Geological Survey of Denmark and Greenland Bulletin 31, 2014, 63-66 63 Aeromagnetic survey in south-eastern Greenland: project Aeromag 2013 Peter Riisager and Thorkild M. Rasmussen Aeromagnetic surveys are nowadays used at a wide range of scales and purposes. In frontier and under-explored areas, where data are otherwise sparse or non-existent, aeromagnet- ic acquisition remains the cheapest and easiest way to obtain or refine a picture of the structural setting. Aeromagnetic data are also useful for strategic planning of geological map- ping campaigns and detailed geophysical data acquisition. Moreover, aeromagnetic data are of importance for prospect- ing, helping to define prospects. Large aeromagnetic surveys can be carried out efficiently and safely almost everywhere, in a short period of time and at reasonable cost. In the following we present the newly released Aeromag 2013 aeromagnetic survey that covers a remote and relatively under-explored coastal region in south-eastern Greenland, stretching from 64°45́ N and northward to 67°30´N (Fig. 1). The survey represents a total of 65 492 line km, and cov- ers an area of 30 100 km2, adding a significant new dataset to the already existing database of government-financed geo- physical surveys in Greenland. With the completion of the Aeromag 2013 project, the database presently contains a total of c. 633 500 line km of high-resolution aeromagnetic data and c. 75 000 line km of multi-parameter data (electromag- netic, magnetic and partly radiometric data). Further details on previous surveys on Greenland and the database of avail- able aeromagnetic data are summarised in Rasmussen et al. (2013; see also Fig. 1). Details of the Aeromag 2013 survey EON Geosciences Inc. flew the Aeromag 2013 survey be- tween 13 June and 2 September 2013, using two Piper Nava- jo PA-31 aircraft equipped with geophysical instruments (de- tailed below), and operating out of the airport at Kulusuk. The magnetic base stations used for correction of diurnal magnetic variations were installed at two different locations in Kulusuk. The survey was carried out by flying along a gently draped surface 300 m above the ground or sea level. Due to the severe topography ranging from sea level to c. 2450 m, the gentle drape resulted in an average height above ground of 711 m. The survey lines were NE–SW-oriented, parallel to the coastline with a separation of 500 m while orthogonal tie-lines were flown with a separation of 5000 m. Total mag- netic field data were recorded with a sampling interval of 0.1 sec. which corresponds to a sample distance of c. 7 m. The magnetic field at the base station was recorded with a 1 sec. sampling interval. Aircraft positional data from differential GPS measurements were recorded with a 1 sec. sampling interval, and aircraft altitude measurements obtained from barometric altimeter and radar were recorded with a sam- pling interval of 0.1 sec. A continuous digital video recording of the terrain passing below was also produced. Further de- tails on the survey operation and equipment can be found in a report by EON Geosciences Inc. (2013), which is available at the online DODEX database at the Geological Survey of Denmark and Greenland (Riisager et al. 2011). © 2014 GEUS. Geological Survey of Denmark and Greenland Bulletin 31, 63–67. Open access: www.geus.dk/publications/bull AEM 1998 AEM 1994 AEM 1995 AEM 1996 AEM 1997 AEM 1998 Aeromag 2001 Aeromag 2013 Aeromag 2012 Aeromag 1997 Aeromag 1992 Aeromag 1998 Aeromag 1996 Aeromag 1995 Aeromag 1999 80°N 75°N 65°N 60°N 80°W 60°W 40°W 50°W 40°W 20°W 20°W 0°W Greenland 500 km 60°N 75°N 65°N 70°N 70°N Fig. 1. Map of Greenland showing the location of government-financed high-resolution airborne geophysical surveys conducted from 1992 to 2014. Red: aeromagnetic surveys (Aeromag). Blue: combined electromag- netic and magnetic surveys (AEM). 6464 Results and products Aeromag 2013 survey dataset can be obtained as line Geomagnetic Reference Field corresponding to the date and location of the measurement has been subtracted from the data leaving the magnetic anomalies caused by the variation of magnetisation in the uppermost crust. Superimposed on the magnetic anomaly data in Fig. 2 is a shaded relief mod- elled by using a light-source illumination inclination of 45° and a declination of 45° (i.e. a light-source from the north- west). Release of data Aeromag 2013 project was marked obtained for free from the Ministry of Industry and Min- eral Resources in Greenland by submitting a form available at the Greenland Mineral Resources portal (http://www. greenmin.gl/). Magnetic anomaly maps and geological implications Nagssugtoqidian orogen that mainly consists of reworked Archaean gneisses with minor supracrustal rocks and several Palaeoproterozoic intrusives (Bridgwater et al. 1990; Kolb in press). Palaeogene intrusions and coast-parallel dykes are found in the northern part of the survey area (Tegner et al. 1998). Aeromag 2013 survey area range in amplitudes between –1318 nT and +3270 nT, with both the most negative and positive values relating to mapped intrusions (Figs 2, 3). In the southern part of the survey area, several NNE–SSW-trending subpar- allel linear and positive anomalies (marked I in Fig. 2) are anomalies can be traced in the survey area over more than Fig. 2. Aeromagnetic anomaly map of the survey area in south-eastern Greenland covered dur- ing the Aeromag 2013 project. I–V: magnetic anomalies discussed in the text. I: Possible large dykes NNE–SSW. II: Magnetic E–W low. III: Ammassalik igneous complex. IV: 1.9–2.2 Ga di- orite intrusion. V: Kruse Fjord gabbro complex. http://www.greenmin.gl/ http://www.greenmin.gl/ 65 - eral of these anomalies appear to be truncated by a magnetic low (II; Fig. 2) just north of the Ammassalik igneous com- have amplitudes in the order of 300 nT, with a magnetisation - ing a magnetisation dominated by induced magnetisation or a remanent magnetisation with a direction in the same gen- (Bridgwater et al. 1990). In order to estimate the depth of the source of the magnetic anomalies, Euler deconvolution (Reid et al. 1990) was carried out using the Standard Euler - od is based on Euler’s homogeneity equation that relates the Euler’s equation simultaneously for each grid position within a window and then determines the anomaly position, depth, parameter in the Euler deconvolution is the structural index, which enters as an exponential factor corresponding to the a given geometry. For our analysis we used a structural index value of 1, which is suitable for a dyke, and a window size of 2 generally below 400 m. Given the uncertainty of the depth area (Becker et al. 2009) where the magnetic anomalies are the coast. Finally, we note that the Kangâmiut dykes found in West Greenland, in a similar location on the North At- lantic craton (south of the central part of the Nagssugtoq- idian orogen in West Greenland) are much less prominent magnetically (Rasmussen & van Gool 2000; Korstgård et al. 2006) than the linear magnetic anomalies seen in the Aero- mag 2013 survey area. We tentatively interpret the linear magnetic anomalies as massive dykes. - massalik igneous complex marked II in Fig. 2 coincides with a suggested suture zone of the Nagssugtoqidian orogen, where the Rae craton to the north-east in an oblique WSW- directed subduction collided with the North Atlantic craton south of the suture zone at c. 1870–1885 Ma (Kolb in press). by the apparent abrupt termination of several of the positive Fig. 3. Geological map of the survey area in south-eastern Greenland (modi�ed from Escher 1990). Red: the Aeromag 2013 survey area. I–V: magnetic anomalies discussed in the text. 6666 NNE–SSW-trending anomalies marked I on Fig. 2 and dis- cussed above. North of the Ammassalik igneous complex the c. 1.7 Ga post-orogenic granodiorite intrusion marked III in Fig. 2 is clearly defined by positive magnetic anomalies with ampli- tudes up to almost 2000 nT. Similarly, the older 1.9–2.2 Ga diorite intrusion farther north marked IV in Fig. 2 is associ- ated with a strong (c. 2500 nT) positive magnetic anomaly. The strongest magnetic anomalies in the Aeromag 2013 sur- vey area are found in the northern part of the survey area and can be related to the Palaeogene intrusions in the area. The Kruuse Fjord gabbro complex marked V in Fig. 2 is related to a negative anomaly, hence having a magnetisation dominated by reversely magnetised remanent magnetisation, which is in excellent accordance with an Ar-Ar isochron age of 48.0 ± 1.2 Ma, and emplacement of the intrusive complex during the reverse C21R chron (Cande & Kent 1995). Conclusions In this paper we present the newly released Aeromag 2013 survey that adds new and exciting data to the already exten- sive database of Greenland aeromagnetic data. The paper focuses on magnetic anomalies of regional extent, including sub-parallel linear and positive anomalies trending NNE– SSW (marked I on Fig. 2) that we suggest stem from hith- erto undiscovered very large dykes. The suggested location of the suture zone of the Nagssugtoqidian just north of the Ammassalik igneous complex (II; Fig. 2) is supported by the aeromagnetic data. Finally, we note a general good cor- respondence between the mapped surface geology of the re- gion and the aeromagnetic data; in particular, the intrusions which are clearly discernible. The magnetic data provide a basis for further analysis and modelling of the 3D geometry of the igneous intrusions. Many more local anomalies can be identified in the Aeromag 2013 dataset but interpretations require further analyses. Acknowledgements Funding of the Aeromag 2013 project was provided by the Ministry of In- dustry and Mineral Resources, Government of Greenland. Thanks are due to EON Geosciences Inc. for fulfilling all aspects of their contracts in a professional manner. References Becker, J.J. et al. 2009: Global bathymetry and elevation data at 30 arc seconds resolution: SRTM30_PLUS. Marine Geodesy 32, 355–371. Bridgwater, D., Austrheim, H., Hansen, B.T., Mengel, F., Pedersen, S. & Winter, J. 1990: The Proterozoic Nagssugtoqidian mobile belt of south- east Greenland: a link between the eastern Canadian and Baltic shields. Geoscience Canada 17, 305–310. Cande, S.C. & Kent, D.V. 1995: Revised calibration of the geomagnetic polarity timescale for the Late Cretaceous and Cenozoic.  Journal of Geophysical Research, Solid Earth 100, 6093–6095. Eon Geosciences Inc. 2013: Final survey report. 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Authors’ address Geological Survey of Denmark and Greenland, Øster Voldgade 10, DK-1350 Copenhagen K, Denmark. E-mail: pri@geus.dk.