Aeromagnetic Map Research Articles

The Effect of Adjacent Magnetic Sources on the Interpretation of Magnetic Anomalies

In interpreting magnetic profiles from the aeromagnetic map of Iraq, it is usually assumed that the observed anomalies are caused by homogeneous magnetic bodies forming the basement that is concealed beneath a thick cover of sediments. Many unknowns are faced in interpreting magnetic anomalies related to such deep sources. Among these: direction of magnetization in these sources, magnetic susceptibility and the contributions of the adjacent sources are the most important unknowns. Therefore, it is to be assumed that these anomalies are related to igneous bodies within the basement rocks and magnetized by induction only from the present earth’s magnetic field. In other word, the study is focused on one of the unknowns mentioned above, which is the effect of contribution of adjacent igneous body at different depths and sizes on the shape of main magnetic anomaly. Previous studies related to interpretation of aeromagnetic anomalies in Iraq, have assumed that these basement anomalies are not affected by adjacent igneous masses and as such not taken in consideration, (Al Mashhadani, 2000, Najar, 1999, Mohammed 1981, Mahmood, 1981, Aziz, 1981). In this paper ,the effects of such close magnetic sources will be considered . The suggested method is designed at estimating the amount of change in the -ve part of the anomaly occurring between two igneous sources, which in turn depends on how close the adjacent igneous sources to the main one.

Application Of Laplace Interpolation In The Analysis Of Geopotential Data

Geophysical data is often collected at irregular intervals along a profile or over a surface area. But most methods for the treatment of geophysical data often require that any data collected at irregular intervals have to be interpolated to obtain values at regular grid. Unlike the common 2-dimensional interpolation procedures, the Laplace (finite-difference) method can be applied to regions of high data gradients without distortions and smoothing. However, by itself, this method is not convenient for the interpolation of geophysical data, which often consists of regions of widely variable data densities. In this paper a procedure is developed which allows that by combining it with the method of quadratic weighting, the Laplace method can be successfully applied to interpolate two-dimensional geophysical data. These methods were applied to some geopotential data. The results show that there is no significant difference between aeromagnetic maps derived from data as observed and maps obtained when the data is interpolated in a region of thick sedimentary formation. This is attributed to the fact that the magnetic body in such region are deeply buried. However, interpolated aeromagnetic map over a region of outcropping granitic bodies exposes more shallow features which are otherwise not seen on the map derived from the observed data that are not interpolated. Similar observation was recorded for a gravity anomaly map produced from data collected in basement terrain. It was concluded that since it is impossible to observe every point in a given surface area in order to produce an accurate map that will reflect the distribution of the various shallow subsurface anomalies, it is better to interpolate the observed data prior to the production of the desired geopotential map. Keywords: Geophysical, Geopotential, Lapalce interpretation, finite difference, quadratic, weighting. Nigerian Journal of Physics Vol. 19 (1) 2007: pp. 129-138

Magnetic Evidence for Volcanism at Eastern Continental Margin of India: Juxtaposition with Elan Bank (Southern Indian Ocean)

The rifted Eastern Continental Margin of India (ECMI) has evolved as a result of breakup of East Gondwanaland. Previous geophysical studies of the continental margin have not elucidated upon its volcanic nature. Magnetics plays a useful role in the study of continental margins, particularly in identifying the volcanic units. The aeromagnetic map of the offshore Mahanadi basin of ECMI displays a conspicuous linear anomaly along the continental shelf. A comprehensive study of the published aeromagnetic, marine magnetic and gravity data of the offshore Mahanadi basin reveals the existence of a seaward dipping volcanic unit in the offshore Mahanadi basin bordering the Hinge zone. This inference suggests that the ECMI is a volcanic rifted margin. The study further indicates the deepening of the basement towards the sea. In addition, the existing geological studies on the ECMI demarcated the probable limit of the continental crust by studying the basement detached tectonic style of the sedimentation in sub-surface configuration of the East coast basins of India. The probable continental crustal limit, the Hinge zone, and the inner edge of the presently inferred volcanic unit conform to one another spatially in the offshore Mahanadi region. These features characterize the inferred volcanic body as seaward dipping reflectors (SDRs) that usually occur at the rifted continental margins. The deepening of the basement towards the sea and the presence of the volcanic body on the continental margin are indicative of the transitional nature of the crust. It is generally accepted that Antarctica and India were juxtaposed before the breakup of Gondwanaland. But the microcontinents in the southern Indian Ocean are neglected in the reconstruction of Gondwanaland continents. The recent studies of the discovery of continental crust within the Elan Bank (EB) microcontinent show that the EB was contiguous with the East coast of India before the breakup of Gondwanaland. Moreover, it is reported that the upper igneous crust of the EB consists of a 2–3 km thick layer of accumulated lava flows originating from the Kerguelen hotspot. An estimate shows that the total volume of volcanic and plutonic component of the Elan Bank is about 0.3 million cubic kilometers. The present inference of a volcanic body from the offshore Mahanadi basin is in agreement with the above observations of the juxtaposition of EB with ECMI.

Structural trends and spectral depth analysis of the residual field of Pategi Area, Nigeria, using aeromagnetic data

Pategi area is located between longitude 530´E and 600´E and latitude 830´N and 900´N and lies at the southwestern part of the Bida basin in west central Nigeria and is filled with Campanian-Maestrichtian sediments comprising mainly sandstones that are underlain by basement rocks comprising of granite-gneiss and migmatites. The area is covered by one half-degree aeromagnetic map, which was digitized at 1.5 km grid and re-contoured. The compiled magnetic map shows that an approximately NE-SW trending fabric characterizes the upper part of the area down to latitude 8.84N. This trend changes to largely E-W in the central portion between latitudes 8.63N and 8.84N. Below latitude 8.63N the trend changes again to approximately NE-SW particularly at the southwest fringe of the map. A major lineament depicting the Romanche paleofracture zone was found in the area. The Pategi area has complex magnetic signature. Short to moderately long discontinuities were found in the magnetic map. Several magnetic closures of various sizes were also noticed. While the discontinuities indicate the presence of minor to moderately long fault zones in the area, the closures depict the sizes of the anomalies that lie beneath the basin. Spectral depth determination indicates that the area contains average sediments up to a thickness of 3.27km. A sedimentary thickness of this size may be recommended for further investigation for petroleum exploration. The intra-basin fault zones may have hydrocarbon implication. Keywords : Magnetic, Bida basin, fault zones, Pategi area. Nigerian Journal of Physics Vol. 18 (2) 2006: pp. 271-276

Investigations into the tectonic lineaments and thermal structure of Kutahya–Denizli region, western Anatolia, from using aeromagnetic, gravity and seismological data

Residual aeromagnetic map of western Anatolia displays intense anomalies, especially in the NW of the region. Curie point depth (CPD) map obtained from the residual aeromagnetic anomalies is indicative of larger area of shallow Curie levels than previously defined. Two important shallow depth regions located at between Kutahya and Denizli are determined from CPD and thermal gradient (TG) map. The intense aeromagnetic anomalies are located in the west of Kutahya along the E-W direction. These anomalies are resulted from the metamorphic units of Menderes Massif. The analytic signal (AS) map produced from residual aeromagnetic data shows the E-W trending geological bodies causing magnetic anomalies which become intense in the west and south of Kutahya. It is also observed from the AS map that thermal structures or bodies are buried under the non-magnetic surface geology. The maxspot map derived from the location of the horizontal gradient of aeromagnetic anomalies can be well correlated with not only tectonic lineaments, but also the distribution of earthquake data. Bouguer anomalies of the region can also be well correlated with the graben system and thus provide supportive information about the geothermal energy potential of the study area.

An improved tectonic model for the Eocene opening of the Norwegian–Greenland Sea: Use of modern magnetic data

Compilation of new and vintage aeromagnetric data from the Norwegian and Greenland Seas of the NE Atlantic provides evidence for a different interpretation of several tectonic elements. The previously interpreted oceanic fracture zones (Gleipne, Surt, Bivrost, Jenegga and Vesterålen) do not exist; these were artefacts of poor navigation and wide line spacing of the vintage dataset. This reinterpretation impacts our understanding of the early spreading history of the North Atlantic, as the opening of the Norwegian–Greenland Sea between the Jan Mayen and Senja-Greenland fracture zones occurred along a stable axis without offsets of the oceanic spreading anomalies or jumps in spreading axis. These results contradict the hypothesis that a spatial relationship exists between transfer zones and fracture zones on the Lofoten margin, and on the NE Greenland margin to which they have been projected. Simplified palaeogeographic reconstruction of the aeromagnetic map to Anomaly 22 reveals that a c. 50 km wide magnetic anomaly cuts across spreading anomalies 24A, 24B and 23 from the Vøring Marginal High on the Norwegian margin to Traill Ø on the East Greenland coast. The anomaly is interpreted to represent an igneous complex referred to as the Traill-Vøring igneous complex (TVIC). The complex crosscuts anomaly 22 on the Greenland margin, suggesting that the igneous activity was active until c. 50 Ma and can be linked up with the NNE-trending initial magmatic lineament (IML) extending between Traill Ø and Kangerlussuaq. The IML has been suggested to relate to a failed attempt of direct linkage between the Reykjanes and Mohns Ridges. The magnetic response of the TVIC along the Vøring margin has previously been interpreted as representing anomaly 24A and 24B. Such an interpretation required the erroneous introduction of an abandoned spreading ridge.

Enhancement of Fault Anomalies by Application of Steerable Filters: Application to Aeromagnetic Map of Part of Ifewara Fault Zone, Southwestern Nigeria

Enhancement of Fault Anomalies by Application of Steerable Filters: Application to Aeromagnetic Map of Part of Ifewara Fault Zone, Southwestern Nigeria

Pangaean megalineaments: geophysical indications for the existence of Laurentian counterparts of the 6000 km Trans-Saharan Tibesti Lineament, and implications for lithospheric tectonics and mineral deposits

A major 6000-km long west-northwest to east-southeast-trending Trans-Saharan lineament, extending from Kenya to Morocco, referred to as the Tibesti Lineament, has been described by Guiraud et al. (2000). This lineament shows up on satellite images, air-photo mosaics and terrain elevation models, as swarms of parallel faults. It corresponds to a major discontinuity of surface wave velocities in the mantle, suggesting that it deeply affects the lithosphere. The highest heat flow measurements in the West African shield are found adjacent to the Tibesti Lineament in the northern Hoggar uplift. Geophysical, geological, and geomorphological evidence suggests that the Tibesti lineament may have continued into North America (Laurentia), which collided with northwest Africa during the Appalachian-Mauritanide orogeny, coinciding with the late Palaeozoic assembly of the supercontinent Pangaea. The contiguous Laurentian lineaments which may be extensions of the Tibesti Lineament, are the here newly recognised Chesterfield Inlet Lineament and the Cabot Strait Lineament, both of which are situated in Canada. These lineaments are defined as linear pattern breaks in processed aeromagnetic anomaly maps of Canada. Some portions of them also can be detected in refraction and reflection seismic survey data ( e.g. , under Hudon Bay), and to a lesser extent, in the Bouguer gravity anomaly map of Canada. The lineaments have present-day (neotectonic) surface expressions in the form of inlets, submarine channels, straight river segments, and strings of linear lakes, extending over 3700 km, from Nunavut Province northwest of Hudson Bay, across Quebec and Newfoundland, and to the Cabot Strait separating Newfoundland from Nova Scotia. The highest heat flow in the Canadian Maritimes is found in the Cabot Strait. The Tibesti-Cabot-Strait-Chesterfield Inlet lineaments reflect deep-seated fracturing of the North African and North American lithosphere, involving both the crust and rigid upper mantle. These lineaments cut right across the grain of numerous orogenic belts dating from the Palaeoproterozoic, Late Mesoproterozoic, and Palaeozoic. By analogy with other large transcontinental lineaments which contain ore deposits, these newly identified megalineaments may be prospective for economic mineral deposits, such as hydrothermal gold and base metal mineralization, and diamondiferous kimberlites.

Shallow Subsurface Structure of the Vulcano-Lipari Volcanic Complex, Italy, Constrained by Helicopter-Borne Aeromagnetic Surveys

Helicopter-borne aeromagnetic surveys at two different times separated by three years were conducted to better understand the shallow subsurface structure of the Vulcano and Lipari volcanic complex, Aeolian Islands, southern Italy, and also to monitor the volcanic activity of the area. As there was no meaningful difference between the two magnetic datasets to imply an apparent change of the volcanic activity, the datasets were merged to produce an aeromagnetic map with wider coverage than was given by a single dataset. Apparent magnetisation intensity mapping was applied to terrain-corrected magnetic anomalies, and showed local magnetisation highs in and around Fossa Cone, suggesting heterogeneity of the cone. Magnetic modelling was conducted for three of those magnetisation highs.Each model implied the presence of concealed volcanic products overlain by pyroclastic rocks from the Fossa crater. The model for the Fossa crater area suggests a buried trachytic lava flow on the southern edge of the present crater. The magnetic model at Forgia Vecchia suggests that phreatic cones can be interpreted as resulting from a concealed eruptive centre, with thick latitic lavas that fill up Fossa Caldera. However, the distribution of lavas seems to be limited to a smaller area than was expected from drilling results. This can be explained partly by alteration of the lavas by intense hydrothermal activity, as seen at geothermal areas close to Porto Levante. The magnetic model at the north-eastern Fossa Cone implies that thick lavas accumulated as another eruption centre in the early stage of the activity of Fossa. Recent geoelectric surveys showed high-resistivity zones in the areas of the last two magnetic models.

Palinspastic reconstruction of Proterozoic basement-related aeromagnetic features in north-central New Mexico: Implications for Mesoproterozoic to late Cenozoic tectonism

Aeromagnetic maps from north-central New Mexico show east-northeast–trending linear features that are offset dextrally across the north-striking Picuris-Pecos, Tusas-Picuris, and (possibly) Nacimiento fault systems. Geologic structures that correspond to these regional aeromagnetic lineaments, where exposed in Phanerozoic uplifts, are major ductile shear zones of Proterozoic age that juxtapose folded metasedimentary rocks associated with areas of low aeromagnetic value with metavolcanic and metaplutonic rocks associated with areas of high aeromagnetic value. Regional aeromagnetic lineaments serve as piercing lines that suggest at least ∼55 km and perhaps as much as ∼90 km of net dextral separation since 1.4 Ga. The restored aeromagnetic lineaments are interpreted to represent a series of east-northeast–striking, dominantly north-vergent, thrust-sense shear zones that formed initially during the ca. 1.65 Ga Mazatzal orogeny and were variably reactivated during the ca. 1.4 Ga magmatic and deformational event.

イタリア・ブルカノ－リパリ火山地域高分解能空中磁気異常図

イタリア・ブルカノ－リパリ火山地域高分解能空中磁気異常図

A Comparative Study Of Source Location And Depth Estimates From Total Intensity And Reduced-To-The Pole Magnetic Data

Locations and depths to magnetic sources can be estimated from the maxima of the horizontal gradient magnitude (HGM), the analytic signal amplitude (ASA) and the local wave number (LWN) of the total intensity magnetic field. In this study, a synthetic magnetic field due to four buried dipoles was analysed to show that estimates of source location and depth can be improved significantly by reducing the data to the pole prior to the application of the HGM, ASA and LWN functions. Furthermore, results show that estimates from the HGM and the ASA are comparable and better than those from the LWN because the LWN uses second derivatives and therefore, is very susceptible to noise in the data. The aeromagnetic map of part of Ilesha area, southwestern Nigeria was also analysed to demonstrate the effectiveness of the RTP process. Key words: dipole anomaly, horizontal gradient magnitude, analytic signal, local wave number, source location, source depth Ife Journal of Science Vol. 7(1) 2005: 91-100

Radiometric ages, aeromagnetic expression, and general geology of mafic dykes from southeastern Brazil and implications for African–South American correlations

Geological, geochronological, and magnetometric data pertaining to mafic dykes from southeastern Brazil, mainly with tholeiitic affinities, suggest eight distinct swarms. The major swarms, which encompass the giant Pará de Minas, appear in aeromagnetic maps. Seven are markers of important crustal extension events at 2.65, 1.72, approximately 1.25, 1.1–0.9, 0.22–0.17, 0.13–0.12, and 0.05 Ga, and a minor swarm shows intrusion and deformation interrelated with the development of vertical transcurrent shear zones active approximately 2.19 Ga ago. Because at least three or four swarms have African counterparts, they may be helpful in reconstructing paleocontinents.

Interpretation of the Gravity and Magnetic Anomalies of the Cappadocia Region, Central Turkey

The Cappadocia region, located in Central Turkey, is characterized by widespread lava flows and volcanoclastic deposits dating from Miocene to Quaternary. Gravity and aeromagnetic anomalies of the region appear to present similar high and low amplitude regions, although the aeromagnetic anomalies exhibit a rather complex pattern which is thought to be caused by remanent magnetization. The low-pass filtered aeromagnetic map shows a deep-seated magnetic anomaly which may be linked to the widespread volcanic activity at the surface. The pseudogravity transformation of the upward continued anomaly has been constructed. The pseudogravity anomaly demonstrates some form of clockwise rotation. This anomaly was modelled by means of a three-dimensional method. The top and bottom of the body are at 6.3km and 11km (including the flight height) from the ground surface, respectively. This deep body is ellipsoidal and extends along an E-W direction, which is in line with the regional stress direction deduced from GPS measurements. A new mobilistic dynamo-tectonic system appears to explain the body’s E-W elongation. The modelled body may be the source for the inferred geothermal energy of the region. Magnetic measurements were carried out on oriented rock samples collected from outcrops of ignimbrites and basalts, providing directions and intensities of remanent magnetization, susceptibilities and Koeningsberger (Q) ratios. Standard deviations of remanent directions of the Natural Remanent Magnetization (NRM) display a wide scatter implying unreliability of the surface data. Reduction to pole (RTP) transformation of magnetic anomalies was successful with the induced magnetization angle despite the complex pattern of magnetic anomalies.

Tectonomagmatic Evolution of Bimodal Plutons in the Central Anatolian Crystalline Complex, Turkey: A Discussion

Figure 1. Aeromagnetic anomaly map (fig. 4 of Kadioglu et al. 2003). Kadioglu et al. (2003) say that this map is the digitized map of Hutchison et al. (1962). I have read the article by Kadioglu et al. (2003) and have several serious concerns about the geophysical section, “Magnetic Anomaly of the Agacoren Intrusive Suite.” First, the aeromagnetic anomaly map shown in figure 4 of Kadioglu et al. (2003) and reproduced here as figure 1 cannot

Earth analog for Martian magnetic anomalies: remanence properties of hemo-ilmenite norites in the Bjerkreim-Sokndal intrusion, Rogaland, Norway

To explain the very large remanent magnetic anomalies on Mars, which no longer has a global magnetic field, it is important to evaluate rocks on Earth with the necessary properties of high natural remanent magnetization (NRM) and coercivity. Here, we describe a possible analog from the 230-km 2 930 Ma Bjerkreim-Sokndal layered intrusion (BKS) in Rogaland, Norway. In the layered series of the BKS, fractional crystallization of jotunitic magma was punctuated by influx and mixing of more primitive magmas, producing six megacyclic units, each typically with early plagioclase-rich norites, intermediate hemo-ilmenite-rich norites and late magnetite norites with subordinate near end-member ilmenite. Following each influx, the magma resumed normal crystallization and, following the last, near the base of Megacyclic Unit IV, crystallization continued until norites gave way to massive fayalite-magnetite mangerites and quartz mangerites in the upper part of the intrusion. The Megacycles are marked on a regional aeromagnetic map by remanent-controlled negative anomalies over ilmenite norites and induced positive anomalies over magnetite norites and mangerites. A prominent negative anomaly (with amplitude −13,000 nT in a high-resolution helicopter survey, down to −27,000 nT below background in ground magnetic profiles) occurs over the central part of Megacyclic Unit IV. The anomaly is centered on ilmenite norite Unit IVe and is most intense where cumulate layering is near vertical at the southeast edge of the Bjerkreim Lobe of the intrusion at Heskestad. Here, Unit IVe is flanked to the east by magnetite norite of Unit IVc and country-rock gneisses (group E) and to the west by Unit IVf magnetite norite and mangerites (group W). Magnetic properties were measured on 128 oriented samples. Susceptibilities are similar for all three sample groups at ∼8×10 −2, but Koenigsberger ratios are very different, with average values of 7.7 for IVe, and <1 for groups E and W. The IVe samples, with only a few percent of oxides, have the highest NRMs measured from the BKS, up to 74 A/m, with an average of 30.6 A/m, making them prime candidates for consideration as Mars analogs. The mean direction for IVe samples is D=17.6°, I=−79.9, a 95=10°, almost opposite the present field. Evidence on origin of the strong NRM in IVe as compared to groups E and W, include greater abundance of hemo-ilmenite and of orthopyroxene with hemo-ilmenite exsolution, and the strong lattice-preferred orientation of both in a relationship favorable for “lamellar magnetism”. Massive magnetite-free hemo-ilmenite ores in anorthosite from the same district also produce negative magnetic anomalies. They have a substantial but much lower NRM, suggesting that there are special oxide properties in the IVe rocks at Heskestad.

Structural model of the San Bernardino basin, California, from analysis of gravity, aeromagnetic, and seismicity data

The San Bernardino basin is an area of Quaternary extension between the San Jacinto and San Andreas Fault zones in southern California. New gravity data are combined with aeromagnetic data to produce two‐ and three‐dimensional models of the basin floor. These models are used to identify specific faults that have normal displacements. In addition, aeromagnetic maps of the basin constrain strike‐slip offset on many faults. Relocated seismicity, focal mechanisms, and a seismic reflection profile for the basin area support interpretations of the gravity and magnetic anomalies. The shape of the basin revealed by our interpretations is different from past interpretations, broadening its areal extent while confining the deepest parts to an area along the modern San Jacinto fault, west of the city of San Bernardino. Through these geophysical observations and related geologic information, we propose a model for the development of the basin. The San Jacinto fault‐related strike‐slip displacements started on fault strands in the basin having a stepping geometry thus forming a pull‐apart graben, and finally cut through the graben in a simpler, bending geometry. In this model, the San Bernardino strand of the San Andreas Fault has little influence on the formation of the basin. The deep, central part of the basin resembles classic pull‐apart structures and our model describes a high level of detail for this structure that can be compared to other pull‐apart structures as well as analog and numerical models in order to better understand timing and kinematics of pull‐apart basin formation.

Skeletonization of aeromagnetic data

Skeletonization is a syntactic pattern‐recognition method that is applied to gridded data to produce an automatic line drawing, with an associated event catalog. Previous implementations of skeletonization have been tailored for seismic data. Here, we modify that technique to render it more suitable for other types of gridded data, with particular emphasis on aeromagnetic maps. A modification from previous schemes is the use of a two‐pass approach, to reduce the effects of an otherwise problematic directional bias that discriminates against events oriented parallel to columns of the grid. The method can be used effectively for filtering aeromagnetic data on the basis of strike direction, event linearity, event amplitude, and polarity. It is based on the delineation of peak‐trough pairs (cycles), which are traced throughout the grid to form contiguous events. Cycles and events are characterized by attributes that include amplitude, polarity, and pulse width. Events are further characterized by length, average strike direction, and linearity. The event attributes are stored in a catalog, thus enabling one to perform attribute‐based analysis and data filtering. We illustrate our algorithm using two regional aeromagnetic examples from different parts of the Canadian Shield. The first, from the Great Slave Lake shear zone, is dominated by linear anomaly trends produced by faults and mafic dikes. The second, from the Manicouagan region of northeastern Quebec, contains abundant subcircular and arcuate anomaly patterns caused by large intrusive complexes and a meteorite impact structure.

Deduction of tectonic structure of North and Central Sinai from aeromagnetic and Bouguer anomaly maps

The aim of the present work is to deduce the tectonic structure of the area and to show the role of the separation methods in the resolution of the multisource of magnetic and gravity implications. The aeromagnetic total intensity map was transformed to the reduced pole aeromagnetic map (RTP). This step was followed by linear wavelength filtering of the RTP aeromagnetic map, and of the Bouguer anomaly map, in order to isolate shallower residual components using different grid intervals. A comparative analysis was carried out for the different filtered RTP aeromagnetic and gravity maps. Statistical trend analysis was applied for the magnetic and gravity data. The study revealed that the area has been affected by three significant tectonic trends, namely; NE-SW, NW-SE and E-W. The RTP aeromagnetic map and Bouguer anomaly maps provided detailed data for 2d modeling of the source. The magnetic and gravity cross section indicates possible sources of the observed anomalies. Only depth of the anomaly sources (mostly in the basement) were determined from magnetic and gravity data to reveal a tectonic zone and structure which leads to a fluid migration. The result of depth estimation to the basement complex revealed that the depth of the basement ranges between 3.2 km and 4.3 km.

The Precambrian supracrustal rocks in the Naternaq (Lersletten) and Ikamiut areas, central West Greenland

Naternaq, or Lersletten, in central West Greenland is an extensive Quaternary outwash plain characterised by light grey, silty sediments. Scattered low hills with outcrops of crystalline Precambrian basement rocks protrude from the outwash plain and form the northern part of the Nagssugtoqidian orogen (e.g. Connelly et al. 2000). The prominent Naternaq supracrustal belt, at least 25 km long and up to c. 2 km wide, occurs along the north-western margin of Lersletten, bordered on both sides by Archaean orthogneisses and granitic rocks; the supracrustal rocks outline a major fold structure with an irregular and sporadically exposed hinge zone (Fig. 1). The supracrustal rocks, including the fold closure, exhibit a negative signature on the regional aeromagnetic map (Fig. 2). The belt is known for its disseminated and massive iron sulphide mineralisation with minor copper and zinc, which is common in the south-eastern part of the belt.