Large Magnetic Anomalies Research Articles

Formation of Australasian tektites from gravity and magnetic indicators

The parent impact crater of Australasian tektites has not been discovered so far, but a consensus has been accepted on its location in a wider area of Indochina. Recently, an alternative location has been suggested in the Badain Jaran Desert (BJD), Northwest China. Employing gravity and magnetic data derived from satellites, possible presence of an impact structure in BJD is investigated. The gravity parameters include the free air gravity disturbance, its vertical derivative component and total horizontal gradient (THG), strike alignment (SA), and Bouguer anomaly with its first vertical derivative and tilt angle. The magnetic parameters include the anomalous total magnetic field (TMF), its reduced to the pole transformation (RTP), the first vertical derivative of the TMF vertical component (Bzz), tilt angle (TA), and logistic total horizontal gradient (LTHG). Both the gravity and magnetic indicators support the presence of the impact structure. Gravity parameters display typical annular gravity highs circumscribing a gravity low. SA analysis reveals preferred parallel directions, implying the susceptibility of special zones to the impact shock waves, both within and beyond the rim. TMF reveals a large magnetic anomaly in the southern part of the proposed crater, and RTP displaces and restricts it further into the rim. Bzz weakens the long wavelength anomalies, amplifies the superficial ones, and separates them horizontally. TA and LTHG delineate the deep-seated and shallow magnetic signals related to the peak and border magnetization, respectively.

Modeling and interpretation of geomagnetic data related to geothermal sources, Northwest of Delijan

Several hot springs and the geology of Northwest of Delijan, Iran, suggest that there is a large geothermal system beneath the area. In 2021, magnetometry methods were used to determine the geothermal structures causing magnetic anomalies of the 300 km2 area surrounding Delijan hot springs. In this research the data are corrected and processed to identify the structural characteristics of the geothermal system in the region and the depth of the geothermal source using the resulting magnetic anomaly maps. After detecting the primary anomalies, the depth and the structural index of the region are estimated using the Euler method. The main source of primary magnetic anomalies in the region is a large pseudo-conical magnetic mass, more the 2000 m deep. The forward model and 3D inversion of data are conducted using ModelVision software and Mag3D computer code, based on the results of the Euler method. The Mag3D is based on the Li-Oldenburg algorithm, by which the best possible solution for magnetic masses is iteratively achieved. The results of modeling and inversion revealed that there is a large magnetic anomaly at 2500–5000 below the surface that is interpreted as a geothermal system source. The results of the current study are in good agreement with the geological information of the region.

Inversion of Magnetic Measurements of the Swarm A Satellite of the Bangui Magnetic Anomaly

We wanted to make a satellite altitude magnetic anomaly map of the large magnetic anomaly in the Central African Republic, the Bangui magnetic anomaly, with data from the Swarm satellites. In the first part of our study, we summarize the earlier investigations and their interpretation. In the second we discuss our data processing applied to produce a magnetic anomaly map. We used the IGRF

Geophysical imaging of multiple volcanic structures at the Huichapan Caldera Complex, Mexico

Detailed processing and analyses of aeromagnetic data supported by geological constraints shed new light on the structural framework and plumbing system of Huichapan Caldera and Nopala volcano in the northern sector of the Mexican Volcanic Belt. The interpretation of magnetic anomalies is complex due to the bipolar nature of the magnetic field so we applied reduction to the pole using a double fast Fourier transform to convert the magnetic anomalies into simplified patterns. Short optimal continuous analytical field filters enhanced subsurface and surface structures. The reduction to pole analysis filtered for the upward/downward continuation of potential field data improved both regional and detailed imaging since it was applied to different depths. Shallow subsurface structures as well as at depths of 1.25 km and 0.65 km, were correlated with the surface and regional features.The Huichapan Caldera has at least two contrasting ignimbrites. Results show that there are two large magnetic anomalies associated with different volcanic structures. The first one, a circular positive anomaly corresponds to the caldera rim and the other to the structure of the younger Nopala volcano to the west. Strong magnetic sources indicate the feeder dikes. This study provides geophysical evidence for the location of the head wall of the Huichapan caldera as well as a second rim on the south but E-W and NE-SW regional faulting has destroyed this older caldera on the north. Results show that the structures are tectonically controlled and that the caldera is open on the east and southwest. The trap door structure was associated with this regional fault.

Internal structure of the buried Suwałki Anorthosite Massif (East European Craton, NE Poland) based on borehole, magnetic and gravity data combined with new petrological results

Advanced magnetic and gravity data analysis has been used to acquire geophysical constraints providing new insights into the geological structure of the Suwalki Anorthosite Massif (SAM). The large negative magnetic anomaly of the SAM anorthosite intrusion is a result of the negative inclination of remanent magnetization, directed antiparallel to the present Earth’s magnetic field. Several filtering processes were applied to the magnetic and gravity maps to better understand the subsurface geology of the SAM area. The geological analysis of residual magnetic and gravity anomaly maps reveals the presence of different rock units, reflecting variation in petrological composition of the crystalline basement rocks. The 2-D modelling of magnetic and gravity data delineate the location and extent of the anorthosite-norite massif. The data is consistent with a thick upper crustal body with density 2690 kg/m3, low susceptibility (0.005 SI) and natural remanent magnetization (1.95 A/m), having inclination of I = –68°, and declination of D = –177°. The rocks bordering the central anorthosite body consist of norite and gabbronorite, granodiorite, diorite and charnockite. These main crystalline basement crustal units are shown more precisely on a new geological map of the SAM.

Marea/Northern Hawwariya in northern Egypt: Integrated results of non‐invasive and excavation works

AbstractMarea/Northern Hawwariya is one of the largest and most important archaeological sites dating between the 3rd century BC and the 8th century AD located at Lake Mareotis in northern Egypt. Even though excavations have been carried out since the end of the 1970s, the settlement is still largely unexplored. To rectify this situation, non‐invasive research has been carried out on 21.5 ha of the site using a precise positioning system and magnetometer measurements. Exact topographic and magnetic maps were prepared based on the data that were obtained. Simultaneously, verification research were carried out, including archaeological excavations, registering surface findings and satellite image analysis. In the south‐eastern part of the site, electrical resistivity measurements were taken as well. This approach allowed for the identification of the densely built areas of the Byzantine and Early Islamic town. On the south‐western side, some magnetic anomalies were registered, following regular patterns with clear linear borders and surrounded by wetlands without clear signs of habitation. On the northern side, the settlement was limited by an artificial coastline, forming straight waterfronts made of stone blocks. As for the town itself, a 260‐m street with adjacent regular built‐up areas was discovered. This regularity points to an order of planning, at least for the Byzantine part of the settlement. Recording of large amplitude magnetic anomalies proves the presence of heavily burnt objects. They may be interpreted as remains of kilns, perhaps used to manufacture amphorae, glass vessels and lime. Electrical resistivity measurements in the south‐eastern part of the site localized cavities in the natural limestone rock uplift. These may have been rock‐cut tombs. Multidisciplinary, non‐invasive research in Marea/Northern Hawwariya allowed for the quick generation of new knowledge on the topography of the site, significantly enriching information on its form and the functions of its various parts.

Magnetic Survey for Iron-Oxide-Copper-Gold (IOCG) and Alkali Calcic Alteration Signatures in Gadarwara, M.P, India: Implications on Copper Metallogeny

A government airborne geophysical survey flown in the late 1970s detected a large Magnetic anomaly at Gadarwara, Madhya Pradesh, in north-central India. Deep drilling indicates that the oval-shaped Magnetic anomaly is caused by underlying Magnetite-bearing banded iron formation belonging to the Mahakoshal Formation of Archean to Early Proterozoic age. The anomaly is hosted in a tectonic rift zone (Narmada-Son Lineament). After drilling alluvium up to 312 m thick, rocks intersected to depths of 612 m provided core samples for research. Broadly speaking, the samples contain banded hematite jaspilite (BHJ) and banded Magnetite (BM) iron formation with pervasive carbonate alterations. Three vertical diamond drill holes were drilled along a 1.4 km long N-S transect across the center of the geophysical anomaly. DDH-1, near the northern edge of the anomaly, went through 309 m of alluvium before intersecting bedrock and then cored 303 m of bedrock for a total depth of 612 m. Copper mineralization with appreciable amounts of cobalt, zinc, molybdenum, silver, rare earth elements, uranium and other elements was intersected. The litho-units are highly oxidised and intensely brecciated with hydrothermal overprinting of Na-K metasomatism alteration mineralogy. The second borehole, DDH-2 failed as the core drilling bit stuck in the alluvium and further drilling was abandoned, whereas the third borehole DDH-3 didnot intersect a Magnetite-hematite association and cored only siltstone. Two-dimensional model studies suggest that the signature of high Magnetic anomaly is at a depth of 0.4 km from the surface, with a width of 3.5 km, dipping at 45∘ in a northerly direction. The causative body has a Magnetic susceptibility of 0.0052 C.G.S. units, suggestive of a hematite with quartz veinlets lithology. Based on predictive Magnetic exploration models for Iron-Oxide-Copper-Gold (IOCG), such deposits can be inferred from geological observations combined with petrophysical data and forward modelling of the observed Magnetic signatures. This paper reports a prospective IOCG-like mineralization style hosted in a rift (Narmada-Son) type of tectonic environment.

Two-phase magnetohydrodynamics: Theory and applications to planetesimal cores

Core freezing and resultant compositional convection are likely important drivers for dynamo activity in large terrestrial bodies like the Earth. The solidification of compositional mixtures, such as iron and sulfur, generates mush zones of partial melt at the freezing front, which can eject chemically buoyant or heavy liquid that then drives convection. For smaller bodies such as planetesimals in the asteroid belt, conditions for generating a dynamo are harder to achieve. Nevertheless, evidence for magnetization of achondrite meteorites is abundant, suggesting that many planetesimal cores were somehow magnetized. As such small bodies cool rapidly under low gravity they likely spend much of their evolution with a large poorly compacted partial melt mushy zone. The magneto-hydrodynamic behavior of a deformable partial melt zone can induce magnetism via separation of solid and liquid phases, and conversely magnetism can impose extra forces on the phase separation in the mush zone. To this end, we have developed a new two-phase magneto-hydrodynamic theory for deformable mushes and slurries. The model includes the standard effects of Lorentz forces, and the competition between magnetic field stretching and diffusion. There are additional effects at the liquid pore or solid grain scale, which involve the interaction between phases, akin to Darcy or Stokes drag; these include Lorentz drag, as well as pore/grain-scale diffusive exchange of magnetism between solid and fluid phases, and field stretching due to relative motion between phases. Magnetic induction by gravitational phase separation is most significant after extensive mixing of liquid and solid phases, such as induced by vigorous mechanical stirring due to, for example, tidal and elliptic instabilities, or impacts with other bodies, all of which are conceivably common in the asteroid belt, especially in the early solar system. Gravitational phase separation following such events can induce significant magnetism in the liquid and solid phases, and much more rapidly than can magnetization by large-scale circulation. Magnetic field variances can be at first orders of magnitude larger than an imposed background field, during initial gravitational phase separation of the well-mixed slurry. As the phases separate toward the top or bottom, the solid phase compacts, the separation velocity decreases, and the magnetic field variance likewise diminishes. However, solitary waves in the compacting region can cause an additional large magnetic induction in the liquid, taking the form of strongly magnetized wave packets that can be trapped in the solid. Thus, phase separation, segregation and compaction potentially induce large magnetic field anomalies. A linear stability analysis for convection in a porous medium (with a rigid matrix) is also explored. Pore and grain scale effects, such as field stretching due to phase separation, are found to enhance the influence of the magnetic field on convective instability.

Constrained 3D modelling and geochemical analyses of the Horseshoe Range BIF: tools for evaluating magnetic signatures under cover

The Horseshoe Range is a banded iron formation (BIF) in the Southern Capricorn Orogen, WA and is associated with a large horseshoe-shaped positive magnetic anomaly. Electron microscope mineralogy identified ubiquitous goethite and magnetite/hematite. This study focussed on measuring the magnetic properties of the rocks at Horseshoe Range in order to accurately predict their geophysical responses when buried beneath cover. Remanent magnetisation intensities of the rocks were high (up to 1300 A/m) and vectors measured in the rocks were oriented predominately downward which typically result in negative anomalies which is inconsistent with the observed anomaly. Due to the positive nature of the magnetic anomaly and the ability to accurately model the response without remanent magnetisation it appears that the high intensity remanent magnetisations may be volumetrically insignificant and likely limited to the near surface. The remanence may be caused by near surface formation of maghemite during bushfires and/or induced by lightning strikes. The BIF can be modelled using a single homogenous layer with a susceptibility of 0.8 SI. However, this is not geologically consistent with BIFs which typically display variable iron-oxide mineralogy and associated petrophysical properties. One way to more accurately model BIFs is to use the first vertical derivative as the model input. Using this approach, a 4 layer model was generated which matched the anomaly to an RMS of ~1%. Modelled susceptibilities ranged from 0.01 – 0.55 SI which are consistent with the measured properties. However, this model did not take into account the measured high intensity downward magnetisation vectors.

Modeling of the Foca-Uzunada magnetic anomaly and thermal structure in the gulf of Izmir, western Turkey

The Gulf of Izmir (GoI) is one of the largest gulfs in the Aegean Sea, Turkey. There is a large magnetic anomaly extending in the NE-SW direction between Foca and Uzunada (Uzun Island) in the gulf. Previously, Curie Point Depth (CPD), geothermal gradient, heat-flow and radiogenic heat production maps of the onshore part of the Aegean region were constructed from the aeromagnetic data. In this study, the same maps except radiogenic heat production map are presented for the offshore part and the largest magnetic anomaly in the northern part of the gulf is focused, particularly. As a result, the thermal structure of GoI is clearly defined and according to the results of this study, CPD values were found from 7 km in the NE of Foca to 10 km through the south of the gulf. The geothermal gradient values vary between 50 and 80 °C/km. Maximum heat flow values around the anomaly are calculated as 200 and 215 mW/m2 according to the thermal conductivity coefficients of 2.5 W m−1 K−1 and 2.7 W m−1 K−1, respectively. Although the anomaly is located in the Izmir Gulf; CPD, geothermic gradient, heat flow anomalies are shifted through the north of Foca and Aliaga towns in the Candarli Bay. This prominent anomaly in the Gulf of Izmir is associated with the magmatics that were encountered at 969 m in the Foca-1 well although it was drilled about 2 km away from the outermost closed contour of the magnetic anomaly. The anomaly is also modeled three dimensionally (3D) in this study. In the model map, the top of the causative body is completely located in the outer part of the gulf, and is very shallow at about 0.5 km while its bottom is inclined through the west of Cigli and Menemen. From this viewpoint, it is possible to suggest that the causative body is inclined through the Foca Peninsula. However, its closed contours are in the NE direction, through the Candarli Bay. Top depth of the causative body is also calculated from the basement horizon on the seismic sections crossing this anomaly. Depth calculations are consistent in these sections and confirm the top depths from the modeling study. The basement geometry in the seismic sections also reflects the shape of 3D model geometry, and bottom depth of the magmatics is also verified by the basement depth calculations in seismic sections.

Tectonics and thermal structure in the Gulf of Iskenderun (southern Turkey) from the aeromagnetic, borehole and seismic data

The Gulf of Iskenderun is one of the largest gulfs in the Eastern Mediterranean Sea. Particularly, it is the easternmost and third largest gulf in the southern Anatolian shoreline of Turkey. It contains the Miocene Iskenderun Basin surrounded by the Misis-Andirin uplift to the west, Hatay-Kizildag ophiolites and Dead Sea Fault to the east. There are two large magnetic anomalies within the basin. The largest one extends in the NE-SW direction, almost parallel to the shoreline of the gulf, and the smaller one is located in the far offshore having a relatively circular shape. In this study, relationship between the ophiolites, basalts (and other basement units) and the magnetic anomalies are investigated by constructing 2D models after the depth estimation from the spectral analysis. Additionally, thermal structure of the gulf and vicinity is also investigated by calculating the Curie Point Depth (CPD) and by constructing the geothermal gradient and heat flow maps from the magnetic anomaly data. CPDs are in the range of 8–13km where the shallow CPDs are represented by closed contours of 8km around the town of Karatas and deeper ones (13km) are located to the NE of Yumurtalik, close to the NE edge of shoreline. Another area indicating deep CPDs (13km) is located between Adana and Kozan towns. However, the area from Kozan through the northeast is a large, shallow CPD region. Trends in the gradient and heat flow maps are also consistent with previous heat flow map in literature obtained from the hot springs and borehole temperatures. Only the large magnetic anomaly in the inner part of the gulf is modeled in this study, because it is completely located in the gulf. There are seismic lines crossing this anomaly and there is a wellbore that penetrated causative body. The borehole and seismic data were used to correlate the models and spectral analysis results, and a reasonable consistency was observed. The Birten-1 well was drilled on the eastern flank of the causative body creating this prominent anomaly. Ophiolites and the main causative body (basalt and peridotite alternation) were encountered at 1306m and 1454m, respectively in the wellbore. These units are found at 1260m and 1640m in the 2D model of the profile P-Isk-A. Similarly, they are observed at 1248m and 1596m in the 2D model of the profile P-Isk-B. Top depths of the main body in 2D models (809m in the P-Isk-A and 857m in the P-Isk-B) are also consistent with the depth from the power spectrum (890m).

Geology and tectonics of the northeast Russian Arctic region, based on seismic data

The structure of the sedimentary cover and acoustic basement in the northeastern Russian Arctic region is analyzed. Beneath the western continuation of the North Chukchi trough and Vil’kitskii trough, a Late Caledonian (Ellesmere) folded and metamorphozed basement is discovered. It is supposed that Caledonides continue further into the Podvodnikov Basin until the Geofizikov branch. A large magnetic anomaly in the Central Arctic zone has been verified by seismostratigraphic data: the acoustic basement beneath the Mendeleev (and partially Alpha) Ridge is overlain by trapps. Wave field analysis showed that the acoustic basement of the Lomonosov Ridge has folded structure, whereas beneath the Mendeleev Ridge, the sporadic presence of a weakly folded stratum of Paleozoic platform deposits is interpreted. It is supposed that the Caledonian and Late Cimmerian fold belts in the periphery of the Arctida paleocontinent appeared as a result of collision between arctic continental masses and southern ones. After Miocene extension and block displacements identified from appearance of horsts, grabens, and transverse rises both on the shelf and in the ocean, a general subsidence took place and the present-day shelf, slope, and the deepwater part of the Arctic Ocean formed.

A volcanic province near the western termination of the Charlie-Gibbs Fracture Zone at the rifted margin, offshore northeast Newfoundland

A mid-Cretaceous to Late Cretaceous volcanic province, named here the Charlie-Gibbs Volcanic Province, is described near the western termination of the Charlie-Gibbs Fracture Zone, against the rifted continental margin northeast of Newfoundland. We used seismic data to map 14 volcanic seamounts, now buried below younger sediments. They rise 0.7 to 2 s two-way time (twt) above the surrounding basement level and are about 8–30 km wide. Some are conical while others are more flat-topped. Underlying igneous units resembling flows and sills are also observed. Based on magnetic modeling of the large positive magnetic anomalies associated with the seamounts, the total thickness of igneous rocks can locally reach about 8 km. This magmatism occurred in the vicinity of the Charlie-Gibbs Fracture Zone and extends about 150 km to the north along the rifted continental margin. The volcanic province also forms the northern boundary of the Jurassic-Early Cretaceous Orphan Basin, along a major transform margin there. Truncation of rift-related structures which extend to deep crustal levels is observed at the transform, along trends similar to those of prerift Appalachian terrane boundaries on the adjacent shelf. This suggests the existence of a preexisting weak zone in the continental lithosphere within which a complex strike-slip fault system developed and may have controlled the location of final continental breakup between the Rockall and North American plates in the Late Cretaceous.

New insights into regional tectonics of the Sunda–Banda Arcs region from integrated magnetic and gravity modelling

The Lombok Island (Indonesia), situated along the Sunda–Banda Arcs transition region, lies between zones characterized by very large intensity magnetic anomalies. These anomalies have been noted as being part of the eight most important ones seen on the 1st edition of the World Digital Magnetic Anomaly Map. Unfortunately, only marine magnetic data of surroundings are available, and no regional high-resolution magnetic surveys of this island had been done. Since 2004, we have carried out several geomagnetic ground surveys in this region. Here, we present modelling results for a new geological and tectonic interpretation of the magnetic anomalies. The new results confirm the general pattern of contiguous negative–positive anomalies, revealing two active Quaternary normal faults and a magmatic arc related to a subduction region. This outcome agrees with earlier results in surrounding areas obtained by satellite, aeromagnetic, and marine platforms, and provides a more detailed picture of strong anomalies on this island. Considering both magnetic anomalies and gravity models, it is possible to suggest an extension of the Flores Thrust zone (reaching the northwest off the Lombok Island). The Flores Thrust zone may be considered as a mature subduction in the back arc region, showing a tendency of progressive subduction during last decades.

Geophysical signatures over and around the northern segment of the 85°E Ridge, Mahanadi offshore, Eastern Continental Margin of India: Tectonic implications

The nature and origin of the subsurface 85°E Ridge in the Bay of Bengal has remained enigmatic till date despite several theories proposed by earlier researchers. We reinterpreted the recently acquired high quality multichannel seismic reflection data over the northern segment of the ridge that traverses through the Mahanadi offshore, Eastern Continental Margin of India and mapped the ridge boundary and its northward continuity. The ridge is characterized by complex topography, multilayer composition, intrusive bodies and discrete nature of underlying crust. The ridge is associated with large amplitude negative magnetic and gravity anomalies. The negative gravity response across the ridge is probably due to emplacement of relatively low density material as well as ∼2–3km flexure of the Moho. The observed broad shelf margin basin gravity anomaly in the northern Mahanadi offshore is due to the amalgamation of the 85°E Ridge material with that of continental and oceanic crust. The negative magnetic anomaly signature over the ridge indicates its evolution in the southern hemisphere when the Earth’s magnetic field was normally polarized. The presence of ∼5s TWT thick sediments over the acoustic basement west of the ridge indicates that the underlying crust is relatively old, Early Cretaceous age.The present study indicates that the probable palaeo-location of Elan Bank is not between the Krishna–Godavari and Mahanadi offshores, but north of Mahanadi. Further, the study suggests that the northern segment of the 85°E Ridge may have emplaced along a pseudo fault during the Mid Cretaceous due to Kerguelen mantle plume activity. The shallow basement east of the ridge may have formed due to the later movement of the microcontinents Elan Bank and Southern Kerguelen Plateau along with the Antarctica plate.

Gravity response of igneous rocks in the northwestern part of the Bohemian Massif

A new cross-border gravity map on the scale of 1:200,000 covering 14,900 km2 of the SE Saxony and NW Bohemia was compiled. It is limited by the sites of Grimma (NW), Karlovy Vary (SW), Neratovice (SE) and Bautzen (NE). Three positive gravity regions – (a) Lusatian Anticline, (b) SE part of the North Saxon Syncline and (c) Tepla–Barrandian Unit were delimited. Separation of the Bouguer anomalies into the regional and residual components together with the Linsser filtering provided three types of derived gravity maps (regional, residual and density boundaries) for geological interpretation. Eighteen negative residual anomalies mostly pertaining to partially buried granite or acid volcanic bodies and ten positive residual anomalies mostly caused by metamorphic complexes were identified. The map of the Linsser indications showing the density boundaries at three depth levels (1, 3 and 6 km) introduces not only numerous disjunctions but also indicates an internal structure of the individual regions. A new cross-border magnetic map covering the same area is also presented. A “central” circular gravity low (−61 mGal) delineates the Altenberg–Teplice Caldera extending to 10 km depth. Variscan igneous bodies produce only negative gravity anomalies regardless their size. Pre-Variscan igneous bodies cause either weak negative or positive anomalies. A chain of gravity and magnetic anomalies follows the Litoměřice Deep Fault and a large pronounced magnetic anomaly between Doupov volcanic complex (SW) and the Elbe Zone (NE) delineates the Saxothuringian/Tepla–Barrandian Suture Zone.

A POSSIBLE MESOARCHAEAN IMPACT STRUCTURE AT SETLAGOLE, NORTH WEST PROVINCE, SOUTH AFRICA: AEROMAGNETIC AND FIELD EVIDENCE

A 25 to 30 km wide magnetic anomaly within the >2.79 Ga granite-greenstone rocks of the northwestern Kaapvaal Craton is spatially associated with megabreccia outcrops near the village of Setlagole in the North West Province, South Africa. The breccia comprises angular to rounded clasts of TTG gneisses, granites and granodiorites, with lesser amounts of amphibolite, calc-silicate rock and banded iron-formation as well as unusual dark grey to black, irregular, centimetre- to decimetre-sized clasts that show evidence of fluidal behaviour and plastic deformation during incorporation into the breccia. The largest clasts reach up to several metres in size. Evidence of fluvial transport is found in rare thin sandy to gritty layers that show crude bedding and upward-fining with layers dipping gently to the northeast. The breccia matrix is highly variable but is dominated by angular mineral clasts (mainly quartz and feldspar, with subsidiary biotite, amphibole and epidote) with interstitial chlorite. The clasts show variable amounts of alteration (saussuritization, sericitization, chloritization of biotite and amphibole). The dark clasts contain angular quartz and feldspar and small biotite fragments in a cryptocrystalline chlorite-dominant matrix. Textures indicate a lower greenschist facies metamorphic overprint. The absence of lava, dolomite or quartzite clasts suggests that the breccia formed prior to the deposition of the Neoarchaean Ventersdorp and Eoproterozoic Transvaal Supergroups, whereas the metamorphic grade indicates that it postdates the ca. 2.79 Ga amphibolite-facies metamorphic peak in the region. This suggests a late Mesoarchaean or early Neoarchaean (ca. 2.79 to 2.71 Ga) age for the breccia. A similar age is inferred for the magnetic anomaly based on postulated cross-cutting dyke ages. Despite a comprehensive search, unequivocal shock-diagnostic microdeformation features have not yet been found in either the breccia or the highly-weathered granitic gneiss outcrops in the central parts of the anomaly. The unusual plastically-deformed dark clasts may represent chloritized mud clasts or impact melt clasts. Geochemical data on these clasts and other components of the megabreccia provide no conclusive support for a meteoritic origin, but the unparalleled composition of the clasts and their high trace element abundances of Ni, Cr, V, Zn and Co relative to rocks of the Kraaipan granite-greenstone basement, suggests an unusual origin for this matrix material. Given the distinctive nature of the breccia and its proximity to a large circular magnetic anomaly, it is postulated that the megabreccia could represent a mass or debris flow in a marine setting triggered by an impact tsunami or resurge. Subsequent faulting may have led to the preferential preservation of these deposits. This interpretation of the Setlagole megabreccia and geophysical anomaly is evaluated in terms of other possible modes of origin and it is concluded that a meteoritic source best fits the available data.

Relationship between characteristics of gravity and magnetic anomalies and the earthquakes in the Longmenshan range and adjacent areas

The 2008 Wenchuan earthquake and aftershocks occurred along the northeast-trending Longmenshan fault zone in the eastern margin of the Tibetan plateau. The Tibetan plateau has the strongest negative Bouguer gravity anomaly zone in China and is surrounded by the great gravity horizontal gradient belt. The horizontal gradient belt of the observed gravity anomaly in the Longmenshan area is a part of this giant gravity gradient belt. The Longmenshan fault zone is located to the east of this belt. The horizontal gradient belt of the residual gravity anomaly, obtained by removing large effects of sedimentary basin and variations in the crustal thickness, well matches the Longmenshan fault zone. But this belt is located to the east of the horizontal gradient belt of the observed gravity anomalies. The deviation of the two horizontal gradient belts increases from the southwest to the northeast with a maximum of about 40–50 km. A significant difference in density exists in the lower crust and the uppermost mantle between the Songpan-Ganzê block and the Sichuan basin block. The Songpan-Ganzê block is less dense than the Sichuan basin block in the lower crust as well as in the uppermost mantle. The boundary between the two blocks is located to the west of the Wenchuan-Maoxian, Yinxiu-Beichuan, and Anxian-Guanxian faults approximately. The fault plane crosses the lower crust and uppermost mantle. The rigid Sichuan basin block acts as a resistant for the pushing from the Songpan-Ganzê block. Far-field effects of the collision between the Indian and Eurasian plates, might lead to thrust of some brittle layers in the upper crust along the detachment, in the middle crust of the Songpan-Ganzê block. When movement on a large and deep crustal mega-thrust occurs, earthquakes strike the Longmen Shan margin of the Tibetan Plateau. In the Guanxian-Beichuan segment in the southern Longmenshan fault zone, push from the Songpan-Ganzê block is perpendicular to the density boundary, which corresponds to the location of the horizontal gradient belt of the residual gravity. But in the Guanxian-Beichuan segment in the northern Longmenshan fault zone, the push is oblique to the location of this gradient belt. Such a relationship could lead to nearly equal amounts of thrust and right-lateral slip along the Beichuan-Qingchuan segment, and in contrast to nearly pure thrust along the Guanxian-Beichuan segment. An unusually large negative magnetic anomaly occurs around Qingchuan-Wenchuan-Guanxian. The computed magnetic model shows that some crystalline complexes with reversal magnetization might exist in the crust, which might be related to extrusion of the metamorphic basement rocks in the lower to middle crust.

The implications of long-lived asymmetry of remanent magnetization across the North Pacific fracture zones

article i nfo Large marine magnetic anomalies accompany the Pacific fracture zones (FZs) for thousands of kilometers. Although the origin of these anomalies is poorly understood, their underlying magnetization contrasts should reflect the temporal record of crustal accretion as well as geomagnetic field variations. Here we present an analysis of archival and newly collected magnetic anomaly profiles measured across three FZs from the North Pacific Cretaceous Quiet Zone (120.6 to 83 Ma) that are characterized by a remarkably uniform shape. Forward and inverse modeling indicate that these anomalies arise from remanent magnetization, with enhanced remanence located on one side of each FZ along the entire studied area. A comparison of geochemical and magnetic data from active ridge discontinuities and transform faults suggests that elevated iron content near segment ends is likely responsible for the observed anomalies in the Cretaceous Quiet Zone as well. A more complex magnetization setting is observed where the FZs contain multiple faults. There, the simple model of one-sided enhancements is only partly valid. Comparison between 3D forward modeling of the Quiet Zone magnetization and the calculated magnetization contrasts found across the Pioneer and Pau FZs suggests that the intensity of the geomagnetic field during the Cretaceous superchron had less than 50 percent variability about its average value. No major trends in the strength of the geomagnetic field during the superchron are observed. The presence of long-duration (>30 m.y.) zones of enhanced magnetization along the young/old sides of the Pioneer/Pau FZs (both left- stepping) requires some long-lived asymmetry in crustal construction processes near ridge-transform intersections. Although the underlying mechanism that controls this long-lived asymmetry remains unclear, absolute plate motions might explain this asymmetry. Shorter period (few m.y.) variations in the amplitudes of the enhancements probably result from oscillations in crustal construction.

High efficiency of natural lamellar remanent magnetisation in single grains of ilmeno-hematite calculated using Mössbauer spectroscopy

Rocks from large remanent magnetic anomalies on Earth have been found to contain exsolved rhombohedral oxides, which also have been proposed as earth analogues for the rocks creating the observed large remanent anomalies on Mars. Theoretical considerations and previous case studies of natural rocks have shown that the natural magnetisation is carried by lamellar magnetism due to uncompensated moments at the interface between antiferromagnetic hematite and paramagnetic ilmenite. Here, single grains (≈ 250 µm) of titanohematite with ferri-ilmenite exsolution lamellae from Mesoproterozoic metamorphic rock samples in southwest Sweden and the Adirondack Mountains, USA, are studied using room-temperature Mössbauer spectroscopy to identify possible characteristic magnetic signatures of lamellar magnetism. Mössbauer spectra of synthetic samples of titanohematite (Fe 0.95Ti 0.05O 3 and Fe 0.9Ti 0.1O 3) were collected for comparison and showed a dominant six-line magnetic spectrum due to Fe 3+ in titanohematite with a weak sextet due to Fe 2 +–Fe 3+ charge transfer. Mössbauer spectra of the natural ilmeno-hematite grains are similar to those for synthetic titanohematite, but contain additionally two paramagnetic doublets corresponding to Fe 2+ and Fe 3+ in ilmenite. However, several grains also contain an additional weak, broad magnetic component that we assign to iron in contact layers according to the lamellar magnetism model. Similar to the previous Mössbauer results for natural hemo-ilmenite, there is no evidence for superparamagnetic behaviour of the nanoscale titanohematite lamellae contained within coarser ferri-ilmenite lamellae, and no evidence for single-domain or superparamagnetic magnetite. The compositions of titanohematite and ferri-ilmenite in the individual ilmeno-hematite grains calculated from the Mössbauer area ratios show that compositions are closer to end-member values than the compositions inferred from electron microprobe and transmission electron microscopy on the same grains, consistent with observations that lamella thicknesses are as small as a few nm. Bulk compositions of ilmeno-hematite grains calculated from the Mössbauer data confirm that the Swedish samples are significantly more ilmenite-rich than the Adirondack samples, and agree with an estimation of bulk composition through point counting of electron backscatter images. The Mössbauer data allow a quantitative estimation of contact layer abundance, which is used to determine the efficiency of lamellar natural remanent magnetisation acquisition. Because minerals with a high natural remanent magnetisation efficiency are expected to create larger remanent magnetic anomalies, contact layer abundance determination by Mössbauer spectroscopy provides a valuable new tool for mineral-based magnetic anomaly interpretation on Earth and other planetary bodies.