Gravity anomalies and crustal structure at the southeast Greenland margin

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.