Aeromagnetic maps of parts of southern and central West Greenland: acquisition, compilation and general analysis of data

Approximately 52 000 line km of aeromagnetic data were acquired in 1975 and 1976. The data are mainly compiled from six survey areas between MON and 68°15'N, and from reconnaissance lines flown over several regions of West Greenland between 62°30'N and nON. The compilation involved editing and cleaning of data, first order correction of diurnal variation by use of a filtered base station magnetic fieid, further magnetic levelling using tie lines, and finally gridding of data. From the gridded data a coloured contour map has been produced at a scale of 1:500 000 together with black and white contoured maps at 1:250000. The report contains a general discusion of the major anomalies. The major elements in the discusssion are the relationships between the aeromagnetic field and geological features such as metamorphic facies boundaries, lithological units and structural lineaments.

, and radiometric data (Secher, 1976(Secher, , 1977, were acquired in a joint operation using the same aircraft. Since then the aeromagnetic data have been cleaned and compiled into preliminary versions of contoured maps. FolIowing the airborne operations magnetic anomalies were investigated on the ground to determine the source of the anomalies (Thorning et al., 1978;Thorning, 1979;Mielby & Svendsen, 1979;Secher & Thorning, 1981).
At the time of acquisition of the aeromagnetic data few facilities were available at GGU for the compilation and presentation of the data. These facilities were developed parallel with the use of the data in such a way that later aeromagnetic data acquired from other areas using other techniques could also be handled (e.g. Larsen & Thorning, 1980). The data from West Greenland served as test material for the development of computer programs. The first programs were described in Thorning (1977b), and since then an increasing number of computer programs have been produced and integrated into a flexible system for compilation, interpretation and presentation of aeromagnetic data (Thorning, 1982).
The aeromagnetic data are published as a colour map at a scale of 1:500 000, and in the near future more detailed contoured maps at 1:250 000 will be published.
This report briefly reviews the acquisition of the data and gives background information on the compilation techniques used in the preparation of the maps. Some of the major magnetic trends and anomalies and their geological significance are discussed; detailed and quantitative interpretation of the data in terms of geological structures will be the subject of separate reports.

SURVEY OPERATIONS
The acquisition of the aeromagnetic data in the field was carried out as a cooperative venture between the Sections for are Geology and for Geophysics of GGU, responsibie for the radiometric and the aeromagnetic work respectively, and a group from the Electronics Department, Risø National Laboratory, responsibie for the instrumentation of the aircraft.
The survey was restricted to a number of selected areas ( fig. 1 and Table 1) as it was not possibie to carry out a systematic coverage of the total area which could be reached from the base of operations. Additional reconnaissance lines were flown over areas which could not be surveyed systematically ( fig. 2).
The airborne oprations were based in Søndre Strømfjord. A few natural landing strips were used for refuelling to increase the operational range of the aircraft. An average of 210 flight hours/year were used, including ferry flights.
. The instrumentation was fitted in a twin engine (STOL) Britten-Norman Islander, char-  tered from Vaengir Airtransport, Iceland, and fitted with Risø/GGU instrumentation. The aircraft had an endurance of 8-9 hours and was operated at an air speed of 200-210 km/h. The magnetometer was a Geometrics G-80l proton precession magnetometer with a sensitivity of 1 nanotesia (nT) at a sampling interval of 1 sec., corresponding to a distance of 40-60 m between sampling points. The instrument was fitted for digital recording and in-flight analog display of data. A similar magnetometer, also with digital recording of data, was used as a ground station magnetometer, placed at exactly the same position in Søndre Strømfjord both years. This was operated at a 10 sec. sampling interval. Time was the common parameter for the two systems, obtained from synchronously working quartz clocks. Magnetic data, time, barometric and radar altitudes were digitally recorded. The surveys were flown after preplotted flight lines by visual navigation with the flight path registered by single frame photography.
Diurnal variations of the geomagnetic field poses a severe problem at the latitudes of southern and central West Greenland. The short period of time available for the operation did not allow strict adherence to some diurnal variation specification. However, there was no flying during severe magnetic disturbances, and ongoing flights were abandoned on the onset of such disturbances. It was attempted, in every way possible, to keep to a minimum the effects of diurnal variations on the data.

SURVEY AREAS
Six areas ( fig. 1) were selected for systematic aeromagnetic coverage. Theyencompass most of the major geological boundaries and structures between MON and 68°l5'N. Logistic considerations were taken into account in the selection of the areas such as distance to the area from the airport and the topography of the areas. Different combinations of survey parameters were applied (Table 1), chosento give information on geological structures and at the same time test the applicability of a certain set of survey parameters in relation to the geology and topography. Some general comments on each of the areas are given below. Roman numbers in brackets refer to fig. 1. The northernmost survey area (I) was intended to cover some of the main features of the Nagssugtoqidian mobile belt (Escher & Watt, 1976;Korstgård, 1979). The survey was flown in 1976, and based on experience of the previous year a fixed barometric altitude of 915 m (3000 ft) was maintained throughout the area. A small area north-east of Holsteinsborg could not be flown at this altitude, and there is a gap in the data coverage at this location. The constant flight altitude of 915 m means that the distance to the sources of the magnetic anomalies varies considerably. Over the north-west corner of the area, around Agto, the surfaee is at a distance of 900-1000 metres depending on height of terrain and depth of water. Over the south-east corner, especiaIly in the highlands around Kingatsiaq, the distance is a couple of hundred metres, less over individual peaks. This is noticeable in the data, and should be taken into account when using the map on a regional scale. Similar comments are applicable to the other survey areas.
The region just south of 67°N has been covered by two separate surveys in 1975. The survey of the part west of 51°45'W, the western Søndre Strømfjord survey (II), has been extended southward along the coast to Sukkertoppen to obtain a continuous N-S coverage here. This area was flown at a constant barometric altitude of 1830 m (6000 ft) clearing the peaks in the mountainous region around Søndre Strømfjord. The eastern part, the eastern Søndre Strømfjord survey (III) east of 51°45'W, and north of the small ice arm of the Inland Ice north of Majorqaq, was flown using a technique often referred to as 'drape-flying'. The northern end of the flight lines was flown at 915 m (3000 ft), the southern end at up to 1830 m (6000 ft) altitude.
Two surveys cover the major part of the Godthåbsfjord region. The peninsula of Nordlandet (IV) was covered in 1975 and 1976 by a relatively detailed survey at 450 m (1500 ft) altitude, partly designed to see how low and close it was practical to operate. The survey of the area east of 51°20' from MON to 65°20'N (V) flown in 1975 at an altitude of 1525 m (5000 ft) is of a more regional type.
an ferry flights to these two areas the existence of a large anomaly just south of Majorqaq (65°40'N, 51°lO'W) was noticed. Part of the available resources in 1975 were redirected in the field to extend the 1525 m survey over the Godthåbsfjord region north into the region around Majorqaq (VI) using similar survey parameters there.
A number of reconnaissance lines were flown in areas not surveyed in a systematical manner including some long N-S profiles together reaching from Fiskenaesset to Nugssuaq ( fig. 2). Severallines were acquired over three main regions: Nugssuaq, the Sukkertoppen Iskappe, and arou~d Fiskenaesset. Since no aeromagnetic contour maps will be produced from these areas the data have been inluded in this report in the form of profile maps and will be discussed together with the aeromagnetic maps.

COMPILATION OF DATA
The aeromagnetic data of this report served as test material for the development of software for the compilation and presentation of aeromagnetic data. The folIowing description of the compilation procedure only includes the methods of processing accepted for general use, and contained in the system of programs now being used (Thorning, 1982). The process diagram of fig. 3 illustrates the main steps in the compilation process.

Navigational data
Flight paths were reeorded by single frame photography, and later plotted on stable 1:100 000 topographic maps (enlargements of 1:250000 maps).
The aeeuraey with whieh individual points ean be identified on the photographs depends on the altitude above ground and thus vanes eonsiderably. For the surveys described in this report the position accuracy is usually a few metres. Such points could be transferred to the base map with an accuracy corresponding to approximately 50 m.
Some caution was necessary in the plotting of points: aircraft groundspeed was calculated along flight lines and abrupt changes in speed were otten related to inaccuracies in the base maps. Most points were plotted with sufficient accuracy for the scale of the work and were then digitized. Linear interpolation was used between the plotted points. Taking all inaccuracies into account the digitized flight path positions can be expected to Iie within 100 m and probably better along flight lines.
The digitized flight path data were merged with the digital magnetic data, using time as a common parameter, and the total bulk of data organized in a magnetic tape data base (ADT, Thorning, 1982).

Correction of magnetic data
It is advantageous that geologically interesting anomalies are of high frequency and considerable magnitude, and thus dominate the smaller low frequency 'anomalies' caused by diurnal effects. This diurnal noise has to be eliminated during compilation of the magnetic data. Atter the data had gone through a simple editing for spikes, erroneous time or date etc., at least two steps were necessary to remove diurnal effects. The first deals with long period variations often causing the average levelof the geomagnetic field to change hundreds of nanotesia from day to day, superimposing a similar bias on the profile data, also on days where there are no or few short period variations. The second deals with the shorter period variations superimposing lesser variations in the levelof the profile data.
The tirst correction to the magnetic data was the subtraction of a smoothed geomagnetic fieid, calculated as the difference between the base station magnetic field and an all summer average of the geomagnetic field at the base station. This procedure was only adopted atter a considerable number of tests as the distance between some of the survey areas and the base station is such that lateral amplitude and phase variations of the geomagnetic field could be expected.
The effect of this correction was checked profile by protile by superimposed plots of profile data before and atter correction, and the corresponding base station data using time as one axis. In most cases the result was a significant improvement of the correspondance between neighbouring profiles, e.g. fig. 4.
The distribution of the intersection difference data also improved statistically, even in the areas farthest away from Søndre Strømfjord. In fig. 5 histograms of intersection differences from the Godthåbsfjord survey (175-300 km from Søndre Strømfjord) before and after correction are shown to iIIustrate this. A final, more subjective test, was also applied by contouring the data before and after the first correction.
This method of a first correction of the data worked well for most of the data, and in all survey areas a considerable improvement in the appearance of the contour maps was noted atter correction. In a few cases, however, the process introduced noticeable errors into the profile data. These errors were eliminated by careful editing in each individual case, and if this was not possibie the profile in question was not used in the compilation process.
Undetected errors of small magnitude may have been introduced, but the second step in the correction procedure eliminated these. The second correction of the data utilized the tie lines flown in each survey area. These were flown during the best possibie diurnal conditions, but even so some diurnal variation remained in the tie lines. Due to this and the inaccuracies in positioning mentioned above it was not possibie to attempt a perfect levelling aimed at zero intersection differences at all intersections between tie lines and profiles.
A technique by Yarger et al. (1978) was adopted after a number of tests and has been used in all areas. The method reduces in a statisticai sense the overall intersection differences through a correction of the profile data, calculated by polynomial approximation of intersec-  tion differences. In some cases it was necessary to except single intersection points from the analysis, because they were so much in error (placed at a gradient, or through incorrect positioning) that they seriously hindered the method in producing good results. The method resulted in significantly reduced intersection difference values in all survey areas and greatly improved the appearance of the contour maps. The tie lines have not been used in the contouring, which is therefore based only on the corrected profiles. Figure 6 shows an example of the effect of this correction. A more detailed explanation of the use of this method is given in the description of the programs NIVELl and NIVEL2 in Thorning (1982).

Reference field
The International Geomagnetic Reference Field (IGRF) has been used with the 1975 coefficients, subtracted on a point to point basis along individual profiles. Several sets of coefficients were tested, but the IGRF 1975 set was used, because it seemed satisfactory in these areas and was internationally accepted. The DGRF 1975(e.g. Peddie, 1983 came toa late to be of use in this study.

Gridding and contouring
The corrected data were used for contouring and construction of profile maps. The gridding of the data was done by one Dr other of the two methods described in Thorning (1982) producing very similar results. Contouring of the gridded data into contour line maps was used as a last quality control. In some areas traces of diurnal or navigational disturbances not completely eliminated by the processing carried out so far, was removed by a light ellipsoidal filtering of the gridded data (different cut-off wave1engths in two perpendicular directions). The final accepted grid files were transferred into a raster type format used directly for the production of colour contour maps (Applicon Jet Ink plotter) of each survey area, later joined together to form the 1:500 000 map in colour.

Pro/ile maps
Reconnaissance lines not suitable for contouring were subjected to the same compilation process and the data plotted as profile maps (TRACK, Thorning, 1982). Note that both the horizontal and magnetic scales vary from area to area.

REGIONAL SURVEYS
A qualitative discussion of the main aeromagnetic features and their relation to the geology of the area is presented below. Recent accounts of the geology ean be found in Escher & Watt (1976), Korstgård (1979) and Kalsbeek (1981). The area is covered by GGU's 1:500000 geological map sheets Frederikshåb Isblink -Søndre Strømfjord and Søndre Strømfjord -Nugssuaq. The geological features treated in this discussion are best illustrated by the 1:500000 magnetic anomaly map in colour, but for easy reference profile maps of the magnetic data have been included in this discussion.

Regional aeromagnetic profile
The long aeromagnetic profile from Hel!efiskeøer near Fiskenaesset to Boyes Sø on Nugssuaq (c. 800 km) is presented in fig. 7. Numbers in brackets refer to the numbers on fig. 7.
The magnetic profile (eomposite of two) has been plotted along a topographie section and a geological section (taken from the 1:500000 geological maps) for easy comparison. The profiles were flown at an altitude of 1980 m (6500 ft) along the 51°W longitude, but slightly higher over the ice cap around 66°N. Thus the anomalies are mostly fairly smooth in appearance.
The anomaly at the northern end of the profile over the Atå Sund area (1) is associated with the supracrustal rocks in the area (Escher & Pulvertaft, 1976). The anomaly is situated over metasediments and metavolcanics on the northern flank of the Talorssuit dome, and its source is a smal! banded ironstone occurrence (L. Keto, personal communication, 1983). The anomaly can also be seen on the profiles from Nugssuaq ( fig. 8) where it is seen to extend into an easterly direction along the coast. The Talorssuit gneiss dome itseif leaves little impression on the magnetic fieid.
The sinistral transcurrent fault zone at Påkitsoq, north of Jakobshavn, taken to be the boundary between the Nagssugtoqidian and Rinkian mobile belts is not readily apparent in the magnetic data, although variations are present (2) which may be attributed to the fault zone.
A significant magnetic maximum (3) can be seen near Christianshåb. This is an area with various granodioritic gneisses, and mica schists with garnet, sillimanite and muscovite in Magnetic measurements solid black curve (relative to IGRF75). Numbers in brackets refer to anomalies discussed in the text. Granulite (gra) and amphibolite (amph) metamorphic facies rocks are indicated. Rock types are those given on 1:500000 geological map. amphibolite facies. The anomaly may be an indication of other rock types at depth, or there may be an as yet undetected variation in metamorphic fades. Further investigation is warranted here.
Many of the geological features of the Nagssugtoqidian mobile belt can be seen on the profile. The Nordre Strømfjord shear zone is distinguished as a slight minimum (4), although the effect from a nearby quartz diorite intrusion obscures the anomaly making it less obvious here than further to the west. The effect of the various metamorphic and deformational boundaries dominates the field further south. Sometimes there is good agreement, as at the southern boundary of the Isortoq complex (6), and at other localities the relationship seems to be of a different nature (e.g. 5). These features will be discussed later.
The boundary between the Nagssugtoqidian mobile belt and the Archaean craton, a few kilometres south of Søndre Strømfjord, can be seen in the magnetic data as a well-defined change in magnetic level and expression (7).
The variations in the aeromagnetic field south of the boundary are of higher amplitude and shorter wave length, culminating under the Sukkertoppen Iskappe (8), partly because of the proximity of the magnetic sources here. The belt of granulite facies rocks just south of Majorqaq results in a significant anomaly (9), and further southwards variations in composition and metamorphic facies of the basement are reflected in the magnetic anomalies (discussed further later). The magnetic field is very smooth over the Godthåbsfjord region, but south of Ameralik two anomalies (10 and 11) correlate well with the position of known metamorphic amphibolite granulite fades boundaries. It these anomalies, and the considerably higher magnetic leveIs south of both of them, really are caused by granulite facies rocks then the amphibolite facies rocks mapped in the Sermilik region may be a thin layer on top of granulite facies rocks below.
There is thus evidence that metamorphic facies boundaries playan important role for the magnetic field in parts of West Greenland. In a number of cases there seems to be good agreement between mapped fades boundaries and anomalies (6,7,9,10,11), in other regions discrepancies exist (5, south of 9, between 10 and 11). Some possibie reasons for this will be discussed in the folIowing sections. Reconnaissance profiles over Disko (not shown here) exhibit similar negative anomalies associated with the reversely magnetized basalts.

Profiles over NOgssuaq
Where the anomalies of the Tertiary lavas do not dominate, the anomalies reflect variations in the Precambrian basement, partly below the Cretaceous-Tertiary sediments. The fault structures limiting the Cretaceous basin to the east, Sarqaqdalen, leave no impression 52°20' 52°10' 52°00' 51°50' 51°40' 51°40' 1-500nT on the magnetic field at this altitude. The anomaly near Torssukåtak has already been discussed ( fig. 7 around l).

Profiles over Sukkertoppen Iskappe
High level profiles over Sukkertoppen Iskappe ( fig. 9) show the magnetically significant boundary between the Nagssugtoqidian mobile belt and the Archaean craton in a manner similar to that observed further east along the boundary in the Søndre Strømfjord survey area (also seen in fig. 7, anomaly 7).

Profiles over the Fiskenaesset region
Several profiles were flown in the Fiskenaesset region ( fig. 10) at various altitudes depending on the terrain. Therefore, the shape of corresponding anomalies varies from line to line. The most significant feature is the increase in the magnetic fjeld just south of Graedefjord, the same as anomaly (11) in fig. 7. This corresponds exactly to a mapped facies boundary separating amphibolite facies rocks in the northern part from granulite facies rocks further south (Kalsbeek, 1976).

The Nagssugtoqidian mobile belt survey
The data coverage in this area is gaad. The profile map af fig. Ila cantains anly half af the profiles flown, yet there is good continuation of both major and minor anomalies from line to line, and there is more detail available than ean be treated in the context of this report.
The northern part of the survey area is dominated by a linear minimum which ean be followed from the coast just north of Nordre Strømfjord (67°34'N, 53°45'W) to the edge of the Inland Ice (68°10'N, 50 0 10'W). This magnetic anamaly correlates with the mapped part af the Nordre Strømfjord shear belt, (Bak et al., 1975;Olesen & Sørensen, 1976), and it ean be assumed that the continuation of the shear belt to the east ean be traced by the narrowing magnetic minimum. This demonstrates that the magnetite content of the rocks in the shear belt is less than that of the rocks outside the shear belt in agreement with the suggestion of Bridgwater & Myers (1979) that titanomagnetite becomes unstable in the shear zone and less magnetic hydrated ferrous oxides form. The variation in magnetite content is most clearly defined in the western part, where the rocks in the shear belt contrast with the surrounding rocks in granulite facies. Further east the surrounding rocks are in amphibolite facies, at least at the present erosion surface, and consequently the difference in magnetic properties is less pronounced.
The aeromagnetic field exhibits a number of positive and negative linear anomalies semiparallel with the Nordre Strømfjord shear belt, and it is likely they reflect the smaller shear zones occurring in the area (see Korstgård, 1979).
South of the eastern end of the shear zone an isolated magnetic maximum correlates with the southern part of the quartz diorite intrusion, just north of and partly below the glacier UsugdlUp serrnia (67°58'N, 50 0 15'W), indicating that this part of the Proterozoic intrusion (Kalsbeek, personal communication, 1983) is significantly different from the remaining part of the intrusion. This is supported by the high magnetic susceptibilities reported from this particular area contrasting with lower susceptibility values in the northern part of the intrusion (Thorning et al., 1978).
In the north-west corner of the survey area near Agto the magnetic anomalies correlate well with metamorphic facies boundaries although the boundaries are not so clearly defined as in some examples described later. Two magnetic minima (67°50'N, 53°42'W, and 68°02'N, 53°06'W) are situated over granodioritic gneisses (amphibolite facies), and the magnetic level is significantly higher over the nearby hypersthene gneisses (granulite facies). Further east (68°01'N, 52°1O'W) a magnetic minimum corresponds to a small area with granodioritic gneiss, but the maximum (distorted by Alangordleq and Arfersiorfik fjords), just east ofthis, is also situated over the same gneiss. The reason for this 'inverted' relationship is not known.
The general trend of the magnetic anomalies in the southern part of the survey area follows the dominant trend of the geology. A band of highly magnetic rock types ean be followed from the coast south of Holsteinsborg (see discussion of the Søndre Strømfjord surveys) to the Inland Ice, although a gap in data coverage prevents direct observation in part of the area. The southern limit of this magnetic band corresponds to the Holsteinsborg thrust zone or the boundary between the Ikertoq and Isortoq complexes. This boundary has been placed at various positions. The 1:500 000 geological map places its intersection with the edge of the Inland Ice approximately 10 km north of the Isunguata serrnia glacier at Akuliaruserssuk (67°20'N, 49°45'W), whereas the more recent work of  and Korstgård (1979) puts it at the northern limit of the Isunguata sermia glacier. Trends corresponding to both these positions ean clearly be observed in the magnetic map, but the latter position is favoured by the magnetic data. Furthermore, an even more southerly position of the geological boundary may be indicated for the easternmost part of the boundary. The detailed structure of this deformational and metamorphic boundary at the coast south of Holsteinsborg has been discussed by Grocott (1979). He demonstrates that facies and deformation boundaries do not necessarily coincide, although the different types of boundaries are usually subparallel. The details in the aeromagnetic field over the approximate position of the boundary near the Inland lee may be related to a similar structure of the boundary. Arrows indicatc approxilll<ltc position af the Nordre SlrømlJord shcar belt.
The northern limit af tlle band af highly magnetic rocks is cxceedingly wcJI defincd and euts tlle Kuk valley at 67°33'N. 50 0 3YW, but at a position whcrc the geologicai map gives 110 indication af a significant geological boundary.
Thc geological map indicatcs a more nonhcrly position ol' tlle bounJary between granulitc and amphibolitc faejes, and ir this is COrrect an alternative explanalion is neccssary for the magnetic anomaly pattern. However. it is also possibie that the magnetically dcfined boundary is the bcies boundary and that tlle geologi cal map conscquently is in error here. In the profilc map of fig. 11 the form of tne anomalics at tlle boundary indicates a fairly sharp magnetic boundary, i.e. a fault or sharp lithological boundary. From the dctaits in the anomalies over {hc northern parI offlle band af highly magnetic rocks it appears that there is an clongated magnetic bod Ysituateu at or near the boundary. The anomaly eulminates in the soulhern pan of Eq,dungmiut nllllfit (67°35' N , 50°15'\\'), partJy over <in area of granodioritic gncisscs which are in amphibolite faeies. A fcw days reconnaissance work was carried uut in the mea in 1977 (Thorning et al., 1978), but rcsuits were ineunc1usive. Thc anomaly may bc caused by a decp-seated and as yet unknown body.
A general feature of the aeromagnetic field in the Nagssugtoqidian mobile belt survey is ; U!!s~Llil.
WQrlh mentioning. It cannOl ca:)ily bc sccn in the profile map of fig. 11. but ean he observcd Oll Cl ddailcd magnetic contour 1l18p. Tile presence uf the Inland Ice has resutted in a tiLting of the ice-free land leading to a higher rate of erosion nea( thc CQasl. Thc presenr surfaee cOllsequently rcprcscnts an inciined section through lhc crust. exposing ;)fllphibo!itc fades rocks in the castcrn part and the mure dccpcr-scated granulite f<lcies rocks in thc western part of tlle are" amund Nordre Strømfjord-l\rfcl'siorfik-Ugssuil. Conscqucntly lhe N-S cxposcd (-"cics bOl.muary is posifioncd where tile indincd facies boundary is eul by thc prcsenl Ut-\Y surface. This boundary ean bc sten in the magnetic fieid, bul in a manner distincrly different from what has been onsen'cd abour tlle mainly E-W facies boundaries associatcd wirh shear belts etc. Thc N~S boundary is difficult to placc cxacrly from rhe magnetic data. It is gradual wirh many minor and major magnetic anomalies euning across. and ean onl)' bc pcrccived ir the different character af lhe magnetic field as il whole is t"ken inlo account with a general tendency towards higher amplitudes anu sllOrtcr wavelenglh anomalies in the west. and a more smooth pattcrn in the east. The Søndre Strømfjord surveys Thc two survey Jreas making up the Søndre Str0mfjord surveys cover the major geologieal houndary between tlle Archaean craton and the Nagssugtoqidian mobile belt. The boundary is an obvious feature 011 the aeromagnetic map and the profile maps af figs 12 and 13. In the eastern survcy area ( fig. 12) the ArchaC31l granulitc facies gneisses to the south ol' the boundary are mostl)' highly magnetic. and the cffeel ol' topography is significant becausc (he magnetic SOU1-cCS are at the surrace. Tlle amphibolilC facies gllcisscs in the Ikertoq shear zone af rlTe Nagssugtoqidian mobile belt north af the boundary are an/y wcakly rnagnetized, and the topography hardly affects the magnetic fieid. It is no{ easy to placc thc bOllndary exactly using the magnetic data, but there is a gaod correlation with lhe geologi cal boundary, and hoth th<: mapped thmst zone and tJle lransjtjon~·tl bOlJlldary belWccll amphiboJire and gratlulite [aeies gncisses seem to affcct {ile magnetic fieid. NO details were obtained from the aerornagnetic data from north ol' the boundary in the castern Søndre Strømfjord survey. Soulh of thc boundary. in lhc Archaean cTaton, a number of features are visible in the magnetic data. Thc Cam brian Sarfårtoq carbonatite (Larsen el al.,19S3) can bc seen just 011 the boundary as a smal! maximum surrounded by a zone of relatively non-magnetic rocks (66OJO'N. SlOtS/W). A delailed magnetic investigation was carried out by Sechcr & Thorning (1981).
Thc arnphibolitc facies gncisscs around (he western end of {he lakc ·hlsersiaq (60 0 1S'N. SIOIO'W) extending along the lake to the eas t is rcvcaled as a magnetic minimulll. il crosses Søndre SIr0mfjord and north to the Itivdleq Jjord. The magnetic highs in the western Søndre Sfrømfjord survey all correlate with granulite facics gneisses. whcre al50 the lOpography is clearly reflcclcu in the magnetic data. The ttivdleq she"r bell amphibo\ilc facies gneisscs create a linear minimum herc. The are;] of granulite facics gneisses north of Iti\'dleq and the small arca of similar character j usl Horth uf the anorthositc between Itivdleq and Søndre Strømfjord are c1early deIincatcd by magnetic maxima. The fault zone and faeies boundary just south uf Holsteinsborg. the western continuation uf the silllilar boundary discussed abovc. are 3150 visible.
The southern part of the western survcy area reachcs south to Sukkertoppen and cxhihilS a number of positive anomaJies which can be correJated with the tupography. The source af the annmafies is mnstly !he granulite faeies rocks exposed at the surfaee in this area.

The Majorqaq survey
The data from this area also demonstrate the relaliunship betwecn the amphibolitcgranulile melamorphic boundary and the aeromagnetic anomaIies (see fig. 14). The area with granuIite faeies rocks just south of the 'fajurqaq valley is perfeetl)' delineated by the large positive magnetic anomctly. investigated in some detail by Thorning el al. (1978) and Mielby & Svendsen (1979). North of this at Ihe cdge of tlle iee cap granulite facies rocks are exposed at lhe surface, and along most of the northern limit of thc survey area there are corresponding magnetic anomalies, from which tile arnphibolite granulite facies boundary ean be placcd, also whcre j( is not cxposed. East of the iee dammed lake lluliagdlup tasia there is a positive magnetic anomaly in an area mappcd as being in amphibolitc facies (65°45'N. 51°30'W). The cause af Ihis anomaly is not known, but it may he an undeteetcd ar uncxposcd area af granuiite faeies rocks. Thc relative minimum bctween this anomaly and the Majorqaq anomaly is prohably a topo· graphic effeet from the Majorqaq valley, and not an indication ef two separate magnetic bodics.
South af the valley af IsuitSllp kua iso la ted spots af granulitc faeies gneisses oecur in an afC3 af amphiboJite facies gneisses. Thc aeromagnetic anomalies indicate that thc granulite [aeies rocks are more widcspread and interconnectcd below the surface. A possibie explanalion for this is that rhe amphibolite gneisscs mappcd at the surface may be only a relatively thin layer. The granulitc facies gncisses occur al same af the topographicaJly highest areas, e.g. the Majorqaq anomaly. and the amphibolite gneisscs al the topographically low areas, e.g. JsuitslIp k(w, and it is therefore ncccssary to pieture fhe granuJite-amphibolite boundary as an undulating surfaee only parti}' exposed. An alternative cxplanation, assuming a morc horizontal boundary and retrogrcssive metamorphisrn along tectonicaJly aClive trends, e.g.
the Majorqaq and lsuitsup k(la valleys. is. also compatibie with the magnetic data. The Qaqarssuk carbonalitc intrusion appears as a cireular magnetic anomaly over the carhonatite bod Y (65°22'N, 5P38'W). Thc anomaly is situated in a linear magnetic low which trends SW-NE and cross euts the boundary ro thc Finnefjeld gneiss. The cause of the linear minimum is unknown.
In the south-east corner of the survey area the effect of the Taserssuaq tonalite can he seen as a magnetic maximum. Tlle amphibolites and metasedimeots near the boundary of the Inland Ice leave 00 impression in tlle aeromagoetic field.
The Godthåbst]ord slJrvey Tlle northcrn half of the aeromagnetic map af this survey area is dominated by a large positive anornaly associaced with the Tascrssuaq tonalite ( fig. 15). Topographic effccts from e.g. (he glacier Sarqap scrmerssua, tlle lake Tascrssuaq, and the large Narssarssuaq plain, creatc relative minima over these Jocalities, but athenvise the Tascrssuaq tonalile, onl)' affected by granulite faeies metamorphisnl in some parts (Allaart el 0.1.,1977), is characlerized by a fairl}' high magnetic level caused by {;ompositionaJ differences belween lhe IOnalite and the gneisses. The magnctic anomaly follows the limits of the tonalite along the Ataneq fault in the area between lIulialik and Isukasia, studicd in detail by Mielby & Svendsen (1979), but in the northcrn part af the Godthåbsfjord the positive anomaly seems to cxtend southwards across the south-east continuation of the Ataneq fault and Out into the fjord.
Smaller isolated anomalies are locatcd over the suite of supracrustal rocks at lvisårtoq and lsukasia. At Isukasia the are of supracrustal rocks is dearly delineated by the positjve anomalies associated with the banded iranstone. The main ironstone dcposit of Tsukasia is at the limit of the aeromagnetic map, and here the amplitude of the magnetic residual anomaly is aTOund 30000 oT at ao altitude of 30G-400 m over tlle deposit. The source of the linear anomal y at. lvisanoq has been shown to be a suite of amphibolites and ul1rabasic rocks with magnctite and chlorite (Mielby & Svendsen, 1979). In the Ujaragssuit nunåt area similar anomalies have been detected near other bands of amphibolite, where small occurrences of uItrabasic rocks have also been mapped.
The most common rock types in the Godthåbsfjord area are the Amitsoq (3700 Ma) and Nfik (3000 Ma) gneisses. Field investigations (Thorning, 1978) have shown that these do not differ noticeable in magnetic properties, and consequently the magnetic anomaly map reveals littie about the relative distribution of these rock types. In one region of mainly Amitsoq type gneisses between Isukasia and Ivisartoq the average magnetic level is lower than e.g. that of the dominantly Nuk type gneisses to the south of Ivisartoq, but this low magnetic level also continues to the north of Isukasia across the Ataneq fauIt, where the gneiss is of the Nfik type. Thus compositional differences between the Nl1k and Amitsoq gneisses of this region are hardly the cause of the anomaly pattern.
An alternative explanation for the fairly high magnetic level over the area of the inner Godthåbsfjord (Nunatarssuaq, Kangiussap nuna, Kapisigdlit timat) must be sought. The magnetic field is smooth, and the rugged topography with deep fjords and peaks up to 1500 m a.s.!. usually has only a small effect on the magnetic fieid, indicating that no significant near surface magnetic sources exist. The distribution of anomalies bears little relation to the surface geology with the exception of Qardlit nunat, which is in granulite facies. These observations force the conclusion that the source of the magnetic anomalies is a subsurface body of considerable bulk and extent, perhaps of the same kind as the Taserssuaq tonalite, and that this subsurface magnetic body is absent in the Isukasia-Ivisartoq area. Deeper levels of intrusions related to the Qorqut granite may be an alternative explanation. It is also possibie that the deep sources of the anomalies are related to the domming of the area south of Ivisartoq required by Brewer et al. (1983) and Chadwick et al. (1983) in their structural analysis of the area.
The Qorqut granite occurs in an elongated area around the Omanap suvdlua fjord. A magnetic anomaly with slightly smaller amplitude than the one described in the preceeding paragraph covers the same elongated area. A limited number of field measurements of magnetic susceptibilities (Thorning, 1979) did not allowa statistical separation of Qorqut granite from the Nfik and Amitsoq gneisses based on magnetic susceptibility, but it appears from the aeromagnetic map that the bulk magnetic properties of the Qorqut granite, including deeper levels of the intrusion, add up to a well defined, albeit small, positive regional anomaly.
The magnetic trend corresponding to the Ataneq fauIt is well defined between Isukasia and Ilulialik, and further to the south-west there are subtle variations in the magnetic anomaly pattern indicating an extension of the fauIt in a south-west direction across Godthåbsfjord to the coast of Storø, somewhere north-west of Umanaq. The strike is similar to the strike of the Omanap suvdlua fjord, and to the mylonite zones of James (1975). Another trend in the magnetic anomalies can be picked up on Storø: a fault with a slightly different strike cuts across the Amitsoq gneiss near the central part of Storø. This aligns perfectly with a magnetic trend going from here in an ENE direction across Kangiussap nuna to Ivisartoq, where it is parallel with the fold axial traces of Hall & Friend (1979) or the south-west Ivisårtoq synform of Chadwick et al. (1983), separating the magnetically different regions to the north and south. The trend is also parallel with another fault cutting across Ilulialik and the southern part of the exposed Taserssuaq tonalite. This indicates that the geologicaIly mapped lineaments (mylonites, fauIts and fold axes) may be a partial surface expression of major structures at deeper levels of the crust. The origin of these structures may be related to transcurrent faulting within the West Greenland Archaean craton 2000-1800 Ma ago (e.g. Smith & Dymek, 1983).

The Nordlandet survey
The part of Nordlandet covered by the aeromagnetic survey is shown on the 1:500000 geological map to consist almost exc1usively of hypersthene gneiss, with the exception of an anorthosite body in the north-west corner of the survey area and Niik gneiss in the north-east corner. Nevertheless, the relatively detailed aeromagnetic measurements ( fig. 16) indicate significant variations in physical properties.
The boundary towards the Niik gneiss in the north-east corner leaves little trace in the magnetic data indicating that it may be just a thin superficial layer. Along the entire east coast of Nordlandet a major magnetic anomaly exists. It is bordered towards the NNW by an almost linear minimum, and towards the SSE the anomaly disappears 2-3 km away from the coast. The linear band of highly magnetic rock is thus 10-15 km in width. The south-west corner of the survey area is magnetically smoother at a lower level, but no geological explanation is known to account for this. Around M025'N, 5l o 48'W the magnetic anomalies form a pattern, resembling a large 'V' open to the north around a minimum just west of the lake Sardlup taserssua. According to James (1975) an intrusive elIiptical granite body here forms the core (at the magnetic minimum) of a domal antiform. The V-shaped anomaly reflects variations or folds in the gneiss, and the same trend ean be seen further to the northwest. Many detaiIs are visible in the north-west corner of the survey area mapped as variable granulite facies gneiss (Garde & McGregor, 1982). Static retrogression to amphiboIite facies occurs Iocally. Thus, the aeromagnetic field over Nordlandet reflects quite subtle variations in the composition and metamorphic state of the gneisses, and therefore provides a means for their mapping. The anorthosite body in the north-west part of the survey area is revealed by a magnetic minimum and an associated linear maximum to the north-west of this. The minimum is less well developed over the northern part of the anorthosite body.
The dominating magnetic trend on Nordlandet is NNE-SSW. This is also the trend of the mylonite zones described by James (1975).

CONCLUSIONS
The discussion in the previous section has been qualitative. More information is, of course, contained in the data, and detailed results will be reported elsewhere. Whereas a number of conc1usions concerning specific geological problems have been stated in the discussion, conc1usions of a more general type are summarized below.
(1) Aeromagnetic anomalies ean be used to map various rock units defined in terms of different composition (e.g. the Taserssuaq granodiorite) or in terms of tectonic-metamorphic processes (e.g. structures in the Nagssugtoqidian mobile belt). This difference in magnetic pattern has recently been used in a successful attempt to map the course of geological units across the Inland Ice to the east coast of Greenland (Thorning et al., 1984).
(2) Metamorphic facies boundaries playan important role for the pattern of aeromagnetic anomalies, and it has been demonstrated that the type of facies boundary has an influence on the detaiIs in the magnetic expression. However, it still remains to be analysed how details in the original rock compositions and in the metamorphic process, e.g. retrogressive or progressive, influence the content of magnetic minerals, and what is the relative importance of coincident metamorphic and tectonic boundaries.
(3) Shear zones of various types can be mapped over large distances by these aeromagnetics paUerns. Subsurface expressions of such zones are also visible in the magnetic data, and this allows correlation of the partial expressions of these structures mapped at the surface. There are indications that such zones are numerous in the Archaean craton.
(4) In some cases the aeromagnetic data reveallocal features, some of direct economic interest, e.g. the Sarfårtoq and Qaqarssuk carbonatites and the Isukasia banded ironstone. Further detailed geophysical and geological work is nearly always necessary over such local features because of the regional character of the aeromagnetic data acquired.
(5) The aeromagnetic data are thus a useful addition to the geological mapping, and it would be worth while in the future to combine aeromagnetic surveying and geological mapping more closely, as the two methods are complementary.