JAMNORT-Project

Main aims of the JAMNORT-Project, (W. Weigel, Univ. Hamburg, RV VALDIVIA, 1987) were,

1. the study of the deep structure on the Jan Mayen Ridge, and the question, whether the ridge is a continental fragment separated from Greenland,
2. the structure of the fossil Aegir Ridge, an extinct spreading center, and a comparison to active spreading ridges and the variation of the crust dependent on its age.

The crustal structure was investigated by deep seismic sounding (OBS measurements) of explosion and airgun signals and gravity profiling along the strike direction and perpendicular to the ridge axis in cooperation of IfG Hamburg (W. Weigel), IOS, Great Brittain (B. Whitmarsh), and IFREMER Brest, France (F. Avedik). In 1989 additional seismic reflection lines were recorded mainly perpendicular to the ridge axis (VALDIVIA Cruise 84). These investigations are to be seen as presite survey for the ODP-proposal by Whitmarsh, Weigel et al. (1989).
The main result of the Aegir Ridge investigation is a depression of an oceanic crust of about 2000 m by cooling. The Moho also subsided from a formerly updomed stage to a normal oceanic depth of 11 km. The subsided, about 30 km wide zone is filled with sediments of about 800 m thickness. In strike direction a strong variation of layer 2, 3a and 3b has been observed. This indicates different magmatic activity along the ridge axis. In the south of our axis profile (south of 66°ree;N) a more than 4 km thick layer 2 (4.7 km/s) covers directly layer 3b (7.0 km/s).

Iceland-Faeroe-Ridge (IFR) and Aegir-Ridge

Two further projects (Inst. of Geophysics, Kiel University, Fr. Theilen) have been funded by the BMFT on the investigation of the sedimentation and subsidence history of the Iceland-Faeroe-Ridge (IFR) involving also the complex structures in the junction area between the IFR and the Aegir-Ridge. About 3000 miles of multichannel reflection seismic profiles are available on the northern flank of the IFR. The seismic sections show a clear record of the sedimentary sequences as well as the basement structures such as magmatic flows especially in the junction area. The structures of the extinct Aegir-Ridge are compared with those of an active ridge on the Kolbeinsey-Ridge north of Iceland.

The sedimentation and subsidence history of the aseismic Iceland-Faeroe-Ridge is important for the understanding of the paleoceanographic and paleoclimatic situation on the northern hemisphere. It formed a barrier between the Norwegian-Greenland Sea and the North Atlantic Ocean preventing the exchange of water masses between both basins. The opening history of the Norwegian-Greenland Sea is more complex than of other oceanic basins. The opening of the Norwegian-Greenland Seas started in the upper Palaeocene (55 ma). The active Aegir Ridge extinguished 28 ma ago and spreading shifted to the Kolbeinsey-Ridge north of Iceland. Three major areas have been studied with multichannel reflection seismic methods during recent years:

- The northern flank of the Iceland-Faeroe-Ridge
- The junction area between the Iceland-Faeroe-Ridge and the formerly active Aegir-Ridge
- The southern section of the Kolbeinsey-Ridge

In total about 3500 miles of seismic profiles have been acquired in these areas. The measurements were performed with a 24-channel streamer (active length: 600 m), an airgun array of 4.5 liters and a digital data acquisition system. The seismic sections show mainly the sedimentary sequences down to the crystalline basement, but there is only little penetration into the basaltic layers.
The seismic sections, which are connected to DSDP-Site 336, reveal the whole sediment sequence on the northern flank of the Iceland-Faeroe-Ridge. The Eocene covers the basement nearly to the ridge crest indicating that it was submerged with the exception of the crestal area during that epoch. Miocene is missing et the DSDP-Site 336, however, a wedge onlapping on Mid-Oligocene and covered by Pliocene was found furtherto the north down to the Norwegian Basin. This sequence could be of Miocene age. The Iceland-Faeroe-Ridge and the Aegir-Ridge are of similar age. In the junction area the Aegir-Ridge, which shows rather morphologic features of an elongated basin than that of a ridge, affects the Iceland-Faeroe-Ridge by a valley filled with distorted sediment layers which are obviously of Eocene age just above the crystalline basement. Distinct lava flows could be recognized at the lower slope of the Iceland-Plateau below a sediment coverage of more than 1000 m. The southern part of the Kolbeinsey-Ridge is characterized by hydrothermal activities. The morphologic structure is rather flat in the south and becomes more developed to the north. The location of a magma chamber could well be found in the seismic records.
The investigation of these areas cover the structures of an aseismic ridge as well as an extinguished and an active ridge. They represent different stages of the tectonic development of the Greenland-Scotland-Ridge complex.