Interpretation of gravity and magnetic data in the Phlegraean fields geothermal area, Naples, Italy

Available gravity and magnetic data of the Phlegraean Fields geothermal area, Naples, Italy, have been interpreted and the obtained structural models discussed in the light of the other available geological, volcanological and geophysical data.On the basis of the results of a previous seismic reflection survey in the Gulf of Naples and in the Pozzuoli Bay, which delineated a basement characterized by a seismic velocity of 4–6 km/s, it has been possible to evaluate the gravity anomaly connected with the morphology of this horizon ( = 2.7 g/cm³).The residual anomaly map, obtained after subtraction of the regional long-wavelength components relative to mantle and deep crustal structures and the computed components relative to the above-mentioned seismic basement, shows up as a circular low with an amplitude of 10 mgal centred in the Pozzuoli Bay. This gravity low has been interpreted as due to the occurrence, in the centre of Pozzuoli Bay, of light (Δ = −0.2 g/cm³) material with a maximum thickness of about 2 km. However, a contribution to the anomaly due to a narrow magmatic body intruded in the basement, as suggested by volcanological and ground deformation data, cannot be excluded.The aeromagnetic map of the Phlegraean Fields is characterized by three main anomalies which have been fitted by superficial tridimensional parallelepipedic bodies, schematically representing lava flows and domes. Their anomalies have been subsequently subtracted from the observed field, obtaining as a residual a large anomaly centred in the southwestern area of the Pozzuoli Bay. It has been interpreted as being due to a lowmagnetized body which, taking into account the thermal state of the area, should represent that part of the pyroclastic sequence which has lost part of its magnetization by thermo-chemical alteration.     

A gravity and magnetic study of the volcanic island of Ischia, Naples (Italy)

Gravity and magnetic data for the volcanic island of Ischia, Naples, Italy, have been analyzed and interpreted in light of recent geological and volcanological data to define a model of the island's shallow and deep structures. From the interpretation of the gravity data it appears that the shallow structures consist of pyroclastic material (p=2.0 g/cm³). Within these pyroclastics there are domes and lava flows of higher density and eruptive centres filled with lower density material. The basement is a “horst” with the shallowest depth at about 1.0 km, south of the centre of the island, if we assign a density contrast of 0.5 g/cm³ relative to the above pyroclastics.Interpretation of magnetic data measured at 725 stations showed that the basement derived from the gravity interpretation is magnetized. Moreover, this basement is less magnetized on the western side of Ischia which may be caused by the anomalous thermal state of the area, as indicated by surface fumaroles, hot springs etc. and temperature measurements in deep drillings.     

Age of the source of the Jarrafa gravity and magnetic anomalies offshore Libya and its geodynamic implications

The interpretation of the Jarrafa magnetic and gravity highs, NW Libyan offshore, suggests that it may be caused by a body of high-density and high magnetization. Analysis of their power spectra indicates two groups of sources at: (1) 2.7 km depth, probably related to the igneous rocks, some of which were penetrated in the JA-1 borehole, (2) 5 km depth, corresponding to the top of the causative body and (3) 10 km depth, probably referring to the local basement depth. The boundary analysis derived from applied horizontal gradient to both gravity and magnetic data reveals lineaments many of which can be related to geological structures (grabens, horsts and faults).The poor correlation between pseudogravity fields for induced magnetization and observed gravity fields strongly suggests that the causative structure has a remanent magnetization (D = −16°, I = 23°) of Early Cretaceous age, fitting with the opening of the Neo Tethys 3 Ocean.Three-dimensional interpretation techniques indicate that the magnetic source of the Jarrafa magnetic anomaly has a magnetization intensity of 0.46 A/m, which is required to simulate the amplitude of the observed magnetic anomaly. The magnetic model shows that it has a base level at 15 km.The history of the area combined with the analysis and interpretation of the gravity and magnetic data suggests that: (1) the source of the Jarrafa anomaly is a mafic igneous rock and it may have formed during an Early Cretaceous extensional phase and (2) the Jarrafa basin was left-laterally sheared along the WNW Hercynian North Graben Fault Zone, during its reactivation in the Early Cretaceous.     

Geophysical observations on an exposed seamount in the Afar Depression

Topographic, magnetic and gravity surveys have been made over an extinct volcano in the Afar Depression. Previous work showed that this volcano was formed under water. Gravity measurements over the volcano indicate that a positive mass contrast of 1 gm. cm^?3 is required below the volcano, which could be explained by the density contrast between a basalt plug and surrounding sediments. There is no observable magnetic anomaly over the volcano. The hyaloclastites of which the volcano is mainly composed have magnetization which is small in intensity but consistent in direction with the Earth’s magnetic field. Basalt cobbles which are present in small quantities have a fairly high intensity of magnetization but directions which bear no relationship to the direction of the Earth’s field. The absence of a magnetic anomaly suggests that the basalt plug below the volcano must have been sufficiently brecciated so that random rotations of portions of the plug have occurred, thus reducing the mean magnetization, and explaining the absence of a magnetic anomaly.     

Fast mapping of magnetic basement depth,structure and nature using aeromagnetic and gravity data: combined methods and their application in the Paris Basin

Assessment of deep buried basin/basement relationships using geophysical data is a challenge for the energy and mining industries as well as for geothermal or CO₂ storage purposes. In deep environments, few methods can provide geological information; magnetic and gravity data remain among the most informative and cost‐effective methods. Here, in order to derive fast first‐order information on the basement/basin interface, we propose a combination of existing and original approaches devoted to potential field data analysis. Namely, we investigate the geometry (i.e., depth and structure) and the nature of a deep buried basement through a case study SW of the Paris Basin. Joint processing of new high‐resolution magnetic data and up‐to‐date gravity data provides an updated overview of the deep basin. First, the main structures of the magnetic basement are highlighted using Euler deconvolution and are interpreted in a structural sketch map. The new high‐resolution aeromagnetic map actually offers a continuous view of regional basement structures and reveals poorly known and complex deformation at the junction between major domains of the Variscan collision belt. Second, Werner deconvolution and an ad hoc post‐processing analysis allow the extraction of a set of magnetic sources at (or close to) the basin/basement interface. Interpolation of these sources together with the magnetic structural sketch provides a Werner magnetic basement map displaying realistic 3D patterns and basement depths consistent with data available in deep petroleum boreholes. The last step of processing was designed as a way to quickly combine gravity and magnetic information and to simply visualize first‐order petrophysical patterns of the basement lithology. This is achieved through unsupervised classification of suitably selected gravity and magnetic maps and, as compared to previous work, provides a realistic and updated overview of the cartographic distribution of density/magnetization of basement rocks. Altogether, the three steps of processing proposed in this paper quickly provide relevant information on a deep buried basement in terms of structure, geometry and nature (through petrophysics). Notwithstanding, limitations of the proposed procedure are raised: in the case of the Paris Basin for instance, this study does not provide proper information on Pre‐Mesozoic basins, some of which have been sampled in deep boreholes.     

Gravity and magnetic signatures of volcanic plugs related to Deccan volcanism in Saurashtra, India and their physical and geochemical properties

The Bouguer anomaly and the total intensity magnetic maps of Saurashtra have delineated six circular gravity highs and magnetic anomalies of 40-60 mGal (10⁻⁵m/s²) and 800-1000 nT, respectively. Three of them in western Saurashtra coincide with known volcanic plugs associated with Deccan Volcanic Province (DVP), while the other three in SE Saurashtra coincide with rather concealed plugs exposed partially. The DVP represents different phases of eruption during 65.5±2.5 Ma from the Reunion plume. The geochemical data of the exposed rock samples from these plugs exhibit a wide variation in source composition, which varies from ultramafic/mafic to felsic composition of volcanic plugs in western Saurashtra and an alkaline composition for those in SE Saurashtra. Detailed studies of granophyres and alkaline rocks from these volcanic plugs reveal a calc-alkaline differentiation trend and a continental tectonic setting of emplacement. The alkaline plugs of SE Saurashtra are associated with NE-SW oriented structural trends, related to the Gulf of Cambay and the Cambay rift basin along the track of the Reunion plume. This indicates a deeper source for these plugs compared to those in the western part and may represent the primary source magma. The Junagadh plug with well differentiated ring complexes in western Saurashtra shows well defined centers of magnetic anomaly while the magnetic anomalies due to other plugs are diffused though of the same amplitude. This implies that other plugs are also associated with mafic/ultramafic components, which may not be differentiated and may be present at subsurface levels. Paleomagnetic measurements on surface rock samples from DVP in Saurashtra suggest a susceptibility of 5.5×10⁻² SI units with an average Koenigsberger ratio (Q_n) of almost one and average direction of remanent magnetization of D=147.4° and I=+56.1°. The virtual geomagnetic pole (VGP) position computed from the mean direction of magnetization for the volcanic plugs and Deccan basalt of Saurashtra is 30°N and 74°W, which is close to the VGP position corresponding to the early phases of Deccan eruption. Modeling of gravity and magnetic anomalies along two representative profiles across Junagadh and Barda volcanic plugs suggest a bulk density of 2900 and 2880 kg/m³, respectively and susceptibility of 3.14×10⁻² SI units with a Q_n ratio of 0.56 which are within the range of their values obtained from laboratory measurements on exposed rock samples. The same order of gravity and magnetic anomalies observed over the volcanic plugs of Saurashtra indicates almost similar bulk physical properties for them. The inferred directions of magnetization from magnetic anomalies, however, are D=337° and 340° and I=−38° and −50° which represent the bulk direction of magnetization and also indicate a reversal of the magnetic field during the eruption of these plugs. Some of these plugs are associated with seismic activities of magnitude ≤4 at their contacts. Based on this analysis, other circular/semi-circular gravity highs of NW India can be qualitatively attributed to similar subsurface volcanic plugs.     

3D upper crustal density structure of the Deccan Syneclise,Central India

A constrained 3D density model of the upper crust along a part of the Deccan Syneclise is carried out based on the complete Bouguer anomaly data. Spectral analysis of the complete Bouguer gravity anomaly map of the study region suggests two major sources: short wavelength anomalies (<100 km) caused primarily due to the density inhomogeneities at shallow crustal level and long wavelength anomalies (>100 km) produced due to the sources deeper than the upper crust. A residual map of the short wavelength anomalies is prepared from the complete Bouguer anomaly using Butterworth high‐pass filter (100 km cut‐off wavelength). Utilizing the constraints from deep resistivity sounding, magnetotellurics and deep seismic sounding studies, 2.5D density models have been generated along 39 profiles of this region. The mismatch between the calculated response of the a priori 2.5D model with the residual (short wavelength) gravity anomalies is minimized by introducing high‐density intrusive bodies (≥2.81 g/cm³) in the basement. With these 2.5D density models, the initial geometry of our 3D density model, which includes alluvium, Deccan trap, Mesozoic sediment and high‐density intrusive bodies in the basement up to a depth of 7 km (upper crust), is generated. In the final 3D model, Deccan trap extends from 200 m to nearly 1700 m below the 90–150 m thick Quaternary sediment. Further down, the sub‐trappean Mesozoic sediment is present at a depth range of 600–3000 m followed by the basement. The derived 3D density model also indicates six intrusive bodies of density 2.83 g/cm³ in the basement at an average depth of about 4–7 km that best fits the residual gravity anomaly of the study area.     

Three-dimensional structure of the Laguna Salada Basin and its thermal regime

A comprehensive reinterpretation of the available gravity, magnetic, geothermal, geological and borehole information has been made of the Laguna Salada Basin to establish a 3D model of the basement and sedimentary infill. According to statistical spectral analysis, the residual gravity anomaly is due to sources with a mean regional depth of 2.8 km. The topography of the basement was obtained from a three‐dimensional inversion carried out in the wavenumber domain using an iterative scheme. The maximum density contrast of ?300 kg/m³ estimated from previous studies and the mean depth of 2.5 km finally constrained this inversion. The resulting model indicated that the sedimentary infill is up to 4.2 km thick at its deepest point. According to the gravity‐derived basement topography, the basin presents an asymmetry (i.e. it is of the half‐graben type). It is deeper to the east, where it is delimited from the Sierra Cucapah by a step fault. By contrast, the limit with the Sierra de Juarez is a gently sloping fault (i.e. a listric fault). The basement is not even, but it comprises a series of structural highs and lows. N–S to NW–SE and E–W to NE–SW faults delimit these structural units. The magnetic modelling was constrained by (i) the gravity‐derived basement topography; (ii) a Curie isotherm assumed to be between 7 km and 10 km; (iii) assuming induced magnetization only; (iv) the available geological and borehole information. The magnetic anomalies were interpreted successfully using the gravity‐derived basement/sedimentary interface as the top of the magnetic bodies (i.e. the magnetic modelling supports the gravity basement topography). An elongated N–S to NW–SE trending highly magnetized body running from south to north along the basin is observed to the west of the basin. This magnetic anomaly has no gravity signature. Such a feature can be interpreted as an intrusive body emplaced along a fault running through the Laguna Salada Basin. Treatment of the gravity and magnetic information (and of their horizontal gradients) with satellite image processing techniques highlighted lineaments on the basement gravity topography correlating with mapped faults. Based on all this information, we derived detailed geological models along four selected profiles to simulate numerically the heat and fluid flow in the basin. We used a finite‐difference scheme to solve the coupled Darcy and Fourier differential equations. According to our results, we have fluid flow in the sedimentary layers and a redistribution of heat flow from the basin axis toward its rims (Sierra de Juárez and Sierra Cucapah). Our model temperatures agree within an error of 4% with the observed temperature profiles measured at boreholes. Our heat‐flow determinations agree within an error of ±15% with extrapolated observations. The numerical and chemical analyses support the hypothesis of fluid circulation between the clay–lutite layer and the fractured granitic basement. Thermal modelling shows low heat‐flow values along the Laguna Salada Basin. Deep fluid circulation patterns were observed that redistribute such flow at depth. Two patterns were distinguished. One displays the heat flow increasing from the basin axis towards its borders (temperature increase of 20°C). The second pattern shows an increasing heat flow from south to north of the basin. Such behaviour is confirmed by the temperature measurements in the thermometric boreholes.     

Morphology,magnetic anomalies and basalt magnetization at the ends of the Galapagos high-amplitude zone

Detailed bathymetric and magnetic data, complemented by nine dredge stations, define the eastern and western limits of a belt of high-amplitude magnetic anomalies associated with the Galapagos hot spot. The hypothesis of “magnetic telechemistry” was tested and locally confirmed. High amplitudes correspond to high remanence, susceptibility, FeO^T, TiO₂, and presumably titanomagnetite concentration. The average remanence of surface samples in the high-amplitude zone is 0.027 emu/cm³ (range, 0.009–0.085 emu/cm³), about 4 times that of the normal-amplitude zone. Magnetic amplitudes are only 2–2.5 times higher, however. If the greater TiO₂/FeO^T ratio of high-amplitude zone basalts also characterizes the titanomagnetites, remanence in the high-amplitude zone may fall off more rapidly with depth in the crust as a result of reheating. Alternatively, small pillows of high remanence are more common than larger pillows at the top of the high-amplitude zone crust; FeTi basalt may also be concentrated in the upper part of the crust. Anomaly amplitudes are highest at the ends of the zone, particularly in the east. As asthenosphere crystal slushes presumably flow away from the Galapagos plume, progressive crystal fractionation may enrich residual magmas in FeO^T and TiO₂. The Galapagos FeTi zone terminates abruptly against transform fractures at both ends, perhaps because subaxial flow is dammed at the transforms. The FeTi-producing crystal slushes have advanced east and west at speeds up to 10 cm/yr since they first appeared at the spreading axis at least 6.6 m.y. B.P. Their progressive advance was connected with the progressive southward jumps of the spreading axis east of the Galapagos hot spot, and northward jumps to the west.     

Basement morphology and rift geometry near the former junction of India,Australia, and Antarctica

Magnetic and gravity anomaly data, together with features of the basement topography presented here show that the continental margin of western Australia, including the Naturaliste plateau, was shaped by NE-SW-trending rift segments offset by nearly orthogonal transform faults. A steep landward gradient of the isostatic gravity field and a lineated magnetic anomaly which occur together at the continental slope are interpreted as marking the ocean-continent boundary of the rifted margin off Perth and the sheared margin between Perth and the Wallaby plateaus. Anomalies diagnostic of the ocean-continent boundary are not observed at the margins of the Naturaliste plateau; the geometry of the rift zone here is adduced from the disposition of magnetic lineations, fracture zones, and basement features. A geophysical survey of the Naturaliste fracture zone shows it to be a continuous basement trough extending from the Diamantina fracture zone 800 km northwest to Dirck Hartog ridge. Similar basement troughs west of and orthogonal to the fracture zone imply that the region west/southwest of the Naturaliste plateau was, like the region north of it, formerly occupied by Greater India. Marine magnetic anomaly and basement trends suggest that the oceanic crust between the plateau and Diamantina fracture zone could be substantially older than Paleocene, heretofore the oldest crust identified between Australia and Antarctica.     

The Alpha Ridge: Gravity, seismic and magnetic evidence for a homogenous, mafic crust

Gravity and bathymetric results from the 1983 Canadian Expedition to Study the Alpha Ridge (CESAR) have outlined positive free-air anomalies centred on the continental break off Ellesmere Island characteristic of normal Atlantic-type passive margins. These data confirm implications derived from depth-to-magnetic basement calculations that the ridge may not be structurally connected to the continent. Across the Alpha Ridge magnetic and gravity anomalies mimic the bathymetry. The magnetic anomalies apparently are not caused, to any great extent, by internal structures or magnetic reversals, but rather seem to result simply from variations in depths to a homogenous magnetic structure. The gravity anomalies across a 500 km wide section of the Alpha Ridge can be almost completely accounted for by topography, shallow sedimentary fill and a simple two-tier crustal model. This implies an extraordinary lateral density homogeneity unknown in continental structures of comparable size. Gravity models show the crustal thickness to increase gradually from 20 km at the Marvin Spur to 38 km at the ridge crest. A comparison of this model with a gravity model of the continental-type Lomonosov Ridge, which has a thickness of about 25 km, indicates that, at the same thickness of 25 km, the average crustal density of the Alpha Ridge is 0.08 Mg/m³ greater. These gravity constraints, the unusually homogenous seismic velocity structure revealed by the CESAR studies, the homogeneous magnetic structure, and the extraordinary high intensity satellite magnetic anomaly associated with the Alpha Ridge, indicate that the ridge may be composed of a large pile of mafic rock, possibly unique on this planet.     

St. John's Island (Red Sea): a new geophysical model and its implications for the emplacement of ultramafic rocks in fracture zones and at continental margins

New gravity and magnetic data from the northern Red Sea reveal the extent of the large gravity anomaly (164 mgal) and the presence of significant magnetic anomalies over St. John's Island. Spectral transformation and three-dimensional potential-field modelling delineate the surface configuration and vertical extent of the causative body and the enormous density contrast required (1.2 g/cm³) suggests that it is composed of unserpentinised peridotite (density 3.4 g/cm³) to a depth of at least 8 km.St. John's Island is uniquely located, not only at a passive continental margin but also within a fracture zone at the transition from plate separation by seafloor spreading to extension by lithospheric attenuation. This precludes several suggested mechanisms for the emplacement of ultramafic bodies in fracture zones.Thermal contraction, serpentinite diapirism and changes in the poles of rotation do not seem possible mechanisms in this tectonic environment and the emplacement is most probably related to the spreading readjustment necessary to create a continent-to-continent fracture zone. A post-Mesozoic age of emplacement, associated with the onset of continental rifting and the rejuvenation of a pre-existing continental fracture, seems most probable.     

Constraints on the origins of lunar magnetism from electron reflection measurements of surface magnetic fields

Apollo 15 and 16 subsatellite measurements of lunar surface magnetic fields by the electron reflection method are summarized. Patches of strong surface fields ranging from less than $\text{1}\text{4}$° to tens of degrees in size are found distributed over the lunar surface, but in general no obvious correlation is observed between field anomalies and surface geology. In lunar mare regions a positive statistical correlation is found between the surface field strength and the geologic age of the surface as determined from crater erosion studies. However, there is a lack of correlation of surface field with impact craters in the mare, implying that mare do not have a strong large-scale uniform magnetization as might be expected from an ancient lunar dynamo. This lack of correlation also indicates that mare impact processes do not generate strong magnetization coherent over ～ 10 km scale size. In the lunar highlands fields of >100 nT are found in a region of order 10 km wide and >300 km long centered on and paralleling the long linear rille, Rima Sirsalis. These fields imply that the rille has a strong magnetization (>5 × 10^?6 gauss cm³ gm^?1 associated with it, either in the form of intrusive, magnetized rock or as a gap in a uniformly magnetic layer of rock. However, a survey of seven lunar farside magnetic anomalies observed by the Apollo 16 subsatellite suggests a correlation with inner ejecta material from large impact basins. The implications of these results for the origin of lunar magnetism are discussed.     

Constraints on the evolution of the Taranaki volcanoes,New Zealand,based on aeromagnetic data

Aeromagnetic anomalies measured over the three relict andesite volcanoes (Paritutu, Kaitake and Pouakai) in Taranaki are largely dominated by topographic effects. Three-dimensional magnetic modelling, well constrained by both contrasting levels of exposure and previous gravity models, shows that the bulk magnetization of the andesite edifice and edifice core of both Kaitake and Pouakai is 2.5–3.5 A m^-1 in a direction close to that of the present earth's field. However, the large andesite dyke/stock complexes below all three edifices and a localized area of the Kaitake edifice directly above the centre of the subedifice complex have anomalously low bulk magnetizations of 0.3–1.0 A m^-1 in the same direction. These subsurface complexes represent dyke injection from magma chambers situated in the basement, probably below 6 km depth. Here, we deduce that heat from these magma chambers drove hydrothermal convection systems causing widespread demagnetization, especially in the subedifice complexes, but also locally within the edifices themselves. A lesser degree of demagnetization at Pouakai, the youngest of the three volcanoes, may indicate a shorter and consequently more intense period of activity at this centre.     

A preliminary magnetic study of soil samples from west-central Minnesota

Various rock magnetic techniques were applied to characterize magnetically the samples of a soil profile taken from west-central Minnesota. There is a marked change in magnetic properties as a function of depth in the core. X-ray analysis and Curie temperature measurements carried out on the magnetic fractions indicate that magnetite is the dominant iron oxide in both the top soil and the subsoil. The intensity of anhysteretic remanent magnetization (ARM) decreases sharply as the depth increases. In contrast, the stability of ARM was found to be higher for the subsoil. The surface soil sample was capable of acquiring a significant amount of viscous remanent magnetization (VRM). The VRM acquisition coefficient (S_a) of the subsoil (S_{a= 3.18 × 10^?6emu g^?1, 3.18 × 10^?6A m² kg^?1}) was about ten times weaker than that of the top soil sample (S_a = 3.868 × 10^{?7emu g?1, 3.868 × 10?7A m2 kg?1}). The magnetic domain state indicator, the ratio of coercivity of remanence to coercive force, H_cr/H_c, was 1.5 and 3.85 for the top soil and subsoil, respectively. It appears that the observed variations in magnetic properties down the present soil core is due only to a difference in grain size. We conclude that the magnetic grains in surface soil samples were more single-domain (SD) like whereas the magnetite grains in the subsoil samples were more likely in pseudo-single-domain (PSD) or small multidomain (MD) range. The observed lower stability for the surface soil samples is attributed to the presence of superparamagnetic grains whose presence was confirmed by transmission electron micrographs.     

Global electromagnetic induction in the Moon and planets

A summary of experiments and analyses concerning electromagnetic induction in the Moon and other extraterrestrial bodies is presented. Magnetic step-transient measurements made on the lunar dark side show the eddy current response to be the dominant induction mode of the Moon. Analysis of the poloidal field decay of the eddy currents has yielded a range of monotonic conductivity profiles for the lunar interior: the conductivity rises from 3·10^?4 mho/m at a depth of 170 km to 10^?2 mho/m at 1000 km depth. The static magnetization field induction has been measured and the whole-Moon relative magnetic permeability has been calculated to be $\text{μ}\text{μ}{}_0\text{= 1.01 ± 0.06}$. The remanent magnetic fields, measured at Apollo landing sites, range from 3 to 327 γ. Simultaneous magnetometer and solar wind spectrometer measurements show that the 38-γ remanent field at the Apollo 12 site is compressed to 54 γ by a solar wind pressure increase of 7·10^?8 dyn/cm². The solar wind confines the induced lunar poloidal field; the field is compressed to the surface on the lunar subsolar side and extends out into a cylindrical cavity on the lunar antisolar side. This solar wind confinement is modeled in the laboratory by a magnetic dipole enclosed in a superconducting lead cylinder; results show that the induced poloidal field geometry is modified in a manner similar to that measured on the Moon. Induction concepts developed for the Moon are extended to estimate the electromagnetic response of other bodies in the solar system.     

The use of potential field data in revealing the basement structure in sub-basaltic settings: an example from the Møre margin, offshore Norway

This study investigates the utility of the potential fields (gravity and magnetics) in volcanic settings as observed on the Møre margin. Synthetic models are used to investigate the effect of volcanics on the gravity and magnetic fields. The focus is on detecting sub-basaltic basement structures. The methods applied to the models are Euler deconvolution on magnetic data, gravity gradients and integrated 3D gravity and magnetic forward modelling. The same methods are used on the Møre margin and the results compared to the synthetic models. The Euler deconvolution on the magnetic signal does provide limited depth solutions in the volcanic environment and the use of different observation levels does not enhance the results. Forward gravity and magnetic models provide a valuable tool to estimate both the basalt and sub-basaltic sedimentary thickness but are limited by the ambiguity inherent in potential field methods. The use of gravity gradients significantly decreases the available model solutions and provides boundary detection even in sub-basaltic settings.     

The isostatic gravity anomaly: key to the evolution of the ocean-continent boundary at passive continental margins

Isostatic gravity highs bordering the passive continental margins are interpreted as resulting from oceanic basement highs. These basement elevations are relics of the transient phenomenon of a higher ridge axis elevation during early rifting. The steep landward gradient in the isostatic gravity field, generally associated with a magnetic edge effect anomaly, delineates the boundary between oceanic and continental basement.     

Preparation of Magnetic Hydroxyapatite and Their Use as Recyclable Adsorbent for Phenol in Wastewater

Magnetic hydroxyapatite (HAP), which combined superparamagnetic Fe₃O₄ nanoparticles and HAP, composite materials were prepared by ultrasound method in this paper. It has also been found that these materials have the ability to adsorb phenol in wastewater. The magnetic materials were investigated by scanning electron microscope, X‐ray diffraction (XRD), Fourier transform infrared spectroscopy, thermal gravimetric analysis, vibrating sample magnetometer, and N₂ adsorption in order to elucidate the morphology, structure, and other properties. When the prepared magnetic materials were calcined at 200°C, the prepared Fe₃O₄ was oxidized to Fe₂O₃, possessing loose‐shaped holes with a high specific area of 325.2 m²/g, a magnetization intensity of 12.5 emu/g, and the N₂ adsorption isotherm belongs to porous adsorption type I. Moreover, the magnetic HAP can adsorb 90% phenol in wastewater. This means that it is an excellent recyclable phenol sorbent for sewage treatment. Experiments confirmed that the Freundlich adsorption isotherms model applies to lower phenol concentrations (0–50 mg/L), while for high phenol concentrations (50–500 mg/L) the Langmuir adsorption isotherms model fits. The magnetic sorbents have the capacity to regenerate after reaching adsorption saturation using ethanol as eluant and external magnetic field as separation unit. The efficiency of adsorption was reduced only by 10% over a six time use period.     

Gravity profiles across the Cheyenne Belt, a precambrian crustal suture in southeastern Wyoming

Geologic discontinuities across the Cheyenne Belt of southeastern Wyoming have led to interpretations that this boundary is a major crustal suture separating the Archaean Wyoming Province to the north from accreted Proterozoic island arc terrains to the south. Gravity profiles across the Cheyenne Belt in three Precambrian-cored Laramide uplifts show a north to south decrease in gravity values of 50–100 mgal. These data indicate that the Proterozoic crust is more felsic (less dense) and/or thicker than Archaean crust. Seismic refraction data show thicker crust (48–54 km) in Colorado than in Wyoming (37–41 km). We model the gravity profiles in two ways: 1) thicker crust to the south and a south-dipping ramp in the Moho beneath and just south of the Cheyenne Belt; 2) thicker crust to the south combined with a mid-crustal density decrease of about 0.05 g/cm³. Differences in crustal thickness may have originated 1700 Ma ago because: 1) the gravity gradient is spatially related to the Cheyenne Belt which has been immobile since about 1650 Ma ago; 2) the N-S gradient is perpendicular to the trend of gravity gradients associated with local Laramide uplifs and sub-perpendicular to regional long-wavelength Laramide gradients and is therefore probably not a Laramide feature. Thus, gravity data support the interpretation that the Cheyenne Belt is a Proterozoic suture zone separating terrains of different crustal structure. The gravity “signature” of the Cheyenne Belt is different from “S”-shaped gravity anomalies associated with Proterozoic sutures of the Canadian Shield which suggests fundamental differences between continent-continent and island arc-continent collisional processes.