Free-air gravity anomalies across the southeast Greenland margin are invest
igated in conjunction with a well-constrained seismic velocity model to pro
vide a constraint on subsurface density structure. This volcanic rifted mar
gin is characterized by the presence of similar to 30-km-thick igneous crus
t, which correlates with a positive gravity high of similar to 60 mGal. A n
ew systematic approach is adopted for gravity modeling, which consists of (
1) full error propagation from the velocity model to predicted gravity anom
alies through a posteriori model covariance represented by Monte Carlo ense
mbles, (2) the inversion of residual gravity anomalies for density variatio
ns within geological subdomains, and (3) the joint inversion of seismic tra
vel times and gravity anomalies. A density model derived from the velocity
model, using conventional conversion laws for the continental and oceanic c
rust, substantially underpredicts the observed gravity by similar to 70 mGa
l over the continental shelf. Neither errors in the velocity model nor the
uncertainty of the chosen conversion laws are shown to be sufficient for su
ch a large gravity misfit. A possible range of mantle contribution is first
investigated by modeling various thermal evolution and depletion scenarios
, which suggests that the maximum contribution is only similar to 20 mGal,
assuming constant source mantle composition throughout continental rifting
and subsequent seafloor spreading. If most of the residual gravity anomaly
has a crustal origin, applying a conversion law with a denser upper crust i
n the continent-ocean transition zone seems to be the only plausible option
to resolve this difficulty. Contrasting eruption environments for the tran
sition zone crust (subaerial) and the oceanic crust (submarine) probably re
sult in different porosity structures, td which a velocity-density relation
ship is highly sensitive. The wire log and laboratory measurements of plate
au basalts recovered from recent drilling legs on North Atlantic margins se
em to support this explanation. An alternative explanation, which invokes a
strong degree of source mantle heterogeneity, is also plausible on the bas
is of a recent geochemical study of the North Atlantic igneous province.