Tecronophysics, 178 (1990) 241-254 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands 241 Deep crustal structure of the northern North Sea Viking Graben: results from deep reflection seismic and gravity data C. FICHLER and J. HOSPERS Division of Petroleum Engineering and Applied Geophysics, The Norwegian Institute of Technology, N-7034 Trondheim (Norway) (Received September 26.1989; revised accepted December 22.1989) zyxwvutsrqponmlkjihgfedcbaZYXW Abstract Fichler, C. and Hospers, J., 1990. Deep crustal structure of the northern North Sea Viking Graben: results from deep reflection seismic and gravity data. Tecronophysics, 178: 241-254. The combination of deep reflection seismic interpretation (NSDP 84 data set) and 3-D modelling of free-air gravity data resulted in a regional crustal model for the Viking Graben area. The interpretation focuses on the structure of the crystalline crust which is displayed by depth sections, a map of the Moho, a map of the thickness of the crystalline crust and a map of the extension factor 8. The thickness of the crystalline crust varies from 10 km near the centre of the graben to 30 km under the adjacent platforms. The thickness of the sedimentary cover varies from 11 km to 1 km, respectively. The topography of the Moho reflects, in a highly smoothed way, the top of the crystalline basement. The “seismic” Moho coincides with the “gravity” Moho, although some discrepancies are observed in the most extensively stretched areas. High /?-values are observed, but no indications of the presence of oceanic-type crust associated with a crustal rupture have been found. The results are thought to favour a stretching model of graben development. A strong inclined reflection and associated reflection patterns in the East Shetland Platform are tentatively interpreted as a fragment of the Iapetus Suture. The asymmetrical structure inhomogeneities in the crust. of the graben is assumed to be associated with initial Introduction The Viking Graben is the northern arm of the North Sea graben system, situated between south- ern Norway and the Shetland Islands. The study area is shown in Fig. 1. In early Palaeozoic times the area was formed by the collision of the continents of Laurentia and Fennosarmatia (cf. Glennie, 1984). The location of the Iapetus Suture in the North Sea is only ap- proximately known and is discussed here with regard to the Viking Graben area. Two major active rift phases occurred in the Viking Graben area, the first one dated between Permian and early Triassic times, the second one between mid-Jurassic and early Cretaceous times (cf. Badley et al., 1988). Between the rift phases a compressional pulse may have occurred (Frost, 1987). During and after the tectonically active phases the graben formation continued with fur- ther subsidence and deposition of sediments. Whereas the sediments of the Viking Graben have been extensively explored in the past (cf. Ziegler, 1982), the deeper crust has only recently been investigated. Deep seismic profiling was car- ried out along a line crossing the northernmost Viking Graben (Beach et al., 1987) and along the NSDP 84 (North Sea Deep Seismic Profiling 1984) lines (Gibbs, 1987; Klemperer, 1988). These inves- tigations imaged the structure of the deeper crust and of the underlying uppermost mantle and pro- vided some information on the Moho. Gravity modelling has also been used to investigate the position of the Moho (Donato and Tully, 1981; 0040-1951/90/%03.50 0 1990 - Elsevier Science Publishers B.V.