Crustal structure of the continent-ocean transition zone along two deep seismic transects in north-western Spitsbergen

Czuba W, Ritzmann O, Nishimura Y, Grad M, Mjelde R, Guterch A, Jokat W

Polar Polish Research

25(3-4), 2004, 205-221

Deep seismic sounding measurements were performed in the continent-ocean transition zone of north-western Spitsbergen, during the expedition ARKTIS XV/2 of the RV Polarstern and the Polish ship Eltanin in 1999. Profile AWI-99200 is 430 km long and runs from the Molloy Deep in the Northern Atlantic to Nordaustlandet in north-eastern Svalbard. Profile AWI-99400 is 360 km long and runs from the Hovgård Ridge to Billeflorden. Seismic energy (airgun and TNT shots) was recorded by land (on-shore) seismic stations (REF) and ocean bottom seismometers (OBS) and hydrophone systems (OBH). Good quality refracted and reflected P waves were recorded along the two profiles providing an excellent data base for a detailed seismic modelling along the profile tracks. Clear seismic records from airgun shots were obtained up to distances of 200 km at land stations and 50 km at OBSs. TNT explosions were recorded even up to distances of 300 km. A minimum depth of about 6 km of the Moho discontinuity was found east of the Molloy Deep. Here, the upper mantle exhibits P-wave velocity of about 7.9 km/s, and the crustal thickness does not exceed 4 km. The continent-ocean transition zone to the east is characterised by a complex seismic structure. The zone is covered by deep sedimentary basins. The Moho interface dips down to 28 km beneath the continental part of the 99200 profile, and down to 32 km beneath the 99400 profile. The P-wave velocity below the Moho increases up to 8.15 km/s. The continental crust consists of two or three crystalline layers. There is a lowermost crustal continental layer, in the 99400 profile's model, with the P-wave velocity in order of 7 km/s, which does not exist in the continental crust along the 99200 profile. Additionally, along the 99200 profile, we have found two reflectors in the lower lithosphere at depths of 14-42 and 40-50 km dipping eastward, with P-wave velocity contrasts of about 0.2 km/s. The characteristics of the region bears a shear-rift tectonic setting. The continent-ocean transition zone along the 99200 profile is mostly dominated by extension, so the last stage of the development of the margin can be classified as rifting. The uplifted Moho boundary close to the Molloy Deep can be interpreted as a south-western end of the Molloy Ridge. The margin in the 99400 profile area is of transform character.