Ice station bathymetric profiles and submarine echograms reveal a steep escarpment at least 200 km long with up to 1700 m of relief subparallel to the Lomonosov Ridge. This feature, formerly called Marvin Spur, is here interpreted as the Makarov-facing flank of the Alpha Ridge. East of the 160¿ meridian the intervening Makarov Basin is a narrow grabenlike trough between 25 and 60 km wide filled with horizontally stratified sediments that were deposited largely after the Marvin Seamounts were constructed along the basin axis. The association of basement relief with geomagnetic field variations within the trough plus evidence of normal faulting on the Lomonosov Ridge suggest that this segment of the Markarov Basin is an extensional feature produced by other than typical seafloor-spreading processes. Gravity models indicate that the crust below the Makarov flank of the Alpha Ridge ranges in thickness between 16 and 20 km, considerably less than the 26 kkm known from seismic refraction work to exist below the Lomonosov Ridge. It is argued, on the basis of gravity data, that adjacent to the Lomonosov Ridge the Fram Basin crust, which is probably truly oceanic, is 3--4 km thinner than that of the Makarov Basin.