Crustal modeling of the North Atlantic from spectrally correlated free-air and terrain gravity

Abstract

We investigate the use of the correlations between free-air and terrain gravity for extending critical, but spatially restricted seismic constraints on the crust of the North Atlantic region. In this tectonically complicated area, we spectrally correlate spherical coordinate free-air gravity anomalies against computed gravity effects of the terrain at 20 km altitude for regional crustal thickness variations. However, at the Mid-Atlantic Ridge, thermal plume, and rift zones of Iceland, our gravity Moho estimates are deeper than the seismic estimates to suggest local reductions of upper mantle density. Invoking thermal expansion, partial melting, and related mantle depletion effectively accounts for the reductions that range up to a few percent of mantle density. The resultant North Atlantic crustal model conforms well to regional seismic constraints. Crustal thickness estimates for Iceland ranging from 25 to 35 km are consistent with seismic predictions of a thickened crust. However, about the plume and rift zones, temperatures approaching the basalt solidus may reach depths up to about 20 km. Continental crust extending from the Jan Mayen Platform to the eastern Iceland Plateau appears to have rifted off eastern Greenland by magnetic chron 7 (ca. 26–28 Ma). A now obscure transform boundary on the Iceland Plateau that may include a rift or fault separating the southeastern Iceland Plateau from the rest of the plateau transformed Reykjanes Ridge spreading from the extinct Aegir Ridge. Thickened crust extends up to 500 km across the Greenland–Scotland Ridge. Prominent necking of our crustal thickness estimates for the Greenland–Iceland Ridge immediately west and east of the Iceland Plateau may reflect the reduction in the strength of the Icelandic Plume at ca. 26–28 Ma. However, our results also map the broad, thickened crust of the Iceland Plateau and prominent Reykjanes Ridge that developed with the resurgence of the Icelandic Plume at ca. 25 Ma.