part of the Carpathian arc lies on the magnetic crust, and the southwestern one — on its non-magnetic lower part, and between the Rava-Rus’ka and Krakovets and Pre-Carpathian faults, the Earth’s crust is magnetized throughout.
 For the territory of the Transcarpathian Depression, the regional and local components of the geomagnetic field were identified for the first time, geological and geophysical characteristics were provided, and their tectonic interpretation was proposed.
 A three-dimensional magnetic model of the Transcarpathian Depression was created taking into account the magnetization of rocks according to measurements. A detailed magnetic model of the upper part of the crustal section of the Transcarpathian Depression was developed. The connection of magnetic sources with fault zones was analyzed; the results were compared with the deep structure and distribution of ore deposits and hydrocarbon accumulations.
 The regional source with magnetism І=1.0 A/m is located within the Mukachevo Depression and is located at a depth of 6.0 km to 13 km. The local component of the geomagnetic field reflects the magnetic inhomogeneity of the Earth’s crust in the upper 3—4 km of the section and mainly reflects the volcano-tectonic structures and dike formations of the Vyhorlat-Gutyn Ridge, the Chop-Berehove Uplift, and the Velika Dobron’ Uplift. The maximum depths of magnetic sources (up to 3.5 km) and their magnetization (1.22 A/m) are characteristic of the structures of the Vyhorlat-Gutynsky Ridge, intermediate values of depths (2.0—3.0 km) and magnetization (up to 0.93 A/m) belong to the Chop-Berehove Uplift, and the minimum depths (up to 1.1 km) and magnetization (up to 0.7 A/m) are characteristic of the Velika Dobron’ Uplift.
 It is shown that the gas fields of Transcarpathia correspond to local positive magnetic anomalies and are localized above a deep magnetic source. Within the Berehove Uplift, positive anomalies and magnetic sources indicate andesiteporphyrite shafts and andesite domes of Sarmatian age associated with gold, gold-polymetallic and silver mineralization. A zone of antimony mineralization is associated with the Pannonian-Pontic volcanic structures along the southern foot of the Vyhorlat-Gutyn Ridge, and deposits and occurrences of bismuth and mercury correlate well with intrusive formations of the Dacian-Romanian age.

Geological Structures Controlling Au/Ba Mineralization from Aeromagnetic Data: Harrat ad Danun Area, Saudi Arabia

Positive and negative magnetic anomalies occupied the total aeromagnetic (TM) map of the Harrat ad Danun area, Saudi Arabia. Reduction to the pole (RTP) maps display the range of magnetic values (−312.4 to 209.4 nT) that vary in shape, size, and magnitude. These anomalies generally follow the NNW–SSE (Red Sea axis trend), NE–SW, and NNE–SSW trends. The NNW-SSE linear negative and positive magnetic anomalies could be brought on by buried faults, shear zones, or subsurface dikes. In the central part, the position of Au and Ba mineralization was connected to this trend. It is concluded that the principal structures are represented by the NNW–SSE, NE–SW, and NNE–SSW tendencies. Based on gridded RTP magnetic data, the 2-D power spectrum was computed and revealed the frequency of the near-surface and deep magnetic components. It is believed that the depths of the shallow and deep magnetic sources are typically 80 m and 570 m, respectively. Additional negative and positive magnetic anomalies with varied amplitudes and frequencies, trending in the NNW–SSE, ENE–WSW, and NE–SW directions, are seen when the high-pass and low-pass maps are closely examined. Many faults in various directions cut into these anomalies. The occurrence of negative linear magnetic anomalies (−36.6 nT to −137.3 nT) at this depth (80 m) is also confirmed by this map. The TDR filter and the Euler deconvolution method were used to identify the horizontal variations in magnetic susceptibility as well as the source position and depth of magnetic sources. The linear clustering rings are thought to be caused by contacts or faults with depths between 1 m to 474 m that are oriented WNW–ESE, NNE–SSW, and NNW–SSE. These faults or contacts are thought to be particularly prominent in the western, eastern, southern, northern, and central zones. The majority of felsic and mafic dikes are found to be connected to subsurface structures, showing that three structural trends—WNW–ESE, NNE–SSW, and NNW–SSE—affect the studied area. This demonstrates that important features and shear zones control the majority of Saudi Arabia’s gold deposits. A negative magnetic anomaly that is centered in the area, trending NNW–SSE and crossing the NNE–SSW fault, is connected to the plotted gold and barite mineralization in the study area. This may imply that these two tendencies are responsible for mineralization. This result raises the possibility of mineralization in the NNW negative magnetic feature located in the western part of the area. The occurrence of gold and barite was significantly impacted by the NNW–SSE and NNE–SSW structural lineaments.

Late-stage structural evolution from near-bottom topographic and magnetic Surveys of Suda Seamount (West Pacific)

The late-stage evolution of seamounts is widespread in the West Pacific and may control the distribution of manganese nodules on the lower seamount flanks and adjacent debris aprons. However, due to the paucity of near-bottom observational data, little research exists specifically focused on the late-stage evolution of seamount structures. To fill this knowledge gap, we systematically investigated the tectonic features and evolution of Suda Seamount based on rock samples, satellite gravity data, and high-resolution magnetic and topographic data surveyed by Autonomous Underwater Vehicle (AUV). We found that the central area of the seamount summit exhibits a negative magnetic anomaly related to a group of minor volcanic structures, while the collapse of volcanic structures on the northeast seamount flank is associated with an extensive positive magnetic anomaly. Surprisingly, a late-stage intrusive dyke with significantly high magnetic susceptibility originates from the lower part of a minor group of volcanic structures but cannot be clearly distinguished below the main summit volcanic structure (hereinafter referred to as the main mound complex). These results demonstrate that at least two late-stage periods of intense magmatic activity occurred during the evolution of the seamount, one centered on the main mound complex and one on the margin of the summit platform. By combining fine-scale topographic and satellite gravity data, we deduce that gravitational slip caused by magma intrusion along a fracture was the main cause of the seamount flank collapse. The late-stage processes resulted in a change in the seamount morphology and distributed clastic rock debris widely along the flanks, which contributed to the uneven distribution of manganese nodules along the lower seamount flanks and determined the distribution of subsequently formed nodules.

Near-Bottom Magnetic Anomaly Features and Detachment Fault Morphology in Tianxiu Vent Field, Carlsberg Ridge, Northwest Indian Ocean

As a product of hydrothermal mineralization at spreading centers, seafloor massive sulfides (SMS) have become a research hotspot in the field of prospecting and exploring deep-sea mineral resources owing to their enrichment of various strategic metals. Since hydrothermal circulation changes the magnetic properties of host rocks and can generate magnetic anomalies, near-bottom magnetic surveying is an effective method to determine magnetic anomaly features of the seafloor. This technology has been applied to the detection of SMS deposits, in addition to its use in understanding hydrothermal fluid flow conduits and associated hydrothermal alterations. The Tianxiu Vent Field (TVF) is a detachment-fault-controlled, ultramafic-associated hydrothermal system located on the Carlsberg Ridge, Northwest Indian Ocean. During China’s DY57th cruise in 2019, near-bottom magnetic data were collected by an autonomous underwater vehicle. In this paper, we use bathymetric and magnetic data, as well as rock sampling information, to analyze and discuss the magnetic anomaly features of the TVF region. Then, we apply 2.5D magnetic anomaly profile forward modeling to determine the shallow magnetic structure and the pattern of detachment faults in the subsurface. Our results show that TVF is characterized by a significant positive magnetic anomaly, where stronger magnetization exists in the area with active hydrothermal vent clusters. The detachment fault has a dip of less than 30° at shallow depths, which steepens to a dip of ~70° at depths of around 300 m.

Magnetic characteristics and tectonic structure of the Earth's crust of the Carpathian oil and gas region as a component of complex hydrocarbon criteria

For the first time, a qualitative and quantitative analysis of the geomagnetic field and oil and gas capacity was performed for the territory of the Carpathian region, which made it possible to reveal the connection of hydrocarbon deposits with the magnetization of the Earth’s crust at the regional and local levels and the features of the deep structure of the Earth’s crust according to seismic data. The regularity of the distribution of oil and gas deposits of the Carpathian oil and gas-bearing region revealed by geomagnetic criteria is confirmed by their connection with regional and local anomalies of the gravity field and heat flow density. According to the analysis of the regional geomagnetic field and magnetic models along the PANCAKE and RomUkrSeis geotraverses, gas fields are located above the magnetic blocks of the middle and lower parts of the Earth’s crust or in their marginal parts, and oil fields are located above practically non-magnetic deep blocks. Under the thrust part of the Carpathian arc, the regional features of the Earth’s crust structures are clearly manifested, namely, starting from the longitude of the city of Rakhiv, the submeridional direction of extension of regional anomalies of magnetic, gravity, and thermal fields and their horizontal gradient zones prevails. The oil and gas potential of the region is in good agreement with this — the gas fields are timed to the minimum values of the regional horizontal gradient of the regional magnetic field, to the increased gradient of Bouguet anomalies, as well as to the minimum values of the horizontal gradient of heat flow density anomalies. In local geophysical fields, gas fields are gravitating: to weakly negative magnetic anomalies and their gradient zones; to negative anomalies of the gravitational field predominantly; to positive anomalies of heat flow density in the northwest, within the area of Trans-European suture zone (without the middle of the Krakovets fault), and negative anomalies in the Carpathian trough. Oil and oil condensate deposits are correlated with a band of positive magnetic field anomalies, negative gravity field anomalies, and increased heat flux density values. The main deep faults of the Carpathian region and the marginal part of the Platform in the first approximation can be considered Velikomostivsko-Chernivetsky, which characterizes the change in the character of magnetic and gravity fields and heat flow density, Rava-Rusky, which reflects the zone of articulation of the Eastern and Western European platforms, Pre-Carpathian — the border of the Pre-Carpathian trough and the Folded Carpathians on the Earth’s surface (as well as the boundary of the Archaean(?)-Paleoproterozoic and Meso-Neoproterozoic crust according to the modern international chronostratigraphic scale), as well as the Transcarpathian fault. On the basis of magnetic and seismic data, the hypothetical depth position of these faults and the possible connection with them of hydrocarbon flows into the upper part of the Earth’s crust are proposed. Therefore, the geomagnetic criteria of the oil and gas bearing capacity of the Earth’s crust of the Carpathian region are agreed based on the results of the study of the gravity field and heat flow density anomalies, which makes it possible to predict the spread of oil and oil condensate and gas and gas condensate deposits according to the regional and local features of these fields in a first approximation.

ПРИМЕНЕНИЕ МЕТОДОВ ЭЛЕКТРОМЕТРИИ ПРИ ИЗУЧЕНИИ РОССЫПНЫХ МЕСТОРОЖДЕНИЙ ЗОЛОТА ХАБАРОВСКОГО КРАЯ

The article presents the results of applying geophysical methods in the study of placers of various genetic types in the Khabarovsk Territory. The main research methods are vertical electrical and remote induction sounding. Geophysical studies of alluvial placers were carried out along river valleys, at the watersheds and small streams of the Turchikansky lode-and-placer gold cluster. The main tasks to be solved are the search for potential structural traps of gold (paleoriverbeds, ravines), determination of the nature of the bedrock surface and the thickness of the overlying loose deposits. The work performed allows us to identify a prospecting indicator for placer gold in a given territory, that is, lower electrical resistivity values in the presence of a local positive magnetic anomaly. The study on the coastal marine placer on the coast of the Gulf of Nicholas (Sea of Okhotsk) focused on the lithological breakdown of the geologic section by physical properties. The results of the work showed strong differences in the electrical conductivity of the bedrock at the base of the section, marine sediments, and continental deposits. The application of electrometry methods is supported by the fact that rocks differ very distinctly by electrical properties. This is confirmed by placer mining activities at the technogenic deposits of the Verkhnyaya Uda and the Sofia mine. Mining and geological factors complicating dredging operations, such as permafrost and bouldery debris, were studied at technogenic placers by induction methods. Research results revealed that the presence of boulders or frozen rocks is manifested in the same way in electrical readings, that is, in a sharp increase of electrical resistivity. Thus, areas of abnormally high resistivity are hazardous areas for the passage of the dredger. These findings indicate the high efficiency of the application of electrical exploration technologies in the study of different types of placer gold deposits.

Using geophysics to follow and model the Precambrian basement terranes beneath the Caledonian nappes in Finnmark, northern Norway: A case study

The bedrock geology of Finnmark, northern Norway, consists primarily of rock complexes dating back to the Precambrian and early Mid Palaeozoic eras and comprises (1) a mid-crustal lithospheric basement which is characterised by granitoid and greenstone complexes dating from Neoarchean to late Paleoproterozoic in age; (2) the Caledonide Orogen, characterised by a series of nappes and thrust sheets. Finnmark forms part of the northern margin of the Fennoscandian Shield, which extends across northern Finland into northwestern Russia. Precambrian rock complexes are also found as thrust slices in Caledonian nappes at several places in Finnmark and Troms, as well as in tectonic windows.Recently acquired high-resolution aeromagnetic data have provided spectacular and confirmatory evidence for the continuation of the diverse Precambrian greenstone belts and granulite terranes beneath the magnetically transparent Caledonian nappes.This study aims at analysing this subsurface continuation of the terranes using potential field data and estimating the depths of the main magnetic sources beneath the nappes. In this study, several geophysical methods are used, including 3D Euler deconvolution and 2D Werner deconvolution. We have focused specifically on the depth-to-basement beneath nappes and purposefully excluded anomalies associated with shallow structures, such as dykes and faults. Different methods have produced similar results and indicate an appreciable increase in the depth of the Precambrian basement rocks from south to north. In the south, Precambrian basement rocks are either exposed on the surface southeast of the Caledonian front or reappear within tectonic windows. A prominent positive magnetic anomaly on the Porsanger Peninsula is thought to represent an extension of the anomaly ascribed to the greenstone belt of the Repparfjord Tectonic Window. A 3D model of this has also been provided. According to estimates, the depths-to-basement below nappes close to the Caledonian front range from less than 500 to 2000 m, whilst in the northern part of the study area the depths generally reach 3000 to 4000 m, and are locally estimated to range between 5000 m and 6000 m.

Extensional structures and Cenozoic magmatism in the northwestern South China Sea

This study investigates the crustal structure and Cenozoic magmatism in the northwestern South China Sea (SCS), based on two long-cable multi-channel seismic reflection profiles, together with gravity and magnetic data, and adjacent wide-angle refraction profiles. Basins/sags are bounded by large listric-normal faults (fault throws ≥ 0.5 km) and massifs are cut off by normal faults with small offsets (fault throws < 0.5 km) in the northwestern SCS. These structures are penetrated by magmatic edifices showing positive gravity and magnetic anomalies. syn-Rift magmatic intrusions/extrusions were intense in the basins/sags and continent-ocean transition zone while post-rift magmatism was widespread from basins/sags to massifs with the most intense stage occurring from 5.5 to 2.6 Ma. Based on previous geophysical and geochemical results, we suggest that syn-rift mantle upwelling from partial melting initiated seafloor spreading magmatic activities, whereas plume-related mantle upwelling contributed to the magmatism during and after seafloor spreading in the northwestern SCS. Stretching factors show that the upper and lower crusts have experienced differential extension from basins/sags to massifs. The non-uniform crustal extension resulted from upper crustal faulting and lower crustal flow. Particularly, the lower crustal flow was probably linked with the combined action of magmatic heating, mantle flow shearing stresses, and sediment loading, resulting in crustal boudinage and reestablishment of an equilibrium state over long distances.

Nova Colinas, Maranhão State: A newly confirmed, complex impact structure in Brazil

AbstractThe Nova Colinas structure is an approximately 7 km wide, nearly circular feature centered at 07°09′33″S/46°06′30″W in Nova Colinas municipality in southwestern Maranhão State, Brazil. The area has been investigated for 40 yr and it has been suggested repeatedly that the structure could be of impact origin—without proof having been furnished. Magnetic anomaly maps depict the structure clearly with a strong, positive magnetic anomaly over the apparent rim zone. The central area is characterized by significant positive K and Th radiometric anomalies. Fieldwork showed that the structure has annular features along the outside and some prominent, structurally dissected hills in the interior. Thirty‐three arenite samples were collected for petrographic analysis, mostly from within the structure. Microdeformation, in the form of cataclasis; concussion fractures related to compaction, and presence of planar fractures, feather features, and planar deformation features in quartz are reported. Three samples with a multitude of quartz grains with these microdeformations were analyzed by universal stage to determine the crystallographic orientations of planar fractures and planar deformation features. The results provide robust evidence that these microdeformation features represent shock metamorphism, with low (approximately 5–10 GPa) to moderate (10–16 and 10–20 GPa) shock levels. Thus, the Nova Colinas structure is now confirmed as a bona fide meteorite impact structure. The structure is moderately eroded, as shown by the absence of stronger shock deformation. The still limited available structural geological field evidence, paired with remote sensing and geophysical data, indicates that the innermost part of the structure may have a sizable remnant of a central uplift. The Nova Colinas impact age is only poorly constrained from stratigraphic inference to an upper limit of about 200–250 Ma.

Magmatic and rifting-related features of the Lomonosov Ridge, and relationships to the continent–ocean transition zone in the Amundsen Basin, Arctic Ocean

SUMMARY The continental Lomonosov Ridge spans across the Arctic Ocean and was the subject of a geophysical study in 2016 with two seismic reflection lines crossing the ridge in proximity to the North Pole, one of which continues across the continent–ocean transition zone into the Amundsen Basin. One seismic station and 15 sonobuoys were deployed along these two lines to record seismic wide-angle reflections and refractions for development of a crustal-scale velocity model. Its viability is checked using gravity data from the experiment which are also used to interpolate crustal structure in areas with poor seismic constraints. On the line extending into the Amundsen Basin, continental crust composed of two layers with velocities of 6.0 and 6.5 km s–1 is encountered beneath the Lomonosov Ridge where the Moho depth is 21 km based on gravity modelling. The crust is overlain by a 1-km-thick layer with velocities of 4.7 km s–1 coinciding with a zone of positive magnetic anomalies of up to 180 nT. This layer is interpreted to include extrusive volcanic rocks related to the Cretaceous High Arctic Large Igneous Province (HALIP). Within the Amundsen Basin, three distinct crustal domains can be distinguished. Closest to the ridge is a 40-km-wide zone with a crustal thickness around 5 km interpreted as thinned continental crust. Five distinctive faulted basement blocks display high-amplitude reflections along their crests with velocities of 4.6 km s–1, representing the continuation of the magmatic rocks further upslope. Brozena et al. (2003) interpreted magnetic Chron C25 to be located in this zone but our data are not consistent with this being a seafloor spreading anomaly. In the adjacent crustal domain, heading basinward, the basement flattens and two layers with velocities of 5.2 and 6.8 km s–1 can be distinguished, where the upper and lower layer have a thickness of 1.5 and 2.0 km, respectively. The upper layer is interpreted as exhumed and highly serpentinized mantle while the lower layer may be less serpentinized mantle with some gabbroic intrusions. This may explain the high-amplitude reflections within the overlying sediments that are interpreted as sill intrusions. Continuing basinward, the last crustal domain represents 4-to 5-km-thick oceanic crust with a variable basement relief and velocities of 4.8 and 6.5 km s–1 at the top of oceanic layers 2 and 3, respectively. It is within this zone that the first true seafloor spreading anomaly Chron C24 is observed, which argues for a similar breakup age in the Eurasia Basin as in the Northeast Atlantic. On the other profile crossing the Lomonosov Ridge, a 60-km-wide intrusion into the lower crust is observed where velocities are increased to 6.9 km s–1. Gravity modelling supports the interpretation of magmatic underplating beneath the intrusion. Seismic data in this region show that the crust is overlain by a 2-km-thick series of high-amplitude reflections with a velocity of 4.8 km s–1 in a 30-km-wide zone where strong magnetic anomalies (&amp;gt;800 nT) are observed, suggesting a composition of basalt flows. This part of the Lomonosov Ridge appears therefore to have a HALIP-related magmatic overprint at all crustal levels.

Aeromagnetic anomalies and magnetic domains of the Jalisco Block, western Mexico

The Jalisco Block (JB) in western Mexico has been considered as a margin terrane being detached from the North American plate along the Tepic-Zacoalco rift and Colima graben. In this study, an aeromagnetic analysis of the JB and surrounding areas is performed applying a series of geophysical filters and VOXI inverse model. The JB is characterized by two aeromagnetic domains (AMD). AMD-Ia located throughout the south-central section that borders the continental margin is characterized by positive magnetic anomalies extending from the Armería river to Banderas Bay. The AMD-Ia corresponds to the Puerto Vallarta batholith emplaced in the late Cretaceous. The AMD-IIa in the northern section is marked by negative magnetic anomalies over volcanic and volcano-sedimentary rocks, extending to the southern Sierra Madre Occidental. We propose that JB tectonic boundaries correspond to the aeromagnetic AMD-Ia eastern limit and AMD-IIa northern limit. Estimated depths from the radially average power spectrum for deep magnetic sources in the AMD-Ia are in the range of 5.4 km, and for shallow sources from 0.8 to 1.2 km. For AMD-IIa depths are 10.45 km for deep sources and 0.13 km for shallow sources. Depths for deep sources from the Euler deconvolution for the AMD-Ia range from 3.3 to 4.5 km and from 3.5 to 4.9 km for the AMD-IIa. Shallow source depths in the AMD-Ia are 0.2–0.3 km and near surface for the AMD-IIa. The integrated analysis shows five aeromagnetic domains, two associated with the JB and three with the surrounding areas. The VOXI inverse model allows to estimate the dimensions and location of magnetic sources in the reduced to the pole and upward continuations.

Magnetic Properties of Impact UHPHT Glasses, Melt Rocks, Suevitic Breccia and Target Rocks of the Giant Kara Meteorite Crater (Pay-Khoy, Arctic Seashore, Russia)

The shock waves can strongly change the physical properties of the target rock minerals including their density and magnetism which determine petrochemical properties of impactites finely as a rule are resulted in astroblemes contours on geophysical maps. Following to the aero-magnetic mapping data the non-magnetic sedimentary rocks of the Kara target create a zero and negative magnetic field with an average intensity of -1 nT, against the background the southwestern region of the Kara astrobleme provides the positive magnetic anomalies with an intensity of 1 to 3 nT which are in a good correspondence with the Pay-Khoy ridge structure general orientation. The Kara dome is characterised with an isometric negative anomaly of intensity -5 nT. Here we present the magnetic properties of the different kinds of the Kara impactites including impact ultra-high pressure high temperature (UHPHT) melt glasses, melt rocks and suevitic breccia compare to sedimentary target rocks. The petrophysical measurements presented the specific magnetic susceptibility of the impactites in the range of 8 to 48×10-8 SI units, where the UHPHT glasses have the limits from 9 to 38×10-8 SI units (15×10-8 SI units, in average). The sedimentary target is characterised with essentially lower level of magnetic susceptibility – no higher than 15×10-8 SI units, where limestone has it about zero. Following to the similar level of the iron content within the impactites and target rocks the magnetism of the Kara impact melts is explained rather by changing of magnetic properties by the impact process. One of the possible source of magnetism can be partially an iron-containing matter of the asteroid component in the form of pyrrhotine accompanied with Ni and Co impurities. Also, we cannot exclude partial presence of magnetic iron component directly within the quenched impact glasses including UHPHT variety.

Integration of ASTER satellite imagery and 3D inversion of aeromagnetic data for deep mineral exploration

Due to high metal values and raised difficulties in finding superficial deposits, the exploration for mineral resources needs to reach greater depths. To achieve this objective, integrating ASTER and 3D inversion of aeromagnetic can be a useful tool in imaging deep magnetic mineralization hosting-structures. In this study, we present such integrated approach for deep mineral exploration in some promising zones of Gabal (G) Semna region, Eastern Desert (ED) of Egypt. Aeromagnetic data was used to delineate the structures and their relation to ore deposits and ASTER data was utilized for mapping lithological units and the hydrothermal zones. The 3D inversion was employed to imagine the depth and geometry of the magnetized ore bodies in the zones that identified by processing ASTER data. The 3D magnetic inversion revealed that mafic rocks and metavolcanics exhibit high-amplitude magnetic anomalies. High amplitude (positive) magnetic anomalies are joined with the large susceptibilities metavolcanics. Alteration zones in the mafic to basic bedrock and alongside major fault systems are high potential zones that revealed by ASTER image analysis. Aster and aeromagnetic data were used to map and alteration zones surface geology that can host mineralization and the 3D inversion was applied to outline the subsurface extent of these promising areas. This approach can be broadly used for deep mineral exploration in the Egyptian ED and identical areas around the world.

Structural controls on the gold mineralization at the eastern margin of the North China Craton: Constraints from gravity and magnetic data from the Liaodong and Jiaodong Peninsulae

The eastern block of the North China Craton (NCC) has been completely decratonized and it hosts numerous early Cretaceous gold deposits dated at about 130–120 Ma. The Jiaodong Peninsula in the easternmost NCC is the largest gold producing district in China, and it hosts more than 150 gold deposits with known gold resources of over 5000 tonnes (t). Also located at the eastern margin of NCC, the Liaodong Peninsula is poorly explored at depths greater than 800 m, with no large concealed gold deposit being discovered yet. In this paper, we carefully analyzed the gravity and magnetic characteristics of the ore-controlling structures in both provinces, including faults, Precambrian basement metamorphic rocks and the Mesozoic granitic intrusions. The deep regional faults are characterized by linear gravity gradient zones or elongated positive magnetic anomalies. The Precambrian basement metamorphic rocks usually have large-amplitude gravity and magnetic highs, while the lithological contacts of Mesozoic granite intrusions are commonly defined by gravity lows or narrow magnetic high anomalies. Interpretations from gravity and magnetic signatures predict two concealed lithological contacts between Mesozoic granites and Precambrian metamorphic rocks beneath Liaodong and Jiaodong.The spatial locations of known gold deposits are then correlated with the concealed structures inferred from the gravity and magnetic data in our paper. Their spatial relationship suggests that the gold mineralization is mainly controlled by oblique regional faults along the structural and lithological contacts between Mesozoic granites and Precambrian metamorphic rocks. Additionally, the structural intersections of different ore-controlling faults, especially the hydrothermal alteration zones at the contacts between Precambrian basement metamorphic rocks and the Mesozoic granites, are most favorable for the gold mineralization in the Liaodong and Jiaodong areas. Many large gold deposits are documented along a deep-seated, but concealed, structure beneath the Jiaodong gold district, while the Qingchengzi ore field and its vicinity in the Liaodong Peninsula still remain underexplored, despite of having a very similar structural setting, and deserve further investigation.

Crustal Density and Susceptibility Structure beneath Achankovil Shear Zone, India

Abstract The Southern Granulite Terrain (SGT) is a large tract of exposed Archean continental crust, divided into the Madurai Block (MB), Trivandrum Block (TB), and Nagercoil Block (NB). These crustal domains are linked with the NW-SE trending Achankovil Shear Zone (AKSZ). We combine gravity and magnetic data with previously published ground observations and geochronological data to re-evaluate the crustal architecture, evolution of the AKSZ, and possible extension of AKSZ into Madagascar. Analyses indicate that the long wavelength trends of the magnetic anomalies originate at ~20 km depth of different SGT blocks. These observations are corroborated with the gravity as well as computed gravity gradient anomalies. The presence of khondalite outcrops in Trivandrum Block implies that high magnetization crust is the main source of positive magnetic anomalies. Such magnetic anomalies advocate that SGT preserves the remanent of Archean crustal blocks in South India, a part due to variation in thermal and geochemical processes. The AKSZ, TB, and MB exhibit contrasting magnetic crustal signatures. The joint modeling results reveal a three-layered crustal configuration with varying Moho ranging from 41 to 34 km in NE to SW, respectively. It is also noted that AKSZ is a narrow and deep structure near to the Western Ghats Escarpment while it is wide and shallow in the far-east, which implies that the evolution of the Western Ghats is a late geological event.

Reconstruction of the Settlement Levoberezhnoe Plan of the Bronze Age (South Ural, Russia)

In the Southern Urals in 2015–2019 research was conducted on the fortified settlement of the Bronze Age Levoberezhnoe (Sintashta II). An asphalt road was drawn through the settlement, during the construction of which about a third of the monument’s area was destroyed. Excavations of the monument have been carried out since 2015. It has been established that the settlement is multi-layered; it functioned in the Late Bronze Age from the turn of the 3rd–2nd millennium BC until the last quarter of the second millennium BC. Detailed magnetic and topographic surveys were performed on the territory that remaining survived the construction of the road. The location of the external moat was reliably determined by linear positive magnetic anomalies. The base of the outer wall of the settlement had a thickness of about 4 m, the width of the outer moat was 2–2.5 m. It was also possible to accurately localize a number of walls of buildings. The settlement had a rectangular shape, inside there were 26 dwellings. As a result of a comprehensive analysis of the data and aerial photographs of the last century, the layout of the entire settlement was reconstructed.