Strong Magnetic Anomalies Research Articles

Evolution of the Tharsis region of Mars: insights from magnetic field observations

Mars Global Surveyor (MGS) observations of crustal magnetic fields over Tharsis provide new constraints on models for the thermal and magmatic evolution of this region. We analyze the distribution of magnetic field anomalies over Tharsis surface units of Noachian, Hesperian and Amazonian age. These data suggest that early Noachian crust underlies the Tharsis province, and formed contemporaneously with the existence of a martian dynamo. This crust either pre-dates the formation of Tharsis, or formed during the earlier phases of Tharsis volcanism. The preservation of strong magnetic field anomalies over some of the earliest Noachian and topographically high units, together with the observation of magnetic field anomalies over Hesperian- and Amazonian-age surface units, indicate that a large fraction of the magnetized crust has remained cool (below the blocking temperature of the magnetic carrier) throughout the construction of Tharsis. Moreover, the distributions of magnetic anomaly amplitudes over Noachian, Hesperian, and Amazonian surface units suggest that the youngest units overlie sites of prolonged intrusion and have undergone a greater extent of thermal demagnetization. The absence of magnetic anomalies around the Tharsis Montes and Olympus Mons argues for strong, localized heating, as would be expected at volcanic centers. We show that end-member models for progressive thermal demagnetization of a Noachian magnetized crustal layer are consistent with the anomaly amplitude distributions. We integrate the magnetic field observations with constraints from tectonics, gravity, and topography, and present a revised scenario for the evolution of the Tharsis region.

Palaeointensity study of the Oshima 1986 lava in Japan: implications for the reliability of the Thellier and LTD-DHT Shaw methods

We have performed a palaeointensity study of the Oshima 1986 lava in order to examine the reliability of the Thellier and LTD-DHT Shaw methods with special reference to high-temperature oxidation states of magnetic grains. Coe’s version of the Thellier method was applied to fifteen specimens from five block samples. The palaeointensities determined by this Thellier method ranged from 44.8 to 59.0 μT with an average of 51.0 ± 4.1 μT ( N = 15). They are systematically higher than the expected intensity (45.5 μT from the DGRF 1985). Since the in situ magnetic survey at the sampling site indicates no strong local magnetic anomaly, these high palaeointensities are overestimation, probably due to some rock magnetic property and/or some thermal alteration during the experiments. Microscopic observations show that the samples, where some titanomagnetites surrounding vesicles were classified with intermediate oxidation indices of III–V, gave 10–30% higher palaeointensity values. Therefore, the erroneously high Thellier palaeointensities have a relation to the high-temperature oxidation state. For eight of the fifteen specimens used in the Thellier experiments, the applied DC field was perpendicular to the primary NRM component for quantitative evaluation of laboratory-induced chemical remanent magnetisation (CRM). Directional changes of NRM thermally demagnetised during the Thellier experiments indicate that most of the samples more or less acquired CRM due to laboratory heating suggesting that some magnetic phase with high blocking temperatures was produced. For the specimens of intermediate oxidation level, the CRM appears to have been gradually acquired at relatively low temperatures (≤400 °C). It would appear to be difficult to detect this gradual thermal alteration with the standard pTRM check, weak-field susceptibility measurements and hysteresis parameter changes. We also applied the double heating technique of the Shaw method combined with low temperature demagnetisation (LTD-DHT Shaw method) to seven specimens from the same block samples. The palaeointensities determined by the LTD-DHT Shaw method yielded an average of 46.4 ± 4.7 μT ( N = 6), which agrees with the expected intensity indicating no dependence on the high-temperature oxidation states.

Anomalous magnetic susceptibility values and traces of subsurface microbial activity in carbonate banks on San Salvador Island, Bahamas

Pure limestones beneath the paleosols on San Salvador Island, Bahamas, contain strong positive magnetic susceptibility anomalies, although the iron content is generally very low. These magnetic phenomena differ from those associated with disconformities, which are marked by accumulation of paramagnetic airborne dust deposits with relatively high iron content. The strength and characters of the magnetic response in these subsurface zones correspond to the presence of magnetite, particularly small single-domain magnetite crystals of microbial origin. These crystals are not present elsewhere in the intergranular rock pores or microvugs. They are preferentially concentrated in capillary microborings, which developed concurrently with formation of calcite cements that have soil-related C and O isotope compositions. These magnetic zones occur several meters below the overlying soil horizons. Very thin and long linear microborings may be attributable to cyanobacterial microborers. The single-domain magnetites in these micrometer-size tunnels plugged by calcite appear to result from later occupation of these tiny holes by magnetotactic bacteria. Inorganic origin of the magnetite seems unlikely. Numerous traces that suggest subsurface microbial activity provide evidence that may be used to develop possible scenarios for subsequent biological studies of the precise bacteria involved.

Timing of the Martian core dynamo

Using the radial component magnetic data acquired by Mars Global Surveyor during its mapping phase, a reliable magnetic anomaly map of Mars is derived. Many weak but reliable magnetic anomalies are now detected in the northern lowlands. Their source bodies have likely been partially demagnetized by the lowlands formation processes, and there has been no core field in the later times to remagnetize the bodies. No consistent anomalies are observed in the Hellas basin, implying that the core dynamo has been inactive at least since the impact event. The topography, the positive Bouguer anomaly at the center, and the strong negative Bouguer anomalies over the outskirts of Thaumasia plateau suggest that the plateau was the site of a large impact basin prior to the formation of the plateau. The lack of magnetic anomalies in the central part and the strong magnetic anomalies over the outskirts of the plateau resemble the magnetic signature of Hellas basin, implying a preexisting giant impact basin beneath the plateau and the absence of the core field since the impact time. The lack of magnetic edge effects of Valles Marineris, the absence of thermal demagnetization of Tharsis plain by the shield volcanoes, and the substantial distance between the paleomagnetic poles and the present rotation axis imply that a major part of Tharsis bulge was formed in the absence of the core field.

The influence of a mini‐magnetopause on the magnetic pileup boundary at Mars

3D single fluid non‐ideal MHD simulations are used to predict positions for the bow shock and magnetic pileup boundary (MPB). The positions are in agreement with those calculated from Mars Global Surveyor data. The simulations were run with the strong southern magnetic anomalies facing into the solar wind, and the IMF in either the northward or southward direction. The simulations show that the strong southern magnetic anomalies create a magnetopause‐like structure (i.e., a mini‐magnetopause) in place of an MPB but plasma signatures for both types of boundaries are similar. Distinguishing characteristics include the magnetic field orientation, pressure, and the current. The current regions associated with a mini‐magnetopause are larger in scale and greater in magnitude than those at the MPB. The simulations also show that including non‐ideal MHD physics is important in resolving both the magnetic pileup boundary and the mini‐magnetopause.

Thermoremanent magnetization of the Martian lithosphere

This paper investigates the thermoremanent magnetization (TRM) of the Martian lithosphere assuming that the upper part of the lithosphere acquired TRM in the early history of the planet and in the presence of the core field (the primary magnetization), whereas the lower part has been gradually magnetized by the magnetic field of the upper part as it has cooled below the Curie temperature (secondary magnetization). The secondary magnetization is relatively weak if the magnetic properties of the lower lithosphere are taken to be similar to that of the extrusive basalt near the oceanic ridge axes on Earth. However, if the Martian lower lithosphere is more magnetic, e.g., by an order of magnitude, the secondary magnetization can be significant, although still weaker than the primary magnetization of the upper part. Such a highly magnetic lower lithosphere does not seem plausible by terrestrial standards, and it remains to be confirmed whether it is geologically plausible for Mars. I also examine a lithospheric model with a strongly magnetic layer beneath the crust, 25 times more magnetic than the extrusive basalt, representing a possible ilmenite‐rich layer with a considerable amount of hematite‐ilmenite lamellae. I assume that the layer was not magnetized by the core field because it was hotter than the Curie temperature of the lamellae when the core dynamo was active. It acquired secondary magnetization later in the presence of the magnetic field of the crust. Despite adopting an extraordinarily high magnetic layer which seems implausible by terrestrial standards, the secondary magnetization of the layer has minor contributions to the observed magnetic anomalies. In all models considered the primary magnetization is ∼20–30 A/m and has the dominant contribution to the observed magnetic anomalies. It is worth noting that the radial components of the primary and secondary magnetization are parallel, whereas the tangential components are in opposite directions. Because the strong magnetic anomalies in the south likely arise from the tangential magnetization, considering the secondary magnetization requires even stronger primary magnetization.

Correlation of a strong lunar magnetic anomaly with a high‐albedo region of the Descartes mountains

Mapping and model simulations of Lunar Prospector magnetometer measurements show that the source of the strongest known magnetic anomaly on the lunar near side (42 nanoTeslas at 18.6 km altitude) coincides approximately with a high‐albedo region of the Descartes mountains centered 60 km south‐southeast of the Apollo 16 landing site. The Descartes mountains represent primary ejecta from one or more basin‐forming events (Imbrium and/or Nectaris), supporting the hypothesis that basin ejecta materials emplaced >3.8 Gyr ago are the main sources of lunar magnetic anomalies. The higher albedo of the surface at this location is consistent with a significant role for solar wind ions in the optical maturation (or “space weathering”) of the lunar surface.

Archaeogeophysical investigations around the Bilge Qagan monument in Khosho Tsaidam, Mongolia

AbstractAncient Turkish monuments in Mongolia are historically important owing to the earliest Turkish and Chinese inscriptions written on them. In this paper, magnetic and conductivity meter surveys conducted in the vicinity of the Bilge Qagan monument are presented. Magnetic and conductivity maps show rectangular shaped anomalies, which could be caused by a buried wall‐like structure or floor tiles to protect the Bilge Qagan monument. The Altar Stone, which is also described as a base for a pagoda, gives a strong magnetic anomaly. This anomaly is processed by advanced geophysical methods and it is demonstrated that the actual rock body has some form of remanent magnetization, which causes the magnetic anomaly. By means of conductivity surveying, it also can be suggested that the near‐surface formations have significant water content. Copyright © 2002 John Wiley & Sons, Ltd.

Initial mapping and interpretation of lunar crustal magnetic anomalies using Lunar Prospector magnetometer data

Maps of relatively strong crustal magnetic field anomalies detected at low altitudes with the magnetometer instrument on Lunar Prospector are presented. On the lunar nearside, relatively strong anomalies are mapped over the Reiner Gamma Formation on western Oceanus Procellarum and over the Rima Sirsalis rille on the southwestern border of Oceanus Procellarum. The main Rima Sirsalis anomaly does not correlate well with the rille itself but is centered over an Imbrian‐aged smooth plains unit interpreted as primary or secondary basin ejecta. The stronger Reiner Gamma anomalies correlate with the locations of both the main Reiner Gamma albedo marking and its northeastward extension. Both the Rima Sirsalis and the Reiner Gamma anomalies are extended in directions approximately radial to the center of the Imbrium basin. This alignment suggests that Imbrium basin ejecta materials (lying in many cases beneath the visible mare surface) are the sources of the nearside anomalies. If so, then the albedo markings associated with the stronger Reiner Gamma anomalies may be consistent with a model involving magnetic shielding of freshly exposed mare materials from the solar wind ion bombardment. Two regions of extensive magnetic anomalies are mapped in regions centered on the Ingenii basin on the south central farside and near the crater Gerasimovic on the southeastern farside. These regions are approximately antipodal to the Imbrium and Crisium basins, respectively. The Imbrium antipode anomaly group is the most areally extensive on the Moon, while the largest anomaly in the Crisium antipode group is the strongest detected by the Lunar Prospector magnetometer. A consideration of the expected antipodal effects of basin‐forming impacts as well as a combination of sample data and orbital measurements on the nearside leads to the conclusion that the most probable sources of magnetic anomalies in these two regions are ejecta materials from the respective impacts. In both regions the strongest individual anomalies correlate with swirl‐like albedo markings of the Reiner Gamma class visible on available orbital photography.

A 50‐degree spherical harmonic model of the magnetic field of Mars

This paper presents a 50‐degree and order spherical harmonic model of the magnetic field of Mars derived at 120‐km altitude using the three orthogonal components of the vector magnetic field data acquired by the Mars Global Surveyor within the 80‐ to 200‐km‐altitude range. The downward continued vector magnetic field components to the surface of Mars delineate details of the Martian magnetic field. The strong magnetic anomalies in the southern hemisphere do not show compelling evidence for magnetic lineations similar to those associated with the seafloor spreading on Earth. The anomalies are more or less equidimensional. The magnetic signatures surrounding the impact basins Hellas, Argyre, and Isidis show that the prominent magnetic anomalies are older than the impact events. An average magnetization of ∼2 A/m is estimated for the upper crust surrounding Hellas basin. Four isolated magnetic anomalies are modeled by vertical prisms with circular cross sections of 170‐ to 190‐km radius and 20‐ to 30‐km thickness. The magnetization of the model prisms is 5 A/m more than that of the surroundings.

Structure and geophysics of the Gåsborn granite, central Sweden: an example of fracture-fed asymmetric pluton emplacement

Abstract The emplacement mechanisms of the Palaeoproterozoic Gåsborn granite, a satellite of the Transscandinavian Igneous Belt (TIB), are investigated using an integrated structural and geophysical approach. The pluton is discordant to c . 1.89 Ga folded supracrustal rocks that were deformed and metamorphosed at c . 1.85–1.80 Ga during the Svecokarelian Orogeny. Emplacement occurred at a depth of c . 10 km, within a regime of late Svecokarelian dextral transpression. Deformation of the pluton during cooling resulted in the formation of a variably developed foliation in the granite and deflection of less competent wall-rock units around its western and eastern contacts. Later E-W Sveconorwegian shortening resulted in the formation of shear zones that affect one of the pluton margins and may contribute to a component of the observed wall-rock distortion. The granite is situated above strong NNW-trending linear magnetic and negative gravity anomalies, which are interpreted to correspond to an important early Svecokarelian shear zone. The geophysical data indicate that the pluton is markedly asymmetric and modelling of the residual gravity field suggests that it consists of a deep root zone in the west and a thin sill-like body, which makes up most of the east and south parts of the body. Emplacement of the sill-like part occurred by lateral flow of magma from the root zone accommodated by downwarping of the underlying units. Intrusion of the thicker, discordant west part may have been accommodated by a combination of roof lifting and floor depression, aided by displacement on an active shear zone.

Medieval iron and lead smelting works: a geophysical comparison

Abstract Geophysical techniques (fluxgate gradiometry and magnetic susceptibility) have been applied to ten iron and two lead smelting sites in the North Yorkshire Moors National Park and the Pennines, respectively. These geophysical techniques, particularly magnetometry, are often regarded as having little application on such sites. Large quantities of iron slag, for example, can mask structures associated with the on-site industrial activity, and this may result in the incomplete or erroneous interpretation of the geophysical data. Geophysical surveys can, however, identify the strong magnetic anomalies associated with iron-smelting furnaces and to a lesser extent those associated with lead smelting. To provide a valid interpretation of such geophysical data it is important to understand the processes that have been conducted on iron and lead smelting sites. This paper compares and contrasts the geophysical responses produced by iron and lead smelting sites and provides examples of the structures that geophysical techniques may reveal on smelting sites.

The geophysical evaluation of an iron-working complex: Rievaulx and environs, North Yorkshire

Geophysical techniques are often considered to have little application on iron-working sites because the responses from large quantities of iron slag can mask structural remains associated with iron production. This may result in the incomplete or erroneous interpretation of the geophysical data. Geophysical surveys, can, however, identify the strong magnetic anomaly associated with a furnace and in these circumstances have worked well. Slag can produce a wide range of responses, which may be attributed to changes in iron-content, depth of burial and thickness of the slag deposit. A more detailed examination of the data, therefore, can often lead to the identification of structures. The geophysical survey work in the Rievaulx area in North Yorkshire, England has examined iron-working activity ranging from a bloomery site to a blast furnace with a finery/chafery complex. Geophysical surveys were undertaken on eight sites to measure magnetic anomalies, magnetic susceptibility and earth resistance. One geophysically surveyed bloomery site was subsequently excavated. The geophysical interpretations can be compared with the historical evidence. Cistercian monks from Rievaulx Abbey operated several of the sites surveyed. After the dissolution of the Abbey in 1538, iron-making continued under the Duke of Rutland, who constructed a charcoal-fired furnace at Rievaulx in ca. 1570. The geophysical work has led to a reassessment of the use of water-power, which was associated with two finery/chafery complexes in the Rievaulx area. Most of the surveys have identified iron-working features that can be matched to topographic changes. The paper covers the results from some of the Rievaulx surveys and details the types of feature than can be identified, including furnace sites, buildings, leats and slag tips. Copyright © 1998 John Wiley & Sons, Ltd.

The role of magnetic susceptibility as a geophysical survey technique: A site assessment at high cayton, north yorkshire

Measurement of magnetic susceptibility is becoming increasingly popular as a method of archaeological prospection, mainly due to the rapidity with which it can be carried out and the relatively inexpensive nature of the equipment required. This study examines the effectiveness of the method, using both field and laboratory-based measurements, compared with magnetometry, resistivity and pulsed induction meter surveys for the investigation of the site of the rural medieval village at High Cayton in North Yorkshire. An investigation is also made of the effect of different magnetic susceptibility sampling intervals on anomaly discovery and delimitation. It is demonstrated that magnetic susceptibility surveys, using a 1 m by 1 m sampling interval, produce complementary but distinctly different information to other geophysical survey methods. It is also shown that laboratory and field measurements of magnetic susceptibility differ as laboratory methods remove some variables whose effects are difficult to quantify, such as stones and soil moisture. However, these variables may be archaeologically relevant. Larger sampling intervals are shown to lead to the progressive distortion of results and loss of anomaly definition, culminating in a 10 m by 10 m sampling interval where even features with extensive, strong magnetic anomalies are difficult to define and delimit, demonstrating the limitations of magnetic susceptibility measurements when used as a rapid technique to detect features within large survey areas.

Alvin magnetic survey of zero‐age crust: CoAxial Segment Eruption, Juan de Fuca Ridge 1993

Magnetic profiles obtained by deep submersible ALVIN across a newly erupted lava flow on the Juan de Fuca Ridge (JDFR) show strong magnetic anomaly fields (>20,000 nT) and high crustal magnetization (>50 A/m contrast) relative to the surrounding older lavas. These results are consistent with previous ideas that extrusive crust is initially highly magnetized. Within the high amplitude magnetic anomaly over the center of the flow, there is a clearly defined deep, “notch‐like” magnetic low. This notch may indicate thermal demagnetization associated with a subsurface feeder dike, weakly magnetized intrusives or altered extrusive rock. If the former mechanism is active, the notch should disappear with time as the dike cools.

Magnetic anomaly near the center of the Vredefort structure: Implications for impact-related magnetic signatures

A strong magnetic anomaly near the center of the ancient and deeply eroded Vredefort structure is attributed to remanent magnetization caused by a large meteorite impact at ∼2.0 Ga. The rocks underlying the anomaly are Archean gneisses thought to represent mid-crustal depths that were uplifted to the surface during the postulated impact event. Measurements of the remanent magnetization of the basement rocks yielded consistent vectors of declination = 25° , inclination = 56° , k = 18, α = 16 that correspond to the paleomagnetic pole position at time of impact. Petrologic studies indicate that during impact, large volumes of rock were heated enough to cause thermal remagnetization in the ambient field. Thermal effects of all large impacts on cratons may induce a remanent magnetization of sufficient intensity to cause anomalies in the geomagnetic field that are detectable even by satellites.

Geophysical investigations of the Lake Lappajärvi impact structure, western Finland

A circular negative Bouguer anomaly approximately −10 mgal in amplitude and 17 km in diameter is associated with the Lappajärvi impact structure. The related mass deficit is −440 × 10 11 kg. There are irregularities in the anomaly pattern due to differentially faulted bedrock blocks and lateral density contrasts between bedrock, shocked bedrock, breccia, suevite, impact melt rock and overburden. The minimum amount of impact melt rock was interpreted, from the positive residual Bouguer anomaly between the centre and the rim in the northwestern part of the crater, as being 1.5–2.0 km 3. There are a few small aeromagnetic anomalies (up to 200 nT at an altitude of 40 m) within the crater, which is mostly characterized by a lack of magnetized material. Strong magnetic anomalies (a few thousand nT) which are abundant in the surroundings disappear in the crater area. The aero-electromagnetic (AEM) data show strong anomalies with inphase /quadrature ratios close to 1, indicating moderately conducting material within the crater. An electromagnetic wide-band (SAMPO EM) profile was measured across the annular depression between the centre and the rim of the crater. Three layers were interpreted. The thickness of the uppermost layer, mostly conductive overburden, is about 100–200 m and the resistivity 60–170 Ωm. In the annular depression, beneath the uppermost layer, there is a 50–250 m thick layer of suevites and breccias with resistivity of about 17–50 Ωm. The resistive lowest layer follows the base of breccia and suevite. Some separate soundings were carried out near the centre of the crater. According to one of these soundings, there is a conductive layer of suevite and breccia, with a resistivity of 30–40 Ωm, beginning at depth of 150 m beneath the exposed impact melt rock. The results of geophysical interpretations, such as the annular depression between the centre and the rim of the crater, the small thickness/diameter ratio of the geophysically anomalous part of the crater, and the calculated amount of impact melt rock, indicate that the Lappajärvi structure belongs to the category of complex craters. Rhomboidal geophysical anomaly patterns (in particular AEM anomalies), parallel to old bedrock structures suggest that the collapse of the transient crater and/or subsequent crater modification were controlled by NW-SE and NE-SW trending pre-existing fracture and fault zones.

Lunar magnetic fields: Implications for utilization and resource extraction

Although the Moon possesses no large‐scale intrinsic magnetic field, locally strong crustal fields exist which must be considered in assessing lunar utilization scenarios. On the basis of available data, orbital anomalies most probably imply the presence of subsurface concentrations of metallic iron in the single‐domain size range. Such concentrations may indicate larger iron volume fractions than are characteristic of sampled materials, allowing more efficient extraction of metal than is now considered likely using nominal regolith compositions. Most other implications relate to the deflection of interplanetary and magnetospheric ions by crustal fields. Numerical simulations are employed to show that solar wind ion deflection by strong lunar magnetic anomalies can produce local increases, as well as decreases, in the implantation rate of solar wind hydrogen. The largest concentrations of implanted solar wind volatiles may therefore be found in local areas near strong anomalies. If water ice exists in permanently shadowed regions of the lunar poles, locally strong magnetic fields can shield these volatiles from sputter erosion losses by interplanetary and magnetospheric ion fluxes. Model simulations indicate that the ability of magnetic anomalies to shield the surface from incident ions increases with the angle of incidence and hence for most particle sources, with selenographic latitude. Finally, the possibility that relatively strong anomalies can provide significant protection of men and materials against major solar flare particle events is examined and found to be unlikely.

Pigeons at Magnetic Anomalies: the Effects of Loft Location

ABSTRACT Homing pigeons from our old lofts at Fox Ridge Farm in Lincoln, MA, were disoriented when released at places where the earth’s magnetic field was irregular - so-called ‘magnetic anomalies’. The orientation of pigeons raised in our lofts at Cornell in Ithaca, NY, was unaffected by anomalies. Further experiments in Lincoln showed that sibling pigeons raised and trained to lofts only 2.5 km apart behaved differently when released at a strong magnetic anomaly. Pigeons from the loft situated in a magnetic gradient of 450 nT km−1 were disoriented at anomalies, whereas birds raised in a loft in a magnetic gradient of 88nTkm−1 were well oriented. This suggests that the location of the home loft may play an important role in determining which cues pigeons use for their navigation, and that these cues are learned sometime after weaning from their parents at 4-6 weeks after hatching.

Geological interpretation of aeromagnetic data in some parts of northcentral Nigeria

Airborne magnetic data and field observations were used to define structural trends in some parts of North Central Nigeria. The aeromagnetic anomalies of the Jurassic Younger Granite complexes are distinctive with relatively high amplitudes and short wave-lengths and are directly associated with the outcrop patterns of the intrusions. The mosaic pattern of magnetic anomalies may be used to identify the Younger Granite ring complexes. The aeromagnetic data also contain evidence of linear structural discontinuities. some of these linear features are long and extend through the survey area. They are identified by a pattern of contour offsets and probably reflect deep crustal features cutting some of the Younger Granite known to be mineralised. In particular, a major fault is mapped which cuts the Ririwai complex to the East. This fault trending NW-SE, appears to extend southwards through the area. The lineaments probably acted as conduit through which mineralized fluid flowed. The regional trends of structures are revealed on the aeromagnetic map, these being mainly northeast-southwest and northwest-southeast. The Northwest-southeast trends are superimposed on the northeast-southwest trends. This overprinting of one magnetic anomaly trend over the other suggests that the Northeast-southwest lineaments are older than the northwest-southeast lineament patterns Three known tin-bearing ring complexes show strong magnetic anomalies which make them more prominent than the other complexes on the coloured magnetic anomaly map. This characteristic should assist in locating new provinces for primary tin deposits.