Long-wavelength Magnetic Anomalies Research Articles

Spreading Magnetic Anomalies Separation of the South China Sea Based on the Nested Elliptical Directional Filters

Spreading magnetic anomalies recorded the paleo-geomagnetic field variation that has great significance in the investigation of the extension process of ocean basins. Interpreting spreading magnetic anomalies under complex geological environments is challenging, especially for marginal sea basins. We proposed nested elliptical directional filters to separate the spreading magnetic anomalies of the South China Sea (SCS). The results show that the spreading magnetic anomalies separated by the nested elliptical directional filters depict the expansion process of the oceanic crust, and the interference magnetic anomalies are effectively suppressed. The separated spreading magnetic anomalies indicate that the expansion process of the SCS is affected by the interactions between the surrounding plates. The spreading magnetic anomalies of the SCS are warped, interrupted, and not strictly parallel. The pattern of the spreading magnetic anomalies reflects multiple ridge jumps during the expansion process and the post-spreading magmatic disturbances. The long-wavelength magnetic anomalies indicate lithospheric fractures and Curie surface variations in the SCS, which are affected by the post-spreading magmatic rejuvenation. The magnetic anomalies of the SCS resulted from the superposition of magnetic anomalies in the ocean crust and the uppermost mantle.

Two Cases of COVID-19 in Adults who Vape

In 2019, a severe acute respiratory disease with coupled multiple organ pathologies, namely, Coronavirus Disease 2019 (COVID-19), emerged. The lungs and other organs in this disease exhibit glass-like (hyaline) tissue pathologies with iron oxide deposits, which mimics rock-related mineralization. The impact of significant geological and geophysical perturbations on aberrant mineralization in living organisms has not been explored. Many Indigenous Knowledge systems posit that unseen forces emitted by orogenic structures and modulated by geophysical dynamics play a significant role in health. Here, it is proposed that carbon dioxide-rich water-lithospheric ferromagnesian silicates interactions generate aberrant lithospheric long-wavelength magnetic anomalies (LWMA) via serpentinization, during geologic periods with polar ice melt-induced hydrosphere redistribution, increased atmospheric carbon dioxide, and weakened geomagnetic field intensity. Here, it is also proposed that COVID-19 in humans (and animals) is a pathogenic manifestation of aberrant LWMA-induced magnetic catalysis of silicate/carbonate-like-iron oxides minerals from tissues (carbon-based molecules), gases (dioxygen and carbon dioxide) and metals (iron, magnesium, silicon), and coronavirus from the genome (endogenous viruses), with the virus capable of replication and limited transmission. These COVID-19-LWMAs are generated in tectonic plates with degraded Proterozoic bedrocks and are linked to the refertilization of iron oxides-silicate/carbonate rocks. Thus, severe COVID-19 outbreaks are/will predominately occur on the Eurasian and North American tectonic plates and governed by the diurnal, semiannual (equinoctial maxima, solstitial minima) and quasi-biannual oscillations of the geomagnetic magnetic field intensity (analogous to a vibrating drum head). Furthermore, abnormally high ferromagnetic iron content in humans is the unifying determinant for COVID-19-induced morbidity and mortality.

WITHDRAWN: Can Traditional Chinese Medicine provide insights into controlling the COVID-19 pandemic: Serpentinization-induced lithospheric long-wavelength magnetic anomalies in Proterozoic bedrocks in a weakened geomagnetic field mediate the aberrant transformation of biogenic molecules in COVID-19 via magnetic catalysis

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The architecture of the Buem Structural Unit: Implications for the tectonic evolution of the Pan-African Dahomeyide Orogen, West Africa

The Buem Structural Unit (BSU) occurs about 50 km west of the high-pressure (HP) rocks of the Pharusian suture zone (PSZ) of the Dahomeyide Orogen, West Africa. Shale and sandstone intercalations form the base of the BSU, followed by serpentinised peridotite, gabbro, massive and pillow basalts (fragment of oceanic crust), jasper/chert, and limestone/dolostone, with sandstone and diamictite forming the top of the BSU. The package was deformed at lower greenschist-facies conditions during the Pan-African orogeny. The N-S-trending BSU has been affected by three deformational events. The main deformation event D2, which transposed bedding-parallel D1 fabrics, produced a steeply ENE-dipping S2 tectonic foliation. S2 is axial planar to upright or west-verging isoclinal and chevron F2 folds. Top-to-the-west D2 shear zones in the sandstone, shale and serpentinised peridotite are associated with a down-dip ENE-plunging L2 stretching lineation. The mafic-ultramafic rocks occur as D2 tectonic slices, marking the boundaries between thrust sheets within the BSU. The D2-related N-S trending structural and short-wavelength (<1 km) magnetic grain of the BSU is superimposed on ENE-trending long-wavelength (>1 km) magnetic anomalies that are continuous along trend with the regional structures of the West African Craton (WAC) and are interpreted as signals from the basement rocks of the BSU. D2 deformation is interpreted to result from overthrusting of the BSU onto the southeastern margin of the WAC during collision of the WAC and Benino-Nigerian Shield (BNS). Meter-scale, steep and open F3 kink and crenulation folds with steeply SE-dipping axial planes are sporadically observed throughout the entire BSU. D3 deformation is inferred to reflect a period of NW-SE-shortening. Deformation history of the BSU shares structural similarities with the one of the PSZ. We, therefore, interpret the BSU as an allochthonous fold and thrust belt with a geodynamic evolution involving subduction of the WAC beneath the BNS and the ensuing collision between the two terranes. The tectonic framework of the BSU suggests a geodynamic evolution of the Dahomeyide Orogen evolving from initial accretion of oceanic crust and deposition of passive margin sedimentary sequences, and their inversion due to continent-continent collision during the Pan-African orogeny.

The potential contribution to long wavelength magnetic anomalies from the lithospheric mantle

Long-wavelength magnetic anomalies (LWMA) are broad scale variations of the magnetic field that are usually observed at high altitudes with amplitudes ranging up to about 20 nT. The source of these anomalies has typically been assumed to reside within the crust with negligible contributions from the lithospheric mantle. Recent work on the magnetization of mantle xenoliths suggests a possible lithospheric mantle contribution to LWMA, depending on tectonic setting and local geotherm. Here, we investigate the potential contribution of the lithospheric mantle to LWMA across two contrasting lithospheric geotherms: cratons and hotspots. To constrain the range of possible mantle contributions to satellite-observable LWMA in these settings, we develop a Monte Carlo suite of forward models of magnetic anomalies patterned after the mantle portions of the Siberian craton, the Hawai'i hotspot, and the French Massif Central plume. The models incorporate the current geotherms, the pressure dependence of the Curie temperature of magnetite, and the statistical distributions of rock magnetic data from mantle xenoliths in the specific regions. The Siberian craton shows the highest potential contribution of the mantle to LWMA with amplitudes of a few nT over wavelengths scaling to the long-axis of the craton. The Hawai'i hotspot region displays a potential contribution from the lithospheric mantle to satellite-measured magnetic anomalies on the order of 1 nT, while the Massif Central plume regions shows an insignificant contribution to satellite magnetic anomalies from lithospheric mantle. When compared with observed lithospheric total field anomalies derived from Swarm satellite data, the results show that the lithospheric mantle may contribute a few tens of percent to LWMA observed at satellite altitudes in specific regions depending on the average remanent magnetization in mantle minerals, the thickness of magnetized mantle (which depends on the geotherm), and the spatial extent of the region under consideration.

Crustal architecture of the Thomson Orogen in Queensland inferred from potential field forward modelling

The basement rocks of the poorly understood Thomson Orogen are concealed by mid-Paleozoic to Upper Cretaceous intra-continental basins and direct information about the orogen is gleaned from sparse geological data. Constrained potential field forward modelling has been undertaken to highlight key features and resolve deeply sourced anomalies within the Thomson Orogen. The Thomson Orogen is characterised by long-wavelength and low-amplitude geophysical anomalies when compared with the northern and western Precambrian terranes of the Australian continent. Prominent NE- and NW-trending gravity anomalies reflect the fault architecture of the region. High-intensity Bouguer gravity anomalies correlate with shallow basement rocks. Bouguer gravity anomalies below –300 µm/s2 define the distribution of the Devonian Adavale Basin and associated troughs. The magnetic grid shows smooth textures, punctuated by short-wavelength, high-intensity anomalies that indicate magnetic contribution at different crustal levels. It is interpreted that meta-sedimentary basement rocks of the Thomson Orogen, intersected in several drill holes, are representative of a seismically non-reflective and non-magnetic upper basement. Short-wavelength, high-intensity magnetic source bodies and colocated negative Bouguer gravity responses are interpreted to represent shallow granitic intrusions. Long-wavelength magnetic anomalies are inferred to reflect the topography of a seismically reflective and magnetic lower basement. Potential field forward modelling indicates that the Thomson Orogen might be a single terrane. We interpret that the lower basement consists of attenuated Precambrian and mafic enriched continental crust, which differs from the oceanic crust of the Lachlan Orogen further south.

Hydrostatic pressure effect on magnetic hysteresis parameters of multidomain magnetite: Implication for crustal magnetization

Hydrostatic pressure effects on magnetic parameters for crustal rock have been poorly investigated yet, while it is important for an understanding of source of long-wavelength magnetic anomaly, which is considered to reside in deep crust. In this study we have conducted the in situ magnetic hysteresis measurements on multidomain (MD) magnetite under high pressure up to 1GPa. With special attention to hydrostatic condition and sample preparation, pressure dependences of its magnetic hysteresis parameters (saturation magnetization, Ms; saturation remanence, Mrs; coercivity, Bc; coercivity of remanence, Bcr) are revealed as follows: (1) Bc monotonically increases with pressure at a rate of +91%/GPa; (2) Ms is constant under high pressure up to 1GPa; (3) Mrs increases with pressure up to 0.5GPa by ∼30% and reaches to saturation above the pressure; (4) Bcr is nearly constant at low pressure, and it increases above ∼0.6GPa; and (5) the changes in ratios Mrs/Ms and Bcr/Bc correlate with each other, resulting in systematic movement on the Day plot. These findings allow us to estimate change in a relaxation time of magnetic remanence carried by MD magnetite as a function of depth in the continental crust. In the model calculation, we consider no effect of plastic deformation on magnetic properties of magnetite, and the relaxation time is calculated using the theoretical thermal gradient. In consequence, the relaxation time monotonously decreases with depth, and primary remanence is considered to be replaced by a viscous remanent magnetization (VRM) over the Brunhes chron. Therefore, it is suggested that MD magnetite in deep crustal rocks can contribute to the source of the anomaly over the continental crust by VRM and induced magnetization.

Eight good reasons why the uppermost mantle could be magnetic

Wasilewski et al. (1979) concluded that no magnetic remanence existed in the uppermost mantle and that even if present, such sources would be at temperatures too high to contribute to long wavelength magnetic anomalies (LWMA). However, new collections of unaltered mantle xenoliths indicate that the uppermost mantle could contain ferromagnetic minerals. 1. The analysis of some LWMA over cratons and forearcs suggest magnetic sources in the uppermost mantle. 2. The most common ferromagnetic phase in the uppermost mantle is stoichiometric magnetite. Assuming a 30. km-thick crust, and crustal and mantle geotherms of 15. °C/km and 5. °C/km, respectively, the 600. °C Curie temperature implies a 30. km-thick layer of mantle. 3. The uppermost mantle is cooler than 600. °C in Archean and Proterozoic shields (>. 350. °C), subduction zones (>. 300. °C) and old oceanic basins (>. 250. °C). 4. Recently investigated sets of unaltered mantle xenoliths contain pure magnetite inclusions in olivine and pyroxene formed in equilibrium with the host silicate. 5. The ascent of mantle xenoliths occurs in less than a day. Diffusion rates in olivine suggest that the growth of magnetite possible within this time frame cannot account for the size and distribution of magnetite particles in our samples. 6. Demagnetization of natural remanent magnetization (NRM) of unaltered mantle xenoliths unambiguously indicates only a single component acquired upon cooling at the Earth's surface. This is most easily explained as a thermoremanent magnetization acquired by pre-existing ferromagnetic minerals as xenoliths cool rapidly at the Earth's surface from magmatic temperatures, acquired during ascent. 7. Modern experimental data suggest that the wustite-magnetite oxygen buffer and the fayalite-magnetite-quartz oxygen buffer extend several tens of km within the uppermost mantle. 8. The magnetic properties of mantle xenoliths vary consistently across tectonic settings. In conclusion, the model of a uniformly non-magnetic mantle should be revisited. © 2014 Elsevier B.V.

Long-wavelength magnetic anomalies as a guide to the deep crustal composition and structure of eastern Australia

Aeromagnetic data over eastern Australia reveal a pattern of domains defined by systematic regional highs and lows, emphasised by low-pass filtering, over which are superimposed shorter (<20 km) wavelength anomalies related to mappable geology and its inferred subsurface continuation. Long-baseline levelling by Geoscience Australia has clarified the definition of these magnetic domains, and confirmed that they are not an artefact of grid merging. Geothermal and teleseismic data indicate that neither variation in Curie depth nor upper mantle magnetisation can produce the long-wavelength pattern. Hence, domain-wide variations in magnetisation at the middle to lower crustal level are presumably the cause of these long-wavelength features. Although reversed polarity remanence could contribute to deeply sourced negative magnetic anomalies, the correspondence of magnetic low domains with the Proterozoic Curnamona Craton and the Ordovician Macquarie Arc, and of a high domain with the western Lachlan Orogen floored by Cambrian ocean crust, suggests that the control may be simply stark contrasts in lower to middle crustal susceptibility. Moho thickness determined by the AusMoho model mimics the pattern of susceptibility domains, suggesting a relationship between tectonic history and mid to lower crustal composition. Implicit in this analysis is a division of the domains into continental and oceanic basement, with implications for the tectonic evolution of the Tasmanides and the distribution of mineral systems in eastern Australia. The mid to lower crust below the Macquarie Arc appears to be continental, and the Thomson Orogen is a compound feature, comprising both attenuated continental/arc crust and accreted oceanic crust.

Inversion of magnetic measurements of the CHAMP satellite over the Pannonian Basin

The Pannonian Basin is a deep intra-continental basin that formed as part of the Alpine orogeny . In order to study the nature of the crustal basement we used the long-wavelength magnetic anomalies acquired by the CHAMP satellite. The anomalies were distributed in a spherical shell, some 107,927 data recorded between January 1 and December 31 of 2008. They covered the Pannonian Basin and its vicinity. These anomaly data were interpolated into a spherical grid of 0.5° × 0.5°, at the elevation of 324 km by the Gaussian weight function. The vertical gradient of these total magnetic anomalies was also computed and mapped to the surface of a sphere at 324 km elevation. The former spherical anomaly data at 425 km altitude continued downward to 324 km. To interpret these data at the elevation of 324 km we used an inversion method. A polygonal prism forward model was used for the inversion. The minimum problem was solved numerically by the Simplex and Simulated annealing methods; a L 2 norm in the case of Gaussian distribution parameters and a L 1 norm was used in the case of Laplace distribution parameters. We interpret that the magnetic anomaly was produced by several sources and the effect of the sable magnetization of the exsolution of hemo-ilmenite minerals in the upper crustal metamorphic rocks . ► Derivation of the total magnetic anomaly field from the CHAMP data over the Pannonian Basin and its vicinity. ► It is supposed the forward model of the source of magnetic anomalies is a triangular prismatic body which has a uniform magnetization. Its coordinates are determined by the numerical solution of an inverse problem. The applied numerical methods are the Simplex and Simulated Annealing procedures. ► Magnetic anomalies are assumed to be produced by the exsolution of the hemo-ilmenite minerals in the crust.

Geophysical applications for Fe-rich emery exploration in the Elmacik area on the Menderes Massif (Turkey)

To determine the continuity of known Fe-rich emery horizons and to explore new deposits in the Elmacik area (Yatagan, Turkey), a geophysical survey was carried out using magnetic and electrical methods. Magnetic measurements were taken in the target area of 5 km2 and a vertical electrical sounding technique was applied at 15 locations in the alluvial/eluvial (A/E) part of the area in order to explore possible placer emery horizons, and to investigate the thickness of the A/E assembly and any probable faults.Significant magnetic anomalies occur in the vicinity of old and abandoned emery pits in the marbles of Mt Ismail. The anomalies in the marbles were caused by Fe-rich emery bodies, which did not crop out and were not more than 10 m deep. The magnetic anomalies in the A/E part of the area were weak in amplitude and may suggest small new placer emery deposits.The result of the vertical electrical soundings indicated two fault zones, one in a N–S direction, and the other approximately in an E–W direction. The thickness of the A/E assembly varies from 2–3 m to 60–100 m. A low resistivity zone, which is located in the mid-east of the A/E part of the study area, correlates well with the long-wavelength magnetic anomaly. According to the survey results, further exploration activities should take place around abandoned emery pits.

Magnetic properties of rocks of the Kapuskasing uplift (Ontario, Canada) and origin of long-wavelength magnetic anomalies

SUMMARY The sources of long wavelength magnetic anomalies (LWA) in the crust are poorly understood. We have measured remanent and induced magnetizations of 210 samples of anorthosite, tonalite and mafic gneiss from the Kapuskasing uplift, an exposed partial crustal crosssection in northwestern Ontario, Canada. Anorthosites have generally high Q ratios of remanent/induced magnetization, in the range 0.3–60, and their natural remanent magnetization (NRM) is resistant to both thermal and alternating-field (AF) demagnetization. However, anorthositeNRMs(0.001–0.3Am −1 )aretooweaktoexplainLWAamplitudes.Maficgneisses also have relatively high Q ratios, peaking in the range 1–10, and the NRM is resistant to thermal and AF demagnetization. NRM and induced magnetization (IM) intensities are in the ranges 0.01–2 and 0.01–0.6 A m −1 , respectively. Tonalites have a bimodal distribution of magnetization. The more strongly magnetic group has both NRM and IM intensities in the range 0.1–5 A m −1 and wide-ranging Q values, from 0.1 to 10 approximately. Some tonalites could be an LWA source, although the long-term stability of their NRMs at high temperature in the crust is questionable because unblocking temperatures are broadly distributed from 100 to 600 ◦ C. In general, Q values measured at surface temperatures overestimate remanence at depth. In the deep crust, IM remains more or less constant but remanence decreases both reversibly and irreversibly, leading to Q ratios of <0.2–0.3 for multidomain grains and ≈1–3 for single-domain grains near their blocking temperatures. Thermoviscous magnetization over the Brunhes chron could add substantially to the effective induced magnetization. Typically induced + thermoviscous magnetization in the direction of the present Earth’s field will outweigh remanence in the direction of an ancient field as a source of LWA originating in the deepest crust. Remanence may play a larger role for mid-crustal sources where single-domain grains are well below their blocking temperatures.

Gravity and magnetic modelling of crustal structure in central Victoria: what lies under the Melbourne Zone?

Three gravity and magnetic profiles through the central and eastern part of the Melbourne Zone, Lachlan Fold Belt, have been modelled, with tight control from surface geology to investigate the deep crust. The largely non-magnetic upper crust (folded Ordovician to Lower Devonian strata of the Melbourne Zone intruded by Devonian granite plutons) has been modelled using recently acquired density measurements, providing a good match with the gravity profiles. The top of the basement is modelled at ∼10–15 km depth in the centre of the zone, and ∼8–10 km depth in the south of the zone, reflecting a thinning of the folded cover sequence toward the south, consistent with outcrops of basement Cambrian rocks on the south coast of Victoria. Long-wavelength magnetic anomalies in the Melbourne Zone are modelled as highly magnetic tabular bodies lying in the basement below the folded strata of the Melbourne Zone. The only rock type that matches the density and magnetic susceptibility is serpentinised ultramafic rock. Thus, a large ophiolitic body is thought to exist in the upper part of the basement to the Melbourne Zone, at depths of 15–25 km in the central part of the zone, and 10–15 km in the south of the zone. This unit is correlated with the Cambrian mafic–ultramafic complex of Tasmania, agreeing with previous correlations of the Selwyn Block with Tasmania.

Anomaly crust fields from MAGSAT satellite measurements:their processing and interpretation

The space distribution of the magnetic anomaly field for the Pacific Ocean is obtained from data of the satellite MAGSAT. A number of long-wavelength magnetic anomalies of the region are identified. A spectrum analysis of a number of profiles of the anomaly field is performed disclosing typical scales of such anomalies. The wave transform of the anomaly magnetic profiles reveals and explicitly exposes the structure of the considered profile. A schematic complex cross-section is constructed, which demonstrates that the satellite data may be used in the study of the magnetic anomaly.

Weakly magnetic crust in the Canadian Cordillera

Current models of continental crust favor an increase in magnetization with depth. Here we report a counter example from the Canadian Cordillera where almost a full thickness of non-magnetic continental crust is suggested by joint interpretation of magnetic and seismic data. The magnetic field over the Cordillera is characterized by complex, short-wavelength (< 100 km) anomalies associated with intrusive, metamorphic and volcanic rocks that occur at shallow depths (< 5 km) within accreted terranes. The long-wavelength (> 100 km) portion of the Cordilleran field is subdued and mainly featureless, and suggests a lack of magnetic sources at greater depths. Seismic reflection and refraction data from three major transects in the Yukon and British Columbia, Canada support this interpretation and indicate that sedimentary-like formations make up the majority of the crust. The dominance of shallow, upper crustal magnetization in the Canadian Cordillera contrasts with the generally-held view that the lower continental crust is the primary source for long-wavelength magnetic anomalies. Sources for these anomalies are often assumed to be located in the lower crust when surface magnetizations are insufficient to produce such anomalies or no correlation exists between the magnetic field and the mapped surface geology. The Canadian Cordillera appears to be an example of a non-magnetic lower crust overlain by a more magnetic upper crust that is, however, not magnetized strongly enough to produce significant long-wavelength magnetic anomalies.

Geophysical evidence for wedging in the San Gorgonio Pass structural knot, southern San Andreas fault zone, southern California

Geophysical data and surface geology define intertonguing thrust wedges that form the upper crust in the San Gorgonio Pass region. This picture serves as the basis for inferring past fault movements within the San Andreas system, which are fundamental to understanding the tectonic evolution of the San Gorgonio Pass region. Interpretation of gravity data indicates that sedimentary rocks have been thrust at least 5 km in the central part of San Gorgonio Pass beneath basement rocks of the southeast San Bernardino Mountains. Subtle, long-wavelength magnetic anomalies indicate that a magnetic body extends in the subsurface north of San Gorgonio Pass and south under Peninsular Ranges basement, and has a southern edge that is roughly parallel to, but 5–6 km south of, the surface trace of the Banning fault. This deep magnetic body is composed either of upper-plate rocks of San Gabriel Mountains basement or rocks of San Bernardino Mountains basement or both. We suggest that transpression across the San Gorgonio Pass region drove a wedge of Peninsular Ranges basement and its overlying sedimentary cover northward into the San Bernardino Mountains during the Neogene, offsetting the Banning fault at shallow depth. Average rates of convergence implied by this offset are broadly consistent with estimates of convergence from other geologic and geodetic data. Seismicity suggests a deeper detachment surface beneath the deep magnetic body. This interpretation suggests that the fault mapped at the surface evolved not only in map but also in cross-sectional view. Given the multilayered nature of deformation, it is unlikely that the San Andreas fault will rupture cleanly through the complex structures in San Gorgonio Pass.

Subduction-zone magnetic anomalies and implications for hydrated forearc mantle

Research Article| June 01, 2005 Subduction-zone magnetic anomalies and implications for hydrated forearc mantle Richard J. Blakely; Richard J. Blakely 1U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025, USA Search for other works by this author on: GSW Google Scholar Thomas M. Brocher; Thomas M. Brocher 1U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025, USA Search for other works by this author on: GSW Google Scholar Ray E. Wells Ray E. Wells 1U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025, USA Search for other works by this author on: GSW Google Scholar Author and Article Information Richard J. Blakely 1U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025, USA Thomas M. Brocher 1U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025, USA Ray E. Wells 1U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025, USA Publisher: Geological Society of America Received: 07 Dec 2004 Revision Received: 22 Feb 2005 Accepted: 01 Mar 2005 First Online: 03 Mar 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 The Geological Society of America, Inc. Geology (2005) 33 (6): 445–448. https://doi.org/10.1130/G21447.1 Article history Received: 07 Dec 2004 Revision Received: 22 Feb 2005 Accepted: 01 Mar 2005 First Online: 03 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Richard J. Blakely, Thomas M. Brocher, Ray E. Wells; Subduction-zone magnetic anomalies and implications for hydrated forearc mantle. Geology 2005;; 33 (6): 445–448. doi: https://doi.org/10.1130/G21447.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Continental mantle in subduction zones is hydrated by release of water from the underlying oceanic plate. Magnetite is a significant byproduct of mantle hydration, and forearc mantle, cooled by subduction, should contribute to long-wavelength magnetic anomalies above subduction zones. We test this hypothesis with a quantitative model of the Cascadia convergent margin, based on gravity and aeromagnetic anomalies and constrained by seismic velocities, and find that hydrated mantle explains an important disparity in potential-field anomalies of Cascadia. A comparison with aeromagnetic data, thermal models, and earthquakes of Cascadia, Japan, and southern Alaska suggests that magnetic mantle may be common in forearc settings and thus magnetic anomalies may be useful in mapping hydrated mantle in convergent margins worldwide. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

New enhancement filters for geological mapping

Two types of filters have been developed for the purpose of enhancing weak magnetic anomalies from near-surface sources while simultaneously enhancing low-amplitude, long-wavelength magnetic anomalies from deep-seated or regional sources. The Edge filter group highlights edges surrounding both shallow and deeper magnetic sources. The results are used to infer the location of the boundaries of magnetised lithologies. The Block filter group has the effect of transforming the data into ``zones' which, similar to image classification systems, segregate anomalous zones into apparent lithological categories. Both filter groups change the textural character of a dataset and thereby facilitate interpretation of geological structures. The effect of each filter is demonstrated using theoretical model studies. The models include both shallow and deep sources with a range of magnetisations. Comparative studies are made with traditional filters using the same theoretical models. In order to simulate real conditions, Gaussian noise has been added to the model response. Techniques for noise reduction and geological signature enhancement are discussed in the paper. The new approaches are applied to actual magnetic survey data covering part of the Goulburn 1:100 000 scale map sheet area, New South Wales. Some new geological inferences revealed by this process are discussed.

Potential field modelling of the Baltica–Avalonia (Thor–Tornquist) suture beneath the southern North Sea

Abstract Magnetic anomaly maps of the Trans-European Suture Zone (TESZ) highlight the contrast between the highly magnetic crust of Baltica and the less magnetic terranes to the SW of the suture. Although the TESZ is imaged on gravity maps, anomalies related to postcollisional rifting and reactivated rift structures tend to dominate. Seismic and potential field data have been used to construct 2 1 2 -D crustal models along three profiles crossing the Baltica–Avalonia suture in the southern North Sea (SNS). The first of these models lies along a transect assembled from reflection line GECO SNST 83-07 and refraction profile EUGENO-S 2; the other two models are coincident with MONA LISA profiles 1 and 2. Additional structural information and density information for the cover sequence is available from released wells, while magnetic susceptibility values are compatible with values measured from borehole core samples. Magnetic anomalies related to the suture are interpreted as due to magnetic Baltican basement of the Ringkobing-Fyn High dipping SW beneath nonmagnetic Avalonian basement underlying the western part of the SNS. Low-amplitude, long-wavelength magnetic anomalies occurring outboard of the suture are interpreted as due to a mid-crustal magnetic body, possibly a buried magmatic complex. This might represent the ‘missing’ arc related to inferred southward subduction of the Tornquist Sea, or an exotic element emplaced during the collision between Avalonia and Baltica. The present model supports an imbricated structure within Baltica as indicated by the latest reprocessing of the MONA LISA seismic data.

Internal structure of Tenerife (Canary Islands) based on gravity, aeromagnetic and volcanological data

Gravity and magnetic methods have been applied to the Tenerife Island, to provide new information about its internal structure. For this study, 365 gravity stations covering the central part of the island have been selected. The anomalous density maps at different depths were obtained by means of an inversion global adjustment, on fixed density contrast, to describe the three-dimensional (3D) geometry of the anomalous bodies. On the other hand, several analysis techniques, such as reduction to the pole, spectral analysis, low-pass filtering, terrain correction and forward modelling, were applied to process the high-resolution data obtained in an aeromagnetic survey, completed with marine and terrestrial data.The joint analysis of gravity and magnetic anomalies has shown tectonic and volcanic features that define some fundamental aspects of the structural framework and volcanic evolution of the island. A strong gravity anomaly produced by a large and deep source has been associated with an uplifted block of the basement beneath the southern part of Tenerife. The sources of the observed gravity highs from 8km b.s.l. may be associated with the growth of the submarine shield stage that was clearly controlled by regional tectonic.The long-wavelength magnetic anomalies reveal highly magnetic sources, interpreted as gabbro-ultramafic cumulates associated with the root zone of a large dyke swarm. This intrusive body could be topped by the emplacement zone of magma chambers that correlate with a magnetic horizon at 5.7±0.8km depth. Rooted in this highly magnetic zone, two dike–like structures can be associated with the magmatic feeding system of large recent basaltic volcanoes. A shallow magnetic horizon (1.4km a.s.l.) can be correlated with the bottom phonolites of the Las Cañadas Edifice.In the central part of the island the coincidence of some gravity and magnetic lows is consistent with the presence of low-density and low-magnetic materials, that infill a collapsed caldera system. The structures close to the surface are characterised by low-density areas connected with the recent volcanism, in particular the minimum over the Teide volcano. Hydrothermal alteration is assumed to be the cause of a short-wavelength magnetic low over the Teide volcano.