The first two traverses of marine multichannel seismic data across the Lomonosov Ridge (central Arctic), by the German research icebreaker Polarstern and the Swedish icebreaker Oden, demonstrate a prograded margin toward the Amerasian side and fault-bounded half grabens toward the Eurasian side of the ridge. Nearly 450m of undisturbed flat-lying strata have been deposited on top of the peneplaned ridge since it rifted from the Barents-Kara Sea margin and subsided below sea level in early Tertiary time (64-56 Ma) . -Authors