We present the results of gravity modelling of the NSDP-84 deep seismic
reflection profiles in the northern North Sea. the Moho depths inferred
from forward modelling of marine Bouguer gravity anomalies, from
independent inversion of free air gravity anomalies derived from
satellite altimetry, and from deep seismic reflection profiling are
in good mutual agreement. This agreement suggests that the Moho discontinuity
interpreted from seismic reflection data corresponds to a major jump
in density and therefore is likely to represent not only the geophysical
but also the petrological boundary between the continental crust
and the upper mantle. In spite of the significant lateral heterogeneity
of the crystalline basement below the northern North Sea area as
documented by seismic reflection profiling, the average density contrast
between the basement and the upper mantle is interpreted to show
rather little variation. This is not true for the deepest part of
the Viking Graben where the nature of the lower crust and upper mantle
may have been changed by magmatic processes accompanying the graben
formation. A relationship between the degree of crustal extension
and the volume of magmatic intrusion into the lower crust is tentatively
supported by our results.