Description:

The continent–ocean transition off East Greenland, between the Jan Mayen and Greenland Senya fracture zones, is less well known than the well explored and studied volcanic rifted margin off Norway. The geophysical and geological data at the Norwegian Vøring and Lofoten margins reveal important vertical and lateral variations in crustal structure and composition. The deeper structure of the East Greenland margin, conjugate to the Vøring Plateau, is therefore of special interest.

One of the main targets is to estimate the amount of magmatic material which intruded into the crust and/or was deposited as a high velocity body beneath the crust which can be interpreted as underplating. The results can be directly compared to the adjacent and conjugate margins, e.g. the Vøring Plateau on the Norwegian conjugate margin, where there is a huge magmatic complex. Existing gravity and magnetic data acquired on- and offshore of the East Greenland margin do not yet support the presence of a feature like the “Vøring Marginal High”. Although multichannel seismic (MCS) and potential field data exist along the East Greenland margin, the data density is insufficient to image lateral variations in the deeper structure. One striking feature of the East Greenland margin, which most likely was connected with the initial break-up in Tertiary times, is marked by a pronounced negative magnetic anomaly. The anomaly extends parallel to the coast over a distance of more than 400 km between Kong Oscar Fjord at 72° N and south of Shannon Island at 75° N. Its shallow and deep structure is not known so far.

Therefore, the Alfred Wegener Institute for Polar and Marine Research started an expedition with the research vessel “RV Polarstern” in 2003. The investigations off East Greenland were done within the framework of the EUROMARGINS project. The main targets of the project are to investigate the crustal architecture and the evolution of the conjugate volcanic margins off mid-Norway and East Greenland in the context of rifting. Deep seismic refraction data were acquired on four 300 – 450 km long profiles (Figure 1). The complete dataset provides new insights into the formation of the entire segment of the continental margin between the Jan Mayen and the Greenland fracture zones. Gathered aeromagnetic data, which cover the area over the two southern seismic refraction profiles, AWI-20030400 and AWI-20030500, will provide constraints on the extent of the magnetic anomaly (Figure 2). In addition, shipborne gravity data were acquired along all transects. All profiles are very well located for comparison of the conjugate Norwegian Vøring and Lofoten margins.

A total of 25 ocean bottom seismometers (OBS) were deployed along the 330 km long northernmost profile, AWI-20030200, from the Greenland shelf into the Greenland basin. The transect, near the Greenland Fracture Zone, crosses an area with very thick sedimentary basins. Typical seismic velocities and thicknesses were found for deep sea sediments and the oceanic crust further offshore. In contrast to the southern profiles, no evidence for underplating could be found. The second profile, AWI-20030300, was acquired in the prolongation of the Ardencaple Fjord south of Shannon Island with 25 OBS and six REFTEK land stations. The transect can be divided into three parts. Continental crust, a diffuse continent-ocean transition zone (COT), and oceanic crust with increasing thickness from the eastern end of the profile towards the COT. Seaward dipping reflectors (SDRs) can be inferred from MCS data and from slow p-wave velocities in the upper oceanic crust between anomalies C24 and C23.

In the prolongation of the Godthåb Gulf the entire profile AWI-20030400 has a total length of 320 km and can also be divided into three parts. Continental crust appears in the west. MCS data and refraction data reveal a wide transitional crust. The profile terminates in the east over thin oceanic crust. Evidence of strong magmatic activity can be assumed from strong variations in the velocity-depth function and a huge high velocity body with velocities of between 7.0 and 7.4 km/s.

Profile AWI-20030500 crosses the Greenland shelf from the Kejser Franz Joseph Fjord, terminates 90 km east of a basement ridge in the deep sea and has a total length of 460 km. P-wave modelling shows considerable similarity to profile AWI-20030400. The thin oceanic crust thickens beneath the basement ridge. The COT is characterised by glacially compacted sediments in the top sediment layers on the shelf and magmatic material underplated with p-wave velocities of 7.0 - 7.3 km/s and with variable thicknesses. The continental crust appears in the western end of this profile with similar thicknesses as on the profile above.

The newly acquired aeromagnetic data show high frequency variations within the pronounced negative anomaly and eastward. The wavelengths of these variations are approx. 5 km and it is assumed that this is due to magmatic intrusions. The p-wave models of the two southern seismic refraction profiles, AWI-20030400 and AWI-20030500, show striking correlations with the potential field data, e.g. the continental basement highs with free air gravity anomalies and the onset of underplating with the negative magnetic anomaly.

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