Sedimentary basins west of the exposed part of the West-Spitsbergen orogenic belt have been outlined
by seismic reflection data. Narrow, fault-bounded grabens are infilled from both sides, with synsedimentary movement and weak compression of the whole sequence. The grabens are probably all of
Tertiary age. Sediment thicknesses are up to 4 km and widths are up to 15-20 km. The main graben starts
in the north with the Forlandsundet graben and is offset outside Isfjorden and possibly south of Bellsund.
We are not able to discriminate between an extensional and a strike-slip model of graben-formation.
Based on lateral velocity variations and thickness of post-rift sediments on the outer shelf and slope west
of Spitsbergen, we assume 1-2 km of Tertiary sediments have been eroded from the offshore basins and
3 km in central Spitsbergen.