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Upwelling or downwelling flow in Earth's mantle is thought to elevate or depress Earth's surface on a continental scale. Direct observation of this 'dynamic topography' on the seafloor, however, has remained elusive because it is obscured by isostatically supported topography caused by near-surface density variations. We calculate the nonisostatic topography of the North Atlantic by correcting seafloor depths for lithospheric cooling and sediment loading, and find that seafloor west of the Mid-Atlantic Ridge is an average of 0.5 km deeper than it is to the east. We are able to reproduce this basic observation in a model of mantle flow driven by tomographically inferred mantle densities. This model shows that the Farallon slab, currently in the lower mantle beneath the east coast of North America, induces downwelling flow that deepens the western North Atlantic relative to the east. Our model also predicts dynamic support of observed topographic highs near Iceland and the Azores, but suggests that the Icelandic high is due to local upper-mantle upwelling, while the Azores high is part of a plate-scale lower-mantle upwelling to the south. An anomalously deep area off the coast of Nova Scotia may be associated with the downwelling component of edge-driven convection at the continental boundary. Thus, several of the seafloor's topographic features can only be understood in terms of dynamic support from flow in Earth's mantle. Keywords: dynamic topography, seafloor depth, mantle flow, North Atlantic, Iceland, Azores, Scotian Basin.

Prestack depth-migrated seismic reflection data collected off Flemish Cap on the Newfoundland margin show a structure of abruptly thinning continental crust that leads into an oceanic accretion system. Within continental crust, there is no clear evidence for detachment surfaces analogous to the S reflection off the conjugate Galicia Bank margin, demonstrating a first-order asymmetry in final rift development. Anomalously thin (3-4 km), magmatically produced oceanic crust abuts very thin continental crust and is highly tectonized. This indicates that initial accretion of the oceanic crust was in a magma-limited setting similar to present-day ultraslow spreading environments. Seaward, oceanic crust thins to Keywords: continental margin, seafloor spreading, extension tectonics, continental breakup.

We present the combined results of deep multichannel reflection and refraction seismic surveys across the Flemish Cap--Goban Spur conjugate margin pair (North Atlantic), which we use to infer rifting style and breakup. Profiles on both margins cross magnetic anomaly 34 and extend into oceanic crust, making it possible to observe the complete history from continental rifting through to the formation of initial oceanic crust. The deep multichannel seismic (MCS) reflection data have previously been used to support a model of symmetric pure shear extension followed by asymmetric breakup and a sharp continent-ocean boundary. Using both types of seismic data, our results indicate instead that asymmetric structures are formed during all stages of rifting, breakup, and complex transition to oceanic spreading. The differing nature of the two ocean-continent transition zones is particularly striking. For Flemish Cap, our reprocessed image of the MCS profile clearly shows tilted fault blocks beneath back-tilted sediment packages, consistent with a wide region of highly thinned continental crust inferred from wide-angle seismic data. In contrast, normal incidence and wide-angle seismic data for the Goban Spur transition zone indicate the presence of exhumed serpentinized mantle. [ABSTRACT FROM AUTHOR]